Personal Account DOI: 10.1002/tcr.201402060
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6,6-Dicyanopentafulvenes: Teaching an Old Dog New Tricks Aaron D. Finke[a] and François Diederich*[a] Laboratory of Organic Chemistry, ETH Zurich, Vladimir-Prelog-Weg 3, 8093 Zurich (Switzerland), E-mail: [email protected], Fax: (+41) 44-632-1109
[a]
Received: July 8, 2014 Published online: ■■
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ABSTRACT: 6,6-Dicyanopentafulvene (DCF) is a fascinating molecular entity that consists of a cyclopentadiene ring conjugated to an exocyclic double bond bearing two cyano groups on its periphery. Herein, we give a brief history of the chemistry of DCFs prior to our arrival to the field in 2011, followed by a summary of our work. We show how substitution on the ring and the exocyclic bond affects the HOMO and LUMO energies of pentafulvenes and how the design of DCFs was exploited computationally for the first time. Shortly after the report of the first rational synthesis of DCFs, we discovered that DCFs had a vast and astonishing array of reactivities to form new molecular entities. Simple, catalyst-free reactions between DCF acceptors and electron-rich donors led to the formation of scaffolds of exceptional complexity. Furthermore, our discovery that DCFs are capable of undergoing mild pentafulvene-to-benzene rearrangements challenges previous conventions of fulvene chemistry. Keywords: chromophores, dicyanopentafulvenes, donor–acceptor systems, radical ions, rearrangement
1. Introduction Pentafulvene (C6H6) is a nonvalence isomer of benzene that displays a rich and fascinating diversity of chemistry. However, after the heydays of pentafulvene chemistry between 1960 and 1980, the scaffold was largely ignored.[1] Recently, there has been a small but vigorous resurgence in interest in pentafulvene chemistry, as the application of sophisticated computational techniques unraveled the electronic properties of pentafulvenes and their potential applicability as functional molecular materials in devices. Particular to their use in electronic devices, it was found that the HOMO–LUMO gap of pentafulvenes was much more susceptible to tuning by chemical substitution than the gap in aromatic compounds. In recent years, we have embarked on a research program to develop new organic donor–acceptor chromophores and explore their optoelectronic properties that result from intramolecular charge-transfer (CT) interactions.[2] We found it appealing to electronically tune pentafulvenes towards strong electron-accepting properties because there is a dearth of organic acceptors relative to the number of their donor counterparts. At the same time, we were looking for new modes of reactivity and ultimately access to new materials unobtainable by other methods. Pentafulvene-based compounds are one of the most surprising and richly varied scaffolds we have studied. Substitution with strong electron acceptors at the exocyclic 6-position with, for example, cyano groups, leads to a dramatic lowering of the LUMO energy, while having less of an effect on the HOMO.[3] Such 6,6-dicyanopentafulvenes (DCFs) have had a recent resurgence in interest for that reason: this property makes them excellent electron acceptors. The frontier orbitals of pentafulvene are at the origin of its remarkable properties. A depiction of the orbitals of unsubstituted DCF (dipole moment 0.60 D) is shown in Figure 1.[3] The unsubstituted pentafulvene moiety displays a dipole moment of 0.44 D due to its nonbenzenoid connectivity.[1c] The positive charge resides on C(6), while the negative charge
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Fig. 1. a) The structure of DCF with the pentafulvene numbering scheme. b) HOMO and c) LUMO of DCF (B3LYP/TZVP).[3]
is dispersed throughout the cyclopentadiene ring. It is clear from the orbital depiction where this dipole moment originates; there is no HOMO isosurface on C(6), while the LUMO is spread throughout. We have calculated the effects of substitution at C(6). Substitution with cyano groups leads to a dramatic lowering of the LUMO energy by 2 eV, with a lowering of the HOMO by only about 1 eV. Unsurprisingly, substitution on the cyclopentadiene ring primarily affects the HOMO. Therefore, we can tune the HOMO–LUMO gap of DCFs by substitution at the ring.[3,4] 1.1. The History of DCFs Before 2011 The first preparation of a DCF was reported in 1974, wherein a dicyanovinyl–molybdenum complex reacted with two equivalents of diphenylacetylene to give a green solid, which turned out to be tetraphenyl-DCF 1 (Figure 2).[5] However, this is the only example, to date, for the preparation of a DCF starting from a metal complex, and the synthetic potential of such methods remains unexplored, despite the well-known analogous preparation of cyclopentadienones from alkynes and metal carbonyls (e.g., in the formation of the tetraphenylcyclopentadienone–Ru dimer by Shvo et al.).[6]
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Fig. 2. The first reported preparation of stable DCF 1.[5]
Fig. 3. The preparation of 1 by Lehnert’s TiCl4/pyridine Knoevenagel condensation conditions. Given are the electrochemical data, recorded in CH2Cl2 containing 0.1 m nBu4PF6 versus the ferrocene couple (Fc/Fc+), and the maximum of the longest-wavelength band, λmax, in CH2Cl2.[12]
In the mid-1980s, Junek et al. reported that, surprisingly, 1,2,3,4-tetrachlorocyclopentadiene reacted with tetracyanoethylene (TCNE) through a metathesis-type transformation to form 1,2,3,4-tetrachloro-DCF 2 (not shown) and malononitrile.[7] However, in our hands this reaction was sadly found not to be general for cyclopentadienes. Later, they investigated the reactivity of 2 with anilines to generate push–pull dyes.[8] Katritzky et al. reported the preparation of DCF-type dyes through various chemistries, but stressed that the preparation of 1 by Knoevenagel condensation of tetrap-
henylcyclopentadienone and malononitrile failed.[9] This appeared to be the death knell for DCF chemistry, and for nearly 20 years no reports on DCFs were published. After synthetic chemists abandoned DCFs, computational chemists picked up interest in them in the early 2000s. In 2006, Michl and co-workers proposed that certain benzo-fused DCF derivatives might have acted as efficient singlet fission materials in organic solar cells, leading to charge-separated states; however, the proposed compounds have yet to be prepared.[10] Ottosson et al. have led the field in the computational
Aaron Finke (born 1984) grew up in Las Vegas, NV, and studied chemistry at the University of Arizona (2002–2006). He received his PhD in chemistry (2006– 2011) from the University of Illinois, Urbana-Champaign, working in the laboratory of Prof. Jeffrey S. Moore. Afterwards, he moved to the laboratory of Prof. François Diederich as a NSFIRFP Postdoctoral Fellow (2011–2014). He is currently a Postdoctoral Fellow at the Swiss Light Source, Paul Scherrer Institute. His research interests include the physical organic chemistry of conjugated molecules and X-ray crystallography.
François Diederich was born in the Grand-Duchy of Luxemburg (1952) and studied chemistry at the University of Heidelberg (1971–1977). He joined Prof. H. A. Staab at the Max-PlanckInstitut für Medizinische Forschung in Heidelberg for his doctoral dissertation (1977–1979). After postdoctoral studies with Prof. O. L. Chapman at UCLA (1979–1981), he returned to Heidelberg for his Habilitation (1981–1985). Subsequently, he joined the Faculty in the Department of Chemistry and Biochemistry at UCLA, where he became Full Professor in 1989. Since 1992, he has been Professor in the Laboratory of Organic Chemistry in the Department of Chemistry and Applied Biosciences at ETH Zurich.
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Fig. 4. Preparation of push–pull DCF 4 by oxidative ring-closure of a bis-donor-substituted pentacyanohexatriene 3E/Z with ceric ammonium nitrate[15] or wet silica gel.[16] Given are the electrochemical data, recorded in CH2Cl2 containing 0.1 m nBu4PF6 versus Fc/Fc+, and the maximum of the longest-wavelength band, λmax, in CH2Cl2.[16]
Fig. 5. Preparation of tetraalkynyl-DCF 5 via the corresponding cyclopentadienone.[18,19] Given are the electrochemical data, recorded in CH2Cl2 containing 0.1 m nBu4PF6 versus Fc/Fc+, and the maximum of the longest-wavelength band, λmax, in CH2Cl2.
synthesis of DCFs that overcame the restrictions reported by Katritzky et al. They found that, even though tetraphenylcyclopentadienone was unreactive under classical Knoevenagel conditions, the use of Lehnert’s TiCl4/pyridine conditions[13] led to the formation of tetraphenyl-DCF 1 in 65% yield (Figure 3). We found these conditions were general for the preparation of a number of DCFs. The Swager group also demonstrated that, even if the parent cyclopentadienone was susceptible to dimerization, the dimer could be “cracked” at elevated temperatures after Knoevenagel condensation with malononitrile. Thus, DCFs tend to be more resistant to dimerization than their cyclopentadienone counterparts. In addition, they showed that DCFs displayed two one-electron reductions in CH2Cl2 (Ered,1 = −0.94 V and Ered,2 = −1.40 V vs. Fc/Fc+) at potentials more positive than that of the parent cyclopentadienones in MeCN (Ered,tetraphenylcyclopentadienone = −1.52 V vs. Fc/Fc+).[12,14] Fig. 6. EPR spectra of the radical anions of a) DCF 1 and b) DCF 5 in THF (296 K).[3]
chemistry of fulvenes and published extensively on the excitedand triplet-state aromaticity of certain fulvene derivatives.[11] In 2010, they proposed that suitably substituted DCFs could exhibit electron affinities greater than that of the linchpin acceptor 7,7,8,8-tetracyano-p-quinodimethane (TCNQ).[4] Clearly, pentafulvenes and DCFs, in particular, still had much to offer to those who were taking on their synthesis and study. That same year, synthetic chemists returned to DCFs. Swager and co-workers,[12] at last, reported a general
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2. Push–Pull Substituted and Perethynylated DCFs At this point, we entered the fray and discovered unexpectedly that a bis-(N,N-dimethylanilino)-substituted pentacyanohexatriene 3E/Z, obtained by the [2+2] cycloaddition– retroelectrocyclization reaction (CA-RE; see below)[2] between 4-ethynyl-N,N-dimethylaniline and N,N-dimethylanilino (DMA)-substituted 1,1,2,4,4-pentacyanobuta-1,3-diene,
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Fig. 7. The formal [2+2] CA-RE of electron-rich alkynes and the electron-poor olefins TCNE (top) and TCNQ (bottom).[2]
Fig. 8. Divergence in regioselectivity in the CA-RE reaction with DCFs.[3] Given are the electrochemical data, recorded in CH2Cl2 containing 0.1 m nBu4PF6 versus Fc/Fc+, and the maximum of the longest-wavelength band, λmax, in CH2Cl2.
underwent oxidative cyclization with (NH4)2Ce(NO3)6 to form push–pull-substituted DCF 4, liberating HCN in the process (Figure 4).[15] Later, we optimized the conditions for the formation of 4 using silica gel in CH2Cl2/H2O.[16] DCF 4 has some remarkable properties, including a strong, (1.59 eV; low-energy CT band (λmax = 782 nm ε = 27500 m−1 cm−1) that extends well into the near-IR (1100 nm), and electrochemical reductions (Ered = −0.45 V and −0.96 V vs. Fc/Fc+ in CH2Cl2) at potentials more positive than those of tetraphenyl-DCF 1.[15] Shoji et al. prepared a push–pull azulenyl DCF similar to 4, but as a byproduct of the formation of an
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octacyano[4]dendralene through a CA-RE reaction of a bis(azulenyl)diyne with TCNE.[17] While the optoelectronic properties of 4 were outstanding, the mechanism of its formation did not appear to be general and we wished to pursue a more rational design of a class of strongly accepting DCFs. Inspired by the work of the groups of Rubin[18] and Tobe[19] in the mid-1990s, we knew that tetraalkynylcyclopentadienones were stable and easy to prepare and handle, providing that the alkynes were terminally substituted with a bulky group, such as tert-butyl or triisopropylsilyl (TIPS). We prepared silyl-protected tetraalkynyl-DCF 5 in the hopes that it would also behave as a strong acceptor, and it
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Fig. 9. Two CA-RE reaction mechanisms leading to the divergence in regioselectivity of DCFs 9 and 1. The RDS was assumed to be the initial attack of 6 on the DCF.[3]
does: 5 displays two one-electron reductions at potentials intermediate to those of 1 and 4 (Ered = −0.57 V and −1.13 V vs, Fc/Fc+, CH2Cl2; Figure 5).[3] It was at this time that we decided to look deeper into the reactivity of DCFs.
Spin localization, and thus, charge stabilization, in 1 and 4 are quite different, and this not only explains the approximate 500 mV difference in their reduction potentials, but also their divergence in reactivity, as described below.
3. Radical Anions of DCFs
4. CA-RE Reactivity of DCFs
Because DCFs are good electron acceptors that have stable and reversible reductions, we decided to study the electron paramagnetic resonance (EPR) properties of the radical anions of 1 and 5.[3] Treatment of a solution of the DCF with sodium mirror in a closed system (THF, 294 K) generated the radical anion, the EPR spectrum of which could be measured. The EPR spectra of 1•– and 5•– are indeed staggeringly different (Figure 6). Tetraalkynyl-DCF 5•– has an EPR spectrum with distinct hyperfine coupling, arising from the cyano nitrogen atoms, and hyperfine coupling of weaker intensity from each silicon atom; thus, the spin of the radical anions is located in these areas of the molecule. The EPR spectrum of tetraphenyl-DCF 1•– is not resolved, but application of the electron nuclear double resonance (ENDOR) technique revealed hyperfine coupling arising from the protons of the phenyl groups.[3] However, unlike in the case of 5•–, the four phenyl groups do not localize spin equally. We know the orientations of the phenyl groups from X-ray crystallographic studies: the phenyl groups attached to C(2) and C(5) are oriented almost perpendicularly to the plane of the DCF, while the phenyl groups at C(3) and C(4) of the DCF are oriented about 50° relative to the DCF;[12] therefore, the last two phenyl groups display more spin localization.
Since 2005, we focused much of our research on the synthesis, reactivity, and optoelectronic properties of nonplanar push– pull chromophores.[2] Key to this program has been the development of the formal [2+2] CA-RE reaction between electronrich alkynes and electron-poor olefins (Figure 7).[17,20] Most of our studies have focused on the reactions of the electron-poor olefins TCNE and TCNQ, but we have demonstrated that less electron-deficient olefins, such as dicyanovinylarenes[21] and cyanoimines,[22] are also competent in this reaction. We presupposed that DCFs would also participate as acceptors in the CA-RE reaction, and they do, but the reality was far from what we expected. Tetralkynyl-DCF 5 was reacted with electron-rich alkyne 6 to give CA-RE adduct 7 in good yield.[3] Owing to the steric bulk and the more negative reduction potential of 5 relative to TCNE and TCNQ, elevated temperatures were required for the transformation to take place. Tetraphenyl-DCF 1 also reacts with 6 to form the CA-RE adduct 8 in good yield, but with the opposite regioselectivity to that of 7 (Figure 8). This is the first example of substituent-directed regioselectivity in the CA-RE reaction. In the case of CA-RE reactions with TCNQ, the donor aniline moiety is conjugated to the quinone moiety (Figure 7); similarly, adduct 7 has the donor aniline conjugated
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Fig. 10. Three adducts formed from the reaction of push–pull-substituted DCF 4 and electron-rich alkynes 6 and 13. Given also are the electrochemical data, recorded in CH2Cl2 containing 0.1 m nBu4PF6 versus Fc/Fc+, and the maximum of the longest-wavelength band, λmax, in CH2Cl2.[16] Quinoid character = δr = (δa + δa′ + δc + δc′)/ 4 − (δb + δb′)/2.[24] For benzene, δr = 0; for fully quinoid rings, δr = 0.08–0.10 Å.[24]
to the fulvene. Both CA-RE adduct 7 and the TCNQ adduct have a “proaromatic” resonance structure, which we believe is a driving force for the observed regioselectivity. So, the question was raised: why do we not observe the formation of the “proaromatic” regioisomer from the CA-RE reaction of 1 with 6, and observe the formation of 8 instead? We tackled this problem with a combined experimental and computational approach.[3] We knew from previous mechanistic experiments that the rate-determining step (RDS) of the CA-RE reaction on sterically rather congested olefins was the nucleophilic attack of the alkyne on the olefin to yield a zwitterionic intermediate.[21] In the case of DCFs, this led to two possibilities: 1) attack at C(6) of the DCF, and 2) attack at C(1) (Figure 9). Thus, understanding the role of the stabilization of charge for each intermediate is critical to understanding the mechanism. It was clear from the EPR studies that the stabilization of electron density was quite different between 1 and 5; however, to understand the role charge stabilization had in the divergence of regioselectivity required computational investigations. Utilization of a density functional with dispersion interactions was critical, so the ωB97XD/6-31G* method[23] was used for our computational studies. The difficulty of working with heavy silicon atoms with DFT was overcome by studying tetraethynyl-DCF 9 instead of 5. The energies of the transition state (TS) in the initial nucleophilic attack of alkyne
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6 onto DCFs 1 and 9 and the intermediate zwitterion were calculated. The computational results corroborated our empirical findings; for tetraalkynyl-DCF 9, Pathway 1 was lower in energy than Pathway 2 for both TS (ΔΔG‡ = 1.3 kcal mol−1) and intermediate formation (ΔΔG = 3.4 kcal mol−1), whereas for tetraphenyl-DCF 1, Pathway 2 was lower in energy than Pathway 1 for both TS (ΔΔG‡ = 4.0 kcal mol−1) and intermediate formation (ΔΔG = 16.9 kcal mol−1) (Figure 9). The results for tetraphenyl-DCF 1 are particularly striking. Although we assumed that the formation of an aromatic, zwitterionic intermediate would lead to a lower-energy species, this was emphatically not the case. As it turns out, such a scenario is ruled out by both steric and electronic considerations, and thus, the “unexpected” regioisomer is formed instead.[3] The unusual results obtained from the CA-RE reactivity of symmetrical DCFs were nothing compared with the CA-RE reactivity of donor–acceptor-substituted DCF 4.[16] This DCF, when reacted with alkyne 6, gave three adducts: CA-RE adduct Z-10, heptafulvene 11, and cyclobutenefusedtetrahydropentalene 12 formed from the reaction with two equivalents of 6 (Figure 10). The staggeringly high introduction of complexity into each scaffold is unprecedented for DCFs. The ratio of formation of each product is sensitive to solvent polarity and stoichiometry (Table 1); the use of more polar MeCN leads to the formation of a higher proportion of CA-RE adduct Z-10 than
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Table 1. Conditions for CA-RE reaction of 4 with 6.
Entry 1 2 3
Conditions
6 [equiv]
CH2Cl2, 20 °C, 60 h CH2Cl2, 20 °C, 24 h CH3CN, 20 °C, 60 h
1 2 1
Ratio [%][a] Z-10
11
12
34 34 32 33 (29) 26 (23) 41 (36) 57 24 19
[a] Ratios determined by HPLC and 1H NMR spectroscopy; isolated yields are given in parentheses.
when CH2Cl2 is used. The use of excess alkyne 6 leads to more tetrahydropentalene 12; however, the mechanism of formation of 12 is distinct from those of Z-10 and 11, which means the monoadducts are not an intermediate for the formation of 12. The proposed mechanisms of 11 and 12 are shown in Figure 11. While the properties of the novel adducts 11 and 12 are unspectacular, those of the CA-RE adduct Z-10 are outstanding. Its absorption spectrum is dispersed throughout the entire visible region and into the near-IR (λmax = 765 nm, λend = 1150 nm (1.59 eV); ε = 15800 m−1 cm−1) due to its strong intramolecular CT band. In addition, adduct Z-10 also displays interesting electrochemical behavior: fairly anodically shifted reduction potentials for a push–pull chromophore (Ered = −0.67 V and −0.89 V vs. Fc/Fc+ in CH2Cl2), and a very low first oxidation potential ((Eox,1 = +0.27 V vs. Fc/Fc+ in CH2Cl2); this suggests a significant change in geometry between neutral 10 and its radical cation. The crystal structure of Z-10 shows the strong quinoidal character[24] of the DMA ring attached to C(6) of the fulvene (δr = 0.05 Å; Figure 10), as well as a very long exocyclic fulvene C=C bond (1.41 Å). This indicates that there is strong conjugation between DMA and the fulvene rings; however, although the long exocyclic C=C bond may suggest isomerization behavior, solution and solidstate studies show that the Z isomer of 10 predominates. By contrast, adduct 14, the exclusive product from the reaction of DCF 4 and alkyne 13, has dynamic E/Z isomerization behavior in solution, with a pronounced solvent dependence (E/Z ratio of 3:1 in CD2Cl2 and 1:1 in CD3CN), but reverts exclusively to the E isomer in the solid state.[16]
5. Polar DCF-to-Benzene Rearrangements Unsubstituted pentafulvene, C6H6, is a nonvalence isomer of benzene; in other words, it cannot rearrange to form benzene through pericyclic reactions. The fulvene-to-benzene rearrangement occurs via diradicaloid intermediates formed from flash vacuum pyrolysis (T > 500 °C)[25] or irradiation.[26] Little was known about the rearrangement chemistry of substituted fulvenes, aside from methyl derivatives.
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Fig. 11. Mechanisms for the formation of 11 and 12.[16]
Despite this, it turns out that many DCFs rearrange spontaneously to their benzene isomers under very mild conditions. We first discovered that push–pull DCF 4 underwent rearrangement to tetracyanobenzene (TCB) 15 in excellent yield through the treatment of 4 with wet silica gel in MeCN.[16] We
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Fig. 12. Proposed mechanism of the SiO2/MeCN-promoted rearrangement of DCF 4 to give TCB 15.[16]
Table 2. Rearrangement of tetraphenyl-DCFs to DCBs.[27]
R
σp[a]
Ered [V][b]
Reaction time
Ratio 1,3:1,4
CN (17) Br (18) H (1) Me (19) OMe (20)
0.66 0.23 0 -0.17 -0.27
−0.57, −1.07 −0.79, −1.07 −0.94, −1.40 −1.01, −1.49 −1.03, −1.48
10 min 1h 6h 8h 16–21 h
82:18 99:1 90:10 95:5 75:25
Entry 1 2 3 4 5 [a]
Hammett sigma parameter for para-substituted benzenes. Values taken from Ref. [28]. versus Fc/Fc+.[27]
believe that MeCN acts as an electrophile in the course of the rearrangement; the postulated mechanism is shown in Figure 12. The rearrangement conditions for 4 seem to be unique because we have yet to find another DCF that undergoes rearrangement under these conditions.
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[b]
Reduction potentials of the parent DCFs in CH2Cl2 + 0.1 M nBu4PF6
However, many other DCFs can undergo rearrangement to dicyanobenzenes (DCBs) under mild conditions without any additional reagents. We found that simply heating tetraphenyl-DCFs, such as 1, in a dipolar aprotic solvent led to the quantitative formation of 1,3-DCB 16a and 1,4-DCB
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16b.[27] The choice of solvent is critical; dipolar aprotic solvents, such as dimethylformamide (DMF), dimethylsulfoxide (DMSO), N,N-dimethylacetamide, and N-methylpyrrolidine (NMP), promote rearrangement, whereas less polar solvents, such as MeCN, CH2Cl2, pyridine, and acetic acid, are ineffec-
Fig. 13. Rearrangement of DCF 21.[27]
tive. Pendant groups on the aryl rings also affect the rearrangement (Table 2); electron-donating groups, such as methyl (19) and methoxy (20), lead to slower rates of rearrangement, whereas electron-accepting groups, such as bromo (18) and cyano (17), lead to enhanced rates. In addition, acenaphthylene-fused DCF 21 also undergoes quantitative rearrangement to 1,3-DCB 22 after 1 h at 160 °C in DMF; no 1,4-DCB isomer is detected (Figure 13). There is also a strong effect of additives: the addition of one equivalent KCN/[18]crown-6 in DMF leads to the rearrangement of DCF 1 to give DCBs 16a/b in a matter of minutes at 160 °C; 13C enrichment of cyano groups was observed when 13C-labeled KCN was used; this indicated replacement of the cyano groups. Indeed, the release of cyanide
Fig. 14. The ring-walk mechanism for the DCF-to-DCB rearrangement.[27]
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appears to be critical for the reaction; mixing 15N-labeled DCF 1 with unlabeled methyl-substituted 19 led to scrambling of the labeled cyano groups, as shown by 15N NMR spectroscopy. We propose a “ring-walk” mechanism that conforms to the mechanistic data we have gathered (Figure 14).
[3]
[4] [5]
6. Summary and Outlook DCFs were largely forgotten after their discovery. However, after their potential in advanced materials applications was recognized, an astonishing array of reactivity and properties associated with them was soon reported. It is clear that the work detailed herein has only scratched the surface of DCF chemistry. The electronic tunability of DCFs and their facile preparation can lead to materials of functional utility, or even scaffolds of unprecedented complexity. In addition, DCFs display a rich reactivity profile exemplified by the CA-RE reactivity of DCFs and their facile rearrangement chemistry. As we learn more about the potential of DCFs, exciting new chemistries will undoubtedly follow.
[6] [7] [8] [9] [10]
[11]
Acknowledgements We are indebted to our colleagues and collaborators who worked with us on these projects, in particular, Dr. Govindasamy Jayamurugan, who discovered the synthesis of 4 and its fascinating properties, and led us into the world of DCF chemistry; Sophie Haberland, who discovered the rearrangement of 1; and Oliver Dumele, who supported our work with DFT calculations. EPR studies on the radical anions were performed in the laboratory of Prof. Georg Gescheidt (TUGraz, Austria) by Dr. Michal Zalibera, Dr. Daria Confortin, and Dr. Pawel Cias. Electrochemistry was measured by Dr. Jean-Paul Gisselbrecht and Prof. Corinne Boudon, Université de Strasbourg, France. A.D.F. acknowledges the NSFInternational Research Fellowship Program (USA) for a fellowship. This project was supported by the ERC Advanced Grant No. 246637 (“OPTELOMAC”).
[12] [13]
[14] [15]
[16]
[17] [18] [19]
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A. D. Finke, O. Dumele, M. Zalibera, D. Confortin, P. Cias, G. Jayamurugan, J.-P. Gisselbrecht, C. Boudon, W. B. Schweizer, G. Gescheidt, F. Diederich, J. Am. Chem. Soc. 2012, 134, 18139–18146. C. Dahlstrand, K. Yamazaki, K. Kilså, H. Ottosson, J. Org. Chem. 2010, 75, 8060–8068. R. B. King, M. S. Saran, J. Chem. Soc. Chem. Commun. 1974, 851–852. Y. Shvo, D. Czarkie, Y. Rahamim, D. F. Chodosh, J. Am. Chem. Soc. 1986, 108, 7400–7402. H. Junek, G. Uray, G. Zuschnig, Liebigs Ann. Chem. 1983, 154–158. H. Junek, G. Uray, G. Zuschnig, Dyes Pigm. 1988, 9, 137– 152. A. R. Katritzky, W.-Q. Fan, D.-S. Liang, Q.-L. Li, J. Heterocycl. Chem. 1989, 26, 1541–1545. I. Paci, J. C. Johnson, X. Chen, G. Rana, D. Popovic´, D. E. David, A. J. Nozik, M. A. Ratner, J. Michl, J. Am. Chem. Soc. 2006, 128, 16546–16553. a) H. Möllerstedt, M. C. Piqueras, R. Crespo, H. Ottosson, J. Am. Chem. Soc. 2004, 126, 13938–13939; b) H. Ottosson, K. Kilså, K. Chajara, M. C. Piqueras, R. Crespo, H. Kato, D. Muthas, Chem. Eur. J. 2007, 13, 6998–7005; c) M. Rosenberg, C. Dahlstrand, K. Kilså, H. Ottosson, Chem. Rev. 2014, 114, 5379–5425. T. L. Andrew, J. R. Cox, T. M. Swager, Org. Lett. 2010, 12, 5302–5305. a) W. Lehnert, Tetrahedron Lett. 1970, 11, 4723–4724; b) W. Lehnert, Tetrahedron 1972, 28, 663–666; c) W. Lehnert, Tetrahedron 1973, 29, 635–638. M. A. Fox, K. Campbell, G. Maier, L. H. Franz, J. Org. Chem. 1983, 48, 1762–1765. G. Jayamurugan, J.-P. Gisselbrecht, C. Boudon, F. Schoenebeck, W. B. Schweizer, B. Bernet, F. Diederich, Chem. Commun. 2011, 47, 4520–4522. G. Jayamurugan, O. Dumele, J.-P. Gisselbrecht, C. Boudon, W. B. Schweizer, B. Bernet, F. Diederich, J. Am. Chem. Soc. 2013, 135, 3599–3606. T. Shoji, S. Ito, T. Okujima, N. Morita, Org. Biomol. Chem. 2012, 10, 8308–8313. N. Jux, K. Holczer, Y. Rubin, Angew. Chem. Int. Ed. Engl. 1996, 35, 1986–1990. Y. Tobe, K. Kubota, K. Naemura, J. Org. Chem. 1997, 62, 3430–3431. a) T. Michinobu, J. C. May, J. H. Lim, C. Boudon, J.-P. Gisselbrecht, P. Seiler, M. Gross, I. Biaggio, F. Diederich, Chem. Commun. 2005, 737–739; b) T. Michinobu, C. Boudon, J.-P. Gisselbrecht, P. Seiler, B. Frank, N. N. P. Moonen, M. Gross, F. Diederich, Chem. Eur. J. 2006, 12, 1889–1905; c) M. I. Bruce, Austr. J. Chem. 2011, 64, 77–103; d) X. Tang, W. Liu, J. Wu, C.-S. Lee, J. You, P. Wang, J. Org. Chem. 2010, 75, 7273–7278; e) M. Morimoto, K. Murata, T. Michinobu, Chem. Commun. 2011, 47, 9819–9821. Y.-L. Wu, P. D. Jarowski, W. B. Schweizer, F. Diederich, Chem. Eur. J. 2010, 16, 202–211.
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M. Chiu, B. Jaun, M. T. R. Beels, I. Biaggio, J.-P. Gisselbrecht, C. Boudon, W. B. Schweizer, M. Kivala, F. Diederich, Org. Lett. 2011, 14, 54–57. J.-D. Chai, M. Head-Gordon, Phys. Chem. Chem. Phys. 2008, 10, 6615–6620. C. Dehu, F. Meyers, J. L. Bredas, J. Am. Chem. Soc. 1993, 115, 6198–6206. a) T. J. Henry, R. G. Bergman, J. Am. Chem. Soc. 1972, 94, 5103–5105; b) B. Hankinson, H. Heaney, A. P. Price, R. P. Sharma, J. Chem. Soc. Perkin Trans. 1973, 1, 2569–2575; c) S. Oikawa, M. Tsuda, Y. Okamura, T. Urabe, J. Am. Chem. Soc.
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6,6-Dicyanopentafulvenes: Teaching an Old Dog New Tricks Aaron D. Finke[a] and François Diederich*[a] Laboratory of Organic Chemistry, ETH Zurich, Vladimir-Prelog-Weg 3, 8093 Zurich (Switzerland), E-mail: [email protected], Fax: (+41) 44-632-1109
[a]
Received: July 8, 2014 Published online: ■■
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ABSTRACT: 6,6-Dicyanopentafulvene (DCF) is a fascinating molecular entity that consists of a cyclopentadiene ring conjugated to an exocyclic double bond bearing two cyano groups on its periphery. Herein, we give a brief history of the chemistry of DCFs prior to our arrival to the field in 2011, followed by a summary of our work. We show how substitution on the ring and the exocyclic bond affects the HOMO and LUMO energies of pentafulvenes and how the design of DCFs was exploited computationally for the first time. Shortly after the report of the first rational synthesis of DCFs, we discovered that DCFs had a vast and astonishing array of reactivities to form new molecular entities. Simple, catalyst-free reactions between DCF acceptors and electron-rich donors led to the formation of scaffolds of exceptional complexity. Furthermore, our discovery that DCFs are capable of undergoing mild pentafulvene-to-benzene rearrangements challenges previous conventions of fulvene chemistry. Keywords: chromophores, dicyanopentafulvenes, donor–acceptor systems, radical ions, rearrangement
1. Introduction Pentafulvene (C6H6) is a nonvalence isomer of benzene that displays a rich and fascinating diversity of chemistry. However, after the heydays of pentafulvene chemistry between 1960 and 1980, the scaffold was largely ignored.[1] Recently, there has been a small but vigorous resurgence in interest in pentafulvene chemistry, as the application of sophisticated computational techniques unraveled the electronic properties of pentafulvenes and their potential applicability as functional molecular materials in devices. Particular to their use in electronic devices, it was found that the HOMO–LUMO gap of pentafulvenes was much more susceptible to tuning by chemical substitution than the gap in aromatic compounds. In recent years, we have embarked on a research program to develop new organic donor–acceptor chromophores and explore their optoelectronic properties that result from intramolecular charge-transfer (CT) interactions.[2] We found it appealing to electronically tune pentafulvenes towards strong electron-accepting properties because there is a dearth of organic acceptors relative to the number of their donor counterparts. At the same time, we were looking for new modes of reactivity and ultimately access to new materials unobtainable by other methods. Pentafulvene-based compounds are one of the most surprising and richly varied scaffolds we have studied. Substitution with strong electron acceptors at the exocyclic 6-position with, for example, cyano groups, leads to a dramatic lowering of the LUMO energy, while having less of an effect on the HOMO.[3] Such 6,6-dicyanopentafulvenes (DCFs) have had a recent resurgence in interest for that reason: this property makes them excellent electron acceptors. The frontier orbitals of pentafulvene are at the origin of its remarkable properties. A depiction of the orbitals of unsubstituted DCF (dipole moment 0.60 D) is shown in Figure 1.[3] The unsubstituted pentafulvene moiety displays a dipole moment of 0.44 D due to its nonbenzenoid connectivity.[1c] The positive charge resides on C(6), while the negative charge
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Fig. 1. a) The structure of DCF with the pentafulvene numbering scheme. b) HOMO and c) LUMO of DCF (B3LYP/TZVP).[3]
is dispersed throughout the cyclopentadiene ring. It is clear from the orbital depiction where this dipole moment originates; there is no HOMO isosurface on C(6), while the LUMO is spread throughout. We have calculated the effects of substitution at C(6). Substitution with cyano groups leads to a dramatic lowering of the LUMO energy by 2 eV, with a lowering of the HOMO by only about 1 eV. Unsurprisingly, substitution on the cyclopentadiene ring primarily affects the HOMO. Therefore, we can tune the HOMO–LUMO gap of DCFs by substitution at the ring.[3,4] 1.1. The History of DCFs Before 2011 The first preparation of a DCF was reported in 1974, wherein a dicyanovinyl–molybdenum complex reacted with two equivalents of diphenylacetylene to give a green solid, which turned out to be tetraphenyl-DCF 1 (Figure 2).[5] However, this is the only example, to date, for the preparation of a DCF starting from a metal complex, and the synthetic potential of such methods remains unexplored, despite the well-known analogous preparation of cyclopentadienones from alkynes and metal carbonyls (e.g., in the formation of the tetraphenylcyclopentadienone–Ru dimer by Shvo et al.).[6]
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Fig. 2. The first reported preparation of stable DCF 1.[5]
Fig. 3. The preparation of 1 by Lehnert’s TiCl4/pyridine Knoevenagel condensation conditions. Given are the electrochemical data, recorded in CH2Cl2 containing 0.1 m nBu4PF6 versus the ferrocene couple (Fc/Fc+), and the maximum of the longest-wavelength band, λmax, in CH2Cl2.[12]
In the mid-1980s, Junek et al. reported that, surprisingly, 1,2,3,4-tetrachlorocyclopentadiene reacted with tetracyanoethylene (TCNE) through a metathesis-type transformation to form 1,2,3,4-tetrachloro-DCF 2 (not shown) and malononitrile.[7] However, in our hands this reaction was sadly found not to be general for cyclopentadienes. Later, they investigated the reactivity of 2 with anilines to generate push–pull dyes.[8] Katritzky et al. reported the preparation of DCF-type dyes through various chemistries, but stressed that the preparation of 1 by Knoevenagel condensation of tetrap-
henylcyclopentadienone and malononitrile failed.[9] This appeared to be the death knell for DCF chemistry, and for nearly 20 years no reports on DCFs were published. After synthetic chemists abandoned DCFs, computational chemists picked up interest in them in the early 2000s. In 2006, Michl and co-workers proposed that certain benzo-fused DCF derivatives might have acted as efficient singlet fission materials in organic solar cells, leading to charge-separated states; however, the proposed compounds have yet to be prepared.[10] Ottosson et al. have led the field in the computational
Aaron Finke (born 1984) grew up in Las Vegas, NV, and studied chemistry at the University of Arizona (2002–2006). He received his PhD in chemistry (2006– 2011) from the University of Illinois, Urbana-Champaign, working in the laboratory of Prof. Jeffrey S. Moore. Afterwards, he moved to the laboratory of Prof. François Diederich as a NSFIRFP Postdoctoral Fellow (2011–2014). He is currently a Postdoctoral Fellow at the Swiss Light Source, Paul Scherrer Institute. His research interests include the physical organic chemistry of conjugated molecules and X-ray crystallography.
François Diederich was born in the Grand-Duchy of Luxemburg (1952) and studied chemistry at the University of Heidelberg (1971–1977). He joined Prof. H. A. Staab at the Max-PlanckInstitut für Medizinische Forschung in Heidelberg for his doctoral dissertation (1977–1979). After postdoctoral studies with Prof. O. L. Chapman at UCLA (1979–1981), he returned to Heidelberg for his Habilitation (1981–1985). Subsequently, he joined the Faculty in the Department of Chemistry and Biochemistry at UCLA, where he became Full Professor in 1989. Since 1992, he has been Professor in the Laboratory of Organic Chemistry in the Department of Chemistry and Applied Biosciences at ETH Zurich.
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Fig. 4. Preparation of push–pull DCF 4 by oxidative ring-closure of a bis-donor-substituted pentacyanohexatriene 3E/Z with ceric ammonium nitrate[15] or wet silica gel.[16] Given are the electrochemical data, recorded in CH2Cl2 containing 0.1 m nBu4PF6 versus Fc/Fc+, and the maximum of the longest-wavelength band, λmax, in CH2Cl2.[16]
Fig. 5. Preparation of tetraalkynyl-DCF 5 via the corresponding cyclopentadienone.[18,19] Given are the electrochemical data, recorded in CH2Cl2 containing 0.1 m nBu4PF6 versus Fc/Fc+, and the maximum of the longest-wavelength band, λmax, in CH2Cl2.
synthesis of DCFs that overcame the restrictions reported by Katritzky et al. They found that, even though tetraphenylcyclopentadienone was unreactive under classical Knoevenagel conditions, the use of Lehnert’s TiCl4/pyridine conditions[13] led to the formation of tetraphenyl-DCF 1 in 65% yield (Figure 3). We found these conditions were general for the preparation of a number of DCFs. The Swager group also demonstrated that, even if the parent cyclopentadienone was susceptible to dimerization, the dimer could be “cracked” at elevated temperatures after Knoevenagel condensation with malononitrile. Thus, DCFs tend to be more resistant to dimerization than their cyclopentadienone counterparts. In addition, they showed that DCFs displayed two one-electron reductions in CH2Cl2 (Ered,1 = −0.94 V and Ered,2 = −1.40 V vs. Fc/Fc+) at potentials more positive than that of the parent cyclopentadienones in MeCN (Ered,tetraphenylcyclopentadienone = −1.52 V vs. Fc/Fc+).[12,14] Fig. 6. EPR spectra of the radical anions of a) DCF 1 and b) DCF 5 in THF (296 K).[3]
chemistry of fulvenes and published extensively on the excitedand triplet-state aromaticity of certain fulvene derivatives.[11] In 2010, they proposed that suitably substituted DCFs could exhibit electron affinities greater than that of the linchpin acceptor 7,7,8,8-tetracyano-p-quinodimethane (TCNQ).[4] Clearly, pentafulvenes and DCFs, in particular, still had much to offer to those who were taking on their synthesis and study. That same year, synthetic chemists returned to DCFs. Swager and co-workers,[12] at last, reported a general
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2. Push–Pull Substituted and Perethynylated DCFs At this point, we entered the fray and discovered unexpectedly that a bis-(N,N-dimethylanilino)-substituted pentacyanohexatriene 3E/Z, obtained by the [2+2] cycloaddition– retroelectrocyclization reaction (CA-RE; see below)[2] between 4-ethynyl-N,N-dimethylaniline and N,N-dimethylanilino (DMA)-substituted 1,1,2,4,4-pentacyanobuta-1,3-diene,
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Fig. 7. The formal [2+2] CA-RE of electron-rich alkynes and the electron-poor olefins TCNE (top) and TCNQ (bottom).[2]
Fig. 8. Divergence in regioselectivity in the CA-RE reaction with DCFs.[3] Given are the electrochemical data, recorded in CH2Cl2 containing 0.1 m nBu4PF6 versus Fc/Fc+, and the maximum of the longest-wavelength band, λmax, in CH2Cl2.
underwent oxidative cyclization with (NH4)2Ce(NO3)6 to form push–pull-substituted DCF 4, liberating HCN in the process (Figure 4).[15] Later, we optimized the conditions for the formation of 4 using silica gel in CH2Cl2/H2O.[16] DCF 4 has some remarkable properties, including a strong, (1.59 eV; low-energy CT band (λmax = 782 nm ε = 27500 m−1 cm−1) that extends well into the near-IR (1100 nm), and electrochemical reductions (Ered = −0.45 V and −0.96 V vs. Fc/Fc+ in CH2Cl2) at potentials more positive than those of tetraphenyl-DCF 1.[15] Shoji et al. prepared a push–pull azulenyl DCF similar to 4, but as a byproduct of the formation of an
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octacyano[4]dendralene through a CA-RE reaction of a bis(azulenyl)diyne with TCNE.[17] While the optoelectronic properties of 4 were outstanding, the mechanism of its formation did not appear to be general and we wished to pursue a more rational design of a class of strongly accepting DCFs. Inspired by the work of the groups of Rubin[18] and Tobe[19] in the mid-1990s, we knew that tetraalkynylcyclopentadienones were stable and easy to prepare and handle, providing that the alkynes were terminally substituted with a bulky group, such as tert-butyl or triisopropylsilyl (TIPS). We prepared silyl-protected tetraalkynyl-DCF 5 in the hopes that it would also behave as a strong acceptor, and it
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Fig. 9. Two CA-RE reaction mechanisms leading to the divergence in regioselectivity of DCFs 9 and 1. The RDS was assumed to be the initial attack of 6 on the DCF.[3]
does: 5 displays two one-electron reductions at potentials intermediate to those of 1 and 4 (Ered = −0.57 V and −1.13 V vs, Fc/Fc+, CH2Cl2; Figure 5).[3] It was at this time that we decided to look deeper into the reactivity of DCFs.
Spin localization, and thus, charge stabilization, in 1 and 4 are quite different, and this not only explains the approximate 500 mV difference in their reduction potentials, but also their divergence in reactivity, as described below.
3. Radical Anions of DCFs
4. CA-RE Reactivity of DCFs
Because DCFs are good electron acceptors that have stable and reversible reductions, we decided to study the electron paramagnetic resonance (EPR) properties of the radical anions of 1 and 5.[3] Treatment of a solution of the DCF with sodium mirror in a closed system (THF, 294 K) generated the radical anion, the EPR spectrum of which could be measured. The EPR spectra of 1•– and 5•– are indeed staggeringly different (Figure 6). Tetraalkynyl-DCF 5•– has an EPR spectrum with distinct hyperfine coupling, arising from the cyano nitrogen atoms, and hyperfine coupling of weaker intensity from each silicon atom; thus, the spin of the radical anions is located in these areas of the molecule. The EPR spectrum of tetraphenyl-DCF 1•– is not resolved, but application of the electron nuclear double resonance (ENDOR) technique revealed hyperfine coupling arising from the protons of the phenyl groups.[3] However, unlike in the case of 5•–, the four phenyl groups do not localize spin equally. We know the orientations of the phenyl groups from X-ray crystallographic studies: the phenyl groups attached to C(2) and C(5) are oriented almost perpendicularly to the plane of the DCF, while the phenyl groups at C(3) and C(4) of the DCF are oriented about 50° relative to the DCF;[12] therefore, the last two phenyl groups display more spin localization.
Since 2005, we focused much of our research on the synthesis, reactivity, and optoelectronic properties of nonplanar push– pull chromophores.[2] Key to this program has been the development of the formal [2+2] CA-RE reaction between electronrich alkynes and electron-poor olefins (Figure 7).[17,20] Most of our studies have focused on the reactions of the electron-poor olefins TCNE and TCNQ, but we have demonstrated that less electron-deficient olefins, such as dicyanovinylarenes[21] and cyanoimines,[22] are also competent in this reaction. We presupposed that DCFs would also participate as acceptors in the CA-RE reaction, and they do, but the reality was far from what we expected. Tetralkynyl-DCF 5 was reacted with electron-rich alkyne 6 to give CA-RE adduct 7 in good yield.[3] Owing to the steric bulk and the more negative reduction potential of 5 relative to TCNE and TCNQ, elevated temperatures were required for the transformation to take place. Tetraphenyl-DCF 1 also reacts with 6 to form the CA-RE adduct 8 in good yield, but with the opposite regioselectivity to that of 7 (Figure 8). This is the first example of substituent-directed regioselectivity in the CA-RE reaction. In the case of CA-RE reactions with TCNQ, the donor aniline moiety is conjugated to the quinone moiety (Figure 7); similarly, adduct 7 has the donor aniline conjugated
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Fig. 10. Three adducts formed from the reaction of push–pull-substituted DCF 4 and electron-rich alkynes 6 and 13. Given also are the electrochemical data, recorded in CH2Cl2 containing 0.1 m nBu4PF6 versus Fc/Fc+, and the maximum of the longest-wavelength band, λmax, in CH2Cl2.[16] Quinoid character = δr = (δa + δa′ + δc + δc′)/ 4 − (δb + δb′)/2.[24] For benzene, δr = 0; for fully quinoid rings, δr = 0.08–0.10 Å.[24]
to the fulvene. Both CA-RE adduct 7 and the TCNQ adduct have a “proaromatic” resonance structure, which we believe is a driving force for the observed regioselectivity. So, the question was raised: why do we not observe the formation of the “proaromatic” regioisomer from the CA-RE reaction of 1 with 6, and observe the formation of 8 instead? We tackled this problem with a combined experimental and computational approach.[3] We knew from previous mechanistic experiments that the rate-determining step (RDS) of the CA-RE reaction on sterically rather congested olefins was the nucleophilic attack of the alkyne on the olefin to yield a zwitterionic intermediate.[21] In the case of DCFs, this led to two possibilities: 1) attack at C(6) of the DCF, and 2) attack at C(1) (Figure 9). Thus, understanding the role of the stabilization of charge for each intermediate is critical to understanding the mechanism. It was clear from the EPR studies that the stabilization of electron density was quite different between 1 and 5; however, to understand the role charge stabilization had in the divergence of regioselectivity required computational investigations. Utilization of a density functional with dispersion interactions was critical, so the ωB97XD/6-31G* method[23] was used for our computational studies. The difficulty of working with heavy silicon atoms with DFT was overcome by studying tetraethynyl-DCF 9 instead of 5. The energies of the transition state (TS) in the initial nucleophilic attack of alkyne
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6 onto DCFs 1 and 9 and the intermediate zwitterion were calculated. The computational results corroborated our empirical findings; for tetraalkynyl-DCF 9, Pathway 1 was lower in energy than Pathway 2 for both TS (ΔΔG‡ = 1.3 kcal mol−1) and intermediate formation (ΔΔG = 3.4 kcal mol−1), whereas for tetraphenyl-DCF 1, Pathway 2 was lower in energy than Pathway 1 for both TS (ΔΔG‡ = 4.0 kcal mol−1) and intermediate formation (ΔΔG = 16.9 kcal mol−1) (Figure 9). The results for tetraphenyl-DCF 1 are particularly striking. Although we assumed that the formation of an aromatic, zwitterionic intermediate would lead to a lower-energy species, this was emphatically not the case. As it turns out, such a scenario is ruled out by both steric and electronic considerations, and thus, the “unexpected” regioisomer is formed instead.[3] The unusual results obtained from the CA-RE reactivity of symmetrical DCFs were nothing compared with the CA-RE reactivity of donor–acceptor-substituted DCF 4.[16] This DCF, when reacted with alkyne 6, gave three adducts: CA-RE adduct Z-10, heptafulvene 11, and cyclobutenefusedtetrahydropentalene 12 formed from the reaction with two equivalents of 6 (Figure 10). The staggeringly high introduction of complexity into each scaffold is unprecedented for DCFs. The ratio of formation of each product is sensitive to solvent polarity and stoichiometry (Table 1); the use of more polar MeCN leads to the formation of a higher proportion of CA-RE adduct Z-10 than
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Table 1. Conditions for CA-RE reaction of 4 with 6.
Entry 1 2 3
Conditions
6 [equiv]
CH2Cl2, 20 °C, 60 h CH2Cl2, 20 °C, 24 h CH3CN, 20 °C, 60 h
1 2 1
Ratio [%][a] Z-10
11
12
34 34 32 33 (29) 26 (23) 41 (36) 57 24 19
[a] Ratios determined by HPLC and 1H NMR spectroscopy; isolated yields are given in parentheses.
when CH2Cl2 is used. The use of excess alkyne 6 leads to more tetrahydropentalene 12; however, the mechanism of formation of 12 is distinct from those of Z-10 and 11, which means the monoadducts are not an intermediate for the formation of 12. The proposed mechanisms of 11 and 12 are shown in Figure 11. While the properties of the novel adducts 11 and 12 are unspectacular, those of the CA-RE adduct Z-10 are outstanding. Its absorption spectrum is dispersed throughout the entire visible region and into the near-IR (λmax = 765 nm, λend = 1150 nm (1.59 eV); ε = 15800 m−1 cm−1) due to its strong intramolecular CT band. In addition, adduct Z-10 also displays interesting electrochemical behavior: fairly anodically shifted reduction potentials for a push–pull chromophore (Ered = −0.67 V and −0.89 V vs. Fc/Fc+ in CH2Cl2), and a very low first oxidation potential ((Eox,1 = +0.27 V vs. Fc/Fc+ in CH2Cl2); this suggests a significant change in geometry between neutral 10 and its radical cation. The crystal structure of Z-10 shows the strong quinoidal character[24] of the DMA ring attached to C(6) of the fulvene (δr = 0.05 Å; Figure 10), as well as a very long exocyclic fulvene C=C bond (1.41 Å). This indicates that there is strong conjugation between DMA and the fulvene rings; however, although the long exocyclic C=C bond may suggest isomerization behavior, solution and solidstate studies show that the Z isomer of 10 predominates. By contrast, adduct 14, the exclusive product from the reaction of DCF 4 and alkyne 13, has dynamic E/Z isomerization behavior in solution, with a pronounced solvent dependence (E/Z ratio of 3:1 in CD2Cl2 and 1:1 in CD3CN), but reverts exclusively to the E isomer in the solid state.[16]
5. Polar DCF-to-Benzene Rearrangements Unsubstituted pentafulvene, C6H6, is a nonvalence isomer of benzene; in other words, it cannot rearrange to form benzene through pericyclic reactions. The fulvene-to-benzene rearrangement occurs via diradicaloid intermediates formed from flash vacuum pyrolysis (T > 500 °C)[25] or irradiation.[26] Little was known about the rearrangement chemistry of substituted fulvenes, aside from methyl derivatives.
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Fig. 11. Mechanisms for the formation of 11 and 12.[16]
Despite this, it turns out that many DCFs rearrange spontaneously to their benzene isomers under very mild conditions. We first discovered that push–pull DCF 4 underwent rearrangement to tetracyanobenzene (TCB) 15 in excellent yield through the treatment of 4 with wet silica gel in MeCN.[16] We
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Fig. 12. Proposed mechanism of the SiO2/MeCN-promoted rearrangement of DCF 4 to give TCB 15.[16]
Table 2. Rearrangement of tetraphenyl-DCFs to DCBs.[27]
R
σp[a]
Ered [V][b]
Reaction time
Ratio 1,3:1,4
CN (17) Br (18) H (1) Me (19) OMe (20)
0.66 0.23 0 -0.17 -0.27
−0.57, −1.07 −0.79, −1.07 −0.94, −1.40 −1.01, −1.49 −1.03, −1.48
10 min 1h 6h 8h 16–21 h
82:18 99:1 90:10 95:5 75:25
Entry 1 2 3 4 5 [a]
Hammett sigma parameter for para-substituted benzenes. Values taken from Ref. [28]. versus Fc/Fc+.[27]
believe that MeCN acts as an electrophile in the course of the rearrangement; the postulated mechanism is shown in Figure 12. The rearrangement conditions for 4 seem to be unique because we have yet to find another DCF that undergoes rearrangement under these conditions.
Chem. Rec. 2014, ••, ••–••
[b]
Reduction potentials of the parent DCFs in CH2Cl2 + 0.1 M nBu4PF6
However, many other DCFs can undergo rearrangement to dicyanobenzenes (DCBs) under mild conditions without any additional reagents. We found that simply heating tetraphenyl-DCFs, such as 1, in a dipolar aprotic solvent led to the quantitative formation of 1,3-DCB 16a and 1,4-DCB
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16b.[27] The choice of solvent is critical; dipolar aprotic solvents, such as dimethylformamide (DMF), dimethylsulfoxide (DMSO), N,N-dimethylacetamide, and N-methylpyrrolidine (NMP), promote rearrangement, whereas less polar solvents, such as MeCN, CH2Cl2, pyridine, and acetic acid, are ineffec-
Fig. 13. Rearrangement of DCF 21.[27]
tive. Pendant groups on the aryl rings also affect the rearrangement (Table 2); electron-donating groups, such as methyl (19) and methoxy (20), lead to slower rates of rearrangement, whereas electron-accepting groups, such as bromo (18) and cyano (17), lead to enhanced rates. In addition, acenaphthylene-fused DCF 21 also undergoes quantitative rearrangement to 1,3-DCB 22 after 1 h at 160 °C in DMF; no 1,4-DCB isomer is detected (Figure 13). There is also a strong effect of additives: the addition of one equivalent KCN/[18]crown-6 in DMF leads to the rearrangement of DCF 1 to give DCBs 16a/b in a matter of minutes at 160 °C; 13C enrichment of cyano groups was observed when 13C-labeled KCN was used; this indicated replacement of the cyano groups. Indeed, the release of cyanide
Fig. 14. The ring-walk mechanism for the DCF-to-DCB rearrangement.[27]
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appears to be critical for the reaction; mixing 15N-labeled DCF 1 with unlabeled methyl-substituted 19 led to scrambling of the labeled cyano groups, as shown by 15N NMR spectroscopy. We propose a “ring-walk” mechanism that conforms to the mechanistic data we have gathered (Figure 14).
[3]
[4] [5]
6. Summary and Outlook DCFs were largely forgotten after their discovery. However, after their potential in advanced materials applications was recognized, an astonishing array of reactivity and properties associated with them was soon reported. It is clear that the work detailed herein has only scratched the surface of DCF chemistry. The electronic tunability of DCFs and their facile preparation can lead to materials of functional utility, or even scaffolds of unprecedented complexity. In addition, DCFs display a rich reactivity profile exemplified by the CA-RE reactivity of DCFs and their facile rearrangement chemistry. As we learn more about the potential of DCFs, exciting new chemistries will undoubtedly follow.
[6] [7] [8] [9] [10]
[11]
Acknowledgements We are indebted to our colleagues and collaborators who worked with us on these projects, in particular, Dr. Govindasamy Jayamurugan, who discovered the synthesis of 4 and its fascinating properties, and led us into the world of DCF chemistry; Sophie Haberland, who discovered the rearrangement of 1; and Oliver Dumele, who supported our work with DFT calculations. EPR studies on the radical anions were performed in the laboratory of Prof. Georg Gescheidt (TUGraz, Austria) by Dr. Michal Zalibera, Dr. Daria Confortin, and Dr. Pawel Cias. Electrochemistry was measured by Dr. Jean-Paul Gisselbrecht and Prof. Corinne Boudon, Université de Strasbourg, France. A.D.F. acknowledges the NSFInternational Research Fellowship Program (USA) for a fellowship. This project was supported by the ERC Advanced Grant No. 246637 (“OPTELOMAC”).
[12] [13]
[14] [15]
[16]
[17] [18] [19]
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