DOI: 10.1002/chem.201302544

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& Siloles

Electrocyclic Reactions of Siloles: A Combined Experimental and Theoretical Study Frank Meyer-Wegner, Josef H. Wender, Konstantin Falahati, Timo Porsch, Tanja Sinke, Michael Bolte, Matthias Wagner, Max C. Holthausen,* and Hans-Wolfram Lerner*[a] Dedicated to Professor Werner Uhl on the occasion of his 60th birthday

Abstract: The reaction of 4-chloro-1,2-dimethyl-4-supersilylsila-1-cyclopentene (2 a) with Li[NiPr2] at 78 8C results in the formation of the formal 1,4-addition product of the silacyclopentadiene derivative 3,4-dimethyl-1-supersilylsila-1,3cyclopentadiene (4 a) with 2,3-dimethyl-4-supersilylsila-1,3cyclopentadiene (5 a). In addition the respective adducts of the Diels–Alder reactions of 4 a + 4 a and 4 a + 5 a were obtained. Compound 4 a, which displays an s-cis-silacyclopentadiene configuration, reacts with cyclohexene to form the racemate of the [4+2] cycloadduct of 4 a and cyclohexene

Introduction The reactivity of unsaturated silicon compounds has been studied extensively.[1, 2] In this context, stable examples with Si=C double bonds, for example, silenes,[2–5] silaallenes,[2, 6] and silaaromatics[2, 7] such as silabenzene or silole anions and dianions,[2, 8] have been synthesized and structurally characterized. In the 1980s, the metastable silenes (Me3Si)2Si=C(1-adamantyl)(OSiMe3)[3] and Me2Si=C(SiMe3)(SiMetBu2)[4] were described by Brook et al. and Wiberg et al. and the X-ray structure determinations of these silenes provided for the first time a clear characterization of the Si=C double bond. The structural features of Wiberg’s silene Me2Si=C(SiMe3)(SiMetBu2) agree closely with those of H2Si=CH2,[9] whereas the double bond in Brook’s silene (Me3Si)2Si=C(1-adamantyl)-

[a] F. Meyer-Wegner, J. H. Wender, K. Falahati, T. Porsch, T. Sinke, Dr. M. Bolte, Prof. Dr. M. Wagner, Prof. Dr. M. C. Holthausen, Dr. H.-W. Lerner Institut fr Anorganische und Analytische Chemie, Goethe-Universitt Max-von-Laue-Strasse 7, 60438 Frankfurt am Main (Germany) Fax: (+ 49) 69-798-29260 E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201302544. It contains synthesis of silacyclopentenes 2 b, 2 c, 3 d, 3 e, 3 g, and 3 h; detailed discussion of the structures of 11, 3 g, 3 h, 13, 14 (monoclinic P21/c), and 14 (monoclinic P21/n); NMR spectra of 9; assignment of 1H and 13C NMR signals of 10 and 11; X-ray crystallographic parameters; computational results for the thermochemistry of conceivable silole dimers; potential energy surface scans of the dimerizations and reaction with DMB; and HOMO–LUMO gaps. Chem. Eur. J. 2014, 20, 4681 – 4690

(9). In the reaction between 4 a and 2,3-dimethylbutadiene, however, 4 a acted as silene as well as silacyclopentadiene to yield the [2+4] and [4+2] cycloadducts 10 and 11, respectively. The constitutions of 9, 10, and 11 were confirmed by NMR spectroscopy and their crystal structures were determined. Reaction of 4-chloro-1,2-dimethyl-4-tert-butyl-4-silacyclopent-1-ene (2 c) with KC8 yielded the corresponding disilane (12), which was characterized by X-ray crystal structure analysis (triclinic, P1¯). DFT calculations are used to unveil the mechanistic scenario underlying the observed reactivity.

(OSiMe3) is significantly elongated. Silenes of the Brook type possess a less polar double bond and are less reactive than silenes of the Wiberg type. In keeping with this general notion, theoretical studies by Apeloig et al. have shown that the former exhibit low or even negative activation energies ( 3– 8 kcal mol 1) for the nucleophilic addition of water, whereas a larger activation barrier of 16 kcal mol 1 was found for the same reaction of a Brook-type silene.[10] The [2+2] cycloreversion of silacyclobutane derivatives at high temperatures has evolved as one of the most important methods to generate transient silenes in the gas phase.[11, 12] Other examples of thermally driven pericyclic routes to silene formation are the [4+2] cycloreversion of silabicyclo[2.2.2]octadiene derivatives or the retro-ene fragmentation of allylsilanes.[13] Further, a [1,5] sigmatropic silyl shift has been discussed as the key elementary step involved in the formation of a transient silene that has been suggested as intermediate in the thermolysis of silyl-substituted 4-silacyclopentadiene derivatives.[14] Most interesting for reactivity studies, however, are processes in which silenes are generated in solution at low temperatures. This includes 1,2-eliminations of MX (M = alkali metal; X = halogens or other good leaving groups), which occur often at temperatures below 100 8C.[2, 15] Accordingly, 1,2-eliminations of MX at low temperatures have preferably been used for reactivity studies of Wiberg-type silenes. Overall, the reactivity of Wiberg-type silenes embraces the following reaction types (Scheme 1): i) adduct formation, ii) dimerization, iii) insertion reactions, iv) [2+2] cycloadditions, v) [2+3] cycloadditions, vi) [2+4] cycloadditions, and vii) ene reactions. Further electrocyclic reactions of disilenes, such as the

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Scheme 2. Synthesis of 1,2-dimethyl-4-silacyclopentene derivatives. i) + NMe3 or NMe2Et in 2,3-dimethyl-1,3-butadiene at RT; ii) 2 a–c: + 1 equivalent of Na[SitBu3] (R = SitBu3, 2 a), Na[SiPhtBu2] (R = SiPhtBu2, 2 b), or LitBu (R = tBu, 2 c) in hexane at 78 8C; iii) for 3 e–g: + 2 equivalents of LiMe (R = Me, 3 e), LiMes (R = Mes, 3 f), or LiPh (R = Ph, 3 g) in Et2O, benzene, or Bu2O at 78 8C. Scheme 1. Reactivity of Wiberg-type silenes.[17]

[2+4] cycloaddition with 1,3-dienes, have also been reported.[16] Herein, we report the pericyclic reactions of 2,3-dimethylbutadiene (DMB) and cyclohexene with 3,4-dimethyl-1-supersilyl1-silacyclopenta-1,3-diene (4 a), which was formed quantitatively by treatment of 4-chloro-1,2-dimethyl-4-supersilyl-4-silacyclopent-1-ene (2 a) with Li[NiPr2] (LDA) at 78 8C. The structural features of trapping products of this 1-silacyclopenta-1,3diene derivative with cyclohexene and DMB have been investigated. In addition we present the structure of a novel disilane, which was obtained from the reaction of 1,2-dimethyl-4-tertbutyl-4-silacyclopent-1-ene with KC8. Furthermore, the fundamental reaction mechanisms of the experimentally observed reaction patterns were explored with DFT methods.

However, the reaction of 1 with an excess of LitBu or Na[SitBu3][21, 22] and Na[SiPhtBu2][21, 23] took another course. When 1 was treated with two molar equivalents of Na[SitBu3], no supersilyl derivative of 3 was formed but instead the NMR signals of compounds 6, 7, and 8 (Scheme 3) were found in a 1:1:1 ratio; the same products resulted from the reactions of 2 b and 2 c with two equivalents of Na[SiPhtBu2] and LitBu, respectively.

Results and Discussion Experimental studies Over the past few decades, the disproportionation reaction of Si2Cl6 leading to the perchlorinated neopentasilane Si(SiCl3)4 and SiCl4 has been the subject of numerous studies.[18] Yet, although this reaction has been known for 60 years and although several tentative mechanistic proposals have been put forward, no solid evidence for the nature of the intermediates involved has been reported. Recently, we have repeated the reaction of Si2Cl6 with NR3 (R = Me, Et) and we have been able to show that amine-complexed dichlorosilylenes represent the key intermediates in this disproportionation reaction.[19, 20] In the course of these studies we isolated the [4+1] cycloadduct of dichlorosilylene SiCl2 with DMB in good yield upon treatment of Si2Cl6 with catalytic amounts of NMe3 or NMe2Et in neat DMB. Reaction of 1 with one equivalent of LitBu, Na[SitBu3],[21, 22] and Na[SiPhtBu2][21, 23] exclusively led to the corresponding monosubstituted derivatives shown in Scheme 2. The use of less bulky nucleophiles, such as LiMe, LiPh, or LiMes (Mes = mesityl),[24] in excess resulted in nearly quantitative yields of the related literature-known, disubstituted derivatives 3. Chem. Eur. J. 2014, 20, 4681 – 4690

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Scheme 3. Reaction of 2 a with Na[SitBu3] or Li[NiPr2] (LDA). i) 2 equivalents of Na[SitBu3] or LDA.

We isolated 6, 7, and 8 after HPLC separation of the reaction mixture and assigned their structures based on NMR spectroscopy. Compound 6 was further characterized by X-ray structure analysis (see below). After HPLC separation, a few single crystals of another minor product were grown from the eluate (3 h, R = SitBu3, R’ = H, see the Supporting Information). However, only the signals of 6, 7, and 8, but no resonances of 3 h were present in the NMR spectra of the reaction solution. Formally, compounds 6, 7, and 8 all represent products of a dimerization between silacyclopentadiene isomers 4 a and 5 a formed as reactive intermediates after deprotonation of 2 a and MCl elimination (M = Li, Na) from the resulting transient

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Full Paper carbanion. Such a scenario is in line with findings of Barton et al. in earlier work.[14] To explain the nature of a dimeric Diels–Alder product obtained by reaction of silole 5 d in a sealed tube, the authors postulated the intermediacy of the isomeric silole 4 d formed through 1,5-trimethylsilyl migration. The rearranged isomer 4 d was suggested to act as a dienophile for a subsequent [4+2] cycloaddition with 5 d. The observation of the related Diels–Alder product for the reaction of 5 d with diphenylacetylene has lent further support to the suggested mechanism (Scheme 4).

Scheme 4. Isomers of 5-trimethylsilyl-1-sila-1,3-cyclopentadiene derivatives (150 8C).

To confirm an occurrence of the postulated intermediate 4 a in the course of the reaction studied here, we conducted trapping studies with cyclohexene and DMB as both compounds have long been known to be excellent trapping agents for silicon–carbon double bonds. Treatment of 2 a with LDA in the presence of a 20-fold excess of cyclohexene at 78 8C afforded a 1:1 mixture of two enantiomers of the [4+2] cycloadduct 9 in quantitative yield (Scheme 5). We identified 9 by its spectral characteristics and by X-ray structure analysis of single crystals, which were obtained as a racemate. The [4+2] cycloadduct 9 has four chiral centers; however, only two diastereomers (i.e., the R,S,R,R and S,R,S,S enantiomers) with boat conformation of the C6 ring are formed (Figure 2). This corresponds to addition of cyclohexene in a concerted reaction to both faces of the silacyclopentadiene scaffold of 4 a.

Scheme 5. Reactions of 4 a with cyclohexene and DMB. i) + cyclohexene at 78 8C; ii) + DMB at 78 8C. Chem. Eur. J. 2014, 20, 4681 – 4690

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As alluded to above, silenes that are generated by thermal salt elimination tend to react with dienes. Accordingly, reactions of silenes with DMB give rise to the corresponding [2+4] cycloadducts. However, the reaction of 4 a, generated in situ from 2 a and LDA, with a 20-fold excess of DMB at 78 8C afforded a 1:1 mixture of the [2+4] cycloadduct 10 and the [4+2] cycloadduct 11 (Scheme 5). Clearly, in contrast to the reaction of 4 a with cyclohexene, in which 4 a acts as the (silabuta)diene partner, 4 a reacts with DMB as a silene as well as a silacyclopentadiene. In the reaction of 4 a with DMB, a pair of enantiomers of 10 is formed, both of which possess a boat conformation of the C6 ring (Figure 3). Apparently, the addition of 4 a with DMB proceeds in a concerted fashion. Notably, the reaction of 2 c with the intercalation complex KC8 proceeds with dehalogenation of 2 c, whereas a deprotonation of 2 c was not observed. When 2 c was treated with KC8, no silacyclopentadiene derivative was formed, but disilane 12 was isolated in 81 % yield (Scheme 6). X-ray quality crystals of 12 were obtained from hexane at room temperature (see Figure 4).

Scheme 6. Synthesis of disilane 12: i) + 2 KC8 in THF at RT.

The molecular structures of the silacyclopentadiene dimer 6, the [4+2] cycloadduct 9, the [2+4] cycloadduct 10, and the disilane 12 are shown in Figures 1–4. A cyclotrisiloxane (13) and a cyclotetrasiloxane (14) (see Figures S4 and S5 in the Supporting Information) were obtained upon hydrolysis of 3 f (the mesityl derivative of 3). We found that decomposition of 3 f occurs during column chromatography. The structures of the silacyclopentene 3 g (R = Ph, Scheme 2) and 3 h (R = SitBu3, R’ = H), the [4+2] cycloadduct 11 (see Figure S1 in the Supporting Information), the cyclotrisiloxane (C6H10SiO)3 (13), and the cyclotetrasiloxane (C6H10SiO)4 (14) are presented in the Supporting Information together with the crystal data and refinement details of all compounds (Table S1). Silole dimer 6 (Figure 1), crystallizes in the orthorhombic space group P212121. It comprises a silacyclopentadiene and a silacyclopentene ring, both connected by a central Si Si bond. The [4+2] cycloadduct 9 crystallizes in the triclinic space group P1¯ as a racemate. Figure 2 represents the structure of one of two enantiomers of 9 (selected bond lengths are reported in the caption of Figure 2). The crystal structure of 9 contains two enantiomeric molecules, which are disordered over two almost equally occupied positions (site occupation factor 0.510(5) for the major occupied site). X-ray structure analysis of 9 indicates a silanorbornene and an annelated sixmembered ring. This tricycle is formed of one Si and ten C atoms. In both isomers the C6 ring of this tricycle possesses

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Figure 3. Solid-state structure of both enantiomers of 10 (monoclinic, P21/n). Displacement ellipsoids are drawn at the 50 % probability level. The H atoms are omitted for clarity. Selected bond lengths [] and angles [8] of the left enantiomer: C1 C2 1.499(5), C2 C3 1.329(5), C3 C4 1.526(5), C4 Si1 1.904(4), Si1 C1 1.881(4), C4 C5 1.551(5), C5 C6 1.503(5), C6 C7 1.341(5), C7 C8 1.510(5), C8 Si1 1.889(3), Si1 Si2 2.3772(13); C1-C2-C3 119.5(3), C2-C3-C4 116.8(3), C3-C4-Si1 104.7(2), C4-Si1-C1 93.59(16), Si1-C1-C2 105.1(2), C4-C5-C6 111.1(3), C5-C6-C7 119.8(3), C6-C7-C8 120.4(3), C7-C8-Si1 108.3(2), C8-Si1-C4 102.53(17), Si1-C4-C5 109.0(2), C1-Si1-C8 107.32(16), C1-Si1-Si2 117.64(12), C4-Si1-Si2 117.68(11), C8-Si1-Si2 115.15(13).

Figure 1. Solid-state structure of the silacyclopentadiene dimer 6 (orthorhombic, P212121). Displacement ellipsoids are drawn at the 50 % probability level. H atoms of methyl groups are omitted for clarity. Selected bond lengths [], bond angles [8], and torsion angle [8]: C1 C2 1.343(4), C2 C3 1.483(5), C3 C4 1.366(5), C4 Si2 1.880(3), Si2 C1 1.865(3), C5 C6 1.509(5), C6 C7 1.527(6), C7 C8 1.318(5), C8 Si3 1.867(3), Si3 C5 1.893(3), Si1 Si2 2.4206(11), Si2 Si3 2.4187(11), Si3 Si4 2.4279(11); C1-C2-C3 116.1(3), C2-C3-C4 114.9(3), C3-C4-Si2 109.0(3), C4-Si2-C1 90.04(15), Si2-C1-C2 109.7(3), C1-Si2-Si1 108.88(9), C1-Si2-Si3 111.89(10), C4-Si2-Si1 111.54(11), C4-Si2-Si3 103.40(10), Si1-Si2-Si3 125.28(4), C5-C6-C7 109.6(3), C6-C7-C8 118.1(3), C7-C8-Si3 112.4(3), C8-Si3-C5 90.39(15), Si3-C5-C6 108.7(3), C5-Si3-Si2 105.96(11), C5-Si3-Si4 111.52(11), C8-Si3-Si2 111.63(10), C8-Si3-Si4 110.59(10), Si2-Si3-Si4 122.08(4); Si1-Si2-Si3-Si4 138.80(5).

a boat conformation. The distance of 2.3643(8)  in 9 is of a characteristic length for Si Si bonds.[25] Crystals of the [2+4] cycloadduct 10 (see Figure 3) were grown from benzene at room temperature, and belong to the monoclinic, P21/n space group as a racemate. Both enantiomers feature a bicyclic structural motif composed of a fiveand a six-membered ring annelated at the former Si=C bond. In both structures, the C6 rings adopt a boat conformation; all C C distances are in the expected range and comparable to those in the X-ray structure of 9. Disilane 12 crystallizes in the triclinic space group P1¯. As shown in Figure 4, its structure features two symmetry-equivalent silapentene moieties in a staggered configuration. The Si Si distance of 2.415(4)  in 12 is somewhat longer than the typical Si Si bond length in disilanes (mean length of Si Si bonds: 2.37 [25]) but the presence of equal dihedral angles as found in 12 indicates that the disilane is not sterically overcrowded.[26] All other structural parameters are in the expected ranges.[25] Quantum-chemical studies

Figure 2. Solid-state structure of one enantiomer of 9 (triclinic, P1¯). Displacement ellipsoids are drawn at the 50 % probability level. The H atoms with the exception of those of the silacyclopentene ring are omitted for clarity. Selected bond lengths [] and angles [8]: C1 C2 1.282(10), C2 C3 1.538(9), C3 C4 1.602(8), C4 Si1 1.868(3), Si1 C1 1.865(2), Si1 C7 1.917(4), C7 C8 1.581(6), C8 C3 1.571(8), C8 C9 1.425(5), C9 C10 1.509(4), C10 C11 1.539(8), C11 C12 1.524(8), C12 C7 1.520(7), Si2 Si1 2.364(1); C1-C2-C3 114.2(8), C2-C3-C4 108.4(7), C3-C4-Si1 93.4(3), C4-Si1-C1 90.8(1), Si1-C1-C2 108.0(4), Si1-C7-C8 102.3(3), C7-C8-C3 109.1(4), C7-C8-C9 113.0(4), C8-C9-C10 117.1(3), C9-C10-C11 113.1(3), C10-C11-C12 112.9(5), C11-C12-C7 112.2(5), C1-Si1-C7 103.2(2), C4-Si1-C7 86.5(2), C1-Si1-Si2 121.2(1), C4-Si1-Si2 121.7(1), C7-Si1-Si2 124.3(1). Chem. Eur. J. 2014, 20, 4681 – 4690

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We have investigated the reaction mechanism underlying the observed reaction patterns by using double-hybrid density functional theory (RI-B2GP-PLYP-D3/def2-QZVPP//RI-B97-D/SVP level; see Computational Details section). Unless noted otherwise, relative energies discussed in the following refer to Gibbs energy differences computed at T = 78 8C. To avoid ambiguities with respect to the numbering scheme used in the experimental part, we use Latin capitals for denoting minimum structures identified along the reaction pathways throughout the theoretical sections of this paper.

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Figure 4. Solid-state structure of the disilane 12 (triclinic, P1¯). Only minor deviations are detected for the corresponding bond lengths and angles of the two crystallographically independent molecules in the asymmetric unit. Displacement ellipsoids are drawn at the 50 % probability level. The H atoms with the exception of those of the silacyclopentene ring are omitted for clarity. Selected bond lengths [] and angles [8] of the displayed molecule: C1 C2 1.525(9), C2 C3 1.351(10), C3 C4 1.509(9), C4 Si1 1.899(6), Si1 C1 1.879(7), Si1 C7 1.910(7), Si1 Si1B 2.415(4); C1-C2-C3 116.5(6), C2-C3-C4 119.2(6), C3-C4-Si1 102.3(4), C4-Si1-C1 94.3(3), Si1-C1-C2 103.6(5), C1-Si1-C7 113.1(3), C1-Si1-Si1B 110.4(3), C4-Si1-C7 112.7(3), C4-Si1-Si1B 110.9(3), C7-Si1-Si1B 113.9(3). Symmetry transformation used to generate equivalent atoms: B: x + 1, y, z + 1.

Assuming that the reaction sequence leading to 6, 7, and 8 commences with the initial formation of A (corresponding to 4 a in the discussion above), we first set out to explore viable isomerization pathways of this species. With a moderate activation barrier of 14 kcal mol 1 the isomeric silole B (corresponding to 5 a) is readily accessible by [1,5] hydrogen shift from A. We find B to be 19 kcal mol 1 more stable than A, which is fully in line with expectations based on the fact that the former isomer contains a conjugated C=C double-bond system, whereas the latter features a Si=C bond in conjugation with a C=C bond. A [1,3] hydrogen shift connects B with C, an only slightly less stable, transoid isomer. Bteille et al.[27] have observed the formation of a similar isomer in small yields upon flash vacuum pyrolysis of 1-methyl-1-allylsilacyclopent-3-ene. However, with an activation barrier exceeding 82 kcal mol 1 for this step, we can safely neglect it here as irrelevant at the temperature ( 78 8C) applied in the experiments (Scheme 7). As an alternative to the formation of B, [1,5] hydrogen shift along the C C framework in A leads to D. With its Si=C double bond, this isomer is also substantially less stable than B and the activation barrier for its formation (DG° = 28 kcal mol 1) is too high for this process to compete with the formation of B. Finally, A can also undergo a ring contraction to yield the bicyclic isomer E. Although such a species has been postulated to play a role in the course of stereospecific additions to siloles,[14] we conclude here that this isomer is unlikely to contribute significantly to the overall reactivity given its high relative energy (21 kcal mol 1 above A) and the large activation barrier separating A and E (DG° = 34 kcal mol 1). We conclude this section by noting that, consistent with tentative mechanistic suggestions by Barton et al.,[14] the initialChem. Eur. J. 2014, 20, 4681 – 4690

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Scheme 7. Computed isomerization pathways of the 1-silacyclopenta-1,3diene A (DG195 in kcal mol 1 with respect to isomer A).

ly formed silole A can isomerize efficiently to build up a significant stationary concentration of B. This step is driven by the strong inclination of A to abolish the intrinsically less stable Si=C double bond by formation of a thermodynamically preferred C=C bond. With a moderate computed activation barrier of DG° = 14 kcal mol 1 this is well feasible even at low temperatures, so that B, once formed, can indeed be seen as a relevant reaction intermediate along the pathway to product formation. The significant exoergicity renders this step irreversible. Proceeding further along the reaction sequence to product formation sketched in Scheme 3, we investigated the [4+2] cycloadditions leading to G and H (corresponding to 7 and 8 in the experimental part) through dimerization of A and reaction of A and B, respectively (Scheme 8). For both steps we were unable to localize transition states, and systematic reaction-coordinate scans on smaller molecular models confirmed that both reactions are barrierless (Figures S15 and S16 in the Supporting Information). Turning to the mechanism of the formation of dimer F (corresponding to 6 in the experimental part), we identified a reaction sequence that starts with the dimerization of two molecules of A by direct Si Si bond formation leading to a formally zwitterionic intermediate I (Scheme 9). This linear head-to-head approach of the two siloles occurs without reaction barrier and the formation of I is exergonic (DGR = 17 kcal mol 1). Yet, I does not represent a stable intermediate but undergoes subsequently an essentially barrierless rotation about the Si Si bond from its initial anti to a gauche conformation,

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Scheme 8. [4+2] cycloadditions leading to the dimers G and H (DG195 R in kcal mol 1 with respect to the reactants).

the preferred conformation of 6 found in the X-ray analysis, see above). As an alternative to the singlet pathway, a second route involving the triplet species is also conceivable; such species have been described as intermediates for reactions of related silenes.[28, 29] Here, the triplet biradical form IT is indeed 5.8 kcal mol 1 (RI-B97-D/SVP) more stable than the corresponding singlet. Starting from the triplet biradical species, two pathways can either lead to the linear head-to-head dimer[28] F or to a 1,2-disilacyclobutane[29] through C C coupling. We identified a singlet–triplet curve crossing on both reaction paths. Comparison of the particular minimum-energy crossing points (MECPs)[30] reveals that the formation of F is favored. Hence, the potential occurrence of triplet biradical intermediates leads to the same product as the reaction on the singlet surface regardless of whether surface crossing is efficient or not. Furthermore, the actual hydrogen shift occurs on the singlet potential energy surface (see the Supporting Information for a presentation of results). To investigate the trapping reaction of A with cyclohexene used in excess, we carried out calculations for the [4+2] cycloaddition of A with cyclohexene (Scheme 10). With a minute activation barrier of DG° = 7 kcal mol 1 the most favorable transi-

Scheme 10. [4+2] cycloaddition of the 1-silacyclopenta-1,3-diene A with cyclohexene (DG195 in kcal mol 1). Conformational isomers are characterized by the indices hc (half-chair), c (chair), and b (boat).

tion state leads to the endo product, in which the cyclohexane moiety assumes a thermodynamically favored chair conformation (DG = 38 kcal mol 1). The corresponding addition from the exo face has a slightly higher barrier with DG° = 9 kcal mol 1. For the various stereochemically relevant combinations of reactants we localized transition-state structures that all feature closer Si C than C C contacts for the bonds being formed (an example is displayed in Figure 5). In other words, the formation of J (corresponding to product 9) from A and

Scheme 9. Computed reaction path for the formation of F in the right ste1 with respect to the reactants). The values reochemistry (DG195 R in kcal mol are calculated with RI-B2GP-PLYP-D3/def2-QZVPP//RI-B97-D/SVP and RI-B97D/SVP (in parentheses). S/T denotes a transition from the singlet to triplet state for which the MECPs have been calculated (Supporting Information).

during which a concomitant hydrogen exchange among the two silolyl moieties occurs and a gauche conformer of product F is formed in a highly exergonic step. Reverse rotation about the Si Si bond interconverts it to the thermodynamically slightly more stable anti conformer of F (which corresponds to Chem. Eur. J. 2014, 20, 4681 – 4690

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Figure 5. Computed RI-B97-D/SVP structures of the endo transition state (left) and product (right) of the [4+2] cycloaddition of silacyclopentadiene A and cyclohexene in boat conformation (selected bond lengths in ; H atoms not located at the silacyclopentene ring are omitted for clarity).

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Full Paper cyclohexene occurs in a concerted, but highly asynchronous step.[31] The relative free energies of conformers resulting for the respective products vary within 3 kcal mol 1 (see Scheme 10, which lists free energies computed at 78 8C). Upon use of thermal corrections to free energies computed for 25 8C, the endo product with its cyclohexyl moiety in a boat conformation represents the most stable isomer (see Table S2 in the Supporting Information), perfectly in line with the crystal structure (Figure 2). We note in passing that the Diels–Alder reaction is kinetically/thermodynamically favored over the competing H migrations, which is in agreement with former theoretical work on the corresponding unsubstituted systems.[31b] We studied two pathways relevant for the [4+2] cycloaddition of DMB. Both routes, that is, A acting as silene to yield K (corresponding to 10) and A acting as diene to yield L (corresponding to 11), are exergonic and we identified the endo isomer of K as the most stable product (DG = 49 kcal mol 1 at 78 8C). Again, we were unable to localize transition states for either step, and a systematic reaction-coordinate scan using a smaller molecular model (see Figures S23 and S24 in the Supporting Information) corroborates our interpretation that both products are formed in barrierless reaction steps. We identified a transition structure connecting the endo isomers of L and K; with an activation barrier of DG° = 20 kcal mol 1 computed for the interconversion of L to the thermodynamically favored K, this path might well contribute to the overall reactivity at least at elevated temperatures (Scheme 11). To qualitatively rationalize the computational results, we performed a frontier orbital analysis for reactants involved in the [4+2] cycloadditions discussed above. Figure 6 displays the molecular orbitals of A and B together with those of both trapping agents and, for comparison, those of cyclopentadiene.

Figure 6. HOMO and LUMO representations (left: RI-B97-D/SVP, right: schematic) of the two isomeric siloles A and B, cyclopentadiene (CpH), cyclohexene (CHE), and (Z)-dimethylbutadiene (DMB) (orbital energies are given in eV, 0.05 au isodensity surfaces).

Consistent with earlier findings,[32] the HOMOs and LUMOs of B and cyclopentadiene are very similar in shape. Yet, the exchange of a CH2 fragment in cyclopentadiene by SiH2 in B results in a significant drop in the LUMO energy level and the frontier-orbital energy gap of the latter is smaller by 0.45 eV. Consequently, silacyclopentadienes can dimerize through cycloaddition reactions much more easily than the corresponding cyclopentadienes, which is in keeping with our computational evidence for a barrierless dimerization of B. For the systems and methods under study here it appears, in fact, that cycloadditions with Si C bond formation are barrierless if the correlating frontier orbitals of the reaction partners exhibit an energy difference below 3 eV (see Table S3 in the Supporting Information).

Conclusion

Scheme 11. [4+2] cycloadditions of the 1-silacyclopenta-1,3-diene A with (Z)-DMB (DG195 in kcal mol 1 at 78 8C with respect to the reactants). Chem. Eur. J. 2014, 20, 4681 – 4690

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The starting point of the present study was the observation that the reaction of 4-chloro-1,2-dimethyl-4-supersilylsilacyclopent-1-ene (2 a) with LDA at 78 8C yields products that imply the formation of transient 1-silabutadiene analogues, such as the 1-silacyclopentadiene derivative 4 a, in the course of the reaction. We were able to obtain crystallographic data for three corresponding products, namely the formal 1,4-addition product of 4 a with 4-silacyclopenta-1,3-diene 5 a as well as the respective adducts of the Diels–Alder reactions of 4 a with 4 a and 4 a with 5 a. In addition, we have investigated the pericyclic reactions of DMB and cyclohexene with 4 a. Interestingly, in the reaction between 4 a and DMB, the former clearly acted as a silene as well as a 1-silabutadiene partner to yield the [2+4] and [4+2] cycloadducts 10 and 11, respectively. The underlying reaction mechanisms have been investigated based on the suggested intermediacy of the two silacyclopen4687

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Full Paper tadiene isomers 4 a and 5 a and our computational exploration of conceivable alternative intermediates and interconnecting reaction paths corroborates this view. The results of our investigations on the fate of the two key intermediates revealed that [4+2] cycloadditions of 4 a + 4 a and 4 a + 5 a as well as a reaction sequence commencing with direct Si Si bond formation between two molecules of 4 a lead to the formation of the experimentally characterized products 9, 10, and 11. All of these subsequent pathways are highly exergonic and occur without activation barriers. The latter finding can be rationalized on the grounds of a qualitative picture derived from frontier-orbital analyses. We have thereby established a mechanistic scenario that straightforwardly explains all experimental observations. Overall, our study sheds light on fundamental mechanistic aspects underlying the rich chemistry of silole reagents and helps to fill some voids in the detailed understanding of the reactivity of unsaturated silicon compounds.

Experimental Section All experiments were carried out under dry argon or nitrogen by using standard Schlenk and glovebox techniques. Alkane solvents were dried over sodium and freshly distilled prior to use. Benzene, toluene, and THF were distilled from sodium/benzophenone. [D6]benzene was distilled from sodium/benzophenone and stored under a nitrogen atmosphere. Na[SitBu3],[21, 22] Na[SiPhtBu2],[23] 1,[19] and 2 a[19] were prepared according to published procedures. All other starting materials were purchased from commercial sources and used without further purification. NMR spectra were recorded on Bruker DPX 250 and Bruker Avance 300, 400, and 800 MHz spectrometers. Elemental analyses were performed at the microanalytical laboratories of the Universitt Frankfurt. For quantitative separations, the dried filtrate was redissolved and separated by HPLC (Reprosil-Pur C18-AQ, 250  20 mm, 10 mm, Maisch GmbH, Germany; Sykam S3310 UV detector, l = 254 nm; Sykam Refractive Index Monitor RI2000) with isocratic elution. Further experimental and crystallographic information is provided in the Supporting Information.

Reaction of 2 a with Li[NiPr2] (LDA) A solution of LDA (110 mg, 0.98 mmol) in THF (15 mL) was dropped into a solution of 2 a (340 mg, 0.98 mmol) in THF (10 mL), which was cooled to 78 8C. The mixture was allowed to warm up to room temperature overnight. The NMR spectra of the reaction mixture revealed that the formal 1,4-addition product of the silacyclopentadiene 4 a with its 1,5-H migration product 5 a, the formal dimer 6 (35 %), the [4+2] cycloadduct 7 (35 %), and the [2+4] cycloadduct 8 (30 %) were formed. After aqueous workup the organic layer (20 mL pentane extract) was dried with MgSO4. The dimer 6, the 1,3-H migration product 7, and the 1,4-addition product 8 were separated by HPLC (Reprosil-pur C18-AQ, 10 mm, 20  250 mm, flow rate: 3 mL min 1, two-solvent system (MeOH/tertbutyl methyl ether (TBME), 1:1), t = 13.8 (8), 19.2 (6), and 31.9 min (7)). Yield: 5 mg (6, 5 %); 5 mg (7, 5 %); 10 mg (8, 11 %). Storing of a solution in MeOH/TBME (1:1) for 1 week at room temperature yielded a few single crystals of 3 h. Compound 3 h was even formed as the result of decomposition reactions of 7 and 8.[33] Reaction of 1 with two equivalents of Na[SitBu3] in THF: A flask was charged with a solution of 1 (0.6 mmol) in THF (0.5 mL) and Chem. Eur. J. 2014, 20, 4681 – 4690

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Na[SitBu3] (1.2 mmol) in THF (3 mL) was added at 78 8C. The NMR spectra of the reaction mixture revealed that in this reaction the same products were formed as were obtained from 2 a and LDA (6 (35 %), 7 (35 %), and 8 (30 %)). The NMR data for 6–8 are given in the Supporting Information. Compound 6: elemental analysis calcd (%) for C36H72Si4 (617.30): C 70.04, H 11.76; found: C 69.87, H 11.43. Compound 7: elemental analysis calcd (%) for C36H72Si4 (617.30): C 70.04, H 11.76; found: C 70.24, H 11.96. Compound 8: elemental analysis calcd (%) for C36H72Si4 (617.30): C 70.04, H 11.76; found: C 70.02, H 11.48.

Reaction of cyclohexene with 4 a, generated from 2 a and LDA A solution of LDA (130 mg, 1.18 mmol) in THF (10 mL) was dropped into a cooled ( 78 8C) mixture of 2 a (410 mg, 1.18 mmol) and cyclohexene (2.38 mL, 1930 mg, 23.53 mmol). The reaction solution was allowed to warm up to room temperature overnight. The NMR spectra of the reaction mixture revealed that the [4+2] cycloadduct 9 was obtained nearly quantitatively. After aqueous workup the organic layer (20 mL pentane) was dried with MgSO4. The [4+2] cycloadduct 9 was finally purified by HPLC (Reprosil-pur C18-AQ, flow rate: 3 mL min 1, two-solvent system (MeOH/TBME, 2:1), t = 29 min (9)). Yield: 170 mg (38 %). The NMR data of 9 are given in the Supporting Information; elemental analysis calcd (%) for C24H64Si2 (390.79): C 73.76, H 11.86; found: C 73.24, H 11.96.

Reaction of DMB with 4 a, generated from 2 a and LDA A solution of LDA (130 mg, 1.25 mmol) in THF (5 mL) was dropped into a mixture of 2 a (430 mg, 1.25 mmol) in DMB (2.82 mL, 2050 mg, 24.92 mmol), which was cooled to 78 8C. The solution was allowed to warm up to room temperature overnight. The NMR spectra of the reaction mixture revealed that the [2+4] cycloadduct 10 and the [4+2] cycloadduct 11 were formed in a ratio of about 1:1. After aqueous workup the organic layer (20 mL hexane extract) was dried with MgSO4. The cycloadducts 10 and 11 were separated by HPLC (Reprosil-pur C18-AQ, 10 mm, 20  250 mm, flow rate: 3 mL min 1, two-solvent system (MeOH/TBME, 10:1), t = 86 (10) and 91 min (11)). Yield: 10: 60 mg (12 %), 11: 20 mg (5 %). The NMR data for 10 and 11 are given in the Supporting Information. Compound 10: elemental analysis calcd (%) for C24H64Si2 (390.79): C 73.76, H 11.86; found: C 73.55, H 11.44. Compound 11: elemental analysis calcd (%) for C24H64Si2 (390.79): C 73.76, H 11.86; found: C 73.39, H 11.65.

Reaction of 2 c with KC8 A solution of 2 c (510 mg, 2.50 mmol) in THF (5 mL) was dropped into a suspension of KC8 (670 mg, 5.00 mmol) in THF (15 mL) at room temperature. The reaction mixture was stirred for 15 h. The solvent was removed in vacuo and hexane (100 mL) was added. After filtering, all volatiles were removed in vacuo. Recrystallization in benzene yielded single crystals of 12. Yield: 340 mg (81 %). The NMR data of 12 are given in the Supporting Information; elemental analysis calcd (%) for C20H38Si2 (334.69): C 71.77, H 11.44; found: C 71.75, H 11.44.

X-ray crystallography of 6, 9–12, 3 g, 3 h, 13, 14(monoclinic P21/c), and 14(monoclinic P21/n) All crystals were measured on a STOE IPDS-II diffractometer with graphite-monochromated MoKa radiation. An empirical absorption correction with the program PLATON was performed for all struc-

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Full Paper tures. The structures were solved by direct methods and refined with full-matrix least-squares on F2 using the program SHELXL97.[34] [8]

CCDC-898722 (6), 898723 (9), 898724 (10), 901931 (11), 898725 (12), 905531 (3 g), 898726 (3 h), 793310 (13), 793311 (14: monoclinic P21/c), and 793312 (14: monoclinic P21/n) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Computational details Geometry optimizations, frequency analyses, and intrinsic reaction coordinate (IRC) following calculations were done with the Gaussian 09 (Rev. C.01) program package[35] using the B97-D functional[36] in combination with the SVP basis set[37] and the corresponding density fitting basis set SVPfit.[38] All stationary points were characterized as minima or first-order transition states by eigenvalue analyses of the computed Hessian matrices. Unscaled zero-point vibrational energies as well as thermal and entropic corrections at 195.18 and 298.15 K, respectively, were calculated at this level of theory by using the standard procedures implemented in Gaussian 09. Improved energies were obtained in single-point calculations performed with the dispersion-corrected (zero-damping)[39] double-hybrid functional B2GP-PLYP[40] and the def2-QZVPP basis set[41] with the ORCA 2.9.1 program package.[42] The RI-JK and RI algorithms were used for the SCF and MP2 part as implemented[43] in ORCA employing the corresponding def2-QZVPP/jk and def2QZVPP/c auxiliary basis sets,[44] respectively. MECP optimizations were performed with the ORCA 3.0.0 program package[42] using the B97-D functional[36] in combination with the SVP basis set[37] employing the RI-J approximation as implemented in ORCA in combination with the corresponding auxiliary basis set SVP/j.[38]

[9] [10] [11] [12] [13]

[14] [15]

[16]

Acknowledgements This work was supported by the Beilstein Institute as part of the NanoBiC research cooperative (project eNet). Computer time was provided by the Center for Scientific Computing (CSC) and the LOEWE-CSC, Frankfurt.

[17]

Keywords: cycloaddition · density functional calculations · electrocyclic reactions · reaction mechanisms · siloles [1] a) G. Raabe, J. Michl, Chem. Rev. 1985, 85, 419 – 509; b) A. G. Brook, M. A. Brook, Adv. Organomet. Chem. 1996, 39, 71 – 158. [2] H.-W. Lerner, Recent Res. Dev. Org. Chem. 2004, 8, 159 – 196. [3] A. G. Brook, S. C. Nyburg, F. Abdesaken, B. Gutekunst, G. Gutekunst, R. K. M. R. Kallury, Y. C. Poon, Y.-M. Chang, W. Wong-Ng, J. Am. Chem. Soc. 1982, 104, 5667 – 5672. [4] N. Wiberg, G. Wagner, G. Mller, Angew. Chem. 1985, 97, 220 – 222; Angew. Chem. Int. Ed. Engl. 1985, 24, 229 – 230. [5] Y. Apeloig, M. Bendikov, M. Yuzefovich, M. Nakash, D. Bravo-Zhivotovskii, J. Am. Chem. Soc. 1996, 118, 12228 – 12229. [6] a) G. E. Miracle, J. L. Ball, D. R. Powell, R. West, J. Am. Chem. Soc. 1993, 115, 11598 – 11599; b) M. Trommer, G. E. Miracle, B. E. Eichler, D. R. Powell, R. West, Organometallics 1997, 16, 5737 – 5747. [7] a) K. Wakita, N. Tokitoh, R. Okazaki, N. Takagi, S. Nagase, J. Am. Chem. Soc. 2000, 122, 5648 – 5649; b) N. Tokitoh, K. Wakita, R. Okazaki, S. Nagase, P. v. R. Schleyer, H. Jiao, J. Am. Chem. Soc. 1997, 119, 6951 – 6952; c) K. Wakita, N. Tokitoh, R. Okazaki, S. Nagase, P. v. R. Schleyer, H. Jiao, J. Am. Chem. Soc. 1999, 121, 11336 – 11344; d) N. Takeda, A. Shinohara, N. Tokitoh, Organometallics 2002, 21, 256 – 258; e) H. Hiratsuka, M. Chem. Eur. J. 2014, 20, 4681 – 4690

www.chemeurj.org

[18]

[19] [20]

4689

Tanaka, T. Okutsu, O. Tetsuo, K. Nishiyama, J. Am. Chem. Soc. Chem. Commun. 1995, 215 – 216. a) W.-C. Joo, J.-H. Hong, S.-B. Choi, H.-E. Son, J. Organomet. Chem. 1990, 391, 27 – 36; b) J.-H. Hong, P. Boudjouk, J. Am. Chem. Soc. 1993, 115, 5883 – 5884; c) J.-H. Hong, P. Boudjouk, S. Castellino, Organometallics 1994, 13, 3387 – 3389; d) U. Bankwitz, H. Sohn, D. R. Powell, R. West, Organomet. Chem. 1995, 499, C7 – C9; e) R. West, H. Sohn, U. Bankwitz, J. Calabrese, Y. Apeloig, T. Mller, J. Am. Chem. Soc. 1995, 117, 11608 – 11609; f) W. P. Freeman, T. D. Tilley, L. M. Liable-Sands, A. L. Rheingold, J. Am. Chem. Soc. 1996, 118, 10457 – 10468; g) W. P. Freeman, T. D. Tilley, G. P. A. Yap, A. L. Rheingold, Angew. Chem. 1996, 108, 960 – 962; Angew. Chem. Int. Ed. Engl. 1996, 35, 882 – 884; h) B. Goldfuss, P. v. R. Schleyer, Organometallics 1997, 16, 1543 – 1552. S. Bailleux, M. Bogey, J. Demaison, H. Burger, M. Senzlober, J. Breidung, W. Thiel, R. Fajgar, J. Pola, J. Chem. Phys. 1997, 106, 10016 – 10026. M. Bendikov, S. R. Quadt, O. Rabin, Y. Apeloig, Organometallics 2002, 21, 3930 – 3939. L. E. Gusel’nikov, N. S. Nametkin, Chem. Rev. 1979, 79, 529 – 577. G. Raabe, J. Michl, The Chemistry of Organic Silicon Compounds, John Wiley & Sons, New York, 1989. a) P. R. Jones, M. E. Lee, L. T. Lin, Organometallics 1983, 2, 1039 – 1042; b) P. R. Jones, M. E. Lee, J. Am. Chem. Soc. 1983, 105, 6725 – 6726; c) A. H.-B. Cheng, P. R. Jones, M. E. Lee, P. Roussi, Organometallics 1985, 4, 581 – 584; d) T. J. Barton, S. A. Burns, I. M. T. Daidson, S. Ijadi-Maghsoodi, I. T. Wood, J. Am. Chem. Soc. 1984, 106, 6367 – 6372. T. J. Barton, W. D. Wulff, E. V. Arnold, J. Clardy, J. Am. Chem. Soc. 1979, 101, 2733 – 2735. a) N. Wiberg, J. Organomet. Chem. 1984, 273, 141 – 177; b) N. Wiberg, G. Preiner, Angew. Chem. 1977, 89, 343 – 344; Angew. Chem. Int. Ed. Engl. 1977, 16, 328 – 330; c) N. Wiberg, G. Preiner, O. Schieda, G. Fischer, Chem. Ber. 1981, 114, 3505 – 3517; d) N. Wiberg, Chem. Ber. 1987, 120, 653 – 655; e) J. Escudie, C. Couret, H. Ranaivonjatovo, Coord. Chem. Rev. 1998, 178, 565 – 592; f) N. Wiberg, G. Wagner, J. Riede, G. Mller, Organometallics 1987, 6, 32 – 35; g) N. Wiberg, G. Wagner, Chem. Ber. 1986, 119, 1455 – 1466; h) N. Wiberg, G. Wagner, J. Riede, G. Mller, Organometallics 1987, 6, 35 – 41; i) N. Wiberg, M. Link, Chem. Ber. 1989, 122, 409 – 418; j) P. R. Jones, T. F. O. Lim, R. A. Pierce, J. Am. Chem. Soc. 1980, 102, 4970 – 4973. a) N. Wiberg, W. Niedermayer, K. Polborn, P. Mayer, Chem. Eur. J. 2002, 8, 2730 – 2739; b) M. Weidenbruch in The Chemistry of Organic Silicon Compounds, Vol. 3 (Eds.: Z. Rappoport, Y. Apeloig), Wiley, New York, 2001; c) R. Okazaki, R. West, Adv. Organomet. Chem. 1996, 39, 231 – 273; d) M. Kira, T. Iwamoto, Adv. Organomet. Chem. 2006, 54, 73 – 148; e) V. Y. Lee, A. Sekiguchi, Organometallic Compounds of Low-Coordinate Si, Ge, Sn and Pb: From Phantom Species to Stable Compounds, Wiley, New York, 2010. A. F. Holleman, E. Wiberg, N. Wiberg in Lehrbuch der Anorganischen Chemie, 102th ed., Walter de Gruyter, Berlin, 2007, p. 943. a) C. J. Wilkins, J. Chem. Soc. 1953, 3409 – 3412; b) A. Kaczmarczyk, G. Urry, J. Am. Chem. Soc. 1960, 82, 751 – 752; c) A. Kaczmarczyk, M. Millaerd, G. Urry, J. Inorg. Nucl. Chem. 1961, 17, 186 – 188; d) J. W. Nuss, G. Urry, J. Inorg. Nucl. Chem. 1964, 26, 435 – 444; e) A. Kaczmarczyk, M. Millaerd, J. W. Nuss, G. Urry, J. Inorg. Nucl. Chem. 1964, 26, 421 – 425; f) G. Urry, Acc. Chem. Res. 1970, 3, 306 – 312; g) E. Wiberg, A. Neumaier, Angew. Chem. 1964, 76, 597; h) D. K. Fleming, Acta Crystallogr. Sect. B 1972, 28, 1233 – 1236; i) U. Herzog, R. Richter, E. Brendler, G. Roewer, J. Organomet. Chem. 1996, 507, 221 – 228; j) A. Zanin, M. Karnop, J. Jeske, P. G. Jones, W.-W. d. Mont, J. Organomet. Chem. 1994, 475, 95 – 98; k) I.P. Mller, W.-W. d. Mont, J. Jeske, P. G. Jones, Chem. Ber. 1995, 128, 615 – 619; l) R. Martens, W.-W. d. Mont, Chem. Ber. 1993, 126, 1115 – 1117; m) R. Martens, W.-W. d. Mont, Chem. Ber. 1992, 125, 657 – 658; n) W.-W. du Mont, L. Mller, R. Martens, P. M. Papathomas, B. A. Smart, H. E. Robertson, D. W. H. Rankin, Eur. J. Inorg. Chem. 1999, 1381 – 1392; o) F. Meyer-Wegner, S. Scholz, I. Snger, F. Schçdel, M. Bolte, M. Wagner, H.-W. Lerner, Organometallics 2009, 28, 6835 – 6837. F. Meyer-Wegner, A. Nadj, M. Bolte, N. Auner, M. Wagner, M. C. Holthausen, H.-W. Lerner, Chem. Eur. J. 2011, 17, 4715 – 4719. However, in this context we note that, depending on the nature of the donor base, other reaction intermediates can also play a decisive role in the disproportionation reactivity. Much to our surprise we observed markedly different reaction products upon use of tetramethylethylene 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

[21] [22] [23]

[24] [25] [26]

[27] [28]

[29]

[30] [31]

[32]

diamine (TMEDA) instead of NR3 as base. By employing equimolar amounts of Si2Cl6 and TMEDA in CH2Cl2 we obtained the dianions [Si6Cl14]2 and [Si8Cl18]2 , both comprising a six-membered cyclic silicon framework coordinated by two Cl ions above and below the ring centers. Most notably, no perchlorinated neopentasilane was formed in this reaction: J. Tillmann, F. Meyer-Wegner, A. Nadj, J. Becker-Baldus, T. Sinke, M. Bolte, M. C. Holthausen, M. Wagner, H.-W. Lerner, Inorg. Chem. 2012, 51, 8599 – 8606. H.-W. Lerner, Coord. Chem. Rev. 2005, 249, 781 – 798. N. Wiberg, K. Amelunxen, H.-W. Lerner, H. Schuster, H. Nçth, I. Krossing, M. Schmidt-Amelunxen, T. Seifert, J. Organomet. Chem. 1997, 542, 1 – 18. a) H.-W. Lerner, S. Scholz, M. Bolte, Z. Anorg. Allg. Chem. 2001, 627, 1638 – 1642; b) H.-W. Lerner, S. Scholz, M. Bolte, M. Wagner, Z. Anorg. Allg. Chem. 2004, 630, 443 – 451. A. Hbner, T. Bernert, I. Snger, E. Alig, M. Bolte, L. Fink, M. Wagner, H.W. Lerner, Dalton Trans. 2010, 39, 7528 – 7533. F. Allen, Acta Crystallogr. Sect. B 2002, 58, 380 – 388. a) M. Bolte, H.-W. Lerner, J. Chem. Crystallogr. 2011, 41, 132 – 136; b) U. W. H. Vitze, A. Murso, M. Bolte, M. Wagner, H.-W. Lerner, Z. Naturforsch. B 2009, 64, 223 – 228. J. P. Bteille, M. P. Clarke, I. M. T. Davidson, J. Dubac, Organometallics 1989, 8, 1292 – 1299. a) D. Bravo-Zhivotovskii, V. Braude, A. Stanger, M. Kapon, Y. Apeloig, Organometallics 1992, 11, 2326 – 2328; b) T. J. Barton, S. K. Hoekman, J. Am. Chem. Soc. 1980, 102, 1584 – 1591; c) A. G. Brook, J. W. Harris, J. Lennon, M. El Sheikh, J. Am. Chem. Soc. 1979, 101, 83 – 95; d) K. M. Baines, A. G. Brook, Organometallics 1987, 6, 692 – 696. a) A. G. Brook, J. W. Harris, J. Am. Chem. Soc. 1976, 98, 3381 – 3383; b) E. T. Seidl, R. S. Grev, H. F. Schaefer III, J. Am. Chem. Soc. 1992, 114, 3643 – 3650; c) A. Venturini, F. Bernardi, M. Olivucci, M. A. Robb, I. Rossi, J. Am. Chem. Soc. 1998, 120, 1912 – 1913; d) D. Bravo-Zhivotovskii, S. Melamed, M. Kapon, Y. Apeloig, Organometallics 2002, 21, 2049 – 2054; e) H. Ottosson, A. M. Eklçf, Coord. Chem. Rev. 2008, 252, 1287 – 1314. a) J. N. Harvey, Phys. Chem. Chem. Phys. 2007, 9, 331 – 343; b) J. N. Harvey, WIREs Comput. Mol. Sci. 2014, 4, 1 – 14. a) N. Wiberg, S. Wagner, S.-K. Vasisht, Chem. Eur. J. 1998, 4, 2571 – 2579; b) T. C. Dinadayalane, K. Geetha, G. N. Sastry, J. Phys. Chem. A 2003, 107, 5479 – 5487. S. Yamaguchi, K. Tamao, J. Chem. Soc. Dalton Trans. 1998, 0, 3693 – 3702.

Chem. Eur. J. 2014, 20, 4681 – 4690

www.chemeurj.org

[33] Sterically overcrowded disilanes such as tBu3SiSitBu3 tend to undergo homolytic cleavage of the Si Si bond to generate the corresponding silyl radicals. Typically, the resulting radicals are transformed to the related silanes as a result of H-abstraction reactions (see reference [21]). [34] G. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112 – 122. [35] Gaussian 09, Revision C.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. J. A. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, . Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. [36] S. Grimme, J. Comput. Chem. 2006, 27, 1787 – 1799. [37] A. Schfer, H. Horn, R. Ahlrichs, J. Chem. Phys. 1992, 97, 2571 – 2577. [38] a) K. Eichkorn, O. Treutler, H. hm, M. Hser, R. Ahlrichs, Chem. Phys. Lett. 1995, 240, 283 – 290; b) K. Eichkorn, O. Treutler, H. hm, M. Hser, R. Ahlrichs, Chem. Phys. Lett. 1995, 242, 652 – 660; c) K. Eichkorn, F. Weigend, O. Treutler, R. Ahlrichs, Theor. Chem. Acc. 1997, 97, 119 – 124. [39] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys. 2010, 132, 154104 – 154119. [40] A. Karton, A. Tarnopolsky, J.-F. Lam re, G. C. Schatz, J. M. L. Martin, J. Phys. Chem. A 2008, 112, 12868 – 12886. [41] F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 2005, 7, 3297 – 3305. [42] F. Neese, WIREs Comput. Mol. Sci. 2012, 2, 73 – 78. [43] S. Kossmann, F. Neese, Chem. Phys. Lett. 2009, 481, 240 – 243. [44] a) C. Httig, Phys. Chem. Chem. Phys. 2005, 7, 59 – 66; b) F. Weigend, J. Comput. Chem. 2008, 29, 167 – 175.

Received: July 2, 2013 Published online on March 11, 2014

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