DOI: 10.1002/chem.201500558
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& Reaction Mechanisms
RhV-Nitrenoid as a Key Intermediate in RhIII-Catalyzed Heterocyclization by C¢H Activation: A Computational Perspective on the Cycloaddition of Benzamide and Diazo Compounds Tao Zhou, Wei Guo, and Yuanzhi Xia*[a] Abstract: A mechanistic study of the substituent-dependent ring formations in RhIII-catalyzed C¢H activation/cycloaddition of benzamide and diazo compounds was carried out by using DFT calculations. The results indicated that the decomposition of the diazo is facilitated upon the formation of the five-membered rhodacycle, in which the RhIII center is more electrophilic. The insertion of carbenoid into Rh¢C(phenyl) bond occurs readily and forms a 6-membered rhodacycle, however, the following C¢N bond formation is difficult both kinetically and thermodynamically by reductive elimination from the RhIII species. Instead, the RhV-nitrenoid intermediate
Introduction Direct transformation of C¢H bonds into C¢C or C¢X bonds provides new strategies for the atom-economical and cost-effective synthesis of natural products and pharmaceuticals and thus represents an area of great interest in organic chemistry.[1] Along with palladium,[2] ruthenium,[3] and other transitionmetal complexes,[4] great attention has been paid to rhodiumcatalyzed C¢H functionalizations in the past years. In this regard, the RhIII precursor [Cp*RhCl2]2 (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) has been found to be effective in a number of cross-coupling and C¢H activation/cyclization < , > reactions.[5] To enhance the reactivity and selectivity, nitrogen-containing directing groups are commonly employed for Rh-catalyzed C¢ H functionalizations.[6] However, in most cases, these groups are retained in the product and are difficult to remove. On the other hand, nitrogen-containing heterocycles are widely distributed in nature and present in bioactive molecules, and the development of new methods for their synthesis has attracted continuous efforts. To this end, recent developments in RhIIIcatalyzed C¢H activation/cyclization reactions[7] have enabled new routes to a variety of heterocycles, which take advantage [a] T. Zhou, W. Guo, Dr. Y. Xia College of Chemistry and Materials Engineering Wenzhou University, Wenzhou 325035 (P.R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500558. Chem. Eur. J. 2015, 21, 9209 – 9218
could be formed by migration of the pivalate from N to Rh, which undergoes the heterocyclization much more easily and complementary ring-formations could be modulated by the nature of the substituent at the a-carbon. When a vinyl is attached, the stepwise 1,3-allylic migration occurs prior to the pivalate migration and the 8-membered ring product will be formed. On the other hand, the pivalate migration becomes more favorable for the phenyl-contained intermediate because of the difficult 1,3-allylic migration accompanied by dearomatization, thus the 5-membered ring product was formed selectively.
of the facile migratory insertion of unsaturated molecules into the Rh¢C bond and the effective C¢N (or C¢X) bond formation through reductive elimination from the rhodacycle intermediate. Except for the directing group, another essential criteria of most C¢H activations is that stoichiometric or excess amounts of oxidant should be used.[8] To overcome this drawback, a new concept of internal oxidant has been introduced in recent years.[9] In this evolutionary strategy, the directing group, which usually contains an N¢O or N¢N moiety,[10] could act as an oxidant for regeneration of the active catalyst, thus no external oxidant is required.[11] Interestingly, these reactions could be generally conducted under milder conditions as compared with those with external oxidants. Among different types of substrates for redox-neutral C¢H activation, the N¢OPiv-containing benzamide has been found a versatile precursor for the syntheses of a variety of lactam derivatives.[12] In this regard, independent research from the groups of Rovis[13] and Cui[14] developed the couplings of benzamides (1) and diazo compounds (2), in which substituent-dependent chemoselectivities were observed (Scheme 1). When aryl-containing diazo 2 a (R = Ph) was used, the g-lactams 3 were obtained for a variety of substrates.[13] On the other hand, the azepinones 4 were formed exclusively when the R group is changed to a vinyl goup in 2 b.[14] The plausible mechanisms for these divergent reactions are shown in Scheme 2. The first generation of the 5-membered ring rhodacycle A from 1 should be a common process for the RhIII-catalyzed C¢H functionalization of benzamides. Upon the incorpo-
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Scheme 1. RhIII-catalyzed cycloadditions of benzamides and diazo compounds.
ration of diazo compound 2, the 6-membered ring rhodacycle B will be formed by dinitrogen elimination and carbene insertion, from which different reactivity will be directed by the nature of the R group. When R = Ph, a sequential C¢N bond formation/N¢O bond cleavage occurs to form 5-membered ring intermediate D, which will finally give rise to the g-lactam 3. When R = vinyl, a key step of 1,3-allylic migration occurs prior to the sequential C¢N bond formation/N¢O bond cleavage. This pathway leads to the 7-membered ring intermediate F and finally forms azepinone 4.
facilely to give the heterocycle product.[15] In light of these results, we envisioned that several mechanistic problems regarding the cyclizations of benzamide and diazo compound are still unsolved. First of all, as the reductive elimination for the C¢N bond formation from C(sp3)¢RhIII-N(sp3) unit is difficult, it was assumed that the heterocyclization processes for reactions of benzamide and diazo compounds should be more complicated than that shown in Scheme 2. More efforts are required to confirm whether the RhV-nitrenoid is a common intermediate for different RhIII-catalyzed heterocyclization reactions.[18] Second, theoretically, both the 5- and 7-membered heterocycles could be possibly formed in the reactions of 1 with 2 a and 2 b, but complementary products were obtained in experiments. The kinetic and thermodynamic properties of key steps that lead to the observed selectivity have not been revealed. Third, the combination of RhIII precursor [Cp*RhCl2]2 with CsOAc additive is an efficient catalytic system for many reactions, thus the [Cp*Rh(OAc)2] was regarded as the catalytic active species, whereas the involvement of other RhIII species could also be possible. Uncovering the real catalytic active species in concerned reactions is helpful for understanding related reactions. Moreover, the proposed mechanism in Scheme 2 suggests the diazo compound 2 is incorporated after the formation of rhodacycle A, whereas the facile decomposition of diazo compound with different transition metals including Rh is known in numerous catalytic and stoichiometric transformations.[19] The priority of dinitrogen elimination of diazo compound versus C¢H activation of benzamide under RhIII catalysis remains unclear.
Scheme 2. Plausible reaction mechanisms.
In regard to the wealth of cycloaddition reactions recently developed based on RhIII-catalyzed C¢H activation of benzamides, relatively little work has been focused on the mechanism of these transformations. Although the RhIII/RhI catalytic cycle was proposed in many reactions, the involvements of RhV intermediates in RhIII-catalyzed C¢H activation were implied in several reports.[15–17] Previously, we studied computationally the mechanisms for the formation of 5 from the reaction of 1 and olefin and found that the direct reductive elimination from 7-membered ring rhodacycle G is very difficult (Scheme 3). Instead, a rare RhV-nitrenoid intermediate I was proposed through migration of the pivaloyl from N to Rh, and this latter intermediate undergoes the reductive elimination Chem. Eur. J. 2015, 21, 9209 – 9218
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Scheme 3. Previous mechanistic insights.[15]
As a continuation of our interest in the mechanistic understanding of RhIII-catalyzed C¢H activation,[15] in the current report a comprehensive DFT study was carried out to better understand the mechanisms for cyclizations of benzamide and diazo compounds.[20, 21] The above mechanistic problems were answered by presenting theoretical details for each step of the reactions concerned.
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Full Paper Results and Discussion Possible initiations of the reactions Because experiments showed that no reaction occurred when only a catalytic amount of [Cp*RhCl2]2 was employed and the presence of CsOAc additive is crucial for efficient transformation,[13, 14] the catalytic active species was investigated first. Calculations showed the formation of [Cp*Rh(OAc)2] from [Cp*RhCl2] and CsOAc is exergonic by 0.4 kcal mol¢1, thus we assumed that all RhIII monomers [Cp*RhCl2], [Cp*RhCl(OAc)], and [Cp*Rh(OAc)2] could be possibly involved in the system and calculations were performed to evaluate their reactivity in promoting both processes of diazo decomposition and benzamide N¢H bond deprotonation/C¢H bond activation. As shown in Table 1, the decomposition of 2 b is most efficient when RhIII is [Cp*RhCl2]. In this process, the formation of reactant complex b-IN1’ is endergonic by 17.1 kcal mol¢1, from which an activation energy of 9.1 kcal mol¢1 is required to surmount b-TS1’. Thus, an overall activation barrier of 26.2 kcal mol¢1 is required for the generation of Rh-carbenoid b-IN2’. Higher activation barriers of 30.1 and 33.7 kcal mol¢1 were calculated for RhIII in the forms of [Cp*RhCl(OAc)] and [Cp*Rh(OAc)2], respectively. A similar energy profile was calculated for 2 a, which is given in the Supporting Information. The energies show that the endergonicity accompanied with the formation of complex b-IN1’ contributes greatly to the barrier height of the diazo decomposition.
Table 1. Energies for decomposition of 2 b with RhIII possible monomers.[a]
X
b-IN1’
b-TS1’
b-IN2’
X1 = X2 = OAc X1 = Cl, X2 = OAc X1 = X2 = Cl
21.3 12.2 17.1
33.7 30.1 26.6
1.5 ¢4.4 ¢5.4
[a] Solvation free energies relative to free 2 b and RhIII catalyst in kcal mol¢1.
In contrast to the decomposition of diazo 2 b, in which the [Cp*Rh(OAc)2] is the least effective, this species is the most efficient for the N¢H bond deprotonation/C¢H bond activation reaction of benzamide 1 (Figure 1 a).[22] The formation of IN1, which is a reactant complex by coordination of the N atom to the Rh center, is endergonic by 7.4 kcal mol¢1. Then, deprotonation of the N¢H bond by one of the acetate ligand is very easy with an activation energy of only 1.1 kcal mol¢1 through a 6-membered ring transition state TS1, forming IN2 and one acetic acid slightly exergonically. The aromatic C¢H bond activation occurs through the well-defined concerted metallation– Chem. Eur. J. 2015, 21, 9209 – 9218
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deprotonation (CMD) process, and the energy of TS2 is 17.9 kcal mol¢1 higher than that of IN2. The acetic acid molecule is associated with the Rh atom in the following intermediate IN3 and its dissociation forms the 5-membered rhodacycle IN4 favorably. This latter intermediate is almost energetically equal to that of the starting materials, being consistent with the deuterium experiments, which found that the C¢H activation is reversible if no diazo compound presents.[13] Instead, much higher activation barriers of 49.1 and 29.2 kcal mol¢1 are required when the [Cp*Rh(OAc)2] and [Cp*RhCl(OAc)] are involved as catalysts, respectively, as shown in Figure 1 b and c. In these latter two processes the energy demanding CMD steps occur through 4-membered ring transition states (TS2’’ and TS2’-I), in which the chloride anion is the hydrogen acceptor. These results demonstrate the importance of anion exchange to generate [Cp*Rh(OAc)2] as an efficient catalyst for N¢H deprotonation and C¢H activation steps. Formations of the 6-membered rhodacycle intermediate The above results show that the C¢H activation of benzamide 1 with [Cp*Rh(OAc)2] occurs more preferentially than the diazo decomposition with [Cp*RhCl2] and forms IN4 as an intermediate. The energy profiles for the following reactions from IN4 are given in Figure 2 and geometric structures for selected species with R = vinyl are shown in Figure 3 (the corresponding structures for species with R = Ph are similar and are given in the Supporting Information). In contrast to the difficult decomposition of diazo compounds with possible RhIII catalysts (Table 1), the reactions of both 2 a and 2 b with intermediate IN4 are much more facile (Figure 2). It was found that the generation of RhIII-diazo complex from IN4 is much less endergonic. Taking 2 b as an example, the b-IN5 (Rh-C3 = 2.37 æ, numbering of the atoms are given in the Figures) is formed with an endergonicity of only 6.9 kcal mol¢1 and an overall activation barrier of 16.2 kcal mol¢1 is required for the dinitrogen elimination via b-TS3 (Rh¢C3 = 2.06 and C3¢N = 1.80 æ). This is 17.5 kcal mol¢1 lower than that via b-TS1’ when [Cp*Rh(OAc)2] is involved. The energy gap between b-TS1’ and b-IN1’ is 12.4 kcal mol¢1 (X1 = X2 = OAc, Table 1), being only 3.1 kcal mol¢1 higher than the energy gap between b-TS3 and b-IN5. However, the much higher endergonicity for generation of complex b-IN1’ (21.3 kcal mol¢1) than b-IN5 (6.7 kcal mol¢1) from the corresponding precursors makes the former process difficult. Also, it was found that the generation of carbenoid bIN6 is much more favorable thermodynamically than the formation of b-IN2’. Thus, it could be proposed that the deprotonated benzamide moiety makes the RhIII center in rhodacycle IN4 more electron deficient than the free catalyst for coordination of the diazo compound and stabilizes the generated RhVcarbenoid species more efficiently. The energies required for the formations of a-IN5 and b-IN5 are the same, but the activation barriers for dinitrogen elimination via TS3 are slightly different for different R groups. When R = phenyl, the energy barrier for decomposition of 2 a via aTS3 is 12.8 kcal mol¢1, being 3.2 kcal mol¢1 lower than that of 2 b via b-TS3 with R = vinyl. This kinetic preference is consis-
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Figure 1. Generation of the 5-membered rhodacycle through sequential N¢H deprotonation/C¢H activation with possible RhIII catalysts.
Figure 2. Potential energy surface for generation of the 6-membered rhodacycle. Chem. Eur. J. 2015, 21, 9209 – 9218
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tent with the stability of the carbenoid intermediate; the exergonicity for the formation of aIN6 is 19.0 kcal mol¢1 whereas that for the formation of b-IN6 is 14.4 kcal mol¢1, implying the stronger ability of the phenyl group than the vinyl group for stabilizing neighboring positive charges in carbenoid IN6. From IN6, the insertion of the carbenoid into the Rh¢C(phenyl) bond occurs readily via TS4, with activation energies of 5.5 and 0.9 kcal mol¢1, respectively, for R = Ph and vinyl. The energy profile shows that the relatively higher barrier via a-TS4 is originated from the more stable nature of intermediate a-IN6. Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 3. Geometric structures for selected species in Figure 2 (methyl groups of the Cp* ligand are hidden for clarity; distances are in æ).
The alternative insertion of carbenoid into the Rh¢N bond was evaluated but much higher activation barrier is required (given in the Supporting Information). The insertion via TS4 forms the 6-memberd rhodacycle intermediate IN7 highly exergonically, in which the Rh atom is coordinated with the phenyl ring of the benzamide moiety. Other conformational isomers (IN8 and IN9) of IN7 containing more flat 6-memberd rhodacycle moieties are slightly more stable, and the intermediate IN9 with the coordination of carbonyl oxygen of the pivalate group to the Rh atom is quite close in energies with IN8.
According to the originally proposed mechanism, the 1,3-allylic migration of b-IN9 is a key step that leads to the 7-membered ring product when R = vinyl.[14] The computed results in Figure 4 indicate that the 1,3-allylic migration is a stepwise
Formation of the azepinone product from the vinyl-substituted intermediate b-IN9 Calculations starting from IN7 showed that the formations of g-lactam products IN16 via TS11 require activation barriers over 40 kcal mol¢1, irrespective of the R group (Scheme 4 a). These are in line with the previous results that show that reductive elimination from the C(sp3)¢RhIII-N(sp3) unit is difficult.[15] The azepinone product should be possible by reductive elimination from the C(sp2)¢RhIII-N(sp3) unit via TS12. However, the energy of a-TS12 is nearly 40 kcal mol¢1 above a-IN7, which could be attributed to the unfavorable dearomatization of the phenyl ring. This is evidenced by the obviously increased endergonicity for the generation of a-IN7. On the other hand, the barrier for reductive elimination via b-TS12 is relatively lower when R = vinyl, but still a barrier of 26.1 kcal mol¢1 is required. Similarly, the reductive elimination from IN9 via TS13 are also energy demanding (Scheme 4 b), suggesting the direct C¢N bond formations from RhIII species are not possible and other steps should occur more favorably prior to the heterocyclization. Chem. Eur. J. 2015, 21, 9209 – 9218
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Scheme 4. Energies for possible C¢N bond formation from RhIII intermediates.
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Full Paper process (geometric structures are shown in Figure 5). The first transformation of b-IN9 into a p-allylic rhodium complex bIN10 is facile with a barrier of 15.6 kcal mol¢1 by passing b-TS5. Complex b-IN10 is the most stable intermediate on the potential energy surface, and no coordination of the pivalate oxygen to Rh could be found in this intermediate. In the second step, the 8-membered rhodacycle b-IN11 is generated endergonically with an activation barrier of 23.6 kcal mol¢1 by passing bTS6.[23] As both b-IN10 and b-IN11 are more stable than b-IN9, the possible reductive eliminations for C¢N bond formation from these two RhIII intermediates were calculated. As expected, such possibilities were ruled out because much higher activation barriers (> 30 kcal mol¢1) were predicted (see the Supporting Information). Thus, the formation of RhV-nitrenoid intermediate through pivalate migration was studied.[15] This is realized through the cyclic transition state b-TS7, which is only slightly higher in energy than b-TS6 but still 3.3 kcal mol¢1 lower than b-TS5 and forms RhV-nitrenoid b-IN12 endergonically.[23] Then, from this latter intermediate, the reductive elimination occurs facilely with a barrier of 6.3 kcal mol¢1, leading to cyclic intermediate b-IN13 irreversibly. Finally, the azepinone product and the active RhIII catalyst will be generated by a further protonation step from b-IN13. Although the above results show how the experimentally observed azepinone was formed through the stepwise 1,3-allylic migration/pivalate migration process, we wondered if the generation of the g-lactam product could be a competitive pathway, given that the RhV-nitrenoid intermediate could also be formed through a pivalate migration from b-IN9. The calculated activation energy for this step (b-TS9) is about 21.4 kcal mol¢1, being 5.8 kcal mol¢1 higher than that via b-TS5, suggesting the 1,3-allylic migration occurs prior to the pivalate migra-
tion. Thus, the pathway leading to g-lactam is kinetically unfavorable due to the high barrier of the pivalate migration via bTS9, although only a low barrier is required for the C¢N bond formation via b-TS10 from RhV-nitrenoid b-IN14. Formation of g-lactam product from phenyl-substituted intermediate a-IN9 Similar to the reaction of b-IN9, the direct reductive elimination from a-IN9 is also very difficult (Scheme 4 b). The potential energy surface in Figure 6 and geometric structures in Figure 7 show how the 5-membered ring heterocycle is afforded by the first formation of RhV-nitrenoid a-IN10 via a-TS5 in which the pivalate group is migrating from N to Rh (Rh¢O1 = 2.09 and N¢O1 = 2.51 æ). This step requires an energy barrier of 20.3 kcal mol¢1 and is endergonic by 12.3 kcal mol¢1. The energy barrier for C¢N bond formation (a-TS6, C3-N = 2.48 æ) via reductive elimination from the RhV-nitrenoid a-IN10 is only 9.2 kcal mol¢1, leading irreversibly to the bicyclic intermediate a-IN11. The possible formation of a 7-membered ring product from a-IN9 was also calculated, however, such a possibility was ruled out due to the difficult 1,3-allylic migration (Figure 6). In this process a slightly more stable p-allyl complex a-IN12 could be possibly formed first, but all attempts to find a transition state that connects a-IN9 and a-IN12 failed. Prior to the C¢N bond formation, the a-IN12 should be transformed into the 8-membered ring intermediate a-IN13 by dearomatization of the phenyl group. As expected, a very high energy barrier of 36.7 kcal mol¢1 was calculated, thus ruling out the 1,3-allylic migration as a possible pathway when a phenyl group is involved.
Figure 4. Potential energy surface for the sequential 1,3-allylic migration/pivalate migration from b-IN9. Chem. Eur. J. 2015, 21, 9209 – 9218
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Figure 5. Geometric structures for the selected species in Figure 4 (methyl groups of the Cp* ligand are hidden for clarity; distances are in æ).
Conclusion The mechanism for the Cp*RhIII-catalyzed cycloaddition of benzamide and diazo compounds was studied by DFT calculations. It was found that the [Cp*Rh(OAc)2]-catalyzed N¢H deprotonation/C¢H activation of benzamide is more favorable than the diazo decomposition, and the formation of the five-membered rhodacycle will promote the dinitrogen elimination of diazo due to the enhanced electrophilicity of the RhIII center. After the facile carbenoid insertion, it is very difficult for heterocyclization to occur by reductive elimination from RhIII species. Instead, the RhV-nitrenoid intermediates could be generated by pivalate migration from N to Rh and undergo the C¢N formation easily, and the regiochemistry in ring-closure is directed by the substituent at the a-carbon. When a vinyl is attached, the stepwise 1,3-allylic migration occurs prior to the pivalate migration and the 8-membered ring product will be formed. Chem. Eur. J. 2015, 21, 9209 – 9218
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On the other hand, the unfavorable dearomatization during 1,3-allylic migration of the phenyl-involved intermediate makes such a process kinetically difficult, thus the 5-membered ring product was formed selectively. The results provided in-depth understanding of the complementary results by the groups of Rovis[13] and Cui[14] and highlight the importance of RhV-nitrenoid intermediate for facile C¢N bond formations when the NOPiv moiety is involved as an internal oxidant.
Computational Section All DFT calculations were carried out with the Gaussian 09 suite of computational programs.[24] The geometries of all stationary points were optimized by using the M06 hybrid functional,[25] and the 631G(d) basis set[26] was used for all atoms except for Rh, which was described by using the LanL2DZ effective core potential and
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Full Paper basis.[27] Frequencies were analytically computed at the same level of theory to obtain the gas phase free energies and to confirm whether the structures are minima (no imaginary frequency) or transition states (only one imaginary frequency). All transition state structures were confirmed to connect the proposed reactants and products by intrinsic reaction coordinate (IRC) calculations.[28] The effect of solvent was examined by performing single-point selfconsistent reaction field (SCRF) calculations based on the polarizable continuum model (PCM) for gas-phase optimized structures at a higher level of M06/6-311 + G(d,p) (SDD for Rh).[29] According to the original experiments, acetonitrile was used as the solvent with the default UFF atomic radii being used in all PCM calculations. Unless stated otherwise, all the energies discussed in the main text are relative solvation-free energies (DGsol), which were obtained by adding the solvation corrections to the computed gas phase relative free energies (DG298). The energies are relative to the energy sum of the free reactant and catalyst, and only the intermediate or transition state that has the lowest energy value among all possi-
ble conformers is used for discussion. Colored Figures and Schemes are available in the Supporting Information.
Acknowledgements This work was financially supported by the Zhejiang Provincial Natural Science Foundation (LY13B020007) and the National Natural Science Foundation of China (21372178). Financial support from the Graduate Innovation Foundation of Wenzhou University to T.Z. and facility support from the High Performance Computation Platform of Wenzhou University are acknowledged. Keywords: C¢H activation · cycloadditions · density functional calculations · diazo compounds · rhodium [1] < For selected reviews on catalytic C¢H functionalizations, see: a) C. Zheng, S.-L. You, RSC Adv. 2014, 4, 6173; b) J. Xie, C. Pan, A. Abdukader, C. Zhu, Chem. Soc. Rev. 2014, 43, 5245; c) K. Gao, N. Yoshikai, Acc. Chem. Res. 2014, 47, 1208; d) J. Yamaguchi, A. D. Yamaguchi, K. Itami, Angew. Chem. Int. Ed. 2012, 51, 8960; Angew. Chem. 2012, 124, 9092; e) B.-J. Li, Z.-J. Shi, Chem. Soc. Rev. 2012, 41, 5588; f) N. Kuhl, M. N. Hopkinson, J. Wencel-Delord, F. Glorius, Angew. Chem. Int. Ed. 2012, 51, 10236; Angew. Chem. 2012, 124, 10382; g) J. F. Hartwig, Acc. Chem. Res. 2012, 45, 864; h) M. P. Doyle, K. I. Goldberg, Acc. Chem. Res. 2012, 45, 777; i) H. M. L. Davies, J. Du Bois, J.-Q. Yu, Chem. Soc. Rev. 2011, 40, 1855; j) H. M. L. Davies, D. Morton, Chem. Soc. Rev. 2011, 40, 1857; k) L. McMurray, F. O’Hara, M. J. Gaunt, Chem. Soc. Rev. 2011, 40, 1885; l) T. C. Boorman, I. Larrosa, Chem. Soc. Rev. 2011, 40, 1910; m) W. R. Gutekunst, P. S. Baran, Chem. Soc. Rev. 2011, 40, 1976; n) J. F. Hartwig, Chem. Soc. Rev. 2011, 40, 1992; o) O. Baudoin, Chem. Soc. Rev. 2011, 40, 4902;
Figure 6. Potential energy surface for the C¢N bond formation from a-IN9.
Figure 7. Geometric structures for the selected species in Figure 6 (methyl groups of the Cp* ligand are hidden for clarity; distances are in æ). Chem. Eur. J. 2015, 21, 9209 – 9218
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Received: February 10, 2015 Published online on May 15, 2015
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& Reaction Mechanisms
RhV-Nitrenoid as a Key Intermediate in RhIII-Catalyzed Heterocyclization by C¢H Activation: A Computational Perspective on the Cycloaddition of Benzamide and Diazo Compounds Tao Zhou, Wei Guo, and Yuanzhi Xia*[a] Abstract: A mechanistic study of the substituent-dependent ring formations in RhIII-catalyzed C¢H activation/cycloaddition of benzamide and diazo compounds was carried out by using DFT calculations. The results indicated that the decomposition of the diazo is facilitated upon the formation of the five-membered rhodacycle, in which the RhIII center is more electrophilic. The insertion of carbenoid into Rh¢C(phenyl) bond occurs readily and forms a 6-membered rhodacycle, however, the following C¢N bond formation is difficult both kinetically and thermodynamically by reductive elimination from the RhIII species. Instead, the RhV-nitrenoid intermediate
Introduction Direct transformation of C¢H bonds into C¢C or C¢X bonds provides new strategies for the atom-economical and cost-effective synthesis of natural products and pharmaceuticals and thus represents an area of great interest in organic chemistry.[1] Along with palladium,[2] ruthenium,[3] and other transitionmetal complexes,[4] great attention has been paid to rhodiumcatalyzed C¢H functionalizations in the past years. In this regard, the RhIII precursor [Cp*RhCl2]2 (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) has been found to be effective in a number of cross-coupling and C¢H activation/cyclization < , > reactions.[5] To enhance the reactivity and selectivity, nitrogen-containing directing groups are commonly employed for Rh-catalyzed C¢ H functionalizations.[6] However, in most cases, these groups are retained in the product and are difficult to remove. On the other hand, nitrogen-containing heterocycles are widely distributed in nature and present in bioactive molecules, and the development of new methods for their synthesis has attracted continuous efforts. To this end, recent developments in RhIIIcatalyzed C¢H activation/cyclization reactions[7] have enabled new routes to a variety of heterocycles, which take advantage [a] T. Zhou, W. Guo, Dr. Y. Xia College of Chemistry and Materials Engineering Wenzhou University, Wenzhou 325035 (P.R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500558. Chem. Eur. J. 2015, 21, 9209 – 9218
could be formed by migration of the pivalate from N to Rh, which undergoes the heterocyclization much more easily and complementary ring-formations could be modulated by the nature of the substituent at the a-carbon. When a vinyl is attached, the stepwise 1,3-allylic migration occurs prior to the pivalate migration and the 8-membered ring product will be formed. On the other hand, the pivalate migration becomes more favorable for the phenyl-contained intermediate because of the difficult 1,3-allylic migration accompanied by dearomatization, thus the 5-membered ring product was formed selectively.
of the facile migratory insertion of unsaturated molecules into the Rh¢C bond and the effective C¢N (or C¢X) bond formation through reductive elimination from the rhodacycle intermediate. Except for the directing group, another essential criteria of most C¢H activations is that stoichiometric or excess amounts of oxidant should be used.[8] To overcome this drawback, a new concept of internal oxidant has been introduced in recent years.[9] In this evolutionary strategy, the directing group, which usually contains an N¢O or N¢N moiety,[10] could act as an oxidant for regeneration of the active catalyst, thus no external oxidant is required.[11] Interestingly, these reactions could be generally conducted under milder conditions as compared with those with external oxidants. Among different types of substrates for redox-neutral C¢H activation, the N¢OPiv-containing benzamide has been found a versatile precursor for the syntheses of a variety of lactam derivatives.[12] In this regard, independent research from the groups of Rovis[13] and Cui[14] developed the couplings of benzamides (1) and diazo compounds (2), in which substituent-dependent chemoselectivities were observed (Scheme 1). When aryl-containing diazo 2 a (R = Ph) was used, the g-lactams 3 were obtained for a variety of substrates.[13] On the other hand, the azepinones 4 were formed exclusively when the R group is changed to a vinyl goup in 2 b.[14] The plausible mechanisms for these divergent reactions are shown in Scheme 2. The first generation of the 5-membered ring rhodacycle A from 1 should be a common process for the RhIII-catalyzed C¢H functionalization of benzamides. Upon the incorpo-
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Scheme 1. RhIII-catalyzed cycloadditions of benzamides and diazo compounds.
ration of diazo compound 2, the 6-membered ring rhodacycle B will be formed by dinitrogen elimination and carbene insertion, from which different reactivity will be directed by the nature of the R group. When R = Ph, a sequential C¢N bond formation/N¢O bond cleavage occurs to form 5-membered ring intermediate D, which will finally give rise to the g-lactam 3. When R = vinyl, a key step of 1,3-allylic migration occurs prior to the sequential C¢N bond formation/N¢O bond cleavage. This pathway leads to the 7-membered ring intermediate F and finally forms azepinone 4.
facilely to give the heterocycle product.[15] In light of these results, we envisioned that several mechanistic problems regarding the cyclizations of benzamide and diazo compound are still unsolved. First of all, as the reductive elimination for the C¢N bond formation from C(sp3)¢RhIII-N(sp3) unit is difficult, it was assumed that the heterocyclization processes for reactions of benzamide and diazo compounds should be more complicated than that shown in Scheme 2. More efforts are required to confirm whether the RhV-nitrenoid is a common intermediate for different RhIII-catalyzed heterocyclization reactions.[18] Second, theoretically, both the 5- and 7-membered heterocycles could be possibly formed in the reactions of 1 with 2 a and 2 b, but complementary products were obtained in experiments. The kinetic and thermodynamic properties of key steps that lead to the observed selectivity have not been revealed. Third, the combination of RhIII precursor [Cp*RhCl2]2 with CsOAc additive is an efficient catalytic system for many reactions, thus the [Cp*Rh(OAc)2] was regarded as the catalytic active species, whereas the involvement of other RhIII species could also be possible. Uncovering the real catalytic active species in concerned reactions is helpful for understanding related reactions. Moreover, the proposed mechanism in Scheme 2 suggests the diazo compound 2 is incorporated after the formation of rhodacycle A, whereas the facile decomposition of diazo compound with different transition metals including Rh is known in numerous catalytic and stoichiometric transformations.[19] The priority of dinitrogen elimination of diazo compound versus C¢H activation of benzamide under RhIII catalysis remains unclear.
Scheme 2. Plausible reaction mechanisms.
In regard to the wealth of cycloaddition reactions recently developed based on RhIII-catalyzed C¢H activation of benzamides, relatively little work has been focused on the mechanism of these transformations. Although the RhIII/RhI catalytic cycle was proposed in many reactions, the involvements of RhV intermediates in RhIII-catalyzed C¢H activation were implied in several reports.[15–17] Previously, we studied computationally the mechanisms for the formation of 5 from the reaction of 1 and olefin and found that the direct reductive elimination from 7-membered ring rhodacycle G is very difficult (Scheme 3). Instead, a rare RhV-nitrenoid intermediate I was proposed through migration of the pivaloyl from N to Rh, and this latter intermediate undergoes the reductive elimination Chem. Eur. J. 2015, 21, 9209 – 9218
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Scheme 3. Previous mechanistic insights.[15]
As a continuation of our interest in the mechanistic understanding of RhIII-catalyzed C¢H activation,[15] in the current report a comprehensive DFT study was carried out to better understand the mechanisms for cyclizations of benzamide and diazo compounds.[20, 21] The above mechanistic problems were answered by presenting theoretical details for each step of the reactions concerned.
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Full Paper Results and Discussion Possible initiations of the reactions Because experiments showed that no reaction occurred when only a catalytic amount of [Cp*RhCl2]2 was employed and the presence of CsOAc additive is crucial for efficient transformation,[13, 14] the catalytic active species was investigated first. Calculations showed the formation of [Cp*Rh(OAc)2] from [Cp*RhCl2] and CsOAc is exergonic by 0.4 kcal mol¢1, thus we assumed that all RhIII monomers [Cp*RhCl2], [Cp*RhCl(OAc)], and [Cp*Rh(OAc)2] could be possibly involved in the system and calculations were performed to evaluate their reactivity in promoting both processes of diazo decomposition and benzamide N¢H bond deprotonation/C¢H bond activation. As shown in Table 1, the decomposition of 2 b is most efficient when RhIII is [Cp*RhCl2]. In this process, the formation of reactant complex b-IN1’ is endergonic by 17.1 kcal mol¢1, from which an activation energy of 9.1 kcal mol¢1 is required to surmount b-TS1’. Thus, an overall activation barrier of 26.2 kcal mol¢1 is required for the generation of Rh-carbenoid b-IN2’. Higher activation barriers of 30.1 and 33.7 kcal mol¢1 were calculated for RhIII in the forms of [Cp*RhCl(OAc)] and [Cp*Rh(OAc)2], respectively. A similar energy profile was calculated for 2 a, which is given in the Supporting Information. The energies show that the endergonicity accompanied with the formation of complex b-IN1’ contributes greatly to the barrier height of the diazo decomposition.
Table 1. Energies for decomposition of 2 b with RhIII possible monomers.[a]
X
b-IN1’
b-TS1’
b-IN2’
X1 = X2 = OAc X1 = Cl, X2 = OAc X1 = X2 = Cl
21.3 12.2 17.1
33.7 30.1 26.6
1.5 ¢4.4 ¢5.4
[a] Solvation free energies relative to free 2 b and RhIII catalyst in kcal mol¢1.
In contrast to the decomposition of diazo 2 b, in which the [Cp*Rh(OAc)2] is the least effective, this species is the most efficient for the N¢H bond deprotonation/C¢H bond activation reaction of benzamide 1 (Figure 1 a).[22] The formation of IN1, which is a reactant complex by coordination of the N atom to the Rh center, is endergonic by 7.4 kcal mol¢1. Then, deprotonation of the N¢H bond by one of the acetate ligand is very easy with an activation energy of only 1.1 kcal mol¢1 through a 6-membered ring transition state TS1, forming IN2 and one acetic acid slightly exergonically. The aromatic C¢H bond activation occurs through the well-defined concerted metallation– Chem. Eur. J. 2015, 21, 9209 – 9218
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deprotonation (CMD) process, and the energy of TS2 is 17.9 kcal mol¢1 higher than that of IN2. The acetic acid molecule is associated with the Rh atom in the following intermediate IN3 and its dissociation forms the 5-membered rhodacycle IN4 favorably. This latter intermediate is almost energetically equal to that of the starting materials, being consistent with the deuterium experiments, which found that the C¢H activation is reversible if no diazo compound presents.[13] Instead, much higher activation barriers of 49.1 and 29.2 kcal mol¢1 are required when the [Cp*Rh(OAc)2] and [Cp*RhCl(OAc)] are involved as catalysts, respectively, as shown in Figure 1 b and c. In these latter two processes the energy demanding CMD steps occur through 4-membered ring transition states (TS2’’ and TS2’-I), in which the chloride anion is the hydrogen acceptor. These results demonstrate the importance of anion exchange to generate [Cp*Rh(OAc)2] as an efficient catalyst for N¢H deprotonation and C¢H activation steps. Formations of the 6-membered rhodacycle intermediate The above results show that the C¢H activation of benzamide 1 with [Cp*Rh(OAc)2] occurs more preferentially than the diazo decomposition with [Cp*RhCl2] and forms IN4 as an intermediate. The energy profiles for the following reactions from IN4 are given in Figure 2 and geometric structures for selected species with R = vinyl are shown in Figure 3 (the corresponding structures for species with R = Ph are similar and are given in the Supporting Information). In contrast to the difficult decomposition of diazo compounds with possible RhIII catalysts (Table 1), the reactions of both 2 a and 2 b with intermediate IN4 are much more facile (Figure 2). It was found that the generation of RhIII-diazo complex from IN4 is much less endergonic. Taking 2 b as an example, the b-IN5 (Rh-C3 = 2.37 æ, numbering of the atoms are given in the Figures) is formed with an endergonicity of only 6.9 kcal mol¢1 and an overall activation barrier of 16.2 kcal mol¢1 is required for the dinitrogen elimination via b-TS3 (Rh¢C3 = 2.06 and C3¢N = 1.80 æ). This is 17.5 kcal mol¢1 lower than that via b-TS1’ when [Cp*Rh(OAc)2] is involved. The energy gap between b-TS1’ and b-IN1’ is 12.4 kcal mol¢1 (X1 = X2 = OAc, Table 1), being only 3.1 kcal mol¢1 higher than the energy gap between b-TS3 and b-IN5. However, the much higher endergonicity for generation of complex b-IN1’ (21.3 kcal mol¢1) than b-IN5 (6.7 kcal mol¢1) from the corresponding precursors makes the former process difficult. Also, it was found that the generation of carbenoid bIN6 is much more favorable thermodynamically than the formation of b-IN2’. Thus, it could be proposed that the deprotonated benzamide moiety makes the RhIII center in rhodacycle IN4 more electron deficient than the free catalyst for coordination of the diazo compound and stabilizes the generated RhVcarbenoid species more efficiently. The energies required for the formations of a-IN5 and b-IN5 are the same, but the activation barriers for dinitrogen elimination via TS3 are slightly different for different R groups. When R = phenyl, the energy barrier for decomposition of 2 a via aTS3 is 12.8 kcal mol¢1, being 3.2 kcal mol¢1 lower than that of 2 b via b-TS3 with R = vinyl. This kinetic preference is consis-
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Figure 1. Generation of the 5-membered rhodacycle through sequential N¢H deprotonation/C¢H activation with possible RhIII catalysts.
Figure 2. Potential energy surface for generation of the 6-membered rhodacycle. Chem. Eur. J. 2015, 21, 9209 – 9218
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tent with the stability of the carbenoid intermediate; the exergonicity for the formation of aIN6 is 19.0 kcal mol¢1 whereas that for the formation of b-IN6 is 14.4 kcal mol¢1, implying the stronger ability of the phenyl group than the vinyl group for stabilizing neighboring positive charges in carbenoid IN6. From IN6, the insertion of the carbenoid into the Rh¢C(phenyl) bond occurs readily via TS4, with activation energies of 5.5 and 0.9 kcal mol¢1, respectively, for R = Ph and vinyl. The energy profile shows that the relatively higher barrier via a-TS4 is originated from the more stable nature of intermediate a-IN6. Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 3. Geometric structures for selected species in Figure 2 (methyl groups of the Cp* ligand are hidden for clarity; distances are in æ).
The alternative insertion of carbenoid into the Rh¢N bond was evaluated but much higher activation barrier is required (given in the Supporting Information). The insertion via TS4 forms the 6-memberd rhodacycle intermediate IN7 highly exergonically, in which the Rh atom is coordinated with the phenyl ring of the benzamide moiety. Other conformational isomers (IN8 and IN9) of IN7 containing more flat 6-memberd rhodacycle moieties are slightly more stable, and the intermediate IN9 with the coordination of carbonyl oxygen of the pivalate group to the Rh atom is quite close in energies with IN8.
According to the originally proposed mechanism, the 1,3-allylic migration of b-IN9 is a key step that leads to the 7-membered ring product when R = vinyl.[14] The computed results in Figure 4 indicate that the 1,3-allylic migration is a stepwise
Formation of the azepinone product from the vinyl-substituted intermediate b-IN9 Calculations starting from IN7 showed that the formations of g-lactam products IN16 via TS11 require activation barriers over 40 kcal mol¢1, irrespective of the R group (Scheme 4 a). These are in line with the previous results that show that reductive elimination from the C(sp3)¢RhIII-N(sp3) unit is difficult.[15] The azepinone product should be possible by reductive elimination from the C(sp2)¢RhIII-N(sp3) unit via TS12. However, the energy of a-TS12 is nearly 40 kcal mol¢1 above a-IN7, which could be attributed to the unfavorable dearomatization of the phenyl ring. This is evidenced by the obviously increased endergonicity for the generation of a-IN7. On the other hand, the barrier for reductive elimination via b-TS12 is relatively lower when R = vinyl, but still a barrier of 26.1 kcal mol¢1 is required. Similarly, the reductive elimination from IN9 via TS13 are also energy demanding (Scheme 4 b), suggesting the direct C¢N bond formations from RhIII species are not possible and other steps should occur more favorably prior to the heterocyclization. Chem. Eur. J. 2015, 21, 9209 – 9218
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Scheme 4. Energies for possible C¢N bond formation from RhIII intermediates.
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Full Paper process (geometric structures are shown in Figure 5). The first transformation of b-IN9 into a p-allylic rhodium complex bIN10 is facile with a barrier of 15.6 kcal mol¢1 by passing b-TS5. Complex b-IN10 is the most stable intermediate on the potential energy surface, and no coordination of the pivalate oxygen to Rh could be found in this intermediate. In the second step, the 8-membered rhodacycle b-IN11 is generated endergonically with an activation barrier of 23.6 kcal mol¢1 by passing bTS6.[23] As both b-IN10 and b-IN11 are more stable than b-IN9, the possible reductive eliminations for C¢N bond formation from these two RhIII intermediates were calculated. As expected, such possibilities were ruled out because much higher activation barriers (> 30 kcal mol¢1) were predicted (see the Supporting Information). Thus, the formation of RhV-nitrenoid intermediate through pivalate migration was studied.[15] This is realized through the cyclic transition state b-TS7, which is only slightly higher in energy than b-TS6 but still 3.3 kcal mol¢1 lower than b-TS5 and forms RhV-nitrenoid b-IN12 endergonically.[23] Then, from this latter intermediate, the reductive elimination occurs facilely with a barrier of 6.3 kcal mol¢1, leading to cyclic intermediate b-IN13 irreversibly. Finally, the azepinone product and the active RhIII catalyst will be generated by a further protonation step from b-IN13. Although the above results show how the experimentally observed azepinone was formed through the stepwise 1,3-allylic migration/pivalate migration process, we wondered if the generation of the g-lactam product could be a competitive pathway, given that the RhV-nitrenoid intermediate could also be formed through a pivalate migration from b-IN9. The calculated activation energy for this step (b-TS9) is about 21.4 kcal mol¢1, being 5.8 kcal mol¢1 higher than that via b-TS5, suggesting the 1,3-allylic migration occurs prior to the pivalate migra-
tion. Thus, the pathway leading to g-lactam is kinetically unfavorable due to the high barrier of the pivalate migration via bTS9, although only a low barrier is required for the C¢N bond formation via b-TS10 from RhV-nitrenoid b-IN14. Formation of g-lactam product from phenyl-substituted intermediate a-IN9 Similar to the reaction of b-IN9, the direct reductive elimination from a-IN9 is also very difficult (Scheme 4 b). The potential energy surface in Figure 6 and geometric structures in Figure 7 show how the 5-membered ring heterocycle is afforded by the first formation of RhV-nitrenoid a-IN10 via a-TS5 in which the pivalate group is migrating from N to Rh (Rh¢O1 = 2.09 and N¢O1 = 2.51 æ). This step requires an energy barrier of 20.3 kcal mol¢1 and is endergonic by 12.3 kcal mol¢1. The energy barrier for C¢N bond formation (a-TS6, C3-N = 2.48 æ) via reductive elimination from the RhV-nitrenoid a-IN10 is only 9.2 kcal mol¢1, leading irreversibly to the bicyclic intermediate a-IN11. The possible formation of a 7-membered ring product from a-IN9 was also calculated, however, such a possibility was ruled out due to the difficult 1,3-allylic migration (Figure 6). In this process a slightly more stable p-allyl complex a-IN12 could be possibly formed first, but all attempts to find a transition state that connects a-IN9 and a-IN12 failed. Prior to the C¢N bond formation, the a-IN12 should be transformed into the 8-membered ring intermediate a-IN13 by dearomatization of the phenyl group. As expected, a very high energy barrier of 36.7 kcal mol¢1 was calculated, thus ruling out the 1,3-allylic migration as a possible pathway when a phenyl group is involved.
Figure 4. Potential energy surface for the sequential 1,3-allylic migration/pivalate migration from b-IN9. Chem. Eur. J. 2015, 21, 9209 – 9218
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Figure 5. Geometric structures for the selected species in Figure 4 (methyl groups of the Cp* ligand are hidden for clarity; distances are in æ).
Conclusion The mechanism for the Cp*RhIII-catalyzed cycloaddition of benzamide and diazo compounds was studied by DFT calculations. It was found that the [Cp*Rh(OAc)2]-catalyzed N¢H deprotonation/C¢H activation of benzamide is more favorable than the diazo decomposition, and the formation of the five-membered rhodacycle will promote the dinitrogen elimination of diazo due to the enhanced electrophilicity of the RhIII center. After the facile carbenoid insertion, it is very difficult for heterocyclization to occur by reductive elimination from RhIII species. Instead, the RhV-nitrenoid intermediates could be generated by pivalate migration from N to Rh and undergo the C¢N formation easily, and the regiochemistry in ring-closure is directed by the substituent at the a-carbon. When a vinyl is attached, the stepwise 1,3-allylic migration occurs prior to the pivalate migration and the 8-membered ring product will be formed. Chem. Eur. J. 2015, 21, 9209 – 9218
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On the other hand, the unfavorable dearomatization during 1,3-allylic migration of the phenyl-involved intermediate makes such a process kinetically difficult, thus the 5-membered ring product was formed selectively. The results provided in-depth understanding of the complementary results by the groups of Rovis[13] and Cui[14] and highlight the importance of RhV-nitrenoid intermediate for facile C¢N bond formations when the NOPiv moiety is involved as an internal oxidant.
Computational Section All DFT calculations were carried out with the Gaussian 09 suite of computational programs.[24] The geometries of all stationary points were optimized by using the M06 hybrid functional,[25] and the 631G(d) basis set[26] was used for all atoms except for Rh, which was described by using the LanL2DZ effective core potential and
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Full Paper basis.[27] Frequencies were analytically computed at the same level of theory to obtain the gas phase free energies and to confirm whether the structures are minima (no imaginary frequency) or transition states (only one imaginary frequency). All transition state structures were confirmed to connect the proposed reactants and products by intrinsic reaction coordinate (IRC) calculations.[28] The effect of solvent was examined by performing single-point selfconsistent reaction field (SCRF) calculations based on the polarizable continuum model (PCM) for gas-phase optimized structures at a higher level of M06/6-311 + G(d,p) (SDD for Rh).[29] According to the original experiments, acetonitrile was used as the solvent with the default UFF atomic radii being used in all PCM calculations. Unless stated otherwise, all the energies discussed in the main text are relative solvation-free energies (DGsol), which were obtained by adding the solvation corrections to the computed gas phase relative free energies (DG298). The energies are relative to the energy sum of the free reactant and catalyst, and only the intermediate or transition state that has the lowest energy value among all possi-
ble conformers is used for discussion. Colored Figures and Schemes are available in the Supporting Information.
Acknowledgements This work was financially supported by the Zhejiang Provincial Natural Science Foundation (LY13B020007) and the National Natural Science Foundation of China (21372178). Financial support from the Graduate Innovation Foundation of Wenzhou University to T.Z. and facility support from the High Performance Computation Platform of Wenzhou University are acknowledged. Keywords: C¢H activation · cycloadditions · density functional calculations · diazo compounds · rhodium [1] < For selected reviews on catalytic C¢H functionalizations, see: a) C. Zheng, S.-L. You, RSC Adv. 2014, 4, 6173; b) J. Xie, C. Pan, A. Abdukader, C. Zhu, Chem. Soc. Rev. 2014, 43, 5245; c) K. Gao, N. Yoshikai, Acc. Chem. Res. 2014, 47, 1208; d) J. Yamaguchi, A. D. Yamaguchi, K. Itami, Angew. Chem. Int. Ed. 2012, 51, 8960; Angew. Chem. 2012, 124, 9092; e) B.-J. Li, Z.-J. Shi, Chem. Soc. Rev. 2012, 41, 5588; f) N. Kuhl, M. N. Hopkinson, J. Wencel-Delord, F. Glorius, Angew. Chem. Int. Ed. 2012, 51, 10236; Angew. Chem. 2012, 124, 10382; g) J. F. Hartwig, Acc. Chem. Res. 2012, 45, 864; h) M. P. Doyle, K. I. Goldberg, Acc. Chem. Res. 2012, 45, 777; i) H. M. L. Davies, J. Du Bois, J.-Q. Yu, Chem. Soc. Rev. 2011, 40, 1855; j) H. M. L. Davies, D. Morton, Chem. Soc. Rev. 2011, 40, 1857; k) L. McMurray, F. O’Hara, M. J. Gaunt, Chem. Soc. Rev. 2011, 40, 1885; l) T. C. Boorman, I. Larrosa, Chem. Soc. Rev. 2011, 40, 1910; m) W. R. Gutekunst, P. S. Baran, Chem. Soc. Rev. 2011, 40, 1976; n) J. F. Hartwig, Chem. Soc. Rev. 2011, 40, 1992; o) O. Baudoin, Chem. Soc. Rev. 2011, 40, 4902;
Figure 6. Potential energy surface for the C¢N bond formation from a-IN9.
Figure 7. Geometric structures for the selected species in Figure 6 (methyl groups of the Cp* ligand are hidden for clarity; distances are in æ). Chem. Eur. J. 2015, 21, 9209 – 9218
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Received: February 10, 2015 Published online on May 15, 2015
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