Research Article pubs.acs.org/acscatalysis
Silver Migration Facilitates Isocyanide-Alkyne [3 + 2] Cycloaddition Reactions: Combined Experimental and Theoretical Study Xiaotian Qi,† Heng Zhang,§ Ailong Shao,‡,§ Lei Zhu,† Ting Xu,† Meng Gao,*,‡ Chao Liu,*,§,∥ and Yu Lan*,† †
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, P. R. China National Research Center for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang, Jiangxi 330022, P. R. China § College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, P. R. China ∥ State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), CAS, Lanzhou 730000, P. R. China
ACS Catal. 2015.5:6640-6647. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/21/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: Silver-mediated isocyanide-alkyne [3 + 2] cycloaddition has been developed as a new method for the synthesis of pyrroles. Density functional theory (DFT) calculations toward this reaction reveal that terminal alkynes participated cycloadditions proceed through two successive 1,5-silver migrations, in which the silver migrates between two carbon atoms and finally returns to original carbon. Natural population analysis (NPA) indicates that silver migration guides the move of charge into a rational way, thereby facilitating the cycloaddition. An analogous silver-migration mechanism is also suitable to explain the reactivity of the cycloaddition between isocyanide and internal alkynes, which shows the generality of the silver-migration process. Moreover, competitive experiments are consistent with the computational results, which provides further support for the mechanism. KEYWORDS: silver migration, [3 + 2] cycloaddition, DFT calculation, NPA charge, competitive experiments
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INTRODUCTION
has the migration of silver been proposed or proved in organic synthesis.9 Recently, a silver catalyzed isocyanide-alkyne [3 + 2] cycloaddition for the synthesis of pyrroles was reported by Lei10 and Bi11 individually (Figure 1). The modularity, broad substrate scope, high efficiency, high selectivity, and high yield of this reaction meet the criteria of “click chemistry”,12 thereby enriches the method for efficient construction of multifunctional heterocycles.10,13 However, the role of silver in this transformation is still unclear, which restricts further application
Transition metal catalyzed or mediated reactions have been developed rapidly for decades.1 When considering the mechanisms, metals are always regarded as the fixed point of these transformations.2 For instance, the elementary reactions in organometallic chemistryligand coordination/dissociation, oxidative addition/reductive elimination, insertion/elimination, and nucleophilic/electrophilic attacks on coordinated ligands, which are put forward to explain reaction pathwayare all defined to occur surround the metal center.3 From another point of view, the migration of metal center also provides an excellent approach to understand transition metal catalyzed or mediated reactions.4 Nevertheless, comprehensive mechanistic study regarding metal migration has rarely been reported,5 although many reactions are deduced to contain metal migration as the key step.6 Among various transition metals, silver catalysis has received increased attention in organic chemistry due to its expanding reactivity recently discovered.7 More and more silver promoted transformations are employed for the construction of heterocycles, as well as the formation of C−C bond.8 In these transformations, silver salts generally facilitate the reactions by its halogenophilicity, alkynophilicity, and Lewis acidity, seldom © 2015 American Chemical Society
Figure 1. Silver-catalyzed isocyanide-alkyne [3 + 2] cycloadditions for the synthesis of pyrroles. Received: September 10, 2015 Revised: September 27, 2015 Published: September 30, 2015 6640
DOI: 10.1021/acscatal.5b02009 ACS Catal. 2015, 5, 6640−6647
Research Article
ACS Catalysis of such an important silver catalysis. Density functional theory (DFT) calculation is thereby utilized to study the silvercatalyzed cycloaddition. To our surprise, the migration of silver in this [3 + 2] cycloaddition is demonstrated to be vital, which is markedly distinct from the mechanism of copper-catalyzed,14 rhodium-catalyzed,15 or iridium-catalyzed azide−alkyne cycloadditions.16 Herein, we would like to share our latest discoveries to bring some fresh ideas for silver catalysis.
Figure 2. Silver catalyzed [3 + 2] cycloaddition of ethyl 2isocyanoacetate and phenylacetylene.
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COMPUTATIONAL METHODS All the DFT calculations were carried out with the GAUSSIAN 09 series of programs.17 DFT method B3-LYP18 with 6-31G(d) basis set (SDD19 basis set for Ag) was used for geometry optimizations. Harmonic vibration frequency calculations were performed for all stationary points to confirm them as a local minima or transition structure and to derive the thermochemical corrections for the enthalpies and free energies. The solvent effects were considered by single point calculations on the gas-phase stationary points with a SMD continuum solvation model.20 The 6-311+G(d,p) basis set (SDD basis set for Ag) was used in the solvation single point calculations. Both M06-L21 and M11-L22 were used to calculate the solvation single point energies to give more accurate energetic information. Because the model reactions for calculation employ N-methyl-2-pyrrolidone (NMP) as the solvent, NMP is used as the ligand of silver in calculation. However, there is no keyword of NMP in solvation option. Considering the ε value of n,n-dimethylformamide (DMF, 37) is very close to that of NMP (32), the keyword n,ndimethylformamide is finally used in the solvation calculation. The energies given in this work are M06-L and M11-L calculated Gibbs free energies in DMF. We have also tried to optimize the structures in solvent, but the calculations did not proceed. However, we performed the optimization at M06-L/6-311+G(d,p) level in gas phase, and calculated the solvation single point energy using the same method in DMF. The obtained data turn out to be very close from the data of M06-L//B3LYP, which suggests that the combination of M06-L and B3LYP can give accurate results. (See Figure S10 in the Supporting Information for details) Since the computational results respectively obtained by M06-L and M11-L show the same trend, we chose the data calculated at M06-L/6-311+G(d,p) (SDD for Ag) level for energy discussion. Besides, the natural population analysis (NPA) charge and the highest occupied molecular orbital (HOMO) were calculated at the M06-L/6-311+G(d,p) (SDD for Ag) level.
Figure 3. Free energy profiles for the deprotonation of phenylacetylene (a) and isocyanide (b). “L” represents for the ligand Nmethyl-2-pyrrolidone (NMP).
These data indicate that intermediates 3, 6, and 8 can be generated easily during the reaction. Thus, silver isocyanide 8 (1.5 kcal/mol more stable than 6) and silver acetylide 3 are set as the relative zero free energy in following free energy profiles. Based on the above information as well as our understanding toward these type reactions, the concerted pathway, which is usually proposed in 1,3-dipolar cycloadditions, is first considered in calculation. As shown in Figure 4, in the presence of silver acetylide 3, either 2,3-disubstituted cyclization product 9 or 2,4-disubstituted 10 could be formed via transition state ts-A or ts-B, respectively. However, the activation energy of transition state ts-A reaches as high as 36.2 kcal/mol, and the corresponding value of ts-B is 4.9 kcal/mol higher. Consequently, the concerted pathway could be safely ruled out. To provide guidance for following calculation, NPA charge analysis toward the reactants was conducted. The NPA charge values shown in Figure 5a indicate that the nucleophilicity of carbon C1 in complex 6 is stronger than carbon C2 and the terminal carbon C2 in complex 8 due to the higher negative charge. The NPA charge of carbon C4 in silver acetylide 3 is −0.451, which suggests the strong nucleophilicity of that carbon. Calculated HOMOs in Figure 5b show that the HOMO of 6 is mainly localized on carbon C1, and most HOMO of 3 are localized on carbon C4, which also imply the strong nucleophilicity. According to these ideas, plausible reaction manners for the cycloaddition between silver isocyanide 6 and silver acetylide 3 is proposed in Scheme 1. In Path-A, when concerted cycloaddition occurs, the negative charge is first moved from carbon C1 to C3, then from carbon C4 to C2. Accordingly, during the formation of 2,4disubstituted pyrrole 10 (Path-B), the charge is first delivered
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RESULTS AND DISCUSSION In the experiment, both terminal and internal alkynes could afford pyrrole products in high yield.10,11 We initially investigated the mechanism of terminal alkynes involved reactions and chose the cycloaddition between ethyl 2isocyanoacetate and phenylacetylene as the model reaction (Figure 2). The in situ IR spectroscopic studies concerning this reaction in Lei’s work have observed silver acetylide and silver isocyanide as intermediates. We have also calculated the deprotonation of phenylacetylene and isocyanide using Ag2CO3. The activation free energies are 11.8 and 22.6 kcal/ mol (Figure 3), respectively. Moreover, the isomerization of generated silver isocyanide 6 via transition state ts-7 could afford intermediate 8; the barrier is 17.7 kcal/mol (Figure 3b). 6641
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competitive pathway Path-F, the nucleophilic attack of carbon C1 toward C4, which generates an 1,4-silver migration intermediate, lead the charge moved to silver Ag2 as well. Furthermore, the straightforward nucleophilic attack of carbon C1 toward silver Ag2 is also an alternative. Two different pathways (Path-G and Path-H) derived from this idea are taken into account. With these plausible reaction manners in mind, DFT calculations for all of these pathways were performed. The free energy profiles of stepwise pathway Path-C and Path-D shown in Figure 6 suggest that through the 1,5-silver migration transition state ts-C1, silver Ag1 is transferred to carbon C4, which generates vinyldisilver intermediate C2 with an activation energy of 26.5 kcal/mol. Subsequently, the nucleophilic attack of carbon C2 toward C4 leads to another 1,5-silver migration via transition state ts-C3 with an activation barrier of 12.2 kcal/ mol. The formation of 2,3-disubstituted pyrrole scaffold 9 is 33.7 kcal/mol exergonic and silver Ag1 ultimately returns to carbon C2. Following protonation of intermediate 9 takes place through transition states ts-11 and ts-13 successively; this process is exergonic by 15.7 kcal/mol and the overall barrier is 25.5 kcal/mol. Finally, the tautomerization of generated complex 14 would release the product pyrrole. In Path-D, the relative free energy of transition state ts-D1 is as high as 39.6 kcal/mol due to the large negative charge localized on both carbon C1 and C4. The cyclization proceeds through transition state ts-D3 with a barrier of 15.4 kcal/mol, generating corresponding 2,4-disubstituted pyrrole scaffold 10. Subsequent protonation via transition states ts-15 and ts-17 generates complex 18; the overall barrier is 24.8 kcal/mol. Compared with Path-D, Path-C is obviously a better approach for the cycloaddition of silver isocyanide 2 and silver acetylide 3, thus only the 2,3-disubstituted pyrrole derived from complex 9 could be observed. Moreover, as the overall barrier of Path-C (29.8 kcal/mol) is 6.4 kcal/mol lower thanthat of Path-A (36.2 kcal/mol), Path-C is demonstrated to be more favorable than Path-A, which is consistent with the NPA charge analysis. Besides, it is particularly noteworthy that during the nucleophilic attacks in Path-C and Path-D, silver Ag1 jumps between carbon C2 and C4, and finally returns to the original carbon. This process is defined as silver-migration to highlight the behavior of silver. Besides, other possible reaction pathways for the cycloaddition of silver isocyanide with silver acetylide have also been considered. As depicted in Figure 7, the relative free energy of transition state ts-E1 in Path-E is 34.7 kcal/mol, which is 4.9 kcal/mol higher than that of transition state ts-C1 in Path-C. In the geometry structure of transition state ts-E1, silver Ag2 is bonded with carbon C2 and C4 simultaneously when the C1− C3 bond is forming. NPA charge analysis shows that carbon C4 is more negative than C3. Therefore, the movement of negative charge from carbon C3 to C4 in transition state ts-E1 is unfavorable, which results in the higher activation free energy. Carbene-silver intermediate E2 is formed with 34.1 kcal/mol endergonic relative to complex 1, followed by cycloaddition occurs via a carbene insertion transition state ts-E3, generating pyrrole scaffold 4 with 55.8 kcal/mol exergonic. As the overall activation free energy of Path-E is 39.7 kcal/mol, this pathway is unfavorable compared to Path-C. In Path-F (Figure 8), the nucleophilic attack of carbon C1 to C4 lead to the high activation energy via transition state ts-F1, which is 3.8 kcal/mol higher than that via transition state ts-E1 in Path-E. Then, the silver-migration occurs from iminodisilver
Figure 4. Free energy profile of the synergetic pathway. “L” represents for the ligand N-methyl-2-pyrrolidone (NMP).
Figure 5. (a) Optimized structure of silver acetylide 3, silver isocyanide complex 8, and its isomer 6. The values in parentheses are corresponding NPA charge of certain atoms. (b) Calculated HOMOs of intermediate 8, 6, and 3.
from carbon C1 to C4, subsequently delivered from C3 to C2. Because of the large negative charge value of carbon C4, the charge move in Path-B is adverse, which results in the higher activation barrier. For stepwise mechanisms, in Path-C, while the nucleophilic attack of carbon C1 to C3 occurs, a 1,5-silver migration takes place at the same time, which lead to the movement of negative charge from carbon C6 to silver Ag1, then to carbon C4. As the NPA charge of Ag1 (0.561) is more positive than C4 (−0.060), the charge move in Path-C would be easier than that in Path-A. Therefore, this pathway is most likely to be more favorable. In contrast, initial step for the formation of 2,4-disubstituted pyrrole is shown in Path-D, the strong nucleophilicity of both carbon C1 and C4 might result in a high activation barrier. Besides, when nucleophilic attack of carbon C1 to C3 takes place, the negative charge could also be delivered to silver Ag2 through π bond, subsequently to carbon C2 (Path-E). In 6642
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Scheme 1. Plausible Reaction Manners for the Cycloaddition between Silver Isocyanide 2 and Silver Acetylide 3a
a
The arrows in blue color represent for the flow of electron. “L” represents the ligand NMP.
Figure 6. Free energy profiles of the stepwise pathway Path-C and Path-D for the cycloaddition of silver isocyanide 6 with silver acetylide 3. “L” represents the ligand NMP.
intermediate F2 via transition state ts-F3, generating intermediate F4, which contains a Ag1−Ag2 interaction. Subsequently, a more stable vinyldisilver intermediate F5 could be produced with 14.6 kcal/mol exergonic from F4 through the rotation of C1−C4 σ-bond. Final cycloaddition via a five-membered ring transition state ts-F6 with a barrier of
14.0 kcal/mol affords the 2,4-disubstituted pyrrole scaffold 10. This pathway is also eliminated because of the higher activation energy. Furthermore, complete reaction pathway for the straightforward nucleophilic attack of carbon C1 toward silver Ag2 is shown in Figure 9 (Path-G) and Figure 10 (Path-H). In Path6643
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Figure 7. Free energy profile of pathway Path-E for the cycloaddition of silver isocyanide 6 and silver acetylide 3. “L” represents the ligand NMP.
Figure 9. Free energy profile of pathway Path-G for the cycloaddition of silver isocyanide 6 and silver acetylide 3. “L” represents the ligand NMP.
Figure 8. Free energy profile of pathway Path-F for the cycloaddition of silver isocyanide 6 and silver acetylide 3. “L” represents the ligand NMP. Figure 10. Free energy profile of pathway Path-H for the cycloaddition of silver isocyanide 6 and silver acetylide 3. “L” represents the ligand NMP.
G, 1,4-silver migration takes place through a five-membered ring transition state ts-G1, forming the iminosilver intermediate G2. The activation energy is 27.1 kcal/mol, which is 0.6 kcal/ mol higher than that via transition state ts-C1 in Path-C. Following 1,5-silver migration via transition state ts-G3 gives intermediate G4 with an increase of free energy by 9.2 kcal/ 6644
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ACS Catalysis mol. After the cycloaddition transition state ts-G5, which contains an 1,2-silver migration, the pyrrole scaffold 9 could be gained. Although this pathway involves the silver-migration as well, the activation free energy for the rate-determining step is 31.4 kcal/mol, which is 1.6 kcal/mol higher than that in Path-C. However, this possibility cannot be excluded thoroughly because of the small energy difference. In addition to the pathway depicted in Figure 9, the transformation from intermediate G2 to cyclization product 9 could also be achieved through the silver-migration (Ag3 in complex G2) among carbon C1, nitrogen N1, carbon C2 and C4. Detailed computational results are listed in the Supporting Information (Figure S1) as this possibility turns out to be unfavorable compared with Path-G. In Path-H (Figure 10), when transition state ts-H1 is formed, the coordination of carbon C1 to silver Ag2 leads the negative charge moved from silver Ag2 to carbon C4, then from carbon C4 to carbon C3 and C2. Due to the large negative NPA charge value (−0.451) of carbon C4, the negative charge move from silver Ag2 to carbon C4 is disadvantageous, which results in the higher relative free energy of transition state ts-H1 (30.4 kcal/ mol) compared with that of ts-C1 (26.5 kcal/mol). Subsequently, the six-membered ring intermediate H2 undergoes transition state ts-H3 could afford the pyrrole scaffold 10. As depicted in the free energy profile, the overall barrier of Path-H is 36.6 kcal/mol, which is 6.8 kcal/mol higher than that of Path-C (29.8 kcal/mol). The mechanism proposed by Bi and co-workers has also been considered in calculation (Figure 11). The activation free
Figure 12. Silver catalyzed [3 + 2] cycloaddition of ethyl 2isocyanoacetate and ethyl 3-phenylpropiolate.
free energy profile for this reaction is obtained and exhibited in Figure 13. The generation of pyrrole scaffold C7 in Path-C, is
Figure 11. 1,1-Insertion of isocyanide into the C−Ag bond of silver acetylide 3.
Figure 13. Free energy profiles of pathway Path-C and Path-D for the cycloaddition of silver isocyanide 6 with ethyl 3-phenylpropiolate 19. “L” represents the ligand NMP.
energy for the 1,1-insertion of isocyanide into the C−Ag bond of silver acetylide 3 is calculated to be 39.6 kcal/mol, which is 9.8 kcal/mol higher than the overall barrier in Path-C (29.8 kcal/mol). Thus, the mechanism proposed by Bi can be safely ruled out (See Figure S11 in the Supporting Information for details). Based on the theoretical studies shown above, Path-C turns out to be the most favorable pathway as the overall barrier therein is the lowest among all these possibilities (Another plausible mechanism in which silver isocyanide 8 reacts with silver acetylide 3 shown in Figure S2 is also eliminated).23 This conclusion is in good accordance with the NPA results, which suggest the silver-migration in Path-C could lead the movement of charge into a rational way, thereby facilitates the cycloaddition. While in other possibilities, the negative charge move during the reaction is disadvantageous. Analogous silvermigration mechanism is further applied to internal alkynes participated reactions to test its validity. As reported in the experiment, internal alkynes could also be employed as the substrate and afford better yields under the same condition. The cycloaddition between ethyl 2-isocyanoacetate 4 and ethyl 3-phenylpropiolate 19 could give 2,3,4trisubstituted product in 89% yield (Figure 12). Corresponding
accomplished through two 1,5-silver migration transition states ts-C4 and ts-C6. Relative free energies of ts-C4 and ts-C6 are 25.2 kca/mol and 18.1 kcal/mol, respectively. While in Path-D, the activation energy for rate-determining step is 30.1 kcal/mol, which is 4.9 kcal/mol higher than that in Path-C as the negative charge move in transition state ts-D4 is unreasonable. Therefore, although the barrier of subsequent cyclization via transition state ts-D6 is only 14.8 kcal/mol, Path-C is supposed to be much more favorable than Path-D. In addition, other conceivable pathways including the synergetic mechanism have also been considered and proved to be irrational for the cycloaddition of silver isocyanide 2 and ethyl 3-phenylpropiolate 6. Computational details for the protonation process and other pathways are listed in the Supporting Information (Figure S3−S6) for clarity. In order to provide further support for the silver-migration mechanism, a series of competitive experiments between internal alkyne 20 and terminal alkynes (1, 1b, and 1c) were carried out (Scheme 2). Both electron-donating and electronwithdrawing group para-substituted phenylacetylenes were employed. When two equivalents of 20 and terminal alkyne (1, 1b, or 1c) were simultaneously added into one reaction under the optimal condition, the major product is found to be 6645
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University (Project 0903005203191). We are also thankful for the project (No.106112015CDJZR228806) supported by the Fundamental Research Funds for the Central Universities (Chongqing University).
Scheme 2. Competitive Experiments between Internal Alkyne and Terminal Alkynesa
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(1) For a review see: de Meijere, A., Diederich, F. Metal-Catalyzed Cross-Coupling Reactions, 2nd completely revised and enlarged ed.; Wiley-VCH: Weinheim, 2004; pp 1−490. (2) (a) Niu, S.; Hall, M. B. Chem. Rev. 2000, 100, 353−406. (b) Lin, Z. Acc. Chem. Res. 2010, 43, 602−611. (3) (a) Lautens, M.; Klute, W.; Tam, W. Chem. Rev. 1996, 96, 49−92. (b) Brown, J. M.; Cooley, N. A. Chem. Rev. 1988, 88, 1031−1046. (c) Zeni, G.; Larock, R. C. Chem. Rev. 2006, 106, 4644−4680. (d) Wu, X. F.; Neumann, H.; Beller, M. Chem. Rev. 2013, 113, 1−35. (e) Tsuji, J. Palladium Reagents and Catalysts: New Perspectives for the 21th Century, 1st ed.; Wiley: New York, 2004; pp 6−97. (4) (a) Ma, S.; Gu, Z. Angew. Chem., Int. Ed. 2005, 44, 7512−7517. (b) Ishida, N.; Shimamoto, Y.; Yano, T.; Murakami, M. J. Am. Chem. Soc. 2013, 135, 19103−19106. (c) Burns, D. J.; Lam, H. W. Angew. Chem., Int. Ed. 2014, 53, 9931−9935. (d) Partridge, B. M.; Solana Gonzalez, J.; Lam, H. W. Angew. Chem., Int. Ed. 2014, 53, 6523−6527. (e) Ikeda, Y.; Takano, K.; Waragai, M.; Kodama, S.; Tsuchida, N.; Takano, K.; Ishii, Y. Organometallics 2014, 33, 2142−2145. (5) Hansen, A. L.; Ebran, J. P.; Ahlquist, M.; Norrby, P. O.; Skrydstrup, T. Angew. Chem., Int. Ed. 2006, 45, 3349−3353. (6) (a) Ikeda, Y.; Takano, K.; Kodama, S.; Ishii, Y. Organometallics 2014, 33, 3998−4004. (b) Karig, G.; Moon, M.-T.; Thasana, N.; Gallagher, T. Org. Lett. 2002, 4, 3115−3118. (c) Hayashi, T.; Inoue, K.; Taniguchi, N.; Ogasawara, M. J. Am. Chem. Soc. 2001, 123, 9918− 9919. (d) Campo, M. A.; Larock, R. C. J. Am. Chem. Soc. 2002, 124, 14326−14327. (e) Zhang, J.; Liu, J. F.; Ugrinov, A.; Pillai, A. F.; Sun, Z. M.; Zhao, P. J. Am. Chem. Soc. 2013, 135, 17270−17273. (f) Yamabe, H.; Mizuno, A.; Kusama, H.; Iwasawa, N. J. Am. Chem. Soc. 2005, 127, 3248−3249. (g) Ikeda, Y.; Takano, K.; Kodama, S.; Ishii, Y. Chem. Commun. 2013, 49, 11104−11106. (h) Piou, T.; Bunescu, A.; Wang, Q.; Neuville, L.; Zhu, J. Angew. Chem., Int. Ed. 2013, 52, 12385−12389. (7) (a) Wang, T.; Chen, S.; Shao, A.; Gao, M.; Huang, Y.; Lei, A. Org. Lett. 2015, 17, 118−121. (b) Chen, Y. R.; Duan, W. L. J. Am. Chem. Soc. 2013, 135, 16754−16757. (c) Sarkar, R.; Mukhopadhyay, C. Eur. J. Org. Chem. 2015, 2015, 1246−1256. (d) Naodovic, M.; Yamamoto, H. Chem. Rev. 2008, 108, 3132−3148. (e) Liu, Y. Y.; Yang, X. H.; Yang, J.; Song, R. J.; Li, J. H. Chem. Commun. 2014, 50, 6906−6908. (f) Unoh, Y.; Hirano, K.; Satoh, T.; Miura, M. Angew. Chem., Int. Ed. 2013, 52, 12975−12979. (8) (a) Liu, W.; Jiang, H.; Huang, L. Org. Lett. 2010, 12, 312−315. (b) He, C.; Guo, S.; Ke, J.; Hao, J.; Xu, H.; Chen, H.; Lei, A. J. Am. Chem. Soc. 2012, 134, 5766−5769. (c) Zhang, X.; Liu, B.; Shu, X.; Gao, Y.; Lv, H.; Zhu, J. J. Org. Chem. 2012, 77, 501−510. (d) Li, C.-J.; Correia, C. A. Heterocycles 2010, 82, 555. (e) Á lvarez-Corral, M.; Muñoz-Dorado, M.; Rodríguez-García, I. Chem. Rev. 2008, 108, 3174− 3198. (9) (a) Halbes-Letinois, U.; Weibel, J. M.; Pale, P. Chem. Soc. Rev. 2007, 36, 759−769. (b) Weibel, J. M.; Blanc, A.; Pale, P. Chem. Rev. 2008, 108, 3149−3173. (10) Gao, M.; He, C.; Chen, H.; Bai, R.; Cheng, B.; Lei, A. Angew. Chem., Int. Ed. 2013, 52, 6958−6961. (11) Liu, J.; Fang, Z.; Zhang, Q.; Liu, Q.; Bi, X. Angew. Chem., Int. Ed. 2013, 52, 6953−6957. (12) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004−2021. (13) (a) Moses, J. E.; Moorhouse, A. D. Chem. Soc. Rev. 2007, 36, 1249−1262. (b) Amblard, F.; Cho, J. H.; Schinazi, R. F. Chem. Rev. 2009, 109, 4207−4220. (14) (a) Hein, J. E.; Fokin, V. V. Chem. Soc. Rev. 2010, 39, 1302− 1315. (b) Meldal, M.; Tornoe, C. W. Chem. Rev. 2008, 108, 2952− 3015. (c) Himo, F.; Lovell, T.; Hilgraf, R.; Rostovtsev, V. V.;
a
Yields shown were determined by NMR analysis with CH2Br2 as the internal standard.
product 21 resulted from internal alkyne 20 and only trace amount of 21a−21c were observed. This phenomenon indicates that internal alkyne 20 is more reactive than terminal alkyne 1, 1b, and 1c in this reaction. On the other hand, the acitvation energies for 20, 1, 1b, and 1c participated reactions were calculated to be 25.4 (20), 29.8 (1), 29.2 (1b), and 30.5 (1c) kcal/mol, respectively, which indicated the overall barrier for the cycloaddition with 20 is the lowest (Figure S7−9). Therefore, the DFT calculations for the reactivity of those alkynes correspond well with the competitive experiments.
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CONCLUSIONS In summary, a combined experimental and theoretical study were conducted to reveal the mechanism of silver catalyzed isocyanide-alkyne [3 + 2] cycloadditions. The most favorable pathway for terminal alkynes participated reactions labeled as Path-C consists of two successive 1,5-silver migration transition states, in which the silver migrates between carbon atoms and finally returns to the first one. Moreover, NPA charge analysis has revealed that the silver-migration plays a significant role in Path-C as it guides the move of negative charge into a rational way, thereby facilitates the entire reaction. An analogous silvermigration mechanism is also suitable for the cycloaddition of isocyanide with internal alkynes, which shows the universality of silver-migration process. Besides, the competitive experiments corresponds well with the computational results, which could provide further support for the mechanism.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02009. Cartesian coordinates and energies of all reported structures and full authorship of Gaussian 09 (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (M.G.). *E-mail:
[email protected] (C.L.). *E-mail:
[email protected] (Y.L.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grants 21372266, 51302327, 21302148 and 21272180), and the Foundation of 100Young Chongqing 6646
DOI: 10.1021/acscatal.5b02009 ACS Catal. 2015, 5, 6640−6647
Research Article
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DOI: 10.1021/acscatal.5b02009 ACS Catal. 2015, 5, 6640−6647
