Mechanistic Comparison of Acid- and Gold(I)-Catalyzed

(21, 22) As we discussed in the Introduction, Hartwig's(16) and He's(17) groups found that TfOH can catalyze the aforementioned reactions with similar...
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Organometallics 2010, 29, 5919–5926 DOI: 10.1021/om1007192

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Mechanistic Comparison of Acid- and Gold(I)-Catalyzed Nucleophilic Addition Reactions to Olefins G abor Kov acs, Agustı´ Lled os, and Gregori Ujaque* Departament de Quı´mica, Universitat Aut onoma de Barcelona, 08193 Bellaterra (Barcelona), Spain Received July 22, 2010

Density functional theory calculations were used to explore the mechanism of Me3PAuOTf (gold)- and TfOH (Brønsted acid)-catalyzed nucleophilic additions to olefins. Addition of N- and O-nucleophiles have been theoretically modeled by the reactions of CbzNH2 (hydroamination) and PhOH (hydroalkoxylation) with ethene. Both gold(I) and Brønsted acids are shown to be active catalysts for the reaction. The gold-catalyzed mechanisms were found to be fully consistent with those proposed earlier for the hydroamination of dienes and hydroalkoxylation of alkenes. The gold(I)-catalyzed hydroamination was found to be stepwise, whereas the hydroalkoxylation was found to proceed via a concerted step. The TfOH-catalyzed process was found to be concerted.

Introduction Addition of N- or O-nucleophiles to olefinic double bonds represents a synthetic process that provides the formation of important starting materials for organic syntheses.1 Hence, it is not surprising that these synthetic processes have been the focus of organic and organometallic catalysis studies. Earlier several metal complexes that involve palladium,2 rhodium,3 ruthenium,4 and platinum5 centers were found to be active catalysts for the aforementioned processes. Catalysis with gold complexes6 has become the center of interest for the past few years, although complexes containing gold(I) or gold(III) metal centers previously had been *To whom correspondence should be addressed. Fax: þ34-93-5812920. E-mail: [email protected]. (1) Some reviews: (a) Schmidt, R. R. Acc. Chem. Res. 1986, 19, 250. (b) Lichtentaler, F. W. Mod. Synth. Methods 1992, 6, 273. (c) M€uller, T. E.; Beller, M. Chem. Rev. 1998, 98, 675. (d) Deiters, A.; Martin, S. F. Chem. Rev. 2004, 104, 2199. (e) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673. (f) Patil, N. T.; Yamamoto, Y. Chem. Rev. 2008, 108, 3395. (g) Muller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M. Chem. Rev. 2008, 108, 3795. (h) Pastori, N.; Gambarotti, C.; Punta, C. Mini-Rev. Org. Chem. 2009, 6, 184. (i) Li, C.-J. Acc. Chem. Res. 2010, 43, 581. (2) See for instance: (a) Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 9546. (b) Utsunomiya, M.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 14286. (c) Stahl, S. S. Angew. Chem., Int. Ed. 2004, 43, 3400. (d) Phua, P. H.; Mathew, S. P.; White, A. J. P.; de Vries, J. G.; Blackmond, D. G.; Hii, K. K. Chem.;Eur. J. 2007, 13, 4602. (3) See for instance: (a) Ahmed, M.; Seayad, A. M.; Jackstell, R.; Beller, M. Angew. Chem., Int. Ed. 2003, 42, 5615. (b) Utsunomiya, M.; Kuwano, R.; Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 5608. (c) Field, L. D.; Messerle, B. A.; Voung, K. Q.; Turner, P. Organometallics 2005, 24, 4241. (d) Takemiya, A.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 6042. (4) See for instance: (a) Oe, Y.; Ohta, T.; Ito, Y. Chem. Commun. 2004, 1620. (b) Utsunomiya, M.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 2702. (c) Oe, Y.; Ohta, T.; Ito, Y. Synlett 2005, 179. (d) Takaya, J.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 5756. (5) See for instance: (a) Quian, H.; Han, X.; Widenhoefer, R. A. J. Am. Chem. Soc. 2004, 126, 9536. (b) Bender, C. F.; Widenhoefer, R. A. J. Am. Chem. Soc. 2005, 127, 1070. (c) Toups, K. L.; Widenhoefer, R. A. Chem. Commun. 2010, 46, 1712. r 2010 American Chemical Society

considered as catalysts of inferior activity compared with other transition metal complexes. Nevertheless, recently many groups have dedicated great efforts to develop efficient catalysts with gold metal centers.7,8 One of the reaction types that has been the focus of investigation with gold complexes is the nucleophilic addition of various functional groups to unsaturated substrates.9,10 Earlier, gold(I) was found to be an active catalyst for the addition of different nucleophiles to alkynes11 and allenes;12 nevertheless the use of gold complexes in activating olefins was more limited.13 Recently He and co-workers have reported the activation of inert olefinic double bonds with Ph3PAuOTf for the addition of both O-nucleophiles (phenols or carboxylic acids)14 and N-nucleophiles.15 More recently Hartwig16 and He17 have independently shown that triflic acid (TfOH) as a Brønsted acid can play the role of the catalyst in both the hydroamination and the hydroalkoxylation of simple olefins. Previously, Brønsted acid-catalyzed hydroamination of imines,18 ketones,19 or activated alkenes20 had already been well known. These recent works, however, represent efficient nucleophilic addition of (6) Reviews on gold catalysis: (a) Hashmi, A. S. K. Gold Bull. 2004, 37, 51. (b) Echavarren, A. M.; Nevado, C. Chem. Soc. Rev. 2004, 33, 431. (c) Hoffmann-R€oder, A.; Krause, N. Org. Biomol. Chem. 2005, 3, 387. (d) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896. (e) Ma, S.; Yu, S.; Gu, Z. Angew. Chem., Int. Ed. 2006, 45, 200. (f) JimenezNu~nez, E.; Echavarren, A. M. Chem. Commun. 2007, 333. (g) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180. (h) Marion, N.; Nolan, S. P. Angew. Chem., Int. Ed. 2007, 46, 2750. (i) Crone, B.; Kirsch, S. F. Chem.;Eur. J. 2008, 14, 3514. (j) Widenhoefer, R. A. Chem.;Eur. J. 2008, 14, 5382. (k) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239. (l) Arcadi, A. Chem. Rev. 2008, 108, 3266. (m) Jimenez-Nu~nez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326. (n) Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem. Rev. 2008, 108, 3351. (o) Shen, H. C. Tetrahedron 2008, 64, 3885. (p) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776. (q) Hashmi, A. S. K.; Rudolph, M. Chem. Soc. Rev. 2008, 37, 1766. (r) Skouta, R.; Li, C. L. Tetrahedron 2008, 64, 4917. (s) Bongers, N.; Krause, N. Angew. Chem., Int. Ed. 2008, 47, 2178. (t) F€urstner, A. Chem. Soc. Rev. 2009, 38, 3208. (u) Nevado, C. Chimia 2010, 64, 247. Published on Web 10/25/2010

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N- and O-nucleophiles to unactivated olefins using reaction conditions that can tolerate even substrates that were formerly considered incompatible with strong Brønsted- acids. Hartwig et al. found that the rates of the addition of sulfonamides were quite similar when TfOH was used instead of metal triflates, and He et al. found that triflic acid had similar activities in the hydroamination of 1,3-dienes with alkyl carbamates (such as CbzNH2, benzyl carbamate) as cationic gold(I) complexes. The aforementioned publications raised the debate on the question of acid- or metal-catalyzed reactions taking place in some of these systems. Hashmi thoroughly reviewed this (7) (a) Nieto-Oberhuber, C.; Perez-Galan, P.; Herrero-G omez, E.; Lauterbach, T.; Rodrı´ guez, C.; L opez, S.; Bour, C.; Rosell on, A.; C ardenas, D. J.; Echavarren, A. M. J. Am. Chem. Soc. 2008, 130, 269. (b) Nieto-Oberhuber, C.; Lopez, S.; Echavarren, A. M. J. Am. Chem. Soc. 2005, 127, 6178. (c) Hashmi, A. S. K.; Rudolph, M.; Weyrauch, J. P.; W€olfle, M.; Frey, W.; Bats, J. W. Angew. Chem., Int. Ed. 2005, 44, 2798. (d) Furstner, A.; Stimson, C. C. Angew. Chem., Int. Ed. 2007, 46, 8845. (e) Zhang, G.; Huang, X.; Li, G.; Zhang, L. J. Am. Chem. Soc. 2008, 130, 1814. (f) Lemiere, G.; Gaudon, V.; Agente, N.; Goddard, J. P.; de Kozak, A.; Aubert, C.; Feusternbank, L.; Malacria, M. Angew. Chem., Int. Ed. 2006, 45, 7596. (g) Xia, Y. Z.; Dudnik, A. S.; Gevorgyan, V.; Li, Y. H. J. Am. Chem. Soc. 2008, 130, 6940. (h) García-Mota, M.; Cabello, N.; Maseras, F.; Echavarren, A. M.; Perez-Ramírez, J.; Lopez, N. ChemPhysChem 2008, 9, 1624. (i) Lemiere, G.; Gandon, V.; Cariou, K.; Hours, A.; Fukuyama, T.; Dhimane, A.-L.; Fensterbank, L.; Malacria, M. J. Am. Chem. Soc. 2009, 131, 2993. (j) Trillo, B.; L opez, F.; Montserrat, S.; Ujaque, G.; Castedo, L.; Lledos, A.; Mascare~ nas, J. L. Chem.;Eur. J. 2009, 15, 3336. (k) Alonso, I.; Trillo, B.; os, A.; Mascare~nas, L opez, F.; Montserrat, S.; Ujaque, G.; Castedo, L.; Lled J. L. J. Am. Chem. Soc. 2009, 131, 13020. (l) Cuenca, A. B.; Montserrat, S.; Hossain, K. M.; Mancha, G.; Lledos, A.; Medio-Simon, M.; Ujaque, G.; Asensio, G. Org. Lett. 2009, 11, 4906. (m) Bandini, M.; Eichholzer, A. Angew. Chem., Int. Ed. 2009, 48, 9533. (n) Bolte, B.; Odabachian, Y.; Gagosz, F. J. Am. Chem. Soc. 2010, 132, 21. (o) Garcia, P.; Malacria, M.; Aubert, C.; Gandon, V.; Fensterbank, L. ChemCatChem 2010, 2, 493. (p) Liu, Y.; Xu, W.; Wang, X. Org. Lett. 2010, 12, 1448. (q) Zhu, R.-X.; Zhang, D. J.; Guo, J.-X.; Mu, J.-L.; Duan, C.-G.; Liu, C.-B. J. Phys. Chem. A 2010, 114, 4689. (r) Dudnik, A. S.; Xia, Y.; Li, Y.; Gevorgyan, V. J. Am. Chem. Soc. 2010, 132, 7645. (8) (a) Hashmi, A. S. K.; Weyrauch, J. P.; Frey, W.; Bats, J. W. Org. Lett. 2004, 6, 4391. (b) Straub, B. F. Chem. Commun. 2004, 15, 1726. (c) Hashmi, A. S. K.; Rudolph, M.; Weyrauch, J. P.; W€olfle, M.; Frey, W.; Bats, J. W. Angew. Chem., Int. Ed. 2005, 44, 2798. (d) Nevado, C.; Echavarren, A. M. Chem.;Eur. J. 2005, 11, 3155. (e) Roithova, J.; Hrusak, J.; Schr€oder, D.; Schwarz, H. Inorg. Chim. Acta 2005, 358, 4287. (f) Fructos, M. R.; Belderrain, T. R.; de Fremont, P.; Scott, N. M.; Nolan, N. M.; Díaz-Requejo, M. M.; Perez, P. J. Angew. Chem., Int. Ed. 2005, 44, 5284. (g) Nieto-Faza,  O.; Silva-L opez, C.; Alvarez, R.; de Lera, A. R. J. Am. Chem. Soc. 2006, 128, 2434. (h) Comas-Vives, A.; Gonzalez-Arellano, C.; Boronat, M.; Corma, A.; Iglesias, M.; Sanchez, F.; Ujaque, G. J. Catal. 2008, 254, 226. (i) Comas-Vives, A.; Gonzalez-Arellano, C.; Corma, A.; Iglesias, M.; Sanchez, F.; Ujaque, G. J. Am. Chem. Soc. 2006, 128, 4756. (j) Hashmi, A. S. K.; Rudolph, M.; Hans, H. U.; Tanaka, M.; Bats, J. W.; Frey, W. Chem.;Eur. J. 2008, 14, 3703. (k) Yang, C. Y.; Lin, G. Y.; Liao, H. Y.; Datta, S.; Liu, R. S. J. Org. Chem. 2008, 73, 4907. (l) Gorin, D. J.; Watson, I. D. G.; Toste, F. D. J. Am. Chem. Soc. 2008, 130, 3736. (m) Sordo, T. L.; Ardura, D. Eur. J. Org. Chem. 2008, 17, 3004. (n) Urbano, J.; Hormigo, A. J.; de Fremont, P.; Nolan, S. P.; Díaz-Requejo, M. M.; Perez, P. J. Chem. Commun. 2008, 6, 759. (o) Zhu, Z. B.; Shi, M. Chem.;Eur. J. 2008, 14, 10219. (p) Correa, A.; Marion, N.; Fensterbank, L.; Malacria, M.; Nolan, S. P.; Cavallo, L. Angew. Chem., Int. Ed. 2008, 47, 718. (q) Davies, P. W.; Martin, N. Org. Lett. 2009, 11, 2293. (r) Cui, L.; Ye, L.; Zhang, L. Chem. Commun. 2010, 46, 3351. (s) Hashmi, A, S. K. Pure Appl. Chem. 2010, 82, 657. (9) Selected examples for N-additions: (a) Zhang, Z. B.; Bender, C. F.; Widenhoefer, R. A. J. Am. Chem. Soc. 2007, 129, 14148. (b) Leseurre, L.; Toullec, P. Y.; Genet, J. P.; Michelet, V. Org. Lett. 2007, 9, 4049. (c) Giner, X.; Najera, C. Org. Lett. 2008, 10, 2919. (d) Bender, C. F.; Widenhoefer, R. A. Chem. Commun. 2008, 24, 2741. (e) Enomoto, T.; Obika, S.; Yasui, Y.; Takemoto, Y. Synlett. 2008, 11, 1647. (f) Lavallo, V.; Frey, G. D.; Donnadieu, B.; Soleilhavoup, M.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 5224. (g) Zhang, X.; Corma, A. Dalton Trans. 2008, 3, 397. (h) Zhang, Z. B.; Lee, S. D.; Widenhoefer, R. A. J. Am. Chem. Soc. 2009, 131, 5372. (i) Yeh, M. C. P.; Pai, H. F.; Lin, Z. J.; Lee, B. R. Tetrahedron 2009, 65, 4789. (j) Zeng, X.; Kinjo, R.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2010, 49, 942. (k) Patil, N. T.; Kavthe, R. D.; Raut, V. S.; Shinde, V. S.; Balasubramanian, S. J. Org. Chem. 2010, 75, 1277.

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question by examining those examples where the gold- and proton-catalyzed processes were studied.21 More recently Hii et al.22 published a comprehensive work on this issue concerning systems with various transition metals, and the conclusion is that we cannot have a general preference for acid versus metal catalysis (or vice versa), as evidence exists for both; conversely, it always depends on the current system. We have recently performed a detailed analysis of the possible mechanistic pathways for the hydroamination of conjugated dienes23 with CbzNH2 and the hydroalkoxylation of alkenes with phenol catalyzed by the Ph3PAuOTf complex.24 The most favorable mechanism for the hydroamination was found to occur through a stepwise process, which includes (i) the attack of the nucleophile on the coordinated double bond, (ii) tautomerization of the Cbz-NH2 moiety, and (iii) proton transfer from the tautomerized moiety to complete the saturation of the double bond. Nevertheless, in the case of the hydroalkoxylation, the most favorable pathway was found to occur in a concerted way (nucleophile attack and proton transfer in one single step) assisted by a proton-transfer agent (phenol, triflate, or water) present in the solution. (10) Selected examples for O-additions: (a) Zhang, Z.; Widenhoefer, R. A. Angew. Chem., Int. Ed. 2007, 46, 283. (b) Dai, L. Z.; Qi, M. J.; Shi, Y. L.; Liu, X. G.; Shi, M. Org. Lett. 2007, 9, 3191. (c) Horino, Y.; Takata, Y.; Hashimoto, K.; Kuroda, S.; Kimura, M.; Tamaru, M. Y. Org. Biol. Chem. 2008, 6, 4105. (d) Kinder, R. E.; Zhang, Z.; Widenhoefer, R. A. Org. Lett. 2008, 10, 3157. (e) Zhang, Z; Widenhoefer, R. A. Org. Lett. 2008, 10, 2079. (f) Fujita, K. I.; Kujime, M.; Muraki, T. Bull. Chem. Soc. Jpn. 2009, 82, 261. (g) Wang, M. Z.; Wong, M. K.; Che, C. M. Chem.;Eur. J. 2008, 14, 8353. (h) Nishina, N.; Yamamoto, Y. Tetrahedron Lett. 2008, 49, 4908. (i) Chao, C. M.; Vitale, M. R.; Toullec, P. Y.; Genet, J. P.; Michelet, V. Chem.;Eur. J. 2009, 15, 1319. (j) Zhang, Z. B.; Lee, S. D.; Fisher, A. S.; Widenhoefer, R. A. Tetrahedron 2009, 65, 1794. (k) Hirai, T.; Hamasaki, A.; Nakamura, A.; Tokunaga, M. Org. Lett. 2009, 11, 5510. (l) Patil, N. T.; Singh, V.; Ashok, K.; Mutyala, A. K. Tetrahedron Lett. 2010, 51, 1493. (11) Some examples: (a) Teles, J. H.; Brode, S.; Chabanas, M. Angew. Chem., Int. Ed. 1998, 37, 1415. (b) Hashmi, A. S. K.; Schwarz, L.; Choi, J.-H.; Frost, T. M. Angew. Chem., Int. Ed. 2000, 39, 2285. (c) Marion, N.; Carlqvist, P.; Gealageas, R.; de Fremont, P.; Maseras, F.; Nolan, S. P. Chem.;Eur. J. 2007, 13, 6437. (d) Hashmi, A. S. K.; Rudolph, M.; Hans, H. U.; Tanaka, M.; Bats, J. W.; Frey, W. Chem.;Eur. J. 2008, 14, 3703. (e) Yang, C. Y.; Lin, G. Y.; Liao, H. Y.; Datta, S.; Liu, R. S. J. Org. Chem. 2008, 73, 4907. (f) Feng, E.; Zhou, Y.; Zhang, D.; Zhang, L.; Sun, H.; Jiang, H.; Liu, H. J. Org. Chem. 2010, 75, 3274. (g) Wang, C.; Han, Z.-Y.; Luo, H.-W.; Gong, L.-Z. Org. Lett. 2010, 12, 2266. (h) Cui, D.-M.; Zheng, J.-Z.; Yang, L.-Y.; Zhang, Z. Synlett. 2010, 5, 809. (12) (a) Li, H.; Widenhoefer, R. A. Org. Lett. 2009, 11, 2671. (b) Manzo, A. M.; Perboni, A. D.; Broggini, G.; Rigamonti, M. Tetrahedron Lett. 2009, 50, 4696. (c) Nishina, N.; Yamamoto, Y. Tetrahedron 2009, 65, 1799. (d) See also refs 7a, 7e, 8d, and 8e. (13) Gold-catalyzed alkene hydroalkoxylation was first postulated by Hashmi et al. See ref 11b. (14) Yang, C.-G.; He, C. J. Am. Chem. Soc. 2005, 127, 6966. (15) (a) Zhang, J.; Yang, C.-G.; He, C. J. Am. Chem. Soc. 2006, 128, 1798. (b) Brouwer, C.; He, C. Angew. Chem., Int. Ed. 2006, 45, 1744. (16) Rosenfeld, D. C.; Shekhar, S.; Takemiya, A.; Utsunomiya, M.; Hartwig, J. F. Org. Lett. 2006, 8, 4179. (17) Li, Z.; Zhang, J.; Brouwer, C.; Yang, C.-G.; Reich, N. W.; He, C. Org. Lett. 2006, 8, 4175. (18) Examples: (a) Uraguchi, D.; Terada, M. J. Am. Chem. Soc. 2004, 126, 5356. (b) Akiyama, T.; Morita, H.; Itoh, J.; Fuchibe, K. Org. Lett. 2005, 7, 2583. (c) Magnus, R.; Sugiono, E.; Azap, C.; Theissmann, T.; Bolte, M. Org. Lett. 2005, 7, 3781. (19) Wabnitz, T. C.; Spencer, J. B. Org. Lett. 2003, 5, 2141. (20) (a) Schlummer, B.; Hartwig, J. F. Org. Lett. 2002, 4, 1471. (b) Anderson, L. L.; Arnold, J.; Bergman, R. G. J. Am. Chem. Soc. 2005, 127, 14542. (21) Hashmi, A. S. K. Catal. Today 2007, 122, 211. (22) Taylor, J. G.; Adrio, L. A.; Hii, K. K. Dalton Trans. 2010, 39, 1171. (23) Kovacs, G.; Ujaque, G.; Lled os, A. J. Am. Chem. Soc. 2008, 130, 853. (24) Kovacs, G.; Ujaque, G.; Lled os, A. Organometallics 2010, 29, 3252.

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Scheme 1. Energy Profile in Toluene for the Mechanism of the Triflate-Assisted Hydroamination of Ethene with CbzNH2 Catalyzed by Me3PAuOTf

Other important publications concerning the theoretical analysis of the Au-catalyzed nucleophilic addition to electrophiles as allenes and alkynes have also been published. The cycloisomerization of bromoallenyl ketones has been evaluated by Li, Gevorgyan, et al.,25 demonstrating that the regiochemistry depends on the used counterion. Paton and Maseras26 carried out a DFT analysis of the hydroalkoxylation of allenes with a gold carbene catalyst, and the observed regioselectivity was found to be due to an allylic ether isomerization subsequent to the nucleophilic addition. Li et al.27 analyzed the cycloaddition of alkynyl cyclopropyl ketones, demonstrating the important effect of the ligand and the counterion on the reaction mechanism. Lein et al. published recently a work concerning the gold-catalyzed addition of water to alkynes, and the importance of hydrogen bonds to guide through the substrate in the reaction was shown.28 We have to mention that Yu and He have recently published a computational work29 concerning the Brønsted acid-catalyzed additions of phenols and protected amines to olefins. They found that the most favorable reaction pathway involved a cyclic concerted transition state and the formation of the syn-product. In the current work we present a comparative study of the Me3PAuOTf (gold)- and TfOH (Brønsted acid)-catalyzed hydroamination and hydroalkoxylation of simple olefins (ethylene was used as model substrate) with CbzNH2 and phenol (being consistent with the previous theoretical works), respectively.

Results and Discussion I. Gold-Catalyzed Nucleophilic Additions. I.1. Hydroamination. First, we present the results for the gold-catalyzed (25) Xia, Y. Z.; Dudnik, A. S.; Gevorgyan, V.; Li, Y. H. J. Am. Chem. Soc. 2008, 130, 6940. (26) Paton, R. S.; Maseras, F. Org. Lett. 2009, 11, 2237. (27) Zhang, J.; Shen, W.; Li, L.; Li, M. Organometallics 2009, 28, 3129. (28) Lein, M.; Rudolph, M.; Hashmi, A. S. K.; Schwerdtfeger, P. Organometallics 2010, 29, 2208. (29) Li, X.; Ye, S.; He, C.; Yu, Z.-X. Eur. J. Org. Chem. 2008, 4296.

Figure 1. Transition states (TS-PA1 and TS-PA2) for the nucleophile-assisted hydroamination of ethene with CbzNH2 catalyzed by Me3PAuOTf.

hydroamination of ethene (selected as model olefin) with CbzNH2. The most favorable mechanistic pathway is completely consistent with the one obtained for the hydroamination of 3-methyl-1,3-pentadiene, previously analyzed by us.23 This pathway involves the same elementary steps as in the case of the diene substrate, which are (i) substitution of triflate anion in Me3PAuOTf with ethene; (ii) nucleophilic addition of CbzNH2 to the double bond; (iii) triflate-assisted “H2N-CdO” to “HNdC-OH” tautomerization of the CbzNH2 moiety; and (iv) subsequent proton transfer from the OH group to the other carbon atom, which leads to product formation and catalyst regeneration (Scheme 1). The global energy barrier for this mechanistic pathway is 18.2 kcal/mol. In addition, on the basis of the results obtained concerning the mechanism for the hydroalkoxylation reaction with the

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Scheme 2. Energy Profile in Toluene for the Mechanism of the Nucleophile-Assisted Hydroamination of Ethene with CbzNH2 Catalyzed by Me3PAuOTf

same catalyst (see below), an alternative mechanism was also considered. It was evaluated that instead of triflate anion, another CbzNH2 molecule can also act as a proton-transfer agent in the reaction, providing feasible energy barriers. In this pathway, the substitution of triflate anion in Me3PAuOTf with ethene and the nucleophilic addition of CbzNH2 to the double bond are the same as for the triflate-assisted pathway. Nevertheless, these preliminary steps are followed by the proton transfer from the NH2 moiety to the NH2 group of a CbzNH2 molecule in the solution through a low (12.5 kcal/mol) energy barrier (TS-PA1, Figure 1). The reaction is completed by the protonation of the unsaturated carbon atom with the CbzNH3þ formed in the solution (TS-PA2, Figure 1); the energy barrier for this pathway is 18.3 kcal/mol, representing the global barrier of the process. The energy profile is shown in Scheme 2. When the nucleophile is acting as a proton-transfer agent, there is no need for the tautomerization process. This could explain how these catalysts still work with substrates, which are not susceptible to tatuomerization, like NH3.9f Similar behavior was found in the hydroamination of alkenes with NH3 studied by Senn et al.30 by means of dynamic ab initio DFT methods. We can conclude that this pathway can compete with the triflate-assisted pathway proposed earlier by us for the hydroamination of dienes with the same gold complex. In both possible pathways the role of a protontransfer agent, either triflate or the nucleophile, is crucial for (30) Senn, H. M.; Bl€ ochl, P. E.; Togni, A. J. Am. Chem. Soc. 2000, 122, 4098. (31) (a) Appelhans, L. N.; Zuccaccia, D.; Kovacevic, A.; Chianese, A. R.; Miecznikowski, J. R.; Macchioni, A; Clot, E.; Eisenstein, O.; Crabtree, R. H. J. Am. Chem. Soc. 2005, 127, 16299. (b) Davies, D. L.; Donald, S. M. A.; Macgregor, S. A. J. Am. Chem. Soc. 2005, 127, 13754. (c) García-Cuadrado, D.; Braga, A. A. C.; Maseras, F.; Echavarren, A. M. J. Am. Chem. Soc. 2006, 128, 1066. (d) Basallote, M. G.; Besora, M.; Duran, J.; Fernandez-Trujillo, M. J.; Lledos, A.; Ma~nez, M. A.; Maseras, F. J. Am. Chem. Soc. 2004, 126, 2320. (e) Balcells, D.; Ujaque, G.; Fernandez, I.; Khiar, N.; Maseras, F. J. Org. Chem. 2006, 71, 6388. (f) Basallote, M. G.; Besora, M.; Castillo, C. E.; Fernandez-Trujillo, M. J.; Lledos, A.; Maseras, F.; Ma~ nez, M. A. J. Am. Chem. Soc. 2007, 129, 6608.

the reaction facilitating the proton transfer after the nucleophile attack. This finding is in accordance with other experimental and theoretical works, showing the important role of proton shuttles in proton-transfer processes.31 I.2. Hydroalkoxylation. The results obtained for the goldcatalyzed hydroalkoxylation of ethene with phenol catalyzed by Me3PAuOTf are completely consistent with the results published for the hydroalkoxylation of 4-phenylbut-1-ene with the same catalyst.24 The first step of the reaction is the replacement of the triflate ligand in Me3PAuOTf with the alkene substrate, similarly to the hydroamination reaction. However, the nucleophilic addition of phenol and the proton-transfer take place simultaneously. The most favorable pathway for the reaction was found to be the participation of another phenol molecule, acting as a proton shuttle. This proton transfer agent plays an important role in lowering the energy barrier for the reaction, similarly to the hydroamination reaction. The concerted transition state for the process (TS-PF), which represents the global energy barrier for the process (28.2 kcal/mol) is shown in Figure 2, and the energy profile is shown in Scheme 3. Moreover, the process was also analyzed in the presence of two more phenol molecules (a total of three PhOH molecules). The energy barrier for the system with the phenol trimer was found to be 32.9 kcal/mol, somewhat higher than that with the dimer form. These results show that the most important effect comes from the presence of a proton-transfer agent in the system. Additional PhOH molecules do not significantly help the reaction.32 In a previous computational study, we have carried out calculations concerning the possible mechanistic pathways for the hydroalkoxylation of 4-phenylbut-1-ene with the same catalyst, and it was found that the reaction has a very high barrier (60.1 kcal/mol) without a proton-transfer agent, as the proton transfer takes place through an unfavorable (32) The dimerization and trimerization energies of phenol are -5.9 and -12.8 kcal/mol, respectively. They show that the addition of a PhOH molecule stabilizes the system to a similar extent.

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Figure 3. Transition state for the proton transfer from TfOH to ethene (TS-P1). Scheme 4. Different Mechanistic Alternatives (syn and anti) for the Acid-Catalyzed Addition of Nucleophiles Figure 2. Concerted transition state for the hydroalkoxylation of ethene with phenol catalyzed by Me3PAuOTf. Scheme 3. Energy Profile in Toluene for the Mechanism for the Phenol-Assisted Hydroalkoxylation of Ethene with PhOH Catalyzed by Me3PAuOTf

four-membered transition state. However, when a protontransfer agent is present, which can be the nucleophile itself (phenol in this case), water molecules (always present when reactions are with alcohols), or the counterion (triflate anion), the barrier for the proton transfer is significantly lowered.24 II. Acid-Catalyzed Nucleophilic Additions. In this study the main goal of our work is comparing the gold vs acid catalysis, which issue continuously appears as a question in the literature, when metal-catalyzed nucleophilic additions are concerned.21,22 As we discussed in the Introduction, Hartwig’s16 and He’s17 groups found that TfOH can catalyze the aforementioned reactions with similar catalytic activities to that of gold complexes. In the following, first we present the theoretical results concerning the hydroamination of ethene with CbzNH2 and then the addition of phenol, both catalyzed by TfOH. II.1. Hydroamination. On the basis of basic chemical considerations it can be assumed that in this reaction the proton coming from TfOH plays a similar role to that of gold in activating the double bond. The classical organic chemistry

picture of this reaction is the protonation of the double bond and formation of the corresponding carbocation, which then reacts with the nucleophile to form the addition product. The transition state for this protonation (TS-P1) was found on the potential energy surface (Figure 3), characterized by a 32.7 kcal/ mol energy barrier. Nevertheless, the analysis of the potential energy surface (by means of IRC calculations) revealed that the separated formation of the carbocation and the triflate species was not feasible; instead, the CH3-CH2OTf (Int-OTf), lying 14.9 kcal/mol below the reactants, was obtained. This is not realistic concerning basic organic chemistry, since CbzNH2 represents a much better nucleophile; thus the formation of the intermediate Int-OTf is quite unlikely. This is probably due to the high tendency to avoid ionic molecules in vacuum. Nevertheless, the optimization in solution does not give separated ionic species either. Hence, the nucleophile was also included in the chemical model of the system during all the calculations. The inclusion of the nucleophile gives rise to two different mechanisms: (i) the syn-addition, with both the acid and the nucleophile on the same side of the olefin, and (ii) the anti-addition, with the acid and the nucleophile being on opposite sides (see Scheme 4). The syn-addition was theoretically analyzed by Yu and He using cyclohexene as alkene, TfOH as acid, and TsNH2 and PhOH as nucleophiles.29 The reaction takes place via a concerted eight-membered-ring transition state for both nucleophiles. The potential energy barriers were found to be 23.1 and 19.3 kcal/mol for TsNH2 and PhOH, respectively. We have performed the analysis of both the syn- and the anti-pathways for the case of benzyl carbamate (CbzNH2). The transition state TS-P2a for the anti-addition (Figure 4) lies 25.6 kcal/mol (representing the global barrier for the reaction) over the van der Waals adduct of the starting

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Kov acs et al.

Figure 4. Transition states (anti and syn) for the TfOH-catalyzed addition of CbzNH2 to ethene. Scheme 5. Energy Profiles in Toluene for the Acid-Catalyzed Hydroamination Reaction with CbzNH2

molecules (vdW-anti-A).33 If we analyze the transition-state geometry, it can be seen that the NH2 group of CbzNH2 is quite far from the carbon atom of the ethene (d(C-N) = 2.181 A˚), and the geometrical position of the transferred proton is similar to that in TS-P1 (d(C-H)TS-P1 = 1.238 A˚ vs d(C-H)TS-P2 = 1.261 A˚). The transition state, however, is concerted, in which the proton is transferred to one of the carbon atoms, and the nitrogen of the CbzNH2 forms a C-N bond with the other carbon atom; the triflate acid is regenerated by deprotonation of the NH2 group. We have performed the calculations concerning the synaddition, which is analogous to the one proposed by Yu et al.29 We carried out calculations with CbzNH2, to have comparable results with the anti-pathway. A concerted mechanism with an eight-membered transition state was (33) The formation of the vdW complex (vdw-anti-B) does not really depend on whether the monomer, dimer, or trimer form of PhOH is involved; their formation energies are -10.3, -10.3, and -10.7 kcal/ mol, respectively. (34) The formation of this vdW-syn-B complex from phenol dimer or phenol trimer is slightly more favorable than in the case of the monomer; the formation energies are -13.4, -13.4, and -10.0,kcal/mol, respectively. These results, however, do not include any modification on the mechanistic analysis.

found (TS-P2s, Figure 4), lying 22.8 kcal/mol over the van der Waals adduct of the substrates (vdW-syn-A).34 The transferred proton lies between the oxygen and carbon, similarly to that found in the case of the anti-addition (d(C-H)TS-P2a = 1.261 A˚ vs d(C-H)TS-P2s = 1.299 A˚), whereas the nitrogen is found far from the carbon atom (d(C-N)TS-P2a=2.181 A˚ vs d(C-N)TS-P2s=2.272 A˚), as seen in the anti-pathway. The obtained results (the overall energy profiles are shown in Scheme 5) show that both syn- and anti-additions have concerted mechanisms. The energy barriers are comparable, although indicating a preference for the synaddition; the energy barrier is 2.8 kcal/mol lower than for the anti-addition. This conclusion may be important for those cases where the additions are performed to prochiral olefins. II.2. Hydroalkoxylation. The reaction mechanism for the acid-catalyzed addition of an O-nucleophile to ethene, specifically PhOH, was also analyzed. As in the case of CbzNH2, the first step of the reaction is the protonation of the double bond with TfOH. Nevertheless, as previously explained, in order to have a proper description of the reaction, the nucleophile needs to be included in the calculations. Once again the addition may go through a syn- or an anti-pathway. The syn-addition was analyzed by Yu and He, finding a concerted eight-membered transition state; the potential energy barrier was found to be 19.3 kcal/mol for PhOH and cyclohexene. We have carried out the analysis for both the syn- and antipathway for the model hydroalkoxylation reaction of ethene with PhOH, similarly to the hydroamination reaction. First, we have analyzed the anti-pathway, which can be described with a concerted mechanism. The concerted transition state TS-P3a (Figure 5) was found on the potential energy surface, lying 25.9 kcal/mol over the van der Waals adduct of the starting molecules (vdW-anti-B). This concerted transition state leads to the formation of the addition product along with the regeneration of the TfOH molecule in a single-step process. The structure is quite similar to the transition state for the anti-mechanism of the acid-catalyzed hydroamination. The transferred proton lies between the oxygen and the (35) (a) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (b) Frisch, M. J.; et al. Gaussian 09, revision A.1; Gaussian, Inc.: Wallingford, CT, 2009.

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Scheme 6. Energy Profiles in Toluene for the Acid-Catalyzed Hydroalkoxylation Reaction with PhOH

Figure 5. Transition states (anti and syn) for the TfOH-catalyzed addition of PhOH to ethene.

carbon at a similar distance to that in the case of the hydroamination (d(C-H)TS-P2a = 1.261 A˚ vs d(C-H)TS-P3a = 1.230 A˚), whereas the nucleophilic oxygen is found a bit farther from the carbon atom, which it reacts with, as in the case of the hydroamination (d(C-N)TS-P2a = 2.181 A˚ vs d(C-O)TS-P3a = 2.129 A˚). Similarly to the hydroamination, the syn-pathway for the model reaction (analogous to the one published by Yu et al.) has also been analyzed. A concerted eight-membered transition state (TS-P3s, Figure 5) for the syn-addition has been found, which is lying 19.6 kcal/mol over the corresponding intermediates (vdW-syn-B). The transition state is analogous to the one found for the syn-pathway in the hydroamination; that is, the transferred proton lies between the oxygen and the carbon at similar distances from the carbon atom (d(C-H)TS-P2s = 1.299 A˚ vs d(C-H)TS-P3s = 1.272 A˚), whereas the oxygen is lying relatively far from the carbon atom in both cases (d(C-N)TS-P2s = 2.272 A˚ vs d(C-O)TS-P3s = 2.234 A˚). The obtained reaction barrier is very similar to that found by Yu et al.29 for the acid-catalyzed hydroalkoxylation of cyclohexene with phenol (19.6 vs 19.3 kcal/mol).29 However, the transition-state geometry is somewhat different; in our work the transferred proton lies farther from the carbon atom (d(C-H)TS-P3s=1.272 vs 1.15 A˚),29 whereas the oxygen lies similarly far from the carbon (d(C-O)TS-P3s = 2.234 vs 2.51 A˚).29 When we consider this transition state, we can remark that TS-P3s has a very similar structure to that found in the goldcatalyzed reaction (TS-PF), in the sense that in both cases a proton-transfer process from an oxygen atom to a carbon atom and the formation of a carbon-oxygen bond take place in a concerted manner. However there are some important differences. In the case of TS-PF the transferred proton lies closer to the proton-transfer agent, quite far from the carbon atom, whereas in the case of the acid catalyst, in TS-P3s the proton is found in the middle between the oxygen and carbon atom (d(C-H)TS-PF = 1.503 A˚ vs d(C-H)TS-P3s = 1.272 A˚). This reflects that the proton coming from TfOH has a strong acidic character, whereas that coming from the PhOH (protontransfer agent) is much less acidic. Nevertheless, in the case of the forming oxygen-carbon bond the trend is the opposite. The bond is almost formed in the case of TS-PF, whereas the oxygen lies far from the carbon atom in the case of TS-P3s (d(C-O)TS-PF = 1.500 A˚ vs d(C-O)TS-P3s = 2.234 A˚).

The obtained results show that the reaction going through the syn-addition is favorable by 6.3 kcal/mol over the antiaddition. Note that the difference between the energy barriers for the syn- and anti-pathways is quite higher in the case of the hydroalkoxylation than in the case of the hydroamination. After analyzing the geometrical structures of the syn-transition states we can find out that in the case of TSP3s a quite strong hydrogen bond is formed between the OH group of phenol and one of the oxygen atoms of triflic acid (d(O-H) = 1.656 A˚), whereas in the case of TS-P2s the analogous interaction between the NH2 group and the oxygen is weaker, d(O-H) = 1.928 A˚. This interaction probably stabilizes more the syn-transition state in the case of the hydroalkoxylation than in the case of the hydroamination, originating a higher syn versus anti energy difference for the first reaction. In the anti-pathway there is no possibility for such kind of interaction, as triflic acid and the nucleophile are on the opposite sides of the double bond. Thus, this H-bond is probably causing the syn-addition pathways to always be the most favorable ones.

Conclusions In the present work we have investigated the reaction mechanism for the PMe3PAuOTf- and TfOH-catalyzed hydroamination of alkenes with CbzNH2 and the hydroalkoxylation of alkenes with PhOH using ethene as model substrate. The comparison of the gold- and acid-catalyzed reactions on this model system showed that the mechanism of the two processes is similar in the sense that the double bond is activated by protonation or by coordination to the metal center. In the acid-catalyzed reactions the double-bond activation and the further reaction with the nucleophile occur in a concerted manner, whereas in the case of the gold-catalyzed reactions the activation precedes the reaction with the nucleophile. In the gold-catalyzed cases, however, there are some significant differences with the nucleophile. For both goldcatalyzed reactions it was shown that a proton-transfer agent is necessary to obtain reasonable energy barriers for the proton-transfer reaction. For the hydroamination reaction

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the proton transfer takes place in a stepwise manner (for both the nucleophile-assited and counterion-assited pathways). Nevertheless, in the gold-catalyzed hydroalkoxylation these two steps are concerted. In the case of the acid-catalyzed reactions, the mechanisms of hydroamination and hydroalkoxylation are analogous. In spite of this, two concerted mechanisms are shown to be reasonable for the TfOH-catalyzed reactions, the anti- and the syn-addition (the latter was also analyzed by Yu, He, and collaborators).29 Both reaction mechanisms are feasible mechanisms, although the syn-pathway represents a more favorable route. As far as the global barriers for the acid versus gold catalysis are concerned, interesting conclusions are obtained. In the case of the hydroamination the overall energy barrier for the gold catalysis was found to be 18.2 (triflate-assisted) or 18.3 (nuclephile-assisted) kcal/mol for the most favorable pathways, but 22.8 (syn-addition) or 25.6 kcal/mol (antiaddition) for the acid-catalyzed mechanism, indicating the preference for gold catalysis in such systems. Nevertheless, for the hydroalkoxylation process the gold-catalyzed barrier was found to be 28.2 kcal/mol, higher than both the anti (25.2 kcal/mol) and the syn (19.6 kcal/mol) in the case of acid catalysis, showing the latter to be more favorable. These results show that both gold(I) and triflic acid are active catalysts for the nucleophilic additions. Thus, both species can be considered as good candidates for performing nucleophilic addition reactions under catalytic conditions. We can state that these energy comparisons for a model system can give some hint concerning the acid- versus goldcatalyzed issues.

Computational Methods The geometry optimizations have been carried by DFT calculations with the program package Gaussian0335a and the B3LYP36 combination of functionals. The SDD37 pseudopotential (36) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (c) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (37) Dolg, M.; Stoll, H.; Preuss, H.; Pitzer, R. M. J. Phys. Chem. 1993, 97, 5852.

Kov acs et al. was employed for the gold center, and the standard 6-31G(d)38 basis set was used for the other atoms. Energies have been calculated by means of single-point calculations using the program package Gaussian 0935b with the M0639 combination of functionals using the same SDD pseudopotential for the metal center and the extended 6-311þþG(d,p) basis set for the other atoms. The effect of the bulk solvent (toluene) was estimated by the application of the polarizable continuum model (PCM)40 as implemented in Gaussian 03 [ε(toluene) = 2.379]. All energies given in the text correspond to those including the effect of the bulk solvent, which was obtained by adding the contribution of the Gibbs energy of solvation to the gas phase total energies. Gas phase electronic energies and Gibbs energies are gathered in the Supporting Information. In the case of the transition states normal coordinate analysis has been used to calculate the imaginary frequencies, and for each transition structure we calculated the intrinsic reaction coordinate (IRC) routes toward the corresponding minima. If the IRC calculations failed to reach the energy minima on the potential energy surface, we performed geometry optimizations from the final phase of the IRC path. Several snapshots for those IRC routes connecting TS-P2a and TS-P3a and their corresponding products are included in the Supporting Information.

Acknowledgment. We are grateful to the Spanish MICINN (Projects CTQ2008-06866-C02-01, ORFEO Consolider Ingenio 2010 CSD2007-00006, and Juan de la Cierva contract to G.K.), as well as to the Generalitat de Catalunya (2009/SGR/68). Supporting Information Available: Cartesian coordinates and absolute energies for the computed structures, relative gas phase energies, gas phase Gibbs energies, and solution energies for all the structures of pathways discussed in the main text; snapshots taken from the IRC and the following pseudo-IRC calculations starting from TS-2a and TS-3a leading toward product formation, and complete ref 35. This material is available free of charge via the Internet at http://pubs.acs.org. (38) (a) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209. (b) Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss, L. A. J. Comput. Chem. 2001, 22, 976. (39) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215. (40) (a) Miertus, S.; Scrocco, E.; Tomasi, J. J. Chem. Phys. 1981, 55, 117. (b) Barone, V.; Cossi, M.; Tomasi, J. J. Chem. Phys. 1997, 107, 3210.