Rh-Catalyzed Enantioselective Diboration of Simple Alkenes: Reaction Development and Substrate Scope Ste´phane Trudeau, Jeremy B. Morgan, Mohanish Shrestha, and James P. Morken* Department of Chemistry, Venable and Kenan Laboratories, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290
[email protected] Received August 5, 2005
The rhodium-catalyzed reaction between bis(catecholato)diboron and simple alkenes results in the syn addition of the diboron across the alkene. The resulting 1,2-bis(boronate) is subsequently oxidized to provide the 1,2-diol. In the presence of enantiomerically enriched Quinap ligand, high enantioselection in the diboration can be achieved. The reaction is highly selective for trans- and trisubstituted alkenes and can be selective for some monosubstituted alkenes as well. The development of this reaction is described as is the substrate scope and experiments that are informative about the reaction mechanism and competing pathways.
Introduction Stereospecific manipulation of carbon-boron bonds is sufficiently well developed that a large number of derivatives may be targeted through stereospecific replacement of the boron atom.1 Amination,2 oxidation,3 catalytic4 and stoichiometric cross-coupling,5 and carbenoid insertions6 all serve to create useful organic assemblies from common (1) Reviews: (a) Brown, H. S.; Singaram, B. Acc. Chem. Res. 1988, 21, 287. (b) Organoboranes for Synthesis; Ramachandran, P. V., Brown, H. C., Eds.; ACS Symposium Series 783; American Chemical Society: Washington, DC 2001. (c) Stereodirected Synthesis with Organoboranes; Matteson, D. S., Ed.; Springer: New York, 1995. (2) (a) Brown, H. C.; Midland, M. M.; Levy, A. B. J. Am. Chem. Soc. 1973, 95, 2394. (b) Brown, H. C.; Kim, K. W.; Cole, T. E.; Singaram, B. J. Am. Chem. Soc. 1986, 108, 6761. (c) Knight, F. I.; Brown, J. M.; Lazzari, D.; Ricci, A.; Blacker, A. J. Tetrahedron 1997, 53, 11411. (d) Fernandez, E.; Maeda, K.; Hooper, M. W.; Brown, J. M. Chem Eur. J. 2000, 6, 1840. (3) (a) Zweifel, G.; Brown, H. C. Org. React. 1963, 13, 1. (b) Brown, H. C.; Snyder, C.; Subba Rao, B. C.; Zweifel, G. Tetrahedron 1986, 42, 5505. (c) Kabalka, G. W.; Wadgaonkar, P. P.; Shoup, T. M. Organometallics 1990, 9, 1316. (4) Reviews: (a) Suzuki, A. J. Organomet. Chem. 1999, 576, 147. (b) Kohta, S.; Lahirir, K.; Kashinath, D. Tetrahedron 2002, 58, 9633. (5) (a) Suzuki, A.; Miyaura, N.; Abiko, S.; Itoh, M.; Brown, H. C.; Sinclair, J. A.; Midland, M. M. J. Am. Chem. Soc. 1973, 95, 3080. (b) Leung, T.; Zweifel, G. J. Am. Chem. Soc. 1974, 96, 5620. (c) Yamada, K.; Miyaura, N.; Itoh, M.; Suzuki, A. Synthesis 1977, 679. (d) Hara, S.; Dojo, H.; Kato, T.; Suzuki, A. Chem. Lett. 1983, 1125. (6) (a) Brown, H. C.; Imai, T. J. Am. Chem. Soc. 1983, 105, 6285. (b) Matteson, D. S.; Majumdar, D. Organometallics 1983, 2, 1529. (c) Sadhu, K.; M.; Matteson, D. S. Organometallics 1985, 4, 1687. (d) Brown, H. C.; Singh, S. M. Organometallics 1986, 5, 994. (e) Brown, H. C.; Singh, S. M. Organometallics 1986, 5, 998. (f) Chen, A. C.; Ren, L.; Crudden, C. M. Chem. Commun. 1999, 611. (g) Ren, L.; Crudden, C. M. Chem. Commun. 2000, 721. (h) O’Donnell, M. J.; Drew, M. D.; Cooper, J. T.; Delgado, F.; Zhou, C. J. Am. Chem. Soc. 2002, 124, 9348.
organoboron precursors. Because of this remarkable breadth, reactions that enable the synthesis of stereodefined carbon-boron bonds have significant value, and this is evidenced by the central role that hydroboration plays in contemporary organic synthesis.7 While stereoselective insertion processes8 also serve to create chiral organoboron compounds, there are few other methods for accessing these motifs in a selective fashion. The catalytic reaction of alkenes with diboron compounds offers one route to organoboron reagents in a conceptually straightforward fashion.9 This transformation furnishes two carbon-boron bonds, both of which might be derivatized in a useful fashion. Along these lines, it has been reported that alkene diboration may be accomplished with the assistance of rhodium,10 platinum,11 gold,12 and silver13 complexes. Each of these complexes is thought to catalyze diboration through a (7) Smith, K.; Pelter, A. In Comprehensive Organic Synthesis; Trost, B. M., Ed.; Pergamon: Oxford, 1995; Vol. 8, p 703. For a review of catalytic asymmetric hydroboration, see: Crudden, C. M.; Edwards, D. Eur. J. Org. Chem. 2003, 4695. (8) (a) Matteson, D. S. Tetrahedron 1998, 54, 10555. (b) Matteson, D. S.; Majumdar, D. J. Am. Chem. Soc. 1980, 102, 7588. (c) Matteson, D. S.; Ray, R. J. Am. Chem. Soc. 1980, 102, 7590. (9) For reviews on catalytic diboration of unsaturated organic compounds, see: (a) Ishiyama, T.; Miyaura, N. Chem. Rec. 2004, 3, 271. (b) Ishiyama, T.; Miyaura, N. J. Organomet. Chem. 2000, 611, 392. (c) Marder, T. B.; Norman, N. C. Top. Catal. 1998 5, 63. (10) (a) Baker, R. T.; Nguyen, P.; Marder, T. B.; Westcott, S. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 1336. (b) Dai, C.; Robins, E. G.; Scott, A. J.; Clegg, W.; Yufit, D. S.; Howard, J. A. K.; Marder, T. B. Chem. Commun. 1998, 1983. (c) Nguyen, P.; Coapes, R. B.; Woodward, A. D.; Taylor, N. J.; Burke, J. M.; Howard, J. A. K.; Marder, T. B. J. Organomet. Chem. 2002, 652, 77. 10.1021/jo051651m CCC: $30.25 © 2005 American Chemical Society
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J. Org. Chem. 2005, 70, 9538-9544
Published on Web 10/20/2005
Enantioselective Diboration of Simple Alkenes SCHEME 1. General Mechanism for Transition-Metal-Catalyzed Reaction by Alkenes and Diboron Compounds
catalytic cycle that involves oxidative addition of the diboron reagent to the metal,14 insertion of the alkene,15 and then reductive elimination of the organodiboron product (Scheme 1).16,17 Significant reaction side products are often observed that appear to arise from β-hydrogen elimination of the intermediate organometallic complex.18 In an effort to develop an asymmetric variant of the alkene diboration process, we initiated studies with chiral transition-metal complexes.19 On the basis of the catalytic cycle described above, exploratory experiments were restricted to those involving bidentate chiral ligands in combination with rhodium complexes since it was expected that bidentate ligands would render the analogous four-coordinate d8 platinum diboryl intermediate inert to olefin coordination and insertion reactions.20 These (11) (a) Iverson, C. N.; Smith, M. R., III Organometallics 1997, 16, 2757. (b) Ishiyama, T.; Yamamoto, M.; Miyaura, N. Chem Commun. 1997, 689. (c) Marder, T. B.; Norman, N. C.; Rice, C. R. Tetrahedron Lett. 1998, 39, 155. (d) Ishiyama, T.; Momota, S.; Miyaura, N. Synlett 1999, 1790. (d) Mann, G.; John, K. D.; Baker, R. T. Org. Lett. 2000, 2, 2105. (12) Baker, R. T.; Nguyen, P.; Marder, T. B.; Westcott, S. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 1336. (13) Ramı´rez, J.; Corbera´n, R.; Sanau´, M.; Peris, E.; Fernandez, E. Chem. Commun. 2005, 3056. (14) Oxidative addition with Rh: (a) Dai, C.; Stringer, G.; Marder, T. B.; Scott, A. J.; Clegg, W.; Norman, N. C. Inorg. Chem. 1997, 36, 272. (b) Nguyen, P.; Lesley, G.; Taylor, N. J.; Marder, T. B.; Pickett, N. L.; Clegg, W.; Elsegood, M. R. J.; Norman, N. C. Inorg. Chem. 1994, 33, 4623. (c) Clegg, W.; Lawlor, F. J.; Marder, T. B.; Nguyen, P.; Norman, N. C.; Orpen, A. G.; Quayle, A. J.; Rice, C. R.; Robins, E. G.; Scott, A. J.; Souza, F. E. S.; Stringer, G.; Whittell, G. R. J. Chem. Soc., Dalton 1998, 301. Oxidative addition with Pt: (d) Iverson, C. N.; Smith, M. R., III J. Am. Chem. Soc. 1995, 117, 4403. (e) Ishiyama, T.; Matsuda, N.; Murata, M.; Ozawa, F.; Suzuki, A.; Miyaura, N. Organometallics 1996, 15, 713. (f) Lesley, G.; Nguyen, P.; Taylor, N. J.; Marder, T. B.; Scott, A. J.; Clegg, W.; Norman, N. C. Organometallics 1996, 15, 5137. (g) Ishiyama, T.; Matsuda, N.; Miyaura, N.; Suzuki, A. J. Am. Chem. Soc. 1993, 115, 11018. (h) Clegg, W.; Lawlor, F. J.; Marder, T. B.; Nguyen, P.; Norman, N. C.; Orpen, A. G.; Quayle, M. J.; Rice, C. R.; Robins, E. G.; Scott, A. J.; Souza, F. E. S.; Stringer, G.; Whittell, G. R. J. Organomet. Chem. 1998, 550, 183. (15) For a documented alkene insertion into a Rh-B bond, see: Baker, R. T.; Calabrese, J. C.; Westcott, S. A.; Nguyen, P.; Marder, T. B. J. Am. Chem. Soc. 1993, 115, 4367. (16) For the organometallic chemistry of transition-metal boryls, see: Irvine, G. J.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.; Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem. Rev. 1998, 98, 2685. (17) For computational studies of the mechanism of alkene diboration, see: Cui, Q.; Musaev, D. G.; Morokuma, K. Organometallics 1997, 16, 1355. (18) Competitive β-hydrogen elimination is catalyst and substrate dependent, see ref 10. (19) For a preliminary report, see: Morgan, J. B.; Miller, S. P.; Morken, J. P. J. Am. Chem. Soc. 2003, 125, 8702. See also, ref 21. (20) Bidentate ligands are known to inhibit the Pt-catalyzed diboration of alkynes and the Pd-catalyzed diboration of allenes. See ref 14f and Pelz, N. F.; Woodward, A. R.; Burks, H. E.; Sieber, J. D.; Morken, J. P. J. Am. Chem. Soc. 2004, 126, 16328.
TABLE 1. Survey of Chiral Ligands in the Rh-Catalyzed Alkene Diboration Reaction
entry
ligand
% yield
syn:anti
% ee syn (anti)
1 2 3 4 5 6 7 8 9
Binap (1) DIOP (2) Chiraphos (3) iPr-PHOX (4) Josiphos (5) Quinap (6) Indane-Pybox (7) MeO-Biphep (8) H-MOP(9)
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