ORGANIC LETTERS
Kinetic Resolution of Racemic 1-Alkyl-2-methylenecyclopropanes via Palladium-Catalyzed Silaborative C-C Cleavage
2009 Vol. 11, No. 13 2880-2883
Toshimichi Ohmura, Hiroki Taniguchi, and Michinori Suginome* Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity, Katsura, Kyoto 615-8510, Japan
[email protected] Received April 16, 2009
ABSTRACT
Kinetic resolution of racemic 1-alkyl-2-methylenecyclopropanes via silaborative C-C cleavage was efficiently catalyzed by a palladium complex bearing a chiral phosphoramidite, affording 2-boryl-3-silylmethyl-1-alkenes as major products with up to 92% ee. Enantioenrichment through parallel kinetic resolution, where the slower reacting enantiomer was converted to the constitutional isomer of the major product, may be involved as the crucial stereodiscriminating step.
Methylenecyclopropanes (MCPs) have become important starting materials in organic synthesis because of the unique reactivity with sufficient stability.1 A variety of reactions of MCPs, including transition-metal-catalyzed bond formation with or without opening of the cyclopropane ring, have been developed,1b-d presenting a potential applicability of MCPs in asymmetric synthesis. However, limited successes have been achieved for asymmetric reactions utilizing MCPs.2 (1) (a) Brandi, A.; Goti, A. Chem. ReV. 1998, 98, 589. (b) Nakamura, I.; Yamamoto, Y. AdV. Synth. Catal. 2002, 344, 111. (c) Brandi, A.; Cicchi, S.; Cordero, F. M.; Goti, A. Chem. ReV. 2003, 103, 1213. (d) Rubin, M.; Rubina, M.; Gevorgyan, V. Chem. ReV. 2007, 107, 3117. (2) (a) Takaya, H.; Hayashi, N.; Ishigami, T.; Noyori, R. Chem. Lett. 1973, 813. (b) Binger, P.; Scha¨fer, B. Tetrahedron Lett. 1988, 29, 529. (c) Binger, P.; Brinkmann, A.; Roefke, P.; Scha¨fer, B. Liebigs Ann. Chem. 1989, 739. (d) Stolle, A.; Becker, H.; Salau¨n, J.; de Meijere, A. Tetrahedron Lett. 1994, 35, 3521. (e) Scott, M. E.; Han, W.; Lautens, M. Org. Lett. 2004, 6, 3309. (f) Taillier, C.; Lautens, M. Org. Lett. 2007, 9, 591. (3) Reviews on kinetic resolution, see: (a) Kagan, H. B. Tetrahedron 2001, 57, 2449. (b) Dehli, J. R.; Gotor, V. Chem. Soc. ReV. 2002, 31, 365. (c) Vedejs, E.; Jure, M. Angew. Chem., Int. Ed. 2005, 44, 3974. (d) Gansa¨uer, A.; Fan, C.-A.; Keller, F.; Karbaum, P. Chem.sEur. J. 2007, 13, 8084. 10.1021/ol900829c CCC: $40.75 Published on Web 06/10/2009
2009 American Chemical Society
Kinetic resolution of racemic MCPs is an unexplored yet potentially attractive method for obtaining enantioenriched chiral products as well as for recovering highly enantioenriched MCPs.3 During the course of our study on the asymmetric silaboration of meso-MCPs,4,5 we became interested in the possibility of the kinetic resolution of racemic MCPs through a transition-metal-catalyzed reaction with silylborane.6 In this paper, we describe a unique kinetic resolution system with palladium-catalyzed silaborative C-C (4) Ohmura, T.; Taniguchi, H.; Kondo, Y.; Suginome, M. J. Am. Chem. Soc. 2007, 129, 3518. (5) For related studies on asymmetric silaboration, see: (a) Suginome, M.; Ohmura, T.; Miyake, Y.; Mitani, S.; Ito, Y.; Murakami, M. J. Am. Chem. Soc. 2003, 125, 11174. (b) Ohmura, T.; Suginome, M. Org. Lett. 2006, 8, 2503. (c) Ohmura, T.; Taniguchi, H.; Suginome, M. J. Am. Chem. Soc. 2006, 128, 13682. For recent reviews on bismetallation of unsaturated hydrocarbons, see: (d) Beletskaya, I.; Moberg, C. Chem. ReV. 2006, 106, 2320. (e) Suginome, M.; Matsuda, T.; Ohmura, T.; Seki, A.; Murakami, M. In ComprehensiVe Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Ojima, I., Vol. Ed.; Elsevier: Oxford, 2007; Vol. 10, p 725. (f) Burks, H. E.; Morken, J. P. Chem. Commun. 2007, 4717. (g) Ohmura, T.; Suginome, M. Bull. Chem. Soc. Jpn. 2009, 82, 29.
Table 1. Silaborative C-C Cleavage of 2a with 1 in the Presence of Pd/(S,S,S)-5 Catalysta
. path selectivitiese entry
equiv of 2a
% yield 3a+4ab
3a:4ac
% ee of 3ad
% ee of 4ad
I:II
a:b
a′:b′
1 2 3 4f
1.0 1.5 3.0 3.0
84 93 96 99 (90)g,h
62:38 70:30 76:24 78:22
71 82 88 91
70 51 30 39
59:41 71:29 80:20 81:19
90:10 90:10 89:11 92:8
78:22 78:22 77:23 81:19
a Pd(dba)2 (3 mol %), (S,S,S)-5 (3 mol %), 1 (0.2 mmol), and 2a (0.2-0.6 mmol) were reacted in toluene (0.1 mL) at 28 °C for 41-95 h. b GC yield based on 1. c Determined by GC. d Determined after conversion to the corresponding β-silyl ketones that analyzed by HPLC with a chiral stationary-phase column. e Calculated from the product distribution. See Scheme 1 and the Supporting Information. f Reaction carried out at 20 °C for 92 h in the presence of 5 mol % of catalyst. g Isolated yield. h Containing dba (
