Enantioselective and Diastereoselective Additions of Allylic Stannanes to Aldehydes Promoted by a Chiral (Acyloxy)borane Catalyst

A modified Yamamoto Lewis acid (CAB), prepared from the 2,6-dimethoxybenzoic ester of ( R, R )-tartaric acid, and 1.5 equiv of BH 3 ·THF was employed in additions of crotyltributyltin ( 6 ) and allyltributyltin ( 9 ) to representative achiral aldehydes in the presence of 2 equiv of (CF 3 CO) 2 O. The crotyltin additions proceeded with good to excellent diastereoselectivity and enantioselectivity affording the syn adducts 7a − e of 70−90% ee as major products (78:22−92:8). Addition of allylstannane 9 to cyclohexanecarboxaldehyde ( 1a ) afforded the ( R )-adduct of only 55% ee. In contrast, the use of Keck's BINOL catalyst gave 10, the allyl adduct of 1a, of 87% ee. However, addition of the crotylstannane to 1a with this catalyst led to a 65:35 mixture of syn and anti adducts 7a and 8a of 95% and 49% ee. Additions of crotylstannane 6 to ( R )- and ( S )-2-methyl-3-ODPS-propanal [( R )- 11 and ( S )- 11 ] promoted by the modified CAB Lewis acid afforded the syn,syn and syn,anti products 12 and 14 in large predominance (98:2 and 90:10), indicative of effective complex control in the transition state. The results are consistent with the Corey H-bonded aldehyde transition-state proposal.