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 BH3.THF was employed in additions of crotyltributyltin (6) and allyltributyltin (9) to representative achiral aldehydes in the presence of 2 equiv of (CF3CO)(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 la, 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.