Solvent-mediated allylation of carbonyl compounds with allylic stannanes

Methanol promotes the addition of allyltrimethylstannane (1a) to isobutyraldehyde (2a, 30 degrees C) yielding the corresponding homoallylic alcohol (3a), without the necessity for added catalyst. The corresponding reaction of aldehydes 2a-e or activated ketone 2f with tetraallyltin (1b, 0.25 equiv) is substantially faster and proceeds in high yield (81-98%) and with easy separation of the product from tin residues. Aliphatic ketones 2g and 2h also react, but require more forcing conditions, Competitive experiments involving equimolar mixtures of selected aldehydes and ketones with 1b indicates very high aldehyde chemoselectivity. The reaction of 1b with aldehydes proceeds slowly at first, followed by a rapid acceleration which may be attributable to a build up of partially soluble tin(TV) methoxide. The increased rate of carbonyl allylation by 1a and 1b in methanol, relative to dimethyl sulfoxide, suggests that the primary activating influence of the solvent is via hydrogen bonding to the carbonyl oxygen. There is no NMR spectroscopic evidence for a significant change in the ground state structure of these allylic stannanes in methanol, relative to other solvents.