Organocatalytic stereoselective α-alkylation of aldehydes with stable carbocations

Article abstract of DOI:10.1002/asia.201000160

The organocatalytic stereoselective alkylation of aldehydes is carried out with the four stable carboca-tions 1-4 in the presence of a catalytic amount (20mol%) of MacMillan imi-dazolidinones 5-6. In all reactions, luti-dine was used as a base. The alkylation reactions are investigated at different temperatures with linear and branched aldehydes. In the case of carbocation tropylium fluoroborate, an interesting reversal of alkylation product configuration was observed, which is driven by entropic effects in the reaction. The absolute configuration of the products obtained is determined by chemical correlation and found to be in general agreement with the model proposed by MacMillan to justify the stereoselectiv-ity obtained in the reactions promoted by catalysts of type 5-6.