Sequence-selective three-component reactions of alkyltrifluoroborates with α,β-unsaturated carbonyl compounds and vinylphosphonium salts

A photocatalytic three-component reaction of alkyltrifluoroborates with two different electron-deficient alkenes has been developed. The addition reaction occurs sequentially with alpha,beta-unsaturated carbonyl compounds and vinyltriphenylphosphonium bromide in this order to produce alpha-branched gamma-phosphoniocarbonyl compounds. The reaction could be followed by stereoselective Wittig olefination with various aldehydes to afford structurally diverse alpha-branched gamma,delta-unsaturated ketones and esters with a Z-configuration. Mechanistic investigations suggested that boron trifluoride generated from the organotrifluoroborate activates the alpha,beta-unsaturated carbonyl compound to facilitate the first chemoselective radical addition. We assume that the high efficiency of the second addition with the vinylphosphonium salt might arise from electrostatic interactions caused by the positive charge of the phosphonio group.