(CDC)–Rhodium-Catalyzed Hydroallylation of Vinylarenes and 1,3-Dienes with AllylTrifluoroborates

Abstract

Catalytic site-selective hydroallylation of vinyl arenes and 1,3-dienes is reported. Transformations are promoted by a readily accessible bidentate carbodicarbene–rhodium complex and involve commercially available allyltrifluoroborates and an alcohol. The reaction is applicable to vinyl arenes and aryl- or alkyl-substituted 1,3-dienes (30 examples). Allyl addition products are generated in 40–78% yield and in up to >98:2 site selectivity. Reaction outcomes are consistent with the intermediacy of a Rh(III)–hydride generated by protonation of Rh(I) by an acid. A number of key mechanistic details of the reaction are presented: (1) Deuterium scrambling into the product and starting alkene indicates reversible Rh(III)–H migratory insertion. (2) A large primary kinetic isotope effect is observed. (3) With substituted allyltrifluoroborates (e.g., crotyl-BF3K) mixtures of site isomers are generated as a result of transmetalation followed by Rh–(allyl) complex equilibration, consequently disproving outer-sphere addition of the allyl nucleophile to Rh(III)–(η3-allyl). (4) Stereochemical analysis of a cyclohexadiene allyl addition product supports a syn Rh(III)–hydride addition. (5) A Hammett plot shows a negative slope. Finally, utility is highlighted by a iodocyclization and cross metathesis.