Catalytic Asymmetric Petasis Reactions of Vinylboronates

Binaphthol-catalyzed asymmetric Petasis reactions of salicylaldehydes with dibutyl vinylboronates and secondary amines in the presence of 4 Å molecular sieves (MS) afforded products with up to 99% ee in isolated yields of 39–94%. The 99% ee of the product indicated that the reaction by the binaphthol-catalyzed pathway was roughly 500 times faster than the uncatalyzed pathway. NMR experiments (¹H and ¹¹B) showed that the amine component played a role in triggering the reaction between the binaphthol catalyst and the vinylboronate in the catalytic reaction sequence. The 4 Å MS enhanced both the rate and enantioselectivity by effective removal of water from the reaction system. A novel rearrangement reaction of the unconjugated allylic amine Petasis reaction product to a conjugated allylic amine was also observed.