Benzylic Boron Reagents Behaving as Allylic Boron Reagents towards Aldehydes: A New Asymmetric Reaction Leading to Homoallylic Alcohols with Concomitant Dearomatisation

Abstract

Retention of stereochemistry: p‐Methoxybenzylic trifluoroborate salts react with aldehydes in the presence of Lewis or Brønsted acids to give homoallylic alcohols with concomitant dearomatisation of the aromatic ring (see scheme). Using enantioenriched benzylic trifluoroborate salts leads to adducts with >98 % retention of the stereochemistry in most cases.

A General Access to Propargylic Ethers through Brønsted Acid Catalyzed Alkynylation of Acetals and Ketals with Trifluoroborates

“…The role of hydrofluoric acid was to sequester boron trifluoride byproduct. More recently, Aggarwal reported trifluoromethanesulfonic acid catalyzed allylation‐like addition of trifluoroborates to aldehydes 20. In that case, however, a Brønsted acid promoted the decomposition of trifluoroborates to difluoroboranes, making this protocol mechanistically similar to Lewis acid catalyzed reactions.…”

Brønsted Acid-Catalyzed Reactions of Trifluoroborate Salts with Benzhydryl Alcohols

“…Both reagents are known to serve as sources of fluoride for the conversion of boronic acids or boronate esters to trifluoroborates . Allylation-like addition of trifluoroborates to aldehydes has been reported in the presence of trifluoromethanesulfonic acid . This protocol, however, was mechanistically similar to Lewis acid-catalyzed transformations due to the formation of a boron dihalide intermediate.…”

Highly Selective Allylborations of Aldehydes Using α,α‐Disubstituted Allylic Pinacol Boronic Esters

“…Treatment with benzaldehyde furnished the homoallylic alcohols 7 with essentially perfect diastereoselectivity and enantioselectivity as before (entries 1–3). We were pleased to observe that this methodology was even capable of effecting dearomatization of a furan ring, again with similarly high selectivity (entry 4) 6b…”