Copper-Catalyzed Asymmetric Mono-borylation of 1,3-Dienes

Abstract

Asymmetric hydroboration of ­alkenes is an important method for preparing enantioenriched α-chiral organoboron compounds. However, these reactions are generally limited to aromatic alkenes, alkenes with directing groups, and cyclopropenes. Asymmetric hydro­boration of 1,3-dienes provides useful homoallylic and allylic boron compounds, but examples giving high enantioselectivities are rare. The authors have reported an asymmetric catalytic monoborylation of 1,3-dienes with a copper(I) catalyst. The reaction produced cyclic optically active homoallylboronates 3 with high regio- and enantioselectivity. Furthermore, product preference between homoallyl- and allylboronates can be switched by changing the reaction conditions for 1,3-cyclopentadiene and 1,3-cycloheptadiene to give enantioenriched cyclic allylboronates, such as 7, which are usually difficult to synthesize.