Asymmetric hydroboration and Matteson homologation for the preparation of fluorinated alpha-phenethanols

摘要

The asymmetric catalytic hydroboration-oxidation of ring-fluorinated styrenes (F-PhCH=CH2) was achieved with catecholborane along with a combination of [Rh(COD)_2]~+BF_4~-and (R)-BINAP providing 81-96% enantioselectivities for the product alcohols for ort/io-unhindered styrenes.A deleterious effect of a 2,6-disubstitution on the enantioselectivity of the product alcohol was observed.2-Trifluoromethylstyrene also provides only 53% ee,probably due to the steric bulk of the CF3 group at the ortho-position of styrene.Asymmetric homologation of fluorophenylmetals (magnesium bromide or lithium) with pinanediol alpha-chloroethylboronate,followed by oxidation readily furnished the desired l-(2,6-difluor-ophenyl)-and l-(perfluorophenyl)ethanols in 94-95% ee.