Homolytic reactions of ligated boranes. Part 17. Amine–boranes as polarity reversal catalysts for radical chain reactions of esters with vinylic epoxides and with allylic tert-butyl peroxides

Abstract

In the presence of an amine-borane catalyst, methyl acetate, dimethyl malonate and dimethyl methylmalonate (HZ) each react with 1 -methylene-2,3-epoxycyclo-hexanes or -pentanes at a C–H group a to the ester function to give allylic alcohols. The reaction proceeds by a radical chain mechanism and is initiated by UV photolysis of added di-tert-butyl peroxide at 30 °C; isolated yields are generally 50–70%. The amine–borane, usually quinuclidine-borane (QNB), acts as a polarity reversal catalyst to facilitate regioselective overall transfer of hydrogen from an α-C–H group of the ester to an allyloxyl radical to give the allylic alcohol. The α-alkoxycarbonylalkyl radical (Z˙) also formed in this reaction adds to the vinyl epoxide to give an oxiranylcarbinyl radical, subsequent ring opening of which regenerates the allyloxyl radical. In the presence of QNB, methyl acetate, dimethyl malonate, triethyl methanetricarboxylate and ethyl cyanoacetate each react at an α–C–H group with an allylic tert-butyl peroxide H₂CC(R)CMe₂OOBu^t(R = H or Me) to give the 2,3-epoxypropanation products ZCH₂[graphic ommitted] in 50–80% yield. Again, a radical chain mechanism is followed and the amine–borane catalyses α-hydrogen-atom transfer from the ester to Bu^tO˙, which is generated by an S_Hi reaction of the β-tert-butylperoxyalkyl radical formed by addition of Z˙ to the allylic peroxide.