A Novel Synthesis of Alkylboranes Using Addition Reaction of Alkyl Radicals

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A novel synthesis of alkylboranes, which can not be obtained regioselectively by hydroboration of alkenes, was investigated in this research. It is well known that the addition reaction of an alkyl radical to an electron deficient alkene, mediated with tributyltin hydride, proceeds regioselectively to carry out carbon-carbon bond formation. Accordingly, the addition reaction was applied to synthesize alkylboranes.2-Phenylethenylborane, prepared by monohydroboration of phenylethyne, was regarded as an electron deficient alkene, and it was subjected to the tributyltin hydride mediated coupling of alkyl halides with electron deficient alkenes. The reaction of some alkyl halides was examined in the presence of a small amount of AIBN under reflux in benzene or under irradiation by a mercury lamp at room temperature. It was found that the reaction proceeded under both reaction conditions to give the corresponding 1-phenyl-2-alkylboranes where the alkyl radical was introduced regioselectively into the alpha-alkenyl carbon atom. The resulting alkylboranes were unable to be prepared regioselectively by usual hydroboration. The reaction of alkyl halides with 2-phenylethenylboronic acid under reflux gave good results, because the reaction using 2-phenylethenylboronic acid did not result in steric hindrance between alkyl radical and boryl group.Next 3-bromo-1-propylborane, prepared by hydroboration of 3-bromo-1-propene, was regarded as an alkyl halide, and the reaction with electron deficient alkenes was examined in a similar manner. 3-Bromo-1-propylborane was reduced to propylborane with tributyltin hydride via radical process, whereas it was difficult to undergo the reaction with electron deficient alkenes. Accordingly, it is needed to examine the reaction using other haloalkenes.