Homologation Strategy for the Generation of 1‑Chloroalkyl Radicals from Organoboranes

The
generation of 1-bromo and 1-chloroalkyl radicals from organoboranes
has been investigated. The direct approach involving the hydroboration
of halogenated alkenes is impeded by partial dehalogenation taking
place during the hydroboration process. An indirect method involving
the generation of B-(1-chloroalkyl)­catecholborane
by homologation of B-alkylcatecholborane with dichloromethyllithium
was developed. A reaction sequence involving a hydroboration reaction,
a Matteson homologation, and a radical allylation process has been
performed as a one-pot process that takes advantage of three different
reactivities of organoboron species. Starting from styrene derivatives,
it was possible to prepare B-(1-chloro-2-arylpropyl)­catecholboranes
that are excellent precursors to 1-chloro-2-arylpropyl radicals. A
concise approach for the synthesis of an optically active α-methylene-γ-lactone
from p-chlorostyrene has been developed on the basis
of a two-step sequence involving an enantioselective hydroboration–homologation–cyclization
reaction followed by a hydrolysis–lactonization process.