Benzyltriboronates: Building Blocks for Diastereoselective Carbon–Carbon Bond Formation

A highly
diastereoselective carbon–carbon bond-forming reaction
involving the tandem coupling of benzyltriboronates, enoates, and
alkyl halides is described. This method was enabled by the discovery
of α-diimine nickel catalysts that promote the chemoselective
triborylation of benzylic C­(sp3)–H bonds using B2Pin2 (Pin = pinacolate). The C–H functionalization
method is effective with methylarenes and for the diborylation of
secondary benzylic C–H bonds, providing direct access to polyboron
building blocks from readily available hydrocarbons. Combination of
the benzylic perborylation with a new deborylative conjugate addition–alkylation
method enables a one-pot procedure in which multiple simple precursors
are combined to generate diastereopure products containing quaternary
stereocenters.