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.