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.

Cobalt-Catalyzed Stereoretentive Hydrogen Isotope Exchange of C(sp3)-H Bonds.

Cobalt dialkyl complexes bearing α-diimine ligands proved to be active precatalysts for the nondirected, C(sp3)-H selective hydrogen isotope exchange (HIE) of alkylarenes using D2 gas as the deuterium source. Alkylarenes with a variety of substitution patterns and heteroatom substituents on the arene ring were successfully labeled, enabling high levels of incorporation into primary, secondary, and tertiary benzylic C(sp3)-H bonds. In some cases, the HIE proceeded with high diastereoselectivity and application of the cobalt-catalyzed method to enantioenriched substrates with benzylic stereocenters provided enantioretentive hydrogen isotope exchange at tertiary carbons.

Cobalt-Catalyzed Benzylic Borylation: Enabling Polyborylation and Functionalization of Remote, Unactivated C(sp(3))-H Bonds.

Cobalt dialkyl and bis(carboxylate) complexes bearing α-diimine ligands have been synthesized and demonstrated as active for the C(sp(3))-H borylation of a range of substituted alkyl arenes using B2Pin2 (Pin = pinacolate) as the boron source. At longer reaction times, rare examples of polyborylation were observed, and in the case of toluene, all three benzylic C-H positions were functionalized. Coupling benzylic C-H activation with alkyl isomerization enabled a base-metal-catalyzed method for the borylation of remote, unactivated C(sp(3))-H bonds.