Cobalt and Nickel Catalysts for Arene Borylation and Alkene Hydrofunctionalizations

Abstract

Understanding and controlling competing reaction pathways is central to the development of selective metal-catalyzed transformations at C=C or C–H bonds. Cobalt and nickel catalysts have been prepared and investigated for C(sp2)–H borylation, hydrofunctionalization, and oligomerization reactions with an emphasis on three main areas: (1) the development of robust and readily-handled catalysts, (2) identification and mitigation of catalyst deactivation pathways, and (3) control of regio- and stereochemical reaction outcomes.

Cobalt bis(carboxylate) complexes supported by 4’-aryl-2,2’:6’,2”-terpyridine were demonstrated to be robust precatalysts for the borylation of C(sp2)–H bonds. Competitive catalyst deactivation by pinacolborane was identified by electron paramagnetic resonance spectroscopic analysis of the reaction under catalytically relevant conditions. Addition of exogenous lithium methoxide to the reaction served to sequester HBPin, mitigated catalyst death, and allowed for synthetically useful yields of the desired aryl boronate esters.

The hydroboration of 3-substituted indenes by terpyridine cobalt alkyl complexes was also demonstrated. Unique regio-, and diastereoselectivity was observed, yielding trans-1,3-disubstituted indane boronate esters, versatile building blocks to diastereopure 1,3-difunctionalized indanes. Selectivity was enabled by chain-walking prior to carbon–boron bond formation. Deuterium labeling experiments and kinetic studies established rate-limiting alkene isomerization and selectivity-determining C–B bond formation at the secondary benzylic position. This study demonstrated the interplay of catalyst and substrate controlled selectivity for remote hydrofunctionalization.

alpha-Diimine nickel bis(carboxylate) complexes were shown to be easily-handled, robust precatalysts for the hydrogenation of unactivated, hindered olefins. Using either an in situ activation method or an isolated nickel hydride complex, [(iPrDI)Ni(µ2-H)]2, the method exhibited unprecedented activity for the hydrogenation of tri- and tetrasubstituted alkenes and improved catalyst handling and synthesis. Mechanistic studies with deuterium gas established an unusual catalytic hydrogenation pathway in which heterolytic cleavage of H2 by pinacolborane and nickel was observed. Kinetic studies supported rate limiting dissociation of [(iPrDI)Ni(µ2-H)]2 to a catalytically active monomeric nickel hydride.

Well-defined -diimine nickel complexes were also investigated for the oligomerization of linear internal olefins. Catalyst activity was highly dependent on both the diimine chelate as well as the presence of aluminum co-catalyst. A monocationic diimine nickel hydride dimer was isolated and its activity described.