Design of alpha-Diimine Cobalt and Nickel Precatalysts for the Synthesis of Alkylpolyboronates

Abstract

Alkylboronates, geminal diboronates and trigeminal triboronates are valuable reagents for C–C and C–heteroatom bond forming reactions. A limitation in their application in synthesis is the lack of methods for their preparation. Synthesis of these privileged reagents by transition-metal catalyzed methods, such as alkene hydroboration or C–H borylation, allows access from minimally-functionalized hydrocarbons and provides avenues for complex molecule synthesis from abundant precursors.

The synthesis and electronic structure characterization of cobalt alkyl complexes supported by redox-active 2,2’:6’,2”-terpyridine and alpha-diimine ligands was carried out, and both classes of cobalt complex proved effective for the isomerization-hydroboration of alkenes, providing terminal alkylboronates. An alpha-diimine cobalt eta3-allyl complex proved an exceptionally active precatalyst for the reduction of hindered di-, tri-, and tetra-substituted alkenes, representing one of the most active homogeneous catalysts known for this reaction.

Cobalt dialkyl and bis(carboxylate) complexes bearing alpha-diimine ligands were prepared and demonstrated as precatalysts for C(sp3)–H polyborylation. These complexes were used for the benzylic and homobenzylic C–H diborylation of alkylarenes, yielding geminal diboronates. Modification of conditions enabled the preparation of substituted benzylmonoboronates and the synthesis of a benzyltriboronate from the functionalization of all three benzylic C–H bonds in toluene.

Nickel dialkyl and bis(carboxylate) complexes supported by alpha-diimines were demonstrated to be robust precatalysts for the polyborylation of benzylic C(sp3)–H bonds. These precatalysts were used for the triborylation of substituted methylarenes to generate benzyltriboronates. The diborylation of secondary benzylic C–H bonds in linear alkylarenes was accomplished, further demonstrating the propensity of the catalysts toward benzylic C–H perborylation.

A deborylative conjugate addition using benzyltriboronates and alpha,beta-unsaturated esters was developed. Consecutive conjugate addition and alkylation enabled selective formation of vicinal stereocenters. Combination of the nickel-catalyzed C(sp3)–H perborylation method and the conjugate addition-alkylation provided a protocol where multiple simple precursors were combined to generate complex, diastereopure products.

alpha-Diimine cobalt dialkyl complexes were also applied to C(sp3)–H hydrogen isotope exchange using deuterium gas. The deuterium labeling of primary, secondary, and tertiary benzylic C–H bonds of alkylarenes was accomplished. Deuteration of chiral substrates revealed a strategy for enantioretentive C(sp3)–H hydrogen isotope exchange.