BASE METAL CATALYSIS FOR THE SYNTHESIS OF ORGANOBORON COMPOUNDS

Abstract

The versatility of organoboron compounds in organic synthesis inspires the discovery of efficient and sustainable methods for their synthesis; hence, providing more inexpensive, less toxic, and Earth-abundant base metal alternatives to more widely used precious metal catalysts for the synthesis of organoboron compounds is highly desirable not only for economic and sustainability reasons, but also to enable new chemistry not previously observed with precious metals. This dissertation will describe the development of pyridinediimine-ligated cobalt catalysts for anti-Markovnikov alkene hydroboration with pinacolborane (HBPin). Further catalyst development resulted to a general method for remote hydrofunctionalization of C-H bonds of hindered alkenes. Modification of the ligand on cobalt resulted to highly active and Z-selective catalysts for the hydroboration of terminal alkynes. Mechanistic studies support a mechanism involving selective insertion of an alkynylboronate ester into a cobalt-hydride bond, a pathway distinct from known precious metal catalysts where metal vinylidine intermediates have been proposed to account for the observed Z-selectivity.

Transition metal-catalyzed arene C(sp2)-H borylation has attracted much attention due to the versatility of the resulting arylboronate products. The design and development of electron-rich pyridine bisphosphine-(“PNP”) ligated cobalt catalysts for arene C(sp2)-H borylation is described. Kinetic data, isolation of relevant intermediates and kinetic isotope effects support a Co(I)-Co(III) redox cycle. When HBPin is used as the boron reagent, H2 reductive elimination from a cobalt(III) dihydride boryl is turnover-limiting, while with B2Pin2, C-H activation from a structurally characterized cobalt(I) boryl intermediate is the slow step. Moreover, borylation of the 4-position of the pincer in the cobalt catalyst prior to the borylation of the arene substrate was observed when B2Pin2 is used as the boron source. Cyclic voltammetry established the electron withdrawing influence of 4-BPin, which speeds up the rate of H2 reductive elimination but slows the rate of C-H oxidative addition. These findings guided the design and synthesis of catalysts with electron-donating methyl and pyrrolidinyl groups with improved activity for the C-H borylation of unactivated arene substrates. Application of the second generation catalysts to the C-H borylation of fluoroarenes revealed high ortho to fluorine selectivity overcoming the consistent steric selectivity observed with iridium catalysts.