Earth-Abundant Transition Metal Catalysts for Alkene Hydrosilylation and Hydroboration: Opportunities and Assessments

Abstract

The addition of a silicon-hydrogen or a boron-hydrogen bond across a carbon-carbon multiple bonds is a well-established method for the introduction of versatile silane and borane functional groups to base hydrocarbon feedstocks. Transition metal catalysis, historically with precious second- and third- row transition metals, has been used to broaden the scope of the hydrofunctionalization reaction, improve reaction rate and enhance selectivity. The anti-Markovnikov selectivity of platinum-catalyzed hydrosilylation of alkenes, for example, is an enabling synthetic technology in the multibillion-dollar silicones industry. Increased emphasis on sustainable catalytic methods and more economic processes has shifted focus to catalysis with more earth-abundant transition metals such as iron, cobalt and nickel. This review describes contemporary approaches and offers a contextual analysis of catalytic alkene hydrosilylation and hydroboration reactions using first-row transition metals. Emphasis is placed on defining advances in the field, what constitutes catalyst cost, safety, and important design features to enable precious metal-like reactivity, as well as new chemistry that is unique to first-row transition metals.

Figure 1 |. Olefin hydrosilylation versus olefin hydroboration: Motivations and mechanism.

a | Transition metal catalysis enable hydrosilylation and hydroboration of readily available olefins for use in commodity chemicals and fine chemicals, respectively. b | Transition metal-catalyzed hydrosilylation and hydroboration are mechanistically similar and may undergo alkene insertion into a metal hydride (Pathway A) or alkene insertion into either a metal silyl or metal boryl (Pathway B). For clarity, oxidative addition of H-FG and reductive elimination of the C-FG bond were not shown.

Figure 2 |. Base metal catalysts for the hydrosilylation of commercially important substrates with rates and selectivities exceeding those associated with platinum.

Complexes 1-4 are symmetrical and the “Ar” group on the imine nitrogen is identical to the one depicted.

Figure 3 |. Overcoming practical limitations in base metal catalyzed hydrosilylation.

a | In situ activation of bench stable iron dihalides with reducing agents and bases poses a safety hazard from base-catalyzed alkoxysilane disproportionation to form pyrophoric SiH₄ gas | b Switching the metal from iron to cobalt allows more modular catalyst synthesis but also switches the reactivity to dehydrogenative sulylation c | Substrate-activated air-stable metal carboxylate precursors provides the advantage of easy pre-catalyst handling without the possibility of hazardous alkoxysilane disproportionation d | The use of commercially-available, inexpensive ligands for base metal precursors significantly decreases catalyst cost but at the expense of catalyst activity.

Figure 4 |. Pyridine(diimine) cobalt catalyzed dehydrogenative silylation.

a | Scope b | Mechanism c | Catalyst design principle for promoting hydrosilylation over dehydrogenative silylation.

Figure 5 |. Proposed mechanism for alkene hydrosilylation with α-diimine nickel complexes.

Figure 6 |. Hydroboration reactivity of precious metal catalysts.

Precious metal catalyzed hydroboration of a | terminal alkenes, b | vinyl arenes, and c | acylic 1,2-disubstituted alkenes. Directed hydroborations of cyclic monosubstituted cyclohexenes furnishing the d | 1,3-anti isomer and the e | 1,3-syn isomer f | directed hydroboration of acyclic internal alkenes g | (E)-selective and f | (Z)-selective hydroboration of terminal alkynes.

Figure 7 |. New reactivity enabled by base metal catalysts.

Remote hydrofunctionalization in the hydroboration of a | hindered tri- and tetra-substituted alkenes, b | internal alkenes containing a reactive functional group c | Accessing branched selectivity in activated substrates via controlled thermodynamic alkene isomerization followed by hydroboration d | Synthesis of homobenzyltriboronate esters via double dehydrogenative borylation followed by hydroboration e | Enantioselective hydroboration of activated 1-substituted vinylarenes and f | unactivated 1,1-disubstituted alkenes g/h | Enantioselective hydroboration/cyclization of 1,6-enynes i | (Z)-selective hydroboration of terminal alkynes with a mechanism distinct from its precious metal counterparts j | Unique alkynylboronate insertion mechanism was leveraged to develop a protocol involving 1,1-diboration of terminal alkynes k | Selective Markovnikov addition of unactivated terminal alkenes and l | unactivated 1,2-disubstituted alkenes are unmet needs in alkene hydroboration chemistry.

Figure 8 |. Proposed mechanisms in base metal catalyzed alkene and terminal alkyne hydroboration explaining new reactivity.

a | Proposed mechanism for remote hydrofunctionalization via isomerization-hydroboration b | Alkynylboronate insertion into a cobalt hydride as the origin of (Z)-selectivity in the hydroboration of terminal alkynes with pre-catalyst 16.