Recent emphasis on the use of visible light opens the possibility for new reactivity in transition metal catalysis, including the potential to unlock mechanistic pathways that are distinct from thermal reactivity. In this way, by changing only the energy source, a single catalyst may follow divergent and distinct pathways, enabling two different outcomes in activity or selectivity. As part of investigations into this concept and the utilization of dihydrogen as a thermodynamically preferred reductant in metal-catalyzed hydrogen atom transfer chemistry, the reactivity of (R,R)-(iPrDuPhos)Co(CO)2H ((R,R)-iPrDuPhos = (+)-1,2-bis[(2R,5R)-2,5-diisopropylphospholano]benzene; BDFE [Co–H] = 54 kcal/mol) was studied for the catalytic hydrogenation of alkenes promoted by both heat and visible light. While efficient turnover was observed with a range of alkenes upon heating to 100 °C, the catalytic performance of the cobalt catalyst was markedly enhanced upon irradiation with blue light at 35 °C. A series of mechanistic experiments established divergent pathways for hydrogenation under the two operating conditions. Specifically, these studies suggested that while the thermal reaction operates via an outer-sphere hydrogen atom transfer pathway, the photochemical reaction proceeds via light-induced CO dissociation, allowing for an inner-sphere alkene-binding and insertion pathway. These results demonstrate a rare example of switchable transition metal catalysis promoted by visible light, as well as the versatility of catalysis with Earth-abundant metals.

Visible-light-enhanced cobalt-catalyzed hydrogenation: Switchable catalysis enabled by divergence between thermal and photochemical pathways

ACS Spring Meeting 2021

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