Visible-Light-Enhanced Cobalt-Catalyzed Hydrogenation: Switchable Catalysis Enabled by Divergence between Thermal and Photochemical Pathways

Abstract

The catalytic hydrogenation activity of the readily prepared, coordinatively saturated cobalt(I) precatalyst, (R,R)-( iPr DuPhos)Co(CO) 2 H ((R,R)-iPr DuPhos = (+)-1,2-bis[(2R,5R)-2,5-diisopropylphospholano]benzene), is described. 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. This improved reactivity enabled hydrogenation of terminal, di-, and trisubstituted alkenes, alkynes, and carbonyl compounds. A combination of deuterium labeling studies, hydrogenation of alkenes containing radical clocks, and experiments probing relative rates supports a hydrogen atom transfer pathway under thermal conditions that is enabled by a relatively weak cobalt−hydrogen bond of 54 kcal/ mol. In contrast, data for the photocatalytic reactions support light-induced dissociation of a carbonyl ligand followed by a coordination-insertion sequence where the product is released by combination of a cobalt alkyl intermediate with the starting hydride, (R,R)-( iPr DuPhos)Co(CO) 2 H. These results demonstrate the versatility of catalysis with Earth-abundant metals as pathways involving open-versus closed-shell intermediates can be switched by the energy source.

Implication of a Silyl Cobalt Dihydride Complex as a Useful Catalyst for the Hydrosilylation of Imines

“…31 Radical pathways, through hydrogen atom transfer (HAT), are normally proposed in the presence of an alkene and a metal hydride, 32 the group of Chirik recently described that HAT mechanism can also occur using a cobalt hydride complex under thermal conditions. 33 In our case, we ruled out a mechanism based on a radical pathway. Indeed, we chose aldimine 4c, ketimines 1x and 6b bearing a cyclopropyl radical-clock substituent in some critical positions (on the nitrogen atom or in the alpha position).…”

Photo‐Initiated Cobalt‐Catalyzed Radical Olefin Hydrogenation

“…A photoactive iridium hydride complex was recently used in photodriven HAT‐OH after reductive excited state quenching [10] . Furthermore, Chirik and co‐workers reported a cobalt catalyst that switches from HAT‐OH to an olefin insertion mechanism upon photolytic dissociation of a CO ligand [3k] …”

Controlling Chemoselectivity of Catalytic Hydroboration with Light

“…[12] Our group has recently exploited light induced modification of the coordination sphere of a cobalt hydride catalyst (Scheme 1a) to control ambidoselectivity in the hydroboration of α,β-unsaturated ketones (Scheme 1b), [13] and this idea has begun to attract more attention for opening up new mechanistic platforms. [14] Given the different reactivity that is observed in dark vs. light, we wondered if it may be possible to develop this into a new concept whereby different functional groups could be targeted depending only on the presence or absence of light. [15] Our plan began by considering hydroboration as a model reaction after we noted that, despite the many reports in recent years, almost none of these publications investigate more complex molecules which contain multiple functionalities, [16] instead focussing on other aspects such as catalyst development, [17] regio- [18] or enantioselectivity.…”