Photocontrolled Cobalt Catalysis for Selective Hydroboration of α,β‐Unsaturated Ketones

Abstract

Selectivity between 1,2 and 1,4 addition of a nucleophile to an α,β‐unsaturated carbonyl compound has classically been modified by the addition of stoichiometric additives to the substrate or reagent to increase their “hard” or “soft” character. Here, we demonstrate a conceptually distinct approach that instead relies on controlling the coordination sphere of a catalyst with visible light. In this way, we bias the reaction down two divergent pathways, giving contrasting products in the catalytic hydroboration of α,β‐unsaturated ketones. This includes direct access to previously elusive cyclic enolborates, via 1,4‐selective hydroboration, providing a straightforward and stereoselective route to rare syn‐aldol products in one‐pot. DFT calculations and mechanistic experiments confirm two different mechanisms are operative, underpinning this unusual photocontrolled selectivity switch.

Diels–Alder Reaction of Photochemically Generated (E)-Cyclohept-2-enones: Diene Scope, Reaction Pathway, and Synthetic Application

“…UV–vis (CH 2 Cl 2 ) λ max (ε) = 232 (8287), 351 nm (178 M –1 cm –1 ). The data obtained matched those reported in the literature …”

“…The data obtained matched those reported in the literature. 44 General Procedure for the Photoinduced Diels−Alder Reaction. The dienophile (1.0 equiv) was transferred as a solution in dry CH 2 Cl 2 (1.5 mL) to a Duran phototube.…”

Efficient β-alkylation of secondary alcohols to α-substituted ketones catalyzed by functionalized Ir complexes via borrowing hydrogen in water

“…It was found that the yield increased gradually from 61% to 99% when the reaction time was increased from 12 to 24 h (entries 9 and 12-14). Potassium hydroxide was found to be more effective than NaOH, Cs 2 CO 3 , and K 2 CO 3 (entries [15][16][17]. Finally, organic solvents such as tert-amyl alcohol, 1,4-dioxane, and toluene were also investigated (entries 18-20).…”

Controlling Chemoselectivity of Catalytic Hydroboration with 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. [19] Using cobalt hydride catalysts of the type shown in Scheme 1a, we hypothesized that in absence of light irradiation, the saturated cobalt centre would not be able to coordinate the substrate directly but, as we previously reported, [13] would be reduced to Co 0 under the proposed conditions, and thus act as a catalyst for intramolecular hydrogen atom transfer (Scheme 1c, left). As such, the initial step would be coordination to the Lewis acidic borane by the more electron-rich carboxylic acid carbonyl, activating it to hydroboration in preference to the ketone (Scheme 1c, left) [20] and enabling a mild catalytic method for carboxylic acid reduction.…”

“…[9] For instance, light responsive motifs can be incorporated into the ligand structure [10] or direct excitation can lead to a change in redox properties [11] amongst other strategies. [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.…”