Controlling Chemoselectivity of Catalytic Hydroboration with Light

Abstract

The ability to selectively react one functional group in the presence of another underpins efficient reaction sequences. Despite many designer catalytic systems being reported for hydroboration reactions, which allow introduction of a functional handle for cross-coupling or act as mild method for reducing polar functionality, these platforms rarely deal with more complex systems where multiple potentially reactive sites exist. Here we demonstrate, for the first time, the ability to use light to distinguish between ketones and carboxylic acids in more complex molecules. By taking advantage of different activation modes, a single catalytic system can be used for hydroboration, with the chemoselectivity dictated only by the presence or absence of visible light.

Mild and Chemoselective Carboxylic Acid Reduction Promoted by Borane Catalysis

“…However, it is notable that most approaches to increase reactivity, for instance with amides, do not do so selectively making them unsuitable for complex substrates [17–19] . In this context, we recently reported a unique cobalt catalysed platform whereby the chemoselectivity of ketoacid hydroboration could be controlled by visible light [20, 21] . In line with our previous work, [22] we suggested that the reactivity observed with carboxylic acid hydroboration was consistent with a Co 0 /Co I catalytic cycle [23] …”

“…[17][18][19] In this context, we recently reported a unique cobalt catalysed platform whereby the chemoselectivity of ketoacid hydroboration could be controlled by visible light. [20,21] In line with our previous work, [22] we suggested that the reactivity observed with carboxylic acid hydroboration was consistent with a Co 0 /Co I catalytic cycle. [23] Upon further mechanistic probes of this carboxylic acid selective hydroboration with in situ NMR (see Supporting Information for more details), we noted key features in the 11 B NMR.…”

Mild and Chemoselective Carboxylic Acid Reduction Promoted by Borane Catalysis

“…[ 17 , 18 , 19 ] In this context, we recently reported a unique cobalt catalysed platform whereby the chemoselectivity of ketoacid hydroboration could be controlled by visible light. [ 20 , 21 ] In line with our previous work, [22] we suggested that the reactivity observed with carboxylic acid hydroboration was consistent with a Co 0 /Co I catalytic cycle. [23] …”

“…When more diluted conditions were applied (Table 1 , entry 6), only starting material was recovered, demonstrating the importance of highly concentrated solution for the reaction to occur. Finally, in the absence of the catalyst and any promoter (Table 1 , entry 7), [20] poor selectivity is observed, with all products obtained in similar proportions. The stability of all reagents, ease of reaction set‐up and exquisite selectivity encouraged us to investigate further the reaction scope of this hydroboration protocol (Scheme 2 ).…”

The transition metal-catalysed hydroboration reaction

“…In many cases for the hydroboration of carbonyls, the catalyst functions mainly as a Lewis acid, activating the carbonyl carbon for nucleophilic attack by the hydride of the borane reagent. A number of different transition metals have been used to catalyse this reaction, 430 including rhodium, 424,431 titanium, 401,402,[432][433][434][435][436][437][438] hafnium, 403 molybdenum, 325 silver, 406 cobalt, 169,[439][440][441][442] ruthenium, 404 palladium, 414,443 zinc, 412,413,444-450 iron, 415,[418][419][420]439,440,451,452 nickel, 409,410 manganese, 408,440,453 copper, 421,422 and yttrium. 454,455 Chiral diols combined with titanium(IV) complexes provide up to 99 : 1 enantiomeric ratios in the hydroboration of simple ketones with catecholborane.…”