Direct deoxygenative borylation of carboxylic acids

Abstract

Carboxylic acids are readily available, structurally diverse and shelf-stable; therefore, converting them to the isoelectronic boronic acids, which play pivotal roles in different settings, would be highly enabling. In contrast to the well-recognised decarboxylative borylation, the chemical space of carboxylic-to-boronic acid transformation via deoxygenation remains underexplored due to the thermodynamic and kinetic inertness of carboxylic C-O bonds. Herein, we report a deoxygenative borylation reaction of free carboxylic acids or their sodium salts to synthesise alkylboronates under metal-free conditions. Promoted by a uniquely Lewis acidic and strongly reducing diboron reagent, bis(catecholato)diboron (B2cat2), a library of aromatic carboxylic acids are converted to the benzylboronates. By leveraging the same borylative manifold, a facile triboration process with aliphatic carboxylic acids is also realised, diversifying the pool of available 1,1,2-alkyl(trisboronates) that were otherwise difficult to access. Detailed mechanistic studies reveal a stepwise C-O cleavage profile, which could inspire and encourage future endeavours on more appealing reductive functionalisation of oxygenated feedstocks.

Development of a Quinolinium/Cobaloxime Dual Photocatalytic System for Oxidative C–C Cross-Couplings via H2 Release

“…Control experiments showed that a photocatalyst, cobalt, acid, light, and an inert atmosphere were all vital for this photoinduced oxidative cross-coupling reaction. Other boron reagents were ineffective, which might be attributed to their prohibitive oxidation potentials ( E red > +2.5 V vs SCE for cyclohexylboronic acid, Cy-B­(OH) 2 , and its pinacol ester, Cy-Bpin). , Notably, among some commercial photocatalysts evaluated, [Ru­(bpy) 3 ]­(PF 6 ) 2 , eosin Y, rose bengal, and rhodamine 6G brought poor results of the Minisci alkylation (<25%), while Nicewicz’s catalyst (Mes-Acr-Ph + ) gave a slightly lower yield (87%).…”

A ligand-enabled metallaphotoredox protocol for Suzuki-Miyaura cross-couplings for the synthesis of diarylmethanes

“…This protocol could apply to the coupling between the other fifteen different potassium benzyltrifluoroborates ( Li et al., 2012 , 2020 , 2021a ; Wang et al., 2018 ) and sixteen aryl halides. For details of the scope of this metallaphotoredox Suzuki-Miyaura coupling, please refer to Li et al.…”

Two-in-one metallaphotoredox cross-couplings enabled by a photoactive ligand

“…Capitalizing on these simplified conditions, a site-selective SM cross-coupling between 37 and 38 was accomplished, affording the mono-debrominated product (39) as the paracyclophane precursor. 86,87 Ni-catalyzed metallaphotoredox cross-couplings Encouraged by the successful application of Ni 2+ /PPQN 2,4-di-OMe in SM cross-couplings, other redox-neutral C-C bond-forming reactions were attempted with the same Ni-based transformative platform (Figure 3). In place of RBF 3 K, Hantzsch's ester (40), which could be easily derived from the corresponding aldehydes, was also shown as an efficacious radical source for our Ni-catalyzed metallaphotoredox C(sp 3 )-C(sp 2 ) cross-coupling (Figure 3; Equation 1).…”