Carboxylation of Alkylboranes by N‐Heterocyclic Carbene Copper Catalysts: Synthesis of Carboxylic Acids from Terminal Alkenes and Carbon Dioxide

Abstract

Caught in the act: N-Heterocyclic carbene copper(I) complexes (1; IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) serve as an excellent catalyst for the carboxylation of alkylboranes (2; R=alkyl) with CO(2) to afford a variety of functionalized carboxylic acids (3) in high yields. A novel copper methoxide/alkylborane adduct (A) and its subsequent CO(2) insertion product (B) have been isolated and shown to be true active catalyst species.

Copper–NHC complexes in catalysis

“…A number of processes have been reported allowing formation of carboxylic acids or esters. However, in the case of alkenes, a tandem reaction with in situ formation of alkylborane followed by the carboxylation has been reported [49]. …”

Making C–C Bonds from Carbon Dioxide via Transition-Metal Catalysis

“…A silver salt likewise promotes reactions of arylboron esters with carbon dioxide [53]. In the same vein, a two-step protocol for alkene hydroboration followed by Cu-catalyzed CO 2 incorporation represents a formal reductive carboxylation (or hydrocarboxylation, vide infra) for the synthesis of linear aliphatic carboxylic acids [54,55].…”

Direct Carboxylation of C(sp3)-H and C(sp2)-H Bonds with CO2 by Transition-Metal-Catalyzed and Base-Mediated Reactions

“…Concerning the mechanism of the Cu(I)-catalyzed reaction, the author proposes that 21 undergo transmetallation with [(IMes)Cu(O t Bu)] affording (η 1 -ally)-Cu(IMes) complex 30. Carboxylation of 30 occurs in extremely mild reaction conditions (P CO 2 = 0.1 MPa, 0 • C) affording 2-aryloxo-3-butenoate anion coordinated to "Cu(I)(IMes)" fragment (31). As final steps, transmetallation between 31 and 21 complexes produces (32) and regenerates the (η 1 -ally)-Cu(IMes) species (30).…”

“…(1) transition-metal-catalysed carboxylation of preactivated substrates including allylstannanes [24][25][26][27], organoboronic esters [28][29][30][31][32], organozinc reagents [33,34] and aryl halides [35][36][37] as exemplified in Scheme 1a-d; (2) transition-metal-catalysed hydrocarboxylation of unsaturated compounds (olefins, allenes, alkynes) requiring AlEt 3 or ZnEt 2 co-reactants to generate a metal-hydride species undergoing insertion of the unsaturated substrate and subsequent carboxylation of the organometallic intermediate [38][39][40][41][42] as exemplified in Scheme 1e. It is worth citing very recent examples of hydrocarboxylation reactions involving substrate reduction by a "formal hydride donor": (i) in Scheme 1f, [43] hydride is formally generated from H 2 O and Mn; (ii) in Scheme 1g, [44] the substrate is reduced by photo-induced transfer of 2e − and 2H + from a sacrificial amine; (iii) in Scheme 1h, [45] the substrate is carboxylated and subsequently reduced with H 2 in a Poly-NHC/Ag/Pd mediated process; (3) use of CO 2 in conjunction with alkenes, alkynes, dienes, allenes, diynes, in oxidative cycloaddition reaction of low valent metal complexes to form five-membered metallacycles [46][47][48][49][50][51][52] as exemplified in Scheme 1i; (4) direct carboxylation of C-H bonds with CO 2 avoiding C-H pre-functionalization, hydrocarboxylation or metallacycle formation as exemplified in Scheme 1j [53]).…”