Carboxylation of Organoboronic Esters Catalyzed by N‐Heterocyclic Carbene Copper(I) Complexes

TLDR: An excellent N-heterocyclic carbene copper(I) catalyst system for the carboxylation of aryland alkenylboronic esters with CO2 is reported, which not only showed higher functional-group tolerance, but could also afford structurally characterizable active catalyst species, thus offering unprecedented insight into the mechanistic aspects of the catalytic process.

Abstract: Carbon dioxide (CO2) is an attractive, cheap, and nontoxic C1 source. However, because of its high thermodynamic stability and low reactivity, the use of CO2 as a C1 source for C C bond formation usually requires highly nucleophilic organometallic reagents, such as alkyllithium compounds and Grignard reagents. Less nucleophilic organoboron compounds, though easily available, usually do not react with CO2. Recently, transition-metal-catalyzed addition of carbon nucleophiles to CO2 has attracted much attention. [2,3] In this context, Iwasawa and co-workers have reported the catalytic carboxylation of aryland alkenylboronic esters with CO2 in the presence of a rhodium(I) compound and additives. This reaction is potentially useful for the synthesis of functionalized carboxylic acid derivatives because of the easy availability of various functionalized organoboronic esters. Unfortunately, however, the Rh catalyst systems showed only limited tolerance toward functional groups. Although carbonyl and cyano groups survived the reaction conditions, more reactive functional moieties, such as bromo, iodo, and vinyl groups, seemed intolerant. Moreover, little information about the active catalyst species and the reaction mechanism was available because of the complexity of the catalyst systems. These difficulties have limited the application scope of the Rh catalyst systems. The search for new catalysts for more efficient, selective CO2 transformation as well as the clarification of the catalytic process is therefore of interest and importance. We report herein an excellent N-heterocyclic carbene copper(I) catalyst system for the carboxylation of aryland alkenylboronic esters with CO2. This Cu catalyst system not only showed higher functional-group tolerance, but could also afford structurally characterizable active catalyst species, thus offering unprecedented insight into the mechanistic aspects of the catalytic process. Copper complexes bearing N-heterocyclic carbene (NHC) ligands have been reported to act as efficient catalysts for the transformation of various carbonyl compounds, such as conjugate reduction of a,b-unsaturated carbonyl compounds, hydrosilylation of ketones, and also for the reduction of CO2. [7] In addition, many copper compounds have also been reported to promote nucleophilic addition of organoboron compounds to electrophiles, such as a,b-unsaturated carbonyls and allylic carbonates. These results encouraged us to examine the carboxylation of organoboronic esters with CO2 by use of N-heterocyclic carbene copper complexes as catalysts. At first we examined the reaction of 4methoxyphenylboronic acid 2,2-dimethyl-1,3-propanediol ester (1a) with CO2 using N-heterocyclic carbene copper species generated in situ from CuCl, IPr·HCland tBuOK. In