Cu(I)-Mediated Alkyl Group Transfer of Alkylboranes to Carboxylic Acid Chlorides

Abstract

The alkyl group transfer reaction of alkylboranes with carboxylic acid chlorides affords the corresponding ketones in moderate yields in the presence of potassium tertbutoxide and copper(I) iodide. We reported previously that the electrochemical alkyl group transfers from trialkylboranes to carbonyl compounds, carboxylic acid compounds and epoxide compounds by implementing an electrochemical procedure using copper as a sacrificial anode in an undivided cell. As soon as it was published, this method drew great attention because of its enormous potential in synthetic utility. Therefore, based on this method, we examined the alkyl group transfer reaction of alkylboranes with carboxylic acid chlorides by the electrochemical procedure and also by the general organic chemistry procedure. Now, we report a new methodology for the alkyl group transfer reaction of alkylboranes with carboxylic acid chlorides via the reactive copper(I) alkylborate complexes in the presence of potassium tert-butoxide and copper(I) iodide by a mild organic chemistry process. Although alkylmagnesium, -zinc, -tin, and aluminum reagents have been successfully used for alkyl transfer reactions with acid chloride, the result comprises an excellent carbon-carbon bond formation synthesis since alkylboranes are readily prepared by hydroboration from alkenes, unlike alkyl transfer reactions reported by Suzuki et al. And this reaction was also successful in carbon-carbon bond formation without expensive palladium catalysts. First of all, a variety of solvents, such as tetrahydrofuran (THF) hexamethylphosphoric triamide (HMPA) (1 : 1, 2 : 1, 5 : 1), tetrahydrofuran (THF) dimethylformamide (DMF) (2 : 1), and tetrahydrofuran (THF) as a solvent, copper(I) halides, such as copper(I) bromide, copper(I) iodide, and copper(I) chloride, and bases, such as potassium tert-butoxide, sodium methoxide, and sodium ethoxide were examined to find the combination that affords the best yield. As a result, we found that THF-HMPA (2 : 1) as a solvent, copper(I) iodide as a copper(I) halide, and potassium tertbutoxide as a base gave the best yield (Table 1). As shown in Table 1, the ethyl group transfer reaction of triethylborane was especially affected not by sodium methoxide (entry 9) and sodium ethoxide (entry 10) but by potassium tert-butoxide (entry 1). Also copper(I) halides