Highly Selective Carboxylative Coupling Of Terminal Alkynes, CO₂ And Allylic Chlorides Catalyzed By N-Heterocyclic Carbene Copper(Ⅰ) Complexes

Catalytic transformations of carbon dioxide (CO2) to useful organic compounds have gained considerable attentions due to its main contribution to global warming and its potential use as an abundant, inexpensive and renewable C1 source. CO2 usually serves as electrophile in the carboxylation of nucleophilic organometallic reagents to synthesize carboxylic acid and esters. Transition-metal-catalyzed carboxylation of less reactive carbon nucleophiles such as organoboron and organozinc reagents represents an effective method to access carboxylic acid and esters bearing various functional groups. Compared with transition-metal-catalyzed carboxylation using CO2 as carboxylative reagent, transition-metal-catalyzed carboxylative coupling of nucleophile, CO2 and electrophile to selectively synthesize carboxylic acid esters was rarely reported because of the easily formed by-product from the cross-coupling. Therefore, highly selective synthesis of carboxylic acid ester by the carboxylative coupling using CO2 as carboxylative reagent is a promising challenge issue.On the basis of the mechanistic insight of carboxylative coupling reaction, this work designed a new reaction system for the carboxylative coupling of terminal alkynes, CO2 and allylic compounds. The carboxylative coupling reaction of phenylacetylene, CO2 and cinnamyl chloride was chosen as a model reaction and N-heterocyclic carbene copper(Ⅰ) complex (IPr)CuCl was identified as best catalyst. Screening of reaction parameters established the optimum conditions:CO2 (1.5 MPa), (IPr)CuCl (10 mol%), K2CO3 (2 equiv.), DMF,60℃,24 h.N-heterocyclic carbene copper(Ⅰ) complex (IPr)CuCl proved to be a highly active catalyst for the carboxylative coupling of various terminal alkynes, CO2 and allylic chlorides to selectively afford various functionalized allylic 2-alkynoates in good yield. The catalyst (IPr)CuCl can be easily recovered in high yield after the reaction by simple chromatography without any loss in activity and product selectivity.