In this study, we have used density functional theory to identify a new mechanism for the formation of carboxylate compounds from heteroarenes, such as benzoxazole, in the presence of copper catalysts. This new mechanism involves the formation of a carbene intermediate that is indirectly stabilized by the electron-releasing copper. This intermediate carbene can isomerize to the experimentally observed resting state of the catalytic cycle, but it is the intermediate carbene itself that has the greater reactivity toward CO 2 and that leads to the final carboxylate product via a lower-energy pathway. Our findings demonstrate the importance of considering metal-stabilized carbenes in such reactions. Our findings also suggest that this carbene intermediate can act as a nucleophile in other organometallic reactions.

Density functional theory (DFT) calculations were utilized to investigate the mechanism of the oxidation of an indolyl propargylic alcohol by a N-oxide in the presence of an imine and a gold­(I) catalyst. The catalytic reaction is proposed to start from regioselective oxidation of the gold­(I)-activated alkyne dictated by a hydrogen bond interaction between the OH group of the propargylic alcohol and the N-oxide. This oxidation was expected to give an α-carbonyl gold carbene complex. In contrast to this expectation, our calculations showed that the corresponding carbene is not a local minimum and the complex undergoes a very fast 1,2 aryl shift to form an alkene complex. Subsequently, an imine is added to the ensuing alkene complex to give an iminium cation from which a cycloaddition process occurs and an indolium is formed. Finally, an N-oxide deprotonates the indolium complex and affords an intermediate which is significantly reactive toward water elimination. Our calculations indicate that the 1,2-aryl shift in α-carbonyl gold carbene complexes is decelerated if the aryl is substituted by an electron-withdrawing group. At the end, we investigated the stability of different gold carbene complexes and found that the identity of the carbene is the determinant of how these carbenes are trapped.

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