Sequential Carbon Monoxide and Isocyanide Insertion into Rhenium–Carbon Bonds

The controlled sequential CO and isocyanide insertions into rhenium–carbon bonds, affording a series of novel acyl, iminoacyl, and carbene complexes, are described. The reaction of Re(III) alkyl and benzyl complexes with CO under mild conditions yields reversible Re–acyl intermediates. These species do not undergo further reaction with CO to yield α-dicarbonyl products. In contrast, the reaction of Re(III) alkyl and benzyl complexes with aryl isocyanides yields double isocyanide insertion products. Sequential exposure of acyl intermediates to CO and isocyanide results in the formation of rare mixed CO–CNR-inserted carbene complexes, as confirmed by multinuclear NMR spectroscopy, single-crystal X-ray diffraction, and DFT (APFD) calculations. Computational studies reveal a clear energetic preference for isocyanide insertion into Re–acyl bonds over CO insertion into Re–iminoacyl bonds, rationalized by the greater nucleophilicity of the iminoacyl nitrogen relative to the acyl oxygen. These findings provide new mechanistic insights into heteroallene activation and demonstrate the cooperative reactivity of CO and isocyanide ligands in the construction of C–C and C–heteroatom bonds at rhenium centers.