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.