An Anionic, Chelating C(sp3)/NHC ligand from the Combination of an N-heterobicyclic Carbene and Barbituric Heterocycle

The coordination chemistry of the anionic NHC 1(-) based on an imidazo[1,5-c]pyridin-3-ylidene (IPy) platform substituted at the CS position by an anionic barbituric heterocycle was studied with d(6) (Ru(II), MnI) and d(8) (Pd(II), Rh(I), Ir(I), Au(III)) transition-metal centers. While the anionic barbituric heterocycle is planar in the zwitterionic NHC precursor 1 center dot H, NMR spectroscopic analyses supplemented by X-ray diffraction studies evidenced the chelating behavior of ligand 1(-) through the carbenic and the malonic carbon atoms in all of the complexes, resulting from a deformation of the lateral barbituric heterocycle. The complexes were obtained by reaction of the free carbene with the appropriate metal precursor, except for the Au(III) complex 10, which was obtained by oxidation of the antecedent gold(I) complex [AuCl(1)](-) with PhICl2 as an external oxidant. During the course of the process, the kinetic gold(I) intermediate 9 resulting from the oxidation of the malonic carbon of the barbituric moiety was isolated upon crystallization from the reaction mixture. The nu(CO) stretching frequencies recorded for complex [Rh(1)(CO)(2)] (5) demonstrated the strong donating character of the malonate-C(sp(3))/NHC ligand 1(-).The ruthenium complex [Ru(1)Cl(p-cymene)] (11) was implemented as a precatalyst in the dehydrogenative synthesis of carboxylic acid derivatives from primary alcohols and exhibited high activities at low catalyst loadings (25-250 ppm) and a large tolerance toward functional groups.

Automated summary

The coordination chemistry of anionic NHC based on an IPy platform substituted at the CS position was studied with transition-metal centers, showing different coordination modes and reaction mechanisms. A gold(I) intermediate was also discovered during the process and structurally characterized. These complexes exhibited high activities and a large tolerance towards functional groups in the synthesis of carboxylic acid derivatives.