The role of silver carbonate as a catalyst in the synthesis of N-phenylbenzamide from benzoic acid and phenyl isocyanate: a mechanistic exploration

The gas-phase extrusion–insertion (ExIn) reactions of a silver complex [(BPS)Ag(O₂CC₆H₅)]²⁻ ([BPS]²⁻ = 4,7-diphenyl-1,10-phenanthroline-disulfonate), generated via electrospray ionisation was investigated by Multistage Mass Spectrometry (MSⁿ) experiments in a linear ion trap combined with density functional theory (DFT) calculations. Extrusion of carbon dioxide under collision-induced dissociation (CID) generates the organosilver intermediate [(BPS)Ag(C₆H₅)]²⁻, which subsequently reacts with phenyl isocyanate via insertion to yield [(BPS)Ag(NPhC(O)C₆H₅)]²⁻. Further CID of the product ion resulted in the formation of [(BPS)Ag(C₆H₅)]²⁻, [(BPS)Ag]⁻ and C₆H₅C(O)NPh⁻. The formation of a coordinated amidate anion is supported by DFT calculations. Heating a mixture of benzoic acid, phenyl isocyanate, silver carbonate (5 mol%) and phenanthroline (20 mol%) in DMSO and heating by microwave irradiation led to the formation N-phenyl-benzamide in an isolated yield of 89%. The yield decreased to 74% without the addition of phenanthroline, while replacing silver carbonate with sodium carbonate gave an isolated yield of 84%, suggesting that the ExIn reaction may not operate in solution. This was confirmed using benzoic acid with a ¹³C-isotopic-label at the carboxylate carbon as the starting material, which, under microwave heating in the presence of phenyl isocyanate, silver carbonate (5 mol%) and phenanthroline (20 mol%) gave N-phenyl-benzamide with retention of the ¹³C isotopic label based on GC-MS experiments under electron ionisation (EI) conditions. DFT calculations using a solvent continuum reveal that the barriers associated with the pathway involving direct attack by the non-coordinated benzoate are below the ExIn pathways for the coordinated silver benzoate.