Disparate Behavior of Carbonyl and Thiocarbonyl Compounds: Acyl Chlorides vs Thiocarbonyl Chlorides and Isocyanates vs Isothiocyanates

The reaction of benzoyl chloride with methanol catalyzed by pyridine is 9 times more rapid than is the same reaction with thiobenzoyl chloride. The difference in reactivity, as well as the dealkylation reactions that occur when the reaction of thiobenzoyl chloride is catalyzed by bases such as Et(3)N, can be understood in terms of the charge distributions in the intermediate acylammonium ions. The reaction of PhNCO with ethanol occurs at a much higher rate (4.8 x 10(4)) than that of PhNCS, corresponding to a difference in activation free energies for the additions of 6 kcal/mol. Transition states for each of these reactions were located, and each involves two alcohol molecules in a hydrogen bonded six-membered ring arrangement. Information concerning differences in reactivity was derived from analysis of Hirshfeld atomic charge distributions and calculated hydrogenolysis reaction energies.