Theoretical Probe to the Mechanism of Pt-catalyzed C-H Acylation Reaction: Possible Pathways for the Key Acylation Reaction of Platinacycles

Abstract

Transition metal-catalyzed C-H bond functionalization offers a variety of desirable transformations of simple hydrocarbons to more complex compounds with applications being applied in pharmaceuticals to material sciences. In 2017, Dr. Huo's research group reported a unique Pt-catalyzed acylation of 2-aryloxypyridines through direct C-H activation. The significance of this reported reaction is represented in the experiment occurring without any oxidants or additives; furthermore, presenting this relatively straight forward reaction mechanism which is initiated only in the presents of platinum. However, through computational analysis the key acylation step of the cyclometalated platinum complex has shown quite intriguing results. Utilization of the nudged elastic band (NEB) method followed by a min-mode optimization of the potential transition states has shown preference towards a nucleophilic substitution over the previously proposed electrophilic addition during the key acylation step. Theoretically, this DFT study at M062X/6-311g**/def2-TZVP-Pt level of theory was unable to simulate the initial formal electrophilic substitution reaction proposed as the arenium ion C could not be minimized. Therefore based on current results the preferred pathway is as follows: (1) nucleophilic substitution of the acyl chloride to form five-coordinate acylplatinum complex with a barrier of 21.7083 k cal/mol (B), followed by (2) 1,2-migration of the acyl group from the platinum to the metalated carbon to form a Pt-arene n-complex or platinacyclopropane with a barrier of 15.0713 kcal/mol (E). Lastly, (3) Re-aromatization of E leads to the acylated product P with a barrier of 35.3138 kcal/mol. Further probing and key mechanistic results will be presented.