Palladium-catalysed directed C–H activation by anilides and ureas; water participation in a general base mechanism

Abstract

C–H activation plays a central role in organometallic catalysis. Concerted metallation–deprotonation (CMD) has been dominant as the pathway for C–H bond cleavage. In the course of studying the mechanism of C–H activation of arylamides and arylureas with Pd complexes as part of catalytic oxidative Heck reactions, DFT calculations were carried out. The turnover-limiting C–H activation is acid-catalysed and can occur readily in the absence of acetate or other coordinating bases. The calculations simulated experiment, so that ligated sulfonate and water, both previously observed by X-ray characterization, were incorporated in the model. A Wheland-type complex between acetanilide and Pd was readily located, but the reactive C–H and the coordinated sulfonate were poorly placed for intramolecular proton transfer. Involvement of a water molecule coordinated to sulfonate provides a low-energy pathway to the palladacycle. The relative reactivity of substituted acetanilides and arylureas according to this model fits well with existing literature. General-base catalysis as described here has broader potential.