The mechanism of ruthenium-catalyzed directed C─H arylation of arenes: the key role of bis-cyclometalated intermediates

The mechanism of Ru-catalyzed N-directed C-H ortho-arylation with haloarenes has been under intense scrutiny over the last decade, with conflicting proposals concerning the relevance of various catalytic intermediates and the nature of the key steps. This work presents experimental and computational studies that address these long-standing questions. Stoichiometric, catalytic, and mechanistic kinetic studies, supported by DFT calculations, reveal that bis-cyclometallated ruthenium species are key intermediates in these reactions. These studies also show that oxidative addition with bromoarenes proceeds via a concerted oxidative addition pathway, as demonstrated by DFT and experimental kinetic orders. Bromoarene activation does not proceed at mono-cyclometalated species. In the catalytic process, zero order kinetics are observed on both reaction substrates, an observation that is rationalized by DFT calculations which predict a rate-limiting step within the product-release stage. These results showcase how detailed experimental and DFT studies can combine to probe mechanistic questions, as well as resolving opposing views around the mechanism of these Ru-catalyzed arylations that form the basis of promising mild C─H functionalizations.