Theoretical elucidation of the mechanism of cleavage of the aromatic C-C bond in quinoxaline by a tungsten-based complex [W(PMe3)4(η2-CH2PMe2)H]

Abstract

The aromatic C-C bond cleavage by a tungsten complex reported recently by Sattler and Parkin offers fresh opportunities for the functionalization of organic molecules. The mechanism of such a process has not yet been determined, which appeals to computational assistance to understand how the unstrained C-C bond is activated at the molecular level. In this work, by performing density functional theory calculations, we studied various possible mechanisms of cleavage of the aromatic C-C bond in quinoxaline (QoxH) by the W-based complex [W(PMe(3))(4)(η(2)-CH(2)PMe(2))H]. The calculated results show that the mechanism proposed by Sattler and Parkin involves an overall barrier of as high as 42.0 kcal mol(-1) and thus does not seem to be consistent with the experimental observation. Alternatively, an improved mechanism has been presented in detail, which involves the removal and recoordination of a second PMe(3) ligand on the tungsten center. In our new mechanism, it is proposed that the C-C cleavage occurs prior to the second C-H bond addition, in contrast to Sattler and Parkin's mechanism in which the C-C bond is broken after the second C-H bond addition. We find that the rate-determining step of the reaction is the ring-opening process of the tungsten complex with an activation barrier of 28.5 kcal mol(-1) after the first PMe(3) ligand dissociation from the metal center. The mono-hydrido species is located as the global minimum on the potential-energy surface, which is in agreement with the experimental observation for this species. The present theoretical results provide new insight into the mechanism of the remarkable C-C bond cleavage.