Room temperature olefination of methane with titanium-carbon multiple bonds

Abstract

C-H activation of methane followed by dehydrocoupling at room temperature led ultimately to the formation of the olefin H2C=(CHBu)-Bu-t via the addition of redox-active ligands (L) such as thioxanthone or 2,2'-bipyridine (bipy) to (PNP)Ti=(CHBu)-Bu-t(CH3) (1). Using both of these exogenous ligand systems, we could trap the titanium fragment via an insertion reaction with these two substrates to afford species of the type (PNP)Ti(L)(LH). A combination of computational and isotopic labeling studies reveals that the L ligand promotes the C-C bond forming step by migration of the methyl moiety in 1 to the alpha-alkylidene carbon by producing a Ti(III) species (PNP)Ti{CH(CH3)Bu-t}(L). In the case of L = thioxanthone, beta-hydrogen abstraction gives an olefin, whereas with 2,2'-bipyridine beta-hydride elimination and migratory insertion lead to (PNP)Ti(L)(LH). These redox-active ligands play two important roles: (i) they accept an electron from the Ti-alkylidene fragment to allow the methyl to approach the alkylidene and (ii) they serve as traps of a hydrogen atom resulting from olefin elimination. These systems represent the first homogeneous models that can activate methane and selectively dehydrocouple it with a carbene to produce an olefin at room temperature.