Electrophilic Metal Complexes

Abstract

Carbon-hydrogen (C-H) bonds are the most abundant bonds in organic chemistry. Activation of the saturated carbon-hydrogen (C-H) bond in alkanes or alkyl groups presents both a fundamental and practical challenge to chemists. The absence of double or triple bonds, lone electron pairs, and the high strength of covalent C-H and C-C σ-bonds in alkanes constitute difficulties in searching for a selective, controllable, reactive system. The Nobel Prize winner, G. A. Olah described the importance of reactions of alkanes with electrophiles and superacids in his Nobel Lecture in 1995, "The realisation of the electron donor ability of shared (bonded) electron pairs (single bonds) could one day rank equal in importance with G.N. Lewis' realisation of electron donor unshared (nonbonded) electron pairs (or for this reason I could add the electron pairs involved in multiple bonding)" The practical challenge arises from the fact that derivatives of saturated hydrocarbons are generally more reactive than the hydrocarbons themselves. Therefore, it is difficult to stop a reaction at the stage yielding primary products. It has been found that certain transition metal complexes can break normally inert hydrogen-carbon bonds under relatively mild condition. Development of reagents capable of selective transformation of C-H bond into more reactive functional groups is a recent and an intriguing objective in organometallic chemistry, and intensive efforts have been made to understand the nature of the activation process. The evidence so far uncovered is described in a number of special reviews on this topic in recent Years. This introduction will summarise current understanding of C-H bond activation.