Activation of carbon [sbnd]hydrogen bonds by metal complexes: Mechanistic, kinetic and thermodynamic considerations

Abstract

The activation and functionalization of carbon hydrogen bonds under homogeneous conditions continues to be an important and challenging objective. Only in recent years have a few examples of such C H bond activation, especially at aliphatic carbon centers, been reported and these involve, for the most part, stoichiometric rather than catalytic reactions; general approaches with widespread applicability still are lacking. Drawing upon the limited insights provided by these examples, and upon the pertinent information derived from related studies on the reverse process ( i.e., metal complex-promoted C H bond formation) and on the catalytic activation of other saturated molecules (notably H 2), as well as from recent determinations of metal carbon and metal hydrogen bond energies, the mechanistic, kinetic and thermodynamic aspects of C H bond activation at metal centers are analyzed. It is concluded that thermodynamic constraints, notably those associated with the characteristic weakness of metal carbon bonds, are of dominant importance in limiting the reactivities of metal complexes toward C H bonds.