Carbon-hydrogen bond activation, aryls

Recently, Gunnoe and co-workers showed in several detailed studies that TpRu (Me)(CO)(NCMe) is a very reactive compound with respect to C-H activation processes giving rise to several unusual transformations of organic substrates. For instance, TpRu(Me)(CO)(NCMe) was found to polymerize styrene and methyl methacrylate in benzene at 90 Furthermore, TpRu(Me)(CO)(NCMe) initiates carbon-hydrogen bond activation of benzene and at the 2-position of furan and thiophene to produce methane and TpRu(aryl)(CO)(NCMe) (aryl = phenyl, 2-furyl, 2-thienyl) (Scheme 24). The solid-state structure has been determined for TpRu(2-thienyl)(CO)(NCMe). The complexes TpRu(aryl)(CO)(NCMe) also serve as catalysts for the formation of ethylbenzene, 2-ethylfuran and 2-ethylthiophene from ethylene and benzene, furan, and thiophene, respectively (vide infra). DFT/B3LYP... 

The feasibility of hydrogen abstraction at the peptidyl a-carbon hydrogen bond by 1,4-aryl diradicals has been determined by examining a model reaction, i.e. abstraction of deuterium from dideuterioglycine by aryl radicals. The results have biological implications for the reactivity of the enediyne anti-tumour antibiotics with proteins. The non-Arrhenius behaviour of hydrogen-abstraction reactions by radicals has been investigated. For a number of reactions studied the enthalpy of activation was found either to increase or to decrease as a function of temperature. 

Carbon-hydrogen bonds are commonly formed by reductive elimination when an alkyl or aryl group and a hydride occupy mutually cis positions. Although intramolecular oxidative additions of C—H bonds and reactions of activated C—H bonds are well known for Ni, Pd, and Pt, additions of C—H bonds in simple alkanes and arenes are less common. 

These results are therefore consistent with a mechanism proceeding by the abstraction of a proton of the aryl ring by the base, in a process in which formation of the metal-carbon bond is concerted with the breaking of the carbon-hydrogen bond. DFT calculations carried out with bicarbonate as a model base for the intramolecular arylation favor two alternatives for the key activation process (i) a mechanism in which the base coordinates to the Pd(II) center and then acts as an... 

Thompson ME, Baxter SM, Bulls AR, et al. a-Bond metathesis for carbon-hydrogen bonds of hydrocarbons and Sc-R (R=H, alkyl, aryl) bonds of permethylscandocene derivatives. Evidence for noninvolvement of the n system in electrophilic activation of aromatic and vinyhc C-H bonds. J vim Chem Soc. 1987 109 203-219. 

---