Benzene C-H activation

SCHEME 11.14 Qualitative energy diagram that explains kinetic isotope effects observed for benzene C—H activation by Cp Rh(PMe3). 

SCHEME 11.42 Calculated transition states for benzene C—H activation by Ru hydroxo and methyl complexes ([Ru] = (Tab)Ru(PH3), Tab = tris(azo)borate calculated bond distances are... 

Fig. 7.29 Thermodynamics of calculated mechanism for the benzene C-H activation (pic)Pt-solid line (bpym)Pt - dashed line.

Similarly, when both the Cp and arene ligands are permethylated, the reaction of 02 with the Fe1 complex leads to C-H activation of the more acidic benzyl bond [57]. When no benzylic hydrogen is present, superoxide reacts as a nucleophile and adds onto the benzene ligand of the FeCp(arene)+ cation to give a peroxocyclohexadienyl radical which couples with a Fe Cp(arene) radical. A symmetrical bridging peroxo complex [(Fe"Cp)2(r 5-C6H60)2] is obtained. The C-H activation reactions of the 19e Fe1 radicals BH can be summarized as follows... 

Rh(TMP)- under these conditions, and in fact the selective activation of methane in benzene solution is a distinctive and unusual feature of this system, given that aryl C—H activation ought to be thermodynamically favored over alkyl C—H activation. The proposed linear transition state proposed in Fig. 8 is the key to this different reactivity. The corresponding trimolecular transition state for an arene would be expected to be bent, and this would be precluded by the bulky TMP... 

For intermolecular hydrocarbon activation, we are not aware of any example where compelling evidence exists for C-H activation directly by square-planar four-coordinate Pt(II), without preceding (or concomitant) ligand loss. In one example, such a direct reaction may take place but the alternative explanation involving ligand loss is also consistent with the data. The compound (dmpe)PtMe(02CCF3) (dmpe = bis(dimethylphosphino)ethane) activates benzene C-H bonds at elevated temperature (125 °C) (23). This reactivity contrasts with that of (dmpe)PtMeCl which is inactive at the same temperature or even at... 

A related base-promoted C-H activation of benzene by Pt(II) was recently reported. With the tridentate monoanionic amido pineer ligand N3 , the triflate complex N3 Pt(OTf), depicted in Scheme 8 was shown to activate benzene in the presence of base (35). It was noted that the chloro complex N3 PtCl was not reactive under these conditions. The activity of the triflate complex again appears to result from the higher lability of triflate which can allow for coordination of the hydrocarbon. 

Iridium-catalyzed formation of B-C bonds from arene C-H bonds was first reported by Smith and coworkers [73]. They demonstrated that the archetypal C-H activation products, Cp lr(PMe3)(H)(R), could mediate B-C bond formation (R = Ph, cyclohexyl) and were able to effect the catalytic borylation of benzene with HBpin (8) to produce CgHsBpin and H2 at 150°C (8). 

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