S-Hydrogen Elimination

It is possible to expect a )S-hydrogen elimination in steps iii and v of the mechanism presented in Scheme 15 producing the same results however, the kinetic data do not corroborate with a )S-hydrogen elimination mechanism. From a thermodynamic point of view, the calculated enthalpy of the reaction for a S-hydrogen elimination was found to be much higher than that of the hydride transfer following a six-centered transition state mechanism (+6 and 47 kcal/mol... 

For example, the acyl-aryloxo bond cleavage (type b) is shown by the reaction of Ni(cod)2 with phenyl propionate in the presence of PPhs (Scheme 3.34) or 2,2 -bipyridine [65]. The reaction products are ethylene, phenol, and (car-bonyl)nickel complex. Formation of these products is conveniently understood by initial oxidative addition of EtC(0)-0Ph followed by decarbonylation, )S-hydrogen elimination and reductive elimination, though (acyl)(aryloxo)nickel(II) intermediate is not isolated. However, such an intermediate is isolated by the selective insertion of CO into the (alkyl)(aryloxo)nickel (or palladium) complexes, which smoothly affords esters by reductive elimination promoted by electron deficient olehns. The results suggest that the oxidative addition involving C-0 bond cleavage is essentially reversible. 

The study of the involvement of hydridopalladium complexes under basic conditions is far more elusive. The involvement of these species in the /S-hydrogen elimination step has never been observed directly and their involvement is only implied based on mechanistic considerations. A good example was provided by studies of the reductive elimination step of the Heck arylation reaction. An alkylpalladium(II) intermediate releases the product methyl cinnamate, while simultaneously capturing a... 

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