Chelating groups 3-hydrogen elimination

The effects of chelating ligand structure on reductive elimination and /3-hydrogen elimination have not been studied extensively. However, one study did evaluate the ratios of products formed from the catalytic process as the ligand bite angle, the aryl group s electronic, and the aryl group s steric properties were systematically perturbed. ... 

Stoichiometric and catalytic reactions that result from attack of nucleophiles onto square-planar palladium(II) and platinum(II) olefin complexes are legion. The o-alkyl products are stable in some cases, but p-hydrogen elimination to generate products from functionalization of the C-H bond of an olefin occurs in other cases. Nucleophihc attack on palladium(II) and platinum(II) complexes of diolefins often generate stable a-alkyl products because the resulting alkyl group is stabilized by chelation. 

There could be a chelation effect of the carbomethoxyl group with the palladium atom in the adduct which preferentially produces the exocyclic elimination product, or there may just be a tendency to eliminate the more electronegative hydrogen atom. 

Of the four chelated structures suggested by the Fiesers, the resonance pair A and B suffice to explain the fact that only the hydrazone residue at C-1 can be methylated or benzoylated, because the imino group of the hydrazone residue at C-2 is involved in hydrogen bonding, Structures C and D would be expected to undergo alkylation on the hydrazone residue at C-2 and must, therefore, either be eliminated from consideration or be regarded as minor constituents. This leaves the resonance pair A and B as the most probable of the Fiesers structures. 

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