Displacement reactions, insertion materials

A different class of electrode materials is based not upon insertion processes but on the displacement reactions of binary metal compounds [260, 261] (Eq. 2.4) ... 

The normal neutral pathway (22 24 25 27) was ruled out by conducting the reaction with monodentate phosphine BINAP ligand mimics (Scheme 12.5). The products obtained were of low enantiomeric excess relative to reactions employing BINAP. The direct cationic pathway (24-> 26) was also eliminated due to the fact that the opposite stereochemistry was obtained under cationic conditions with the addition of silver salts. The switch in stereoselectivity in the presence of silver salts, moreover, indicates that oxidative insertion is not the enantioselective step. j8-Hydride elimination was also discounted as the enantioselective step due to the influence of the double-bond geometry of the starting material on the enantioselectivity of the cyclization. The proposed enantioselective step is the formation of the cationic intermediate 26 by an associative displacement (24-> 28-> 26). In the case of square planar pafladium(n) complexes, substitution chemistry can occur through associative processes. Axial coordination of the alkene would form the pentacoordinate pafladium(II) complex 28. Reports of isolated and characterized pentacoordinate palladium(II) species provide support for this proposed intermediate. 

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