Palladium conjugate base mechanism

Moving to palladium complexes, related early studies on [PdX(Et4-dien)]+ also indicated the possible operation of a CB mechanism. Hydroxide ions reacted differently to other nucleophiles at these steri-cally hindered molecules in showing a distinct bimolecular (A2) dependence as well as a solvolytic pathway (85). This was unexpected because OH" is known to be a poor nucleophile in these systems, and it led to the proposal of the conjugate base contribution. The ions [PdX-(Et4Me-dien)]+, which have no acidic protons, did not show this behavior. The CBs reacted some 30 times faster than their precursors (a reactivity enhancement somewhat less than their gold(III) counterparts with the same ligands), again probably by an associative route. 

The catalytic cycle for stabilised or soft nucleophiles (which are the conjugate bases of acids whose pKi, is lower than 25) involves the direct attack of the nucleophile on the alkene coordinated to palladium. Part of the mechanism for substrate 11 is depicted in Scheme 8.5. 

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