Dynamic stereochemistry of allylic palladium(II) carboxylate complexes

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Abstract
The low-temperature 1H n.m.r. spectra of the π-allylic palladium(II) carboxylate dimers [(π-all)Pd(O2CR)]2(R = Me, CH2Cl, CH2Ph; all = allyl: R = Me; all = 2-methylallyl, 2-t-butylallyl, 2-chloroallyl, syn-1-methylallyl, 1,1-dimethylallyl) have demonstrated the presence of conformational isomers in solution. Variable-temperature studies have shown that the n.m.r. equivalence of the π-allylic ligands occurs via a rapid intramolecular process and, at higher temperatures, via a bimolecular exchange process involving palladium–carboxylate bond breaking. The effect of the weak bridge-splitting ligands HOAc and Me2SO on these processes have been investigated. Studies of π-allylic ligand exchange between [(π-all)PdCl]2 and [(π-all)PdOAc]2 has been shown to occur very rapidly and to involve the intermediacy of mixed species containing both chloride and acetate. N.m.r. studies of the solution species [(π-all)Pd(OAc)PMe2Ph](all = 2-substituted allyl ligand) have shown that rapid syn–anti proton exchange at the terminal allylic carbon atom cis to PMe2Ph occurs via a σ-allylic intermediate. The free energy of activation of this syn–anti exchange decreases in the order 2-neopentylallyl > 2-t-butylallyl > 2-methylallyl > allyl.

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