Dynamic stereochemistry of allylic palladium(II) carboxylate complexes

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.