Mild and selective Pd–Ar protonolysis and C–H activation promoted by a ligand aryloxide group

Abstract

A bidentate nitrogen-donor ligand with an appended phenol group, C₅H₄NCH[double bond, length as m-dash]N-2-C₆H₄OH, H(L1) was treated with a palladium cycloneophyl complex [Pd(CH₂CMe₂C₆H₄)(COD)], with both Pd-aryl and Pd-alkyl bonds, to give a Pd-alkyl complex, [Pd(CH₂CMe₂C₆H₅)(κ³-N,N',O-OC₆H₄N[double bond, length as m-dash]CH(2-C₅H₄N))], 1. The cleavage of the Pd-aryl bond and the deprotonation of the ligand phenol to afford a bound aryloxide, indicates facile Pd-aryl bond protonolysis. Deuterium labelling experiments confirmed that the ligand phenol promotes protonolysis and that the reverse, aryl C-H activation, occurs under very mild reaction conditions (within 10 min at room temperature). An unusual isomerization of the Pd-alkyl complex 1 to a Pd-aryl complex, [Pd(C₆H₄(2-t-Bu))(κ³-N,N',O-OC₆H₄N[double bond, length as m-dash]CH(2-C₅H₄N))], 2, was observed to give an equilibrium with [2]/[1] = 9 after 5 days in methanol. The isomerization requires that both aryl C-H activation and Pd-alkyl protonolysis steps occur. The very large KIE value (k_H/k_D = ca. 40) for isomerization of 1 to 2, suggests a concerted S_E2-type mechanism for the Pd-alkyl protonolysis step.