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, C₅H₄NCHN-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-OC₆H₄NCH(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-OC₆H₄NCH(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.