O,N,N-Pincer ligand effects on oxidatively induced carbon–chlorine coupling reactions at palladium

Abstract

The syntheses of two families of sterically tuneable O,N,N pro-ligands are reported, namely the 2-(phenyl-2′-ol)-6-imine-pyridines, 2-(C₆H₄-2′-OH),6-(CMeNAr)C₅H₃N [Ar = 4-i-PrC₆H₄ (HL1_a), 2,6-i-Pr₂C₆H₃ (HL1_b)] and the 2-(phenyl-2′-ol)-6-(amino-prop-2-yl)pyridines, 2-(C₆H₄-2′-OH),6-(CMe₂NHAr)C₅H₃N [Ar = 4-i-PrC₆H₄ (HL2_a), 2,6-i-Pr₂C₆H₃ (HL2_b)], using straightforward synthetic approaches and in reasonable overall yields. Interaction of HL1_a/c and HL2_a/b with palladium(II) acetate affords the O,N,N-pincer complexes, [{2-(C₆H₄-2′-O)-6-(CMeNAr)C₅H₃N}Pd(OAc)] (Ar = 4-i-PrC₆H₄ (1a), 2,6-i-Pr₂C₆H₃ (1b)) and [{2-(C₆H₄-2′-O)-6-(CMe₂NHAr)C₅H₃N}Pd(OAc)] (Ar = 4-i-PrC₆H₄ (2a), 2,6-i-Pr₂C₆H₃ (2b)), which can be readily converted to their chloride derivatives, [{2-(C₆H₄-2′-O)-6-(CMeNAr)C₅H₃N}PdCl] (Ar = 4-i-PrC₆H₄ (3a), 2,6-i-Pr₂C₆H₃ (3b)) and [{2-(C₆H₄-2′-O)-6-(CMe₂NHAr)C₅H₃N}PdCl] (Ar = 4-i-PrC₆H₄ (4a), 2,6-i-Pr₂C₆H₃ (4b)), respectively, on reaction with an aqueous sodium chloride solution. Treating each of 3a, 3b, 4a and 4b with two equivalents of di-p-tolyliodonium triflate at 100 °C in a toluene/acetonitrile mixture affords varying amounts of 4-chlorotoluene along with the 4-iodotoluene by-product with the conversions highly dependent on the steric and backbone properties of the pincer complex employed (viz.4a > 3a > 4b > 3b); notably, the least sterically bulky and most flexible amine-containing 4a reaches 90% conversion to 4-chlorotoluene in 15 h as opposed to 17% for imine-containing 3b. In the case of 3a, the inorganic palladium species recovered from the reaction has been identified as the Pd(II) salt [{2-(C₆H₄-2′-O)-6-(CMeN(4-i-PrC₆H₄)C₅H₃N}Pd(NCMe)][O₃SCF₃] (5a), which was independently prepared by the reaction of 3a with silver triflate in acetonitrile. Single crystal X-ray structures are reported for HL1_a, HL2_a, 1a, 1b, 2a, 2b, 3a and 5a.