Synthesis and catalytic application of palladium imidazol(in)ium-2-dithiocarboxylate complexes

Abstract

The palladium(II) dimer, [Pd(C,N-C₆H₄CH₂NMe₂)Cl]₂ reacts with two equivalents of the NHC·CS₂ zwitterionic ligands [NHC = IPr (1,3-diisopropylimidazol-2-ylidene), ICy (1,3-dicyclohexylimidazol-2-ylidene), IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IDip (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), SIMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene)] in the presence of NH₄PF₆, to yield the cationic products [Pd(C,N-C₆H₄CH₂NMe₂)(S₂C·NHC)]⁺. In a similar fashion, the compounds [Pd(C,N-bzq)(S₂C·NHC)]⁺ (bzq = benzo[h]quinolinyl, NHC = ICy, IMes, IDip) are obtained from the corresponding dimer [Pd(C,N-bzq)Cl]₂. The bis(phosphine) compounds [Pd(S₂C·NHC)(PPh₃)₂]²⁺ (NHC = ICy, IMes, IDip, SIMes) are obtained on treatment of [PdCl₂(PPh₃)₂] with NHC·CS₂ zwitterions in the presence of NH₄PF₆. The reaction of [PdCl₂(dppf)] with IMes·CS₂ and NH₄PF₆ provides the complex [Pd(S₂C·IMes)(dppf)]²⁺. The complexes [Pd(S₂C·NHC)(PPh₃)₂](PF₆)₂ (NHC = IMes, IDip) were active pre-catalysts (1 mol% loading) for the conversion of benzo[h]quinoline to 10-methoxybenzo[h]quinoline in the presence of PhI(OAc)₂ and methanol. The intermediacy of [Pd(C,N-bzq)(S₂C·NHC)]⁺ was supported by the high yield of 10-methoxybenzo[h]quinoline using [Pd(C,N-bzq)(S₂C·IDip)]⁺ to promote the same reaction. Small amounts of 2,10-dimethoxybenzo[h]quinoline were also isolated from these reactions. Using [Pd(C,N-bzq)(S₂C·IDip)]⁺ and N-chlorosuccinimide as the oxidant led to the formation of 10-chlorobenzo[h]quinoline in moderate yield from benzo[h]quinoline. The molecular structures of [Pd(S₂C·IMes)(PPh₃)₂](PF₆)₂ and [Pd(S₂C·IMes)(dppf)](PF₆)₂ were determined crystallographically.