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