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

Abstract

The palladium(II) dimer, [Pd(C,N-C(6)H(4)CH(2)NMe(2))Cl](2) reacts with two equivalents of the NHC·CS(2) 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 NH(4)PF(6), to yield the cationic products [Pd(C,N-C(6)H(4)CH(2)NMe(2))(S(2)C·NHC)](+). In a similar fashion, the compounds [Pd(C,N-bzq)(S(2)C·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(S(2)C·NHC)(PPh(3))(2)](2+) (NHC = ICy, IMes, IDip, SIMes) are obtained on treatment of [PdCl(2)(PPh(3))(2)] with NHC·CS(2) zwitterions in the presence of NH(4)PF(6). The reaction of [PdCl(2)(dppf)] with IMes·CS(2) and NH(4)PF(6) provides the complex [Pd(S(2)C·IMes)(dppf)](2+). The complexes [Pd(S(2)C·NHC)(PPh(3))(2)](PF(6))(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)(S(2)C·NHC)](+) was supported by the high yield of 10-methoxybenzo[h]quinoline using [Pd(C,N-bzq)(S(2)C·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)(S(2)C·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(S(2)C·IMes)(PPh(3))(2)](PF(6))(2) and [Pd(S(2)C·IMes)(dppf)](PF(6))(2) were determined crystallographically.