Cycloauration of 2-substituted pyridine derivatives. Synthesis, structure and reactivity of six-membered cycloaurated complexes of 2-anilino-, 2-phenoxy- and 2-(phenylsulfanyl)-pyridine

At room temperature and in ethanol 2-anilinopyridine reacted with H[AuCl4]. 4H(2)O as well as Na[AuCl4]. 2H(2)O to give directly the six-membered cycloaurated complex [AuCl2(pap-C-1, N)] 1a [pap = 2-(2-pyridylamino)phenyl], whereas 2-phenoxypyridine (Hpop) and 2-(phenylsulfanyl)pyridine (Hptp) produced only the salts [H(2)pop][AuCl4] 2b and [H(2)ptp][AuCl4] 2c, respectively. The adducts [AuCl3(Hpop)] 3b and [AuCl3(Hptp)] 3c have separately been prepared by the reactions of Na[AuCl4]. 2H(2)O with Hpop and Hptp, respectively, in an acetonitrile-water mixed solvent. The salts 2b and 2c can be converted into the corresponding adducts 3b and 3c when they are stirred in acetonitrile-water at room temperature. The cycloaurated complexes [AuCl2(pop-C-1, N)] [pop = 2-(2-pyridyloxy)phenyl] 1b and [AuCl2(ptp-C-1, N)][ptp = 2-(2-pyridylsulfanyl)phenyl] 1c have been obtained by heating the salts or the adducts in acetonitrile-water, Moreover, complexes 1b and 1c have been synthesized directly by the reaction of H[AuCl4]. 4H(2)O with Hpop and Hptp in refluxing acetonitrile-water, ethanol-water and proyan-2-ol-water. The reaction of 1a with an equimolar amount of PPh3 in the presence of NaBF4 gave the cationic complex [AuCl(pap-C-1, N)(PPh3)]BF4 5a, while two equivalents of PPh3 or PEt3 afforded [AuCl(pap-C-1)(PPh3)(2)]Cl 6a or [AuCl(pap-C-1)(PEt3)(2)]Cl 7a where the pap ligand co-ordinates through only the carbon atom. On the other hand, the C-N chelates in 1b and 1c are easily cleaved with one equimolar amount of PPh3 to give [AuCl2(pop-C-1)(PPh3)] 8b and [AuCl2(ptp-C-1)(PPh3)] 8c, respectively. The boat-form structures of the three six-membered auracycles have been confirmed by X-ray diffraction studies of 1b, 1c and 5a. The crystal structure of 7a has also been determined.