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

Abstract

At room temperature and in ethanol 2-anilinopyridine reacted with H[AuCl₄]·4H₂O as well as Na[AuCl₄]·2H₂O 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 [H₂pop][AuCl₄] 2b and [H₂ptp][AuCl₄] 2c, respectively. The adducts [AuCl₃(Hpop)] 3b and [AuCl₃(Hptp)] 3c have separately been prepared by the reactions of Na[AuCl₄]·2H₂O 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]·4H₂O with Hpop and Hptp in refluxing acetonitrile–water, ethanol–water and propan-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-C¹)(PPh₃)₂]Cl 6a or [AuCl(pap-C¹)(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-C¹)(PPh₃)] 8b and [AuCl₂(ptp-C¹)(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.