Chemoselective methoxycarbonylation of terminal alkynes catalyzed by Pd(ii)-TROPP complexes

摘要

The mono-phosphanes 5-(diphenyl)-phosphanyl-5H-dibenzo[a,d]cycloheptene (TROPP~(Ph)) and 5-((2-methoxy)phenyl)-phosphanyl-5H-dibenzo[a,d] cycloheptene (TROPP~((2-MeOPh))) have been employed to coordinate PdCl_2, yielding [PdCl_2(TROPP~(Ph))] (1a) and [PdCl_2(TROPP~((2-MeOPh)))] (1b), respectively. The corresponding tosylate (OTs) complexes [Pd(OTs)_2(TROPP~(Ph))] (2a) and [Pd(OTs)_2(TROPP~((2-MeOPh)))] (2b) have been successfully applied in the p-benzoquinone (BQ)-assisted methoxycarbonylation of terminal alkynes to give chemoselectively the corresponding alkynylcarboxylic acid methyl ester with high TOF (up to 980 h~(-1)). Unlike 2a/b, the Pd~(II)-(tosylate)-diphosphane complexes [Pd(OTs)(H_2O)(dppp)] (OTs) 2c (dppp = 1,3-bis-di(phenylphosphanyl)propane and [Pd(H _2O)_2(MeO-dppp)](OTs)_22d (MeO-dppp = 1,3-bis(di(2-methoxyphenyl)phosphanyl)propane) preferentially catalyzed a double alkyne insertion. The "in situ" spectroscopic observation of the Pd-methoxycarbonyl compound [Pd(COOMe)(TROPP~(Ph))](OTs) (3a) in conjunction with the evidence of the fast β-hydride elimination reaction of TROPP-based Pd-catalysts in the dimerization reaction of ethene are indicative for a strongly electrophilic metal centre. The X-ray crystal structures of 1a·0.5C_2H_4Cl_2 and of the neutral and cationic Pd-alkyl complexes [Pd(Me)(Cl)(TROPP~(Ph)) (7a·0. 5CH_2Cl_2) and [Pd(CH_2CH_2COMe) (TROPP~(Ph))](PF_6) (10a·0.5C_6H _6), respectively, confirm unambiguously the bidentate coordination mode of TROPP-ligands to Pd~(II), that persists even in the presence of CO pressure.