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

Abstract

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(2)O)(dppp)](OTs) 2c (dppp = 1,3-bis-di(phenylphosphanyl)propane and [Pd(H(2)O)(2)(MeO-dppp)](OTs)(2) 2d (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 beta-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 x 0.5 C(2)H(4)Cl(2) and of the neutral and cationic Pd-alkyl complexes [Pd(Me)(Cl)(TROPP(Ph)) (7a x 0.5 CH(2)Cl(2)) and [Pd(CH(2)CH(2)COMe)(TROPP(Ph))](PF(6)) (10a x 0.5 C(6)H(6)), respectively, confirm unambiguously the bidentate coordination mode of TROPP-ligands to Pd(II), that persists even in the presence of CO pressure.