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 PdCl₂, yielding [PdCl₂(TROPP^Ph)] (1a) and [PdCl₂(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)(H₂O)(dppp)](OTs) 2c (dppp = 1,3-bis-di(phenylphosphanyl)propane and [Pd(H₂O)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·0.5C₂H₄Cl₂ and of the neutral and cationic Pd-alkyl complexes [Pd(Me)(Cl)(TROPP^Ph) (7a·0.5CH₂Cl₂) and [Pd(CH₂CH₂COMe)(TROPP^Ph)](PF₆) (10a·0.5C₆H₆), respectively, confirm unambiguously the bidentate coordination mode of TROPP-ligands to Pd^II, that persists even in the presence of CO pressure.