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

The mono-phosphanes 5-(diphenyl)-phosphanyl-5H-dibenzo[a,d]cycloheptene (TROPPPh) and 5-((2-methoxy)phenyl)-phosphanyl-5H-dibenzo[a,d]cycloheptene (TROPP(2-MeOPh)) have been employed to coordinate PdCl2, yielding [PdCl2(TROPPPh)] (1a) and [PdCl2(TROPP(2-MeOPh))] (1b), respectively. The corresponding tosylate (OTs) complexes [Pd(OTs)2(TROPPPh)] (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 PdII-(tosylate)-diphosphane complexes [Pd(OTs)(H2O)(dppp)](OTs) 2c (dppp = 1,3-bis-di(phenylphosphanyl)propane and [Pd(H2O)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)(TROPPPh)](OTs) (3a) in conjunction with the evidence of the fast b-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)(TROPPPh) (7a·0.5CH2Cl2) and [Pd(CH2CH2COMe)- (TROPPPh)](PF6) (10a·0.5C6H6), respectively, confirm unambiguously the bidentate coordination mode of TROPP-ligands to PdII, that persists even in the presence of CO pressure.