Chemoselective methoxycarbonylation of terminal alkynes catalyzed by Pd(II)-TROPP complexes.
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.
Istituto di Chimica dei Composti Organometallici, Area di Ricerca CNR di Firenze, via Madonna del Piano 10, 50019, Sesto Fiorentino, Italy.
Lorenzo
L
Bettucci
Claudio
C
Bianchini
Werner
W
Oberhauser
Matthias
M
Vogt
Hansjörg
H
Grützmacher