Ligand effects in the hydrogenation of methacycline to doxycycline and epi-doxycycline catalysed by rhodium complexes molecular structure of the key catalyst [closo-3,3-(η2,3-C7H7CH2)-3,1,2-RhC2B9H11]
作者:A. Felekidis、M. Goblet-Stachow、J.F. Liégeois、B. Pirotte、J. Delarge、A. Demonceau、M. Fontaine、A.F. Noels、I.T. Chizhevsky、T.V. Zinevich、V.I. Bregadze、F.M. Dolgushin、A.I. Yanovsky、Yu.T. Struchkov
DOI:10.1016/s0022-328x(96)06743-5
日期:1997.5
The catalytic reduction of the exocyclic methylene group of methacycline (A) leads to the formation of two diastereoisomers, doxycycline (B, the alpha-epimer) and 6-epi-doxycycline (C, the beta-epimer), with a selectivity which markedly depends on the nature of hydrocarbon and carborane ligands of closo-(pi-cyclodienyl)rhodacarborane catalysts. Neutral norbornadienyl complexes with unsubstituted carborane ligands [closo-3,3-(eta(2,3)=C7H7CH2)-3,1,2-RhC2B9H11] (1) and [closo-2,2-(eta(2,3)-C7H7CH2)-2,1,7-RhC2B9H11] (7) are more active and afford higher selectivity in the formation of doxycycline than those having mono- or di-substituents at the carborane cage, [closo-3,3-(cyclodienyl)-1-R-2-R'-3,1,2-RhC2B9H9] (R = H, R' = Me, PhCH2; R = R' = Me; cyclodienyl = eta(2.3)-C7H7CH2 or eta-C10H13) as well as those from the closely related series of eta(5)-cyclopentadienyl complexes [(eta(2.3)-C7H7CH2)Rh(eta(5)-C5Rn)]+PF6- (R-n = H-5, Me-5, or H-2-1,2,4-Ph-3). Mechanistic aspects of the hydrogenation reaction of methacycline are sketched. The results of the X-ray diffraction study of the best catalyst 1 are reported. (C) 1997 Elsevier Science S.A.