LIGAND EFFECTS IN THE HYDROGENATION OF METHACYCLINE TO DOXYCYCLINE AND EPI-DOXYCYCLINE CATALYZED BY RHODIUM COMPLEXES - MOLECULAR-STRUCTUREOF THE KEY CATALYST [CLOSO-3,3-(ETA(2,3)-C7H7CH2)-3,1,2-RHC2B9H11]

The catalytic reduction of the exocyclic methylene group of methacycli ne (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 ca rborane ligands of closo-(pi-cyclodienyl)rhodacarborane catalysts. Neu tral norbornadienyl complexes with unsubstituted carborane ligands [cl oso-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 sel ectivity 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 = et a(2.3)-C7H7CH2 or eta-C10H13) as well as those from the closely relate d series of eta(5)-cyclopentadienyl complexes [(eta(2.3)-C7H7CH2)Rh(et a(5)-C5Rn)]+PF6- (R-n = H-5, Me-5, or H-2-1,2,4-Ph-3). Mechanistic asp ects of the hydrogenation reaction of methacycline are sketched. The r esults of the X-ray diffraction study of the best catalyst 1 are repor ted. (C) 1997 Elsevier Science S.A.