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]

Article abstract of DOI:10.1016/S0022-328X(96)06743-5

The catalytic reduction of the exocyclic methylene group of methacycline (A) leads to the formation of two diastereoisomers, doxycycline (B, the α-epimer) and 6-epi-doxycycline (C, the β-epimer), with a selectivity which markedly depends on the nature of hydrocarbon and carborane ligands of closo-(π-cyclodienyl)rhodacarborane catalysts. Neutral norbornadienyl complexes with unsubstituted carborane ligands [closo-3,3-(η^2,3-C₇H₇CH₂)-3,1,2-RhC₂B₉H₁₁] (1) and [closo-2,2-(η^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 = η^2,3-C₇H₇CH₂ or η-C₁₀H₁₃) as well as those from the closely related series of η⁵-cyclopentadienyl complexes [(η^2,3-C₇H₇CH₂)Rh(η⁵-C₅R_n)]⁺PF^-₆ (R_n = H₅, Me₅, or H₂-1,2,4-Ph₃). 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.