Complexes cationiques η 3-1′,2,3-[(methylene-2γ-butyrolactone)-yl-3]-(η 5-cyclopentadienyl)carbonylnitrosyl molybdene: Synthese et etudes des additions nucleophiles stereoselectives

Abstract

Compound (η 3-CH 2 C(O)OCH 2C H)(η 5-C 5H 5)(CO)(NO)Mo is obtained when a carbonyl ligand of the molybdenum complex (η 3-CH 2 C(O)OCH 2C H)(η 5-C 5H 5(CO) 2Mo is replaced by the isoelectronic nitrosyl cation. In a polar solvent containing the exo-endo conformers of η 3-1′,2,3-(2γ-methylenebutyrolacton-3-yl)(η 5-cyclopentadienyl)dicarbonyl molybdenum (I), the reaction is stereoselective and proceeds with conformational interconversion. The relative exo-endo ratios of I and II have been evaluated by NMR spectroscopy at low temperature. The kinetic conformer II( endo), predominant at low temperature, is the only one that reacts towards nucleophiles. No reaction is observed with the thermodynamic compound II( exo). In all the cases studied, the nucleophilic attack occurs on the methylene carbon atom of the η 2-lactone ligand and yields η 2-olefinic nitrosyl complexes. Isolation of only one diastereoisomer confirms the stereospecificity of this addition step. For the neutral η 2-olefinic complex, a conformational equilibrium has been studied by NMR spectroscopy at low temperature: the ratios of the rotamers about the metal-olefin bond varies with solvent polarity. Oxidation by cerium ions of the η 2-olefinic molybdenum complexes yielding the free organic ligand has been achieved.