Easily Accessible Methylene Complexes of Molybdenum and Tungsten: Structure and Application in Organic Synthesis [1

Abstract

Chlorides such as MoCl5, (EtO)3MoCl2, MoOCl3(thf)2, (EtO)MoOCl2, MoOCl4, MoO2Cl2, WOCl3(thf)2, and WOCl4, (but not WCl6, or halides of Mo(II) and Mo(III)) react with CH3Li (molar ratio 1∶2) in THF or ether to give labile, carbonyl olefinating complexes; approximately 1 mole of CH4 is liberated in every case. Attempts to isolate complexes have so far failed. From NMR data we ascribe to these reagents the structures of bis-μ-methylene complexes (1,3-dimetallacyclobutanes). The complexes appear to be potent in organic synthesis: whereas transition metal complexes with a terminal methylene or alkylidene ligand (e.g. Cp2Ti=CH2, (Me3CCH2)3Ta=CHCMe3) are distinguished by high reactivity in carbonyl olefinating reactions, the merit of the μ-methylene complexes is high chemo- and regio-selectivity in such reactions. Resistance towards hydroxy groups and very low basicity are further advantages; in solvents such as pentane or cyclohexane, the molybdenum and tungsten methylene complexes effect metallation (CH-activation), isomer-isation, dimerisation, cyclisation, and metathesis of olefins. These reactions are suppressed in THF, in contrast to carbonyl olefination reactions. Furthermore, a report is given on similar complexes obtained by reactions of molybdenum and tungsten halides with AlMe3 and ClMgCH2-SiMe3, respectively.