Novel Reactions of Organomolybdenum and Organotungsten Compounds: Additive-Reductive Carbonyl Dimerization, Spontaneous Transformation of Methyl Ligands into mu-Methylene Ligands, and Selective Carbonylmethylenation

摘要

Hitherto there was no reaction known that permits transformations of R~1R~2-CO->0.5 R~1R~2R~3C-CR~1R~2R~3 in one step. This type of additive-reductive carbonyl dimerization is now possible using alkoxy(alkyl)tungsten(v) complexes with aromatic, heteroaromatic, or alpha,beta-unsaturated aldehydes and ke-tones. When a corresponding phenyl complex was employed in a test experiment, it was revealed that an aliphatic ketone could be used as the substrate in this reaction, A second interesting type of reaction is the transformation of CH_3 ligands into H-CH_2 ligands, which occurs during the treatment of MeLi or Me_3Al with molybdenum or tungsten chlorides (oxidation states VI and V, for Mo additionally IV) at low temperatures with liberation of CH_4. Here, the question arises as to whether the intermediate involved has a terminal CH_2 ligand (Schiock carbene complex) or a mu-CH_3 ligand (CH_3 bound by a two-electron three-center bond to two metal atoms) Of all the mu-CH_2 complexes obtained, those whichwere synthesized by the action of MeLi on molybdenum chlorides can be recommended as reagents for car-bonylmethylenation of aldehydes and ketones. They display high selectivity. very low basicity, a surprising resistance to protons, they are readily available, can be easily modified and, as regards their selective behavior, they have been investigated more thoroughly than other readily accessible carbonylmethylena-tion reagents of comparable selectivity. The results of NMR spectroscopic investigationson the structure of the mu-CH_2 complexes, and associcated reaction mechanisms are discussed. A survey of carbonylmethylenation reagents, which have been reported in the literature, permits comparisons to be made with carbonylmethylenating molybdenum andtungsten complexes.