Preparation, Characterisation and Reactivity of Neutral Mono‐η 4 ‐oxadiene Complexes of Molybdenum and Tungsten Containing Bidentate Donor Ligands

Neutral molybdenum and tungsten complexes containing only one η 4 ‐coordinated 1‐oxa‐1,3‐diene ligand are obtained diastereospecifically from the reaction of the corresponding dicarbonylbis(oxadiene) complexes with bidentate donor ligands capable of forming five‐membered ring chelates. Thus, dicarbonylbis[η 4 ‐( R )‐(+)‐pinocarvone]metal complexes react with 1,2‐bis(dimethylphosphanyl)ethane (dmpe) or N,N,N′,N′ ‐tetramethylethylenediamine (tmeda) to give the mono(oxadiene) compounds 1 – 4 in 45–95% yield. The molecular structure of the dmpe‐tungsten complex 3 was determined by X‐ray structural analysis Spectroscopic data indicate that an effective transfer of electron density from the donor atoms occurs. This is also reflected in a decreased reactivity of the carbonyl ligands in the mono(oxadiene) complexes towards nucleophilic alkylation. As a result, the selectivity in the alkylation (a formal “Umpolung” reaction) of anionic intermediates generated from 2 and methyllithium with carbon electrophiles is greatly improved, allowing access to alkylation products, e.g. 5 , that are otherwise difficult to obtain.