| Popis: |
Neutral molybdenum and tungsten complexes containing only one η4-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[η4-(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. |
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