Hyperdistorted Tungsten Allyl Complexes and Their Stereoselective Deprotonation to Form Dihapto-Coordinated Dienes

Several π-allyl complexes of {TpW(NO)(PMe3)} are investigated as possible sources of η2-diene complexes. In order to prepare the diene complex as a single diastereomer, the allyl complex must undergo deprotonation stereoselectively. Allyl complexes of this tungsten system are highly distorted, with the difference between the W−C bond lengths for the two allyl termini being as much as 0.69 A. DFT calculations and several crystal structures are presented that collectively suggest that one terminus, C1, distal to the PMe3 group, tends toward an sp2 carbocation. Consistent with this interpretation, deprotonation preferentially occurs at a carbon adjacent to this allyl terminus for six-membered rings. However, in the presence of base the five-membered cyclic analogue [TpW(NO)(PMe3)(C5H7)]+ fails to form either isomer of the corresponding η2-diene complex.