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 Å. 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.