Fast “Wittig-Like” Reactions As a Consequence of the Inorganic Enamine Effect

The
tungsten alkylidyne [CF3–ONO]­WCC­(CH3)3(THF)2 (3) {where CF3–ONO = (MeC6H3[C­(CF3)2O])2N3–} supported by a
trianionic pincer-type ligand demonstrates enhanced nucleophilicity
in unusually fast “Wittig-like” reactions. Experiments
are designed to provide support for an inorganic enamine effect that
is the origin of the enhanced nucleophilicity. Treating complex 3 with various carbonyl-containing substrates provides tungsten-oxo-vinyl
complexes upon oxygen atom transfer. The rates of reactivity of 3 are compared with the known alkylidyne (DIPP)3WCC­(CH3)3 (DIPP = 2,6-diisopropylphenoxide).
In all cases (except acetone), complex 3 exhibits significantly
faster overall rates than (DIPP)3WCC­(CH3)3. New oxo-vinyl complexes are characterized by NMR,
combustion analysis and single crystal X-ray diffraction. Treating 3 with acid chlorides provides the tungsten oxo chloride species
[CF3–ONO]­W­(O)Cl (4) and disubstituted
alkynes. In the case of acetone the oxo-vinyl complex results in two
rotational isomers 10syn and 10anti. The rate of isomerization was determined for the forward
and reverse directions and was complimented with DFT calculations.