The answer to your question is quite an interesting one. The Bu3P.CS2
adduct is present in solution in equilibrium with the free phosphine and
CS2. With electron poor alkenes such as maleates, fumarates acrylates etc.
the phosphine adds in a Michael fashion and this is followed by a
1,2-proton transfer to give stabilised ylides of the form E-C(=PBu3)-CH2-E
(from ECH=CHE). We reported this very briefly in conference proceedings
(Phosphorus, Sulfur and Silicon, 1993, 77, 172) and recently Konovalov,
Tyryshkin and coworkers in Kazan, Russia have examined it in more detail by
measuring the equilibrium constant for the phosphine CS2 adduct's
dissociation in a range of solvents (Zh. Obshch. Khim., 1995, 65, 1101).
With best wishes
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