strong CH-acids such as malononitrile or ethyl cyanoacetate (2), which leads to the corresponding stable phosphorus ylides (3) in excellent yields (Scheme 1).
Scheme 1
Result and Discussion
The reactions of malononitrile or ethyl cyanoacetate (2) with dialkyl acetylenedicarboxylates (1) in the presence of triphenylphosphine proceeded in a mixture of ethyl acetate and n-hexane (2:1) at room temperature and were complete within 1 hour. The 1H- and 13C- NMR spectra of the crude product clearly indicated the formation of stable phosphorus ylides, 3. No products other than 3 could be detected by NMR spectroscopy. The structures of compounds 3a–f were deduced from their IR-, 1H-, 13C- and 31P- NMR spectra. The mass spectra of the products displayed molecular ion peaks at appropriate m/z values. Any initial fragmentations involved the loss of alkoxy, alkoxycarbonyl, and PPh3 moieties.
The 1H-, 13C-, and 31P- NMR spectra of ylides 3a–f are consistent with the presence of two geometrical isomers. The ylide moiety of these compounds is conjugated with the adjacent carbonyl group, and rotation around the partial double bond of the (E)- 3 and (Z)- 3 geometrical isomers is slow on the NMR timescale at ambient temperature. Selected 1H-, 13C-, and 31P- NMR chemical shifts and coupling constants in the major (M) and minor (m) geometrical isomers of compounds 3a–f are shown in Table 1. On the basis of the well-established chemistry of trivalent phosphorus nucleophiles,1,6,7 it is reasonable to assume that the phosphorus ylides (3) result from the initial addition of triphenylphosphine to the acetylenic ester and subsequent deprotonation of the 1:1 adduct (Scheme 2).
