On the structure and reactivity of a stabilized phosphorane: methyl (triphenylphosphoranylidene) acetate

Abstract

. Can. J. Chem 68, 2123 (1990).In the absence of a source of acidic protons, methyl (triphenylphosphoranylidene)acetate 1 exists as a mlxture of the cis and trans isomers even at room temperature. Acidic protons catalyze the interconverslon of the two forms and facilitate the reaction of the ylide with active carbonyl compounds such as benzaldehyde or phthalic anhydride.

Organometallic Ruthenium Complexes: Application in the Olefination of Carbonyl Compounds

“…One of the main features is the collapse of the peaks of the ylide's cis and trans isomers. This pattern is similar to that observed on addition of benzaldehyde to the ylide. 26a,b The collapse of the peaks indicates that the interconversion between cis and trans isomers is very fast (on the NMR time scale).…”

“…At the same temperature the 31 P NMR shows a steady decrease of the peak of complex 7 (δ P = 41 ppm) while two new peaks arise: a peak at 50.3 ppm for carbene complex 8 and a somewhat broader peak at 17.9 ppm, which splits into two peaks after 20 min at −10 °C. The latter peaks, at δ P = 18.4 and δ P = 16.7 ppm, correspond to the cis and trans isomers of the phosphorus ylide, as further confirmed by the 1 H NMR.…”

“…The same reaction in the presence of benzaldehyde (Figure ) shows some particular features: the ylide has a single sharp peak (δ P = 18.1 ppm) in the 31 P NMR, a result of its interaction with the aldehyde; the formation of OPPh 3 (δ P = 28.0 ppm) is fast and it is present almost from the beginning of the catalysis; the ylide is consumed completely within 15 min, during which phosphazine XIII (δ P = 22.6 ppm) scarcely reacts. After 150 min at RT the reaction is complete, as there is only the peak of OPPh 3 in the 31 P NMR.…”

“…The 31 P NMR has three main peaks besides the free PPh 3 (excess): a singlet at δ P = 21.1 ppm and two broad peaks corresponding to the geometrical isomers of the ylide (δ P = 18.0 and 16.4 ppm), which, as the temperature increases, almost coalesce to a single broad peak at δ P =17.9 ppm . It is necessary to wait 24 h until the phosphazine is almost entirely consumed, leaving ylide as the main product, observable in both the 31 P and 1 H NMR spectra.…”

“…Hooper et al26b suggested that during the Wittig reaction neither the cis nor trans isomers of the phosphorus ylide seem to readily react with the carbonyl group. Instead the reaction in carried out by an intermediate present during the interconversion of the two isomers, which bears a negative charge on the methine carbon (Scheme ) 26b.…”