A-Acylmethylenephosphoranes

We have found it more convenient to prepare (carbethoxymethyl)-triphenylphosphonium bromide in situ in the same reaction vessel from triphenylphosphine and ethyl bromoacetate. This effectively provides a one-pot preparation for the a-acylmethylenephosphorane from triphenylphosphine and allows facile incorporation of various esters, by employing different bromoacetate esters, in the acetylenic product. The yield and purity of the resultant phosphorane is unaffected by the following one-pot procedure. 

For acyl halides or anhydrides which do not afford ketenes in the presence of base (such as perfiuoroacyi halides), however, the a-acylmethylenephosphoranes can be prepared directly in one step from the phosphonium salts by using two equivalents of base by the present procedure (Table I).2 Both tetrahydrofuran and methylene chloride have been used as solvents and in the case of the title compound, tetrahydrofuran provides the best results. Good yields of the phosphoranes are generally obtained when is an electron-withdrawing group such as ester or nitrile. The yields of phosphoranes obtained for the thiomethyl or phenyl cases can be... 

The yellow crystalline solid is analytically pure (>95% by NMR and HPLC analysis), mp 124-127°C. We have found that the yield of the acetylenes can be adversely affected by small amounts of impurities in the a-acylmethylenephosphoranes and prefer to recrystallize the phosphoranes prior to thermolysis. The yield from the recrystallization step is greater than 95%. 

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