E-Selective Wittig reactions using a nonstabilized phosphonium ylide bearing a phosphastibatriptycene skeleton with benzaldehydes under salt-free conditions

摘要

The Wittig reactions of a nonstabilized phosphonium ylide bearing a phosphastibatriptycene skeleton with benzaldehydes provided selectively E-alkenes in the presence of sodium salt. VT-P-31{H-1} NMR spectra showed two signals at -57.3ppm at -80 degrees C and at -55.2ppm at -40 degrees C assigned to cis- and trans-1,2-oxaphosphetanes, respectively, and the existence of the equilibrium between cis- and trans-forms at -40 degrees C. The observed intermediates were confirmed to be 1,2-oxaphosphetanes by deprotonation of the -hydroxyalkylphosphonium salt with TMS2NNa. The crossover reactions have been conducted in deprotonation of the -hydroxyalkylphosphonium salt with TMS2NNa in the presence of p-chlorobenzaldehyde to observe the exchange process between benzaldehyde and p-chlorobenzaldehyde moieties at -50 degrees C. The results clearly indicated that the stereochemical drift occurred at -40 degrees C, showing the reason for the E-selective Wittig reaction under salt-free conditions.