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

GRAPHICAL ABSTRACT

ABSTRACT

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

KEYWORDS:

• Nonstabilized phosphonium ylide
• tridentate aryl ligand
• VT-³¹P{¹H} NMR spectroscopy
• 1,2-oxaphosphetane
• stereochemical drift

Funding

This study was partially supported by a Grant-in-Aid for Education, Culture, Sports, Science and Technology of Japan, the Murata Science Foundation, and a Kitasato University Research Grant for Young Researchers.