(E)‐Selective Wittig Reactions between a Nonstabilized Phosphonium Ylide Bearing a Phosphastibatriptycene Skeleton and Benzaldehydes

Abstract

Wittig reactions between benzaldehyde derivatives and a nonstabilized phosphonium ylide bearing a phosphastibatriptycene skeleton, regarded as a tridentate aryl ligand, gave (E)‐alkenes with high selectivity in the presence of both lithium and sodium salts. As previously reported, reactions between a triphenylphosphonium ylide and benzaldehyde derivatives under the same conditions afforded mainly (Z)‐alkenes. Variable‐temperature (VT) 31P{1H} NMR spectra showed two signals, assigned to cis‐ and trans‐1,2‐oxaphosphetanes, which were observed at different temperatures (–80 °C and –40 °C, respectively) in the Wittig reaction between benzaldehyde and the nonstabilized phosphonium ylide bearing the phosphastibatriptycene skeleton, in the presence of both lithium and sodium salts, and showed the existence of equilibrium between these products at –40 °C. On the other hand, this equilibrium was not clearly observed in the reaction between the triphenylphosphonium ylide and benzaldehyde, for which only one signal was detected. The observed intermediates were confirmed to be 1,2‐oxaphosphetanes by deprotonation of the isolated β‐hydroxyalkylphosphonium salts bearing a phosphastibatriptycene skeleton and a triphenylphosphine moiety, respectively. Crossover reactions were conducted in the deprotonations of β‐hydroxyalkylphosphonium salts with TMS2NNa in the presence of p‐chlorobenzaldehyde, resulting in the observation of signals corresponding to 1,2‐oxaphosphetanes containing phenyl and p‐chlorophenyl groups at the 4‐positions, indicating the exchange process between benzaldehyde and p‐chlorobenzaldehyde at –40 °C for the phosphastibatriptycene system and at 0 °C for triphenyl derivatives. These results clearly indicated that stereochemical drift occurred at those temperatures, even in reactions using nonstabilized phosphonium ylides. The stereochemical drift in the phosphastibatriptycene system occurred at a lower temperature than in the case of the triphenyl derivative, thus explaining the (E)‐selective Wittig reactions between the benzaldehyde derivatives and the nonstabilized phosphastibatriptycene‐based phosphonium ylide in the presence of lithium and sodium salts.