Heteroatom effects toward isomerization of intermediates in Wittig reactions of non‐stabilized phosphonium ylides bearing a phosphaheteratriptycene skeleton with benzaldehyde

Isomerization of intermediates, cis ‐ and trans ‐1,2‐oxaphosphetanes, in Wittig reactions of non‐stabilized phosphonium ylides bearing a phosphaheteratriptycene skeleton containing group 14 (PhSi, PhGe, PhSn, n ‐BuSn) and 15 (P, As, Sb, and Bi) elements with benzaldehyde (Ph CHO ) was investigated by variable‐temperature ( VT ) 31 P{ 1 H} NMR spectroscopy. The isomerization from the cis ‐1,2‐oxaphosphetane to the trans ‐form occurred at lower temperatures as the row number of the same group elements increases. Wittig reactions under the same conditions gave the ( Z )‐olefin as a major product in the cases of period 3 elements (PhSi and P) and the ( E )‐olefin as a major product in the cases of elements from period 4 and below (PhGe, PhSn, n ‐BuSn, As, Sb, and Bi). The selectivity of olefin formation is considered to depend on the isomerization temperature of the intermediates, because each olefin must be obtained from the corresponding 1,2‐oxaphosphetane. The VT ‐ 31 P{ 1 H} NMR spectra showed that the cis ‐1,2‐oxaphosphetanes were the kinetic products in the first step of Wittig reactions and the trans ‐forms were the thermodynamically stable products formed by isomerization from the cis ‐forms via ring‐opening and ring‐closing reactions of phosphonium ylides with Ph CHO . Density functional theory ( DFT ) calculations indicated that cis ‐1,2‐oxaphosphetanes were less stable than the trans ‐forms by ~2 kcal/mol, supporting thermodynamically favorable isomerization from cis ‐forms to trans ‐forms, as observed by VT ‐ 31 P{ 1 H} NMR spectroscopy. Heteroatoms at the bridgehead position of the phosphaheteratriptycene skeleton significantly affected the isomerization temperature as well as the phosphorus‐31 signals in the 31 P{ 1 H} NMR spectra, which were observed at lower field as row number of the same group element increases.