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

Abstract

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 (PhCHO) was investigated by variable-temperature (VT)³¹P{¹H} 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-³¹P{¹H} 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 PhCHO. 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-³¹P{¹H} 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 ³¹P{¹H} NMR spectra, which were observed at lower field as row number of the same group element increases.