Wittig Reactions of Phosphonium Ylides Bearing the Phosphastibatriptycene Skeleton with Carbonyl Compounds

GRAPHICAL ABSTRACT

Abstract

To investigate effects of the tridentate ligand on the reactivity of a 1,2-oxaphosphetane, an intermediate of the Wittig reaction, the reactions of phosphonium ylides 1 and 2 bearing the phosphastibatriptycene skeleton with carbonyl compounds have been conducted and compared with those of triphenyl phosphorus compounds. Non-stabilized and semi-stabilized phosphonium ylides 1 and 2 bearing the tridentate ligand were generated from the corresponding phosphonium salts 3 and 4, which were prepared by the reactions of a phosphastibatriptycene with ethyl iodide and benzyl bromide, respectively. The reactions of ylides 1 and 2 with benzaldehyde showed high E-selectivity, in sharp contrast to those of triphenylphosphonium ylides 5 and 6 giving Z-alkenes as a major product. However, the reaction of 1 with trifluoroacetophenone gave E/Z-alkenes 10 in the ratio of 69:31 together with phosphine oxide 13, which showed slightly higher selective E-alkene formation compared with the reaction of non-stabilized ylide 5 (E/Z ratio of 51:49). The results showed that the reactions did not proceed with high stereoselectivity as same as those of semi-stabilized phosphonium ylides 2 and 6. In the VT-³¹P{¹H} NMR spectra of the reaction mixture of non-stabilized phosphonium ylide 1 with trifluoroacetophenone, the signals due to the corresponding 1,2-oxaphosphetanes were observed as multi signals in high field around −54 ppm at −80°C, which was different from the reaction of ylide 5 showing two signals.

Keywords:

• Wittig reaction
• tridentate ligand
• VT-³¹P{¹H} NMR
• 1,2-oxaphosphetane