ATTEMPTED SYNTHESIS OF TRIMESITYLPHOSPHAETHENE; OBSERVATIONS RELATED TO THE MECHANISM OF ACID CATALYZED NUCLEOPHILIC SUBSTITUTIONS AT PHOSPHORUS (III)

Abstract

Trimesitylphosphaethene (MesP[dbnd]CMes₂, If) is of interest as a sterically protected and presumably very stable phosphaalkene. Its synthesis was attempted along three different routes. The first two routes were modelled after the well-documented syntheses by phosphaalkenes by base catalyzed elimination of hydrogen chloride from MesPCICHMes₂ (3). In the first approach, 3 could not be obtained from the precursor MesP(NEt₂)CHMes₂ (4) by treatment with hydrogen chloride. Instead, the phosphonium salt [MesPH(NEt₂)CHMes₂]^⊕Cl^⊖ (2) was formed; (2) is of interest as a “frozen” intermediate in the acid catalyzed nucleophilic substitution at phosphorus(III). The mechanistic implications of its formation and the reasons for its lack of reactivity are discussed.

In the second approach, 3 was obtained from the reaction of MesPCl₂ (8) with α-potassiodimesityl-methane. However, several attempts to eliminate hydrogen chloride from 3 were unsuccessful. Similarly, the third route, aimed at the preparation of CIP[dbnd]CMes₂ (9) from Cl₂PCHMes₂ (10) was thwarted because hydrogen chloride could not be eliminated from 10. The unusual behavior of 2, 3, and 10 can be explained by steric hindrance in these extremely crowded molecules.