Ring strain and the reactivity of dibenzophospholes and their quaternary salts: crystal and molecular structure of 5-(p-bromobenzyl)-5-phenyldibenzophospholium bromide

Abstract

Crystals of the title compound are monoclinic, a= 25.685(15), b= 10.444(6), c= 18.818(11), β= 119.18(5)°, Z= 8 in space group C2/c, with the oxygens of water molecules present in the crystals lying along two-fold axes. The structure was determined by Patterson and electron-density methods and refined by least-squares techniques to R= 0.066 for 992 independent reflections. The fused ring system of the dibenzophospholium cation is almost planar and while the exocyclic C–P–C angles lie in the range 110–117°, the endocyclic angle is 93.9°. It is suggested that this low value is consistent with the presence of considerable ring strain, which is relieved either on phosphorane formation following nucleophilic attack at phosphorus, or on dealkylation of dibenzophospholium salts. The implications of ring strain in dibenzophospholium salts are discussed in connection with the possible aromaticity of the dibenzophospholes.