The effect of a sterically demanding P-substituent on the reactivity of P-heterocycles: selective transformations during the ring enlargement of a 1-(2,4,6-triisopropylphenyl)-2,5-dihydro-1H-phosphole 1-oxide†

Abstract

Dichlorocyclopropanation of the title dihydrophosphole oxide (5) by CHCl₃–NaOH/H₂O under phase transfer catalysis (PTC) gave adduct 6A in a selectivity of 80%. The use of sodium trichloroacetate as the precursor of dichlorocarbene resulted in, however, the exclusive formation of the other isomer (6B) exhibiting the same stereostructure, as the product formed in the liquid–liquid two-phase dichlorocyclopropanation of the phenyl-dihydrophosphole (1). The base- and thermo-induced cyclopropane ring opening of the adducts (6A and 6B) led, surprisingly, to distinct dihydrophosphinine isomers (7 and 8, respectively) in a fully selective manner. The unusual reactivity of the P-heterocycles (5 and 6) is due to the sterically demanding P-aryl substituent. Semiempirical calculations on the P-aryl dihydrophosphinines (7 and 8) revealed a geometry and electron distribution that explains the unique NMR features observed.