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

Dichlorocyclopropanation of the title dihydrophosphole oxide (5) by CHCl3-N aOH/H2O under phase transfer catalysis (PTC) gave adduct 6A in a selectivit y of 80%. The use of sodium trichloroacetate as the precursor of dichloroca rbene 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 phenyldihydrophosphole (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 substitu ent. Semiempirical calculations on the P-aryl dihydrophosphinines (7 and 8) revealed a geometry and electron distribution that explains the unique NMR features observed.