The chemistry of some quaternary derivatives of the 10,11-dihydrodibenzo[b,f]phosphepin system. The kinetics of alkaline hydrolysis, and products of betaine collapse, and a comparison with other phosphonium salts

Abstract

Alkaline hydrolysis of 10,11-dihydro-5-methyl-5-phenyldibenzo[b,f]phosphepinium iodide (1; R = Ph) proceeds with cleavage of the exocyclic phenyl group at a rate fifty times faster than that observed for the ring-opening, hydrolysis of the 5,5-dimethyl analogue (1; R = Me). Comparison of the rate data and activation energies for hydrolysis of these seven-membered ring salts with those for the acyclic salts [PMePh3][I] and [PMe2Ph2][I] indicates the existence of a ‘seven-membered-ring effect’, which may be due to the preferential occupation by the ring of diequatorial positions in intermediate trigonal-bipyramidal phosphoranes. Hydrolysis of these seven-membered-ring salts proceeds much more slowly than those of related five-membered-ring salts, indicating that relief of ring strain on phosphorane formation is of little importance for the seven-membered-ring compounds. The betaine (9) derived from 10,11-dihydro-5-phenyldibenzo[b,f]phosphepin and styrene oxide collapses in refluxing ethanol to form styrene and the related cyclic phosphine oxide (5; R = Ph); no rearrangement products are formed, in contrast to the reactions of the corresponding betaine derived from triphenylphosphine.