Chlorocyclopropenes, Chloropropenyl Cations, and Cyclopropenon1a

Treatment of tetrachlorocyclopropene with tri-n-butyltin hydride in hydrocarbon solvents effects successive removal of chlorines, leading to 1,2,3- and 1,3,3-trichlorocyclopropenes II and III, 1,3- and 1,1-dichloro-cyclopropenes IV and VI, 3-chlorocyclopropene VII, and cyclopropene. Appropriate conditions were devised for the preparation of a II, III equilibrating mixture, a IV, VI equilibrating mixture, and VII. In ether solvents, or with ether or tin oxide impurities, the product distribution changes and 1,2-dichlorocyclopropene V and 1-chlorocyclopropene VIII are formed. Nmr studies show that in the II, III and IV, VI equilibria the isomer with fewer vinyl chlorines is preferred under some solvent and temperature conditions, in contrast to the situation in other systems. Further, IV equilibrates with its enantiomer and/or congruent isomer more rapidly than with its isomer VI. The nmr spectra of III, IV, VIII, IX, and X show a shielding, not deshielding, effect by chlorine, which is explained using the McConnell equation. 13C coupling constants and infrared spectra of the chlorocyclopropenes are in line with those of other cyclopropene derivatives. With Lewis acids these chlorocyclopropenes have been converted to dichlorocyclopropenyl cation IX and monochlorocyclopropenyl cation X. The IV, VI mixture hydrolyzes to cyclopropenone; the d₂ and 18O derivatives have also been prepared and examined spectroscopically. The II, III mixture hydrolyzes with ring opening to dichloroacrolein. Diels-Alder additions of the chlorocyclopropenes to a number of dienes have been examined. The products involve exo addition, except for VII which goes both exo and endo.