The dehydrochlorination of 1,1-diaryl-2,2,2-trichloroethanes in protic and dipolar aprotic solvents

Abstract

PART I A general introduction is given to the mechanisms of olefin-forming β-elimination. The development of mechanistic criteria and their application to the various reactions is discussed. Conflicting theories concerning the nature of the transition states of E2H reactions in protic solvents as well as the Winstein-Parker E2C-like transitions states for elimination by weak bases in dipolar aprotic solvents are described. PART II The use of primary deuterium isotope effects: structure-reactivity relationships, and rate-acidity correlations as mechanistic criteria for the E2 mechanism and the variants of the E1cB mechanism is reviewed. These criteria have been applied to the dehydrochlorination of the 1,1-diaryl-2, 2, 2-trichloroethanes (DDT-type compounds) with methoxide-methanol and t-butoxide-t-butanol. Literature comparisons indicate that the kinetic evidence for these two systems is in accord with an "irreversible" E1cB elimination pathway. PART III Characteristics of weak base-promoted eliminations in dipolar aprotic advents are reviewed. The evidence is not entirely consistent with either E2C or the E2H mechanism. Deuterium isotope effects and equilibrium constants are reported for the chloride ion-promoted elimination of the 1, 1-diary1-2,2,2-trichloroethanes in dimethylformamide and acetone respectively. The results are in accord with an E2H mechanism.