E1cB and E2cB mechanisms in the elimination of trifluoroethoxide ion from α,α-dinitro-β,β-diphenyl-β-(trifluoroethoxy)ethanide anion and the non-reactivity of the β-cyano-α,α-dinitro-β,β-diphenylethanide anion

Abstract

The reaction of α,α-dinitro-β,β-diphenyl-β-(trifluoroethoxy)ethane (5) with the bases (B) pyridine and collidine in acetonitrile gives the corresponding free ammonium ion BH⁺ and carbanion (6a). The equilibrium constants (K_diss) for the reaction (5)+ B ⇌(6a)+ BH⁺ are 0·05 for collidine and 0·00027 for pyridine at 40°. The elimination of CF₃CH₂O^– from (6a)(k_e) to give 1,1-dinitro-2,2-diphenylethylene (3a) is first order in (6a) and in BH⁺, and a rate-determining BH⁺-assisted expulsion of CF₃CH₂O^–(E2cB mechanism) is suggested. This is verified by the relative reactivities of the BH⁺, the reaction in the presence of oxalic acid, and the isotope effects of 1·27 with pyridine and 1·54 with collidine. With Et₃N the elimination is first order in (6a) and unassisted by BH⁺; the isotope effect is 1·07. Addition of Et₃N to 3,3-dinitro-2,2-diphenylpropionitrile (4a) and its bis-(p-methoxy)-derivative (4b) gives immediately the corresponding carbanions (6b and c). These carbanions do not eliminate CN^–, and essentially do not exchange their cyano-group with added cyanide ion, thus representing an extreme case of E1cB reactions where the anion which is quantitatively formed is too stable to undergo elimination. The effects of the α-activating groups and the leaving group on the rate of the C–X bond cleavage are discussed.