Negative Ion Hyperconjugation in Fluorocarbanions and the Nature of the Borderline between E1cB and E2 Mechanisms. An ab Initio Study.

Ab initio calculations at the MP2/6-31+G//MP2/6-31+G level on the 2,2-difluoroethyl (7), 1,2,2-trifluoroethyl (8), 1,1,2-trifluoroethyl (9), 2,2,2-trifluoroethyl (10), 1,1,2,2-tetrafluoroethyl (11), 1,2,2,2-tetrafluoroethyl (12), and 1,1,2,2,2-pentafluoroethyl (13) carbanions show that the bond to the anti-beta-fluorine is significantly weakened with respect to both the C-F bonds of the conjugate acids and the gauche-beta- fluorine(s) of the carbanion. The extent of bond weakening decreases with increasing fluorination of the carbanion, but even 13 shows some evidence of weakening. The criteria are the carbon-fluorine bond length, the charge on fluorine, and the (18)F/(19)F equilibrium isotope effect (EIE). The TS for proton abstraction by hydroxide ion from 1,1,1-trifluoroethane (3) to give 10 hydrated at the carbanion center shows bonding changes at all centers of a sort normally associated with E2 TSs, judging from isotope effects, bond lengths, and charges. The carbon-hydrated 10 collapses to elimination products only as the water molecule is migrated from carbon to fluorine. Calculations on the E2 reaction of hydroxide ion with ethyl fluoride result in an E1cB-like E2 TS very similar to that for the proton-transfer step in the E1cB reaction of 3. It is suggested that E1cB reactions can show a spectrum of TSs from carbanion-like to E2-like as the stability of the carbanion decreases.