The Competition between Elimination Pathways in the Reactions of a Wide Variety of Bases with 2-Fluoro- and 2-Chlorobutane in the Gas Phase.

Ab initio methods are used to examine the regio- and stereoselectivities of the elimination reactions of 2-fluorobutane and 2-chlorobutane with a series of nucleophiles (F(-), HO(-), CH3O(-), (CH3)3CO(-), NH2(-), CH3(-), H(-), Cl(-), HS(-), and PH2(-)). The data suggest that regiochemistry is most closely related to the nature of the transition state on the E2 spectrum with E1cb-like reactions favoring the least-substituted alkene product and E1-like reactions favoring the most-substituted alkene product. There appears to be no correlation between the extent of π-bond formation (as measured by the Cα-Cß distance) and the preference for forming the more highly substituted alkene. The stereochemistry (E vs Z) is less sensitive to the nucleophile and is relatively constant with the exception of a few systems that appear to have long-range interactions that reduce the bias against the Z product. Comparisons with experimental results in solution show, with a few exceptions, similar reactivity trends in solution and the gas phase.

VBSCF calculations on the bimolecular (E2) elimination reaction. The nature of the transition state.

Valence bond calculations utilizing the Xiamen package have been carried out on the bimolecular (E2) elimination reaction X(-) + HCH(2)CH(2)Y --> XH + CH(2)=CH(2) + Y(-) where X,Y = F,F; F,Cl; Cl,F; Cl,Cl for anti and syn reactant complexes, transition states, and product complexes. The calculations were supplemented by MO-based calculations at MP2/6-311++G**//MP2/6-311++G**. The valence bond calculations give reasonable energies with eight contributors to the resonance hybrid. Charge-localized contributors dominate the transition states. NPA charges from the MO calculations confirm that the transition states possess a significant degree of localized charge and can be described by the key resonance structure X(-)-H(+)-CH(2)(-)-CH(2)(+)-Y(-). At the same time, the MO calculations show that electronically and geometrically the reactions are clearly concerted though not synchronous. Valence bond state correlation diagrams (VBSCD) show that a simple proton transfer such as that in the E1cB irreversible reaction is predicted to have a lower barrier than a synchronous concerted (E2) reaction. The E2 transition state evidently avoids this energetic disadvantage by becoming localized and nonsynchronous, though with important electronic and geometric changes at all of the reacting centers.

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.