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

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journal contribution
posted on 06.11.2015, 00:00 by William H. Saunders, Scott Gronert
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–Cβ 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.