Proton-Transfer Reactions. 1. Partitioning of Carbanion Intermediates Generated by Reactions of Alkenes with Alkoxide Ions in Alcohol

Article abstract of DOI:10.1021/ja00408a023

Nucleophilic reactions with sodium alkoxide in alcohol have been studied with various gem-difluoroalkenes of general structure C6H5CR=CF2.Rates and Arrhenius paramaters 3 (M ^-1 s^-1) at -50 deg C, ΔH^* (kcal mol^-1), and ΔS^*, (eu)> are respectively: R = -CF3 (II), 105, 9, and -23; -CF2Cl (III), 135, 10 and -19; -CF2CF3 (IV), 40.6, 9, and -23; -CF2H (V), 3.63, 11, and -19.Reactions proceed via carbanion intermediates, and the products from reaction with ca. 0.3 N sodium ethoxide in ethanol are: II, 85percent vinyl ether and 15percent saturated ether; III, 100percent allylic ether; IV; 74percent vinyl ether, 22percent allylic ether, and 4percent saturated ether; V, >98percent allylic ether.The observed product distribution suggest the following order of leaving group ability for fluoride in different environments: -CF2H >> -CF2OR > -CF2CF3 >> -CF3.Solvent protonation of the carbanion is apparently slower than fluoride ion ejection from all groups studied other than trifluoromethyl.Product isotope effects (PIE), k^H/k^D, for the protonation of the carbanion generated from II by reaction in ethanol are 1.50 (-78 deg C) and 1.86 (20 deg C) and by reaction in methanol are 1.22 (-78 deg C).