The α-effect in elimination reactions and competing mechanisms in the gas phase

Abstract

Our recent studies of the α-effect (enhanced reactivity of nucleophiles with lone pair electrons adjacent to the attacking group) have shown increased nucleophilicity for HOO− in a series of bimolecular substitution (SN2) reactions in the gas phase supporting an intrinsic origin of the effect. Our current work continues to explore the intrinsic behavior of α-nucleophiles by examining other reaction systems and mechanisms. We evaluate intrinsic reactivity differences in the elimination (E2) reactions of a series of anions with tert-butyl chloride by a Brønsted-type correlation. Additional analysis in the E2 reaction with tert-butyl chloride and the SN2 reaction with methyl chloride is accomplished by reagent pairing of normal and α-nucleophiles of nearly identical proton affinities [FCH2CH2O−, C6H5CH2O−, (CH3)3COO−]. Finally, the relationship between product distributions and the magnitude of the α-effect in the competing reactions of dimethyl methylphosphonate (DMMP) and methyl formate is assessed by reagent pairing (HOO− versus HO−, CH3O−, C2H5O−, and (CH3)2CHO−). While our results do not indicate significant differences between the α-anions and the normal anions for the E2 mechanism, enhanced nucleophilicity is observed for both (CH3)3COO− and HOO−. Unlike condensed phase studies, the magnitude of the α-effect in the methyl formate reactions at the sp3 carbon (nucleophilic substitution, SN2) and the sp2 carbon (addition-elimination at the carbonyl center, BAC2) are nearly identical. In the context of hard and soft acids and bases, the behavior of the α-nucleophiles can be rationalized by “soft” base behavior.