Solvent-Promoted E2 Reaction Competing with SN2 Reaction and Stepwise Solvolytic Elimination and Substitution Reactions

Competing base-promoted E2 and E1 reactions of an acidic tertiary substrateElectronic supplementary information (ESI) available: the effect of base concentration on rate constant and product composition. See http://www.rsc.org/suppdata/p2/b2/b200758d/

“…The solvolysis of 4-chloro-4-(4Ј-nitrophenyl)pentan-2-one (1-Cl) or 4-bromo-4-(4Ј-nitrophenyl)pentan-2-one (1-Br) in aqueous acetonitrile yields the alcohol 1-OH and the elimination products 4-(4Ј-nitrophenyl)-2-oxopent-4-ene (2), (E )-4-(4Ј-nitrophenyl)-2-oxopent-3-ene (3), along with traces of (Z )-4-(4Ј-nitrophenyl)-2-oxopent-3-ene (4). Reaction with the acetonitrile component of the solvent gives rise to a small amount of the substitution product 1-NHCOMe (Scheme 2).…”

Mechanisms of Solvent- and Base-Promoted Imine-Forming Elimination Reactions

“…The base-promoted alkene-forming dehydrochlorination of 9-(1-chloroethyl)fluorene has been concluded to be of E2 type. ,, A nitrogen−chlorine bond is weaker than a bond between carbon and chlorine. Thus, 1-Cl should undergo a facile cleavage of the nitrogen−chlorine bond, and we suggest that this reaction also occurs by an E2 mechanism.…”

“…We are interested in the borderline between stepwise and one-step alkene-forming elimination reactions and have discussed this subject in a number of recent papers. − Such questions as what can induce a change in mechanism from a stepwise route involving an intermediate of carbocationic or carbanionic type to a one-step, concerted process have been addressed.…”

“…In the corresponding alkene-forming elimination reactions of fluorenyl-activated substrates, depending on the substrate structure and the leaving group, E1cB R , E1cB I , and E2 reactions are possible in addition to E1 elimination through an ion-pair intermediate. Moreover, the first conclusive evidence for the solvent-promoted E2 elimination mechanism was reported recently for secondary halides in a highly aqueous solvent. − What are the possible mechanisms of imine-forming elimination reactions? A review by Hofmann, Bartsch, and Cho discusses one-step, concerted base-promoted imine-forming reactions, but there seems to be no report on stepwise carbanionic imine-forming reactions in the literature.…”

Competing solvolytic elimination and substitution reactions via very short-lived ion-pair intermediates

“…The solvent-promoted E2 reaction is significant for the closely related, more slowly ionizing, secondary substrates, 9-(1-X-ethyl)fluorene, which have an acidic hydrogen in the 9-position of the fluorene moiety. 16,38 The incomplete consistency of the measured isotope effects with the common ion-pair mechanisms of Scheme 3 suggests that the ion pair is very unstable and does not have time to choose between dehydronation and addition of a solvent water molecule. This supports the conclusion that the preassociation route is significant for the elimination reaction.…”