The carbanion mechanism of olefin-forming elimination

Abstract

In some cases it has been found that trans elimination from a vinylic halide proceeds by a concerted mechanism whereas the analogous cis elimination is carbanionic (and slower) (See S.

Identification of the Secondary Degradates of L-648,548 in an Animal Health Formulation

“…E1cb elimination usually occurs when a compound has a poor leaving group and a highly acidic proton. 13 With 5-oxo-L-648,548 (3), the elimination can take place by loss of the C2 proton followed by departure of the C7 hydroxyl. Under basic conditions, the deprotonation at C2 takes place to form the carbanion 7 which is stabilized by the adjacent C1 lactone and unsaturated ketone.…”

Isolation and Decomposition of the Intermediate of an ElcB Elimination

“…ElcB reactions are two-step 1,2-eliminations; in the first step the conjugate base (2) is formed from the substrate (1); in the second step (2) decomposes by wayiof a first order reaction to give the elimination product (3) and X e [I].…”

“…Substituents X capable of increasing the basicity at the P atom and substituents Y capable of increasing the acidity at the N atom should favor the bonding of the proton to the phosphorus i.e. favor structure (21; on the other hand, however, they should also weaken the phosphazene bond and the stability of (2). A compound in which X = NR2 and Y = SOzR would represent a typical case in this connection.…”

Kinetics of Proton Transfer Reactions of Carbon Acids. II. Reaction of Di-(4-nitrophenyl)methane with Alkoxide Bases

“…Thus, the E2 reaction may have a symmetrical transition state with equal amounts of pcarbon-hydrogen and ci-carbon-halogen bond breaking, or the transition state may be unsymmetrical, either El-like with more carbon-halogen bond breaking or ElcB-like with more carbon-hydrogen bond breaking. Such different transition states have been postulated several times (12)(13)(14)(15) and correlated with kinetic isotope effects (1 5-1 7).…”