ElcB mechanism irreversible

The carbanion can be destroyed in two ways, k2 or k, and two limiting types of behavior are expected for ElcB mechanisms. If k2 3> fe-i[BH], then the carbanion always decomposes to the alkene product and the rate law simplifies to feobs = fei. In other words, the rate is only dependent on carbanion formation and the rate law has a form that is identical to what would be obtained in a concerted E2 reaction. This has been referred to as an EIcBirr mechanism, where IRR indicates that deprotonation is irreversible. On the other hand, if k2 fc i[BH], then carbanion formation rarely leads to the product and the rate law simplifies to the following ... 

The pH-rate profile for unbuffered hydrolysis of glyceraldehyde-3-phosphate (6-3-P) has been attributed to hydrolysis of the monoanion of the phosphate monoester at pH < 4, spontaneous formation of glyceraldehyde from the phosphate dianion at pH 7-8, and, at higher pH, hydroxide-catalysed methylglyoxal formation. Reaction of the dianion is not subject to a solvent isotope effect and is believed to occur by the irreversible ElcB mechanism whereby an enediolate intermediate, formed on rate-determining C(2) deprotonation, subsequently expels phosphate trianion by C—0 bond breaking. The diethylacetal and 2-methyl-G-3-P do not hydrolyse under the same conditions.5... 

Values of k/k QL) = 1.045,1.044 and 1.049 0.011 and values of k/k = 1.051 0.004,1.036 0.005 and 1.036 0.004 have been obtained for R = / -MeO, H and p-C 1 compounds. They indicate that large changes in bonding both at C(a) and C(/J) take place in the rate-determining steps of these reactions consistent with an Elcb-like E2 mechanism. The observed relatively large C(a) isotope effects are inconsistent with an irreversible Elcb mechanism. The E2 transition state for 388, R = / -MeO is somewhat Elcb-like. As R becomes a better EWG, the transition state becomes more reactant-like (less double-bond character) and more carbanion-like. 

The reaction is an elimination process that has been interpreted through an ElcB mechanism, as described in Scheme 17.2. The only question left is to determine whether the first step (A i, removal of the proton) or the second step k2, elimination of the leaving group) is rate determining. In other words, to distinguish between an (ElcB)jir(ElcB irreversible) and an (E1cB)r (ElcB reversible) mechanism. 

Saunders (1997) used quantum-chemical methods in the most recent attempt to establish the mechanistic importance of negative ion hyperconjugation. Earlier, Saunders (1976) had proposed that one could distinguish between an E2 and an irreversible Elcb(irr) mechanism (Lowry and Richardson, 1987) by measuring the leaving group KIE for the reaction. The... 

The distinction between the irreversible Elcb (equation 258) and E2573 concerted mechanism of equation 259 was possible due to C(flt) and C(y9) carbon-14 K.I.E. determinations carried out for the Me07Me0H promoted dehydrochlorination of 388 to 392574. 

In general, it is impossible to distinguish unequivocally an irreversible Elcb reaction (no reprotonation or deuterium incorporation) from an Elcb-like E2 process. Conclusions can be based on indirect evidence at best. This ambiguity may be circumvented in the organometallic area. Treatment of 2-ethyl-5,6-dihydro-4-methyl-2i/-pyran with LIDA in the presence of catalytic amoimts of potassium tert-butoxide affords, after neutralization, exclusively (Z)-5-methyl-4,6-heptadien-l-ol and of its 3,6-dihydro isomer exclusively (3Z,5E)-4-methyl-3,5-heptadien-l-ol, in both cases, the products required by an E2 mechanism. If instead deprotonation of an allylic methylene would lead to a transient common intermediate 246, the ensuing irreversible Elcb reaction had to furnish both isomeric lithium dienolates in identical proportions (Scheme 1-190). ... 

The elimination of HF from 10.28 is thought to proceed by a conventional E2 mechanism. 10.28 may be reacted with methyl iodide to give 10.29. Elimination of HF from 10.29 is thought to proceed by an ElcB reaction, in which anion formation is irreversible. Draw mechanisms for the processes and explain why the reactions proceed by different... 

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