Intermediates with long lifetimes

The addition of strong nucleophile to reactive electrophile gives a relatively stable anionic addition intermediate with a lifetime long enough to diffuse through the solution and abstract a proton before it reverts to reactants (k > k i) (Scheme 2.2). 

But either tetrahedral intermediate could lose water instead. In one case (top line below) the original starting material is regenerated complete with label. But in the second case, labelled water is lost and unlabelled starting material is formed. This result would be difficult to explain without a tetrahedral intermediate with a lifetime long enough to allow for proton exchange. This addition-elimination mechanism is now universally accepted. 

Jencks has discussed how the gradation from the 8fjl to the 8n2 mechanism is related to the stability and lifetime of the carbocation intermediate, as illustrated in Fig. 5.6. In the 8n 1 mechanism, the carbocation intermediate has a relatively long lifetime and is equilibrated with solvent prior to capture by a nucleophile. The reaction is clearly a stepwise one, and the energy minimxun in which the caibocation mtermediate resides is significant. As the stability of the carbocation decreases, its lifetime becomes shorter. The barrier to capture by a nucleophile becomes less and eventually disappears. This is described as the imcoupled mechanism. Ionization proceeds without nucleophilic... 

Transglycosidation with retention of configuration (Chipman and Sharon, 1969) would be more difficult to explain if an open-chain carbonium ion were formed in lysozyme reactions, necessitating an equilibrium reaction with free aldehyde. It seems unlikely, however, that a cyclic carbonium ion intermediate could have a sufficiently long lifetime to react with a saccharide molecule that can bind to the enzyme only after displacement of the leaving group in a fairly aqueous environment (above discussion). Therefore, the concept of a cyclic carbonium ion also presents difficulties for interpretation and should not be accepted uncritically. 

The relatively long timescales of the ionization, isolation, thermalization, reaction, and detection sequences associated with low-pressure FTICR experiments are generally thought to preclude the use of this technique as a means of examining the unimolecular dissociation of conventional metastable ions occurring on the microsecond to millisecond timescale. Nonetheless, as just demonstrated (Section IIIC), intermediates with this order of magnitude of lifetime are routinely formed in the bimolecular reactions of gaseous ions with neutral molecules at low pressures in the FTICR cell, as in Equation (13). 

Two chapters in this volume describe the generation of carbocations and the characterization of their structure and reactivity in strikingly different milieu. The study of the reactions in water of persistent carbocations generated from aromatic and heteroaromatic compounds has long provided useful models for the reactions of DNA with reactive electrophiles. The chapter by Laali and Borosky on the formation of stable carbocations and onium ions in water describes correlations between structure-reactivity relationships, obtained from wholly chemical studies on these carbocations, and the carcinogenic potency of these carbocations. The landmark studies to characterize reactive carbocations under stable superacidic conditions led to the award of the 1994 Nobel Prize in Chemistry to George Olah. The chapter by Reddy and Prakash describes the creative extension of this earlier work to the study of extremely unstable carbodications under conditions where they show long lifetimes. The chapter provides a lucid description of modern experimental methods to characterize these unusual reactive intermediates and of ab initio calculations to model the results of experimental work. 

The stereochemical results are consistent with a 1,4-diradical intermediate such as 63 with a lifetime sufficiently long to permit loss of stereochemistry by rotation for the cycloaddition reaction. At most, however, the same diradical can be an intermediate in only a portion of the ene reaction. 

A transient intermediate has been observed which decays with k = 12.6 s-1 in a first-order process. In view of its long lifetime the transient is supposed to be the cyclohexadienyl radical. A similar reaction mechanism had earlier been proposed for the photochemical cyclization of 5-(2-chlorophenyl)-1,3-diphenyl-1,2,4[1 //]-triazole561. Intramolecular aryl-aryl coupling has also been described for A2-l,2,4-triazol-5(l//)-one derivatives containing an 0-bromophenyl and a heteroaryl substituent in vicinal positions562. 

Living polymerization—A polymerization in which the reactive intermediates have very long lifetimes. An example is an anionic polymerization with no termination steps. 

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