Carbocations lifetime

A. Williams, Concerted Organic and Bioorganic Mechanisms, CRC Press, New York, 2000. W. P. Jencks, How Does a Reaction Choose Its Mechanism , Chem. Soc. Rev. 1981,10, 345. J. P. Richard, Simple Relationships between Carbocation Lifetime and the Mechanism for Nucleophilic Substitution at Saturated Carbon, Adv. Carbocation Chem. 1989, 1, 122. T. W. Bentley and G. Llewellyn, Scales of Solvent Ionizing Power, Prog. Phys. Org. Chem. 1990, 17, 121. 

The reaction of azide ions with carbocations is the basis of the azide clock method for estimating carbocation lifetimes in hydroxylic solvents (lifetime = 1 lkiy where lq, is the first-order rate constant for attack of water on the carbocation) this is analogous to the radical clock technique discussed in Chapter 10. In the present case, a rate-product correlation is assumed for the very rapid competing product-forming steps of SN1 reactions (Scheme 2.24). Because the slow step of an SN1 reaction is formation of a carbocation, typical kinetic data do not provide information about this step. Furthermore, the rate constant for the reaction of azide ion with a carbocation (kaz) is assumed to be diffusion controlled (ca. 5 x 109 M 1 s 1). The rate constant for attack by water can then be obtained from the mole ratio of azide product/solvolysis product, and the molar concentrations of azide (Equation 2.18, equivalent to Equation 2.14) [48]. The reliability of the estimated lifetimes was later... 

J. R Richard, Simple Relationships Between Carbocation Lifetime and the Mechanism for Nucleophilic Substitution at Saturated Carbon, in Advances in Carbocation Chemistry (X. Creary, Ed.) 1989,1, JAI Press, Greenwich, CT. 

There are less data related to carbocation lifetimes as compared to radical lifetimes. Yet, some extensive studies by Mayr, Richards, and others have provided much insight into substituent effects on their lifetimes. In general, the lifetimes are extremely short in water. For example, Toteva found that the f-butyl carbocation has a lifetime of only lO" s in water. Hence, although we consider tertiary carbocations stable, they are clearly not persistent in this medium. Secondary carbocations are even more reactive toward addition of water, and many secondary derivatives undergo concerted hydrolysis in water that avoids formation of the carbocation reactive intermediate. The primary 4-methoxybenzyl carbocation inter-... 

For many secondary sulfonates, nucleophilic substitution seems to be best explained by a concerted mechanism with a high degree of carbocation character at the transition state. This has been described as an exploded transition state. Both the breaking and forming bonds are relatively weak so that the carbon has a substantial positive charge. However, the carbocation per se has no lifetime because bond breaking and fonnadon occur concurrently."... 

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... 

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

It is normally not possible to detect the carbocation intermediate of an SnI reaction directly, because its lifetime is very short. However, in the case of 3,4-dimethoxy-diphenylmethyl acetate (7) and certain other substrates in polar solvents it was possible to initiate the reaction photolytically, and under these conditions the UV spectra of the intermediate carbocations could be obtained, providing additional evidence for the SnI mechanism. 

A common feature of these intermediates is that they are of high energy, compared to structures with completely filled valence shells. Their lifetimes are usually very short. Bond formation involving carbocations, carbenes, and radicals often occurs with low activation energies. This is particularly true for addition reactions with alkenes and other systems having it bonds. These reactions replace a tt bond with a ct bond and are usually exothermic. 

Owing to the low barriers to bond formation, reactant conformation often plays a decisive role in the outcome of these reactions. Carbocations, carbene, and radicals frequently undergo very efficient intramolecular reactions that depend on the proximity of the reaction centers. Conversely, because of the short lifetimes of the intermediates, reactions through unfavorable conformations are unusual. Mechanistic analyses and synthetic designs that involve carbocations, carbenes, and radicals must pay particularly close attention to conformational factors. 

Solvent-dependent lifetimes and ion-pairing of these intermediates can be responsible for the observed variations in the stereo- and chemo-selectivity. Assuming that bromonium ions and carbocations are formed in discrete pathways, the influence of these factors can be readily understood. On the one hand, bridged ions react stereospecifically whatever the medium the... 

Moss and coworkers provided an early example of the way in which micellization can control the stereochemical course of a reaction. Deamination of chiral primary aliphatic amines in water proceeds with net inversion and extensive racemization, and the extent of racemization depends upon the lifetime of the carbocation-like intermediate. The situation changes dramatically if the salts of the primary amine can self-micellize, because now the nucleophile, typically water, is directed in from the front-side so that there is extensive retention of configuration (Moss et al., 1973). 

The generation of a-ferrocenyl-P-silyl substituted vinyl cations of type 28 does not require superacidic conditions, they can be generated by protonation of l-ferrocenyl-2-trialkylsilyl alkynes with trifluoroacetic acid at room temperature. The SiR3-groups with larger alkyl substituents increase the lifetime of this type of carbocations. 

A few nucleophiles either did not give any spin adduct with PBN or directly gave the benzoyl nitrone [9], Bromide ion did not give any spin adduct, explicable by the very short lifetime of Br-PBN (Rehorek and Janzen, 1984) and trifluoroacetate, nitrate, phenylsulfinate and chloride ion produced [9]. This can either be explained by the rapid further oxidation of the spin adduct formed [similar to reaction (29) see Table 4] or a rapid solvolysis reaction of the latter (Scheme 2), forming [9] by reaction of the intermediate carbocation... 

Fig. 1 for stepwise solvolysis of R-X is due to the increase in ks (s ), with decreasing stability of the carbocation intermediate, relative to the constant value of az (M s ) for the diffusion-limited addition of azide anion. The lifetime for the carbocation intermediate eventually becomes so short that essentially no azide ion adduct forms by diffusion-controlled trapping, because addition of solvent to R occurs faster than escape of the carbocation from the solvent cage followed by addition of azide ion (k Now, the nucleophile adduct must form through a... 

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