Free carbocations

Alkyl halides and sulfonates are the most frequently used alkylating acceptor synthons. The carbonyl group is used as the classical a -synthon. O-Silylated hemithioacetals (T.H. Chan, 1976) and fomic acid orthoesters are examples for less common a -synthons. In most synthetic reactions carbon atoms with a partial positive charge (= positively polarized carbon) are involved. More reactive, "free carbocations as occurring in Friedel-Crafts type alkylations and acylations are of comparably limited synthetic value, because they tend to react non-selectively. 

These reactions do not appear to involve free carbocations, because they proceed effectively in nucleophilic solvents that would successfully compete with halide or similar anions for free carbocations. Also, rearrangements are unusual under these conditions, although they have been observed in special cases. 

Like the kinetic evidence, the stereochemical evidence for the SnI mechanism is less clear-cut than it is for the Sn2 mechanism. If there is a free carbocation, it is planar (p. 224), and the nucleophile should attack with equal facility from either side of the plane, resulting in complete racemization. Although many first-order substitutions do give complete racemization, many others do not. Typically there is 5-20% inversion, though in a few cases, a small amount of retention of configuration has been found. These and other results have led to the conclusion that in many SnI reactions at least some of the products are not formed from free carbocations but rather from ion pairs. According to this concept," SnI reactions proceed in this manner ... 

As with other SnI reactions, there is clear evidence that SnI reactions can involve ion pairs. If the intermediate attacked by the nucleophile is a completely free carbocation, then, say, should give the same mixture of alcohols when reacting with... 

No matter how produced, RN2 are usually too unstable to be isolable, reacting presumably by the SnI or Sn2 mechanism. Actually, the exact mechanisms are in doubt because the rate laws, stereochemistry, and products have proved difficult to interpret. If there are free carbocations, they should give the same ratio of substitution to elimination to rearrangements, and so on, as carbocations generated in other SnI reactions, but they often do not. Hot carbocations (unsolvated and/or chemically activated) that can hold their configuration have been postulated, as have ion pairs, in which OH (or OAc , etc., depending on how the diazonium ion is generated) is the coun-... 

For El eliminations, if there is a free carbocation (25), it is free to rotate, and no matter what the geometry of the original compound, the more stable situation is the one where the larger of the D-E pair is opposite the smaller of the A-B pair and the corresponding alkene should form. If the carbocation is not completely free, then to that extent, E2 type products are formed. Similar considerations apply in ElcB eliminations. ... 

Zeolites are the main catalyst in the petrochemical industry. The importance of these aluminosilicates is due to their capacity to promote many important reactions. By analogy with superacid media (1), carbocations are believed to be key intermediates in these reactions. However, simple carbocationic species are seldom observed on the zeolite surface as persistent intermediates within the time-scale of spectroscopic techniques. Indeed, only some conjugated cyclic carbocations were observed as long living species, but covalent intermediates, namely alkyl-aluminumsilyl oxonium ions (2) (scheme 1), where the organic moiety is bonded to the zeolite structure, are usually thermodynamically more stable than the free carbocations (3,4). 

Gas-phase intracomplex substitution in (R)-(- -)-l-arylethanol/CHs OH2 adducts. It is well established that bimolecular Sn2 reactions generally involve predominant inversion of configuration of the reaction center. Unimolecular SnI displacements instead proceed through the intermediacy of free carbocations and, therefore, usually lead to racemates. However, many alleged SnI solvolyses do not give fully racemized products. The enantiomer in excess often, but not always, corresponds to inversion. Furthermore, the stereochemical distribution of products may be highly sensitive to the solvolytic conditions.These observations have led to the concept of competing ° or mixed SNl-SN2 mechanisms. More recently, the existence itself of SnI reactions has been put into question. ... 

Contact- and solvent-separated ion pairs form whenever solvolysis proceeds to the free carbocation. However, these intermediates are generally only thought of as significant when their formation can be detected by experiment. We have focused on several different reactions of ion pairs that leave detectable signatures. 

The addition of water to a free carbocation intermediate of solvolysis can be distinguished from addition to an ion-pair intermediate by an examination of common ion inhibition of solvolysis. Common leaving group inhibition of solvolysis is observed when the leaving group ion (X ) acts, by mass action, to convert the free carbocation (R , Scheme 5A) to substrate (R-X). This results in a decrease in the steady-state concentration of R that leads directly to a decrease in the velocity of solvolysis. Some fraction of the solvolysis reaction products form by direct addition of solvent to the carbocation-anion pair intermediate. The external... 

The value of (feobsd caic) at a given concentration of bromide anion depends on the association constant for formation of the ion pair from free ions (Ai s = and on the relative reactivity of the ion-pair and free carbocation toward addition of solvent For example, if is small, then the concentration of the ion-pair... 

The value of 0.05 M from equation (7) is consistent with values of A as < 1 and ( /fes) < 1 for reactions in water. For example, = 0.3 gives (Jdjki ) = 0.17 for the relative rate constants for addition of solvent to the carbocation-anion pair and free carbocation. By comparison, the three-fold smaller rate constant for addition of water to an intramolecular trityl carbocation-sulfonate ion pair compared with addition to the analogous substituted trityl carbocation o-sulfonyl methyl ester has been used to estimate a value of (kjk ) = 0.33. " ... 

The rate constant for addition of solvent to the free carbocation, which is assumed to be equal to for addition of solvent to the ion pair [Ref. 32]. Estimated rate constant for diffusional separation of the ion pair [Ref. 20]. 

While providing structural details and rates of intramolecular rearrangements-fragmentations, stable ion studies do not address questions relating to the reactivity of a carbocation with a second reagent, either with a solvent such as water or with some added nucleophile (Nu). Shown in Scheme 1.4 is the kinetic scheme for a limiting SnI solvolysis, that is, for a solvolysis that proceeds by way of a free carbocation. As is true for any reaction in which a reactive intermediate is formed as a stationary-state species, studies of this system cannot provide absolute rate con-... 

The reaction is also stereospecific because different stereoisomers give stereochemically different products, e.g. CIS— racemic and transmeso. Because of this stereospecificity, the intermediate cannot be the free carbocation CH,CHBrCHCH,. The same carbocation would arise from either cis-or /rans-2-butene, and the product distribution from both reactants would be identical. 

For El eliminations, if there is a free carbocation (25), it is free to rotate, and no matter... 

To study the possible stabilizing effect of [3-silyl cations, Olah and co-workers334 prepared the 2- [(1 -trimethylsilyl)vinyl]-2-adamantyl cation 132 [Eq. (3.43)] as well as the parent silicon-free carbocation. In contrast to the above observations, NMR data [the (Cl ), (C2), and (C2 ) carbons are more deshielded in 132 than in the parent ion] showed that cation 132 is destabilized compared with the silicon-free analog. Furthermore, at — 100°C the C(l) and C(3) carbons were found to be equivalent, whereas in the parent ion they were nonequivalent. This indicates a rapid rotation about the C(l)-C(3) bond in 132, which can be rationalized by assuming the intermediacy of the [3-silyl-stabilized cation 133. The difference between cation 132 and those having [3-silyl-stabilization discussed above may be the orthogonal arrangement of the [3-C-Si bond and the p-orbital of the carbocation center. 

Mechanistic arguments based on product yields have been used to attempt to decide whether a free carbocation is the reaction intermediate in Sn 1 reactions. If a free carbocation is formed... 

A detailed and elegant study of the SnI solvolysis reactions of several substituted 1-phenylethyl tosylates in 50% aqueous TEE has enabled the rates of (1) separation of the carbocation-ion pair to the free carbocation, (2) internal return with the scrambling of oxygen isotopes in the leaving group, (3) racemization of the chiral substrate that formed the carbocation-ion pair, and (4) attack by solvent to be determined.122... 

The nucleophile can approach the resulting free carbocation equally well from either side, resulting in the formation of equal amounts of product with retained and inverted configuration (42% of each). The final result is 42% retention and 16% + 42% = 58% inversion. The reaction has occurred with racemization and some excess inversion. 

The mercurinium ion reacts with water in the same manner as the bromonium ion. The nucleophile attaches to the more highly substituted carbon from the side opposite the leaving mercury. Because a free carbocation is not involved in the mechanism, the reaction is not prone to reairangement. 

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