Carbocations isomerization

An ambident anion is one with different kinds of atoms which may become attached to a carbocation. Isomerization of the anionic portion permits detection of ion pair return in close analogy to the equilibration of 180 labeled benzoate or sulfonate esters. The isomerization of thiocyanates (112) to isothiocyanates (113) is the most studied of these ambident rearrangements 5, "6 In some respects these reactions differ from the return processes of carboxylate or sulfonate ion pairs ... 

In the field of cationic polymerization, on the other hand, a number of examples are known in which a propagating carbocation isomerizes (rearranges) into an energitically more stable carbocation that in turn propagates This isomerization polymerization should be clearly distinguished from the monomer-isomerization polymerization that involves isomerization of the starting monomer, not the propagating species. 

The isomerization of butane to iso butane in superacids is illustrative of a protolytic isomerization, where no intermediate olefins are present in equilibrium with carbocations. 

It must be emphasized that we are not dealing with an equilibrium between two isomeric carbocations There is only one carbocation Its structure is not adequately represented by either of the individual resonance forms but is a hybrid having qualities of both of them The carbocation has more of the character of A than B because resonance struc ture A IS more stable than B Water attacks faster at the tertiary carbon because it bears a greater share of the positive charge... 

These reactions involve the intermediate formation of thiols, followed by condensation to the sulfides. The observation of isomerized products in suitable cases indicates the intermediate formation of carbocations, either by protolysis of alkanes by the superacid or reversible ionisation of the thiol products (149). 

Carbocations generated from alkanes using superacids react with carbon monoxide under mild conditions to form carboxyUc acid (188). In this process isomeric carboxyUc acids are produced as a mixture. However, when the reaction is mn with catalytic amounts of bromine (0.3 mmol eq) in HF-SbF solution, regio-selective carboxylation is obtained. / -Propane was converted almost exclusively to isobutyric acid under these conditions. 

Molecular orbital calculations predict that oxirane forms the cyclic conjugate acid (39), which is 30 kJ moF stabler than the open carbocation (40) and must surmount a barrier of 105kJmoF to isomerize to (40) (78MI50500). The proton affinity of oxirane was calculated (78JA1398) to be 807 kJ mol (cf. the experimental values of 773 kJ moF for oxirane and 777-823 kJ moF for dimethyl ether (80MI50503)). The basicity of cyclic ethers is discussed in (B-67MI50504). 

An interpretation of activation parameters has led to the conclusion that the bromination transition state resembles a three-membered ring, even in the case of alkenes that eventually react via open carbocation intermediates. It was foimd that for cis trans pairs of alkenes tiie difference in enthalpy at the transition state for bromination was greater than the enthalpy difference for the isomeric alkenes, as shown in Fig. 6.2. This... 

Free radicals, unlike carbocations, do not normally undergo isomerization by methyl or hydrogen migration. However, hydrogen transfer (chain transfer) occurs when a free radical reacts with other hydrocarbons. 

Isomerization reactions occur frequently in catalytic cracking, and infrequently in thermal cracking. In both, breaking of a bond is via beta-scission. However, in catalytic cracking, carbocations tend to rearrange to form tertiary ions. Tertiary ions are more stable than secondary and primary ions they shift around and crack to produce branched molecules (Equation 4-10). (In thermal cracking, free radicals yield normal or straight chain compounds.)... 

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

The substitution reaction of toluene with Br2 can, in principle, lead to the formation of three isomeric bromotoluene products. In practice, however, only o- and jp-bromotoluene are formed in substantial amounts. The meta isomer is not formed. Draw the structures of the three possible carbocation intermediates (Problem 15.48), and explain why ortho and para products predominate over meta. 

The isomerization of isopentenyl diphosphate to dimethylally diphos phate is catalyzed by JPP isomerase and occurs through a carbocation pathway Protonation of the IPP double bond by a hydrogen-bonded cysteine residue ir the enzyme gives a tertiary carbocation intermediate, which is deprotonated b a glutamate residue as base to yield DMAPP. X-ray structural studies on the enzyme show that it holds the substrate in an unusually deep, well-protectec pocket to shield the highly reactive carbocation from reaction with solvent 01 other external substances. 

The rearrangement of the intermediate alkyl cation by hydrogen or methyl shift and the cyclization to a cyclopropane by a CH-insertion has been studied by deuterium labelling [298]. The electrolysis of cyclopropylacetic acid, allylacetic acid or cyclo-butanecarboxylic acid leads to mixtures of cyclopropylcarbinyl-, cyclobutyl- and butenylacetamides [299]. The results are interpreted in terms of a rapid isomerization of the carbocation as long as it is adsorbed at the electrode, whilst isomerization is inhibited by desorption, which is followed by fast solvolysis. 

As in the case of the base-catalyzed reaction, the thermodynamically most stable alkene is the one predominantly formed. However, the acid-catalyzed reaction is much less synthetically useful because carbocations give rise to many side products. If the substrate has several possible locations for a double bond, mixtures of all possible isomers are usually obtained. Isomerization of 1-decene, for example, gives a mixture that contains not only 1-decene and cis- and franj-2-decene but also the cis and trans isomers of 3-, 4-, and 5-decene as well as branched alkenes resulting from rearrangement of carbocations. It is true that the most stable alkenes predominate, but many of them have stabilities that are close together. Acid-catalyzed migration of triple bonds (with allene intermediates) can be accomplished if very strong acids (e.g., HF—PF5) are used. If the mechanism is the same as that for double bonds, vinyl cations are intermediates. 

Figure 1. Isomerization selectivity on Pt/WOx-Zr02 (0.3% wt. Pt, 12.7% wt. W, H-atoms formed from H2 or adamantane on Pt, 073 calcination) and Pt/SOx-ZrOj sites are involved in carbocation hydrogen (0.4% wt. Pt, 4.5% wt. S, 723 K...

With the renaissance in alkene chemistry engendered by the rising versatility of olefin metathesis in both fine chemical and commodity production, new methods for alkene isomerization are of increasing interest and importance. Alkene isomerization can be performed using Bronsted-Lowry acid or base catalysis (1). However, these reactions are limited to substrates which tolerate carbanionic or carbocation intermediates, and are susceptible to undesired side reactions. 

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