Carbocations cyclic

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Such a carbocation however has been demonstrated to be less stable than an alterna tive structure called a cyclic bromonium ion, m which the positive charge resides on bromine not carbon... 

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). 

Measurement of (R /R ) can be accomplished by cyclic voltammetry for relatively Stable species and by other methods for less stable cations. The values obtained for AG -range from 83kcal/mol for the aromatic tropylium ion to 130kcal/mol for destabilized betizylic cations. For stable carbocations, the results obtained by this method correlate with cation stabiUty as measured by pKf.+. Some of these data are presented in Table 5.3. 

The stereochemistry of both chlorination and bromination of several cyclic and acyclic dienes has been determined. The results show that bromination is often stereo-specifically anti for the 1,2-addition process, whereas syn addition is preferred for 1,4-addition. Comparable results for chlorination show much less stereospeciftcity. It appears that chlorination proceeds primarily through ion-pair intermediates, whereas in bromina-hon a stereospecific anfi-l,2-addition may compete with a process involving a carbocation mtermediate. The latter can presumably give syn or anti product. 

When this sequence is applied to carbohydrates, the first step takes place intramoleai-larly to yield a cyclic hemiacetal. The second step is intermolecular, requires an alcohol R"OH as a reactant, and proceeds readily only in the presence of an acid catalyst. An oxygen-stabilized carbocation is an intermediate. 

The propagating species involved in the polymerization of cyclic formal seem to resemble carbocations, and random copolymers are formed in the copolymerization of cyclic formals with styrene. For the copolymerization of DOL with styrene, the DOL-St cross-sequence was estimated, by NMR or by chemical methods, from the decrease of the formal unit in the copolymer and the formation of nearly random copolymer was confirmed132. ... 

The existence of open carbocation intermediates is supported by the contrast in the pattern of alkyl substituent effects with that found in brominations, where cyclic intermediates are involved. In the latter case substitution of alkyl groups on H2C=CH2 causes a cumulative rate acceleration until all four... 

The mechanism of these reactions is often El. However, in at least some cases, an E2 mechanism operates.It has been shown that stereoisomers of cyclic y-amino halides and tosylates in which the two leaving groups can assume an anti-periplanar conformation react by the E2 mechanism, while those isomers in which the groups cannot assume such a conformation either fragment by the El mechanism or do not undergo fragmentation at all, but in either case give rise to side products characteristic of carbocations. " ... 

There is evidence that the configuration of the molecule may be important even where the leaving group is gone long before migration takes place. For example, the 1-adamantyl cation (17) does not equilibrate intramolecularly, even at temperatures up to 130°C, though open-chain (e.g., 5 50 and cyclic tertiary carbocations... 

Carbocation-carbanion zwitterionic intermediates were proposed for the thermal cleavage of several cyclic compounds. In most of these reactions the ionically dissociating bond belongs to one of four strained ring systems, i.e. cyclopropane (13), cyclobutane (14), cyclobutene (15) or norbornadiene (16). The mechanism is distinguished from the formation of a diradical intermediate through homolysis in terms of solvent and substituent effects... 

Fig. 8 Reactions of various carbocations with Kuhn s anion [2 ] as compared with their reduction potentials (peak potentials measured vs. Ag/Ag in acetonitrile by cyclic voltammetry cf. Tables 1 and 8 and Okamoto et al., 1983). SALT, salt formation COV, covalent bond formation ET, single-electron transfer. 

The structure of nonclassical carbocations, such as norbomenyl 3, has been the subject of debate since the 1950s when Saul Winstein published his milestone studies on the solvolysis of tosylated norbomenyl compounds. It was proposed that the norbomenyl cation should be represented as the nonclassical structure 4+, with a 3-center, 2-electron cyclic system (3c-2e), rather than as the classical equilibrium... 

Despite the cyclic character of these TSs, both the bond distances and charge distribution are characteristic of a high degree of charge separation, with the butenyl fragment assuming the character of an allylic carbocation. 

A particularly interesting case of carbocation stabilisation occurs with Hiickel 4n + 2 systems when n = 0, i.e. cyclic systems with 2-7re (p. 18). Thus derivatives of 1,2,3-tripropylcyclopropene (8) are found to yield ion pairs containing the corresponding cyclopropenyl cation (9) extremely readily,... 

These observations are explainable by a pathway in which one end of a bromine molecule becomes positively polarised through electron repulsion by the n electrons of the alkene, thereby forming a n complex with it (8 cf. Br2 + benzene, p. 131). This then breaks down to form a cyclic bromonium ion (9)—an alternative canonical form of the carbocation (10). Addition is completed through nucleophilic attack by the residual Br (or added Ye) on either of the original double bond carbon atoms, from the side opposite to the large bromonium ion Br , to yield the meso dibromide (6) ... 

Addition is initiated by the positively polarised end (the less electronegative halogen atom) of the unsymmetrical molecule, and a cyclic halonium ion intermediate probably results. Addition of I—Cl is particularly stereoselective (ANTI) because of the ease of formation (and relative stability compared with carbocations) of cyclic iodonium ions. With an unsymmetrical alkene, e.g. 2-methylpropene (32), the more heavily alkyl-substituted carbon will be the more carbocationic (i.e. the less bonded to Br in 33), and will therefore be attacked preferentially by the residual nucleophile, Cle. The overall orientation of addition will thus be Markownikov to yield (34) ... 

Initial attack will always take place on a terminal carbon atom of the conjugated system, otherwise the carbocationic intermediate (64), that is stabilised by delocalisation, would not be obtained. It is because of this stabilisation that a carbocation intermediate is formed rather than a cyclic trromonium ion (cf. 66). Completion of overall addition by nucleophilic attack of Bre on (64) can then take place at C2 [1,2-addition, (a) ->(68)] or C4 [1,4-addition, (b) — (69) J ... 

The cyclic phosphonium salts 140,141,143,145, and 146 so obtained are evidence for the mechanism of the oxaphospholic cyclization and especially for the main role of the tertiary carbocation formation during the process. The additional data which support this assumption, come from the investigation of the same reaction, but with different substrate, i.e., dimethyl(l,2-hexadienyl)phosphine oxide 147. In this case, the reaction mechanism involved formation of secondary carbocation that gives oxaphosphole product 148 only in 10% yield (Scheme 60) [124],... 

There is also substantial stabilization of [4+] by electron delocalization from the cyclic a-vinyl group. This is shown by a comparison of the thermodynamic driving force (p Tr lies between —7.8 and —8.5) and absolute rate constant (ks = 1 -6 x 107 s 1) for the reaction of [4+] in 25% acetonitrile in water with the corresponding parameters for reaction of the resonance-stabilized l-(4-methoxyphenyl)ethyl carbocation in water (p Tr = — 9.4and s= 1 x 108 s Table 5). 

A comparison of rate and equilibrium constants for partitioning of the cyclic carbocation [18+] with those for the l-(4-methylphenyl)ethyl carbocation Me-[6+] (Table 5) shows that placement of the cationic benzylic carbon in a five-membered ring results in the following complex changes in the reactivity of the carbocation towards deprotonation and nucleophilic addition of solvent (Scheme 15). 

Table 5. Rate and equilibrium constants for the formation and reaction of cyclic benzylic carbocations [18 + ] and [20+ ] and analogous ring-substituted 1-phenylethyl carbocations (Scheme 15)°... 

Two alternative mechanisms were considered for the reaction (a) synchronous mechanism involving a cyclic transition state 91, and (b) stepwise electrophilic addition proceeding through formation of a carbocation 92 (Scheme 35). 

Hydride transfer from alkenes was also proposed to occur during sulfuric acid-catalyzed alkylation modified with anthracene (77). Then the butene loses a hydride and forms a cyclic carbocation intermediate, yielding—on reaction with isobutene—trimethylpentyl cations. This conclusion was drawn from the observation of a sharp decrease in 2,2,3-TMP selectivity upon addition of anthracene to the acid. 

The carbocations so far studied are called classical carbocations in which the positive charge is localized on one carbon atom or delocalized by resonance involving an unshared pair of electrons or a double or triple bond in the allylic positions (resonance in phenols or aniline). In a non-classical carbocation the positive charged is delocalized by double or triple bond that is not in the allylic position or by a single bond. These carbocations are cyclic, bridged ions and possess a three centre bond in which three atoms share two electrons. The examples are 7-norbomenyl cation, norbomyl cation and cyclopropylmethyl cation. 

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