Secondary cations

When 2-butyl tosylate is solvolyzed in the less nucleophilic solvent triiluoroacetic acid, a different result emerges. The extent of migration approaches the 50% that would result from equilibration of the two possible secondary cations. 

These results indicate an energy profile for the 3-methyl-2-butyl cation to 2-methyl-2-butyl cation rearrangement in which the open secondary cations are transition states, rather than intermediates, with the secondary cations represented as methyl-bridged species (comer-protonated cyclopropanes) (Fig. 5.10). 

Step 3 of Figure 27.14 Third Cyclization The third cationic cyclization is somewhat unusual because it occurs with non-Markovnikov regiochemistry and gives a secondary cation at C13 rather than the alternative tertiary cation at C14. There is growing evidence, however, that the tertiary carbocation may in fact be formed initially and that the secondary cation arises by subsequent rearrangement. The secondary cation is probably stabilized in the enzyme pocket by the proximity of an electron-rich aromatic ring. 

It is likely that protonated cyclopropane transition states or intermediates are also responsible for certain non-1,2 rearrangements. For example, in superacid solution, the ions 14 and 16 are in equilibrium. It is not possible for these to interconvert solely by 1,2 alkyl or hydride shifts unless primary carbocations (which are highly unlikely) are intermediates. However, the reaction can be explained " by postulating that (in the forward reaction) it is the 1,2 bond of the intermediate or transition state 15 that opens up rather than the 2,3 bond, which is the one that would open if the reaction were a normal 1,2 shift of a methyl group. In this case, opening of the 1,2 bond produces a tertiary cation, while opening of the 2,3 bond would give a secondary cation. (In the reaction 16 14, it is of course the 1,3 bond that opens). 

A mechamism closely related to that of (29) can be viewed for the loss of methyl radicals from terminal positions of ionized n-alkanes54. For example, based on experiments with 2H-labelled hydrocarbons and on careful analysis of energetic data it was shown convicingly that CD3 loss from 128 gives only the secondary cation 130. From the fact that 131 is not found to be formed at all, it must be concluded... 

Numerous adsorption complexes of CO and AN in Na-A and in Na-FER were investigated only some of these adsorption complexes (giving an example of each type) are summarized in Table 1. First we discuss the effect from the top due to the interaction with the secondary cation(s). The CO molecule adsorbs on the primary cation (via C end) and when the secondary extra-framework cation is at a suitable distance from the primary cation CO forms a bridged adsorption complex between the... 

Yoshida has studied anodic oxidations in methanol containing cyanide to elucidate the electrode processes themselves.288 He finds that, under controlled potential ( 1.2 V), 2,5-dimethylfuran gives a methoxynitrile as well as a dimethoxy compound (Scheme 57). Cyanide competes for the primary cation radical but not for the secondary cations so that the product always contains at least one methoxy group. On a platinum electrode the cis-trans ratio in the methoxynitrile fraction is affected by the substrate concentration and by the addition of aromatic substances suggesting that adsorption on the electrode helps determine the stereochemistry. On a vitreous carbon electrode, which does not strongly adsorb aromatic species, the ratio always approaches the equilibrium value. 

The behavior of the isomeric dihydronaphthalenes emphasizes the importance of the relative stabilities of carbocation intermediates in ionic hydrogenations. Treatment of 1,2-dihydronaphthalene with Et3SiH/TFA at 50-60° gives a 90% yield of tetralin after one hour. Under the same conditions, the 1,4-dihydronaphthalene isomer gives less than 5% of tetralin after 70 hours.224 This difference in reactivity is clearly related to the relatively accessible benzylic cation formed upon protonation of the 1,2-isomer compared to the less stable secondary cation formed from the 1,4-isomer.224... 

The 2-butyl cation is the smallest secondary cation that can be stabilized either by C-C or C-H hyperconjugation. Experimental results give evidence for two equilibrating isomers.33 MP2/6-311G(d,p) calculations show that the symmetrically hydrido-bridged structure 11 is marginally more stable than the partially methyl-bridged structure 10.34 35... 

Ph-CH2+, whereas during ethyl transfer it is a more stable secondary cation, - h-CH3/ which is easier to form. It is also apparent, that ethylbenzene is a better acceptor than xylene. We suggest that this is largely a consequence of the larger steric requirement of the bulky diphenylmethane intermediate for alkyl transfer to xylene vs to ethylbenzene. 

The 3- and 4-heptyl cations cannot form a 1,6-p-H-bridge. The 3-heptyl cation could in principle form a primary-secondary 1-5-p-H-bridged structure with an ethyl group at the secondary cation terminus. 

Primary cations are about 15 kcal/mol less stable than secondary cations and are unlikely to make a significant contribution to the overall reactions, even at high temperatures, and so these have been excluded from consideration. Isomerization between the normal extended structures of the 2-, 3-, and 4-... 

This transition-state, 22, has a calculated energy only slightly higher (0.48 kcal/mol at B3LYP/6-31G ) than the transition-state for isomer 18, and the geometry is also slightly different, for example, a somewhat shorter H—H bond. Loss of dihydrogen from this transition-state will lead to a secondary cation intermediate, and after a 1,2-hydride shift, to the same 1-methylcyclohexyl cation product that is directly produced from the other route. 

Figure 8. Alternative optimized transition-state for H2 loss in 1-6-p-H-bridged 2-heptyl cation, 22, leading to a cyclized secondary cation intermediate at the...

We have not carried out calculations starting with secondary cations derived from the title alkanes because at a computational level, these will have ground-states and transition-states similar to heptane itself (previously discussed). This will be true even though the most stable carbocations in these branched alkanes will be the corresponding tertiary ions, which in themselves will not be directly involved in dehydrocyclization processes. However, one has to keep in mind that the thermodynamic ground-states in these real catalytic reactions will be the alkanes themselves, and in this regard secondary cations formed from n-octane or 2- (or 3-) methylheptane will not differ much in absolute energy. As shown earlier, a 1,6-closure of 2-methylheptane leads eventually to m-xylene, while 3-methylheptane has eventual routes to both o- and p-xylene. 

The author s theory which has been used here was developed in detail to explain the polymerisations by ionising radiations of some alkyl vinyl ethers, the polymerisations of which proceed by secondary ions. Although it was shown that the theory is also perfectly serviceable for the tertiary carbenium ions considered here, it must be realised that there is a fundamental difference between these two types of carbenium ions. When one of the bonds of the carbenium ion is a C—H bond, the solvators, especially of course an ion, can get much closer to the positive centre, and they are therefore correspondingly more firmly held to which effect is added that of a smaller steric hindrance. The most researched monomer propagating by secondary cations, apart from the alkyl vinyl ethers, is, of course, styrene. Thus, Mayr s many studies with diaryl methylium cations are directly relevant to the polymerisation of styrene. 

As observed, aromatic hydrocarbons gave products of protonation on dissolution in hydrofluoric acid. Oxidation into aromatic cation-radicals did not take place (Kon and Blois 1958). Trifluoro-acetic acid is able to transform aromatics into cation-radicals. This acid is considered a middle-powered one-electron oxidant (Eberson and Radnor 1991). Its oxidative ability can be enhanced in the presence of lead tetraacetate. This mixture, however, should be used carefully to avoid oxidation deeper than the one-electron removal. Thus, oxidation of 1,2-phenylenediamine by the system Pb(OCOCH3)4 -I- CE3COOH -P CH2CI2 leads to the formation of either primary or secondary cation-radicals. The primary product is the cation radical of initial phenylenediamine, whereas the secondary product is the cation radical of dihydrophenazine (Omelka et al. 2001). Sulfuric acid is also used as an one-electron oxidant, especially for aromatic hydrocarbons. In this case, generation of cation radicals proceeds simultaneously with the hydrocarbon protonation and sulfonation (Weissmann et al. 1957). 

The presence of a protonated cyclopropyl ring distinguishes the two norbomyl ions and one mi t hope to characterize a stabilized ion by its proton exchange behaviour. To do this one would have to investigate the exchange behaviour of protonated alkylcyclopropanes and tertiary and secondary cations in an acid sufficiently strong to stabilize all species for some time. 

Accordingly, if the shift quantitatively reflects charge density, one would expect the isopropyl centre to be significantly downfield of the tertiary in t-butyl. The fact that it is not suggests that the C shift in this ion cannot provide a general model for secondary cations and its use to predict the shifts of the classical norbomyl ion is highly dubious. 

The mechanism of the reaction involves electrophilic attack of the catalyst on the double bond of propylene to form the more stable secondary cation, which reacts with the ti cloud of benzene to give a delocalized ion. Deprotonation rearomatizes the ring. 

Lifetimes are longer in the more weakly nucleophilic TFE and HFIP, and cations whose existence is on the borderline in water and simple alkanols can become quite long lived, especially in HFIP. Benzenium ions such as protonated mesitylene can also be observed in HFIP, and there is an estimate for a simple secondary cation 77 60 pjjgj.g Qjjg estimate of the lifetime of an acylium ion (72), based upon the clock approach. Even with the powerful electron-donor 4-Me2N on the aromatic ring, this cation appears to be very short lived in water. 

Oxaspiropentanes generally rearrange with inversion at the migrating terminus (see Section 3.2.2.2.). However, if a primary cation is involved, cyclopropylmethyl to cyclopropyl-methyl rearrangement with formation of a more stable secondary cation may precede the ring enlargement, and the stereochemistry of substituents in the cyclopropane part of the oxaspiropentane may be lost. This was found to be true for /ram-4,5-dimethyl-l-oxaspiropentane (5) where a considerable amount of m-2,3-dimethylcyclobutanone (m-6) was formed.47... 

Other alkenes, however, may not react selectively. Product distribution in the transformation of 32 [Eq. (11.70)] is consistent with the equilibrium of the tertiary and secondary cations 33 and 34, or the presence of hydrogen-bridged ion 35 221... 

The latter, however, is more reactive then secondary cation 4 and reacts with propylene more rapidly than it eliminates a proton. 

It is therefore apparent that the catalysts which produce isotactic polypropylene have an ionicity which, rather than anionic, is cationic, as shown in Fig. 8. The propagation in polypropylene involves the secondary cation which is closely associated and balanced with its catalyst gegen ion. This balance is dependent on the electron releasing effect of the methyl group at the propagating end of the double bond. 

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