Carbonium compounds

Basically, compounds with a central C atom, which are considered electrophilic reaction centers, are treated with a nucleophilic aromatic compound. Subsequent oxidation affords the desired carbonium compound ... 

Apparently the alkoxy radical, R O , abstracts a hydrogen from the substrate, H, and the resulting radical, R" , is oxidized by Cu " (one-electron transfer) to form a carbonium ion that reacts with the carboxylate ion, RCO - The overall process is a chain reaction in which copper ion cycles between + 1 and +2 oxidation states. Suitable substrates include olefins, alcohols, mercaptans, ethers, dienes, sulfides, amines, amides, and various active methylene compounds (44). This reaction can also be used with tert-huty peroxycarbamates to introduce carbamoyloxy groups to these substrates (243). 

Hydroxypyrroles. Pyrroles with nitrogen-substituted side chains containing hydroxyl groups are best prepared by the Paal-Knorr cyclization. Pyrroles with hydroxyl groups on carbon side chains can be made by reduction of the appropriate carbonyl compound with hydrides, by Grignard synthesis, or by iasertion of ethylene oxide or formaldehyde. For example, pyrrole plus formaldehyde gives 2-hydroxymethylpyrrole [27472-36-2] (24). The hydroxymethylpyrroles do not act as normal primary alcohols because of resonance stabilization of carbonium ions formed by loss of water. 

When dihydromyrcene is treated with formic acid at higher temperatures (50°C) than that required to produce dihydromyrcenol and its formate, an unexpected rearrangement occurs to produce a,3,3-trimethylcyclohexane methanol and its formate (106). The product is formed by cyclization of dihydromyrcene to the cycloheptyl carbonium ion, which rearranges to give the more stable cyclohexyl compound (107). The formate ester, a,3,3-trimethylcyclohexane methanol formate [25225-08-5] (57) is a commercially avaUable product known as Aphermate. 

The protonated azirine system has also been utilized for the synthesis of heterocyclic compounds (67JA44S6). Thus, treatment of (199) with anhydrous perchloric acid and acetone or acetonitrile gave the oxazolinium perchlorate (207) and the imidazolinium perchlorate (209), respectively. The mechanism of these reactions involves 1,3-bond cleavage of the protonated azirine and reaction with the carbonyl group (or nitrile) to produce a resonance-stabilized carbonium-oxonium ion (or carbonium-nitrilium ion), followed by attack of the nitrogen unshared pair jf electrons to complete the cyclization. 

The Bamford-Stevens decomposition of tosylhydrazones by base has been applied to steroids, although not extensively. It has been demonstrated that the reaction proceeds via a diazo compound which undergoes rapid decomposition. The course of this decomposition depends upon the conditions in proton-donating solvents the reaction has the characteristics of a process involving carbonium ions, and olefins are formed, often accompanied by Wagner-Meerwein-type rearrangement. In aprotic solvents the diazo compound appears to give carbene intermediates which form olefins and insertion products ... 

Studies of solvolysis of similar polyfluonnated polycyclic aromatic systems, such as 2,3-(tetrafluorobenzo)bicyclo[2 2 2]octadienes and related compounds, proved the ionic mechanism of this rearrangement [55, 36, 37] (equation 9) Possible nonclassical carbonium ion involvement has been discussed [5S, 39, 40, 41]... 

The formation of 88 is postulated to be occurring by the nucleophilic attack of a hydride ion (47), abstracted from the secondary amine, on the a-carbon atom of the iminium salt (89). The resulting carbonium ion (90) then loses a proton to give the imine (91), which could not be separated because of its instability (4H). In the case of 2-methyIhexamethylenimine, however, the corresponding dehydro compound /l -2-methylazacyclo-heptene (92) was isolated. The hydride addition to the iminium ion occurs from the less hindered exo side. 

Through a study of the influence of thiophene and other aromatic compounds on the retardation and chain transfer on the polymerization of styrene by stannic chloride, the relative rates of attack of a carbonium-ion pair could be obtained. It was found that thiophene in this reaction was about 100 times more reactive than p-xylene and somewhat less reactive than anisole. ... 

It will be observed that most syntheses yield pyrylium salts in which positions 2,4, and 6 are substituted. Since according to formulas Ib-lc these positions have a partial positive charge, it can readily be understood why electron-donating substituents (hydroxy, alkoxy, alkyl, or aryl) in these positions stabilize the pyrylium salts. Only three pyrylium salts which do not have substituents in either a-position have been reported and few unsubstituted in y or in one a-position they are less stable toward hydrolysis, and in the case of perchlorates they explode more easily, than 2,4,6-trisubstituted compounds. In fact, the former are secondary, the latter tertiary carbonium ions. This fact also explains why the parent compound (1) was prepared only in 1953. 

Thane et al. [46] reported that in pentane suspension, alkylaluminum compounds efficiently alkylate labile chlorines in PVC, and alternatively, PVC carbonium ions could alkylate aromatic compounds to give rise to polymers of increased stability. The values of 2-3% for labile chlorines estimated by them were considerably higher than now generally believed. 

The retrosynthetic operations that we have addressed thus far have not resulted in significant structural simplification. After all, intermediate 6 still possesses a linear fusion of four rings and six contiguous asymmetric carbon atoms. But, nevertheless, intermediate 6 could potentially be derived in one step from intermediate 8, a polyunsaturated monocyclic compound containing only one stereogenic center. Under conditions that would be conducive to a heterolytic cleavage of the C-OH bond in 8, it is conceivable that the resultant tertiary allylic carbonium ion 7 would participate in a... 

Very powerful initiators of carbonium-ion polymerization were recently reported by Plesch.32b They belong to the salt-like class of organic compounds and dissociate readily into C104 ions and carboxonium positive ions. The latter are sufficiently reactive to initiate carbonium-ion polymerization of styrene. 

FIGURE 6.6. The type of model compounds that were used to estimate the electrostatic stabilization in lysozyme (the only hydrogen atom shown, is the one bonded to the oxygen). Such molecules do not show a large rate acceleration due to electrostatic stabilization of the positively charged carbonium transition state. However, the reaction occurs in solution and not in a protein-active site, and the dielectric effect is expected to be very different in the two cases. 

This reaction exemplifies the important process of methyl removal, which becomes even more significant in the case of multiply branched paraffins. The rather large exothermicity of Reaction 11 results from the fact that a secondary carbonium ion is formed. The beta fission process can be illustrated using reactions in 8-ra-hexylpentadecane (compound 2) as an example. Table II shows that the ions formed by C-C fission at a branch point (Ci4+, m/e = 197 and Ci5+, m/e = 211) have intensities appreciably larger than the other alkyl ions in the same region... 

Several types of proton transfer reactions can be studied conveniently by a neutral product analysis. Until now, the most extensive investigations have been concerned with (1) proton transfer from H3+ and CH5 + to various hydrocarbon molecules, and (2) the transfer of a proton from carbonium ions to larger olefins or other organic compounds. 

We have seen that carbonium ions can undergo a variety of photoreactions, affording products which often vary considerably from those obtained in the photolysis of the corresponding uncharged compounds. The predominant mode of reaction encountered would seem to be isomerization to one or more valence bond isomers, which occurs via a symmetry-allowed disrotatory electrocyclic closure, rather than a [<72a-f 7r2a] cycloaddition in the case of alkylbenzenium ions and pro-... 

The equilibrium (1) at the electrode surface will lie to the right, i.e. the reduction of O will occur if the electrode potential is set at a value more cathodic than E. Conversely, the oxidation of R would require the potential to be more anodic than F/ . Since the potential range in certain solvents can extend from — 3-0 V to + 3-5 V, the driving force for an oxidation or a reduction is of the order of 3 eV or 260 kJ moR and experience shows that this is sufficient for the oxidation and reduction of most organic compounds, including many which are resistant to chemical redox reagents. For example, the electrochemical oxidation of alkanes and alkenes to carbonium ions is possible in several systems... 

The effect of structure of the alkyl group on the stability of monoalkyl-thallium(III) compounds can best be understood by reference to the different mechanisms by which these compounds undergo decomposition. A number of authors have attributed the instability of monoalkylthallium(III) compounds to facile C—T1 bond heterolysis and formation of carbonium ions [Eq. (25)] (52, 66, 79). This explanation is, however, somewhat suspect in cases where primary carbonium ions would be involved and either the two-step sequence shown in Eqs. (26), (27), or the fully synchronous 8 2 displacement shown in Eq. (28), is more compatible with the known facts. Examination of the oxythallation reactions that have been described reveals that Eq. (27) [or, for concerted reactions, Eq. (28)] can be elaborated, and that five major types of decomposition can be recognized for RTlXj compounds. These are outlined in Scheme 8, where Y, the nucleophile... 

From the above discussion it follows that the probability of carbonium ion formation during decomposition of RTIX2 compounds by a Type 5 process is low when X is carboxylate, but significantly higher when X is nitrate, sulfate, perchlorate, or fluoroborate. The important role played by the anion of the metal salt in oxymetallation has in fact been recognized only very recently for both oxymercuration 11, 12) and oxythallation (92). The... 

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