Rearrangements Involving Carbonium Ions

A number of such molecular rearrangements involving carbonium ions are known. They will be discussed in the next section. 

Pinacol Rearrangement.12 Certainly one of the best-known examples of a carbonium ion rearrangement is the pinacol transformation in which polysubstituted ethylene glycols are converted into substituted ketones by the action of acidic reagents such as mineral acid, acetyl chloride, or acetic acid and iodine. In accordance with Whitmore s theory of carbonium ion rearrangements,11 the mechanism of the reaction can be outlined as follows, with pinacol itself as an example  

It is evident that the crucial step for the reaction is the generation of the transitory carbonium ion (XIII). Further support of this general scheme for the reaction is that other processes which would be expected to produce the same carbonium ion give exactly the same reaction. Thus pinacol bromohydrin (XIY) leads to pinacolone when it is treated with silver nitrate or silver oxide,13 and the same product is obtained by the action of nitrous acid on 2,3-dimethyl-3-amino-2-butanol (XV).14 

Actually the reaction does not appear to proceed by the discrete steps coordination, carbonium ion formation, and rearrangement. Rather, the transformation involves the simultaneous separation of the hydroxyl group (or nitrogen molecule or halogen atom), together with a shift of one of the groups to the backside of the carbon atom from which the separation occurred. Thus the semipinacolic deamination of (+)-l,l-diphenyl-2-amino-l-propanol (XVIII). takes place with a Walden inversion of the carbon atom to which the amino group was attached.16 

The tendency of various groups to migrate in the pinacol reaction has received considerable attention. It has been found with the symmetrical pinacols (XX) that this tendency, or migrational aptitude, may be expressed by a numerical value which will enable us to predict with remarkable accuracy the proportions of the two products, XXI and XXII, which might be obtained.16 17 18 

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A further advantage is that the reaction appears to be free from rearrangements involving carbonium ion intermediates.41 Thus norbomene (9) is converted in close to quantitative yield into exo-norborneol (10) of > 99.8% purity. 

For best results, thionyl chloride is conveniently purified by distillation from dipentene. l,l,2-Trifluoro-2-chloro-triethylamine (Et2NCF2CHClF) converts many alcohols into fluoro-compounds, e.g. (22) — (23) (24) — (25), although eliminations and rearrangements involving carbonium ions are not infrequent. A novel variant uses the reagent in the presence of lithium chloride or... 

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 ... 

Neither C5- nor C6-cyclization involve carbonium-ion intermediates over platinum metal. The rates of the -propylbenzene - indan reaction (where the new bond is formed between a primary carbon atom and the aromatic ring) and the n-butylbenzene- 1-methylindan reaction (which involves a secondary carbon atom) are quite similar (13). Furthermore, comparison of the C6-cyclization rates of -butylbenzene and n-pentylbenzene (forming naphthalene and methylnaphthalene, respectively) over platinum-on-silica catalyst shows that in this reaction a primary carbon has higher reactivity than a secondary carbon (Table IV) (29). Lester postulated that platinum acts as a weak Lewis acid for adsorbed cyclopentenes, creating electron-deficient species that can rearrange like carbonium ions (55). The relative cyclization rates discussed above strongly contradict Lester s cyclization mechanism for platinum metal. 

From these experiments, some general conclusions can be drawn concerning the behavior of small alkanes in the strongest HF-SbF5 system. (1) The reversible protonation of the alkanes (i) is very fast in comparison with the ionization step (ii) it takes place on all cr-bonds independently of the subsequent reactivity of the alkane (iii) it involves carbonium ions (transition states), which do not undergo molecular rearrangements. (2) Protonation of an alkane is atypical acid-base reaction and carbon monoxide has no effect on this step. 

An unusual cationic ring opening polymerization involving electron rearrangement was discovered recently by Mukaiyama et al. (176). These authors found that when cyclic imino carbonates were interacted with Lewis acids (BF8> TiCl4, etc.) polyurethanes were obtained. The mechanism involves carbonium ions and it represents the first cationic polyurethane synthesis. The polymerization can be visualized as follows ... 

This subject has already arisen in connection with the stabilities of olefinic bonds at various positions in the steroid nucleus (p. 14) and as a side-reaction in certain hydrogenation processes. Rearrangements of allylic systems will be covered in Chapter 9, and photolytic reactions in Chapter ii. We are concerned here with the double-bond migrations which involve carbonium ions generated by protonation of simple olehns. 

Cooper, C. N., Jenner, P. J., Perry, N. B., Russell-King, J., Storesund, H. J., Whiting, M. C. Classical carbonium ions. Part 13. Rearrangements from secondary to primary alkyl groups during reactions involving carbonium ions. J. Chem. Soc., Perkin Trans. 21982, 605-611. 

This alkyl carbonium ion, an electrophile, attacks the electron rich aromatic ring to give the alkylbenzene. The fact that carbonium ions are involved means it is to be expected they would undergo rearrangement to the most stable carbonium ion. It is this rearrangement of carbonium ions that accounts for the production of tert-pentylbenzene as the major product in (a)-(d), In each case, the tert-pentyl CH,... 

It is very important in a degradation or synthesis used in structure determination to try to employ reactions whose course is highly predictable. In particular, reactions involving carbonium ion intermediates should be avoided whenever possible because of the great ease with which many carbonium ions rearrange. Thus, the best way to replace the hydroxyl group in a secondary alcohol by a hydrogen atom is not to dehydrate with sulfuric... 

Since intramolecular 1,2-hydrogen rearrangements occur commonly in reactions involving carbonium ion intermediates 157a), there is no... 

The nature and stereospecificities observed in the rearrangement of (68b) to (69a) and (69a) to (70a) suggests that these rearrangements involve two discrete carbonium ion intermediates A and B (see Chart II). 

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 third mode of termination which occurs in some carbonium ion polymerizations involves rearrangement of the active carbonium ion into an inactive one which cannot continue the propagation. These reactions can be avoided to a great extent by working at sufficiently low temperatures, and on the whole, they only contribute significantly to the termination reaction in a few systems. 

This synthetic approach involves rearrangement of the incipient carbonium ion derived from the readily available six-membered ring compounds. Acid catalyzed 15,16) and solvolysis 17) reactions of 17a and 17b, respectively, afforded dibenzo[6/]-thiepin (9) which was also obtained by reaction of thioxanthylium ion (18) with diazomethane 18,19). 

Although the Wolf rearrangement of diazoketones in the presence of silver benzoate and triethylamine in methanol could involve rearrangement of an intermediate diradical or carbonium ion conjugate... 

The focus of the next four chapters (Chapters 14-17) is mainly on the theoretical/computational aspects. Chapter 14 by T. S. Sorensen and E. C. F. Yang examines the involvement of p-hydrido cation intermediates in the context of the industrially important heptane to toluene dehydrocyclization process. Chapter 15 by P. M. Esteves et al. is devoted to theoretical studies of carbonium ions. Chapter 16 by G. L. Borosky and K. K. Laali presents a computational study on aza-PAH carbocations as models for the oxidized metabolites of Aza-PAHs. Chapter 17 by S. C. Ammal and H. Yamataka examines the borderline Beckmann rearrangement-fragmentation mechanism and explores the influence of carbocation stability on the reaction mechanism. 

Hammond postulate has been used to explain the effect of substituents on the rate of benzilic acid rearrangements, mechanism of electrophillic aromatic substitution reactions and reactions involving highly reactive intermediates such as carbonium ions and carbon ions. 

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