Carbenium ions rearrangements

In comparison to carbanions, which maintain a full octet of valence electrons, carbenium ions are deficient by two electrons and are much less stable. Therefore, the controlled cationic polymerization requires specialized systems. The instability or high reactivity of the carbenium ions facilitates undesirable side reactions such as bimolecular chain transfer to monomer, /1-proton elimination, and carbenium ion rearrangement. All of that limits the control over the cationic polymerization. 

Fig. 11.17. First semipinacol rearrangement of a glycol monotosylate. The reaction involves three steps in neutral media formation of a carbenium ion, rearrangement to a carboxonium ion, and deprotonation to the ketone.

Ail carbcne rearrangements (as well as carbenium ion rearrangements, such as pinacol type isomerisations) require that the o-bond of the migrating group and the empty orbital of the carbene should be parallel. TNs has been dcmunstiated, for example, by the isotope labeling of hydrogens in norbornane derivatives (33] ... 

The highly unstable primary carbenium ion rearranges rapidly to give the relatively stable tertiary butyl cation, which gives isobutylene by proton abstraction ... 

Epoxides may be generated as reactive intermediates when unsaturated materials are treated with BPs/trifluoroperacetic acid, a powerful oxidation/rearrangement mixture developed by Hart. ° The products from numerous alkenes and aromatic substrates, to the extent that comparisons can be made, are those expected from epoxide/carbenium ion rearrangements. 

It is one example of an important group of reactions between triazolediones and polycyclic alkenes. Related syntheses in this group are summarized in Table 6. In each case, carbenium ion rearrangements, like that for benzvalene, lead to urazoles which on conversion to the five-membered ring diazene and deazetization generated a three-membered ring. 

Addition of chlorosulfonyl isocyanate to bicyclo[l. 1,0]butanes proceeds rapidly to afford mainly 3-azabicyclo[3.1.0]hexan-2-ones 30. The course of the reaction is believed to involve initial SE2-like attack at the less hindered bridgehead carbon atom followed by cyclobutyl to cyclo-propylmethyl carbenium ion rearrangement or conformational ring inversion, and collapse of the zwitterion. 

A carbenium-ion rearrangement of the 1,1-diphenylethylene system has been described (p. 282). A rearrangement involving a pentachlorophenyl group shift occurs in the treatment of perchlorotolane with oleum, which gives the carbinol [112] by oxidative degradation (109) (Ballester et ai, 1986a). 

Such an interaction is probable. However, according to the available data (see Sect. IV.2.Q, a change in the acid medium does not usually cause great changes in the rate constants of carbenium ion rearrangements (cf., however, the rearrangement of hydroxytenzenium ions in aqueous acids. Sect. IV.2.C). One can, therefore, assume the equilibrium between the isomeric ions not to be very sensitive to the nature of acid medium either. The equilibrium constants (K) for isomeric isopropyl-trimethylcyclopentenyl cations at 25 "C in different acids i - i > are as follows ... 

A similar extremely low sensitivity to the acid m ium is demonstrated for the rearrangements of the 6-hydroxy-1,1,2,3,4,5-hexamethylbenzenium ion >, substituted cyclopentyl cations and alkylcarbenium ions (see also These observations allow us to compare the carbenium ion rearrangements obtained in different acid media. 

The addition of HCl to 3,3-dimethyl-l-butene in acetic acid gives the three products shown in Eq. 10.11 in the ratio indicated. Protonation is rate-determining as indicated by a kinetic analysis that is first order in both acid and alkene, and an isotope effect of 1.15 is found (comparing HCl in AcOH with DCl in AcOD). A carbenium ion rearrangement has occurred to give the second product. To verify that the rearrangement occurs during the reaction, the other two products were subjected to the reaction conditions, and they are stable. 

The outcome of the Ritter reaction can also be determine by kinetic control. For example, conflicting reports have appeared in the literature regarding the behavior of alcohol (9), the alternative products (10) or (11) both having been reported. These differences result from the order of addition of the re-agents. The amides (10) result if the alcohol is mixed first with the nitrile and acetic acid, the sulfuric acid being added last. In this instance (kinetic control), the initial catirms are reacted to (10) as they are produced. Conversely, if (9) is mixed t with the acids, and then the nitrile is added, the products (11) result. In this case (thermodynamic control) carbenium ion rearrangement precedes Ritter reaction (Scheme 5). For less clear-cut cases, the use of less acidic conditions and/or lower temperature results in greater isomer selectivity, but at the cost of lower overall yields. ... 

Tautomerizations are also subject to the dictates of equation (16), with the more polar form increasingly favored in more polar media. This has been another successful target for FEP calculations, with applications ranging from 2-hydroxypyridine and histamine to nucleotide bases. In addition, the influence of solvation on simple carbenium ion rearrangements has been investigated by MC-FEP simulations. For the conversion of the classical to the non-classical 2-norbomyl cation in water and that of a tertiary cyclopentylcarbinyl cation to a secondary cyclohexyl cation in methylene chloride and THF, solvation differences of only ca, 1 kcal mol were obtained. Such ions are all relatively charge-delocalized and are consequently immune to significant differential solvation. 

This involves the formation of a carbenium ion which is best described as a hybrid of the two structures shown. This then rearranges by migration of a bond, and in so doing forms a more stable tertiary carbenium ion. Elimination of a proton yields camphene. 

These examples serve to illustrate the fact that, in reactions in which carbenium ions are formed in proximity to the acetal lone pairs, unexpected rearrangements may occur. 

From 5 the formation of alkene 2 is possible through loss of a proton. However, carbenium ions can easily undergo a Wagner-Meerwein rearrangement, and the corresponding rearrangement products may be thus obtained. In case of the Bamford-Stevens reaction under protic conditions, the yield of non-rearranged olefins may be low, which is why this reaction is applied only if other methods (e.g. dehydration of alcohols under acidic conditions) are not practicable. 

The reaction mechanism has been confirmed by trapping of intermediates 13, 14 and 15. Because of the fact that neither a carbene nor a carbenium ion species is involved, generally good yields of non-rearranged alkenes 2 are obtained. Together with the easy preparation and use of tosylhydrazones, this explains well the importance of the Shapiro reaction as a synthetic method. 

The reaction is strictly intramolecular the migrating group R is never completely released from the substrate. The driving force is the formation of the more stable rearranged carbenium ion 4, that is stabilized by the hydroxy substituent. The... 

When a cyclic /3-amino alcohol—e.g. 1—is treated with nitrous acid, a deamination reaction can take place, to give a carbenium ion species 2, which in turn can undergo a rearrangement and subsequent loss of a proton to yield a ring-enlarged cyclic ketone 3. This reaction is called the Tiffeneau-Demjanov reactionit is of wider scope than the original Demjanov reaction ... 

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