Rearrangements protons equilibration

Core electron spectroscopy for chemical analysis (ESCA) is perhaps the most definitive technique applied to the differentiation between nonclassical carbocations from equilibrating classical species. The time scale of the measured ionization process is of the order of 10 16 s so that definite species are characterized, regardless of (much slower) intra- and intermolecular exchange reactions—for example, hydride shifts, Wagner-Meerwein rearrangements, proton exchange, and so on. 

Protonation and alkylation of arenes afford cyclohexadienyl cations (arenium ions) which are also of importance in electrophilic aromatic substitution. The hepta-methylbenzenium ion (16) is a very stable species30-, but even the parent benzenium ion (17a) has been observed as have most of its alkyl, halo, and alkoxy derivatives51. The benzenium ion (17a) undergoes a rapid degenerate rearrangement which equilibrates the seven protons over six carbons. Data for the monosubstituted benzenium ions show that (17) is the most stable of the possible isomeric forms. Positive charge... 

The acidity of the propargylic proton of the starting compound 18 allows the equilibration with the allene 19 induced by bases such as tertiary amines or alcoholates (Scheme 7.4). Such prototropic rearrangements furnish the title compounds 19 with at least one proton at the terminal carbon atom, often in good yields. The EWG group involves carboxylic acids [33], esters [34], ketones [35, 36], isonitriles [37], sul-fones [38], sulfoxides [39, 40] and phosphonates [41], The oxidation of easily accessi-... 

The data can be rationalized by assuming that 2,3,4-trimethyl-pentane is initially converted to an ion by hydride transfer to either a carbonium ion or an oxidizing agent in the acid. The TUP" " ion then rearranges and cleaves to a t-C Hg ion and IC Hg. Rapid proton exchange with the acid equilibrates the ion with isobutylene before the cation extracts a hydride ion from another... 

If rearrangement proceeded only by 1,3-hydrogen shifts there is no way in which deuterium could appear in the 1-position. Even an unlikely series of 1,2-shifts can be ruled out by the greater amount of C2H5CD2OH formed than of (C2H4D)CHDOH. However, the results are easily accommodated by the intermediacy of partially equilibrating protonated cyclopropanes. A number of other solvolytic and deamination studies also support the idea that protonated cyclopropanes are reactive intermediates 28,29)... 

The stereochemistry of 3-C-nitro glycals has been studied in some detail [210]. For example, when nitroanhydroglucitol 106 was subjected to reaction with triethylamine, it gave the elimination product 107, which was then rearranged to an equilibrated mixture of glycals 108 and 109 (O Scheme 36) [211,212]. Some researchers have tried to explain this equilibrium shift by arguing that the quasi-equatorial anomeric proton is made more acidic by the stereoelectronic effect [213]. 

Since in electron spectroscopy the time scale of the measured ionization processes is on the order of 10 16 sec, definite ionic species are characterized, regardless on their possible intra- and intermolecular interactions (e. g., Wagner-Meerwein rearrangements, hydride shifts, proton exchange, etc.). Thus, electron spectroscopy gives an undisputible, direct answer to the long debated question of the non-classical nature of the norbornyl cation independent of any possible equilibration process. 

Therefore it was suggested 95> that rearrangement must be faster than ylid equilibration and that the initial proton abstraction step is the rate determining step which controls the product distribution. 

A walk rearrangement of hydroxybicyclohexenyl cation 69 is involved in the acid catalyzed isomerization of bicyclohexanone 68 to cyclohexadienone 70. The reversible nature of this walk rearrangement has been demonstrated by the equilibration 69-4-CD3s=t 69-5-CD3, which precedes the ring opening to the protonated cyclohexadienone derivative (Figure 12) (109). 

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