Anchimeric assistance

Winstein, one of the most brilliant chemists of his time, concluded that it is attractive to account for these results by way of the bridged (non-classical) formulation for the norbornyl cation involving accelerated rate of formation from the exo precursor [by anchimeric assistance His formulation of the norbornyl cation as a cr-bridged species stimulated other workers in the solvolysis field to interpret results in a variety of systems in similar terms of rr-delocalized, bridged carbonium... 

Table 15 shows that peroxyester stabiUty decreases for the alkyl groups in the following order tert — butyl > tert — amyl > tert — octyl > tert — cumyl > 3 — hydroxy — 1,1 dimethylbutyl. The order of activity of the R group in peroxyesters is also observed in other alkyl peroxides. Peroxyesters derived from benzoic acids and non-abranched carboxyUc acids are more stable than those derived from mono-a-branched acids which are more stable than those derived from di-a-branched acids (19,21,168). The size of the a-branch also is important, since steric acceleration of homolysis occurs with increasing branch size (236). Suitably substituted peroxyesters show rate enhancements because of anchimeric assistance (168,213,237). 

The behaviour of /S-oxovinylazides is quite similar to those above. The Z isomer (556), formed from the /S-halo carbonyl compound and sodium azide, is unstable losing N2 and forming the isoxazole (557) in an anchimerically assisted concerted reaction (75AG(E)775, 78H(9)1207). At moderate temperatures (50-80 °C) the E isomer formed acylazirines which at higher temperatures rearranged to oxazoles and isoxazoles. 

Anchimeric assistance by oxirane oxygen may accelerate the solvolysis of (44 Scheme 33) (by a factor of ca. 10 -l6 ) compared to the exo isomer (45 Scheme 34) (81JOC3512). 

The SEM ester was used to protect a carboxyl group where DCC-mediated esterification caused destruction of the substrate. It was formed from the acid and SEM chloride (THF, 0°, 80% yield) and was removed solvolytically. The ease of removal in this case was attributed to anchimeric assistance by the phosphate group. Normally SEM groups are cleaved by treatment with fluoride ion. Note that in this case the SEM group is removed considerably faster than the phenyl groups from the phosphate. ... 

Based on kinetic investigations the solvolysis of ochloroalkyl sulphoxides 506 in 80% ethanol was found to proceed via a cyclic intermediate formed via anchimeric assistance of the sulphinyl oxygen atom601,602. For a solvolysis of 4-halogenothian-l-oxides see Reference 603 (equation 309). 

At present, this rule fails only when functional neighboring substituents, capable of anchimeric assistance and in a convenient position with respect to the developing positive charge, can compete with bromine in the charge stabilization of the cationic intermediate (ref. 15). For example, the reaction of some unsaturated alcohols (ref. 16) goes through five- or six-membered cyclic oxonium ions, rather than through bromonium ions. 

It is thus clear that P-aryl groups can function as neighboring groups. Much less work has been done on aryl groups located in positions farther away from the leaving group, but there is evidence that these too can lend anchimeric assistance. ... 

The question as to whether 9 is an intermediate or a transition state has been much debated. When R is aryl or vinyl, then 9 is probably an intermediate and the migrating group lends anchimeric assistance (see p. 414 for resonance stabilization of this intermediate when R is aryl). When R is alkyl, 9 is a... 

However, the rates of hydration of these compounds are about a factor of ten to one hundred faster than predicted by means of the Taft relation. Anchimeric assistance by the hydroxyl group in the transition state for protonation, 9, has... 

Anchimeric assistance in the solvolysis of /3-arylthiovinyl sulfonates was demonstrated by means of kinetic studies on model compounds (182). In a variety of solvents ranging from nitromethane and methanol to acetic acid, the /3-arylthiovinyl sulfonate 216 was shown to react 20 to 33 times faster than the triphenylvinyl sulfonate 217. Different accelerating factors were... 

Recently, anchimeric assistance was also reported in the solvolysis of dialkyl-j3-thiovinyl sulfonates (184). In particular, vinyl ester 221 reacts 3.8 X 10 times faster and vinyl ester 222, 4.2 x 10 times faster, respectively, than model compound 223 in 9 1 CH3N03 CH3 0H at 25°. The large anchimeric effects, 10 to 10, in the solvolysis of dialkyl- 3-thiovinyl sulfonates... 

Rearrangement across the double bond and possible anchimeric assistance involving carbon also has been investigated. Rappoport and Gal (134) have reported that the solvolysis of l-anisyl-2,2-diphenyl bromide, 224, in aqueous ethanol as well as in formic acid gave a,a-diphenyl-p-methoxyacetophenone, 225, as the only product i.e., no rearrangement product 226 was observed. 

More mechanistic work undoubtedly will be done on the stereochemistry of solvolysis, particularly of simple alkyl-substituted acyclic vinyl systems. The questions of concerted anchimerically assisted or nonconcerted rearrangements both to the double bond and across the double bond need further development. 

An interesting alcoholysis of epoxides has been reported by Masaki and coworkers <96BCSJ195>, who examined the behavior of epoxides in the presence of a catalytic amount of the Tt-acid tetracyanoethylene (TCNE, 85) in alcoholic media. Ring-opening is very facile under these conditions, typically proceeding via normal C-2 attack, as exemplified by styrene oxide (86). Certain epoxy ethers (e.g., 89) undergo C-1 attack due to anchimeric assistance. Analysis of the reaction mixtures revealed the presence of captodative ethylenes (e.g., 85) formed in situ, whieh were shown to be aetive in eatalyzing the reaction. The proposed mode of catalysis is represented by the intermediate 87. The affinity of these captodative olefins for... 

Epimerization of 4 at C-2 provided 5a-carba-a-DL-galactopyranose (6). When the pentaacetate IS was heated in acetic acid containing sulfuric acid, epimerization occurred at C-2 through an intermediary cyclic acetoxonium ion (18), with anchimeric assistance of the vicinal, axial acetoxyl group. After acetylation, 5a-carba-a-DL-galactopyranose pentaacetate (19) was obtained in a yield of 14% it was converted into 6 by hydrolysis. The antimicrobial activity of the racemate 6 was found to be about half that of the natural antibiotic 7 in the same assay system, indicating that the L-antipode is probably inactive. " ... 

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