Anchimeric assistance, also effects

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... 

Anchimeric assistance may also explain slight changes in mechanism resulting from the presence of a neighboring group. One likely case is the acid-catalyzed hydrolysis of phenylglycosides. Raftery s group" found that the secondary kinetic isotope effect (/tr/ d) was 1.13 for acid hydrolysis of phenyl-4-0-(2-acetamido-2-deoxy-j8-D-glu-... 

For example, anchimerically assisted solvolyses may show the special salt effect , implicating solvent-separated ion pairs, and solvolyses of highly stabilized substrates (e.g. diphenylmethyl compounds) show the common ion effect , implicating kinetically free cations (Raber et al., 1974). Also, Hammett (1970a) has drawn attention to an inconsistency in the widely accepted interpretation of the special salt effect. 

The large amounts of five- and six-membered cyclic tosylates, products of internal return (49% yield in one case 12a of Table 3) points also to a highly selective intermediate, although the reason for such a remarkable selectivity towards external nucleophiles is not clear. On the other hand, acetolysis of 6-phenyl-5-hexynyl brosylate, which is apparently anchimerically assisted by the triple bond (4>bs./ caic. = 1 6) yields only the five-membered ring product in addition to open-chain solvolysis products (13 of Table 3). The effect of the phenyl group in orienting the oyclization reaction would indicate that intermediate species like 43 may become important when R =Ph. 

Thus, two types of active esters are of interest those formed from an acid and a substituted phenol (12-15) and those formed from an acid and a substituted hydroxylamine (16-19). Both types are reactive by virtue of the electron-withdrawing properties of the OR moiety in 2. The level of activation of the substituted phenyl esters varies directly with the electronic effect going from 4-nitrophenyl to 2,4,5-trichlorophenyl, pentachlorophenyl, and pentafluorophenyl, which corresponds with the increasing acidity of the phenols. A diminution in the rate of aminolysis is caused by the presence of a substituent in the ortho position of the ring.f l An additional phenomenon contributes to the reactivity of the esters formed from substituted hydroxylamines, namely anchimeric assistance. Since the anoinolysis of active esters is a bimolecular reaction, it is dependent on concentration and can be forced to completion by an excess of one of the reactants. Aminolysis is also characterized by a pronounced dependence on the polarity of the solvent in particular for the esters formed from substituted phenols, the half-life of a 2,4,5-trichlorophenyl ester in the presence of benzylamine being one hundred times less in dimethylformamide than in benzene. Furthermore, aminolysis is catalyzed by mild acid such as acetic acid. The rate of anoinolysis is slowed if the side chain of the active ester contains a P-methyl substituent. 

Brown has shown that on introduction of a double bond to isomeric p-nitro-benzoates 52 and 53 the rate of solvolysis in either case decreases equally (about fivefold) which is due to the inductive double-bond effect and clearly points to the lacking anchimeric assistance in the solvolysis of206 this is also evidenced by the absence of tricyclic compounds among the reaction prcKiucts. Tlte isomers 206 and 207 are characterized by a high exotendo rate ratio and pr iominant formation of exo alcohol. 

Subsequently the same authors established that the solvolysis of optically active 6,7-dimethoxy-l,2-dimethyl-2-exo-norbornyl chloride or -p-nitrobenzoate is accompanied by anchimeric acceleration as distinct from the unsul tituted analogues 275 and 276 the resulting products are optically active Thus, anchimeric assistance leads to the formation of an unsymmetrical cation which may be the homobenzylic ion 278 while the partial loss of activity is due to the interconversion of enantiomeric ions. The authors have also shown that the accelerating effect of the homopara-methoxyl group is practically the same for the solvolysis in methanol, 50% acetone... 

Another widely used comparison, especially in bicyclic systems, is the exojendo rate ratio. This comparison is also fraught with pitfalls. Problems arise because the endo and exo isomers of a polycyclic system have unique configurations and hence different steric requirements. This can affect anchimeric assistance by neighboring groups. In addition there are inherent differences in field effects and nucleophilic solvent assistance. The 350-fold difference in rate constant for acetolysis of exo- and endo-2-norbornyl p-bromobenzenesulfonates [(11) and (12), respectively] was taken as evidence for a participation in the solvolysis of (11) by Winstein... 

The solvent may also drastically affect the anchimerically assisted fraction in nucleophilically assisted ionization reactions. As the solvent becomes more nucleophilic, there is greater solvent participation, if possible, at the expense of neighboring group assistance. The data in Table 4 are illustrative of this effect. It is obvious that in both formic acid and acetic acid there is considerable MeO-5 and MeO-6 nucleophilic participation. In ethanol, however, the ratio kjk is diminished as solvent attack (fcj becomes more effective. These effects may be quantitatively assessed. Winstein et alP used a Taft treatment for the system RCH2OBS in estimating the kj portion of the solvolytic rate constant [Eq. (14)]. Values for K/K where /c, is the titrimetrically observed rate constant, were determined. 

The relative rate data (Table 7) for solvolysis of (17), (19), and (21) are consistent with anchimeric assistance in (19), but the mechanims for (21) is in doubt. The Arrhenius plot for the solvolysis of (21) in 60% aqueous acetone is nonlinear, and its solvolysis rate is considerably less than that for either (17) or (19). On the basis of the rate data, one may agree that the cis derivative (19) may be accelerated owing only to anchimeric assistance by nitrogen. The relative rate data may also reflect steric effects. Schleyer et al. " found that cis- and trun5-3-r-butylcyclobutyl toluene-p-sulfonate undergo hydrolysis in 60% aqueous acetone considerably slower than cyclobutyl... 

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