Anchimeric assistance rate acceleration

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 direct interaction of the reaction center of a molecular entity with a lone pair of electrons of an atom within that same molecular entity that is not associated with the reaction center or, interaction of the reaction center with the reaction center nor conjugated with the reaction center. Rate acceleration by such a process is referred to as anchimeric assistance. See Intramolecular Catalysis Synartetic Acceleration. 

For a number of years, a storm of controversy raged over this proposal, with H. C. Brown as the chief opponent. Brown ruled out anchimeric assistance as an explanation for the rate acceleration of the exo derivative, arguing that exo was normal, but that endo was unusually slow because of a steric effect. The racemization and isotopic tracer results, he proposed, could be explained by a rapid equilibrium between the classical ions 15 and 17 (see Scheme 1.3), with a steric effect responsible for the exo addition of nucleophiles. In terms of the cation, the question revolves around the issue whether the classical ions 15 and 17 should be joined by the equilibrium depiction (the rapidly rearranging scenario) or with a... 

The concept of an intermediate phenonium ion was, at first, controversial, and its chief detractor was H. C. Brown.25 Although 3-phenyl-2-butyl tosylate showed the stereochemical behavior expected if an intermediate phenonium ion were formed, it did not, in his opinion, show the rate acceleration that should attend anchimeric assistance to ionization of the tosylate.26 Brown said that the stereochemical results could be accounted for by invoking rapidly equilibrating open carbocations (15). According to his explanation, ionization of the tosylate... 

A much more spectacular driving force was found in the acetolysis of anti-1-norbornenyl tosylate (51). This compound solvolyzes 1011 times faster than the saturated analog and gives as the sole product the anti-acetate, 52.68 Winstein attributed the enormously accelerated rate to powerful anchimeric assistance of both p orbitals of the 2,3-double bond. 

Winstein estimated the rate acceleration due to anchimeric assistance in ionic perester decompositions as follows. By using Equation 6.66, in which the first term on the right-hand side is the difference in homolytic dissociation... 

The experimental dependency of the /J-silyl effect on 0 in solvolysis reactions is sketched in Figure 764. Obviously, it differs from that anticipated for a k mechanism with rate-determining formation of siliconium ion or from the cosine-squared function expected for the pure hyperconjugative stabilization model. Apparently, the /J-silyl effect is operative in the solvolysis of both the syn- and rmt/ -periplanar conformations. The rate acceleration in the latter might be ascribed to a more favourable geometry for the (T-anchimeric assistance. 

In 1960, Winstein reported a record-breaking anchimerically assisted solvolysis rate acceleration 7-norbomadienyl precursors reacted lO times faster than their saturated 7-norbomyl analogs [47]. The cation favors a C, rather than a Qv structure, [47, 48] since interaction between the cationic center and... 

Shoppee s suggestion [ig] in 1946 that retention of configuration at C<3) was a consequence of jr-electron participation was supported by kinetic studies when Winstein [20] two years later, demonstrated a marked accelerating effect of the 5,6-double bond in the acetolysis of cholesteryl tosylate. The acetolysis followed strictly first order kinetics, the rate being only slightly affected by added salts, including alkali-met acetates. Further demonstrations of anchimeric assist-... 

In the controversy that developed, the point under attack was not so much the existence of the intermediate bridged ion—although this was questioned, too— as its mode of formation. The 3-phenyl-2-butyl tosylates undergo solvolysis at much the same rate as does unsubstituted e c-butyl tosylate formolysis a little faster, acetolysis a little slower. Yet, as depicted by Cram, phenyl gives anchimeric assistance to the reaction. Why, then, is there no rate acceleration ... 

If the migrating group does provide anchimeric assistance, certain consequences should follow. One is kinetic The rate should be faster than the rate of an exactly analogous, but unassisted, reaction. Another is stereochemical The migration terminus should be inverted by the rearrangement. There are obvious problems in the experimental evaluation of these criteria. Rate acceleration is often difficult to ascertain because the rate of the non-assisted reaction cannot be reliably predicted (cf. Section 7.2.). Inversion of the migration terminus may also be a result of ion pairing (Section 4.) or of conformational control (Section 6.). Combined kinetic and stereochemical evidence provides the most powerful support for anchimeric assistance. 

In contrast to Brown s assertions and in accord with Winstein s and Trifan s assumption, the solvolysis of these secondary systems proceeds with anchimeric acceleration. This is concluded from the following facts a) the exo endo rate ratio for 2-norbomyl systems is 10 -10 as the reaction rate of the endo isomer is not anomalous (see above), hende the exo isomer reacts at an elevated rate b) the rate of solvolysis of exo isomers is 10 to 10 times as high as that calculated according to the semiempirical scheme from only steric effects c) the ratio of the reaction rate of secondary 2-exo-norbomyl systems to the solvolysis rate of secondary cyclopentyl analogues is 100 times as great as that of tert-2-exo-norbomyl derivatives and tert-cyclopentyl analogues since tert-2-norbomyl derivatives are solvolyzed without anchimeric assistance, the factor of 100 characterizes tentatively the amount of anchimeric assistance in the secondary 2-exo-norbornyl systems d) exo- and endo-6-substituents decrease the solvolysis rate of 2-exo-norbomyl tosylate this cannot be accounted for without participation of the electrons of the 1,6 bond in the transition state their participation increases the non-bonded interaction due to a decrease in the C -C distance. 

In a study of sulfur participation in perester decomposition it was of interest to eliminate steric acceleration as a factor. Hence the rates of thermolysis of (15) and (16) were compared as evidence that anchimeric assistance by sulfur rather than steric acceleration was responsible for the high relative rates observed for the o-phenylthioperesters (see Chapter 5). 

The hydrolysis of 2,2 -bischloroethyl sulfide (mustard gas) was one of the first neighboring-group-accelerated reactions to get widespread attention. Initially the hydrolysis of this compound is purely first order in the sulfide, and the rate is unaffected by added alkali or other nucleophiles the increasing amount of chloride ion, however, slows the rate with time. Since formation of the primary carbocation is not expected, these data suggest an anchimerically assisted solvolysis [Eq. (1)], leading to the formation of the intermediate sulfonium ion (1), which may react with water, chloride ion, or any other nucleophile present. A second anchimerically assisted process that leads to displacement of the second chloro group can occur. 

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... 

The u-methoxy-group on PMegCo-MeOCeHi) obviously has little effect on the rates whereas an acceleration of 100 times is observed in oxidative additions involving methyl iodide. This effect is attributed to anchimeric assistance due to interaction of the methoxy oxygen with iridium. However, in the present case the lack of effect is probably a reflection of the much lower polarity of the transition state (2) compared with that for methyl iodide addition (3). 

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