Isopropyl tosylate, solvolysis

Solvolysis rates of isopropyl tosylate and 2-adamantyl tosylate (28, p. 243) in 80 percent ethanol are measured with and without added azide. Define rate enhancement, R.E., as the ratio of rate with azide to rate without, and designate by /bn3 the fraction of alkyl azide in the product. Explain the significance of the fact that the isopropyl results fit the equation... 

Fig. 9. Absence of a free-energy correlation for the solvolysis of isopropyl tosylate in representative solvents. I Reproduced from Brown, H. C. The nonclassical ion problem. New York Plenum Press 1977.1...

The parameter Y is defined by the solvolysis of 2-adamantyl tosylate and Figure 11 illustrates the fit to Equation (32) for the solvolysis of isopropyl tosylate. The additional term introduces a better correlation and indicates that the solvolysis has substantial nucleophilic assistance (/ = 0.4) in the transition structure as would be expected. 

Figure 11 Extended Grunwald-Winstein plot for the solvolysis of isopropyl tosylate , plot of the data against , improved plot when bilinear equation is...

Finally, strong solvation of the carbenium ion intermediate also causes a marked reduction in secondary (5-deuterium KIE.84 For example, the (5-deuterium KIE in the solvolysis of isopropyl tosylate is reduced from 1.13/(5-D in trifluoroethanol, which does not solvate the developing carbenium ion strongly, to 1.08/ p-D in water where the carbenium ion is strongly solvated. Again, hyperconjugation is reduced because the solvent stabilizes the developing carbenium ion in the transition state. 

The value of m from the Winstein-Grunwald equation (for 2-endo-tosylate 0.69, i.e. between 2-adamantyl tosylate, 0.91, and isopropyl tosylate, m = 0.44)also points to some participation of the solvent. Finally, as shown by Nordlander et al. kjikj, = 30 for the solvolysis of 2-endo-norbornyl tosylate. If this ester was solvolyzed with considerable steric hindrance to ionization the solvent participation would be unusually great while the ratio observed coincides with that for trans-2-methylcyclopentyl tosylate (kjikj = 30). 

In contrast to typical mono- or acyclic substrates (e. g.,isopropyl), 2-adaman-tyl derivatives are also found to be insensitive to changes in solvent nucleophilicity. A variety of criteria, summarized in Table 13, establish this point. In all cases, the behavior of 2-adamantyl tosylate is comparable to that observed for its tertiary isomer but quite unlike that observed for the isopropyl derivative. Significant nucleophilic solvent participation is indicated in the solvolysis reactions of the isopropyl system. The 2-adamantyl system, on the other hand, appears to be a unique case of limiting solvolysis in a secondary substrate 296). The 2-adamantyl/ isopropyl ratios in various solvents therefore provide a measure of the minimum rate enhancement due to nucleophilic solvent assistance in the isopropyl system 297). 

The nitrous acid deamination appears to be as complex as the nitrosoamide reaction, and in a common solvent it appears that similar mechanisms are followed. For example, the nitrosonaphthamide of 1-phenylethylamine in acetic acid, and the reaction of 1-phenyl-ethylamine with nitrous acid in the same solvent yield 1-phenylethyl naphthoate (Table 7) and 1-phenylethanol respectively, both with 79-81% retention of configuration. These products are the intramolecular products from the respective reactions, and thte stereochemical results suggest that they are formed by a common path. Similar results were also found in the deaminations of S-cholC stanylamine 2-phenyl-2-butylamine ° and 1,2,2-triphenylethyl amine Also pertinent, is the finding that the acetates fi-om the deamination of n-propylamine in acetic acid, by the two methods, contained similar amounts of the isomeric product, isopropyl acetate the amount of isomerization observed was far greater than in the solvolysis of n-propyl tosylate in the same solvent... 

The nitrous acid deamination of n-propylamine in acetic acid yields an acetate fraction containing far more isopropyl acetate (32%) than the acetate from the solvolysis of n-propyl tosylate in the same solvent (2-8%) Also, the deamination of n-butylamine in acetic acid yields a butanol fraction containing 35% of 2-butanol, whereas the solvolysis of n-butyl -nitrobenzenesulfonate in the same solvent was reported to give exclusively n-butyl acetate . These results are reasonable in terms of the larger positive charge on the cation formed in deamination, relative to the charge on the more solvated ion formed in solvolysis. 

These effects, determined with tertiary chlorides, unquestionably refer to 8n1 reactions so also do Lewis and Boozer s effects on acetolysis and formolysis—-if we can rely on the constancy of the effect of a-deuteration (Table VIII). The /3-effects then suggest that formolysis of the secondary bromide and solvolysis in aqueous ethanol of the tosylate are also SnI—or nearly so, unless bimolecular displacement reactions are also subject to similar isotope effects. Shiner (117) had however already shown that this was not so, since deuteration in the two methyl groups of isopropyl bromide did not lead to an experimentally significant effect on the displacement reaction with ethoxide ion in ethanol. It is thus also reasonable to interpret the very small effect (AAF" = 6 cal.) cited by Lewis (74f) for acetolysis of ethyl-2d8 brosylate as evidence that acetolysis of primary sulfonate esters is borderline if not Sn2. This conclusion, already suggested by the abnormally low a-effect for unassisted acetolysis of phenylethyl tosylate [Table VIII and text of Sec. VA, 2(b) 1 is supported by a similarly... 

Secondary isotope effects, as we have seen particularly in cormec-tion with eqs. (III-9) and (III-15), are regarded as being predomi nantly zero-point enei y effects. It is thus something of a shock to learn, from the careful investigation of the temperature dependence of the j3-effect on water solvolysis of the isopropyl sulfonates and bromide, performed by Leffek et al. (151), that virtually all of the effect is on the entropy of activation. At 50 C., AAF /n = 46, 47, and 29 cal., respectively, for the tosylate, methanesulfonate, and bromide, of which 45, 50, and 35 cal. are due to This is temperature... 

---