Acetolysis, substituent effects

The effects of substituents on the symmetrically disubstituted diarylethyl tosylates, [27(X = Y)j, can be described accurately in terms of the Y-T relationship with p = —4.44 and r = 0.53. The Y-T plot against the Y-T ascale with an appropriate r of 0.53 gives an excellent linear correlation for the whole set of substituents, indicating a uniform mechanism for all of them. When Y X, the overall solvolysis rate constant ki corresponds to the sum of the rate constants, k + kj, and hence k, cannot be employed directly in the Y-T analysis. The acetolysis of monosubstituted diphenylethyl tosylates gave a non-linear Y-T correlation, which is ascribed to a competitive X-substituted aryl-assisted pathway k and the phenyl-assisted k pathway. By application of an iterative non-linear least-squares method to (9), where the terms k and ks are now replaced by k and k, respectively, the substituent effect on kt can be dissected into a correlation with = —3.53, = 0.60, and an... 

It is somewhat surprising that the rate constants for acetolysis and ethanolysis are so similar for (11) and (12). These similarities have been rationalized in terms of the blending of and substituent effects. Since (11) and (12) are interconverted during acetolysis, attack on (18) may be partly rate limiting. 

The influence of bulky substituents in the 3 positions of vinyl substrates also appears to increase the selectivity of the vinyl cation (Rappoport and Gal, 1973a, 1970). This effect was attributed to shielding of the cationic intermediate from the incoming nucleophile. Thus, in the acetolysis of vinyl bromides, common ion return for two )3 substituents decreases in the order anisyl, anisyl > anisyl, H > methyl, methyl > H, H (Rappoport and Gal, 1970). 

Various studies have been carried out to determine the effects of changes in a-substituents on the chemistry of cyclopropyl derivatives " An a-phenyl substituent was observed to accelerate the rate of acetolysis of cyclopropyl tosylate by a factor of 10. Even though the product was 2-phenylallyl acetate, it was suggested that the high sensitivity of a-arylcyclopropyl tosylates to the introduction of substituents onto the aromatic ring is due to the fact that the activated complex is an aryl-stabilized cyclopropyl cation, not a ring-opened allyl cation species. 

The rate of acetolysis of 5a-cholestan-6a-yl tosylate is influenced in the expected manner by 3-chloro-substituents. Although each of the C-3 isomers retards carbonium ion formation, the 3qj-chloro-group is less effective than 3 -chloro. It is argued that the data reffect a through-space field effect, with the negative pole of the 3a-Cl bond closer to the reaction centre than in the equatorial 3fi-isomer. The acetolysis products comprise the 5-ene and the 6a-acetate. in proportions influenced by the configuration at C-3. No 6 -acetate was found cf. ref. 99). 

Solvolysis of the p-nitrobenzoate of (196) is ten times faster than that of (197) at 100 °C in aqueous acetone. Both give the same wide range of products, suggesting that a common ion is involved, but this intermediate must differ from the one involved in acetolysis of (198), which gives different products. Solvolyses of exo- and eni/o-dinitrobenzoates (199) (R = H, Me, Ph, or p-anisyl) have been compared. Although methyl substitution increased the rate of the less reactive exo-ester by 5 x 10 but of the endo-isomers by only 20, it has little effect on the type of product formed. The aryl substituents, unlike H and Me, led to considerable amounts of products with unrearranged skeletons. It is concluded that cyclobutyl cations will only be stable... 

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