Structural effects in solvolytic reactions. XI. Rates and products of solvolysis of 2-aryl- and 2-methyl-2-norbornenyl p-nitrobenzoates. Exo:endo rate ratio as a function of increasing electron demand

Correlation of rates of solvolysis of secondary tosylates with tertiary benzylic derivatives

“…As shown in Figure 3 phenyl, and p-trifluoromethylphenyl derivatives gave a linear py+ plot with a p value of -4.21;22 however, the m,m'-bis-(trifluoromethyl)phenyl derivative deviates from this slope indicating the onset of participation. 23 The exact amount of anchimeric assistance can be determined by calculating an unassisted rate for 2-exo-norbornenyl tosylate (from the p value of the unassisted series,22 y+ for hydrogen, and eq 2) and comparing it with the observed rate. Thus a factor of approximately 18 was attributable to it participation.…”

Using the tool of increasing electron demand to evaluate the Yukawa-Tsuno equation and for identifying nonclassical carbocations

“…Consequently p was chosen to fit the p-OMe point, so again the derived + value is almost exactly that (-0. (iii) The data for compounds 7 and 8 appear at first sight to be marginally more valid, with rate coefficients included for the m,m/-(CF3)2 and P-NO2 derivatives, respectively. Alas this is illusory since these compounds have not been made either, the quoted "rate coefficients" being merely calculated assuming that the Hammett-Brown equation applies perfectly.7'8 Again the derived + _0 ß values will be virtually those chosen in the first place, and compounds 7 and 8 cannot be considered further.…”

Structural effects in solvolytic reactions. 15. Solvolysis of 1-arylcyclohexyl and 1-arylcyclohex-2-enyl p-nitrobenzoates. Effect of increasing electron demand on the contributions of an allylic double bond in the solvolysis of tertiary cyclohex-2-enyl derivatives

“…The effect of certain homoallylic double bonds in enhancing the rates of solvolysis reactions is well documented.2-3 For example, the rate factor of 1011 observed in anti -7-norbornenyl tosylate (2) is attributed to participation. Rel rate (25°) 1.0 10u the solvolysis of 7-aryl-a«n'-norbornenyl p-nitrobenzoates (3) and the corresponding saturated derivatives (4), Gassman and Ar^XDPNB Ar^__.OPNB b 3 (p+ -2.30) b 4(p+ -527) Fentiman established that the contribution of the electrons to the stabilization of the electron-deficient center increases with increasing electron demand.3 On the other hand, the tool of increasing electron demand establishes the absence of participation in the cyclopent-3-enyl system4 (5) In contrast, the 2-aryl-2-norbornenyl system (6) appears to be a borderline case. 5 With the less electron-demanding substituents (p-CHsO, p-H, P-CF3) the exoxndo rate ratio remains sensibly constant, indicating the absence of participation.…”

“…Rel rate (25°) 1.0 10u the solvolysis of 7-aryl-a«n'-norbornenyl p-nitrobenzoates (3) and the corresponding saturated derivatives (4), Gassman and Ar^XDPNB Ar^__.OPNB b 3 (p+ -2.30) b 4(p+ -527) Fentiman established that the contribution of the electrons to the stabilization of the electron-deficient center increases with increasing electron demand.3 On the other hand, the tool of increasing electron demand establishes the absence of participation in the cyclopent-3-enyl system4 (5) In contrast, the 2-aryl-2-norbornenyl system (6) appears to be a borderline case. 5 With the less electron-demanding substituents (p-CHsO, p-H, P-CF3) the exoxndo rate ratio remains sensibly constant, indicating the absence of participation. However, with the more deactivating 3,5-(CF3)2 substituent, there is observed a modest increase in the exoxndo rate ratio, suggesting the initiation of it participation.…”

“…Steric hindrance to ionization of the endo isomers arising from the cloud of the double bond has been proposed for the relatively high, essentially constant exoxndo rate ratio observed in the 2-aryl-2-norbornenyl derivatives. [5][6] Activation of the double bond by a methyl group (7) converts the system (6) from a borderline case to one that clearly involves participation.7 7 (exo: p+ -3.27) (endo: p+ -4.19) It was of interest to examine the question of whether the tool of increasing electron demand could detect conjugation as well as it participation. To this end we undertook a study of the solvolysis of the 1-arylcyclohexyl (8) and the 1-arylcyclohex-2-enyl (9) p-nitrobenzoates.…”