Solvolysis substituted 1-phenylethyl

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

Studies of the solvolysis of 1-phenylethyl chloride and its p-substituted derivatives in aqueous trifluorethanol containing azide anion as a potential nucleophile provide details relative to the mechanism of nucleophilic substitution in this system. 

Steady-state approximation competition experiments. The solvolysis of a-phenylethyl bromide (RX) in ethanol-ethoxide ion solution produces the ether, a substitution product. 

We have examined the competing isomerization and solvolysis reactions of 1-4-(methylphenyl)ethyl pentafluorobenzoate with two goals in mind (1) We wanted to use the increased sensitivity of modern analytical methods to extend oxygen-18 scrambling studies to mostly aqueous solutions, where we have obtained extensive data for nucleophilic substitution reactions of 1-phenylethyl derivatives. (2) We were interested in comparing the first-order rate constant for internal return of a carbocation-carboxylate anion pair with the corresponding second-order rate constant for the bimolecular combination of the same carbocation with a carboxylate anion, in order to examine the effect of aqueous solvation of free carboxylate anions on their reactivity toward addition to carbocations. 

The specific rates of solvolysis of benzyl p-toluenesulfonate and nine benzylic-ring-substituted derivatives (324) have been satisfactorily correlated using Aij and Tots scales within the extended Grunwald-Winstein equation. The reactions of Z-phenylethyl X-benzenesulfonates (325) with Y-pyridines (326) in acetonitrile at 60 °C have been studied at high pressures. The results indicated that the mechanism of the reaction moves from a dissociative 5)vr2 to an early-type concerted 5)vr2 with increasing pressure. 

A detailed and elegant study of the SnI solvolysis reactions of several substituted 1-phenylethyl tosylates in 50% aqueous TEE has enabled the rates of (1) separation of the carbocation-ion pair to the free carbocation, (2) internal return with the scrambling of oxygen isotopes in the leaving group, (3) racemization of the chiral substrate that formed the carbocation-ion pair, and (4) attack by solvent to be determined.122... 

For the recombination of a-phenylethyl cations with strong nucleophiles, Richard and Jencks (1984a,b,c) obtained good Y-T correlations with the same r value of 1.15 as observed for the solvolysis (Fig. 35). The p value of -2.7 for the bimolecular substitution reaction of azide ion with 1-phenylethyl derivatives is significantly more positive than the value of p = -5.7 for the solvolysis reaction. This shows that there is a smaller development of positive charge in the transition state for the reaction of azide ion than for solvolysis. It is consistent with a coupled concerted reaction with a transition state in which positive charge development at the benzylic carbon is neutralized by bonding to azide ion. 

It is known that for SnI ewaction (substitution-nucleophilic-unimolecular) [140-141] a key intermediate is a carbocation, therefore the more reactive substrate will be the one that can produce the most stable carbocation. The reactivity of methyl, ethyl, 2-propyl, and 2-methyl-2-propyl tosylates under SnI reaction conditions is inversely proportional to the calculated hydride affinity of the corresponding carbocations. The calculated values were in agreement with the experimental findings which were obtained through solvolysis rate measurement of these tosylates under SnI conditions [142, 143]. Correlation of the cation stability-hydride and affinity-solvolytic rate of the reaction under Sn 1 reaction conditions was observed for the allyl cation (allyl, 3-penten-2-yl, and 2-methyl-3-butene-2-yl cations)[144] and the benzyl cation (benzyl, 1-phenylethyl, and 2-phenyl-2-propyl cations) [145] series. The most reactive substrates were the ones that formed the carbocations with the lowest hydride affinity. 

That C -X heterolysis is less developed in the transition state for thermolysis of esters than alkyl halides is supported by recent studies on substituted 1-phenylethyl chlorides . At 608°K a Hammett reaction constant of—1.36 was observed, this being much smaller than that of—4.95 at 318°K observed for the solvolysis of the same substrates in 80% aqueous acetone. Although part of the difference between these latter two values is attributable to the difference in reaction temperature, the results suggest that C -X bond breaking is less developed in pyrolyses than in solvolytic reactions. 

Stabilization of cationic intermediates by conjugation with an aromatic ring, as in the 1-phenylethyl system, leads to nucleophilic substitution with diminished stereospecificity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. For the ion-pair equilibria... 

Both primary and secondary carbocations with )8-phenyl substituents usually give evidence of aryl participation. For example, isotopically labeled carbons are scrambled to some extent during solvolysis of j8-phenylethyl tosylates, A bridged-ion intermediate or rapidly reversible rearrangement of a primary carbocation could account for the randomization of the label. The extent of label scrambling increases as solvent nucleophilicity decreases. The data are shown in Table 5.19. This trend can be attributed to competition between Sn2 displacement by solvent and ionization with participation of the aryl group. While substitution in more nucleophilic solvents such as ethanol proceeds almost exclusively by direct displacement, the non-nucleophilic solvent trifluoroacetic acid leads to complete randomization of the label. 

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