Isotopic scrambling, solvolysis

A detailed study of the solvolysis of L has suggested the following mechanism, with the reactivity of the intermediate M being comparable to that of L. Evidence for the existence of steps ki and k 2 was obtained fiom isotopic scrambling in the sulfonate M when it was separately solvolyzed and by detailed kinetic analysis. Derive a rate expression which correctly describes the non-first-order kinetics for the solvolysis of L. 

Compounds labeled with multiple isotopes of the same atom provide the opportunity to observe virtual isomerization reactions that go undetected in conventional studies.There have been relatively few modern studies of these virtual isomerization reactions," " because the reactions themselves are not common and because of difficulties in drawing general conclusions from the observation of the scrambling of isotopic label during solvolysis (Scheme 8). 

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... 

Isotopically labeled carbons are scrambled to some extent during solvolysis of )8-phenylethyl tosylates. A bridged-ion intermediate or rapid rearrangement of a primary carbonium ion could account for the rearrangement. The extent of rearrangement increases as solvent nucleophilicity decreases. This increase is attributed... 

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. 

The low values obtained for the secondary deuterium kinetic isotope effects in the comparative solvolyses of (141) and (142) in formic acid (kn/ko = 1.154 at 29.9 °C) and in aqueous ethanol kji/ko = 1.116 at 64.4 °C) are regarded as evidence for extensive bridging (143) in the solvolytic transition state the deuterium atom at the C-1 bridgehead in (142) obviates the scrambling problem.As the contribution from the P-deuterium is unknown, then assuming the effects of a- and -deuterium are cumulative the tt-kinetic isotope effect is further decreased from the value expected for reaction occurring without nucleophilic assistance. The products of solvolysis of ent-methyl-12p-p-tolylsulphonyloxybeyeran-19-oate, which is a related bicyclo [3,2,1 ] -octan-2-yl system, have been discussed in terms of intermediate carbocation stabilities and lifetimes in the solvent employed. ... 

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