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to our colleagues, Profs. D. Locke (Queens), G. Quigley, and M. Blumenstein (Hunter) for their invaluable technical assistance.Supplementary Material Available: X-ray crystallography material for urethane 15 (10 pages). Ordering information is given on any current masthead page.

Solvolysis as a Steric PhenomenonSir : We wish to report that in three representative Ushaped systems examined, cis-bicyclo[3.3.0]octane, norbornane, and endo-5,6-trimethylenenorbornane, noncarbonium ion reactions exhibit preferences for reaction at the exo face that are comparable in magnitude to the exo:endo rate ratios observed in the solvolyses of the tertiary methyl p-nitrobenzoates. This common pattern of reactivity suggests that steric factors must contribute to the greater reactivity from the exo direction exhibited by all reactions, carbonium ion as well as noncarbonium ion.It is well known thst exo-norbornyl derivatives undergo solvolysis at rates that are considerably greater than those of the corresponding endo isomers. Moreover, the resulting ion or ion pair undergoes substitution to give the exo isomer predominantly. This preference for exo reactivity and exo attack has been attributed to a participation leading to the formation of a a-bridged cation.'It has become evident in recent years that this exo preference is not restricted to carbonium ion reactions. Many reactions not involving carbonium ions exhibit a similar preference for exo attack, although the magnitude of the effect may vary considerably from reaction to rea~tion.~,3 With data now available for the solvolysis of the tertiary methylp-nitrobenzoates of the cisbicyclo[3.3.0]octane4 (X), norbornane5 (XI), and endo-5,6-trirnethylenenorb~rnane~ (XII) systems, we decided to examine a number of typical reactions of these systems in order to ascertain whether the parallelism previously noted for norbornane extends to these additional U-shaped structures.OPNB the 4-5 bond is in a very poor position geometrically to participate. Participation would involve resonance with a very highly strained tetracyclic cation. No rearrangements to this tetracyclic structure are observed in the solvolysis of the tertiary (Table 11) or the secondary derivatives, The high exo : endo ratio arises not as a result of any very high rate for the exo isomer, compared to I, but because of a greatly reduced rate for the endo. Table 11. Products in the Solvolysis of VIII-OPNB ~ VIII-X-OPNB 100 0%. 24% 22% 51% VIII-N-OPNB 125 1 % 19% 19% 44% a <0.2%, the limit of detectability.It follows that the exo:endo rate ratio of 4300 for the VIII and of 19 for the I X derivatives must be primarily steric in origin.It should be noted that VIII-X-OPNB and IX-X-OPNB exhibit rates that are very similar to that of I. This may be fortuitous. It was pointed out earlier that decreased flexibility in the ring might be expected to result in a moderate decrease in rate.6 Possibly relief of steric strain, engendered by the rotation of the methyl group out of the crowded endo environment, provides a driving force that compensates for the anticipated effect of reduced flexibility.It follows from the similarity in the steric requirements of the methyl and acyloxy groups' that the same strains must be present in the endo isomers. The slow rate of the endo isomers can onIy mean that, in contrast...

The generally accepted position that the exo:endo rate ratio in the solvolysis of 2-norbornyl derivatives is the result of a ka process for exo and a k , process for endo was subjected to critical examination by investigating the exo:endo rate ratio as a function of solvent. These mesylates were synthesized, along with several representative derivatives (isopropyl, cyclopentyl, and cyclohexyl), and solvolyzed in pure water and in 60% aqueous acetone. The tosylates were solvolyzed in methanol and in formic acid. These data, as well as other data, including those for solvolysis in trifluoroacetic acid and hexafluoroisopropyl alcohol, were then subjected to critical examination. Over the entire range of solvents, log k,,, plots linearly against log kendo. This is not consistent with the usual interpretation that the solvolysis of the exo isomer involves a k A process, insensitive to the nucleophilicity of the solvent, whereas the solvolysis of the endo isomer involves a k , process, sensitive to the nucleophilicity of the solvent. The solvolysis of 2-adamantyl tosylate has been proposed as a representative k , process. Both e m -and endonorbornyl tosylates are nicely correlated with the rates of solvolysis of 2-adamantyl tosylates over the exceptionally wide range of solvents here examined. On the other hand, isopropyl, cyclopentyl, and cyclohexyl derivatives, postulated to involve k , processes, are not so correlated. Clearly it is necessary to reconsider the previous interpretation of the solvolysis of endo-norbornyl derivatives as involving a k , process and the exo:endo rate ratio as arising from the operation of k a / k , processes. The data are in better agreement with the solvolyses of both e m -and endo-norbornyl proceeding through processes that are essentially kc.0002-7863 I 7 8 I1 500-3143$01 .OO/O