Preparative Solvolysis of 2-OPNB. in 25 ml of 50x aqueous acetone containing 1.5 equiv of 2,6lutidine was sealed in 2 test tubes under nitrogen and heated for 24 hr at 125'. The cooled solution was concentrated and the product (15 mg, 44%,1) isolated by ether extraction was identified as %OH by nmr comparison. Acknowledgment. We are grateful for financial support from Eli Lilly and Company through an unrestricted research grant, the National Institutes of Health, and the A. P. Sloan Foundation. measured (room temperature) at 272 mp.28 The absorbance generally decreased by about 6 O z during the acetolysis. For runs at higher temperatures, aliquots of tosylate solutions similarly prepared were heated individually in sealed ampoules. The rate constants were determined graphically.B. Hydrolysis. The p-nitrobenzoates were hydrolyzed in 50% aqueous acetone (volume per cent before mixing) and the rates measured as previously described.5bThe kinetic data are summarized in Table 111.Abstract: Various cyclic additions to norbornene (1) and 7,7-dimethylnorbornene (2) have been studied to determine the effects of 7,7-dimethyl substituents on the stereochemistry and rates of additions to norbornyl systems. The 7,7-dimethyls exerted very large steric hindrance to exo attack, equal to or even greater than the hindrance to endo attack arising from the endo-5,6-hydrogen atoms. Certain reactions, such as silver ion complexation, addition of nitrosyl chloride, and addition of dichlorocarbene, proceed quite satisfactorily with 1, but fail with 2, presumably because the attack of the adding moiety is severely hindered by both the 7,7-dimethyl groups and the endo-5,6hydrogen atoms. Comparative rate studies of exo attack of 1 us. 2 indicate substantial rate retardations for reactions involving exo addition via cyclic processes in such reactions as epoxidation (lOOO), hydroboration with 9-BBN (480), diimide reduction (950), and addition of benzenesulfenyl chloride (1 820), whereas the retardation factor is smaller for additions not involving cyclic species, such as the free radical reaction of thiophenol (30). The importance of the steric influence of 7,7-dimethyl substituents is also revealed by the stereochemistry of addition. For all known additions to 1, the adding moieties come in preferentially from the exo side. Even the introduction of 7,7-dimethyl substituents does not reverse this exo stereoselectivity for additions proceeding through noncyclic processes. Thus, the two-stage addition of thiophenolis 99.5% exo with 1, and 95% exo with 2. HOWever, for additions involving three-and four-membered ring cyclic processes, the preference for exo reaction is not retained in 2, presumably because of the large steric crowding by the 7,7-dimethyl groups. For instance, hydro-Brown, Kawakami, Liu / Cyclic Additions to Norbornenss 9 10

A systematic study of the stereochemistry of epoxidation of norbornene, 7,7-dimethylnorbornene, and related bicyclic olefins was undertaken in order to establish more precisely the importance of steric effects in controlling the direction of concerted additions to such systems. The epoxidation of norbornene proceeds almost exclusively exo, 99%, whereas the corresponding epoxidation of 7,7-dimethylnorbornene takes place preferentially from the opposite direction, 88-94% endo. 1 -Methylnorbornene, 2-methylnorbornene, and 1,7,7-trimethylnorbornene exhibit the same steric pattern of reaction. Similarly, epoxidation of the exocyclic double bond in 2-methy-

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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...

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