Exo-endo relative reactivities in three representative U-shaped systems. Exo-endo rate ratio in solvolysis as a steric phenomenon

Abstract

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...

Synthesis of novel epibatidine-related derivatives through 1,3-dipolar cycloaddition of pyridinenitrile oxides

“…The structural assignment to adducts 11a-c was unequivocally established by 1 H-NMR spectroscopy, since the absence of vicinal coupling constants between protons H-1 and H-2 as well as between protons H-3 and H-4 (Scheme 1) is a clear indication of their exo configuration. 49,50 By taking into account the outcome of reactions carried out on olefins structurally related to 8, 51,52 we assigned the exo configuration even to spirocyclic intermediates 9a-c, which were the only isomers isolated in each cycloaddition step. Finally, the pericyclic reaction of pyridinenitrile oxides with olefin 12 yielded bicyclic cycloadducts 13a-c.…”

Epoxidation of substituted norbornenes. Stereochemical aspects and accompanying intramolecular transformations

“…122 ± 125 Earlier, the configuration of the epoxides has been studied by converting them into known norbornanols; thus, it was found that the epoxynorbornane formed contains only a slight amount of the endo-epoxide (6%, 126 0.5% 127 ). Brown et al 127 attributed the exo-attack to the predominant steric hindrance of the endo-area by the norbornene fragment. As model compounds, they chose norbornene (1), cisbicyclo[3.3.0]octene, and endo-5,6-trimethylenonorbornene 10.…”

“…The increase in the steric hindrance of the endo-area of the ring was judged by the increase in the exo-stereoselectivity. 127 Later, the selectivity of the attack and the high reactivity of norbornene have been explained by the influence of hyperconjugation and out-of-plane deformation. Theoretical studies showed that the =C±H bonds in norbornene, unlike those in cyclohexene or bicyclo[2.2.2]octene, are bent by 2 ± 5 8 to the endo-area.…”

Additions to bicyclic olefins. III. Stereochemistry of the epoxidation of norbornene, 7,7-dimethylnorbornene, and related bicyclic olefins. Steric effects in the 7,7-dimethylnorbornyl system

“…(9) T. T. Tidwell, ibid., 92, 1448Tidwell, ibid., 92, (1970. < 10) H. C. Brown andJ. H. Kawakami, ibid., 92, 1990 (1970).…”

“…Epoxidation-Reduction of Norbornene. The epoxidation of 0.94 g (10 mmol) of norbornene with 2.20 g (10 mmol) of 78% m-chloroperbenzoic acid was complete within 20 min, but stirred at room temperature for 1 hr. Reduction with lithium-ethylenediamine and analysis on UCON LB 550X at 100°indicated 99.0% exonorbornanol, 0.3% endo-norbornanol, 0.5% nortricyclanol, and 0.2% 7-norbornanol in 87% yield in order of increasing retention time.…”