Additions to bicyclic olefins. V. Effect of 7,7-dimethyl substituents on the stereochemistry and rates of cyclic additions to norbornenes

Abstract

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

Torsional control of stereoselectivities in electrophilic additions and cycloadditions to alkenes

“…The control of π-facial selectivity in organic reactions has been a subject of both experimental 1 and theoretical studies. 2 The origin of π-facial selectivity has been attributed to several factors such as electrostatics, 3 hyperconjugation, 4 steric 5 and torsional effects. 6 In the absence of charged or highly polar groups, torsional effects can have a remarkably large influence on stereoselectivity.…”

Stereoretentive Copper(II)‐Catalyzed Ritter Reactions of Secondary Cycloalkanols

“…[25] Ritter reactions of bornyl and isobornyl chloride in the presence of SbCl 5 in acetonitrile have been shown to give the same exo -amide product [26] suggesting a common carbocation intermediate and a preference for exo -attack. [27] In the case of isoborneol (entry 4), the present method gave only the stereoretentive amide ( exo -product) product 15 albeit in low yield (21%). However, the main byproduct is the chloride but, here as well, we observe a preference for stereoretention (i.e.…”

Additions to bicyclic olefins. IX. Electrophilic addition of acetic acid and perdeuterioacetic acid to norbornene and 7,7-dimethylnorbornene. Remarkable stereoselectivity for the addition process with evidence for the capture of the 2-norbornyl cation in an unsymmetrical (classical) state

“…Clearly, the 7,7-dimethyl substituents do not exert a major influence on the stereochemistry of the addition process, such as is generally observed for cyclic addition processes. 11,21 Competitive experiments revealed knorbornenel k7.7-dimethyInorbornene to be 2.8. The relative rates of exo addition are very much higher, in the range of 480-1820, for representative cyclic additions.…”