Photochemical reductions of norbornenes

TetrahedronLettersNo.47, pp. 4763-4763,1967. PergamenRem

Ltd. Printed in Great Britain.

PHOlCCHRMICALREDUCTIONSOF NORRORNgNES R. R. Sauers,W. Schinslci, and M. M. Mason Dep&$ent

of Chemistry,Rutgers, 'lheState University New Brunswick,New Jersey 08903 (Receivedin USA 21 Auguet 1967)

We would like to report aoam preliminaryremltr involvingphotochemically-induced reductionof severalnorbornenes(I) to norbornanes(II) (1). Irradiation(2) of various derivativesof I ( X=H, Y=CN; X=CN, Y=E; X=Y=H, X,Y=H,OH;X,Y=H,CO&&) in dilute ( 0.1-2 $) solutionsof ether, cyclohexane,trimethylpentane, or mixtures of these (3) led to 16-25 $ yields of the correepondingnorbornanes(4). The reductioncould also be effectedby irradiationof acetone solutionsof I with pyrexfilteredlight. For example,norbornene( I, X,Y=H,H)was convertedto norbornane( II, X,Y =H,H) in 25 $ yield under these conditions. Other productsof this reactionwere 2,2'-binorbornyl,2-exo-a etonylnorbornane, norbornenedimers and 2,5-hexanedione(5). h Attempted intramolecularsensitizationof the reduction(6) by irradiationof 2-exo-acetyl5-norbornenein cyclohexaneled to high molecularweight productsonly. Similartreatmentof the corresponding

isomer producedan oxetane,but no reduction(Y).

Free radicalswould appear to be the moat likely intermediatesin these reductions. Very likely, the exited double bonds initiatethe reactionsby abstractionof hydrogen atoms from the solvents(l). The reductionsin acetone probably involvenorbornenetripletsproducedvia energy transfer. Abstractionof hdyrogen from acetonewould lead to norbornylradicalsand acetonyl radicals. In dilute solution,norbornylradicalscould abstracta secondhydrogen atom in ccxpetitionwith the various radical re-combinationreactions(8,g).

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While photochemicallyinducedreductionsof conjugatedand homoconjugatcddouble bonds have been reported (lo), these results and those of Kropp (1) representthe first examplesof reductionsof simple olefins. That this novel reactionhas not been reportedpreviouslyis attributablein part to the low concentrations used in this study. In addition,many of the earlier studieswith norborneneshave dealt with systemswhich were able to undergounimolecular reactions,e.g. intramolecular cycloadditionor rearrangement,in preferenceto hydrogen abstraction(11). Investigations on the scope of these reactionsare continuing. Acknowledwrents.-We are gratefulto the donors of the PetroleumResearchFund, administered by the American Chemical Society,for generousfinancialsupportof this research. M. H. M. gratefullyacknowledgesthe American CyanamidCompany for a Junior EducationAward. R. R. S. gratefullyacknowledgesthe National Institutesof Health for a SpecialFellowship. REFERENCES 1. During the preparationof this manuscript,P. J. Kropp, J. Am. Chem. Sot.,&,

3650(1%7)

reported some closely relatedresults. 2. A 450-w Hanovia inemrsionlamp was used throughout. Vycor or no filterswere used in the direct irradiations. 3.

Typically,6 g. of I (X=H, Y=CN) in 1.2 1 of a 1:l mixture of ether and cyclohexanewas consumed in 17 hrs.

4. Numeroushigher molecularweight productswere formedwhich ware not identified. Norbornene dimers and 2,2'-binorbornyl were identifiedby comparisonsof retentiontimes on two columns (Ucon and Apieson L) with those of authenticsamples. Reductionproductswere isolatedand identifiedby comparisonsof infraredand/or nun spectrawith known samples. 5. D. Scharf and F. Korte, TetrahedronLetters,No. 13, 821(1%3). W. Reusch,J._Org. Chem., 2'& 1882(1%2). D. R. Arnold, D. J. Trecker,and E. B. Whipple, J. Am. Chem. Sot.,a, 25%(1965). 6. H. Morrison,u.,

8&

932(1%5).

7. The structureof this oxetanewill be discussedseparately. Dr. P. J. Kropp has informed us that he has also isolatedthis product. 8. the possibilitythat ketyl radicalsinitiatethe reductionsin acetone cannot be eliminated. 9. Photolysesin diethyl ether led to formationof 2,3-diethoxybutanes.Cf. M. K. N. Ng and G. R. Freeman,e.,

3,

1635(1%5). A free radicalmechanic has been postulatedfor

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light-catalyzed addition8of llcobolr to olefinm. La W. H. Urry, F. W. Stacey,

g. S. Huyoer, and 0. 0. Jweland, E., 10. C. W. Griffin md E. J. O'Connell,m., and Il.I4.Coopor,x., 257(1*);

450(1954).

& _, &

4198(1962);R. L. Cargill, 3. 8. Dmeuood,

88, 1330(1966); I. A. Williama and P. Bladon, TetrahedronLetters,

Proc. Clmm. &,,.J32 XI.Keller, G. P. Rabold, K. Weim, muI T. K. Mokhorjee,--

(1964);D. A. Ben-Efraim,Tetrahedronkttorr, g57(1%7). 11. 6. 0. !khonckand R. Steimeta, s.,

g,

5m(l%3); J. R. E-9

Jo br* G&am.Spc.,Et

3454(1%6);R. C. Cookson,E. Cnmduel1, 1. R. Bill, and J. Hudec, J. Ckem. Sot., 3062 (1%).