A Valence Isomer Trapping Procedure for Introductory Organic Laboratory Synthesis of a Homobarrelene Derivative David W. Kurtz Ohio Northern University. Ada, OH 45810 Richard P. Johnsar University of New Hampshire, Durham, NH 03824
In spite of its conceptual importance and the large volume of research devoted to i t over the last 30 years, valence isomerism is scantily presented in most organic texts and rarely presented a t all in laboratory courses. This may he due to the scarcity of illustrative experiments feasible for the undergraduate laboratory. We describe here a procedure that has been in use a t Ohio Northern for several years in which norcaradiene is trapped out of its cycloheptatriene valence isomer in a Diels-Alder reaction with maleic anhydride. The procedure illustrates not only valence isomerization but also the trapping of a reactive intermediate and retrosynthetic analysis applied t o the Diels-Alder reaction. Background Dlscusslon Valence isomerism is usually defmed as a thermal equilibrium between two isomeric species of different bond lengths and angles achieved without passing through an electronic excited state or intermediate loss of any atom. Species displaying such a thermal equilibrium are termed fluctional molecules (I). Well-known cases of fluctional hehavior include cycloheptatriene-norcaradiene (eq I), cyclooctatetraene-bicyclo]4.2.0]octa-2,4,7-triene (eq 2), and fluctionally degenerate molecules such as semihullvalene (2)and bullvalene (3) (eqs 3 and 4 below).
cyclohepatriene
norcaradiene
Although the photoconversion of benzene to benzvalene and Dewar benzene is sometimes termed valence isomerization, being nonthermal, i t is not a case of fluctional hehavior. Nor are ket-no1 tautomerizations considered valence isomerizations, since their equilibria are generally mediated through intermolecular proton shifts. However, many common sigmatropic shifts such,as the Claisen, Cope, and vinylcyclopropane rearrangements are essentially valence isomerizations with large enthalpy differences between the isomers. The rearrangements and cycloadditions of cyclohepta-
trienes have been scrutinized for over a century and continue to be investiaated (41. l'he cycloheptatriene system seems almost profligate in the variety OF its reactibns; the ene reaction (5a), 2+2 (5b), 2+4 (5c), and even 4+6 cycloadditions ( 5 d ) are documented. l'he earliest investigations of cycloheptatriene reactions by Buchner revealed difficulties in seoaratine and identifving the reaction nroducts of cvclohep;triene &h diazoac&icVesters.In 193'9 the ~iels-Alder reaction between cycloheptatriene and maleic anhydride was first studied, but the major adduct defied structure elucidation until 1953,when Alder and Jacobs demonstrated that it contained a cyclopropane ring (61. Ry that time Doerina (7) and Corey ( 8 ) had formulated the hvpothesis that cfcloheptatriene is in rapid equilihrium w i t h bicyclo[4.1.0]hepta-2,4-diene (norcaradiene, eq 1) and used this hypothesis to explain the presence of a three-membered ring in many of the cycloaddition products of cycloheptatriene. The most thoroughly investigated system displaying this equilihrium is the parent hydrocarbon, cycloheptatriene. Cvclohe~tatrieneDre~onderatesin the eauilibrium. with norcaradiene estiiateh to be less thano.l%of the mixture a t ambient temperature (9). None of the numerous direct mettroscopic attempts to detect norcaradiene in the equilibrium mixture have succeeded: thus, although norcaradiene has been photolytically produced and dirertly observed in iowtemperature argon matrix (101, its experimental beat of formagon and e ~ &percentage in equilibrium with cycloheptatriene remain conjectural. Studv of 7-substituted cvclohentatrienes has shown that placingelectron-withdrawihgsuhstituents on the sp3 hybrid carbon shifts the eauilibrium toward the norcaradiene tautomer (Il),
In the cases of 7,7-dicyano-, 7.7-dicarboethoxy, and a few other systems, the shift toward the norcaradiene form is so complete that no spectroscopic evidence of the cycloheptatriene tautomer is found. The effect of electron withdrawers a t C, is to stabilize the norcaradiene form by lowering electron density in the HOMO of the cyclopropane ring, which is antibonding hetween C1 and C,j (12). In the absence of such electron withdrawers the cycloheptatriene form is estimated t o he 4 4 kcal/mol more stable than the norcaradiene form. In view of the small amount of norcaradiene tautomer present in unsuhstituted cycloheptatriene, i t is somewhat surorisine that most dienoohiles eive Diels-Alder adducts on& with-the norcaradiene k m ) & r . Indeed, the only renctants~resentlvknown toeiveanv"normal"2t4 adduct with the open form of cyclohebtatrieie are nitroso benzene (13) and maleic anhydride (14). Volume 66 Number 10 October 1989
873
their methyl esten and chromatography of these on QF-1at 2M)*C. Crystals of adduct (50 mg) are refluxed 30 min in 5.0 mL of methanol to which a catalytic amount of tosic acid has been added; the mixture poured into 25 mL of 3% NaHC03 solution and extracted with diethyl ether. The ether extract is dried and analyzed by GC (10 ft, 10%QF-1.18 mllmin). Under these conditions tA3) = 5 min 40 s; t,(4) = 4 min 30 s. Discussion
me Diels-Alder reaction W e e n cycloheptatrieneand maleic anhydride. A thorough reinvestigation of the DielgAlder reaction hetween cycloheptatriene and maleic anhydride established that three 2+4 adducts are formed (see figure) (14). The major product, 3, composing slightly more than 90% of the product mixture, is the adduct from endo addition of maleic anhydride to norcaradiene in which the cyclopropane ring is syn to the double bond. Next most plentiful is 4, the exo adduct of norcaradiene with its cyclopropane ring again syn t o the alkene; 4 is about 9% of the total product. The adduct 5 from maleic anhydride and open cycloheptatriene is 0 . S 1% of the product mixture; this minor product might be called the "expected" or "normal" Diels-Alder adduct. Experimental non-6-enendo,endoPreparation of anti-Tricyc10[3.2.2.0~.~] 8,9dicarboxyiic Anhydride Since cyclaheptotriene is toxic and flammable this procedure should he performed i n an efficient fume hood.
Cycloheptatriene (15 mmol. 1.38 g. 1.55 mL) (Caution: toxic, hood!l and finelv oowdered mabic anhvdride (14 mmol. 1.37 a ) are refluxid in 10 of mixed xylenes & a 50-mI. round-bottomed flask. After 2 h of reflux 10 drops of the reaction mixture are removed and set aside for derivatization and TLC analysis. The eondensor is reset for distillation of the pot, and the mixture is distilled until 6 mL of the xylene has heen removed. The distillation mixture is then poured into a 50-mL Erlenmeyer far crystallization, the reaction vessel rinsed with 3.0 mL of ethyl acetate, and the rinsings added to the Erlenmeyer. Addition to 10 mL of hexane followed by chilling affords largeneedles of product as creamcolored 3, mp98.S 99.5 OC (lit 101 "C) (14). These crystals still contain 4% of the exo adduct 4, which can he removed by further recrystallization from hexane-ethylacetate. TLC Analysis of the Reaction Mixture As the aylene is distilling,the 10-dropsample is prepared for TLC examination. Aniline (1drop, 50-70 mg) is added to the beaker, and the beaker is heated to boiling in a hot plate for 3 min, then cooled to
room temperature. This mixture of 3,4, and 5 as their N-phenylimides is now diluted with 5.0 mL of pentane and spotted on phosphor-impregnated silica gel TLC sheet. Development with ethyl acetate and viewing under a UV lamp shows the derivatized adducts: 3,RI = 0.41; 4, Rf = 0.32; 5,Rf = 0.27 (very faint).
GC Analysis of the Crystals
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Gas chromatoeraohv .. . an a OF-1 . ..elass column at 225 OC seomates the adduch satisfacudy; however, there is some iuumerization on metal columns.A preferable procedure ir converdion d l ,4, and 5 lo ~
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874
Journal of Chemical Education
The procedure is highly reliable and easy to perform, but is challeneing for students to understand. A portion of the reflux per~odshouldbe used to bring students into discussion groups in which they suggest answers to questions such as these: (1) Given the structure of the maior adduct. what diene a c t u h y reacted with the maleic anhydride? (2j How does this diene (norcaradiene) arise from the cvcloheotatriene reactant? (3) How might one check whether norc&adiene or cycloheptatriene is dominant? (4) Given that cycloheptatriene is the major isomer, what can be said about the ability of maleic anhydride to "trap" the minor isomer? While other approaches might be suggested, the retrosynthetic analvsis irn~licitin question 1 seems t o be the crucial step in b;inging&udents t o grasp this conceptually complex procedure. Thin-layer detection of the reaction products is complicated by their lack of any chromophore that absorbs above 2200 A. Accordinelv the nroduct mixture is converted to Nt o ckromatography. We have suggsted phenylimides that analvsis of the reaction mixture be done bv thin-laver and anal& of the crystalline product he done by GC. ~ c k ally either method can be used a t any point in the reaction. The product, recrystallized as directed above, gives a clean 60-mHz PMR spectrum; unfortunately the cyclopropane hydrogens appear as three non-first-order multiplets. This rules out asking students to infer the adduct structure from PMR data. The 100-mHz P M R is first-order and is discussed by Itoh e t al. (14). While many good DielgAlder sophomore lab procedures already exist, we are unaware of any others that in a single period introduce valence isomerization and the trapping of a reactive species. Furthermore, this procedure is inexpensive, since onlv small quantities of cvclohe~tatrieneare used. ~echnicai-grade~ ~ > l o h e ~ t a t r i e n ~good ~ i v results es without further purification if stored under refrigeration. Finally, it is a lab in which discussion and analysis during the procedure are crucial, since one really is "making a white powder and proving that i t is [not] what you expect" (15). Literature Cited
h r , n g . w v E . ~ ~ t wh .R ~s,rohedronI%J.I~.-I'I. 4 tat Hurhncr. E:Cunws.'l' Her Is-. IR.2377. tb, Harm. W Rurhner. . F Arr 1911. Jl.'ld.'.tcl Lciltch..l..Sprnlal~n~k, (: Brr 1986. 119. 1610 5 (alC~nnamm..lI .H'o#r-.K .IOr,, Chrm 1961.M.~II.ll~~Kevl.M..Okamnvl.Y.: V h ~ k s m o l o . 7. M . l u a . l ' Bd1. Chem S.r Jdpnn 197(1.51.1161.tr# K a n ~ m a l i l l . K MI r ~ t r . SFbku.h~nra. . 'i.Oraws. E.J Am. ('hem. Sor. 1981.103.5211: ~ d Hl~acr. , 1 ) I1 Howden. \I F I1 .I Ch.m S r P.rk.n Trum 119XO.C-2. 6 . la1 Knhlcr. B I':l'#uhlcr M . I'ntrcr. H :lhrrnosrn. H J Am Chsm l r 1929.61.
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