An Easy Synthesis of Two Cage Hydrocarbons Dao Cong Dongi Erindale College, University of Toronto, Mississauga, Ontario, Canada L5L 1C6 Interest in the synthesis of strained cage molecules has accelerated greatly during the last two decades ( I ) , but most synthetic procedures leading to cage molecules are either lengthy or in poor yield. For example: in the synthesis of prismane, yield in the last step is only 4-6% (2); the first synthesis of cuhane consisted of fifteen steps in 2.3% overall ,yield (3);and dodecahedrane remains an unknown compound ). desnite reoeated efforts (.4~. We describe here a simple, three-step synthesis of two caCe molecules, hirdcagr hydrucarhon (VIIIJ ( 5 , 6 j and its homoIoyue, the homobirdcage hydrocarhon 1X (71,suitable for an advanced undergraduate laboratory course. The polychlorinated compounds involved in the first two steps have the great advantage of being highly crystalline, high melting solids, so that isolation of intermediates is easy. Additionally, the steric and electronic properties of chlorine are important in directing the course of the reactions (see the questions below), so that every step takes place with very high yield. Having served its function, the chlorine is removed in the final step by highly efficient procedure due to Winstein et al. (8). Because all products are easily purified and are formed in high yields, and because of the unusual structure of birdcage (VIII) and its homologue (IX), students will enjoy their preparation.
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1)iscuasion for Students: Rirdcage hydrocarhon Vlll has been knownsince 1954 (Fjoj. Whilestudying thesulvoly~isot' the brosylate X, Winstein discovered that could rearrange to VIII (eqn. 1). Discuss the rearrangement pathway.
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Reaction Sequence (see structures) The first step is a stereospecific Diels-Alder reaction of hexachloronorbomadiene (I) with cyclopentadiene (11) giving exclusively endo-endo adduct (IV) in 73% yield. Similarly, reaction of I with 1,3-cyclohexadiene (111) gave adduct V in 87% ~ l-r.l ~ - . .",i.-.
Discussion for Students: What are othrr possible isomers of IV and V? Why are they not formed in Dirls-Alder reaction" W r r i c effects of ('CI? rrouD. serondarv orbital effvuts only one in concerted cycloaddition Feaction.) double bond of I react? Why is hydroquinone added in the preparation of V? The second step involves ( 2 ~ 2a)photocyclization (9)of isodrin (IV) and of homoisodrin (V) in acetone to give respectively the chlorinated cage compounds VI and VII in quantitative yield. Discussion for students: What are the differences between thermally allowed (first step) and photochemically allowed reactions (second step)? The last step introduces to students
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' Present address: UNIROYAL Research Laboratories, 120 Huron
St.. Gueloh. NIH 6N3. Ontario. Canada.
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a n efficient and generalized method of dechlorination by lithium, t-butyl alcohol in dry T H F (Winstein's method) in
Journal of Chemical Education
Experimental Safety precaution:It has been known that many chlorinated compounds (e.g., DDT, chlordane, aldrin, dieldrin, isodrin) are toxic to insects, many others (e.g., dioxin, the PCB's) are also highly toxic to people. It is highly recommended to wear a pair of rubber gloves in handling compounds I, IV-VII, and to carry out all reactions in the fume hood.
In a round-bottomed flask equipped with a condenser, a mixture of hexachloronorbornadiene2(I) (3.0g, 10mmol) and freshly distilled cyclopentadiene (2.0 z. 30 mmol) is heated at70DCwith stirring for 2 hr. A further amount of cyclopentadiene (2.0 g) is added anithe solution is heated to 80'C for 2 hr. The operation is repeated. Acetone
(30 ml), then methanol (30 ml) are added to the mixture. Concentration of the solution in uacuo gives a colorless solid (2.7 g, 73%) mp 235-238OC. Recrystallization from ethyl ether affords colorless crystals, mp 240-24Z°C; IR (KBr): 3080,1608 cm-I; NMR (CDC13) 6.03 (t, 2H), 3.40 (t, 2H), 3.0-3.2 (m, 2H), 1.5-2.0 (m, 2H).
Homoisodrin ( V) PRECAUTION: This reaction should be carried out behind a n explosion-proof shield i n the fume hood. A mixture of hexachloronorhornadiene (I) (3.0 g, 10 mmol), 1, 3cyclohexadiene (111)(1.6 g, 20 mmol) and hydroquinone (10 mg) in a pressure bottle is stirred and heated a t 120'-130°C (oil bath temnerature) for 48 hr. The nroduct is crvstallized from ethvl ether as
the solution is poured into ice water (200 g). The product is extracted into ether (3 X 50 ml), and the ether solution is washed with water (2 X 50 ml). After drying over anhydrous sodium sulfate, the ether is removed carefully (under slightly reduced pressure, a t room temperature) leaving hirdcage hydrocarbon as a colorless volatile solid which is sublimed at water pressure on a steam bath to give colorless, waxy crystals (0.59 g, 75%),mp 160-163'C; NMR (CDC13) 6 2.2-2.7 (unresolved m, lOH), 1.80 (d, J = 10 Hz, 2H), 1.40 (d, J = 10 Hz, 2H).
Homobirdcage Hydrocarbon IX [Jnder similar ronditims as described abow photocyclizarion of V (:(.I)g ) in acetone I,i(IOmll gives YU'b uf VII, m.p. 172-1'C; NhlR d 2.94 (1. ~hnmd5 2111. I.Hb tm. . . 4H1.2.61) .. . . IHI. Derhlorinarion uf\'II tI.!)Og,5mmol, i n d r y 1 ' ~ ~ l ~ n ' m wirhf-burylalrohol lr (10 + 5 m l 1 and ltrh~umwire (1.0 + U5g1 gwrs I); osrolorless, waxy crystals t O 60 g, YO?,, mp 9(1-91°C; N M H d 2.0 -2.7 (unresulrrd m, 8Hl. 1.0-1.8 (unresolved m, 8H). ~~~
1.44 (m, 4H).
Hexachlorobirdcage Hydrocarbon Vl A solution of isodrin (IV) (2.5 g) in acetone (300 ml) is irradiated with a Hanovia 450 W medium pressure Hg lamp for 1hr. Evaporation of acetone solution in uacuo gives a colorless solid (2.25 g, 90%) mp 260-265°C. Recrystallization from ethyl ether gives VI as colorless plates (2.15 g, 86%)mp 295-297-C: no absorption in olefinic region; NMR (CDCld 3.15 (broad, s, 6H), 1.70,1.90 (AB type, J = 12 Hz, 2H). Birdcage Hydrocarbon ( VIIO A three-necked, round-bottomed flask is fitted with a nitrogen inlet, a reflux condenser provided with a gas outlet connected to a gasbubbler, and a magnetic stirrer. After being charged with dry THF 30 (ml), hexachlorohirdcage XI (1.63 g, 5 mmol), and t-butyl alcohol (8 ml), the flask is flushed with nitrogen, and thecontentsstirred. To this solution is added lithium wire (1.0 g) cut into small pieces. The solution is maintained at reflux (using oil-bath heating when necessary) for 2 hr, then a further 5 ml of t-butyl alcohol and 0.5 g of lithium are added, and refluningis continued for another 1hr. After cooling,
Acknowledgment T h e author wishes to thank Professor J. T. Edward for helpful suggestions. Grateful acknowledgments are d u e to the reviewers for their kind remarks. Literature Cited
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