In the Laboratory
Photodimerization of Anthracene: A [4ps + 4ps] Photochemical Cycloaddition Gary W. Breton* and Xoua Vang Department of Chemistry, Berry College, Mount Berry, GA 30149–5016
The Diels–Alder cycloaddition is an often exemplified reaction in organic laboratory courses because it is an important synthetic procedure, and it also serves as a basis for discussion of orbital symmetry rules.1 Symmetry rules dictate the plausibility of many organic reactions, including cycloadditions, electrocyclic reactions, and sigmatropic rearrangements. An often discussed—but rarely demonstrated—aspect of orbital symmetry is the reversal of the allowed or forbidden nature of a given thermal process under photochemical conditions (1). Some reactions that are “forbidden” thermally, such as [4πs + 4πs] cycloadditions, are “allowed” under photochemical conditions. Although such processes are a fascinating extension of thermal reactions, they are often avoided in laboratory courses, presumably as a result of the perceived need for costly photochemical equipment (quartz glassware, Hg lamps, etc.). The photodimerization of anthracene (1) is an interesting photochemical [4π s + 4π s] cycloaddition reaction that requires only common laboratory equipment.2 We have performed this reaction in our Advanced Organic Chemistry course in conjunction with the conventional thermally allowed [4πs + 2π s] cycloaddition of maleic anhydride with anthracene. Dimerization and Product Characterization 2
1
3 4
1'
hν 2'
(1)
∆
4' 3'
1
IR spectrum of 2 exhibits saturated C–H stretching bands, which are absent in 1, and a diagnostic pattern for aryl ortho-substitution in the 1600–2000 cm {1 region. Thermolysis Thermolysis of dimer 2 was conducted in a capillary tube in the heated port of a melting-point apparatus. The temperature of the sample was raised to its melting point (~280 °C), and a pale yellow liquid formed. The liquid was maintained at this temperature for one minute to effect the thermolysis. The sample was then removed and cooled. The resulting pale yellow crystalline compound was identified as anthracene by its melting point and by TLC analysis (see Experimental Procedure). Quantitative conversion of 2 to anthracene was observed. Discussion Compound 2, resulting from dimerization across the 9,10-positions of anthracene, is the only product observed in the reaction despite the fact that isomers 3, 4, and 5 might also be formed. The isomers differ in that the structure of 2 contains four intact benzene rings, whereas 3 contains two benzene rings and one naphthalene ring, and structures 4 and 5 contain two naphthalene rings apiece. It has been determined that resonance stabilization per pi electron is higher for a benzene ring than a naphthalene ring (4). Thus, compound 2, which contains the maximum possible number of benzene rings, enjoys the most stable arrangement of aromatic systems, and is formed preferentially.
2
When irradiated, anthracene (1) undergoes a photochemically allowed [4π s+ 4πs ] dimerization to form compound 2 (eq 1, note the numbered positions designating the π–electrons involved in the reaction). 3 The dimerization occurs simply by exposure of a saturated benzene solution of anthracene in Pyrex glassware to sunlight for several days, or by irradiation with a 300-W incandescent lamp for 24 hours (2, 3). Pure crystals of anthracene photodimer (2) form, which are easily isolated via filtration. The dimeric nature of 2 may be discerned in several ways depending upon the availability of instrumentation. The low solubility of 2 in common organic solvents makes it difficult to obtain samples of sufficiently high concentration for 1H NMR analysis unless high-field FT instruments are used. If available, however, the bridgehead C–H signal (absent in the monomer) is readily identified as a singlet at δ 4.53 in CDCl3. The UV spectrum (Fig. 1) is also indicative of 2, since the absorptions at 220 and 272 nm are more similar to those of an ortho-substituted benzene (corresponding absorptions at 212 and 262 nm for o-xylene) than to those of anthracene (absorptions at 250 and 356 nm). Finally, the
*Corresponding author.
1.0 Anthracene (1) Photodimer (2)
A b s o r b 0.5 a n c e
0.0 200
250
300
350
Wavelength (nm) Figure 1. Comparative UV spectra of 1 (4 ×10{ 6 M) and 2 (5 ×10{5 M) as solutions in CH3CN.
JChemEd.chem.wisc.edu • Vol. 75 No. 1 January 1998 • Journal of Chemical Education
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In the Laboratory Formation of 2 via a concerted [4πs + 4πs] cycloaddition is thermally forbidden because conservation of orbital symmetry from reactants to product predicts formation of the product in an electronically excited state (5). Since under photochemical conditions the reactant 1* (a molecule of photochemically excited 1) begins in an excited state, formation of the product in an excited state is “allowed”, and the reaction proceeds (eq 2). Initially, a molecule of 1* combines with a nonexcited molecule of 1 to form a bimolecular complex (termed an “excimer”) (6, 7). Excimer decay via cycloaddition affords 2. hν
1 → 1* 1 + 1* → [1? 1]*
(2)
excimer
[1? 1]* → 2 Photoexcited anthracenes have been reported to add to other dienes (e.g., cyclohexadiene and 2,5-dimethyl-2,4hexadiene) to form [4πs + 4πs] cycloadducts in good yields (8). However, by employing anthracene as both the electrondeficient 4π component and the diene component, the experimental procedure is considerably simplified. Trapping of anthracene as a diene in this manner may be compared to the often illustrated trapping of anthracene by conventional dieneophiles (e.g., maleic anhydride) in thermally allowed [4π s + 2π s] cycloadditions. Heating dimer 2 at its melting point results in monomerization to afford 1 quantitatively. The principle of microscopic reversibility dictates that just as the [4π s + 4πs] cycloaddition is thermally forbidden, so must be the reverse process (9). The anthracene formed upon heating of the photodimer is therefore not accounted for by a simple cycloreversion process. A thermally allowed homolytic bond scission of one of the strained bridgehead–bridgehead bonds to form a transient diradical intermediate followed by rupture of the second bridgehead–bridgehead bond (and rearomatization) is the most probable mechanism for formation of 1 from 2 under these conditions.
cm). The heat from the lamp was sufficient to reflux the solution gently. After 24 h, the lamp was removed and the solution was cooled to room temperature. The crystals that formed were isolated as described above to afford 0.128 g (64% yield) of 2.
Thermolysis A few crystals of 2 were collected in a capillary tube and the temperature of the sample was rapidly raised to the melting point of the crystals in a melting-point apparatus. The resulting pale yellow liquid was maintained at this temperature for 1 min, and then the capillary was removed. Upon cooling, the melt solidified to afford a pale yellow crystalline material. A second melting point determination gave 212 °C (the melting point of anthracene). The capillary was crushed into a vial and its contents dissolved with a few tenths of a milliliter of benzene. TLC analysis (SiO2 plates, 1:1 hexane/ C6H 6 as eluent) of this material versus anthracene (Rf = .71), and 2 (Rf = .45), demonstrated that only anthracene was present. Acknowledgments Acknowledgment is made to generous financial support provided by Berry College. Acknowledgment is also made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Notes 1. A brief survey of commonly used organic chemistry laboratory texts reveals that most include at least one [4πs + 2πs] Diels– Alder cycloaddition experiment. 2.The fluorescence spectral characteristics of 9-cyanoanthracene photodimer have been reported earlier in this Journal (see ref 3). No characterization data for the dimer were included. 3. We thank a referee for pointing out that in the case of the dimerization of anthracene, it would be difficult to distinguish a [4πs + 4πs] pathway from either a [4πs + 8πs], or a [8πs + 8πs ] pathway. However, for simplicity, we classify the reaction as a [4πs + 4πs] cycloaddition in this paper.
Experimental Procedure
Preparation A solution of anthracene in benzene was prepared by swirling 200 mg (1.12 mmol) of anthracene with 30 mL of dry benzene until most of the solid dissolved. The mixture was filtered through a plug of glass wool into a clean, dry 50-mL Erlenmeyer flask (Pyrex). The flask was sealed, set on a window sill, and exposed to sunlight for 1 week. The resulting fine, starlike crystals were isolated via vacuum filtration, washed with a few milliliters of ice-cold benzene, and dried under vacuum to afford 0.111 g (56% yield) of 2, mp 265–275 °C (rapid heating) (lit. mp 270–280 °C [rapid heating]) (10); IR (KBr) cm{1 3016, 2924, 1473, 1454, 765; 1H NMR (250 MHz, CDCl3 ) δ 6.87 (m, 8 H), 6.80 (m, 8 H), 4.53 (s, 4 H). Alternatively, a solution of anthracene in benzene (prepared as described above) in a 50-mL round-bottom flask fitted with a reflux condenser was irradiated with a 300-W clear incandescent lamp positioned close to it (approx. 0.5
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Literature Cited 1. Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper and Row: New York, 1987; Chapter 10. 2. Greene, F. D.; Misrock, S. L.; Wolfe, J. R. J. Am. Chem. Soc. 1955, 77, 3852–3855. 3. Ebeid, El-Z. M. J. Chem. Educ. 1985, 62, 164–165. 4. Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, 3rd ed., Part A; Plenum: New York, 1993; pp 521–531. 5. Lowry, T. H.; Richardson, K. S. Op. cit.; pp 875–879 and 891– 894. 6. For a discussion of the nature and reactivity of excimers, see Lowry, T. H.; Richardson, K. S. Op. cit.; pp 1000–1009. 7. Yang, N. C.; Shou, H.; Wang, T.; Masnovi, J. J. Am. Chem. Soc. 1980, 102, 6653–6654. 8. Yang, N. C.; Libman, J. J. Am. Chem. Soc. 1972, 94, 1405–1406. 9. Carey, F. A.; Sundberg, R. J. Op. cit.; p 193. 10. Dictionary of Organic Compounds, 5th ed.; Buckingham, J., Ed.; Chapman and Hall: New York, 1982; p 383.
Journal of Chemical Education • Vol. 75 No. 1 January 1998 • JChemEd.chem.wisc.edu
