In the Laboratory
A Simple 2+2 Photocycloaddition That Poses an Interesting NMR Problem A New Photochemistry Experiment John T. Magner,1 Matthias Selke, Arlene A. Russell, Orville L. Chapman Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA 90024-1569
A theoretical discussion of the thermal 4+2 Diels–Alder cycloaddition and the photochemical 4+4 and 2+2 cycloaddition reactions can, ideally, be supported by having students actually perform these reactions in the laboratory. The 2+2 photocycloaddition, a theoretically important reaction of great synthetic utility, poses experimental difficulties that make it difficult to adapt to an undergraduate laboratory. Specifically, this reaction often requires expensive quartz Hanovia photoreactors, long periods of irradiation, and short-wavelength UV light. We could only find mention of one 2+2 photocycloaddition reaction designed for the undergraduate instructional laboratory2 (1). The 2+2 photocycloaddition reaction shown in Figure 1 circumvents these difficulties. The only specialized piece of equipment is a simple 300-nm Rayonet photoreactor.3 The irradiation is carried out in hexane or chloroform in a regular borosilicate glass test tube4, and a nitrogen or argon source is used to degas the solution. The reaction goes to completion in one 3-h lab period. The irradiation of 2,3-dimethyl-1,3-butadiene with βnitrostyrene produces two isomeric cyclobutane products (2), which can be isolated by simple evaporation of the solvent and excess diene. The yellow liquid product is obtained cleanly enough to characterize it by 1H NMR. The 1H NMR spectrum of the two chiral products, in fact, poses a challenging and instructive NMR problem and demonstrates a number of important concepts in NMR. Additional exercises such as the assignment of the 1 Author to whom correspondence should be addressed: John Magner, Montgomery College, Department of Chemistry, 3200 Hwy 242W, College Park Drive, Conroe, TX 77384. 2 A search of ERIC (Education Article and Report Citations, a database on the Stanford system) yielded 34 citations for instructional experiments related to photochemistry. Only one of these citations involved a 2+2 photocycloaddition experiment. 3 A Rayonet photoreactor consists of a circular well containing a series of UV tubes around the perimeter of the well and a cooling fan at the bottom of the well. The sample to be irradiated is suspended in the well and the top of the well is then covered. The UV tubes used in this experiment were broad-band germicidal fluorescent tubes with a λmax= 300 nm. A similar photoreactor can be easily constructed with a series of these 300-nm tubes and aluminum foil. 4 Photochemical experiments, including many 2+2 photocycloadditions, often require expensive quartz photoreactors because normal glass cannot be counted upon to transmit light of wavelength below 300 nm, which is often necessary in such experiments. In this experiment normal borosilicate glass test tubes can be used because the λ max for the n,π* transition for β-nitrostyrene is 310 nm, which is above the cutoff for this glass.
854
hv NO2
Ph
NO2
+
+ Ph
1
NO2
2
Figure 1. The 2+2 photocycloaddition of β-nitrostyrene and 2,3dimethyl-1,3-butadiene.
protons of the products and the determination of the regiochemical outcome of the reaction from the NMR are logically incorporated into the experiment. Both β-nitrostyrene and 2,3-dimethyl-1,3-butadiene are reasonably priced and commercially available. βNitrostyrene can also be prepared by the condensation of nitromethane and benzaldehyde (3). The experiment was performed by chemistry and biochemistry majors at the end of their first quarter of organic laboratory at UCLA. The students worked in groups of three. The experiment was completed in one 4-h period, except for the NMR analyses, which were performed after the laboratory by the TA on a Brucker 200-MHz NMR. All students obtained spectra that were satisfactory for analysis, though in a few cases the removal of solvent was not quite complete. Experimental β-Nitrostyrene (10 mg, 0.067 mmol) and 2,3-dimethyl1,3-butadiene (62 mg, 0.75 mmol) were dissolved in 2.0 mL of hexane in a borosilicate test tube. Argon or N2 was bubbled through a needle into the solution for 10 min and then the test tube was immediately sealed with a septum to maintain an oxygen-free environment. The test tube was inserted into the top of a Rayonet photoreactor (equipped with 300-nm bulbs) to a depth such that only the test tube (and not the septum) was exposed to the light. The solution was then irradiated for 3 h. (Caution: do not look at the light from the photoreactor!) Following the irradiation, the solvent and excess 2,3dimethyl-1,3-butadiene were removed under reduced pressure. Alternatively, the solvent and excess diene may be effectively removed by warming to 40 °C under a stream of nitrogen for about a minute. A clear, oily, yellow liquid remains in the tube. After dissolving this material in CDCl3, proton NMR shows the crude product to consist of a mixture of two isomers of the 2+2 photoadduct. Proton NMR Interpretation The different protons on isomer 1 in Figure 2 are iden-
Journal of Chemical Education • Vol. 73 No. 9 September 1996
In the Laboratory
A1
(a)
G1
H1
Isomer
NO2 E1 C1 F1
A2
B1
Ph D1
1 (minor, 44%)
G2
H2
F2
B2
Ph C2
NO2 E2
D2
2 (major, 56%)
(b)
(c)
Figure 2. (a) Full 1H NMR spectrum (360 MHz, CDCl3), with expansions, of the crude product mixture consisting of the two isomeric products. (b) Expansion of the methyl signals. (c) Expansion of the methylene and methyne signals.
tified by letters A through H with the subscript 1, and the corresponding protons on isomer 2 are denoted by the same letters with the subscript 2. The protons on the allylic methyl groups of both products can be distinguished from the protons on the nonallylic methyl groups by the small allylic splitting by the vinylic protons. The allylic methyl protons of isomer 1 resonate at δ 1.79, standard for an allylic methyl group, whereas the allylic methyl protons of the second isomer resonate at δ 1.07, which is unusually far upfield owing to the shielding effect by the adjacent phenyl ring. The protons on the nonallylic methyl group of isomer 1 are likewise shielded by the adjacent phenyl ring and resonate at δ 1.04. This again is substantially upfield from the corresponding protons on isomer 2, which are on the opposite side of the cyclobutane ring to the phenyl ring and resonate at δ 1.52. The shielding by the phenyl ring experienced by the protons on the methyl groups gives conclusive evidence of the regiochemical outcome of the reaction. These methyl groups must be alpha to the phenyl ring in the cyclobutane. The other possible regioisomer that could have been produced in the cycloaddition would have the nitro group alpha to the carbon bearing the methyl groups, and the shielding, therefore, would not have been observed. The integrations of the methyl protons of isomer 2 are approximately 20% greater than the integrations of the corresponding protons of isomer 1. This indicates that 2 is the major isomer produced in the reaction. Another striking feature of this 1H NMR spectrum is that the vinylic protons of both isomers not only resonate at the same frequency, but they also show virtually no splitting. The coupling constant (J) for nonequivalent, geminal vinylic protons normally ranges from –3 to +3 Hz. In this spectrum J is approximately 0 for the vinylic protons of both isomers. The protons alpha to the phenyl ring appear as doublets, split by the proton alpha to the nitro group, at δ 3.88 and δ 4.25. The doublet at δ 3.88 may be assigned to isomer 2, since its integration is greater than the doublet at δ 4.25. The protons alpha to the nitro group of both isomers appear as overlapping multiplets at δ 5.23. Finally, the four separate multiplets at δ 2.38, δ 2.50, δ 2.74, and δ 2.92 are assigned to the methylene protons. The multiplets at δ 2.50 and δ 2.92 can be assigned to isomer 2 once again because their integrations are greater than the multiplets at δ 2.38 and δ 2.74. We speculate that the lower field multiplet at δ 2.92 corresponds to the pro-
Vol. 73 No. 9 September 1996 • Journal of Chemical Education
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In the Laboratory
ton trans to the vicinal nitro group of isomer 2 because the inductive deshielding by the nitro group experienced by this proton should be greater than the deshielding experienced by the proton that is cis to the nitro group. By the same reasoning we assign the multiplet at δ 2.74 to the proton trans to the nitro group of isomer 1. By default, therefore, the multiplet at δ 2.38 is assigned to the methylene proton cis to the nitro group of isomer 1and the multiplet at δ 2.50 is assigned to the methylene proton cis to the nitro group of isomer 2. Discussion The 2+2 photocycloaddition of β-nitrostyrene with 2,3dimethyl-1,3-butadiene cleanly generates two isomeric cyclobutane products. The reaction probably proceeds via a biradical intermediate generated by the addition of the triplet n,π* excited nitrostyrene to the diene. The resulting formation of the most stable biradical, in which one radical is allylic and the other benzylic, accounts for the observed regiochemistry. Cyclobutane formation could also result from a concerted cycloaddition of nitrostyrene in an excited singlet state to the diene (2, 4, 5). The high yield, based on nitrostyrene, and clean production of the 2+2 photoadducts may be explained by the
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use of a large excess of the diene, which drives the reaction to completion. Following the reaction, the volatile 2,3dimethyl-1,3-butadiene is easily removed leaving the pure products. Interestingly, cis-β-nitrostyrene, which is produced by the photoisomerization of trans-β-nitrostyrene under the reaction conditions, seems to be unreactive toward the diene. Acknowledgment We thank all the students in Chemistry 132BLH during the winter quarter of 1994 at UCLA for their part in performing this experiment. Particularly we appreciate the extra effort of Lisa Rosenberg and Carolyn Alexander. We also acknowledge E. D. Hoganson, who originally investigated this 2+2 photocycloaddition reaction and characterized the products. Literature Cited 1. Nash, E. G. J.Chem. Educ. 1974, 51, 619. 2. Hoganson, E. D. Ph.D. Thesis, Iowa State University, 1965. 3. Worrall, D. E. Organic Syntheses Collective Volume 1, 2nd ed.; Gilman, H.; Blatt, A. H., Eds.; Wiley: New York, 1956; Vol. 1, p 413. 4. Fleming, I. Frontier Orbitals and Organic Chemical Reactions; Wiley: New York, 1976. 5. Lowry, T. H.; Richardson, K. S. Mechamism and Theory in Organic Chemistry; Harper and Row: New York, 1987.
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