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Photochemical Dimerization of Dibenzylideneacetone A Convenient Exercise in [2+2] Cycloaddition Using Chemical Ionization Mass Spectrometry G. Nageswara Rao,* Chelli Janardhana, V. Ramanathan, T. Rajesh, and P. Harish Kumar Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prasanthi Nilayam – 515 134, Andhrapradesh, India; *
[email protected] Chemical reactions induced by light have been utilized for synthesizing highly strained, thermodynamically unstable compounds, which are inaccessible through non-photochemical methods. Photochemical cycloaddition reactions, especially those leading to the formation of four-membered rings, constitute a convenient route to compounds that are difficult to prepare by other methods. For example, the irradiation of cyclopentenone (1) or cyclohexenone (2) each produces a mixture of two dimers (one head-to-head and the other head-to-tail). The conjugated enone–alkene [2+2] photocycloaddition is useful particularly in the synthesis of cage compounds (3, 4) and natural products (5–7). Although the theory of cycloaddition reactions1 is encountered in advanced texts on organic chemistry most practical illustrations of the theory restrict themselves to [4+2] thermally mediated Diels–Alder cycloadditions. Here we present a [2+2] cycloaddition that is relatively easy to carry out using sunlight for photochemical mediation. The dimerization is conveniently shown by the chemical ionization mass spectroscopy (CI–MS).2 Moreover, a second product may be isolated in significant quantities and more ambitious students can be invited to apply detective work to elucidate its structure and account for its formation. The starting material, dibenzylideneacetone, is easily prepared from benzaldehyde and acetone (8) and also is commercially available. Enones such as this undergo n–π* photoreactions with light λ > 300 nm (9). Thus Pyrex containers, essentially transparent to the radiation above 300 nm, are inexpensive and effective reaction vessels. Experimental Procedure Dibenzylideneacetone (1 g) was dissolved in dry toluene (10 mL), transferred to a Pyrex flask, stoppered, and kept in bright sunlight. The initial formation of white solid was observed after ca. 16–24 h exposure to light and maximum
yield (0.15 g, 15%) was obtained in ca. 40 h. The product was filtered and washed with toluene. TLC analysis of the product3 (on silica gel plates, irrigating solvent, 4:1 petroleum ether:ethyl acetate) showed the presence of two compounds with R f values 0.73 (compound A) and 0.25 (compound B). Compound A (a colorless solid, yield 110 mg) could be obtained by extracting the crude product with hot chloroform (4 × 5 mL). The remaining residue contained mainly compound B (a colorless solid, yield 30 mg) with trace quantities of A in it. Alternatively the mixture could be subjected to silica gel column chromatography using petroleum ether and ethyl acetate as the eluent to obtain compound A and B (see the Supplemental MaterialW). Both compound A (mp 218–220 ⬚C) and compound B (mp 226–228 ⬚C) were recrystallized from tetrahydrofuran. Spectroscopic properties4,5 of compounds A and B are recorded in Table 1. Hazards Petroleum ether and toluene are moderately toxic and highly flammable. Chloroform and deuterated chloroform are eye irritants and suspected carcinogens. Dimethyl sulfoxide (DMSO-d6) readily passes through the skin and takes solutes with it. Acetone and ethyl acetate are moderate irritants and highly flammable. Tetrahydrofuran forms explosive peroxides with air and is flammable. Ethanol and methanol are flammable. Benzaldehyde is a cancer suspect agent, mutagen, and a skin irritant. Sulfuric acid is a highly toxic oxidizer and is corrosive. The methanol兾sulfuric acid spray should be used in a fume hood. Dibenzylideneacetone is a skin irritant. Discussion Cycloaddition reactions of the type [2+2] give rise to four-membered rings. In photochemical reactions the reactive excited state of a saturated ketone is the n–π* state. But
Table 1. Spectral Data of Compound A and Compound B Method
Compound A
UV–vis λmax(MeOH)/nm
260, 280
258, 280
IR (KBr)/cm᎑1
3050, 3025, 2921, 1718(s), 1600, 1495, 1448, 1419, 1384, 751, 697
3050, 3025, 2920, 1715(s), 1590,1495,1448,1385, 751, 697
CI-MS m/z (rel abundance)
469(50) [P+1 peak], 287(12), 235(40), 211(52), 169(100), 131(8)
703(20) [P+1 peak], 685(8), 505(4), 469(44)
H NMR (CDCl3) δ
3.33 (4H, m), 3.72 (4H, m), 7.03–7.35 (20H, m)
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H NMR (DMSO-d6) δ
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3.35–3.9 (12H, multiplets), 7.15–7.40 (30H, m)
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Compound B
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In the Laboratory
Scheme I. [2+2] Reactions of dibenzylideneacetone: formation of dimers A1, A2, and A3 and trimer B.
a π–π* transition is most likely to occur when the ketone group is conjugated with an extensive π bonding system. The initial excitation is probably S1 (n–π*), followed by intersystem crossing to T1 (n–π*) or a twisted (π–π*) state, which is apparently the reactive state (10). The mass spectrum of compound A showed a parent molecular ion (P+1) at m兾z 469 that corresponds to dimeric dibenzylideneacetone. It is possible to suggest dimers of the type A1 and A2. However such structures are not supported by the IR spectroscopic evidence since the strong peak at 1718 cm᎑1 indicates that there is no α, β unsaturation. Conjugation would decrease the stretching frequency of the carbonyl group to about 1665 cm᎑1. Moreover, its UV–vis spectrum shows a weak absorption at 280 nm, which is typical of a saturated cyclohexanone and shows the absence of conjugation. A predictably strong absorption at 260 nm in the UV– vis spectrum indicates the presence of monosubstituted benzene rings. The spectroscopic details are consistent with a dimeric product A3 arising from addition across both the double bonds of the two dibenzylideneacetone molecules, that is, two simultaneous [2+2] additions. Structure A3 is supported by the 1H NMR data: the cyclobutane ring protons are seen as a pair of incompletely resolved multiplets centered at δ = 3.72 (4H) and δ = 3.33 (4H) ppm. The signal at δ = 3.33 is assignable to the protons α to the carbonyl and the one at δ = 3.72 to the benzylic protons. The mass spectrum of compound B showed the P+1 molecular ion at m兾z 703 that corresponds to the trimeric dibenzylideneacetone. The UV–vis and IR spectra of compound B (UV–vis: 280, 258 nm and IR: 1715 cm᎑1) are simi1668
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lar to those of the dimer A indicating the presence of saturated ketone groups and absence of any unsaturation. Hence the trimer must be formed by three [2+2] cycloadditions of all the six double bond units of three dibenzylideneacetone molecules. This leads to a compound possessing three cyclobutane units and a nine-membered ring. The 1H NMR spectrum includes a 12H multiplet between δ = 3.35 and δ = 3.9 ppm. Again, these can be assigned to the protons on the cyclobutane rings that are α to the carbonyls and the benzylic protons. The aromatic protons’ signals occur between δ = 7.15 and δ = 7.40 ppm. On the basis of these data the trimer has been assigned the structure shown in Scheme I. Acknowledgment We express our sincere gratitude to N. R. Krishnaswamy for helpful advice. WSupplemental
Material
Instructions for the students, notes for the instructor, and details of the chromatographic separation of the dimer A and trimer B are available in this issue of JCE Online. Notes 1. The theory of cycloaddition reaction is essentially the symmetry of the highest occupied molecular orbital of one component connects with that of the lowest unoccupied molecular orbital of the other component.
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In the Laboratory 2. Electron impact mass spectroscopy (EI–MS) is not helpful as it does not provide parent ion information. 3. Compound A and compound B gave a pink color on TLC when sprayed with MeOH兾H2SO4 followed by heating. 4. NMR spectra were recorded on Varian FT 80A, 200 MHz instrument. 5. CI–MS was carried out on a CEC-21-110 instrument with methane supplementing the helium carrier gas.
Literature Cited 1. Wagner, P. J.; Buchech, D. J. J. Am. Chem. Soc. 1969, 91, 5090. 2. Lam, E. Y. Y.; Valentine, E.; Hammond, G. S. J. Am. Chem. Soc. 1967, 89, 3482. 3. Mehta, G.; Padma, S.; Osawa, E.; Barbaric, D. A.; Mochizuki, Y. Tetrahedron Lett. 1987, 28, 1295.
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4. Marchand, A. P.; Annapurna, P.; Reddy, S. P.; Watson, W. H.; Nagl, A. J. Org. Chem. 1989, 54, 187. 5. Paquette, L. A.; Lin, H. S.; Gun, B. P.; Coghlan, M. J. J. Am. Chem. Soc. 1988, 110, 5818. 6. Winkler, J. D.; Muller, C. L.; Scott, R. D. J. Am. Chem. Soc. 1988, 110, 4831. 7. Crimmins, M. T.; Gould, L. D. J. Am. Chem. Soc. 1987, 109, 6199. 8. Furniss, B. S.; Hannaford, A. J; Smith, P. W. G.; Tatchell, A. R. Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Addison Wesley: Harlow, United Kingdom, 1996; pp 1033– 1034. 9. Smith, M. B. Organic Synthesis; McGraw Hill: New York, 1994; p 1197. 10. Schuster, D. I.; Bonneau, R. R.; Dunn, D. A.; Rao, J. M.; Joussiest–Dudieu, J. J. Am. Chem. Soc. 1984, 106, 2706.
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