Double Stereochemical Labeling in Stilbene Photochemistry: Tracing Phenyl Rotation Reveals Stereochemically Distinct Reaction Pathways for Formation of the E Photoproducts James E. Gano,* Patricia A. Garry, Padmanabhan Sekher, Jason Schliesser, and Yong Wah Kim Bowman-Oddy Laboratories, Department of Chemistry UniVersity of Toledo, Toledo, Ohio 43606 Dieter Lenoir GSF-Forschungszentrum fuer Umwelt und Gesundheit Institut fuer Oekologische Chemie D-85758 Oberschleissheim, Germany ReceiVed July 8, 1996 The mechanistic details of the Z/E photoisomerization of stilbenes 1a and 2a are thought to be generally understood. 1-3 They differ little from the original proposal put forth by Saltiel in 1967. 4,5 Currently, there is a lively interest in this system as finer details of the excited state reaction coordinate are slowly unveiled through spectroscopic investigations employing modern techniques. 6-11 Although stereochemistry was utilized in the 1,2-dinaphthylethene systems, 12,13 it has not been used to study stilbene photochemistry beyond the level of Stoermer’s original discovery of E/Z photoisomerization. 14,15 The rapid intercon- version of stereoisomers prevented traditional stereochemical labeling experiments. The first double stereochemical labeling experiments, rotation about θ and φ in stilbene photochemistry, are reported below (Scheme 1). Cautious extensions of these results to other stilbenes are very instructive. Studies in our laboratories have focused on the sterically congested stilbenes 3 and 4 (2,2,4,4-tetramethyl-3,4-diphenylhex- 3-enes). 16-22 Their geometries are distinctly different from planar 1 and slightly twisted 2. 3 Both 3 and 4 are planar about the C e dC e bond; however, steric repulsion rotates the phenyl groups 90° out of the molecular plane so their planes are perpendicular to the plane of the central C e dC e bond. 20,23 Due to steric congestion, rotation about the C e -Ph bond in 3 and 4 is sufficiently slow to study the stereochemistry of the phenyl rotation during the Z/E photoisomerization. A mixture of E isomers 3b syn and 3b anti (∼1:1) was prepared from 3-methylbenzonitrile. 24 Distinction of the isomers and their separation were difficult. Ultimately, 3b anti was caused to crystallize selectively from a methanol solution containing 3b anti and 3b syn through slow solvent evaporation over a period of 6 months at room temperature. 1 H NMR analysis of 15 crystals of 3b anti showed each contained 8-12% of 3b syn . X-ray diffraction analysis of one half of a single crystal determined the structure to be 3b anti . NMR analysis of the remaining half of the same crystal made the correlation with the observed methyl chemical shift. Attempts to crystallize 3b syn always resulted in cocrystallization (∼1:1) with 3b anti . 25 Excitation of 3b at 229 nm produced an ∼1:1:1:1 mixture of 3b syn , 3b anti , 4b syn , and 4b anti from which an ∼1:1 mixture of 4b syn and 4b anti was obtained by column chromatography as described for the parent compound. 16 Slow crystallization of the 4b syn and 4b anti mixture from methanol gave crystals of two distinct forms, long triangular prisms and diamond-shaped plates, which could be individually selected under a microscope. X-ray diffraction analysis of the triangular prism-shaped crystals determined them to be 4b anti . These had the methyl resonance at δ ) 2.07. 26 Both 3b syn and 3b anti were stable indefinitely at room temperature. Although 4b syn and 4b anti were stable in the solid, slow syn/anti interconversion occurred in solution. The half lives for 4b syn f 4b anti and 3b anti f 3b syn were 3.7 × 10 3 and 1.0 × 10 5 s, respectively, at 58 °C. The Z/E isomerization, 4b f 3b, is unimportant under the conditions of this study. 16a Photolyses were performed with 0.005 M solutions of 3b anti , 4b syn , or 4b anti in hexane purged of oxygen and continually stirred. Normally, irradiation used 229 nm light. The distribu- tion of stereoisomers was monitored by 1 H NMR in benzene- d 6 to distinguish 3b anti from 3b syn . 16b The results for photolysis of 3b anti are shown in Figure 1. Similar results were obtained for 4b anti and 4b syn . Also, similar results were obtained for 3b anti when the 229 nm light was replaced with 254 nm light. By combining the (3b anti + 3b syn )/(4b anti + 4b syn ) ratio at the photostationary state with the UV spectra in the usual manner, “excited state partitioning ratios” are found to be the same for irradiation at 229 and 254 nm, 1.8 and 1.6, respectively. (1) Meier, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 1399-420. (2) Goerner, H.; Kuhn, H. J. In AdVances in Photochemistry; Neckers, D. C., Volman, D. H., Von Buenau, G. V., Eds.; Wiley: New York, 1995; Vol. 19, pp 1-118. (3) Waldeck, D. H. Chem. ReV. 1991, 91, 415-36. (4) Saltiel, J. J. Am. Chem. Soc. 1967, 89, 1036-7. (5) Saltiel, J. J. Am. Chem. Soc. 1968, 90, 6394-400. (6) Abrash, S.; Repinec, S.; Hochstrasser, R. M. J. Chem. Phys. 1990, 93, 1041-53. (7) Sandros, K.; Sundahl, M.; Wennerstroem, O.; Norinder, U. J. Am. Chem. Soc. 1990, 112, 3082-6. (8) Dutt, G. B.; Konitsky, W.; Waldeck, D. H. Chem. Phys. Lett. 1995, 245, 437-40. (9) Petek, H.; Yoshihara, K.; Fujiwara, Y.; Lin, Z.; Penn, J. H.; Frederick, J. H. J. Phys. Chem. 1990, 94, 7539-43. (10) Saltiel, J.; Waller, A.; Sun, Y.-P.; Sears, D. F. J. Am. Chem. Soc. 1990, 112, 4580-1. (11) Polanyi, J. C.; Zewail, A. H. Acc. Chem. Res. 1995, 28, 119-32. (12) Sun, Y.-P.; Sears; J. D. F.; Saltiel, J.; Mallory, F. B.; Mallory, C. W.; Buser, C. A. J. Am. Chem. Soc. 1988, 110, 6974-88. (13) Shim, S. C.; Lee, K. T.; Kim, M. S. J. Org. Chem. 1990, 55, 4316- 21. (14) Stoermer, R. Ber. Dtsch. Chem. Ges. 1909, 42, 4865-71. (15) Fischer, E. J. Mol. Struct. 1982, 84, 219-26. (16) (a) Lenoir, D.; Gano, J. E.; McTague, J. A. Tetrahedron Lett. 1986, 27, 5339-42. (b) For NMR analysis, see: Supporting Information. (17) Gano, J. E.; Park, B.-S.; Pinkerton, A. A.; Lenoir, D. J. Org. Chem. 1990, 55, 2688-93. (18) Gano, J. E.; Subramaniam, G.; Birnbaum, R. J. Org. Chem. 1990, 55, 4760-3. (19) Gano, J. E.; Park, B.-S.; Subramaniam, G.; Lenoir, D.; Gleiter, R. J. Org. Chem. 1991, 56, 4806-8. (20) Gano, J. E.; Park, B.-S.; Pinkerton, A. A.; Lenoir, D. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1991, 47, 162-4. (21) Gano, J. E.; Skrzypczak-Jankun, E.; Kirschbaum, K.; Lenoir, D.; Frank, R. Acta Cryst., Sect. C: Crystallogr. Struct. Commun. 1993, 1985- 8. (22) Laali, K. K.; Gano, J. E.; Lenoir, D.; Gundlach, C. W. I. J. Chem. Soc., Perkin Trans. 2 1994, 2169-73. (23) Gano, J. E.; Subramaniam, G.; Skrypczak-Jankun, E.; Kirschbaum, K.; Kluwe, C.; Sekher, P. Manuscript in preparation. (24) Lenoir, D.; Burghard, H. J. Chem. Res. (S) 1980, (S) 396-7, (M) 4715-25. (25) For spectral data of compounds 3b syn and 3banti, see: Supporting Information. (26) For spectral data of compounds 4bsyn and 4banti, see: Supporting Information. Scheme 1 3826 J. Am. Chem. Soc. 1997, 119, 3826-3827 S0002-7863(96)02334-7 CCC: $14.00 © 1997 American Chemical Society