Dihydrophenanthrene-Type Intermediates during Photoreaction of trans-4′-Benzyl-5-styrylfuran Shingo Samori, Michihiro Hara, Tong-Ing Ho, † Sachiko Tojo, Kiyohiko Kawai, Masayuki Endo, Mamoru Fujitsuka, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan, and Department of Chemistry, Faculty of Science, National Taiwan University, Taipei, Taiwan majima@sanken.osaka-u.ac.jp Received November 15, 2004 Photoreaction of trans-4′-benzyl-5-styrylfuran (trans-BSF) has been studied by the 355-nm laser flash photolysis (LFP) in CH 2 Cl 2 using a Nd 3+ :YAG laser (30 ps, 5 mJ pulse -1 or 5 ns, 30 mJ pulse -1 ). Transient fluorescence and absorption spectra assigned to the singlet excited trans-BSF were observed during the 30-ps LFP, whereas a transient absorption spectrum with two peaks at 400 and 510 nm, assigned to the trans-fused dihydrophenanthrene (DHP)-type intermediate (DP1), was observed during the 5-ns LFP. It is clearly suggested that a two-photon absorption process is involved in the formation of DP1. The first photoreaction is the photoisomerization of trans-BSF, which occurs to give cis-BSF. The second photoreaction process is photocyclization of cis-BSF, which occurs to give DP1 decaying with the half lifetime (τ 1/2 ) of 2.8-4.0 μs to produce another DHP-type intermediate (DP2) with an absorption peak at 400 nm in the absence of O 2 , through [1,9]-hydrogen shift. DP2 decayed with τ 1/2 > 500 μs to give the product through aromatization. In O 2 -saturated CH 2 Cl 2 , DP1 decayed with τ 1/2 ) 250 ns to give a radical intermediate (X) with two peaks at 410 and 510 nm, through hydrogen abstraction of DP1 by O 2 . X decayed with τ 1/2 ) 150 μs to give the product through successive hydrogen abstraction. Introduction Photochemical reactions of stilbene-type compounds are well documented. The oxidative cyclization occurs to form phenanthrene-type compounds as one of the major pathways 1-3 and can be utilized for organic synthesis. 4 The mechanistic studies have not been fully carried out, although the transient absorption of the intermediates dihydrophenanthrenes (DHPs) is suggested to be detected in solution in the 400-500 nm region. The isolation of DHPs is very difficult because of the rapid oxidative hydrogen abstraction to give phenanthrene-type com- pounds or reverse ring opening to the starting stilbene- type compounds. Several attempts to isolate the stable intermediates have been reported. Muszkat and Fischer photolyzed 1,1′,3,3′,5,5′-hexamethylstilbene and 1,2- diphenylcyclopentene and obtained the corresponding DHPs in 21% and 67% yields, respectively. 5 Blackburn et al. reported that DHPs may be produced from photo- * To whom correspondence should be addressed. Tel: Japan +6- 6879-8495. FAX: Japan +6-6879-8499. † National Taiwan University. (1) Waldeck, D. H. Chem. Rev. 1991, 91, 415. (2) Hammond, G. S.; Saltiel, J.; Lamola, A. A.; Turro, N. J.; Bradshaw, J. S.; Cowan, D. O.; Counsell, R. C.; Vogt, V.; Dalton, C. J. Am. Chem. Soc. 1964, 86, 3197. (3) Mallory, F. B.; Wood, C. S.; Gordon, J. T. J. Am. Chem. Soc. 1964, 86, 3094. (4) Blackburn, E. V.; Timmons, C. J. Q. Rev. Chem. Soc. 1969, 23, 482. (5) Muszkat, K. A.; Fischer, E. J. Chem. Soc. B 1967, 662. 2708 J. Org. Chem. 2005, 70, 2708-2712 10.1021/jo047977x CCC: $30.25 © 2005 American Chemical Society Published on Web 03/08/2005