Abstract
Moderate to high enantiomeric excesses for the cis and trans olefinic products of a Norrish type II cleavage reaction have been obtained for the first time through the use of the solid-state ionic chiral auxiliary approach.
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Notes and references
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M. Vaida, R. Popvitz-Biro, L. Leiserowitz and M. Lahav, Probing reaction pathways via asymmetric transformations in chiral and centrosymmetric crystals, in Photochemistry in Organized and Constrained Media, ed. V. Ramamurthy, VCH, New York, 1991, p. 247.
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For an introduction to the use of ionic auxiliaries in solid-state organic photochemistry, see: J. N. Gamlin, R. Jones, M. Leibovitch, B. Patrick, J. R. Scheffer, J. Trotter, The ionic auxiliary concept in solid state organic photochemistry, Acc. Chem. Res., 1996, 29, 203.
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For a recent review in this topic, see: C. Scott and J. R. Scheffer, Crystal structure-solid state reactivity relationships: toward a greater understanding of Norrish/Yang type II photochemistry, in CRC Handbook of Organic Photochemistry and Photobiology, ed. W. M. Horspool, CRC Press, Boca Raton, in press.
K. C. W. Chong and J. R. Scheffer, Thermal and photochemical transformation of conformational chirality into configurational chirality in the crystalline state, J. Am. Chem. Soc., 2003, 125, 4040.
Owing to the relief of cyclopropane ring strain, Norrish type II cleavage is favored over Yang cyclization in the case of compounds of general structure 1. For a discussion of the Norrish type II chemistry of cyclopropyl ketones, see: W. G. Dauben, L. Schutte, R. E. Wolf, Solution photolysis of cis- and trans-2-methylcyclopropyl methyl ketone, J. Org. Chem., 1969, 34, 1849.
For a review of the Norrish/Yang type II reaction, see: P. J. Wagner and B.-S. Park, Photoinduced hydrogen atom abstraction by carbonyl compounds, in Organic Photochemistry, ed. A. Padwa, Marcel Dekker, New York, 1991, vol. 11, ch. 4.
For the synthesis of compound 1a, see ESI†. For the synthesis of closely related compounds, see: A. B. Charette, A. Gagnon, J. F. Fournier J. Am. Chem. Soc., 2002, 124, 386.
The preference for abstraction of Ha over Hb in the case of ketones of general structure 1 is in accord with previous work from our laboratory on the geometric requirements for type II hydrogen transfer. See: H. Ihmels and J. R. Scheffer, The Norrish type II reaction in the crystalline state: toward a better understanding of the geometric requirements for gamma-hydrogen atom abstraction, Tetrahedron, 1999, 55, 885.
A. Sevin and P. Chaquin, Photochemical three-membered-ring cleavage of α-cyclopropyl ketones: a theoretical study, J. Org. Chem., 1982, 47, 4145.
The lowest energy conformation of ester 1b calculated by MM+ molecular mechanics is close to the actual solid-state conformation. See ESI† for details. Crystal structure determination of ester 1b: C28H28O5, M = 444.50, a = 17.271(2), b = 5.7140(5), c = 23.476(3) Å, β = 98.234(5)°, monoclinic, P21/n (no. 14), Z = 4, μ(Mo-Kα) = 0.088 mm−1, reflections measured = 17434, unique = 4013 (Rint = 0.10). Rall = 0.096. CCDC reference number 216688. See http://www.rsc.org/suppdata/pp/b3/b309256a/ for crystallographic data in CIF or other electronic format.
In a separate study, crystal structures of derivatives of 2,3-diphenyl-1-benzoylcyclopropyl ketones revealed that α1 varied from 164–178°
For a review on the “Hula-twist” mechanism in photochemical isomerization, see: R. S. H. Liu and G. S. Hammond, Examples of hula-twist in photochemical cis-trans isomerization, Chem. Eur. J., 2001, 7, 4536.
T. Ohhara, S. Ikeda, H. Imura, H. Uekusa, Y. Ohashi, I. Tanaka and N. Niimura, Direct observation of deuterium migration in crystalline-state reaction by single crystal neutron diffraction IV. “Hula-Twist” rotation of a long alkyl radical produced by photoirradiation, J. Am. Chem. Soc., 2002, 124, 14–736.
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This paper is dedicated to Professor Fred Lewis on the event of his 60th birthday.
Electronic supplementary information (ESI) available: experimental procedures and characterization data. See http://www.rsc.org/suppdata/pp/b3/b309256a/
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Charette, A.B., Chong, K.C.W., Fournier, JF. et al. Asymmetric synthesis in a Norrish type II cleavage reaction induced by crystal chirality. Photochem Photobiol Sci 2, 1052–1055 (2003). https://doi.org/10.1039/b309256a
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DOI: https://doi.org/10.1039/b309256a