Organic & Biomolecular Chemistry Dynamic Article Links Cite this: Org. Biomol. Chem., 2011, 9, 7105 www.rsc.org/obc PAPER Chiral ionic liquid-mediated photochirogenesis. Enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylic acid† Gaku Fukuhara,* a Takahiro Okazaki, a Marco Lessi, b Masaki Nishijima, c Cheng Yang, a Tadashi Mori, a Andrea Mele, d ,e Fabio Bellina, b Cinzia Chiappe* b and Yoshihisa Inoue* a Received 6th May 2011, Accepted 20th July 2011 DOI: 10.1039/c1ob05716b Enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylic acid (AC-H) and its lithium salt (AC-Li) in chiral ionic liquid (CIL), (R)-1-(2,3-dihydroxypropyl)-3-methylimidazolium acetate {[(R)-GLYMI][AcO]}, gave a mixture of two head-to-tail (HT) and two head-to-head (HH) cyclodimers in HT/HH ratios of 1.3–1.7 (for AC-H) and 2.2–4.3 (for AC-Li) with low enantiomeric excesses (ee) of 0–3% for chiral syn-HT and anti-HH dimers. In contrast, irradiation of AC-H in an aqueous solution, containing cucurbit[8]uril (CB[8]) as a host and [(R)-GLYMI][AcO] or [(R)-GLYMI][Tf 2 N] as a modifier of CB portals, afforded the HH dimers in 91–99% selectivity, although the anti-HH dimer was totally racemic. Interestingly, irradiation of AC-H in a dichloromethane solution, containing [(R)-GLYMI][AcO] as a chiral template, led to the formation of the HH-dimers in 98% selectivity with chiral anti-HH dimer in -14% ee, presumably by the dual ligation of two ACs to a CIL through electrostatic and hydrogen-bonding interactions. Introduction Photochemical asymmetric synthesis, or photochirogenesis, 1 pro- vides us with unique, versatile routes to a variety of optically active compounds, which are alternatives to the conventional catalytic and enzymatic asymmetric synthesis, 2 but this still remains a challenge in current photochemistry. Of several approaches to photochirogenesis examined so far, the use of chiral solvent does not appear to be very attractive, necessitating a large excess amount of chiral source yet affording modest enantiomeric excesses (ee) in general. 3 Ionic liquids (ILs), room-temperature molten salts of unique physical and chemical properties, have attracted much interest in recent years as environmentally benign media for various organic reactions. 4 Chiral ionic liquids (CILs) have also been prepared for conventional (thermochemical) asymmetric syntheses. 4,5 In contrast, photochemical reactions in ILs have been explored only a Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita, 565-0871, Japan. E-mail: gaku@chem.eng.osaka-u.ac.jp, inoue@ chem.eng.osaka-u.ac.jp; Fax: (+81) 6 6879 7923; Tel: (+81) 6 6879 7922 b Dipartimento di Chimica e Chimica Industriale, Universit` a di Pisa, via Risorgimento 35, 56126 Pisa, Italy. E-mail: cinziac@farm.unipi.it; Fax: (+39) 050 2219669; Tel: (+39) 050 2219660 c Center for Advanced Science and Innovation, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan d Dipartmento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, via Mancinelli 7, 20131 Milano, Italy e CNR -Istituto di Chimica del Riconoscimento Molecolare, via L. Mancinelli 7, 20131 Milano, Italy †Electronic supplementary information (ESI) available: NMR charts of [(R)-GLYMI][AcO], titration experiment of (R)-(-)-1,2-propanediol and the characterization of cyclodimers 1–4. See DOI: 10.1039/c1ob05716b recently, 6 and CILs have rarely been employed for photochirogenic reactions. In their pioneering work on photochirogenesis in CIL, Armstrong and co-workers performed the photochemical di-p- methane rearrangement of dibenzobarrelenedicarboxylic acid in CILs to give the corresponding tricyclic acid in 3–12% ee. 7 More recently, we performed the [4 + 4] photocyclodimerization of 2- anthracenecarboxylic acid and its alkali metal salts (AC-X; X = H, Li, K, Cs) in (R)-1-(2,3-dihydroxypropyl)-3-methylimidazolium bistriflimide {[(R)-GLYMI][Tf 2 N]} (Scheme 1) to obtain syn-HT (2*) in 41% ee for AC-H but in 4–10% ee for AC-M (M = Li, K, Cs) at -50 ◦ C. 8 The contrasting results for AC-H versus AC-M led us to a conclusion that the hydrogen-bonding interaction of AC- H with the chiral diol moiety of CIL, rather than the ion-pairing interaction of AC anion with the imidazolium cation in CIL, is responsible for the higher ee obtained for AC-H. 8 This mechanistic Scheme 1 Photocyclodimerization of 2-anthracenecarboxylic acid and its lithium salt (AC-X) in (R)-1-(2,3-dihydroxypropyl)-3-methylimidazolium acetate {[(R)-GLYMI][AcO]} and in the corresponding bistriflimide {[(R)-GLYMI][Tf 2 N]}. This journal is © The Royal Society of Chemistry 2011 Org. Biomol. Chem., 2011, 9, 7105–7112 | 7105 Downloaded by Universitaire d'Angers on 12 February 2012 Published on 21 July 2011 on http://pubs.rsc.org | doi:10.1039/C1OB05716B View Online / Journal Homepage / Table of Contents for this issue