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/lp/de-gruyter/catalytic-supramolecular-photochirogenesis-H6VtdkIewK
- Publisher
- de Gruyter
- Copyright
- Copyright © 2015 by the
- ISSN
- 2084-7246
- eISSN
- 2084-7246
- DOI
- 10.1515/supcat-2015-0002
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Abstract
Supramolecular ptochirogenesis is a new strategy for circumventing the inherent difficulties encountered in conventional ptochirogenesis, i.e. the interactions associated with geometrically less-defined, srt-lived excited states, by confining a prochiral substrate(s) in a chiral supramolecular environment(s) prior to ptoexcitation. This rather simple, but very successful, strategy has been applied to a variety of chiral ptoreactions. wever, a stoichiometric, or even excess amount of supramolecular st is often needed to ensure full complexation of the substrate, and achieve the optimum stereochemical outcome. This apparent drawback has recently been removed by introducing a sensitizing moiety to the supramolecular st, or by batchromically shifting the absorption band of substrate through Lewis acid, or charge-transfer complexation. Recent progress in catalytic supramolecular ptochirogenesis will be reviewed. Keywords: ptochirogenesis, supuramolecular chemistry, chiral ptochemistry, catalysis 1 Introduction Ptochemical asymmetric synthesis, or ptochirogenesis, has long been a highly challenging task in modern ptochemistry, but has achieved significant progress in the last two decades.[1-4] Traditionally, (nonsupramolecular) catalytic ptochirogenesis has been accomplished by exploiting chiral ptosensitizers, as *Corresponding autr: Yoshihisa Inoue: Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan, E-mail: inoue@chem.eng.osaka-u.ac.jp Zhiqiang Yan, Wanhua Wu, Cheng Yang:Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry and State Key Laboratory of Biotherapy, West China Medical Scol, Sichuan University, 29 Wangjiang Road, Chengdu 610064, China exemplified by the first report by Hammond and Cole in 1965 on the enantiodifferentiating ptoisomerization of 1,2-diphenylcyclopropane sensitized by a chiral naphthylethylamine derivative.[5] The systematic study on the enantiodifferentiating ptoisomerization of cyclooctenes sensitized by chiral (poly)alkyl benzene(poly)carboxylates, as well as the critical role of entropy played at the most crucial stage of enantiodifferentiation, appears to have remarkably boosted research on molecular ptochirogenesis.[6-8] wever, conventional ptochirogenesis with chiral sensitizers is not very efficient in general, affording modest to good enantioselectivities in most cases, except for the one reported recently.[9] This is due to the generally weak interactions, and relatively large conformational freedoms in the exciplex intermediate formed upon interaction of an excited chiral sensitizer with a ground-state substrate. In this context, supramolecular ptosensitization, which combines the catalytic nature of ptosensitization with the intimate interactions of the supramolecular complex, appears to be highly promising. Indeed, the most prominent, yet practical advance in this field has been attained in the new interdisciplinary area of supramolecular ptochirogenesis, where the supramolecular interactions in both ground and excited states are exploited for manipulating the stereochemical outcomes of ptochirogenic reactions.[5,10,11] The ptochemical chirality transfer in supramolecular systems can be synergistically controlled not only by reaction kinetics through weak, srt-lived excitedstate interactions also available in conventional molecular ptochirogenesis, but also by complexation thermodynamics through the long-lasting ground-state interactions in the chiral environment. Thus, many of the highly enantioselective ptoreactions reported so far were achieved through supramolecular ptochirogenic approaches, where cyclodextrins,[12,13] chiral hydrogenbonding templates,[14] chirally modified zeolites,[15] chiral molecular cages,[16] and chiral aggregates[17,18] were employed as chiral sts. wever, these supramolecular ptochirogenic reactions were carried out in the presence of an excess amount of chiral st to gain the optimal © 2015 Zhiqiang Yan et al., licensee De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. stereochemical outcomes. This is due to the non-covalent nature of supramolecular complexation, which requires at least a stoichiometric, but often an excess, amount of chiral st to ensure the full complexation of guest substrate and prevent the formation of racemic product from uncomplexed substrate in the bulk solution. More sensible strategies to accelerate an enantiodifferentiating ptoreaction with a substoichiometric amount of chiral st have recently been proposed to achieve catalytic supramolecular ptochirogenesis. Complexation of a substrate with a supramolecular st frequently alters the ptophysical properties, and ptochemical reactivities of the system significantly. Many ptoreactions are known to be remarkably expedited upon confinement of substrate in a hydropbic st cavity due to pre-arrangement, confinement, and/or concentration effects. Furthermore, the non-covalent nature and generally moderate affinity of supramolecular complexation allows the chiral st site/cavity to turn-over by releasing ptoproduct, which introduces new substrate, and thus suppresses product-inhibition. wever, in supramolecular ptochirogenesis, it is more crucial to prevent the nonchirogenic ptoreactions that occur to give racemic products outside the chiral cavity. In this review, we will focus on such supramolecular ptochirogenic systems that satisfy the following criteria: (1) use of a chiral st in sub-stoichiometric quantity, (2) acceleration of a ptochirogenic reaction, and (3) the invention to suppress the non-chirogenic ptoreactions that occur outside the chiral supramolecular site/cavity. racemic 1E in 0.51.5% enantiomeric excess (ee), revealing the poor ptochirogenic ability of the native b-CD cavity for 1Z.[22] In contrast, the supramolecular ptoisomerization of 1Z included and sensitized by a 0.1 equivalent of benzoyl-CD (4a) affords 1E in up to 11% ee.[21] Since then, a series of sensitizer-modified a-, band g-CD derivatives 4b-m (Figure 1) have been prepared for investigating the supramolecular ptoisomerization of 1Z, and cyclooctadienes (2ZZ and 3ZZ) (Scheme 1).[2331] h 1Z (R)-1E + (S)-1E + (R)-2EZ (S)-2EZ 2ZZ 3ZZ (R)-3EZ (S)-3EZ Scheme 1. Enantiodifferentiating ptoisomerization of (Z)-cyclooctene (1Z) and (Z,Z)-1,3- and (Z,Z)-1,5-cyclooctadienes (2ZZ and 3ZZ). R O O O OR' O R'O R'O O OR' O 2 Catalytic Supramolecular Ptochirogenesis with Cyclodextrins Cyclodextrin (CD), a cyclic oligosaccharide with a hydropbic cavity and hydrophilic exterior, is one of the most widely studied st molecules. This is due to its immediate availability, good water-solubility, facile chemical modification, and complexation ability for a wide variety of organic guests.[19] Furthermore, CDs are UV-transparent and inherently chiral, facilitating their application to supramolecular ptochirogenesis. Indeed, a variety of chiral ptoreactions have been studied by using native and modified CDs as chiral sts.[20] Ptoisomerization of (Z)-cyclooctene (1Z) mediated by sensitizer-modified CDs (Scheme 1) is the earliest example of an enantiodifferentiating supramolecular ptosensitization.[21] Direct ptoirradiation of a 1:1 complex of 1Z with native b-CD at 185 nm gives nearly R'O 4a: n = 6, R = R' = H 4b: n = 5, R = R' = H 4c: n = 6, R = o-CO2Me, R' = H 4d: n = 6, R = m-CO2Me, R' = H 4e: n = 6, R = p-CO2Me, R' = H 4f : n = 6, R = o-OMe, R' = H 4g: n = 6, R = m-OMe, R' = H 4h: n = 6, R = p-OMe, R' = H 4i : n = 6, R = H, R' = Me 4j : n = 7, R = R' = H O O O O O O O 4k : n = 5 4l : n = 6 4m: n = 7 n Figure 1. Sensitizer-appended a-, b-, and g-CDs (4a-m) for supramolecular ptosensitization. A nice trick employed in this supramolecular ptochirogenic sensitization system is that the sensitizing ability is turned on only when a guest substrate is included in the st cavity. In the absence of guest, the hydropbic sensitizer moiety is self-included in the CD cavity and cannot transfer excitation energy to substrates outside the cavity, and hence no racemic product is formed in bulk solution. Once a guest substrate is included, the sensitizer moiety is moved to the portal area but is in close contact with the included substrate to facilitate energy transfer and subsequent ptoreactions in the chiral environment. This mechanism, in particular its absence of the sensitized ptoisomerization in bulk solution, renders this supramolecular ptosensitization system catalytic. In general, b-CD-based sensitizers offer higher enantioselectivities than tse based on a- or g-CD due to a cavity size better fitted to cyclooctenes.[23,30] Besides the cavity size, substituent(s) introduced to the benzoate moiety critically affect the supramolecular ptosensitization. Thus, the ee of 1E is significantly enhanced from modest 11% for 6-O-benzoyl-b-CD 4a to 24% for 6-O-(methyl phthaloyl)-b-CD 4c and then to 46% for 6-O-(m-metxybenzoyl)-b-CD 4g, which are in sharp contrast to the much lower ee values obtained with the o- and p-analogues, 4f and 4h.[26,27] These results reveal that even an apparently small difference in structure of the supramolecular st may lead to a significant change in stereochemical outcome and hence the structural optimization of sensitizing st is desirable for obtaining optimal results. Another interesting aspect of supramolecular ptosensitization is the finding that the product's ee is relatively insensitive to change in temperature. This is in keen contrast to the highly temperature-dependent ee values observed upon the conventional ptosensitization in isotropic media,[6,32] for which the relatively rigid CD skeleton fixed by the inter-glucoside hydrogen-bonds around the secondary rim is likely to be responsible at least in part. In support of this rationalization, when ptosensitization is performed by using permethylated CD derivative 4i, the skeleton of which is much more flexible due to the brokeydrogen-bonding network, the ee of 1E greatly varies with temperature to give an antipodal product.[29,33] Controlling enantiodifferentiating ptoisomerization of 1,3-cyclooctadiene 2ZZ appears to be more difficult, compared to the 1Z case mentioned above. Thus, the highest ee of 2EZ reported for the ptoisomerization of 2ZZ with a variety of conventional chiral sensitizers is only 17% even at 110 °C.[34] For better stereochemical control, supramolecular ptosensitization of 2ZZ with naphthalene-appended a-, b- and g-CDs 4k-m has been examined in aqueous solutions. b-CD-based sensitizing st 4l exhibits a much higher affinity to 2ZZ than a- and g-CD analogues 4k and 4m, due to the better size-matching, but affords 2EZ in mere 4.6% ee upon ptoirradiation in aqueous solution,[30] revealing that supramolecular ptosensitization does not always lead to better results than conventional molecular ptosensitization, and the chiral environment of sensitizing st plays a decisive role. Recently, a series of polymeric CDs crosslinked by pyromellitic dianhydride (5-7) (Figure 2), called CD nanosponge (CDNS), have been employed as sensitizing sts for the enantiodifferentiating ptoisomerization of 1Z and 2ZZ.[28,35] Upon gradual increase of its concentration in water, CDNS evolves several phases from sol to suspension, flowing gel, and finally rigid gel. Possessing pyromellitate moieties as crosslinkers, CDNS can sensitize the enantiodifferentiating ptoisomerizations of 1Z and 2ZZ revealing that the ee values of obtained 1E and 2EZ critically depend on the phase condition of CDNS, but are consistently optimized at the border of the flowing and rigid gel regions. For example, the ee value of 2EZ obtained upon ptosensitization of 2ZZ by b-CDNS 6 decreases from 4.7% in sol phase to nearly zero in suspension but revives in flowing gel to reach the maximum of 6.1% at the border to rigid gel. g-CDNS 7 behaves similarly, affording almost racemic 2EZ in both sol and suspension regions, but the highest 13.3% ee near the border of flowing and rigid gels, is the best ee value ever reported for the supramolecular ptoisomerization of 2ZZ. This phasedependent ptochirogenic behavior is presumably attributed to the alternation of the chiral binding and reaction site for cyclooctenes from the CD cavity in sol phase to the void space surrounded by the exterior walls of CD in gels. This void space mechanism has been supported by a recent study.[36] Thus, analogous nanosponges prepared from cyclic nigerosylnigerose, a saucer-shaped cyclic tetrasaccharide with a shallow concave surface, have been examined as sensitizing supramolecular media for the same ptosensitization to sw very similar ptochirogenic behavior, affording generally low ee values of 4% to 6% in aqueous solutions and suspensions but much higher ee values of 22-24% at the border of flowing and rigid gels. These results confirm the positive roles of chiral void space formed upon gelation of the crosslinked saccharide polymer. O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 5-7 5: -CD:PDA = 1 : 2 6: -CD:PDA = 1 : 4 7: -CD:PDA = 1 : 4 Figure 2. CD nanosponges 5-7 prepared by cross-linking b- and g-CDs with pyromellitic dianhydride (PDA) in 1:2 and/or 1:4 ratios. Enantio- and diastereodifferentiating ptoisomerizations of cis-1,2- diphenylcyclopropanes (8a-d) mediated by b-CD have been studied comparatively in the solid state and in aqueous solutions with and witut a triplet sensitizer added to the system (Scheme 2).[37] Ptoirradiation of a mixture of 8a and 4-metxyacetophenone (added as a triplet sensitizer) in the solid state affords the corresponding trans-isomer 9a in 13% ee, while direct irradiation of 8b in the solid state gives 9b in comparable 13% ee. Direct irradiation of solid-state complexes of chiral 8c and 8d with b-CD affords 9c and 9d in better, 28%, and 30% diastereomeric excess (de), respectively, which are much higher than the de of <2% obtained for the same reactions in an aqueous solution, suggesting the crucial role of the conformational freedoms of included substrate in determining the stereochemical outcome. The enantiodifferentiating [4+4] ptocyclodimerization of 2-anthracenecarboxylate (AC) is one of the most comprehensively investigated supramolecular ptochirogenic reactions. This reaction gives four stereoisomeric 9,10-bridged cyclodimers 10-13, of which syn-head-to-tail (syn-HT) dimer 11 and anti-headto-head (anti-HH) dimer 12 are chiral (Scheme 3). Inclusion complexation of two AC molecules with native g-CD in aqueous solution greatly accelerates the ptocyclodimerization as a result of the prearrangement of ACs in a CD cavity to give syn-HT-dimer 11 in 46% yield and 41% ee.[12] For better chemical and optical yields, a series of g-CD derivatives have been prepared and the effects of external factors, such R h or h/Sens + 9 a: R = H b: R = CO2Et c: R = H N ent-9 d: R = H N Scheme 2. Enantio- and diastereodifferentiating ptoisomerization of cis-1,2-diphenylcyclopropane derivatives 8a-d. h > 320 nm -O C 2 -O C 2 anti-HH CO2 syn-HH CO2 anti-HT syn-HT 11* 12* Scheme 3. Enantiodifferentiating [4+4] ptocyclodimerization of AC. as solvent, temperature, pressure, and irradiation wavelength, have also been investigated in considerable detail.[38-45] All of these internal and external factors critically affect the stereochemical outcomes of AC ptocyclodimerizations, affording chiral cyclodimers 11 and 12 in good to excellent enantioselectivities. Despite the significant acceleration of ptocyclodimerization upon complexation, an excess amount of g-CD derivative is necessary in most cases for obtaining the best enantioselectivity by minimizing both the amount of free AC in the bulk solution, and the formation of racemic product therefrom. wever, this does not immediately mean that catalytic supramolecular ptochirogenesis is infeasible for AC ptocyclodimerization witut using the sensitization technique. Recently, the non-sensitized ptocyclodimerization of AC was achieved for the first time by using a catalytic amount (0.1 equivalent) of g-CD derivative with a Cu(II)-chelate sidearm.[46] Thus, the Cu2+ ion chelated by the diamino sidearm of 14-16 (Figure 3) pre-organizes two ACs iH orientation in the CD cavity H2N NH O O O O O O O O O O O O O O O O O O O O O O O N NH O O O O through coordination to the AC's carboxylate anion, while unchelated Cu2+ in the bulk solution efficiently quenches excited free AC located outside the cavity to prevent the formation of racemic products. The trick in this system is that the chelated Cu2+ does not quench the excited AC in the CD cavity but manipulates the stereochemical consequence of AC ptocyclodimerization to dramatically enhance the chemical yield of the sterically hindered HH-dimers (12 and 13) to 80% and the optical yield of 12 to 70% ee.[46] The competitive enantiodifferentiating polar ptoaddition of methanol and water to 1,1-diphenylpropene (17) has expanded the scope of sensitized supramolecular ptochirogenesis from the unimolecular to bimolecular regime. Upon supramolecular ptosensitization by cyanonaphthyl-modified b-CD (20) in aqueous methanol, 17 affords anti-Markovnikov adducts 18 and 19 via the competitive nucleophilic attack of methanol and water, respectively, to radical cationic 17 (Scheme 4). Methanol-adduct 18 is favored by a factor of NH N O O O O O O O O O O O O O O O O O O O O Figure 3. Modified g-CDs with a diamino sidearm that chelates to Cu2+. OMe + H2O/Me 17 h /Sens* + 18 19 Sens*: O NH Scheme 4. Ptosensitized anti-Markovnikov addition of methanol and water to 1,1-diphenylpropene 17 mediated by cyanonaphthyl-b-CD 20. 2.5 over water-adduct 19 due to the higher nucleophilicity of methanolic oxygen. In 10% aqueous methanol solution at -10 °C, modest enantioselectivities of 13% and 18% ee are achieved for 18 and 19, respectively. Interestingly, the ee values and the chiral sense of the ptoadducts are also sensitive to environmental variants such as temperature and solvent. Thus, by lowering the temperature, the absolute configuration of product 18 is inverted with the ee value varying from -2.1% at 45 °C to +5.8% at -40 °C. 3 Catalytic Supramolecular Ptochirogenesis with Chirally-Modified Zeolite Zeolite supercages have widely been exploited as inorganic sts for accommodating various organic guests, and for mediating corresponding guest (pto) reactions. Altugh conventional zeolites are not chiral, chiral nanospace can be created by immobilizing chiral inductors in the supercage. Analogous to CDs, zeolites are UV-transparent (tugh somewhat scattering) and therefore allow a variety of ptoreactions to occur in their supercages. Chiral modification of zeolites is realized by replacing the counter cations on the interior surface of supercage with chiral organic ammonium ions, or by adsorbing neutral chiral organic molecules. Since zeolites are insoluble in most solvents, ptoreactions are usually carried out in the solid state or in suspension. The ptochirogenesis with chirally modified zeolites often requires an excess amount of chiral inductor to make all the supercages fully modified so as to avoid the formation of racemic ptoproduct in the unmodified supercages, while supercages located deep inside the zeolite are relatively difficult to access. The latter factor makes the exchange of ptoproduct with substrate slow, leading to H OMe O the product inhibition behavior. Therefore, the catalytic ptochirogenesis with zeolite st is achieved only by immobilizing chiral sensitizers inside zeolite supercages. Ptoreactions mediated by chirally modified zeolites afford modest to good diastereo- and enantioselectivities. Asymmetric induction upon intramolecular [2+2] ptocycloaddition of 1-cyano-2-(1,5-dimethyl-2-oxa-4hexenyl)naphthalene 21 has been examined in zeolites modified by chiral amine or alcol (Scheme 5).[47] Irradiation of a slurry sample of 21 adsorbed on dry NaY zeolite modified with L-(+)-diethyl tartrate gives cycloadduct 22 in 10% ee, while the use of L-phenylalaninol as chiral inductor immobilized on the zeolite improved the ee to 15%. O NC O h Chirally modified zeolite Scheme 5. Intramolecular [2+2] ptocycloaddition of 1-cyano-2-(5methyl-2-oxa- 4-hexenyl) naphthalene. Turro et al. have studied the deracemization of benzoin methyl ether 23 upon ptolysis in chirally modified zeolites (Scheme 6). The geminate radical pair produced upon the Norrish type I (a-cleavage) reaction of 23 recombines in situ in the chiral supercage to give enantiomerically enriched starting material 23 and para-substituted 26, or alternatively the benzoyl and metxybenzyl radicals generated each escape the initial supercage and modimerize to 24 and 25. The product distribution obtained upon ptolysis in zeolite supercages significantly differs from that in isotropic solutions. In the ptolysis of racemic 23 in L-diethyl tartrate-modified NaY zeolite, the formation of geminate recombination products 23 rac-23 H OMe + O O O + OMe OMe + OMe O Scheme 6. Norrish type I (a-cleavage) reaction of racemic benzoin methyl ether 23 in chirally modified NaY zeolite. and 26 is favored, due to the decelerated escape from the supercage, while the ptochemical deracemization in the chiral supercage affords 23 in 9.2% ee.[48] Ramamurthy and coworkers have investigated the diastereodifferentiating cis-trans ptoisomerization of 2b,3b-diphenylcyclopropane-1a-carboxylates 27 with chiral ester auxiliaries in triplet sensitizer-immobilized LiY, NaY, KY, RbY, and CsY zeolites (Scheme 7).[49] Triplet-sensitized ptoisomerization of 27a adsorbed in 4-metxyacetophenone-immobilized NaY zeolite affords 28a/29a in up to 55% de, which is much higher than that obtained in solution phase (5% de).[50] Benzonorbornadiene 30 is ptochemically inert upon direct irradiation, but smoothly undergoes the di-pmethane rearrangement to 31 upon triplet sensitization (Scheme 8).[51,52] The use of TlY zeolite as a st accelerates the intersystem crossing of singlet-excited ptosubstrate due to the heavy atom effect, facilitating the rearrangement to 31 via the triplet excited state.[53] Thus, the ptolysis of 30 within TlY zeolite chirally modified by (+)-ephedrine hydrochloride gives 31 in 14% ee. To realize the catalytic ptochirogenesis with chiral zeolites, Inoue and coworkers have examined the enantiodifferentiating ptoisomerization of 1Z in chiral sensitizer-modified zeolite. In sharp contrast to the formation of racemic 1E upon conventional chiral ptosensitization with enantiopure 1-methylheptyl benzoate in isotropic solution, the supramolecular ptosensitization with NaY zeolite modified with the same chiral benzoate affords 1E in a modest but significantly higher ee of 4.5%.[54] 4 Catalytic Supramolecular Ptochirogenesis with Chiral Templates The use of a chiral template is a relatively recently emerging supramolecular strategy for mediating asymmetric ptochemical reactions. Chiral templates are generally much simpler in chemical structure than conventional chiral sts, and need to be more sophisticatedly designed for a reasonable balance between binding affinity and steric conflict. The concise structure of chiral templates, as well as the limited number of supramolecular interactions to be considered, allow rational or even de novo design, and also the elucidation the chiral differentiation mechanisms in considerable detail. Of the many weak non-covalent interactions, the hydrogen-bonding interaction, which is R* h or 3Sens zeolite 27 R*: R* R* Scheme 7. Diastereodifferentiating cis-trans ptoisomerization of 2b,3b-diphenylcyclopropane-1a-carboxylates with chiral auxiliary (R*). h 30 31 + rac-31 Scheme 8. Di-p-methane rearrangement of benzonorbornadiene. strong and directional, is most frequently employed as a tool for capturing the guest substrate and controlling its orientation, configuration and/or conformation in the supramolecular complex formed.[55,56] Multiple hydrogen-bonding interactions provided by chiral templates can also confine and preorganize substrate(s) to drive ptoreactions towards the specific stereochemistry desired.[57-59] Bach et al. have developed a series of Kemp's acidbased chiral templates, such as 32 and 33, which possess a lactam moiety as a hydrogen-bond donor/acceptor, and an aromatic moiety as both a built-in ptosensitizer and a fence to shield one of the enantiotopic faces of an amidecarrying prochiral substrate bound to the template. This dual hydrogen-bonding motif with a sensitizing/shielding aromatic plane enables efficient enantiotopic faceselective binding, and the subsequent ptosensitization or ptoreaction. Thus, the benzophenone and xantne moieties in 32 and 33 play dual roles of shielding one of the enantiotopic faces of the complexed substrate and accepting an electron from the bound substrate upon ptoinduced electron transfer (PET).[60,61] Ptoirradiation of 34 bound to 32 (Scheme 9) leads to PET from the pyrrolidine nitrogen to the benzophenone moiety of 32, which is followed by migration of a hydrogen from the carbon a to the radical cationic pyrrolidine nitrogen. Since one of the enantiotopic faces of quinolone is blocked by the benzophenone moiety, the pyrrolidyl radical can attack the ipso-carbon, only from the open face of quinolone to form the spiro skeleton. As a result, the ptocyclization of 32 in the presence of 0.1 equivalent of chiral template 32, gives 35 in 52-64% yield, and in up to 70% ee. These chiral templates have also been applied to intramolecular [2+2] ptocycloaddition of w-alkenyl(oxy)-2-quinolones 36a-f.[62] Ptoirradiation of 4-(3'-butenyloxy)quinolone 36a in the presence of 0.1 equivalent of 32 results in tricyclic adducts 37a and 38a in 75:25 ratio and in 39% and 17% ee, respectively.[63] With xantne-bearing template 33, which possesses O Chiral template O O R R rac-35 R R X H H H O R R O Chiral template + 38a-f 36a-f 37a-f a: X = O, R = H; b: X = OCH2, R = H; c: X = O, R = Me; d: X = S, R = H; e: X = SO2, R = H; f: X = CH2, R = H O NH O O N O NH O N O O Chiral template: Scheme 9. Ptocyclization sensitized by chiral templates. a higher triplet energy and a more effective shielding ability, 37a and 38a are given in an excellent ee of 94%. For ptocycloaddition of 36b-f, xantne-based organocatalyst 33 sws a better enantioselective performance than benzophenone-based 32, to afford the corresponding cycloadducts in 7287% ee under the optimized conditions. Sivaguru and coworkers have recently reported enantioselective intramolecular ptocycloadditions of 4-alkenyl-substituted coumarins 39 catalyzed by a series of chiral binaphthyl-based thioureas 41 (Scheme 10).[64] The thiourea moiety hydrogen-bonds to the carbonyl group of 39, while the binaphthyl part functions as a built-in chiral ptosensitizer. The binding affinities of 39 to 41 are modest, with the combination of 39a and 41a swing an association constant of 84 M-1 in methylcyclexane. Ptoreaction of 39a with one equivalent of 41b in a 1:1 toluene/m-xylene mixture at 78 °C affords 40a in 96% ee. The enantioselectivity of 40a is held high at 94% ee even when the quantity of catalyst is reduced to 0.1 equivalent but is reasonably reduced to 77% ee with a 0.01 equivalent of 41b. 5 Catalytic Supramolecular Ptochirogenesis with Chirally Coordinated Transition Metal Ions Ptochirogenesis via ptoredox reactioas attracted much attention in recent years.[65,66] In such a system, the enantioselective ptoredox reaction is induced by visible light excitation of a chirally coordinated transition metal ion interacting also with a substrate. For example, intramolecular [2+2] ptocycloaddition of a,w-alkadienes is greatly accelerated in the presence of copper ion through coordination of the two double bonds to the metal ion. Bach and coworkers have synthesized enantiomeric bicyclo[4.2.0]octane derivatives by using Cu(II)-catalyzed [2+2] ptocycloaddition of the corresponding 1,7-heptadienes and the subsequent enantiodifferentiating Baeyer-Villiger oxidations.[67] Mattay et al. have examined the intramolecular [2+2] ptocycloadditions of 1,6-dienes 44 catalyzed by Cu(I) with chiral oxazoline ligands 42 and 43 (Scheme 11) to obtain 45 in less than 5% ee, most likely due to the low affinity of the Cu(I) complex with the substrate.[68] Yoon and coworkers have recently reported a dual catalyst approach to the enantiodifferentiating [2+2] ptocycloaddition of a,b-unsaturated ketones 46 and 47 (Scheme 12) using a Ru(II) tris(2,2'-bipyridine) complex as a visible light-absorbing PET sensitizer, and a chiral Lewis acid (Gd(III) or Eu(III) triflate) as a stereocontrolling cocatalyst to give the corresponding cyclobutane 48 in 89-93% ee with chiral ligand 50a and epimeric 49 in 84-97% ee with 50b.[69] Meggers and coworkers have reported the enantiodifferentiating ptoalkylation of acylimidazole 51 with benzyl bromide 52 catalyzed by a ptoredox transition metal complex (Scheme 13).[70] In this O O Chiral template X H Y O O 39a-f 40a-f a: X = Y = H; b: X = Me, Y = H; c: X = F, Y = H; d: X = OMe, Y = H; e: X = H, Y = Me; f: X = H, Y = F Chiral template: R R CF3 S CF3 41a: R= H 41b: R= CF3 Scheme 10.Ptocycloadditon of coumarin derivatives using thioureas derived from atropisomeric binaphthyl. R h Chiral copper(I) complex 44 Chiral ligand for Cu(I): O N O N O N O N Scheme 11. Ptocycloaddition of 1,6-dienes 44 to bicyclo[3.2.0]heptanes 45 mediated by Cu(I) complexes with chiral ligands 42 and 43. O + Me 46 47 O Me h Ru(bpy)3/Eu(OTf)3á 50 /i-Pr2NEt O Me Me 48 O Me + Me O Me Me 49 N O N O NHBun N O O NHBun 50a 50b Scheme 12. Enantiodifferentiating [2+2] ptocycloaddition of 46 and 47 catalyzed by chiral metal complex. O N N Ph Br N N tBu Ph X N Ir N X N N Me Me tBu X = O, S Scheme 13. Enantioselective ptoredox alkylation of acylimidazole 51. ptoreaction, Ir(III) complex 54 coordinated by two bidentate ligands in chiral configuration plays triple roles, acting as chiral center, catalyst and ptoredox mediator, to afford alkylation product 53 in up to 99% ee upon visible light irradiation. 6 Catalytic Supramolecular Ptochirogenesis with Miscellaneous Chiral Sources Besides the supramolecular ptochirogenic systems described above, several interesting chiral supramolecular sensitizers that can catalyze ptoreactions have been reported. Inoue and coworkers have studied the enantiodifferentiating ptoisomerization of (Z)-cyclooctene 1Z and (Z,Z)-1,5-cyclooctadiene 3ZZ sensitized by planar-chiral paracyclophanes 55a,b (Figure 4)[9] to realize the planar-to-planar chirality transfer in the excited state. Differing from the conventional pointchiral sensitizers studied previously, the decamethylene bridge in planar-chiral sensitizer 55 effectively shields one of the enantiotopic faces of the paracyclophane sensitizer to render the excited-state interaction with the more enantiotopic face-selective substrate. Under optimized conditions, ptosensitizations with 55a and 55b afford 1E in 42% and 43% ee at 110 °C and 3EZ in 87% and 85% ee at 140 °C, respectively. (CH2)10 CO2R 55a: R = Me b: R = i-Pr Figure 4. Planar-chiral cyclophane sensitizers 55. Enantiodifferentiating ptoisomerization of 2ZZ has been investigated using naphtyl-curdlan as the supramolecular ptosensitizer.[71] Curdlan is a linear glucan composed of 1>3-linked b-D-glucose units, which is known to form a triple helical structure OAc AcO in aqueous solutions. The ptoisomerization of 2ZZ sensitized by naphtyl-curdran critically depends on the irradiation time, solvent, phase, temperature, and guest concentration to give 2EZ in up to 8.7% ee in solution and 11.7% ee in the solid state. Liquid crystals, including lyotropic poly(g-benzylL-glutamate) (PBLG) and thermotropic clesteryl oleyl carbonate, have been employed as chiral ptosensitizing supramolecular media for the enantiodifferentiating ptoisomerization of 1Z to evaluate the ptochirogenic abilities of the moderately organized media.[72] Altugh the ptoisomerization of 1Z in liquid crystals affords 1E mostly in low ee (<5%), it suld be noted that lyotropic PBLG gives contrasting results in clesteric and nematic mesophases, revealing the importance of the mesophase structure. The generally low ee values obtained may indicate wever that the chiral 3D structure of the liquid crystal is too large to be sensed by the substrate at the molecular level, and hence the existence of a chiral sensitizer is necessary to afford an appreciable enantioselectivity. The supramolecular enantiodifferentiating ptoisomerization of 2ZZ and the polar ptoaddition of methanol to 1,1-diphenylpropene 17 have been studied by using chiral molecular clips 56 and 57 (Figure 5) as sensitizing sts. Chiral clip 56 strongly binds the substrates with association constants of 8800 M-1 for 2ZZ and 27000 M-1 for 17 in 4:1:5 THF-Me-H2O.[33] The supramolecular complexation by chiral clip facilitates the ptoisomerization of 2ZZ to give a good E/Z ratio of 0.19, while the ee of 2EZ thus obtained is low (0.7% ee). This result suggests that the inherent chirality of the molecular clip is not effectively transferred to the substrate captured by the two naphthalene moieties. On the other hand, the ptoaddition of methanol to 17 sensitized by molecular clips afford only a negligible amount of methanol adduct, suggesting that the electron-transfer ptosensitization is discouraged in this sandwiched structure. OAc AcO *ROOC COOR* COOR* *ROOC R* = (-)-menthyl Figure 5. Chiral molecular clips 56 and 57. H N Br3Al O N O CF3 Scheme 14. Intramolecular [2+2] ptocycloaddition of 58 through complexation with chiral Lewis acid 60. Recently, Bach and coworkers have proposed a new strategy for catalytic intramolecular [2+2] ptocycloaddition of 58 using chiral Lewis acid 60 (Scheme 14).[73] The significant batchromic shift of >50 nm caused upon complexation of substrate 58 with 60 enables exclusive excitation of the complexed substrate at longer wavelengths, while suppressing the unfavorable ptoreaction of uncomplexed 58 to racemic 59. This strategy is widely applicable to a series of enone derivatives to afford cycloadducts in 80-90% ee at high conversions. Melchiorre and coworkers have reported a catalytic ptochemical a-alkylation of aldehydes 61, which is driven by the excitation of a donor-acceptor complex of the chiral enamine derived from the aldehyde with benzyl bromide 62.[74] As illustrated in Scheme 15, aldehyde 61 first reacts with chiral amine 64 or 65 added as a catalyst (0.2 equivalent) to give the corresponding enamine, which forms a donor-acceptor complex with electron-deficient benzyl bromide 62 in the ground state. Selective ptoexcitation of the charge-transfer band of the complex in the visible region induces electron transfer to give radical cationic enamine and 62 radical anion, the latter of which spontaneously decomposes to give a benzyl radical. The radical attacks the radical cationic enamine to afford an iminium cation, which eventually decomposes to the alkylation product in 83%-94% ee, regenerating chiral amine in the presence of one equivalent of 2,6-lutidine. 7 Summary and Perspective Catalytic supramolecular ptochirogenesis has experienced a rapid progress in strategy and metdology particularly in the last decade. The use of a chiral supramolecular st/assembly significantly enhances the local concentration of substrate and the rate and stereoselectivity of the subsequent chiral ptoreactions, as a consequence of the complex, and long-lasting supramolecular interactions in the ground and excited states. One of the major strategies for achieving catalytic ptochirogenesis in a supramolecular system is to append a hydropbic ptosensitizing moiety to a transparent chiral st molecule. In such a sensitizing st system, the hydropbic sensitizer, being embedded in the st cavity, is masked or disabled in the absence of guest substrate. Upon guest inclusion into the st cavity, the sensitizer moiety moves to the st portal but stays in close contact with the included guest, which allows for highly efficient energy or electron transfer, and the subsequent stereodifferentiating chemical process to take place in the chiral supramolecular environment. This metd is of high quantum efficiency in principle, and affords good to excellent stereoselectivities as described above. Non-sensitizing metds have also been developed for catalytic supramolecular ptochirogenesis. The most crucial issue in this strategy is to avoid or quench the background ptoreactions occurring outside the chiral environment. Background ptoreactions can usually be suppressed by adding a transition or heavy metal ion, which coordinates to free substrate in the bulk solution, and quenches its ptoreaction by facilitating some intersystem crossing with excited substrate. This metd, which consumes part of the available excitation energy, produces inherently low quantum efficiencies. Better quantum efficiencies unique to this ptochemistry can be achieved by increasing the excitation wavelength, which effectively avoids the background reactions of free substrate, through coordination to chiral Lewis acid catalyst, or through charge-transfer complexation with a chiral electron donor/ acceptor, both of which cause batchromic shifts of the original absorption band of the substrate, to allow exclusive excitation or recovery of the coordinated or complexed substrate, even in the presence of excess substrate. From a catalytic point of view, if the affinity of ptoproduct to the chiral st employed is higher than that of the substrate, product inhibition suld occur O + Br Ar Ar OTMS Ar Ar OTMS Ar = 3,5-(CF3)2-C6H3 O H R EWG O H R EWG R Thermal reactivity Ptochemical reactivity EWG R LG LG EWG R Colorless donor LG EWG Acceptor LG e- e- transfer h EWG R Colored EDA complex Chiral radical ion pair Scheme 15. Catalytic ptochemical a-alkylation of aldehyde. to hinder the catalytic use of the supramolecular st. Hence, a modest affinity to substrate, which is lower than that for ptoproduct, is favorable in general, or some other technique for efficient removal of ptoproduct is needed to design an efficient catalytic supramolecular ptochirogenic system. The catalytic supramolecular ptochirogenic systems examined are still limited in number and variety, and the optical (and/or chemical) yields reported are not necessarily satisfactory in some cases, leaving much space for future endeavors. The research in this intriguing interdisciplinary field of ptochemistry, catalysis, asymmetric synthesis, and supramolecular chemistry is certainly fast-growing in number and expanding in area, but we still need to better understand the factors and mechanisms operative in ptoreactions occurring in chiral supramolecular environments to construct more efficient chiral supramolecular systems that are applicable to a wider variety of ptochirogenic reactions. Acknowledgment: This work was supported by the grants from National Natural Science Foundation of [15] Sivaguru J., Natarajan A., Kaanumalle L.S., Shailaja J., Uppili S., Joy A., Ramamurthy V., Asymmetric ptoreactions within zeolites: role of confinement and alkali metal ions, Acc. Chem. Res., 2003, 36, 509-521. [16] Nishioka Y., Yamaguchi T., Kawano M., Fujita M., Asymmetric [2+2] olefin cross ptoaddition in a self-assembled st with remote chiral auxiliaries, J. Am. Chem. Soc., 2008, 130, 8160-8161. 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Chem. Soc., 1995, 117, 11033-11034. [22] Inoue Y., Kosaka S., Matsumoto K., Tsuneishi H., Hakushi T., Tai A., Nakagawa K., Tong L.-H., Vacuum UV ptochemistry in cyclodextrin cavities. Solid state Z-E ptoisomerization of a cyclooctene-b-cyclodextrin inclusion complex, J. Ptochem. Ptobiol., A, 1993, 71, 61-64. [23] Inoue Y., Wada T., Sugahara N., Yamamoto K., Kimura K., Tong L.-H., Gao X.-M., u Z.-J., Liu| Y., Supramolecular ptochirogenesis. 2. enantiodifferentiating ptoisomerization of cyclooctene included and sensitized by 6-O-modified cyclodextrins, J. Org. Chem., 2000, 65, 8041-8050. [24] Gao Y., Inoue M., Wada T., Inoue Y., Supramolecular ptochirogenesis. 3. enantiodifferentiating ptoisomerization of cyclooctene included and sensitized by 6-O-mono(o-metxybenzoyl)cyclodextrin, J. Incl. Phenom. Macrocycl. Chem., 2004, 50, 111-118. [25] Gao Y., Wada T., Yang K., Kim K., Inoue Y., Supramolecular ptochirogenesis in sensitizing chiral nanopore: Enantiodifferentiating ptoisomerization of (Z)-cyclooctene included and sensitized by POST-1, Chirality, 2005, 17, S19-S23. [26] Lu R., Yang C., Cao Y., Tong L., Jiao W., Wada T., Wang Z., Mori T., Inoue Y., Enantiodifferentiating ptoisomerization of cyclooctene included and sensitized by aroyl--cyclodextrins: a critical enantioselectivity control by substituents, J. Org. Chem., 2008, 73, 7695-7701. [27] Lu R., Yang C., Cao Y., Wang Z., Wada T., Jiao W., Mori T., Inoue Y., Supramolecular enantiodifferentiating ptoisomerization of cyclooctene with modified b-cyclodextrins: critical control by a st structure, Chem. Commun., 2008, 374-376. [28] Liang W., Yang C., Nishijima M., Fukuhara G., Mori T., Mele A., Castiglione F., Caldera F., Trotta F., Inoue Y., Cyclodextrin nanosponge-sensitized enantiodifferentiating ptoisomerization of cyclooctene and 1,3-cyclooctadiene, Beilstein J. Org. Chem., 2012, 8, 1305-1311. China (No. 21372165 and 21321061), State Key Laboratory of Polymer Materials Engineering (No. sklpme2014-206) (both for CY), and Japan Society for the Promotion of Science (No. 21245011 and 26620030) (YI), which are gratefully appreciated.
Journal
Supramolecular Catalysis – de Gruyter
Published: Jul 3, 2015