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Photophysical Properties of Cinchona Organocatalysts in Organic Solvents† Wenwu Qin,‡ Alessandro Vozza, and Albert M. Brouwer* UniVersity of Amsterdam, Nieuwe Achtergracht 129, 1018 WS Amsterdam, The Netherlands ReceiVed: February 28, 2009; ReVised Manuscript ReceiVed: April 22, 2009
The photophysical properties of cinchona organocatalysts 1 (dehydroquinidine) and 2 (9-benzylcupreidine) in several organic solvents have been studied by means of absorption and fluorescence spectroscopy. In addition to a locally excited (LE) state emission near 360 nm for both compounds, broad emission bands were observed at longer wavelengths. For 1, the long-wavelength emission bands are pronounced in polar solvents, and their appearance is accompanied by a strong reduction of the LE emission. Electron transfer from the quinuclidine amino group to the locally excited quinoline ring is a likely mechanism that accounts for these observations. For 2, on the other hand, the long-wavelength emission is relatively strong even in nonpolar solvents. Protonation of the quinuclidine amino group by a strong acid in acetonitrile solution leads to an increase of the LE emission at the expense of the long wavelength bands, both for 1 and 2. Upon addition of water to 1 and 2 in acetonitrile, however, the emission spectra change in a very different way: the long wavelength emission of 1 is suppressed, but that of 2 is enhanced. This indicates that the long-wavelength emission of 2 is to be attributed to proton transfer rather than to electron transfer. 1. Introduction
CHART 1: Compounds Studied
Cinchona alkaloids were among the first organocatalysts applied in asymmetric synthesis,1,2 and many highly enantioselective reactions catalyzed by a wide variety of cinchona alkaloid derivatives have been reported more recently.3-15 For further design and development of new organocatalysts with high enantioselectivity, it is important to get more insight into the mechanisms of the catalytic reactions. Although density functional theory (DFT) and NMR studies have been reported for cinchona bifunctional organocatalysts,16 the understanding of the processes governing enantioselectivity is still not satisfactory. In particular the role of the individual active sites of the catalyst molecules is still unclear.17,18 Fluorescence spectroscopy is an attractive technique that has proven to be a powerful research tool in many fields of science including chemistry, biology, medical sciences, and materials science.19-23 In recent years, applications of fluorescence spectroscopy for catalyst screening and chemical reaction monitoring have been reported.24-27 These applications are based on the change of the fluorescence parameters of the substrate and the product of the catalytic reaction. For example, a method to screen for transition metalcatalyzed reactions based on fluorescence resonance energy transfer (FRET) and the use of this assay to identify catalysts for room-temperature Heck reactions of aryl bromides has been reported. In this case, a strongly fluorescent substrate was converted to a weakly fluorescent product.27 Single molecule fluorescence spectroscopy has recently been used for studying enzymatic catalysis using fluorogenic substrates. Dynamic disorder in enzymatic turnover rates has been observed by real time single-molecule fluorescence experiments.28,29 Alternatively, the changes of the state of the catalyst during the reaction steps can be addressed.30,31 †
Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed. E-mail: a.m.brouwer@ uva.nl. ‡ Present address: College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 73000, P. R. China. E-Mail: qinww@ lzu.edu.cn.
In the work presented in this paper, we have investigated the fluorescence of dehydroquinidine 1 and 9-benzylcupreidine 2 (Chart 1) as representative examples of the group of cinchona organocatalysts. The common fluorophore of this class of molecules is the quinoline group. The main difference between the two compounds is that compound 2 has an aromatic OH group, which is very acidic in the excited state. The secondary OH group at the 9 position was protected as a benzyl ether because the parent cupreidine was found to be very poorly soluble in organic solvents. Compound 2 is an active catalyst, however,10 which indicates that the 9-OH group is not required in the catalytic mechanism. Organocatalytic reactions using cinchona alkaloids are typically carried out in organic solvents, varying from the nonpolar toluene to polar solvents such as acetonitrile. Most fluorescence studies of cinchona alkaloids, however, focus on aqueous acidic solutions32 and reports on the fluorescence of cinchona alkaloids in organic solvents are extremely rare.33 More studies are available on the aromatic fluorophores of the cinchona-type organocatalysts, 6-methoxyquinoline (6MQ) and 6-hydroxyquinoline (6HQ), which are active excited state bases and acids, respectively. Since most studies on these compounds are limited to polar solvents and protic solvents, reference experiments on
10.1021/jp901867h CCC: $40.75 2009 American Chemical Society Published on Web 05/11/2009
Properties of Cinchona Organocatalysts
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6MQ and 6HQ in the same solvents in which we studied 1 and 2 were performed (Chart 1). In the present work we included the interaction of organocatalysts 1 and 2 with acid and water in acetonitrile. This provides a first insight into the interactions of the catalysts with simple electrophilic and hydrogen bonding agents. The effects of the interaction with substrates of catalytic reactions on the fluorescence behavior of 1 and 2 will be reported in due course. 2. Experimental Section Compounds 1 and 2 were obtained from the bio-organic synthesis group of our institute.34 Butyronitrile (PrCN) and acetonitrile (MeCN) were of spectroscopic grade, and were distilled from CaH2 before use. Other solvents for the spectroscopic measurements were of spectroscopic grade and were used without further purification. p-Toluenesulfonic acid monohydrate (pTsOH) (>98.5%), 6-methoxyquinoline (6MQ) (98%), and 6-hydroxyquinoline (6HQ) (95%) were purchased from SigmaAldrich. 6HQ was recrystallized from ethyl acetate (mp ) 192-194 °C). UV/vis absorption spectra were recorded on a Cary 3 (Varian) UV/vis spectrophotometer, while for the fully corrected steady-state excitation and emission spectra a SPEX Fluorolog 3 was employed. Excitation stray light was suppressed using long-pass filters (320 or 335 nm). For the determination of the fluorescence quantum yields (φf), dilute solutions with an absorbance below 0.1 at the excitation wavelength λex were used. Quinine bisulfate in 1 M H2SO4 (λex ) 320 or 330 nm, φf ) 0.55) was used as the fluorescence quantum yield standard.35 In all cases, correction for the refractive index was applied. The Φf values reported are the averages of multiple (generally 3), fully independent measurements. All spectra were recorded at room temperature (23 °C) using nondegassed samples. The titration experiments with acid were carried out by adding small quantities (20 µL) of a stock solution of pTsOH in MeCN to a much larger volume (25 mL) of solutions of 1 or 2. Time-resolved fluorescence data were acquired using the timecorrelated single-photon counting (SPC) technique by an inhouse assembled SPC, which has been described elsewhere.36 Briefly, laser excitation (320 nm) was achieved by frequency doubling the output of a cavity dumped DCM dye laser (Coherent model 700) pumped by a mode-locked Ar+ laser (Coherent 486 AS Mode Locker, Coherent Innova 200 laser). To exclude polarization effects, fluorescence was collected under the magic angle (54.7°). A microchannel plate (Hamamatsu R3809) was used as the detector. The overall instrumental response function (IRF), measured from Raman scattering of doubly deionized water, has a full width at half-maximum of ∼19 ps. All measurements were done at 23 °C. 3. Results and Discussion a. Excited State Behavior of 6MQ and 6HQ. Figure 1 shows the UV/vis absorption and fluorescence emission spectra of 6MQ in several solvents. The absorption spectra have a main band of similar shape as that of 6MQ in other solvents shown in the literature,37 with an absorption maximum at ∼330 nm (S0-S1 transition) and a second peak at the short wavelength side, presumably due to a vibrational transition. A normal, nearly mirror-image, emission band (∼355 nm) could be observed upon excitation at 320 nm. The fluorescence quantum yield Φf is ∼1% in toluene vs ∼7% in MeCN and PrCN. Table 1 summarizes the photophysical data of compounds 6MQ and 6HQ in several solvents. The absorption spectra of 6HQ (not shown) are almost the same as those of 6MQ in all the solvents used. The emission
Figure 1. Normalized absorbance and emission spectra (λex ) 320 nm) of 6MQ in several solvents.
TABLE 1: Photophysical Properties of 6MQ and 6HQ compound
solvent
λabs (nm)
λem (nm)
φfa
6MQ
toluene PrCN MeCN toluene CH2Cl2 PrCN MeCN MeOH
331 329 329 332 330 333 331 331
353 355 358 353 354 358 358 371
0.014 0.075 0.071 0.008 0.043 0.086 0.088 0.107
6HQ
a Fluorescence quantum yield (reference: quinine bisulfate in 1 M H2SO4, φf ) 0.5535).
spectra of 6HQ in toluene and butyronitrile are of similar shape as those of 6MQ in the same solvents. A fluorescence emission band at ∼355 nm was observed. The quantum yield φf increases with solvent polarity (Table 1). In methanol the emissions of 6HQ and 6MQ are significantly red-shifted.37,38 We can conclude that the excited state behavior of 6MQ and 6HQ in most solvents is very similar. b. Absorption and Emission Spectra of Catalysts 1 and 2. UV/vis absorption and fluorescence emission spectra of 1 and 2 in several solvents are depicted in Figure 2. The absorption spectra are of similar shape as those of 6MQ and 6HQ. Table 2 summarizes the photophysical data of compounds 1 and 2. For both catalysts 1 and 2, fluorescence emission shows a strong solvent dependence. The common feature for both compounds is the emission centered near 360 nm, originating from the locally excited (LE) state of the quinoline ring and resembling the emission of the neutral forms of 6MQ and 6HQ. The second, red-shifted band is only apparent in polar solvents in the case of 1, but in all solvents for 2. The second emission band maximum steadily shifts to longer wavelengths in going from nonpolar to polar solvents. Whereas the total fluorescence quantum yields of 6MQ and 6HQ increase with increasing solvent polarity, those of 1 are relatively low in all solvents used (0.01 e φf e 0.06), and first increase but then decrease again upon increasing solvent polarity. A similar trend can be observed for 2, although the total quantum yields are a little higher and reach a maximum of 0.12 in THF. The increase can be related to the same tendency of 6MQ and 6HQ, and the decrease is due to a stronger quenching of the LE emission in more polar solvents. The long wavelength emission band can be attributed to different phenomena. The quinuclidine amino group being a good electron donor and the quinoline a reasonable electron acceptor, the possibility of intramolecular charge transfer (ICT) may be considered. The amino group is also a strong base, and in the case of 2 an aromatic OH group is present which is a
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Figure 2. Normalized absorption spectra and emission spectra (λex ) 320 nm) of 1 (A) and 2 (B) in several solvents. The emission band in THF was scaled to a maximum intensity of 1.0, and the other emission spectra were scaled with the same factor, so that their integrals reflect the relative quantum yields.
TABLE 2: Photophysical Properties of Catalysts 1 and 2 compound
solvent
λabs (nm)
1
toluene CH2Cl2 THF PrCN MeCN toluene CH2Cl2 THF PrCN MeCN
332 333 332 332 332 335 334 338 337 335
2
a
λem (nm) 360 363 361 359, 358, 355, 363, 358 361, 362,
485 520 434 470 490 500
φfa 0.023 0.033 0.063 0.032 0.008 0.06 0.035 0.12 0.021 0.016
Total fluorescence quantum yield.
strong excited state acid, so proton transfer is a likely option in this case. Excited state proton transfer from the 9-OH group in 1 is unlikely because neither this OH group nor the quinuclidine amino group forms part of the excited chromophore. The energy of a pure charge-transfer state in a polar solvent can be estimated from the one-electron standard redox potentials Ered of the electron donor and the electron acceptor according to eq 1.
EICT ≈ e(Ered(D+ /D) - Ered(Α/Α-))
(1)
Unfortunately, the redox potentials are not known, because the electrochemical reduction of quinolines and the electrochemical oxidation of quinuclidines are irreversible processes. In order to assess the likelihood of electron transfer, we resort to a quantum chemical estimation in which energies are evaluated of the neutral and radical ion species in the redox couple using solvation energies from the polarizable continuum model.39 Using the approach of Fu et al.39 we calculated standard
potentials for the oxidation of quinuclidine of 1.3 V and for the reduction of 6-methoxyquinoline of -1.9 V vs NHE, so the energy of the ICT state can be estimated as ca. 3.2 eV (details given in the Supporting Information). The excitation energy corresponding to the LE state is about 3.6 eV, so a significant driving force for charge separation is likely to exist. In less polar solvents, the solvation energy of the ICT species is smaller, favoring the LE emission. The emission band shifts to longer wavelengths with increasing solvent polarity, which is characteristic of a charge-transfer transition from a state with large dipole moment to one with a small dipole moment. In 9-benzylcupreidine 2, the long wavelength emission could be due to ICT as well, but the rather different quantum yields, the different solvent effect, and the evidence below suggest another possibility, namely, intramolecular proton transfer (IPT). c. Time Resolved Fluorescence Spectroscopy. To directly probe the excited state dynamics of 1 and2, fluorescence decay traces in different solvents were collected as a function of emission wavelength λem by the single-photon timing technique. Each fluorescence decay trace was analyzed individually as a sum of three exponential functions in terms of decay times τi and associated preexponential factors ai (i ) 1-3). Details are given in Tables S2 and S3 in the Supporting Information. All attempts to perform global analyses, linking the decay times of different wavelengths, were unsuccessful. Before an attempt is made to analyze the results, it is worth recapitulating the literature data on quinolines. In organic solvents, the fluorescence decays of 6MQ and 6HQ are often single exponential. Their fluorescence lifetimes tend to increase in solvents with higher polarity, especially for 6MQ.37,40 For 6MQ the fluorescence lifetimes are between 0.3 ns in methylcyclohexane and ca. 2.5 ns in methanol.37 For 6HQ, fluorescence lifetimes are close to 1.5 ns in ethylacetate, acetonitrile and methanol.40 Given the complicated fluorescence spectra of 1, we expected the decay kinetics of 1 also to be more complex than that of 6MQ in the same solvents. Moreover, it is wellknown that cinchona alkaloids can adopt different conformations, which may possess different photophysical and photochemical properties.41-43 In toluene, the fluorescence decays of 1 show two main components, with time constants of 0.7 and 0.3 ns. The fluorescence quantum yield of 1 in toluene is higher than that of 6MQ, which indicates that no additional decay channels are accessible. The two decay times can be tentatively attributed to the presence of different conformers. The fluorescence decay in dichloromethane is remarkably different from that in toluene. A fast component (decay time τ3 ≈ 70 ps) is present with large amplitude at short wavelengths (360-400 nm), where the LE emission predominates. It is likely that the more rapid decay compared to the situation in toluene is due to the transition to the ICT state, which is responsible for the red tail of the fluorescence band. If the conversion from the LE to the ICT state is irreversible the short decay time τ3 ≈ 70 ps can be considered as the lifetime of the LE state, which is substantially shortened by the new decay channel opened. Then the slower decay times τ1 ≈ 1.8 ns or τ2 ≈ 0.6 ns could be attributed to the lifetime of the lower energy state. The two additional components, however, also have substantial amplitudes in the range 440 nm. The 5 ns component is actually responsible for most of the emission intensity at long wavelengths. Interestingly, at 490 and 530 nm, a fast rise of the emission could be detected, indicating that the state emitting at long wavelength originates from the LE state. In acetonitrile, at all wavelengths τ3 with a decay time 5.0 ns. The results of fluorescence decay in acetonitrile are similar to those in butyronitrile. The short decay time τ3 ) 30 ps can be considered as the lifetime of the LE state whereas the slow decay with τ1 of ca. 5 ns can be attributed to the lifetime of the IPT state emitting at long wavelength. Faster components, however, are also present in the emission in this range. In general, the decay profiles are complicated due to the involvement of at least two different emitting electronic states, and the likely presence of multiple conformers. A simple compartmental model that could directly link the different emitting species could therefore not be established. d. Protonation. The obvious difference between 6MQ and 6HQ on the one hand and the cinchona alkaloids on the other is the presence of the strongly basic and potentially electron donating amino group of the quinuclidine ring system. In the diprotonated state in water, quinine and analogous cinchona alkaloids have a high fluorescence quantum yield.35,45-47 Quinine (Q) in aqueous solution has two pH dependent acid-base equilibria, characterized by pKa values of 9.7 and 5.7.48 Η+
Η+
-Η+
-Η+
Q {\} QΗ+ {\} QΗ22+ Recently, Takemoto and co-workers proposed two reaction models of the catalytic action of catalysts related to the cinchona alkaloids.49 Protonation of the quinuclidine nitrogen is an important step in both proposed mechanisms and thus we turned to examine the spectral changes of 1 and 2 upon protonation in acetonitrile. The fluorescence emission spectra of 1 in acetonitrile solution at room temperature as a function of pTsOH concentration (Figure 3B) exhibit two prominent bands with maxima at ∼360 and ∼450 nm. Upon increasing [pTsOH], the weak 520 nm (ICT) band of 1 is decreased in intensity, while the 360 nm emission (LE) band strongly increases. The fluorescence quantum yield of the monoprotonated form of 1 (φf ∼ 0.068)
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Figure 4. Emission (λexc ) 320 nm) of 2.5 × 10-5 M of 1 in acetonitrile containing various concentrations of water (%, in v/v).
is almost the same as that of 6MQ in acetonitrile solution (φf ∼ 0.071), whereas the quantum yield of the neutral form (φf ∼ 0.008) is much lower than that of the monocationic form. Upon increasing [pTsOH] further, the quinoline ring is also protonated, leading to the doubly protonated form, which exhibits a fluorescence maximum at ∼450 nm and a much higher intensity, with a fluorescence quantum yield of 0.31 for 1H22+. This approaches the value of QH22+ in acidic solutions. In the range of concentrations in which the neutral and monoprotonated forms of 1 coexist, the UV/vis absorption spectra (figure 3A) do not change much with [pTsOH] while the dication form has a radically different absorption spectrum, with maxima at 350 and 315 nm. Monoprotonation of 2 also leads to the recovery of the LE emission, at the expense of the long wavelength band (Figure 3D). This demonstrates clearly that the quinuclidine nitrogen atom is directly involved in the process that gives rise to the quenching of the LE state and the formation of the long wavelength emitting species. Upon further protonation of the quinoline nitrogen, the absorption and emission spectra change in a way similar to that observed for 1. The emission quantum yield of 2H22+, however, is rather low. The fluorescence decays (Table S2) of 1 in acidified acetonitrile (2.5 × 10-5 M 1 and 5.2 × 10-5 M pTsOH) are almost all biphasic. Under these conditions, the monoprotonated and diprotonated forms coexist (Figure 3A). The lifetimes change with the emission wavelength, as was also found by Pant et al.45 for 6MQ in acidic media. At 480 nm, the emission of 1H22+ is monoexponential, with a decay time of 14 ns, somewhat shorter than those of 6MQH+ or quinine bisulfate.46 The fluorescence decays of 2 in acidified acetonitrile (2.0 × 10-5 M catalyst 2 and 2.0 × 10-5 M pTsOH), could be described by a biexponential decay with τ1 in the 1.8 ns range (slow component) and τ2 of about 0.50 ns (fast component). Under these conditions, the solution mainly contains neutral and monocationic forms of the catalyst 2. In contrast to the situation in pure acetonitrile, the major contribution to the fluorescence emission is the longer-lived species, attributable to the monocation (cf. 1.5 ns for 6HQ in MeCN38). e. Effect of Water. The fluorescence emission spectra of quinine in acetonitrile and dioxane upon addition of water were reported by Fletcher in 1968,33 but no other literature about this phenomenon could be found after that. The absorption spectrum of 1 in acetonitrile does not change upon addition of water. The fluorescence emission spectra, however, change considerably, as shown in Figure 4. In water free acetonitrile solution two bands are observed with maxima at 360 nm and ∼520 nm, which we attributed to emission from the LE state and the ICT state, respectively. Upon addition of water to the solution
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Figure 5. Fluorescence emission spectra (λex ) 330 nm) of 1.1 × 10-5 M of 2 in acetonitrile containing different concentrations of water (% v/v).
SCHEME 1: Proposed Enhanced Excited State Proton Transfer in a Water-Bridged Arrangement in 2
(keeping the concentration of catalyst 1 constant) the 520 nm (ICT) band decreases its intensity, while the 360 nm emission (LE) band increases and shifts to slightly longer wavelengths. This change in emission spectra of 1 is similar to that upon monoprotonation by pTsOH, shown in the previous section. The decrease in emission intensity of the ICT band correlates with the increase of 360 nm LE emission band, which shifts slightly to longer wavelengths with increasing water content. The formation of hydrogen bonds between the quinuclidine amino group and water reduces its availability as an electron donor. At higher water concentration, the amino group (pKa 9.7) is likely to be fully protonated. Catalyst 2 responds more promptly than 1 to water addition in acetonitrile, as can be seen in figure 5. A large increase in emission intensity at long wavelength is observed at even low water content. The clearly different response to water addition hints at a fundamental difference in the mode of interaction between catalysts 1 and 2 and their substrates. In 1, charge transfer occurs, which is suppressed by protonation and by hydrogen bonding to water. In 2, proton transfer occurs, which is suppressed by protonation of the basic amino group, but enhanced by complexation with water. The distance between the 6′-OH and the quinuclidine nitrogen is too large for a direct hydrogen bond to be formed. Excited state proton transfer therefore is not very fast. When a bridging water molecule is inserted, however, proton transfer can occur much more smoothly (Scheme 1). At higher water concentrations, the excited state protonation of the quinoline nitrogen also comes into play.38 4. Conclusions The fluorescence of quinidine-type organocatalysts 1 and2 in organic solvents shows remarkably rich dynamics. The shapes of the absorption bands of the compounds are barely affected by changing the 6′-substituent (MeO in 1, OH in 2) or by the solvents. The fluorescence emissions, on the other hand, are strongly solvent and structure dependent. Dehydroquinidine 1 shows a strong quenching of the fluorescence of the locally excited state in solvents more polar than toluene. With increasing
Properties of Cinchona Organocatalysts solvent polarity, a long-wavelength shoulder develops into a pronounced emission band. This can be attributed to intramolecular electron transfer between the photoexcited quinoline fluorophore and the quinuclidine nitrogen atom. Protonation of the latter by strong acid and deactivation by interaction with water suppress the electron transfer process and restore the LE emission. 9-Benzylcupreidine 2, on the other hand, shows a longwavelength emission band in all solvents. This contrasts with the behavior of 1 and 6HQ: both the 6′-OH and the quinuclidine are required. Upon protonation of the quinuclidine nitrogen, the long-wavelength band disappears and the LE emission is restored. The addition of water has a very different effect on the emission of 2 than on that of 1. If electron transfer would be the cause of the long-wavelength emission band we would expect similar trends in the fluorescence of 1 and 2 in the series of solvents. On the basis of the results we propose that the long wavelength band is due to proton transfer from the 6′-OH to the quinuclidine nitrogen atom. This process is enhanced by small amounts of water. At higher water concentrations, a proton will probably also be transferred rapidly to the quinoline nitrogen, which is an extremely strong excited state base. In catalysis, interactions of the organocatalyst with proton donors and other electrophiles and with hydrogen bond donors and acceptors play a key role. The results obtained here with the simple electrophile H+ and the simple hydrogen bonding water indicate that the fluorescence of the catalysts may strongly respond to this kind of interaction, which implies that fluorescence modulation is likely to occur during the catalytic cycle. The challenge to be addressed in future work is to observe such effects while the catalyst is in action. Acknowledgment. The authors are grateful to Dr. Tommaso Marcelli and Prof. Henk Hiemstra for providing us with the compounds studied and Dr. Junhong Qian for helpful discussions and for carrying out some fluorescence measurements. This research was supported by The Netherlands Research School Combination Catalysis (NRSCC) and in part by NanoNed, a national nanotechnology program coordinated by the Dutch Ministry of Economic Affairs. Supporting Information Available: Brief description of the computational estimation of redox potentials of quinuclidine and 6MQ as models for the electron donor and acceptor units in 1. Tables S2 and S3 provide the detailed results of the fluorescence decay measurements of compounds 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bredig, G.; Fiske, P. S. Biochem. Z. 1913, 46, 7. (2) Hiemstra, H.; Wynberg, H. J. Am. Chem. Soc. 1981, 103, 417. (3) Berkessel, A.; Gro¨ger, H. Asymmetric Organocatalysis; WileyVCH: Weinheim, Germany, 2005. (4) Li, H. M.; Wang, B. M.; Deng, L. J. Am. Chem. Soc. 2006, 128, 732. (5) Vakulya, B.; Varga, S.; Csampai, A.; Soos, T. Org. Lett. 2005, 7, 1967. (6) Wang, Y. Q.; Song, J.; Hong, R.; Li, H. M.; Deng, L. J. Am. Chem. Soc. 2006, 128, 8156. (7) Song, J.; Wang, Y.; Deng, L. J. Am. Chem. Soc. 2006, 128, 6048. (8) Li, H. M.; Wang, Y. Q.; Deng, L. Org. Lett. 2006, 8, 4063. (9) Bella, M.; Jorgensen, K. A. J. Am. Chem. Soc. 2004, 126, 5672. (10) Marcelli, T.; van der Haas, R. N. S.; van Maarseveen, J. H.; Hiemstra, H. Angew. Chem., Int. Ed. 2006, 45, 929.
J. Phys. Chem. C, Vol. 113, No. 27, 2009 11795 (11) Marcelli, T.; van Maarseveen, J. H.; Hiemstra, H. Angew. Chem., Int. Ed. 2006, 45, 7496. (12) Hammar, P.; Marcelli, T.; Hiemstra, H.; Himo, F. AdV. Synth. Catal. 2007, 349, 2537. (13) van Steenis, D.; Marcelli, T.; Lutz, M.; Spek, A. L.; van Maarseveen, J. H.; Hiemstra, H. AdV. Synth. Catal. 2007, 349, 281. (14) Dondoni, A.; Massi, A. Angew. Chem., Int. Ed. 2008, 47, 4638. (15) Connon, S. J. Chem. Commun. 2008, 2499. (16) Bassan, A.; Zou, W. B.; Reyes, E.; Himo, F.; Cordova, A. Angew. Chem., Int. Ed. 2005, 44, 7028. (17) Fu, A. P.; List, B.; Thiel, W. J. Org. Chem. 2006, 71, 320. (18) Clemente, F. R.; Houk, K. N. Angew. Chem., Int. Ed. 2004, 43, 5766. (19) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer-Verlag: New York, 2006. (20) Haugland, R. P. The Handbook. A Guide to Fluorescent Probes and Labeling Technologies, 10th ed.; Molecular Probes, Inc: Eugene, OR, 2005. (21) Lakowicz, J. R. E. Topics in Fluorescence Spectroscopy, Vol. 1, Techniques; Plenum Press: New York, 1991. (22) Baeyens, W. G. R.; de Keukeleire, D.; Korkidis, K. Luminescence Techniques in Chemical and Biochemical Analysis; Marcel Dekker: New York, 1991. (23) Valeur, B. Molecular Fluorescence. Principles and Applications; Wiley-VCH: Weinheim, Germany, 2002. (24) Tanaka, F.; Thayumanavan, R.; Barbas, C. F. J. Am. Chem. Soc. 2003, 125, 8523. (25) Copeland, G. T.; Miller, S. J. J. Am. Chem. Soc. 1999, 121, 4306. (26) Stauffer, S. R.; Beare, N. A.; Stambuli, J. P.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 4641. (27) Stambuli, J. P.; Stauffer, S. R.; Shaughnessy, K. H.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 2677. (28) De Cremer, G.; Roeffaers, M. B. J.; Baruah, M.; Sliwa, M.; Sels, B. F.; Hofkens, J.; De Vos, D. E. J. Am. Chem. Soc. 2007, 129, 15458. (29) Martinez, V. M.; De Cremer, G.; Roeffaers, M. B. J.; Sliwa, M.; Baruah, M.; De Vos, D. E.; Hofkens, J.; Sels, B. F. J. Am. Chem. Soc. 2008, 130, 13192. (30) Xie, X. S.; Lu, H. P. J. Biol. Chem. 1999, 274, 15967. (31) Xie, X. S. J. Chem. Phys. 2002, 117, 11024. (32) Aloisi, G. G.; Barbafina, A.; Canton, M.; Dall’Acqua, F.; Elisei, F.; Facciolo, L.; Latterini, L.; Viola, G. Photochem. Photobiol. 2004, 79, 248. (33) Fletcher, A. N. J. Phys. Chem. 1968, 72, 2742. (34) Marcelli, T. Cinchona-derived organocatalysts for asymmetric carbon-carbon bond formation. Ph.D. Thesis, University of Amsterdam: Amsterdam, 2006. (35) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229. (36) van Dijk, S. I.; Wiering, P. G.; Groen, C. P.; Brouwer, A. M.; Verhoeven, J. W.; Schuddeboom, W.; Warman, J. M. J. Chem. Soc. Faraday Trans. 1995, 91, 2107. (37) Pant, D.; Tripathi, H. B.; Pant, D. D. J. Photochem. Photobiol. A-Chem. 1991, 56, 207. (38) Mehata, M. S.; Joshi, H. C.; Tripathi, H. B. Chem. Phys. Lett. 2002, 359, 314. (39) Fu, Y.; Liu, L.; Yu, H. Z.; Wang, Y. M.; Guo, Q. X. J. Am. Chem. Soc. 2005, 127, 7227. (40) Mehata, M. S.; Joshi, H. C.; Tripathi, H. B. Chem. Phys. Lett. 2002, 366, 628. (41) Burgi, T.; Baiker, A. J. Am. Chem. Soc. 1998, 120, 12920. (42) Dijkstra, G. D. H.; Kellogg, R. M.; Wynberg, H.; Svendsen, J. S.; Marko, I.; Sharpless, K. B. J. Am. Chem. Soc. 1989, 111, 8069. (43) Hamza, A.; Schubert, G.; Soos, T.; Papai, I. J. Am. Chem. Soc. 2006, 128, 13151. (44) Poizat, O.; Bardez, E.; Buntinx, G.; Alain, V. J. Phys. Chem. A 2004, 108, 1873. (45) Pant, D.; Tripathi, H. B.; Pant, D. D. J. Lumin. 1991, 50, 249. (46) Meech, S. R.; Phillips, D. J. Photochem. 1983, 23, 193. (47) Velapoldi, R. A.; Mielenz, K. D. Standard Reference Materials: A Fluorescence Standard Reference Material: Quinine Sulfate Dihydrate; National Bureau of Standards Special Publication 260-64; National Bureau of Standards: Washington, DC, 1980 http://ts.nist.gov/MeasurementServices/ ReferenceMaterials/upload/SP260-64.pdf. (48) Pedros, J.; Porcar, I.; Gomez, C. M.; Campos, A.; Abad, C. Spectroc. Acta Part A 1997, 53, 421. (49) Okino, T.; Hoashi, Y.; Furukawa, T.; Xu, X. N.; Takemoto, Y. J. Am. Chem. Soc. 2005, 127, 119.
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