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NMR analysis of photochromism of bisthiazolylindenols
Tetrahedron Letters 54 (2013) 6366–6369
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
NMR analysis of photochromism of bisthiazolylindenols F. G. Erko a, J. Berthet a, H. Ogawa b, Y. Yokoyama b, S. Delbaere a,⇑ a b
Université Lille Nord de France, UDSL, CNRS UMR 8516, BP83, F-59006 Lille Cedex, France Graduate School of Chemistry, Yokohama National University, Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
a r t i c l e
i n f o
Article history: Received 29 July 2013 Revised 5 September 2013 Accepted 13 September 2013 Available online 24 September 2013 Keywords: Photochromism NMR spectroscopy Diarylethenes Diastereoselectivity Solvent polarity
a b s t r a c t The photochromic reaction of two derivatives of bisthiazolylindenols was investigated in three solvents of various polarities (C6D12, THF-d8 and CD3CN) by NMR spectroscopy. Photoirradiation can generate two diastereomeric closed forms. High conversion ratio and large diastereomer excess were obtained and the reasons of the excellent properties were discussed in terms of intramolecular hydrogen bonds, steric bulkiness of substituents, and solvent polarities. Two sets of intramolecular nitrogen–hydrogen interactions fix the conformation of the two investigated bisthiazolylindenols in favor of cyclization, which also lead to high diastereoselectivity. The N–H interactions are partially disrupted when polarity of the solvent increases. Ó 2013 Elsevier Ltd. All rights reserved.
Diarylethenes are one of the most promising photochromic compounds to apply to optical devices and photoresponsive functional materials because of their excellent fatigue-resistance and their thermal irreversibility.1,2 Their photochromism is based on the conrotatory 6p-electrocyclization between a hexatriene and a cyclohexadiene, the latter being a pair of enantiomers generated from the photoreactive anti-parallel conformers of the hexatriene counterpart.3 When the molecule possesses at least one chiral center, the photochromic ring-closure generates a pair of diastereomers instead of enantiomers.4 Stereoselectivity of up to 100% in cyclization reactions have been realized with several diarylethenes5,6 with the concept to eliminate the ground state conformation of the compound which would generate the minor diastereomer. On a different but related system, a high quantum yield for the photocyclization has been achieved by fixing the molecular conformation in favor of cyclization.7 In this way, the fixation of the conformation inducing not only the high quantum yield for photocyclization but also the high diastereoselectivity was recently reported for bisthiazolylindenols.8 Here we present the structural aspects of the photochromic behavior of two bisthiazolylindenols 1o and 2o (Scheme 1) in three solvents (cyclohexane, C6D12, tetrahydrofuran, THF-d8, and acetonitrile, CD3CN) investigated by NMR spectroscopy. All 1H NMR spectra of both 2,3-bis(5-methyl-2-phenyl-4-thiazolyl)-1-phenyl-1-indenol (1o) and 1-tert-butyl-2,3-bis(5-methyl2-phenyl-4-thiazolyl)-1-indenol (2o) in the three deuterated solvents show only one set of signals. This could indicate either the ⇑ Corresponding author. Tel.: +33 2 096 4013; fax: +33 2 095 9009. E-mail address: [email protected] (S. Delbaere). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.09.063
presence of only one conformer as a result of the locking of antiparallel conformation by hydrogen bonding or the presence of two conformers (antiparallel and parallel ones) but they are in fast equilibrium. It is known that both antiparallel and parallel conformations cannot be separated by NMR due to the fast interconversion between them unless the substituents or the aromatic rings themselves are large enough.9,10 Since the thiazole ring is not large, the last case seems to be the most plausible. Successive irradiation with 313-nm light was applied to solutions of 1o in C6D12, THF-d8, and CD3CN. The photochemical process was followed by 1H NMR spectra.11 Signals of initial 1o decreased while new signals were detected. In C6D12, 1o was converted into the sole cyclized form 1c-1, whereas two cyclized isomers, 1c-1 and 1c-2 were observed when the photoirradiation was carried out in CD3CN and THF-d8. Figure 1 illustrates the evolution of signals belonging to 1o and diastereomers of 1c in THF-d8 before and after irradiation of 313-nm light. One can observe the decrease in intensity of OH signal at 6.1 ppm and the two methyl signals at 1.87 and 2.03 ppm in 1o, and the appearance of new resonances characterizing two photoproducts in quite different ratios. By measuring the peak-intensities of well-separated signals in spectra recorded at regular time of irradiation, the time-evolution of concentrations of isomers of 1 can be plotted (Fig. 2-left) for the three solutions. In C6D12, 97.5% of 1o was converted into cyclized 1c-1. In THFd8, 82% of 1c-1 was produced in addition to 14.5 % of the second cyclized isomer 1c-2. Similarly, in CD3CN, 79.8% of 1c-1 and 11.9% of 1c-2 were detected after photoirradiation. The same procedures were applied to 2o. Irradiation with 313nm light generated only one cyclized compound (2c-1) in the three
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2o in C6D12
1o in C6D12 100
100 80
80
1o
1c-1
60
2o
60
2c-1
%
% 40
40
20
20
Scheme 1. Photochromic reaction of bisthiazolylindenols.
0
0 0
100 time / mn
200
1o in THF-d8
100
0
40
100
80 120 160 200 time / mn
2o in THF-d8
80
80 1o
60 %
%
1c-2
40
2o
60
1c-1
2c-1
40 20
20 1
Figure 1. H NMR spectra of 1o in THF-d8. (a) before irradiation; (b) after irradiation.
solvents used, that is C6D12, THF-d8, and CD3CN. The photochemical process was followed by recording 1H NMR spectra between each irradiation period. Signals of initial 2o decreased while new signals were detected. In Figure 3, the NMR spectra recorded before and after 313-nm light irradiation of 2o in THF-d8 are depicted. The time-evolution of concentrations of 2 in the three solvents used is illustrated in Figure 2-right. The conversion of 2c-1 reaches 98.6% in C6D12, 91% in THF-d8, and 89.1% in CD3CN. Since the photoproducts resulting from 313-nm light irradiation of 1o and 2o returned to their original states upon visible light irradiation, it was confirmed that they have the photocyclized structures 1c and 2c, respectively. To identify them precisely, 1D and 2D NMR experiments were carried out. Since the photochemical cyclization to form a cyclohexadienetype structure is the carbon–carbon bond formation, the hybridization of carbon atoms from sp2 into sp3 occurs. 1H-13C HMBC evidences long-range correlations in 1c-1 in THF-d8 (Fig. 4) between the methyl signal at 1.86 ppm with carbon signals at 69.1 and 154.1.4 ppm, and between the methyl signal at 2.00 ppm and carbon signals at 69.2 and 151.3 ppm. The second cyclized compound 1c-2 presents long-range correlations between the methyl signal at 1.84 ppm with carbon signals at 69.6 and 150.8 ppm, and between the methyl signal at 1.96 ppm and carbon signals at 69.2 and 152.7 ppm. Dipolar contacts between protons were investigated by acquiring NOESY. In 1c-1, the aromatic protons at 7.52 ppm (in THF-d8) on the ortho position of the phenyl attached to indenol are in close proximity with the methyl group at 1.86 ppm, while the hydroxy proton at 5.17 ppm is in dipolar contact with the methyl at 2.00 ppm (Fig. 5). For 2c-1 in THF-d8, long-range correlations (Fig. 6) between the methyl signal at 2.11 ppm with carbon signal at 69.6 and 154.4 ppm, and between the methyl signal at 1.89 ppm and carbon signals at 68.9 and 150.7 ppm are observed.
0
0
1o in CD3CN
100
0
40
80 120 160 200 time / mn
2o in CD3CN
100
80
80 1o
60
60
1c-1
%
1c-2
40
2o 2c-1
% 40 20
20 0
0
30 60 90 120 150 time / mn
0 0
30 60 90 time / mn
120
0
20 40 time / mn
60
Figure 2. Time-evolution of concentrations of isomers upon UV irradiation.
Dipolar correlations are measured between the sharp tert-butyl signal at 1.10 ppm with the aromatic proton at 7.52 ppm, the hydroxy signal at 4.46 ppm, and the methyl signal at 2.11 ppm (Fig. 7). The full assignments for the two major cyclized compounds, 1c1 and 2c-1 are displayed in Figure 8. From the NMR experiments, it is apparent that the major diastereomers generated by the photochemical ring closure from 1o and 2o are 1c-1 and 2c-1, respectively. This means that the compounds have taken the conformation in favor of conrotatory cyclization by the hydrogen bonds between the hydroxy group and the nitrogen atom on the thiazole ring in close proximity in their open forms. The conversion ratio (cr) and the diastereomer excess (de) of 1 and 2 investigated by UV–vis spectroscopy and HPLC in hexane, ethyl ether and acetonitrile8 are shown in Table 1 with the data obtained by NMR this time.
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Figure 3. 1H NMR spectra of 2o in THF-d8. (a) Before irradiation; (b) after irradiation.
Figure 6. HMBC of 2c-1 in THF-d8.
Figure 4. HMBC of 1c-1 and 1c-2 in THF-d8.
Figure 7. 1H–1H NOESY of 2c-1 in THF-d8.
Figure 5. Part of 1H–1H NOESY of 1c-1 in THF-d8.
Although the solvents used in both studies are not the same except acetonitrile, they can be classified by their polarities. Indeed,
C6D12 and hexane are the less polar, the dielectric constant e = 2.0 and 1.88, respectively, while THF-d8 and ethyl ether have medium values, e = 7.6 and 4.33, respectively, and acetonitrile the most polar (e = 37.5). It can be concluded that the diastereoselectivity is dependent on the solvent polarity and the nature of the substituents. In C6D12, the photocylization of phenyl derivative 1o occurs with the highest cr to give only one diastereomer, 1c-1. When a more polar solvent is used, the cr is slightly lower and a second photocyclised diastereomer is formed, that is the de decreased.
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The second molecule with a tert-butyl substituent, 2o, follows the same trend in cr. It diminished when changing the solvent from less polar to more polar. As for the de, however, only one diastereomer was formed in all solvents. The bulky tert-butyl group overwhelmed the solvent polarity effect, as only one diastereomer was detected whatever the solvent. To conclude, photochromism of two derivatives of indenols was investigated by NMR in three solvents with different polarities. The diastereomer excess of 1c in cyclohexane proved to be the largest, and the lowest in acetonitrile. This can be explained by that the two sets of intramolecular nitrogen hydrogen interactions (Na Ha–Ar and Nb HbO– in Scheme 1) fix the conformation of 1o and 2o in favor of cyclization, and also in favor of diasteroselective manner. The polar N H interactions are partially disrupted when polarity of the solvent increases. However, the substituent bulkiness is also dominant since 2o with the bulky tert-butyl group showed the generation of only one diastereomer by NMR in any solvents used. Acknowledgements The 300 and 500 MHz NMR facilities were funded by the Région Nord-Pas de Calais (France), the Ministère de la Jeunesse de l’Education Nationale et de la Recherche (MJENR) and the Fonds Européens de Développement Régional (FEDER). Part of this collaborative work was performed within the framework of GDRI CNRS 93 ‘Phenics’ (Photoswitchable Organic Molecular Systems & Devices). References and notes 1. 2. 3. 4. 5. Figure 8. 1H and 13C NMR assignments of 1c-1 and 2c-1 (data obtained in THF-d8). 6. Table 1 Conversion ratio (cr) and diastereomer excess (de) of indenols 1o and 2o at the photostationary state
a b
NMR solvents (e) cr de HPLC solvents (e) cr de
1o/1c C6D12 (2.0) THF-d8 (7.6) 97.5% 96.5% a — 85/15 Hexane (1.88) Ethyl ether (4.33) 96.6b% 97.3b% 99.1/0.9b 92/8b
CD3CN (37.5) 91.7% 87/13 CH3CN (37.5) 92.9b% 86.7/13.3b
NMR solvents (e) cr de HPLC solvents (e) cr de
2o/2c C6D12 (2.0) THF-d8 (7.6) 98.6% 91.0% —a —a Hexane (1.88) Ethyl ether (4.33) 97.9%b 97.0%b >99.9/<0.1b 99.8/0.2b
CD3CN (37.5) 89.1% —a CH3CN (37.5) 94.9%b 99.7/0.3b
Another diastereomer not detected by 1H NMR. Values from Ref. 8, obtained by HPLC and UV–vis spectra.
7. 8. 9. 10. 11.
Irie, M.; Uchida, K. Bull. Chem. Soc. Jpn. 1988, 71, 985–996. Irie, M.; Mohri, M. J. Org. Chem. 1988, 53, 803–808. Irie, M. Chem. Rev. 2000, 100, 1685–1716. Yokoyama, Y. New J. Chem. 2009, 33, 1314–1319. Tani, Y.; Ubukata, T.; Yokoyama, Y.; Yokoyama, Y. J. Org. Chem. 2007, 72, 1639– 1644. Shiozawa, T.; Hossain, M. K.; Ubukata, T.; Yokoyama, Y. Chem. Commun. 2010. 4785-4723. Morinaka, K.; Ubukata, T.; Yokoyama, Y. Org. Lett. 2009, 11, 3890–3893. Ogawa, H.; Takagi, K.; Ubukata, T.; Okamoto, A.; Yonezawa, N.; Delbaere, S.; Yokoyama, Y. Chem. Commun. 2012, 11838–11840. Uchida, K.; Nakayama, Y.; Irie, M. Bull. Chem. Soc. Jpn. 1990, 63, 1311–1315. Irie, M.; Sakemura, K.; Okinaka, M.; Uchida, K. J. Org. Chem. 1995, 60, 8305– 8309. NMR spectra were recorded on Bruker 500 or 300 MHz spectrometer equipped with TXI or QNP probe. Samples were irradiated, directly in the NMR tube (5 mm) using a 1000 W Xe–Hg high-pressure filtered short-arc lamp (Oriel). The light was filtered by passing through a filter first (Schott 11FG09: 259 < k < 388 nm with kmax = 330 nm, T = 79%), then through an interferential one (k = 313 nm and T = 16%).
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
NMR analysis of photochromism of bisthiazolylindenols F. G. Erko a, J. Berthet a, H. Ogawa b, Y. Yokoyama b, S. Delbaere a,⇑ a b
Université Lille Nord de France, UDSL, CNRS UMR 8516, BP83, F-59006 Lille Cedex, France Graduate School of Chemistry, Yokohama National University, Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
a r t i c l e
i n f o
Article history: Received 29 July 2013 Revised 5 September 2013 Accepted 13 September 2013 Available online 24 September 2013 Keywords: Photochromism NMR spectroscopy Diarylethenes Diastereoselectivity Solvent polarity
a b s t r a c t The photochromic reaction of two derivatives of bisthiazolylindenols was investigated in three solvents of various polarities (C6D12, THF-d8 and CD3CN) by NMR spectroscopy. Photoirradiation can generate two diastereomeric closed forms. High conversion ratio and large diastereomer excess were obtained and the reasons of the excellent properties were discussed in terms of intramolecular hydrogen bonds, steric bulkiness of substituents, and solvent polarities. Two sets of intramolecular nitrogen–hydrogen interactions fix the conformation of the two investigated bisthiazolylindenols in favor of cyclization, which also lead to high diastereoselectivity. The N–H interactions are partially disrupted when polarity of the solvent increases. Ó 2013 Elsevier Ltd. All rights reserved.
Diarylethenes are one of the most promising photochromic compounds to apply to optical devices and photoresponsive functional materials because of their excellent fatigue-resistance and their thermal irreversibility.1,2 Their photochromism is based on the conrotatory 6p-electrocyclization between a hexatriene and a cyclohexadiene, the latter being a pair of enantiomers generated from the photoreactive anti-parallel conformers of the hexatriene counterpart.3 When the molecule possesses at least one chiral center, the photochromic ring-closure generates a pair of diastereomers instead of enantiomers.4 Stereoselectivity of up to 100% in cyclization reactions have been realized with several diarylethenes5,6 with the concept to eliminate the ground state conformation of the compound which would generate the minor diastereomer. On a different but related system, a high quantum yield for the photocyclization has been achieved by fixing the molecular conformation in favor of cyclization.7 In this way, the fixation of the conformation inducing not only the high quantum yield for photocyclization but also the high diastereoselectivity was recently reported for bisthiazolylindenols.8 Here we present the structural aspects of the photochromic behavior of two bisthiazolylindenols 1o and 2o (Scheme 1) in three solvents (cyclohexane, C6D12, tetrahydrofuran, THF-d8, and acetonitrile, CD3CN) investigated by NMR spectroscopy. All 1H NMR spectra of both 2,3-bis(5-methyl-2-phenyl-4-thiazolyl)-1-phenyl-1-indenol (1o) and 1-tert-butyl-2,3-bis(5-methyl2-phenyl-4-thiazolyl)-1-indenol (2o) in the three deuterated solvents show only one set of signals. This could indicate either the ⇑ Corresponding author. Tel.: +33 2 096 4013; fax: +33 2 095 9009. E-mail address: [email protected] (S. Delbaere). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.09.063
presence of only one conformer as a result of the locking of antiparallel conformation by hydrogen bonding or the presence of two conformers (antiparallel and parallel ones) but they are in fast equilibrium. It is known that both antiparallel and parallel conformations cannot be separated by NMR due to the fast interconversion between them unless the substituents or the aromatic rings themselves are large enough.9,10 Since the thiazole ring is not large, the last case seems to be the most plausible. Successive irradiation with 313-nm light was applied to solutions of 1o in C6D12, THF-d8, and CD3CN. The photochemical process was followed by 1H NMR spectra.11 Signals of initial 1o decreased while new signals were detected. In C6D12, 1o was converted into the sole cyclized form 1c-1, whereas two cyclized isomers, 1c-1 and 1c-2 were observed when the photoirradiation was carried out in CD3CN and THF-d8. Figure 1 illustrates the evolution of signals belonging to 1o and diastereomers of 1c in THF-d8 before and after irradiation of 313-nm light. One can observe the decrease in intensity of OH signal at 6.1 ppm and the two methyl signals at 1.87 and 2.03 ppm in 1o, and the appearance of new resonances characterizing two photoproducts in quite different ratios. By measuring the peak-intensities of well-separated signals in spectra recorded at regular time of irradiation, the time-evolution of concentrations of isomers of 1 can be plotted (Fig. 2-left) for the three solutions. In C6D12, 97.5% of 1o was converted into cyclized 1c-1. In THFd8, 82% of 1c-1 was produced in addition to 14.5 % of the second cyclized isomer 1c-2. Similarly, in CD3CN, 79.8% of 1c-1 and 11.9% of 1c-2 were detected after photoirradiation. The same procedures were applied to 2o. Irradiation with 313nm light generated only one cyclized compound (2c-1) in the three
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2o in C6D12
1o in C6D12 100
100 80
80
1o
1c-1
60
2o
60
2c-1
%
% 40
40
20
20
Scheme 1. Photochromic reaction of bisthiazolylindenols.
0
0 0
100 time / mn
200
1o in THF-d8
100
0
40
100
80 120 160 200 time / mn
2o in THF-d8
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80 1o
60 %
%
1c-2
40
2o
60
1c-1
2c-1
40 20
20 1
Figure 1. H NMR spectra of 1o in THF-d8. (a) before irradiation; (b) after irradiation.
solvents used, that is C6D12, THF-d8, and CD3CN. The photochemical process was followed by recording 1H NMR spectra between each irradiation period. Signals of initial 2o decreased while new signals were detected. In Figure 3, the NMR spectra recorded before and after 313-nm light irradiation of 2o in THF-d8 are depicted. The time-evolution of concentrations of 2 in the three solvents used is illustrated in Figure 2-right. The conversion of 2c-1 reaches 98.6% in C6D12, 91% in THF-d8, and 89.1% in CD3CN. Since the photoproducts resulting from 313-nm light irradiation of 1o and 2o returned to their original states upon visible light irradiation, it was confirmed that they have the photocyclized structures 1c and 2c, respectively. To identify them precisely, 1D and 2D NMR experiments were carried out. Since the photochemical cyclization to form a cyclohexadienetype structure is the carbon–carbon bond formation, the hybridization of carbon atoms from sp2 into sp3 occurs. 1H-13C HMBC evidences long-range correlations in 1c-1 in THF-d8 (Fig. 4) between the methyl signal at 1.86 ppm with carbon signals at 69.1 and 154.1.4 ppm, and between the methyl signal at 2.00 ppm and carbon signals at 69.2 and 151.3 ppm. The second cyclized compound 1c-2 presents long-range correlations between the methyl signal at 1.84 ppm with carbon signals at 69.6 and 150.8 ppm, and between the methyl signal at 1.96 ppm and carbon signals at 69.2 and 152.7 ppm. Dipolar contacts between protons were investigated by acquiring NOESY. In 1c-1, the aromatic protons at 7.52 ppm (in THF-d8) on the ortho position of the phenyl attached to indenol are in close proximity with the methyl group at 1.86 ppm, while the hydroxy proton at 5.17 ppm is in dipolar contact with the methyl at 2.00 ppm (Fig. 5). For 2c-1 in THF-d8, long-range correlations (Fig. 6) between the methyl signal at 2.11 ppm with carbon signal at 69.6 and 154.4 ppm, and between the methyl signal at 1.89 ppm and carbon signals at 68.9 and 150.7 ppm are observed.
0
0
1o in CD3CN
100
0
40
80 120 160 200 time / mn
2o in CD3CN
100
80
80 1o
60
60
1c-1
%
1c-2
40
2o 2c-1
% 40 20
20 0
0
30 60 90 120 150 time / mn
0 0
30 60 90 time / mn
120
0
20 40 time / mn
60
Figure 2. Time-evolution of concentrations of isomers upon UV irradiation.
Dipolar correlations are measured between the sharp tert-butyl signal at 1.10 ppm with the aromatic proton at 7.52 ppm, the hydroxy signal at 4.46 ppm, and the methyl signal at 2.11 ppm (Fig. 7). The full assignments for the two major cyclized compounds, 1c1 and 2c-1 are displayed in Figure 8. From the NMR experiments, it is apparent that the major diastereomers generated by the photochemical ring closure from 1o and 2o are 1c-1 and 2c-1, respectively. This means that the compounds have taken the conformation in favor of conrotatory cyclization by the hydrogen bonds between the hydroxy group and the nitrogen atom on the thiazole ring in close proximity in their open forms. The conversion ratio (cr) and the diastereomer excess (de) of 1 and 2 investigated by UV–vis spectroscopy and HPLC in hexane, ethyl ether and acetonitrile8 are shown in Table 1 with the data obtained by NMR this time.
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Figure 3. 1H NMR spectra of 2o in THF-d8. (a) Before irradiation; (b) after irradiation.
Figure 6. HMBC of 2c-1 in THF-d8.
Figure 4. HMBC of 1c-1 and 1c-2 in THF-d8.
Figure 7. 1H–1H NOESY of 2c-1 in THF-d8.
Figure 5. Part of 1H–1H NOESY of 1c-1 in THF-d8.
Although the solvents used in both studies are not the same except acetonitrile, they can be classified by their polarities. Indeed,
C6D12 and hexane are the less polar, the dielectric constant e = 2.0 and 1.88, respectively, while THF-d8 and ethyl ether have medium values, e = 7.6 and 4.33, respectively, and acetonitrile the most polar (e = 37.5). It can be concluded that the diastereoselectivity is dependent on the solvent polarity and the nature of the substituents. In C6D12, the photocylization of phenyl derivative 1o occurs with the highest cr to give only one diastereomer, 1c-1. When a more polar solvent is used, the cr is slightly lower and a second photocyclised diastereomer is formed, that is the de decreased.
F. G. Erko et al. / Tetrahedron Letters 54 (2013) 6366–6369
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The second molecule with a tert-butyl substituent, 2o, follows the same trend in cr. It diminished when changing the solvent from less polar to more polar. As for the de, however, only one diastereomer was formed in all solvents. The bulky tert-butyl group overwhelmed the solvent polarity effect, as only one diastereomer was detected whatever the solvent. To conclude, photochromism of two derivatives of indenols was investigated by NMR in three solvents with different polarities. The diastereomer excess of 1c in cyclohexane proved to be the largest, and the lowest in acetonitrile. This can be explained by that the two sets of intramolecular nitrogen hydrogen interactions (Na Ha–Ar and Nb HbO– in Scheme 1) fix the conformation of 1o and 2o in favor of cyclization, and also in favor of diasteroselective manner. The polar N H interactions are partially disrupted when polarity of the solvent increases. However, the substituent bulkiness is also dominant since 2o with the bulky tert-butyl group showed the generation of only one diastereomer by NMR in any solvents used. Acknowledgements The 300 and 500 MHz NMR facilities were funded by the Région Nord-Pas de Calais (France), the Ministère de la Jeunesse de l’Education Nationale et de la Recherche (MJENR) and the Fonds Européens de Développement Régional (FEDER). Part of this collaborative work was performed within the framework of GDRI CNRS 93 ‘Phenics’ (Photoswitchable Organic Molecular Systems & Devices). References and notes 1. 2. 3. 4. 5. Figure 8. 1H and 13C NMR assignments of 1c-1 and 2c-1 (data obtained in THF-d8). 6. Table 1 Conversion ratio (cr) and diastereomer excess (de) of indenols 1o and 2o at the photostationary state
a b
NMR solvents (e) cr de HPLC solvents (e) cr de
1o/1c C6D12 (2.0) THF-d8 (7.6) 97.5% 96.5% a — 85/15 Hexane (1.88) Ethyl ether (4.33) 96.6b% 97.3b% 99.1/0.9b 92/8b
CD3CN (37.5) 91.7% 87/13 CH3CN (37.5) 92.9b% 86.7/13.3b
NMR solvents (e) cr de HPLC solvents (e) cr de
2o/2c C6D12 (2.0) THF-d8 (7.6) 98.6% 91.0% —a —a Hexane (1.88) Ethyl ether (4.33) 97.9%b 97.0%b >99.9/<0.1b 99.8/0.2b
CD3CN (37.5) 89.1% —a CH3CN (37.5) 94.9%b 99.7/0.3b
Another diastereomer not detected by 1H NMR. Values from Ref. 8, obtained by HPLC and UV–vis spectra.
7. 8. 9. 10. 11.
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