720
N. Ma et al. / Tetrahedron Letters 52 (2011) 718–720
Table 3
References and notes
Combination influence of chiral amine and b-CD to the photoreaction in H2O
1. (a) Inoue, Y.; Ramavurthy, V. Chiral Photochemistry; New York: Marcel Dekker
Press, 2004; (b) Inoue, Y. Chem. Rev. 1992, 92, 741–770; (c) Renaud, P.; Leong, P.
Science 2008, 322, 55–56.
Molar ratio of inclusion complex
Yields (%)
dl/meso
b-CD
Chiral amine 9
Acetophenone
2. Lattanzi, A.; Scettri, A. Curr. Org. Chem. 2004, 8, 607–621.
1
1
1
2
0
0.5
1
1
1
1
1
79
81
85
80
68:32
71:29
82:18
79:21
3. Matsushita, Y.; Ichimura, T.; Ohba, N.; Kumada, S.; Sakeda, K.; Suzuki, T.;
Tanibata, H.; Murata, T. Pure Appl. Chem. 2007, 79, 1959–1968.
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1
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Q. Youji Huaxue 2006, 26, 599–609.
Table 4
Enfluence of loading NaY zeolites on the reactions
7. Toda, F.; Tanaka, K. Chem. Commun. 1986, 1429–1430.
Entry
The ratio of loaded compounds
Yields (%)
dl/meso
8. Nakamura, A.; Inoue, Y. J. Am. Chem. Soc. 2003, 125, 966–972.
9. Koodanjeri, S.; Ramamurthy, V. Tetrahedron Lett. 2002, 43, 9229–9232.
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12. Zhao, H.; Hsu, D. C.; Carlier, P. R. Synthesis 2005, 1–16.
Chiral amine 9
Acetophenone
1
2
3
4
0.5
1.0
2.0
5.0
1.0
1.0
1.0
1.0
69
70
77
80
67:33
69:31
69:31
70:30
13. Griesbeck, A. G.; Uwe, J. M. Angew. Chem., Int. Ed. 2002, 41, 3147–3154.
14. (a) Nicolaou, K. C.; Yang, Z.; Liu, J. J.; Ueno, H.; Nantermet, P. G.; Guy, R. K.;
Claiborne, C. F.; Renaud, J.; Couladouros, E. A.; Paulvannan, K.; Sorensen, E. J.
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J. Chem. Lett. 2004, 33, 1412–1413; (c) Gracia, I. S.; Dietrich, H.; Bobo, S.; Chiara,
J. L. J. Org. Chem. 1998, 63, 5883–5889; (d) Mühlman, A.; Lindberg, J.; Classon,
B.; Unge, T.; Hallberg, A.; Samuelsson, B. J. Med. Chem. 2001, 44, 3407–3416.
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16. McMurry, J. E.; Fleming, M. P. J. Am. Chem. Soc. 1974, 96, 4708–4709.
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19. (a) Cossy, J.; Belotti, D. Tetrahedron 2006, 62, 6459–6470; (b) Synthetic Organic
Photochemistry; Horspool, W. M., Ed.; New York: Plenum Press, 1984. Chapter
3.
(60–78%), whereas b-CD gave much better results. As shown in Ta-
ble 3, dl/meso ratio increased to 68:32 with the molar ratio of b-CD
and acetophenone as 1:1. Hence, the data indicate that the main
function of b-CD is its special confined spaces, where the move-
ment (rotation, vibration, etc.) of substrate (excited state, reactive
intermediate) is restricted, causing diastereoselectivity.
Interestingly, irradiation of the b-CD–acetophenone inclusion
complex in the presence of chiral tertiary amine 9 provided the
best result. The dl/meso ratio was improved up to 82:18 when
the molar ratio of b-CD, chiral amine, and acetophenone was
1:1:1. The coupling product mixture was further analyzed by HPLC
by a chiral column chromatography (CHIRALCEL AD-H) and 30% ee
was obtained. To the best of our knowledge, this result has not
been reported in the photo-induced pinacol coupling reaction up
to date.
The supramolecular channels and windows of zeolites are an-
other powerful tool in asymmetric photochemical reactions by
their recognition ability. The NaY zeolites were known to induce
diastereoselectivity.1a The photo-induced pinacol coupling reac-
tion of acetophenone loaded on NaY zeolites was studied, and
the results were shown in Table 4. It showed that increasing the
molar ratio of chiral amine 9, slightly improved the dl/meso ratio
from 65:35 to 70:30 while the yields remained in 70–80%.
In summary, a novel and facile electron transfer photo-induced
pinacol coupling reaction of acetophenones in various systems was
developed. We have demonstrated that the substituents of ace-
tophenones and chiral tertiary amines had considerable influence
on both the yield and the ratio of dl/meso. The reactions in b-CD
and NaY zeolites improved the diastereoselectivity as well. The
best result of dl/meso (82:18) was obtained when the title reaction
was carried out in b-CD with combination of the chiral tertiary
amine 9. Further study on the reaction scope and stereoselectivity
is under way.
20. Typical experimental procedure: (a) Procedure of the coupling reactions of
acetophenone in solvent: Triethylamine (0.1 g, 1 mmol) was added to
a
solution of acetophenone (0.12 g, 1 mmol) in methanol (50 mL) in a sealed
Pyrex-glass tube. The solution was purged with nitrogen bubbling for 30 min.
The solution was irradiated under a 500 W medium pressure mercury lamp for
6 h. The reaction was monitored by TLC. After removal of solvent, 30 mL of
ethyl acetate was added to the residue, washed with 5 mL of 10% HCl, saturated
aqueous solutions of NaHCO3 and NaCl for three times, respectively. The
organic layer of solution was dried with anhydrous sodium sulfate and the
solvent was evaporated. The sample mixture was isolated on silica gel column
with elution of petroleum ether–ethyl acetate (4:1) to give of the coupling
product 10 (0.11 g, 92%). The ratios of dl/meso of the product were determined
to be 53:47 by HPLC. 1H NMR d = 1.49 (s, 6H, CH3, dl), 1.58 (s, 6H, CH3, meso),
2.36 (s, 2H, OH, meso), 2.67 (s, 2H, OH, dl), 7.18–7.16 (m, 10H, PhH); ESIMS: m/
z 265 [M+Na]+ (100%).
(b) Procedure of the coupling reactions of acetophenone in b-CD: Preparation
of the inclusion complex of substrates in b-CD: 1.13 g (1 mmol) of b-CD was
dissolved in 50 mL of water with stirring. When 0.12 g of acetophenone
(1 mmol) and 0.15 g of
9 (1 mmol) were added to the solution, white
precipitate appeared immediately. The mixture was heated until the solution
became clear, the heating was maintained for 6 h. The solution was cooled and
the formed white solid was filtrated, washed with n-hexane, and dried. The
suspended solution of the above inclusion complex in 100 mL of water was
transferred to a sealed Pyrex-glass tube. The solution was purged with nitrogen
bubbling for 30 min and irradiated under a 500 W medium pressure mercury
lamp for 6 h. After completion of the reaction, the solution was extracted with
20 mL of ethyl acetate. The organic layer was washed with saturated aqueous
solutions of NaHCO3 and NaCl for three times, respectively, and dried with
anhydrous sodium sulfate. After removal of solvent, the residue was purified
on a silica gel column with elution of petroleum ether–ethyl acetate (4:1) to
give the coupling product 10 (0.106 g, 85%). The ratios of dl/meso of the product
were determined to be 82:18 by HPLC. Compound 10 was further analyzed by
HPLC on a chiral column (CHIRALCEL AD-H) and 30% ee was obtained.
Acknowledgment
This work was supported by the National Science Foundation of
China (20972113/B020502).