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N. Watanabe et al.
LETTER
(2) Smith, M. B. March’s Advanced Organic Chemistry; John
Wiley: Hoboken, 2013, 7th ed., 609-615.
2-Tritylfuran (6)
Colorless needles; yield: 186 mg, (60%); mp 201–202 °C
(hexane–Et2O); Rf = 0.24 (hexane). FTIR (KBr): 1595, 1490,
1442 cm–1. 1H NMR (500 MHz, CDCl3): δ = 6.04 (dd, J =
2.0, 1.0 Hz, 1 H), 6.32 (dd, J = 2.0, 1.5 Hz, 1 H), 7.06–7.11
(m, 6 H), 7.22–7.28 (m, 9 H), 7.45 (br s, 1 H). 13C NMR
(125.8 MHz, CDCl3): δ = 60.84, 109.78, 111.31, 126.58
(×3), 127.60 (×6), 130.20 (×6), 142.25, 145.14 (×3), 159.28.
Anal. Calcd for C23H18O: C, 89.00; H, 5.85. Found: C, 89.15;
H, 5.47.
(3) For reviews, see: (a) Bandini, M.; Melloni, A.; Tommasi, S.;
Umani-Ronchi, A. Synlett 2005, 1199. (b) Almasi, D.;
Alonso, D. A.; Nájera, C. Tetrahedron: Asymmetry 2007, 18,
299. (c) Vicario, J. L.; Badía, D.; Carrillo, L. Synthesis 2007,
2065. (d) You, S.-L.; Cai, Q.; Zeng, M. Chem. Soc. Rev.
2009, 38, 2190. (e) Terrasson, V.; Marcia de Figueiredo, R.;
Campagne, J. M. Eur. J. Org. Chem. 2010, 2635. (f) Lu,
H.-H.; Tan, F.; Xiao, W.-J. Curr. Org. Chem. 2011, 15,
4022. (g) Lu, L.-Q.; Chen, J.-R.; Xiao, W.-J. Acc. Chem.
Res. 2012, 45, 1278.
(4) For monographs, see: (a) Berkessel, A.; Gröger, H.
Asymmetric Organocatalysis: From Biomimetic Concepts to
Applications in Asymmetric Synthesis; Wiley-VCH:
Weinheim, 2005. (b) Enantioselective Organocatalysis:
Reactions and Experimental Procedures; Dalko, P. I., Ed.;
Wiley-VCH: Weinheim, 2007. (c) Science of Synthesis:
Asymmetric Organocatalysis: Workbench Edition; Vol. 1;
List, B., Ed.; Thieme: Stuttgart, 2012. (d) Science of
Synthesis: Asymmetric Organocatalysis: Workbench
Edition; Vol. 2; Maruoka, K., Ed.; Thieme: Stuttgart, 2012.
(e) Comprehensive Enantioselective Organocatalysis;
Dalko, P. I., Ed.; Wiley-VCH: Weinheim, 2013.
(5) (a) McCubbin, J. A.; Hosseini, H.; Krokhin, O. V. J. Org.
Chem. 2010, 75, 959. (b) McCubbin, J. A.; Krokhin, O. V.
Tetrahedron Lett. 2010, 51, 2447. (c) McCubbin, J. A.;
Nassar, C.; Krokhin, O. V. Synthesis 2011, 3152. See also:
(d) Zheng, H.; Ghanbari, S.; Nakamura, S.; Hall, D. G.
Angew. Chem. Int. Ed. 2012, 51, 6187.
(6) (a) Westmark, P. R.; Gardiner, S. J.; Smith, B. D. J. Am.
Chem. Soc. 1996, 118, 11093. (b) Kheirjou, S.; Abedin, A.;
Fattahi, A. Comput. Theor. Chem. 2012, 1000, 1.
(7) For a review, see: (a) Zhang, Z.; Schreiner, P. R. Chem. Soc.
Rev. 2009, 38, 1187. For some recent examples on halide
anion capture in thiourea catalysis, see: (b) Raheem, I. T.;
Thiara, P. S.; Peterson, E. A.; Jacobsen, E. N. J. Am. Chem.
Soc. 2007, 129, 13404. (c) Reisman, S. E.; Doyle, A. G.;
Jacobsen, E. N. J. Am. Chem. Soc. 2008, 130, 7198. (d) De,
C. K.; Klauber, E. G.; Seidel, D. J. Am. Chem. Soc. 2009,
131, 17060. (e) Knowles, R. R.; Lin, S.; Jacobsen, E. N.
J. Am. Chem. Soc. 2010, 132, 5030. (f) Brown, A. R.; Kuo,
W.-H.; Jacobsen, E. N. J. Am. Chem. Soc. 2010, 132, 9286.
(g) Birrell, J. A.; Desrosiers, J.-N.; Jacobsen, E. N. J. Am.
Chem. Soc. 2011, 133, 13872. (h) Min, C.; Mittal, N.; De, C.
K.; Seidel, D. Chem. Commun. 2012, 48, 10853.
(8) For our previous works on thiourea-based organocatalysis,
see: (a) Mori, K.; Maddaluno, J.; Nakano, K.; Ichikawa, Y.;
Kotsuki, H. Synlett 2009, 2346. (b) Mori, K.; Maddaluno, J.;
Nakano, K.; Ichikawa, Y.; Kotsuki, H. Synlett 2011, 2080.
(c) Sasakura, N.; Nakano, K.; Ichikawa, Y.; Kotsuki, H. RSC
Adv. 2012, 2, 6135. (d) Moritaka, M.; Miyamae, N.; Nakano,
K.; Ichikawa, Y.; Kotsuki, H. Synlett 2012, 23, 2554.
(9) Jakab, G.; Tancon, C.; Zhang, Z.; Lippert, K. M.; Schreiner,
P. R. Org. Lett. 2012, 14, 1724.
2,5-Ditritylfuran (7)
Colorless needles; yield: 108 mg (39%); mp >280 °C
(sublimed, CHCl3); Rf = 0.15 (hexane). FTIR (KBr): 1594,
1492, 1444 cm–1. 1H NMR (500 MHz, CDCl3): δ = 5.96 (s,
2 H), 6.98–7.05 (m, 12 H), 7.15–7.25 (m, 18 H). 13C NMR
(125.8 MHz, CDCl3): δ = 60.92 (×2), 110.92 (×2), 126.35
(×6), 127.52 (×12), 130.27 (×12), 144.85 (×6), 159.51 (×2).
Anal. Calcd for C42H32O·H2O: C, 88.39; H, 6.00. Found: C,
88.58; H, 5.75.
(12) (a) The use of molecular sieves as acid scavengers is well
established; see: Encyclopedia of Reagents for Organic
Synthesis; Vol. 9; Paquette, L. A.; Crich, D.; Fuchs, P. L.;
Molander, G. A., Eds.; Wiley: Chichester, 2009, 2nd ed.,
7162–7165. (b) Unfortunately, the same reaction in the
presence of a tertiary amine base such as Et3N as an acid
scavenger gave a rather complex mixture of products, and
the yields were not determined.
(13) When the same reaction was conducted by adding 10 mol%
of TBACl, the reaction was completely suppressed,
indicating the important contribution of the common-ion
effect.
(14) Judging from this result, there is some possibility that 3 Ǻ
MS not only acted as an acid scavenger, but also an
accelerator.
(15) Judging from the result shown in entry 9 of Table 1, we were
obliged to conclude that diphenylthiourea 3 acted as a
catalyst poison in this system..
(16) We did not check the recyclability of catalyst 1. However,
after the reaction was complete, in most cases 1 could be
clearly detected by TLC and should be recoverable.
(17) In a control experiment (Table 2, entry 1), only a trace
amount of products was formed in the absence of catalyst 1
(36 h in refluxing DCE).
(18) Unfortunately, the product distribution did not change when
the reaction was performed in the presence of a radical
inhibitor such as hydroquinone (6 h in refluxing DCE): 14
(44%), 15 (17%), and Ph3CH (20%). This product
distribution is similar to that obtained the presence of a
radical initiator such as AIBN: 14 (45%), 15 (8%), and
Ph3CH(26%). Importantly, the formation of furanylmethyl
radicals has only been noted under conditions of photo-
irradiation. See, for example: Cantrell, T. S.; Allen, A. C.;
Ziffer, H. J. Org. Chem. 1989, 54, 140.
(19) All attempts to perform the tritylation of 1,4-dimethoxy-
benzene in the presence of catalyst 1 failed for reasons that
are not yet clear.
(20) For example, see: Zhang, J.; Bellomo, A.; Creamer, A. D.;
Dreher, S. D.; Walsh, P. J. J. Am. Chem. Soc. 2012, 134,
13765.
(10) As expected, the use of TrOH in place of TrCl was
ineffective in this transformation.
(11) 2-Tritylfuran (6) and 2,5-Ditritylfuran (7); Typical
Procedure (Table 1, Entry 4)
Catalyst 1 (50.0 mg) and powdered activated 3 Ǻ MS (10
mg) were added to a colorless solution of Ph3CCl (5, 279 mg,
1.0 mmol) and freshly distilled furan (340 mg, 5.0 mmol) in
dry CH2Cl2 (2.0 mL), and the mixture was stirred for 4 h at
r.t. At the end of the reaction, the solution became reddish
purple. The mixture was concentrated and the residue was
purified by column chromatography [silica gel, hexane–
Et2O (50:1 to 10:1)] to give 6 and 7.
(21) In a control experiment (Table 3, entry 2), the reaction was
significantly retarded in the absence of catalyst 1 (6 h in
refluxing DCE), and gave 25% of 27 along with 7% of 28.
(22) We cannot exclude the possibility that an SN2-type
mechanism might also operate in reactions with 8 as the
electrophile. At present, we have no clear idea of the
halophilic strength (Br versus Cl) of catalyst 1.
Synlett 2014, 25, 438–442
© Georg Thieme Verlag Stuttgart · New York