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Figure 1. 1H NMR spectra of 2a (600 MHz) in CDCl3 containing: (a) 0 equiv; (b) 1 equiv; (c) 3 equiv and (d) 5 equiv of TfOH.
O
O
O
O
O
MeO
MeO
+ TfOH
+
HO N
N
O
OTf
O
5
2a
Scheme 2. Equilibrium observed in the 2a–TfOH system.
1985, 1, 352–355; (c) Ellison, E. H.; Moodley, D.; Hime, J. J. Phys. Chem., B. 2006,
110, 4772–4781; (d) Sheng, X.; Peng, A.; Fu, H.; Yao, J. Colloids Surf. A:
Physicochem. Eng. Aspects 2007, 308, 136–140; (e) Ambruster, C.; Knapp, M.;
Rechthaler, K.; Schamschule, R.; Parusel, A. B. J.; Köhler, G.; Wehrman, W. J.
intermediacy of p-methoxybenzoic acid or its protonated form was
excluded since we did not observe the acylation of ferrocene by
this acid in the presence of TfOH under the same reaction
conditions.
In conclusion, we have demonstrated that N-hydroxysuccinim-
idyl, tetrafluorophenyl and phenyl esters of benzoic and p-methoxy-
benzoic acid are activated by triflic acid under milder conditions
than an alkyl ester (methyl benzoate), and can be used for efficient
Friedel–Crafts acylation of electron-rich arenes such as ferrocene
and pyrene. To the best of our knowledge, the reactions described
in this work constitute the first example of the use of active esters
for the formation of C–C bonds. Its scope and limitations are cur-
rently under study and the results will be published in due course.
ˇ
Photochem. Photobiol., A: Chem. 1999, 125, 28–29; (f) Búcsiová, L.; Hrdlovic, P.;
Chmela, Š. J. Photochem. Photobiol., A: Chem. 2001, 143, 59–68; (g) Yao, C.;
Kraatz, H.-B.; Steer, R. P. Photochem. Photobiol. Sci. 2005, 4, 191–199.
10. N-Hydroxysuccinimidyl esters 2a,b were prepared according to published
procedures.11 Preparation of tetrafluorophenyl esters 2c,d: To a solution of a
carboxylic acid (10 mmol) and 2,3,5,6-tetrafluorophenol (10 mmol) in CH2Cl2
(100 ml), was added a solution of DCC (20 mmol) in CH2Cl2 (30 ml) and the
mixture stirred at rt for 24 h. The resulting precipitate was filtered and the
filtrate concentrated in vacuo to ꢀ25% of its initial volume and the product
precipitated with pentane, filtered and dried. The esters 2c,d were used in the
Friedel–Crafts reactions without further purification. Spectroscopic data: 2c:
1H NMR (600 MHz, acetone-d6) d: 3.96 (s, 3H); 7.17 (m, 2H); 7.58 (m, 1H); 8.19
(m, 2H); 13C NMR (150 MHz, CDCl3) d: 56.36 (s); 104.68 (m); 115.56 (s); 119.83
(s); 133.77 (s); 141.15 (m); 142.74 (m); 146.39 (m); 148.02 (m); 163.03 (s);
166.23 (s). 2d: d: 7.05 (m, 1H, Ar); 7.55 (m, 2H,); 7.70 (m, 1H); 8.22 (m, 2H). 13
C
References and notes
NMR (150 MHz, CDCl3) d: 103.25 (m); 127.25 (s); 128.86 (s); 130.72 (s); 134.58
(s); 140.02 (m); 141.75 (m); 145.29 (m); 146.96 (m); 162.59 (s).
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active esters: Arene (0.5 mmol) and ester (1 mmol) were dissolved in
CH2Cl2(1 ml). To this solution was added triflic acid (132 ll, 1.5 mmol) at
room temperature with vigorous stirring. Stirring was continued for 1 h and
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product.
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and IR spectra with those described in the literature.14
_
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ˇ
ˇ
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