7 (a) K. Aikawa, M. Kojima and K. Mikami, Angew. Chem., Int.
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9 Review on binol-derived phosphoric acids and amides as Brønsted
acid catalysts: M. Rueping, B. J. Nachtsheim, W. Ieawsuwan and
I. Atodiresei, Angew. Chem., Int. Ed., 2011, 50, 6706–6720.
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and I. Atodiresei, Chem.–Eur. J., 2010, 16, 9350–9365.
11 S. M. Kim, J. H. Park and Y. K. Chung, Chem. Commun., 2011,
47, 6719–6721.
Fig. 1 Cationic and neutral gold(I)–phosphate complexes 9b–c.
12 C. Nieto-Oberhuber, S. Lopez, M. P. Munoz, D. J. Cardenas,
E. Bunuel, C. Nevado and A. M. Echavarren, Angew. Chem., Int.
Ed., 2005, 44, 6146–6148.
Scheme 5 General pathways in the gold(I)-activation of terminal
alkynes.
13 C. Nieto-Oberhuber, M. P. Munoz, S. Lopez, E. Jimenez-Nunez,
C. Nevado, E. Herrero-Gomez, M. Raducan and A. M. Echavarren,
Chem.–Eur. J., 2006, 12, 1677–1693.
14 N. Mezailles, L. Ricard and F. Gagosz, Org. Lett., 2005, 7,
4133–4136.
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Soc., 2005, 127, 6178–6179; (b) C. Nieto-Oberhuber, P. Perez-Galan,
E. Herrero-Gomez, T. Lauterbach, C. Rodrıguez, S. Lopez, C. Bour,
A. Rosellon, D. J. Cardenas and A. M. Echavarren, J. Am. Chem. Soc.,
2008, 130, 269–279.
16 M. Raducan, C. Rodrıguez-Escrich, X. C. Cambeiro, E. Escudero-Adan,
M. A. Pericas and A. M. Echavarren, Chem. Commun., 2011, 47,
4893–4895.
In contrast, these species probably play a minor role in
catalysis with complexes [AuX(L)] or [Au(L)(L0)]X whose
anionic ligands are the conjugate bases of very strong acids,
such as HSbF6, HBF4, or Tf2NH. In these cases, the released
strong Brønsted acid HX shifts the equilibrium towards I and
does not act as a catalyst in the cyclisation as demonstrated by
control experiments carried out with HBF4, TfOH, and
Tf2NH and enynes 10 and 13.25
In the case of gold(I)–phosphate complexes the use of a
protic solvent such as methanol restores the catalytic activity
presumably by facilitating the associative ligand substitution
step12 through activation of the phosphate ligand by an
H-bond and by lowering the basicity of the phosphate anions
by solvation. The lack of reactivity of enyne 13 in the presence
of 7 and 8 shows that the first step of the catalytic cycle that
forms cationic species I is much slower with these neutral
complexes than with cationic gold(I) catalysts [Au(L)(L0)]+Xꢀ
(L0 = weakly coordinating ligand).
17 (a) N. Marion, R. S. Ramon and S. P. Nolan, J. Am. Chem. Soc.,
2009, 131, 448–449; (b) A. Leyva and A. Corma, J. Org. Chem.,
2009, 74, 2067–2074.
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and F. D. Toste, J. Am. Chem. Soc., 2008, 130, 4517–4526;
(b) Y. Odabachian, X.-F. Le Goff and F. Gagosz, Chem.–Eur.
J., 2009, 15, 8966–8970; (c) S. Gaillard, A. M. Z. Slawin and
S. P. Nolan, Chem. Commun., 2010, 46, 2742–2744;
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2010, 46, 2313–2315; (e) G. C. Fortman, A. Poater, J. W. Levell,
S. Gaillard, A. M. Z. Slawin, I. D. W. Samuel, L. Cavallo and
S. P. Nolan, Dalton Trans., 2010, 39, 10382–10390;
(f) A. Simonneau, F. Jaroschik, D. Lesage, M. Karanik,
R. Guillot, M. Malacria, J.-C. Tabet, J.-P. Goddard,
L. Fensterbank, V. Gandon and Y. Gimbert, Chem. Sci., 2011,
DOI: 10.1039/c1sc00478f; (g) M. C. Blanco, J. Camara,
M. C. Gimeno, P. G. Jones, A. Laguna, J. M. Lopez de Luzuriaga,
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10.1021/om200397t; (h) T. J. Brown and R. A. Widenhoefer,
Organometallics, 2011, DOI: 10.1021/om200840g.
This work shows that in order to extend the chiral counter-
ion concept to gold(I)-catalysed activation of alkynes, anionic
ligands less basic than phosphates should be used.
We thank the MICINN (CTQ2010-16088/BQU and Con-
solider Ingenio 2010, Grant CSD2006-0003), the AGAUR
(2009SGR47), Dr E. Escudero-Adan (ICIQ X-Ray Diffraction
unit), and the ICIQ Foundation for financial support.
19 C. Nieto-Oberhuber, M. Paz Munoz, S. Lopez, E. Jimenez-Nunez,
C. Nevado, E. Herrero-Gomez, M. Raducan and A. M. Echavarren,
Chem.–Eur. J., 2006, 12, 1677–1693.
Notes and references
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20 In addition, products assigned to the addition of TfOH or Tf2NH
to the 1,6-enyne were also observed in the crude reaction mixtures.
21 Complex 19 shows two signals in the 31P NMR (CDCl3) at d 30.54
(s, PPh3) and ꢀ10.28 (t, J(31P–19F) = 973.0 Hz, PO2F2).
22 (a) A pKa of 0.3 has been reported for difluorophosphoric acid in
water: J. W. Larson and B. Su, J. Chem. Eng. Data, 1994, 39, 33–35;
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P. Christ, A. G. Lindsay, S. S. Vormittag, J.-M. Neudorf, A. Berkessel
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23 See ESIw for details.
24 (a) A pKa of 4.2 has been determined for phosphoric acid 6 in DMSO:
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and A. C. O’Donoghoue, Chem.–Eur. J., 2011, 17, 8524–8528; (b) The
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25 No reaction was observed with 10 or 13 using HBF4, TfOH, and
Tf2NH (1 and 5 mol%, respectively, 24 h in CDCl3).
c
54 Chem. Commun., 2012, 48, 52–54
This journal is The Royal Society of Chemistry 2012