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Gold Catalysis The Benefits of N and N O Ligands.

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Angewandte
Chemie
Homogeneous Catalysis
Gold Catalysis: The Benefits of N and N,O
Ligands**
A. Stephen K. Hashmi,* Jan P. Weyrauch,
Matthias Rudolph, and Elzen Kurpejović
Dedicated to Professor Johann Mulzer
on the occasion of his 60th birthday
The catalysis of organic reactions by gold compounds has
been recently shown to be a powerful tool in synthesis.[1, 2]
When gold(i) compounds are used as precatalysts, ligands
such as phosphanes[3–8] or phosphites[9] can be applied. The
gold(iii) precatalysts are mainly simple halides;[2] other
examples include one thioether-containing,[10] one phosphite-containing,[9] and organogold(iii)[11] compounds.
For the gold-catalyzed phenol synthesis,[12] AuCl3 usually
delivers good results with simple substrates, but with more
complicated ones a significant loss of activity is observed. At
lower temperature, kinetic studies with our most simple testsubstrate 1 (see Scheme 1) showed that the problem with
Figure 1. Reaction of 1 with AuCl3. Reaction conditions: CD3CN, 10 8C,
a) 3.3 mol % AuCl3 ; b) 5.0 mol % AuCl3.
Scheme 1. Conversion of the test-substrate 1 with AuCl3 as the precatalyst; Ts = tosyl (reaction conditions see Figure 1).
regard to the loss of activity occurs even with as much as
5 mol % of catalyst (Figure 1). With small amounts of catalyst,
the conversion remains incomplete.
We have now tested several gold(i) and gold(iii) complexes
with different ligands as catalysts for this reaction. Gold(i)
complexes showed low selectivity and led to several side
products. Satisfactory results in terms of activity, long-term
stability and product-selectivity were obtained only with
gold(iii) complexes with pyridine derivatives, some of which
contained chelating oxygen functionalities. The most interesting complexes were precatalysts 3–6.[13] The complexes did
not suffer deactivation, as shown in Figure 2 for 3—the
activity even holds in a second catalytic run. Unlike with
AuCl3, a mechanistically interesting induction period was
[*] Prof. Dr. A. S. K. Hashmi, Dipl.-Chem. J. P. Weyrauch,
Dipl.-Chem. M. Rudolph, Dipl.-Chem. E. Kurpejović
Institut f9r Organische Chemie
Universit<t Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Fax: (+ 49) 711-685-4321
E-mail: hashmi@hashmi.de
[**] This work was supported by the Fonds der Chemischen Industrie,
J.P.W. is grateful for a Doktoranden-Stipendium. A number of gold
complexes were generously provided by Johnson Matthey.
Angew. Chem. Int. Ed. 2004, 43, 6545 –6547
Figure 2. Reaction of 1 with the precatalyst 3. Reaction conditions:
CD3CN, 45 8C, 5 mol % 3. After 25 h the same amount of 1 was added.
observed for 3–6, clearly proving that here the complexes are
precatalysts. This is also the reason for the higher activity in
the second run, since the catalytically active species is already
present and does not have to be formed in a slow reaction.
With as little as 0.07 mol % of 3 a complete conversion
could be achieved; this corresponds to 1180 instead of the
usual 20–50 turnovers. The complexes 4–6 are also highly
stable catalysts; a comparison of their activity is depicted in
Figure 3: the acceptor-substituted pyridine carboxylate 5 is
the most reactive one, followed by the unsubstituted 4 and the
donor-substituted 6. Nevertheless, the initial activity of 3, 4,
and 6 is lower than that of AuCl3. In part, this problem can be
solved by switching to dichloromethane/acetonitrile mixtures
or even pure dichloromethane (as shown for 3 in Figure 4). In
DOI: 10.1002/anie.200460232
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6545
Communications
Figure 3. Comparison of the activities of 4–6 (with test-substrate 1).
Reaction conditions: room temperature, 5 mol % of catalyst, MeCN.
The major benefit in preparative terms of using these
precatalysts in this reaction is that they enable the preparation
of benzoannelated heterocycles with a phenolic hydroxy
group in a position which is difficult to achieve by employing
classic synthetic methodology.[12d] The most significant targets
are antitumor antibiotics from the tetrahydroisoquinoline
family,[15] which—with this substitution pattern—are not
readily accessible by the Pictet–Spengler or related reactions.[16] As reported previously, with 5 mol % or less AuCl3 no
complete conversion could be achieved,[12a] but 6 gave a
quantitative conversion of 7 a at room temperature
(Scheme 2). And even the Z-protected substrate 7 b (Z =
CO2CH2Ph) gave a clean reaction with 6. With cationic
gold(i) complexes we could not achieve the product selectivity
reported here.
Scheme 2. Conversion of the substrates 7 a and 7 b with the
precatalyst 6.
Figure 4. Activity of 3 in the conversion of 1 in different solvents. Reaction conditions: room temperature, 5 mol % 3, a) CH2Cl2/
MeCN = 80:20; b) CH2Cl2/MeCN = 100:0; c) CH2Cl2/MeCN = 90:10.
spite of the slower conversion, these complexes are superior
to other transition-metal complexes known to catalyze this
reaction (Figure 5).[12c, 14]
In conclusion, the development of the gold(iii) precatalysts 3–6 represents a first step in catalyst-tuning by ligand
design, a major focus of research for most other transition
metals. These results provide good evidence for the establishment of significantly more effective gold catalysts.
Experimental Section
Complex 7 a (300 mg, 861 mmol) and complex 6 (15.7 mg, 38.7 mmol;
4.5 mol %) in CH2Cl2 (1.50 mL) were stirred at room temperature for
4 h. The solvent was removed in vacuo, and column chromatography
yielded 8 a (284 mg; 95 %) .
8 a: M.p. 166–167 8C. Rf (petroleum ether/dichloromethane/ethyl
acetate, 10:3:1) = 0.10; IR (neat): ñ = 3514, 1587, 1530, 1467, 1346,
1311, 1260, 1235, 1216, 1161, 992, 958, 873, 854, 805, 763, 737, 691, 619,
600, 580 cm 1; 1H NMR (CDCl3, 250 MHz): d = 2.20 (s, 3 H), 2.88 (t,
J = 5.9 Hz, 2 H), 3.43 (t, J = 5.9 Hz, 2 H), 4.31 (s, 2 H), 4.82 (s, 1 H), 6.60
(d, J = 7.7 Hz, 1 H), 6.93 (d, J = 7.7 Hz, 1 H), 8.04 (d, J = 9.0 Hz, 2 H),
8.36 ppm (d, J = 9.0 Hz, 2 H); 13C NMR (CDCl3, 126 MHz): d = 15.57
(q), 28.93 (t), 43.82 (t), 43.85 (t), 118.63 (s), 120.12 (s), 121.06 (d),
124.74 (d, 2C), 129.12 (d, 2C), 129.29 (d), 132.53 (s), 143.38 (s), 150.51
(s), 150.53 ppm (s); MS (70 eV): m/z (%): 348 (31)[M+], 161 (100),
134 (41), 91 (10). C, H, N analysis calcd (%) for C16H16N2O5S (348.38):
C 55.16, H 4.63, N 8.04; found: C 55.16, H 4.66, N 8.07.
Received: April 6, 2004
.
Keywords: alkynes · gold · heterocycles · homogeneous
catalysis · N,O ligands
Figure 5. Comparison of the activity of 3 with the activities of different
transition-metal catalysts (product 2). Reaction conditions: 5 mol %
catalyst, AuCl3 : 10 8C, MeCN; 3: 45 8C, MeCN; PdCl2 : 45 8C, MeCN;
[{IrCl(cod)}2]: 45 8C, MeCN; [Ru3(CO)12]: 45 8C, benzene; PtCl2 : 45 8C,
MeCN; [{RhCl(cod)}2]: 45 8C, MeCN.
6546
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2004, 43, 6545 –6547
Angewandte
Chemie
[2] a) A. S. K. Hashmi, Gold Bull. 2003, 36, 3 – 9; b) A. S. K.
Hashmi, Gold Bull. 2004, 37, 51 – 65.
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[13] Complexes 4 and 5 were kindly provided by Johnson Matthey
from their polyarthritis research. Complex 3 was prepared
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Usually the annelation is directed to the para position of an
oxygen substituent on the arene and not to the ortho position.
Angew. Chem. Int. Ed. 2004, 43, 6545 –6547
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6547
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