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The Catalysis Gold Rush New Claims.

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Highlights
DOI: 10.1002/anie.200502735
Gold Catalysis
The Catalysis Gold Rush: New Claims**
A. Stephen K. Hashmi*
Keywords:
hydrogenation · alkenes · alkynes · gold ·
homogeneous catalysis
1. Introduction
Homogeneous catalysis by gold is
not new: the roots reach back at least
70 years.[1] The catalysts are quite robust; an exclusion of water or oxygen in
most cases is not necessary.[1, 2] Nevertheless, only very few publications in this
field appeared up to the end of the year
2003. In a review on homogenous goldcatalyzed reactions from that year,[1] I
concluded with the sentence “I am
confident that these developments will
accelerate in the near future, and that
most people working in the field of
homogeneous transition-metal catalysis
will also give gold a chance on a regular
basis”. Obviously many investigators
had the same thought: since 2004 a
sharp and continuing increase of the
publications on homogeneous catalysis
by gold complexes can be observed.
Here, I would like to “highlight” some
of the exciting new “claims” in this
emerging field (excluding aspects recently reviewed elsewhere).[3–5]
2. Nucleophilic Additions to
Unactivated Olefins as
Substrates: The Gate to Catalytic
Asymmetric Synthesis
This field has been reviewed,[3c] and twofold addition of alcohols to alkynes[8]
further examples for that type of reac- indicated that even the intermediate
tion have been reported recently.[7] In enol ethers can be activated for the
particular, the addition of water and attack of a second alcohol nucleophile
alcohols to alkynes, which in the case of by the gold catalyst.
The first examples for such goldterminal alkynes cleanly delivers the
Markovnikov product, has become a catalyzed nucleophilic addition reackind of benchmark for gold catalysts.[8] tions were provided by Li et al.[15] In
Intramolecular reactions investigated by Kozmin and
Genet have proven to be
useful tools for organic synthesis.[9]
In 2000 we described the
formation of compound 3
from 1 and suggested the Scheme 2. Addition of pronucleophiles to activated alkenes.
intramolecular addition of a R1, R2 = alkyl, aryl; R3 = aryl.
hydroxy group to a furylsubstituted, styrene-like alkene 2 as the final step in its formation 2004, they successfully used 1,3-dicar(Scheme 1).[10] This was not straightfor- bonyl compounds 4 as pronucleophiles
and activated alkenes 5 (styrene derivatives; in one example also norbornene;
Scheme 2). Li et al.[16] could also extend
that principle to the intermolecular reaction of 4 with cyclic enol ethers 7 and
cyclic 1,3-dienes 9 (Scheme 3). In these
examples the authors propose that the
gold catalyst might activate the C H
Hutchings was the first to recognize
the superiority of gold catalysts for the
electrophilic activation of alkynes.[6]
Scheme 1. Proposed pathway for the formation of 3.
[*] Prof. Dr. A. S. K. Hashmi
Institut f/r Organische Chemie
Universit2t Stuttgart
Pfaffenwaldring 55
70569 Stuttgart (Germany)
Fax: (+ 49) 711-685-4321
E-mail: hashmi@hashmi.de
[**] A.S.K.H. thanks the European Union for
financial support (AURICAT EU-RTN,
HPRN-CT-2002-00174).
6990
ward: Teles et al. had shown that alkynes were highly reactive,[8b] but they
also claimed that alkenes do not react.[11]
Our work,[10] work of Krause et al.,[12]
and a beautiful application of Lee
et al.[13] subsequently demonstrated that
strained allenes also react.[14] But the
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 3. Addition of pronucleophiles to enol
ethers and dienes. R1, R2 = alkyl, aryl.
Angew. Chem. Int. Ed. 2005, 44, 6990 – 6993
Angewandte
Chemie
bond of the nucleophile instead of
activating the alkene for a reaction with
the enol tautomer of 4, but so far there is
no experimental data to support either
model.
Just recently, in a remarkable paper
He et al.[17] reported that at 85 8C in
toluene even unactivated alkenes 12
reacted readily with weak nucleophiles
like phenols 11 or carboxylic acids 14
(Scheme 4)! All these additions follow
Scheme 6. Catalytic asymmetric hydrogenation. Napht = naphthyl.
Scheme 4. Addition of weak nucleophiles to
unactivated alkenes. R1 = OMe, tBu, CO2Me,
NO2 ; R2 = alkyl, aryl; R3 = benzyl, alkyl;
R4 = alkyl, aryl.
Markovnikov?s rule. This regioselectivity is, in contrast to the additions to
alkynes, connected with the formation
of new stereogenic centers even when
terminal alkenes are the starting compounds. Now the possibility for the
development of stereoselective catalysis
was opened.
Ito and Hayashi[18] had provided the
first example for asymmetric gold catalysts almost 20 years ago, and Echavarren et al. recently revived this aspect
with the enyne cyclization catalyzed by
17 (Scheme 5).[7e] Although the ee values are still low, if we extrapolate from
the Ito–Hayashi asymmetric aldol reac-
tion better enantioselectivity can be
expected.
One first example documenting that
trend was reported by Corma et al.:[19]
The [(AuCl)2{(R,R)-Me-Duphos}] complex
(Me-Duphos = 1,2-bis(2,5-dimethylphospholanyl)benzene) can be used
in catalytic enantioselective hydrogenations of esters 19 and imines 22
(Scheme 6). The activities were comparable to the activities of the analoguous
Pt and Ir complexes, but with gold the ee
values were superior. Rather than a
dihydride formed by oxidative addition,
the authors suggest a unique gold(i)
monohydride complex as an intermediate.
3. The First Auraoxetane: Model
Compound for an Organometallic
Intermediate in Gold-Catalyzed
Reactions
Despite significant effort, so far only
two intermediates of gold-catalyzed re-
Scheme 5. Catalytic asymmetric cyclization of an enyne.
Angew. Chem. Int. Ed. 2005, 44, 6990 – 6993
actions have ever been proven experimentally.[20] Unfortunately, these intermediates, a 5-alkylidene-4,5-dihydrooxazole and an arene oxide, were not
organometallic but merely organic compounds. Now, in a spectacular paper
Cinellu et al. managed to isolate and
characterize the very first auraoxetanes[21] as stable organometallic analogues of intermediates in gold-catalyzed reactions. The reaction of the
bridged m-oxo dimer 24 with norbornene
25 led to the alkene complex 26, also a
representative of a rare species, and the
unique auraoxetane 27 (Scheme 7). In
addition, the epoxynorbornane 28 was
identified as the final product of the
reaction. A related reaction of a platinum(ii) compound had been reported
before, but there the oxygen always
coordinated to two platinum centers
and the formation of oxiranes was not
observed.[22]
While Teles et al.[8b] initially suggested a syn oxyauration for the gold(i)catalyzed additions of alcohols to alkynes, we have proven an anti oxyauration for the gold(iii)-catalyzed oxazole
synthesis.[20a] It will be a future challenge
for both theoreticians and experimentalists to verify whether Cinellu?s synaddition mode is a general principle or
the outcome of the preferred exo addition at the strained norbornene. The
ligand used by Cinellu et al. might also
serve as a lead structure for the detection of intermediates in other goldcatalyzed reactions. The diastereoselec-
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6991
Highlights
Scheme 7. Formation of auraoxetane 27.
tivity is also of importance for asymmetric catalysis (see the end of the
Section 2). The prognosis or understanding of the stereoselectivity observed is intrinsically linked to knowing
whether a syn or an anti addition is
occurring.
4. The Oxidation State of the
Active Species: New Pieces of the
Puzzle
Often the same reaction can be
catalyzed by gold(i) as well as gold(iii).[1]
Since it is conceivable that the active
species is formed in situ from the
precatalyst by a change of the oxidation
state (for example, either by reduction
of gold(iii) or by disproportionation of
gold(i) to gold(iii) and gold(0)), the same
catalyst might be operating in both
cases. Periana et al. and Straub set out
to resolve this question by theoretical
calculations, but they could not rule out
simultaneous catalysis by gold(i) and
gold(iii).[23, 24] Nevertheless, all gold-catalyzed reactions discussed so far probably do not involve a change in the
oxidation state of the gold catalyst
(classical steps like oxidative addition
or reductive elimination do not occur) in
the catalytic cycle.
Arcadi et al.[25] developed a synthesis of pyridines 32 from carbonyl compounds 29 and propargylamine 30
(Scheme 8). The initial isomerization
product is the dihydropyridine 31, which
must be dehydrogenated somehow.
Whether H2, which potentially should
reduce the catalyst, is released or a
transfer hydrogenation to the alkyne
occurs, was not investigated.
Corma et al. now have reported the
remarkable observation that gold on
supported nanocrystalline CeO2 efficiently catalyzes the homocoupling of
boronic acids (Scheme 9).[26] Since the
reactivity correlates with the amount of
gold(iii) on the surface, it is assumed that
Scheme 9. Catalytic homocoupling of boronic
acids.
Scheme 8. Dehydrogenation as part of
the gold-catalyzed synthesis of pyridine
derivatives. R1 = H, alkyl, aryl; R2 = H, alkyl,
aryl, heteroaryl.
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the reaction is initiated by a twofold
transmetalation from boron to gold(iii)
followed by reductive elimination of the
biaryl compound. Now the catalytic
cycle can be completed only when
gold(i) is reoxidized. The reaction proceeds in the absence of oxygen, and H2
could be detected by Raman spectroscopy. This suggests that possibly nanoparticles containing gold(i) are directly
oxidized by protons. Corma et al. were
also able to extend this principle to the
reaction of homogeneous gold catalysts
with N,O ligands and similar systems
anchored in zeolites.[27] Unlike with
palladium catalysts in the Suzuki coupling, cross-coupling products were not
observed even in the presence of aryl
iodides.
Ito, Sawamura et al.[28] discovered
the chemoselective dehydrogenative silylation of alcohols (Scheme 10). The
[AuCl(xantphos)] complex (36; xantphos = 9,9-dimethyl-4,5-bis(diphenylphosphanyl)xanthene) allows the formation of the silyl ethers from alcohols
like 35 and triethylsilane in the presence
of other functional groups like alkenes,
alkynes, alkyl halides, tertiary alcohols,
aldehydes, ketones, and carbamates. The
mechanistic proposal, in analogy to
Corma?s asymmetric hydrogenation
(see Section 2) also involves a gold
monohydride.
The latest contribution addressing
the oxidation state of the catalyst was
delivered from Gevorgyan et al.[29] They
used the haloallenyl ketones 38 in goldcatalyzed cycloisomerizations yielding
furans (Scheme 11). With AuCl3 they
observed a preference for the product
40, probably formed by carbonyl coordination and via a bromoirenium zwitterion intermediate, while Au(PR3)Cl
exclusively delivered 42, presumably by
means of initial p coordination. This
concept for the synthesis of halofurans
had previously failed with PdII catalysts
Scheme 10. Silyl ethers prepared by dehydrogenative coupling.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 6990 – 6993
Angewandte
Chemie
[4]
[5]
Scheme 11. Products resulting from precatalysts of different oxidation states. R = C8H17.
[6]
[7]
and a bromoallenyl compound. Only
with a chloroallenyl substrate was a low
yield (2 %) of the corresponding 2chlorofuran obtained.[30]
Gevorgyan et al. presents the very
first example for a gold-catalyzed reaction in which different oxidation states
of the precatalyst lead to different
products. This is a very important contribution to the discussion of the oxidation state of the active gold species.
5. Outlook
Gold catalysis has become a hot spot
in organometallic chemistry. The many
examples published for transformations
that were previously possible only with
other reagents or catalysts, or even
multistep syntheses, demonstrate the
benefits of the gold catalyst, for example, higher activity, higher selectivity,
and milder conditions. It is no longer
sufficient to simply reinvestigate known
reactions and to show that gold has
some activity there. Stereoselectivity
will probably be a major synthetic focus
in the future. The fact that in many
reactions the gold catalysts are superior
to other catalysts is still not understood.
Here further mechanistic insight is a
challenge for experiment and theory.
And the digging for new, unprecedented
reactions certainly continues.
[8]
[9]
[10]
[1] A. S. K. Hashmi, Gold Bull. 2004, 37,
51 – 65.
[2] A. S. K. Hashmi, T. M. Frost, J. W. Bats,
J. Am. Chem. Soc. 2000, 122, 11 553 –
11 554.
[3] Recent reviews: a) A. Hoffmann-RKder,
N. Krause, Org. Biomol. Chem. 2005, 3,
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[11]
387 – 391; b) A. Arcadi, S. Di Giuseppe,
Curr. Org. Chem. 2004, 8, 795 – 812;
c) A. S. K. Hashmi, Gold Bull. 2003,
36, 3 – 9.
C,H activation of methane: D. E. De Vos, B. F. Sels, Angew. Chem. 2005, 117,
30 – 32; Angew. Chem. Int. Ed. 2005, 44,
30 – 32.
Involvement of carbenoids in the enyne
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Chem. 2005, 53, 420 – 423; b) C. Bruneau, Angew. Chem. 2005, 117, 2380 –
2386; Angew. Chem. Int. Ed. 2005, 44,
2328 – 2334.
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295.
a) S. T. Staben, J. J. Kennedy-Smith,
F. D. Toste, Angew. Chem. 2004, 116,
5464 – 5466; Angew. Chem. Int. Ed.
2004, 43, 5350 – 5352; b) T. Yao, X.
Zhang, R. C. Larock, J. Am. Chem.
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2004,
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c) A. S. K. Hashmi, P. Sinha, Adv. Synth.
Catal. 2004, 346, 432 – 438; d) N. Asao,
K. Sato, Meggenbateer, Y. Yamamoto,
J. Org. Chem. 2005, 70, 3682 – 3685;
e) M. P. Munoz, J. Adrio, J. C. Carretero,
A. M. Echavarren, Organometallics
2005, 24, 1293 – 1300; f) M. Alfonsi, A.
Arcadi, M. Aschi, G. Bianchi, F. Marinelli, J. Org. Chem. 2005, 70, 2265 – 2273;
g) A. S. K. Hashmi, L. Grundl, Tetrahedron 2005, 61, 6231 – 6236; h) C. NietoOberhuber, S. LPpez, A. M. Echavarren, J. Am. Chem. Soc. 2005, 127, 6178 –
6179; i) J. P. Markham, S. T. Staben, F. D.
Toste, J. Am. Chem. Soc. 2005, 127,
9708 – 9709.
a) Y. Fukuda, K. Utimoto, J. Org. Chem.
1991, 56, 3729 – 3731; b) J. H. Teles, S.
Brode, M. Chabanas, Angew. Chem.
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Int. Ed. 1998, 37, 1415 – 1418; c) E.
Mizushima, K. Sato, T. Hayashi, M.
Tanaka, Angew. Chem. 2002, 114,
4745 – 4747; Angew. Chem. Int. Ed.
2002, 41, 4563 – 4565; d) R. Casado, M.
Contel, M. Laguna, P. Romero, S. Sanz,
J. Am. Chem. Soc. 2003, 125, 11 925 –
11 935; e) P. Roembke, H. Schmidbaur,
S. Cronje, H. Raubenheimer, J. Mol.
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a) L. Zhang, S. A. Kozmin, J. Am. Chem.
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J. Am. Chem. Soc. 2005, 127, 9976 –
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A. S. K. Hashmi, L. Schwarz, J.-H. Choi,
T. M. Frost, Angew. Chem. 2000, 112,
2382 – 2385; Angew. Chem. Int. Ed.
2000, 39, 2285 – 2288.
J. H. Teles, personal communication. For
this reason allenes but not alkenes were
covered in a patent: J. H. Teles, M.
Schulz (BASF AG), WO-AI 9721648,
1997 [Chem. Abstr. 1997, 127, 121499].
[12] a) A. Hoffmann-RKder, N. Krause, Org.
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[13] P. H. Lee, H. Kim, K. Lee, M. Kim, K.
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Chem. 2005, 117, 1874 – 1877; Angew.
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[14] There is one report of a gold-catalyzed
propargyl Claisen rearrangement/reduction to form b-hydroxy allenes and ghydroxy alkynes as stable products without further intramolecular addition to
the C C multiple bonds: B. D. Sherry,
F. D. Toste, J. Am. Chem. Soc. 2004, 126,
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[15] X. Yao, C.-J. Li, J. Am. Chem. Soc. 2004,
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[20] a) A. S. K. Hashmi, J. P. Weyrauch, W.
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[21] M. A. Cinellu, G. Minghetti, F. Cocco, S.
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[25] G. Abbiati, A. Arcadi, G. Bianchi, S.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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