Gold and palladium combined for cross-coupling

Article abstract of DOI:10.1002/anie.200902942

Gold and palladium-a unique liason: A study of the transmetalation abilities of organogold compounds builds the basis for a new class of cross-coupling reactions. Stable intermediates of gold catalysis deliver new complex products by a palladium-catalyzed coupling reaction, (see Scheme)

Full text of DOI:10.1002/anie.200902942

Angewandte
Chemie
DOI: 10.1002/anie.200902942
Cross-Coupling
Gold and Palladium Combined for Cross-Coupling**
A. Stephen K. Hashmi,* Christian Lothschꢀtz, Renꢁ Dꢂpp, Matthias Rudolph,
Tanuja D. Ramamurthi, and Frank Rominger
The use of gold catalysts as highly active tools for efficient and
atom-economic transformations continues to grow exponen-
tially.[1] In contrast to other transition metals, the most
significant limitation of homogeneous gold catalysts seems
to be the inferior ability of gold to change oxidation states
during catalytic cycles. Of the few reported homogeneous
gold-catalyzed coupling reactions in which changes in oxida-
tion states in catalytic cycles are presumed, they are all
accomplished at elevated temperatures wherein heterogene-
ous catalysis should at least be considered.[2] One approach to
broadening the scope of gold-catalyzed coupling reactions
was accomplished by the use of external stoichiometric
oxidants instead of oxidation by the substrate.[3,4] Whereas
recently only symmetric molecules could be synthesized by
the oxidative dimerization of gold(III) intermediates, Zhang
and co-workers reported an impressive oxidative cross-
coupling reaction using Selectfluor as reoxidizing reagent.[5]
In our opinion, there is another option to extend the scope
of homogeneous gold chemistry: the transmetalation of the
in situ generated organogold species A to other transition
metals such as palladium species B (Scheme 1). In these
reactions strong stoichiometric oxidizing reagents could be
avoided, and the reluctance of the gold species to undergo an
oxidation state change could become an advantage as the
orthogonal reactivity of the two metals could guarantee
highly selective reactions. Our initial experiments to achieve a
double catalytic conversion gave only low yields, potentially
caused by the ligand exchange processes between the two
different metal centers. Therefore we decided to turn to
transmetalation experiments using stoichiometric amounts
organogold compounds to simplify the reaction conditions. To
the best of our knowledge, a general study of the trans-
metalation abilities of organogold compounds with catalytic
amounts of palladium is lacking. So far there are only few
examples for transmetalation and gold.[6]
Herein we present a study of the gold/palladium system by
using stoichiometric amounts of organogold compounds and
catalytic amounts of palladium complexes in cross-coupling
reactions. Our initial foray into the cross-coupling of organo-
gold intermediates began with an assessment of palladium
catalysts 1–7 (Figure 1)[7] for the model reaction of iodoben-
zene and triphenylphosphine vinyl gold 8a. Of the different
Figure 1. Palladium catalysts. Cy=cyclohexyl, dppf=1,1’-bis(diphenyl-
phosphino)ferrocene, MTBE=tert-butylmetyl ether.
catalysts used, 2 delivered the highest conversion rates
(Figure 2). The positive effect of a bidentate ligand on the
palladium lies in the prevention of ligand exchange reactions
between the two metal centers, as these are not chelating
ligands for gold(I) through a linear coordination geometry.
Switching to the use of the monodentate N-hetereocyclic
carbene (NHC) complex 5 delivered only poor results, even
after several days. Encouraged by the results obtained with
the phosphane ligands, we performed a solvent screening
using the optimized catalyst system (2). Changing the type of
solvent seemed to have only a minor impact on transmeta-
lation, the efficiency of which decreased in the following
order: CH3CN ꢀ DMF > dioxane ꢀ toluene > THF > Et2O.
We did not use protic solvents so as to avoid possible
problems caused by protodeauration. To gain insight into the
variability of the cross-coupling, we expanded our procedure
to different organogold species as well as different aryl halides
(Table 1).
Scheme 1. Transmetalation of organogold intermediates.
[*] Prof. Dr. A. S. K. Hashmi, Dipl.-Chem. C. Lothschꢀtz,
Dipl.-Chem. R. Dꢁpp, Dr. M. Rudolph, M. Sc. T. D. Ramamurthi,
Dr. F. Rominger
Organisch-Chemisches Institut
Ruprecht-Karls-Universitꢂt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax: (+49)6221-54-4205
E-mail: hashmi@hashmi.de
Homepage: http://www.hashmi.de
[**] This work was supported by the Chinesisch–Deutsches Zentrum
(GZ 419 (362/3)) and the Deutsche Forschungsgemeinschaft
(HA 1932/11-1); we thank Umicore AG & Co. KG for the generous
donation of noble metal salts; C.L. is thankful for financial support
by the Studienstiftung des dt. Volkes e.V.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902942.
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Communications
Figure 3. Molecular structure of coupling product 9n. Thermal ellip-
soids shown at 50% probability.
Reactivation of the gold halide with silver tosylate allowed us
to run a second cycle of this transformation (Scheme 2). As in
other cross-couplings, the in situ reduction of PdII pre-
catalysts, also shown in Scheme 2, probably proceeds by a
two transmetalation steps and subsequent reductive elimina-
tion.
Figure 2. Catalyst screening.
For a series of different aryl halides and triphenylphos-
phinevinylgold (Table 1, entries 1–5), it turned out that the
use of electron-rich substrates (Table 1, entry 4) led to lower
yields, thus indicating that the oxidative addition step might
be rate determining. By using the diastereomerically pure
trans-styrene gold compound 8 f (Table 1, entry 6), complete
conservation of double bond geometry was observed. Com-
parable results were achieved by the use of triphenylphos-
phine alkynylgold compounds (Table 1, entries 7 and 8), as
well as arylgold compounds (Table 1, entries 9–12). In most
cases our yields were lower when compared to those of
existing cross-coupling methods, however in a number of
cases our yields were higher.[9]
Usually organogold compounds were synthesized by
transmetalation reactions from electropositive metals such
as lithium, magnesium, or boron;[10] recently, Hammond and
co-workers were able to isolate a stable vinylgold compound
as the product of an allene cycloisomerization reaction with
stoichiometric amounts of gold.[11] Naturally, we were excited
to test these substrates for a possible coupling reaction. On
the basis of the unprecedented thermal stability of the gold
furanones used (Table 1, entries 13–19) we were able to
perform these transformations at elevated temperatures.
Under these conditions unactivated aryl halides (Table 1,
entries 16–18) gave moderate to good yields. To prove the
potential of the reaction, we carried out the transformation on
a 2 mmol scale (Table 1, entry 15) with an excellent yield. The
structure of the product 9n was confirmed by X-ray crystallo-
graphic analysis (Figure 3).[8] The latter example might clarify
the scope of the procedure developed.
Scheme 2. Reaction pathway and re-activation of gold(I).
A limitation of this transformation is the use of sterically
hindered aryl halides. By using o,o-disubstituted iodoben-
zene, only the starting organogold compound could be
isolated (Table 1, entry 17).
To investigate the ligand dependency of the transmetala-
tion step, we introduced NHCs as well as noncyclic amino
carbenes (NCAC) as ligands on the organogold compounds
(Table 2). In the case of the mesityl compounds (Table 2,
entry 1 and 2) the conformationally more flexible NCAC
ligand delivered a moderate yield, whereas only poor
conversion was detected in the case of the more sterically
shielding NHC ligand at room temperature (Table 2, entry 2).
Not surprisingly, decreasing steric bulk at the organogold
compound delivered better results (Table 2, entry 3 and 4).
For additional experimental insight into the elemental
steps of the gold/palladium system, we used trans-[PdII-
(PPh3)2I(4-CNC6H4)] to probe reversibility of the transmeta-
lation step (Scheme 3). However, the transmetalation turned
out to be an irreversible process; as shown by the 31P NMR
spectrum which remained unchanged.
By combining the high reactivity of gold for cycloisome-
rization reactions together with its transmetalation ability, it is
possible to generate complex organic compounds which could
not be easily prepared by any other procedure. Even though
this procedure still demands stoichiometric amounts of the
gold compound, a nearly quantitative reisolation (> 99%) of
the gold as triphenylphosphine gold iodide, should make this
procedure attractive for preparative use in organic synthesis.
Scheme 3. The transmetalation is irreversible.
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Angewandte
Chemie
Table 1: Cross-coupling of organogold compounds.
Entry
1
8, R
t [h]
Product
Yield [%]
85[a]
Entry
11
8, R
t [h]
Product
Yield [%]
73
24
8d
48
2
3
8a
8a
24
24
18[a]
12
13
24
4
73
86
88[a]
4
8a
8a
24
24
39[a]
50[a]
14
15
8 f
8 f
4
4
91[c]
95[d]
5[f]
9n
6
24
54
16
8 f
4
84
7
8
24
24
84
46
17
8 f
8 f
8 f
72
72
48
92
37
0[e]
8c
18[f]
9p
9
24
24
74
19
10
8d
76[b]
[a] Yield determined by GC methods; average of two runs. [b] See the Supporting Information for the solid-state structure analysis of 8d.[8] [c] [Ph3PAuI]
isolated in 94% yield. [d] Reaction carried out on 2 mmol scale. 1.1 Equivalent of 4-iodobenzonitrile; isolated 99% of [Ph3PAuI]. [e] The starting
material, [Ph3PAuR], was reisolated in 97% yield. [f] Br instead of I.
In conclusion, we have shown that the transmetalation
from organogold(I) compounds to palladium is a generally
applicable methodology. All the reactions examined could be
carried out under very mild reaction conditions with no
significant solvent influence. In contrast to other coupling
procedures, the addition of any additives was not necessary.
Furthermore, the remarkable stability of the gold compounds
is worth mentioning. Currently, we are working on a version
of this method which is catalytic in both metals by using
substrates which having anions with a less pronounced affinity
to gold relative to the halides, as well as a potential
transmetalation from organogold(III) compounds.
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Communications
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Entry
1
LAuR
Product
Yield [%]
32
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2
9r
2[b]
3
4
82[b]
96
[a] Reaction time was 24 h. [b] Yield was determined by GC methods.
Received: June 1, 2009
Revised: July 13, 2009
Published online: September 29, 2009
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crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Keywords: carbene ligands · cross-coupling · gold · palladium ·
.
transmetalation
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