Scope and limitations of palladium-catalyzed cross-coupling reactions with organogold compounds

Article abstract of DOI:10.1002/adsc.201000159

Five different alkenylgold(I) phosphane complexes were prepared and then investigated in [1,1′-bis(diphenylphosphino)ferrocene]palladium(II) dichloride-catalyzed cross-coupling reactions with different aryl halides, heterocyclic halides, an alkenyl halide, an alkynyl halide, allylic substrates, benzyl bromide and an acid chloride. With regard to the halides, the iodides were highly reactive, bromides or chlorides gave significantly reduced yields or failed, allylic acetates failed, too. The cross-coupling partners contained a number of different functional groups, while free carboxylic acids did not deliver cross-coupling products and o,o-disubstituted arenes failed as well, a broad range of other functional groups like nitro groups, nitrile groups, ester groups, α,β-unsaturated ester groups and lactones, aldehydes, alkoxy groups, pyridyl groups, thienyl groups, unprotected phenols and anilines, even aryl azides were tolerated. The structures of one alkenylgold(I) species and of four of the cross-coupling products were proved by crystal structure analyses.

Full text of DOI:10.1002/adsc.201000159

FULL PAPERS
DOI: 10.1002/adsc.201000159
Scope and Limitations of Palladium-Catalyzed Cross-Coupling
Reactions with Organogold Compounds
A. Stephen K. Hashmi,a,* Renꢀ Dçpp,a Christian Lothschꢁtz,a Matthias Rudolph,a
Dominic Riedel,a and Frank Romingera
a
Organisch-Chemisches Institut, Ruprecht-Karls-Universitꢂt Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg,
Germany
Fax : (+49)-(0)6221-54-4205; phone: (+49)-(0)6221-54-8413; e-mail: hashmi@hashmi.de
Received: February 28, 2010; Published online: May 5, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000159.
Abstract: Five different alkenylgold(I) phosphane cross-coupling products and o,o-disubstituted arenes
complexes were prepared and then investigated in failed as well, a broad range of other functional
[1,1’-bis(diphenylphosphino)ferrocene]palladium(II)
groups like nitro groups, nitrile groups, ester groups,
dichloride-catalyzed cross-coupling reactions with a,b-unsaturated ester groups and lactones, aldehydes,
different aryl halides, heterocyclic halides, an alkenyl alkoxy groups, pyridyl groups, thienyl groups, unpro-
halide, an alkynyl halide, allylic substrates, benzyl tected phenols and anilines, even aryl azides were
bromide and an acid chloride. With regard to the tolerated. The structures of one alkenylgold(I) spe-
halides, the iodides were highly reactive, bromides or cies and of four of the cross-coupling products were
chlorides gave significantly reduced yields or failed, proved by crystal structure analyses.
allylic acetates failed, too. The cross-coupling part-
ners contained a number of different functional Keywords: alkenes; allenes; cross-coupling; gold;
groups, while free carboxylic acids did not deliver heterocycles; palladium
Introduction
clude cross-couplings[3] and homocouplings,[4] homo-
coupling using external stoichiometric oxidants[5] like
To a large extent modern organic synthesis uses tran- BAIB,[5a] t-BuOOH,[5b] or Selectfluor[5c] and cross-cou-
sition metal-catalyzed cross-coupling reactions for C
C bond formation. Among the transition metals em-
pling using stoichiometric Selectfluor.[6]
À
For a long time, alkenylgold species have been pro-
ployed for cross-coupling, palladium is still the most posed as intermediates of gold-catalyzed reactions
frequently used,[1] which includes the synthesis of nat- with allenes or alkynes.[7] These alkenylgold species
ural products and pharmaceuticals.[2] The mechanism usually regenerate the active catalyst by a fast proto-
of these palladium-catalyzed cross-coupling reactions deauration step, more seldom by trapping with alter-
consists of three different steps, the oxidative addition native electrophiles.[8] Last year Hammond et al. suc-
of aryl or alkenyl halides, triflates etc., the transmeta- ceeded in isolating a stabile alkenylgold(I) species de-
lation step and the reductive elimination which regen- rived from allenic substrates bearing an acceptor sub-
erates the active catalyst species and sets free the stituent on the allene unit.[9] Other alkenylgold(I) spe-
product of the C C bond formation (Scheme 1). A cies obtained recently from allenes[10] or alkynes were
À
number of organometallic compounds derived from less stable.[11] A new dimension for gold catalysis
electropositive metals has been used for the transme- would be the use of alkenylgold intermediates for a
talation step; the name reactions shown in Scheme 1 transmetalation to different transition metals like pal-
go along with the use of B, Sn, Zn and Mg. Unlike ladium, linking two catalytic cycles. The reluctance of
palladium, gold is not a good catalyst for cross-cou- gold to undergo a change in oxidation state would
pling reactions, it usually does not readily undergo a then become an advantage as the orthogonal reactivi-
change in oxidation state (especially oxidative addi- ty of both metals should guarantee highly selective
tion) under mild reaction conditions. Still, in the last conversions. Although a few examples of the transme-
years some examples have been reported, which in- talation of different organogold compounds to various
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A. Stephen K. Hashmi et al.
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Scheme 1. Transmetalation as a key step in palladium-catalyzed cross-coupling.
transition metals were known,[12] no general study in (Scheme 3). Moderate to good yields were obtained.
combination with a cross-coupling catalytic cycle ex- The only exception was 2f, with R1 =Ph no cyclization
isted prior to our first communication on this sub- took place. This is in accordance with Hammondꢄs re-
ject.[13] Although these reactions were catalytic in Pd sults. We were able to crystallize compound 3b and
but stoichiometric in gold, a nearly quantitative reiso- performed a crystal structure analysis (Figure 1). Two
lation of gold as Ph3PAuI could already render this independent molecules are linked by an aurophilic in-
À
À
method attractive. While the potential combination of teraction (Au Au distance 3.1783 ꢅ, Au P distances
gold and palladium catalysis is under investigation,[14] 2.301 ꢅ and 2.292 ꢅ). This is in accordance with the
in the context of the known literature the interpreta- results reported by Hammond et al. for 3d.[9] A minor
tion has to be done with great care.[15]
aberration from linearity is observed for the C Au P
Here we present the scope and limitations of the angles: 173.08 and 169.68.
À
À
gold/palladium transmetalation in palladium-cata-
lyzed cross-coupling.
Based on our previous results, the cross-coupling
reactions of alkenylgold compounds 3 with various C-
electrophiles have been carried out at 608C under a
nitrogen atmosphere in dry and degassed MeCN
Results and Discussion
using 1 mol% of [PdCl
In general, the use of a nitrogen atmosphere was
(dppf)].[13]
2ACHTUNGTRENNUNG
Allenoates 2a–f were prepared according to literature found to be crucial. For example, a cross-coupling
protocols by a Wittig reaction of acyl chlorides with product was not detectable by GC-MS analysis of the
the Wittig reagents (Scheme 2).[16] The cyclization/ reaction of triphenylphosphine-alkenylgold(I) and io-
elimination to the alkenylgold species was conducted dobenzene under an atmosphere of air, an otherwise
in analogy to a protocol by Hammond et al.[9] with quite successful coupling. This has been attributed to
stoichiometric amounts of Ph3PAuCl and AgOTf
Scheme 3. Synthesis of organogold compounds by gold-in-
duced cyclization/elimination.
Scheme 2. Synthesis of allenoates by a Wittig reaction.
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Scope and Limitations of Palladium-Catalyzed Cross-Coupling Reactions
Table 1. (Continued)
À
Entry R X
Equiv. of
À
R X
Time
[h]
4a
Yield
4
5
6
1.50
1.50
1.50
4
4ac 85%
4ad 83%
4ae 0%[c]
4
26
7
8
9
1.50
0.45
0.45
48
4
4ae 0%
4af 73%[d]
4ag 95%[e]
Figure 1. Molecular structure of 3b in the solid state. Ther-
mal ellipsoids shown at 50% probability.
18
the catalytically active Pd(0) species being sensitive
towards oxygen. However, water is tolerated. The
cross-coupling reaction between 3a and 4-methyliodo-
benzene has been carried out in a 9:1 mixture of
MeCN and water (both degassed) to afford 4ai in
81% yield (Table 1, entry 12).
10
11
12
1.50
1.50
1.50
4
4ah 86%
4ah 0%
10
4
4ai 81%[f]
Table 1. Palladium-catalyzed cross-coupling of 3a with a va-
riety of electrophiles (X=I, Br, Cl, OAc, C(=O)Cl).[a]
13
1.50
48
4aj 0%[g]
14
15
16
17
18
19
1.50
1.50
1.50
1.50
1.50
1.50
4
4ak 84%
4al 89%
4am 78%
4an 71%
4ao 92%
4ao 37%
4
À
Entry R X
Equiv. of
R X
Time
[h]
4a
Yield
4
À
4
1
2
3
1.50
1.50
1.50
6
4aa 77%
4ab 91%[b]
4ab 39%
72
72
4
45
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A. Stephen K. Hashmi et al.
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zonitrile was found to be a less effective coupling
partner (entry 3). Substrate 3a was incompletely con-
sumed even after a prolonged reaction time of 45 h.
Crude 4ab was isolated in a much lower yield (39%)
than in entry 2 (91%). The latter two yields show
clearly that as expected the iodide gives better results
than the bromide, which is also visible in entry 9 and
in the comparison of entries 10 and 11 as well as 18
and 19 or entries 24 and 25.
Further functional groups that were tolerated well
are a formyl group (entry 4), an ester group (entry 5),
but the coupling fails with a carboxylic acid (entries 6
and 7, protodeauration detectable). The correspond-
ing cross-coupling products 4ac (85%) and 4ad (83%)
could be isolated in excellent yields. No addition of
the organogold nucleophile to the aldehyde or ester
functionalities was observed. However, the use of 2-
iodobenzoic acid resulted in incomplete conversion,
partial protodeauration of 3a and formation of a gold
mirror. No cross-coupling product 4ae was observed
by GC-MS analysis in this case.
With a diiodo substrate an efficient two-fold cou-
pling was observed (entry 8). The yield of 73% for 4af
corresponds to 85% yield for each individual cou-
pling. With the bromo-iodo compound a selective
coupling of the iodo position was observed, no two-
fold coupling was detectable, 4ag was isolated in 95%
yield (entry 9).
An alkyl substituent was not a problem (entry 12),
iodobenzene and 1-iodo-4-methylbenzene gave quite
similar yields of 86% of 4ah and 81% of 4ai. Again a
dramatic decrease in reactivity was observed with
bromobenzene compared to iodobenzene (entries 10
and 11), no cross-coupling product 4ah was obtained
even after a prolonged reaction time. But an o,o-di-
Table 1. (Continued)
À
Entry R X
Equiv. of
À
R X
Time
[h]
4a
Yield
20
1.50
48
4ap 0%[h]
21
22
1.50
1.50
4
4
4aq 81%
4ar 73%[i]
23
24
1.50
1.50
4
4as 80%
4at 70%
48
25
26
27
1.50
1.50
1.50
24
4at traces
4au traces
4au 0%[j]
14 d
48
28
1.50
4
4av 87%
29
1.20
4
4aw 82%
[a]
The reactions have been carried out under an inert gas
À
atmosphere; Ph3PAuR (150 mmol), Ar X (225 mmol),
[PdCl
N
[b]
The reaction was also carried out on 2 mmol scale,
1.1 equiv. Ar X, 95% cross-coupling product and 99% of
[Ph3PAuI] were isolated.
Partial protodeauration of starting material but no cross-
coupling (GC-MS).
2 mol% [Pd] referred to halide, only disubstituted prod-
uct was obtained.
Neither substitution of bromide nor substitution of both
halides was observed (GC-MS).
In MeCN/H2O 9:1 (degassed).
Starting material [Ph3PAuR] reisolated in 97% yield.
No conversion (TLC).
[c]
[d]
[e]
AHCTUNGTREGsNNNU ubstitution leads to failure (entry 13), no conversion
of 3a took place with sterically demanding 2-iodo-1,3-
dimethylbenzene. After 48 h the starting material 3a
was reisolated in 97% yield. The o-methoxy group in
entry 14 gives a good yield.
We also applied further electron-rich, deactivated,
aryl halides in our cross-coupling (entry 14). Even an
unprotected phenol is tolerated (entry 15), the same
is true for an aniline (entry 16) and an aryl azide
[f]
[g]
[h]
[i]
3
(Z)-configuration, JH,H cis =12.6 Hz.
No conversion (GC-MS).
[j]
Overall, the functional group tolerance turned out (entry 17)! 2-Iodoanisol, 4-iodophenol and 4-iodoani-
to be very high. Most of the cross-coupling reactions line afforded the corresponding cross-coupling prod-
showed complete conversion of 3 within 4 h. The re- ucts 4ak (84%), 4al (89%), 4am (78%) in excellent
sults of our studies are summarized in Table 1. The yields. A very important point is that no protodeaura-
nitro group is tolerated (entry 1), the cyano group, tion of 3a was observed with 4-iodophenol. From 4-
too (entry 2). 1-Iodo-4-nitrobenzene and 4-iodobenzo- azido-1-iodobenzene 4an was isolated in 71% yield.
nitrile were effectively coupled with 3a to afford 4aa
Different heterocycles also react, pyridines (en-
(77%) and 4ab (91%) in excellent yields. The latter tries 18 and 19), and thiophene (entry 21), but the
reaction has also been carried out on 2 mmol scale chloropyrimidine fails (entry 20). Although a long re-
and 4ab was obtained in 95% yield. Moreover, 99% action time (72 h) was necessary, cross-coupling of 2-
of the gold could be reisolated as Ph3PAuI which can iodopyridine with 3a afforded 4ao in 92% yield, the
also be used for the synthesis of organogold com- corresponding bromide was less reactive (37%). In
pounds 3 after activation with AgOTf. 4-Bromoben- contrast, sulfur-containing 2-iodothiophene smoothly
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Scope and Limitations of Palladium-Catalyzed Cross-Coupling Reactions
Table 2. Palladium-catalyzed cross-coupling of the alkenyl-
gold(I) complexes 3a–e with 4-iodobenzonitrile.[a]
reacted with 3a within only 4 h and the cross-coupling
product 4aq was isolated in 81% yield.
To demonstrate further the scope of our cross-cou-
pling protocol, we also used alkenyl, allyl, alkynyl
electrophiles, benzyl bromide and benzoyl chloride.
An example for a (Z)-alkenyl iodide is shown in
entry 22. The cross-coupling between 3a and ethyl
(2Z)-3-iodoprop-2-enoate diastereoselectively afford-
ed 4ar in 73% yield. A coupling constant of J=
Entry
1
Ph3PAuR
Time [h]
4
4
Yield
91%
1
12.6 Hz for olefinic hydrogen atoms in the H NMR
spectrum indicates a (Z)-configuration for 4ar. An al-
kynyl iodide reacts (entry 23), from the cross-coupling
reaction with 3a and (iodoethynyl)benzene 4as was
isolated in 80% yield. Allylic iodides seem to be more
difficult (entry 24), a slower reaction was observed for
3-iodoprop-1-ene (full conversion of 3a only after
48 h). Cross-coupling product 4at was isolated in 70%
yield. But the allylic bromides and acetate failed (en-
tries 25–27), only traces of the corresponding cross-
coupling products were detected by GC-MS with 3-
bromoprop-1-ene or (E)-(3-bromoprop-1-enyl)ben-
zene (entries 25 and 26). No reaction took place with
(2E)-3-phenylprop-2-en-1-yl acetate (entry 27). In
contrast to other bromides, (bromomethyl)benzene
smoothly reacted with 3a to afford 4av in 87% yield
(entry 28).
The benzoyl chloride furnishes the ketone
(entry 29), 4aw was obtained in 82% yield. No decar-
bonylation was observed by GC-MS. Moreover, no re-
action took place in the absence of a Pd source, which
again underlines the catalytic nature of Pd and the
lack of reactivity of the organogold compound to-
wards an addition to carbonyl groups (entries 4, 5 and
28).
4ab
2
3
4
4
4b
4c
88%
90%
4
4
8
4d
4e
91%
91%
5
[a]
Reactions have been carried out under inert gas atmos-
À
phere; Ph3PAuR (150 mmol), Ar X (225 mmol), [PdCl2
(dppf)] (1 mol%), MeCN (1.5 mL), 608C.
As described above (Scheme 3) we also synthesized Conclusions
organogold compounds 3b–e. These have been used
in the cross-coupling with 4-iodobenzonitrile. Our re- The numerous successful cross-couplings described
sults are summarized in Table 2. Cross-coupling was here show clearly that highly functionalized substrates
successful in each case and products 4b–e were isolat- can be tolerated in combinations of gold and palladi-
ed in excellent yields.
um catalysis in the future. Especially the phenol and
Entry 1 shows the yield obtained with 3a in Table 1 aniline derivatives and the unsaturated lactone rings
for comparison. With the mono-substituted aurated illustrate the possibilities. This investigation also re-
lactone ring in 3b also an excellent 88% yield of 4b veals the limitations when using the current condi-
was isolated (entry 2). The disubstituted aurated lac- tions and thus provides a basis for the future fine-
tones 3c and 3d with the benzyl and the ester func- tuning of the palladium catalyst and the optimal
tionalized side-chain delivered even better yields of transmetalation conditions.
90% and 91% (entries 3 and 4). And even the trisub-
stituted lactone ring of the alkenylgold(I) species pro-
vided 91% of 4e (entry 5).
Experimental Section
The structure of the products has been proven un-
ambiguously by a number of crystal structure analy-
ses. The structures of 4aa, 4ad, 4ai (Table 1) and 4d
(Table 2) are shown in Figure 2.[17]
General Methods
All reagents and solvents were obtained from Fisher Scien-
tific, ABCR, Alfa Aesar, Sigma–Aldrich or VWR and were
used without further purification unless otherwise noted.
Deuterated solvents were purchased from Euriso-Top. Ab-
solute solvents were dried by a MB SPS-800 using drying
columns. Preparation of air- and moisture-sensitive materials
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A. Stephen K. Hashmi et al.
FULL PAPERS
Figure 2. Molecular structures of a) 4aa, b) 4ad, c) 4ai and d) 4d. Thermal ellipsoids shown at 50% probability.
was carried out in flame-dried flasks under an atmosphere
of nitrogen using Schlenk techniques. Cross-coupling reac-
tions were performed in dry and degassed solvents. Thin
layer chromatography (TLC) was performed using Poly-
gramꢆ precoated plastic sheets SIL G/UV254 (SiO2, 0.20 mm
thickness) from Macherey–Nagel. Column chromatography
was performed using silica gel (40.0–63.0 nm particle size)
from Macherey–Nagel. NMR spectra were recorded on
Bruker Avance 500, Bruker Avance 300 and Bruker ARX-
250 spectrometers at room temperature. Chemical shifts (in
ppm) were referenced to residual solvent protons.[18] Signal
multiplicity was determined as s (singlet), d (doublet), t
(triplet), q (quartet) or m (multiplet). 13C NMR assignment
was achieved via DEPT 90, DEPT 135 or HSQC spectra.
Mass spectra were recorded on a Vacuum Generators ZAB-
2F, Finnigan MAT TSQ 700 or JEOL JMS-700 spectrome-
ter. GC-MS were recorded on an Agilent 5890 Series II Plus
with a HP 5972 mass analyzator. IR spectra were recorded
on a Bruker Vector 22 FT-IR. Crystal structure analysis was
accomplished on Bruker Smart CCD or Bruker APEX dif-
fractometers. Elemental analysis was performed on an Ele-
mentar Vario EL.
5 min. The acyl chloride (1.05 equiv.) was added dropwise
and the mixture was stirred at room temperature overnight.
The solvent was removed under reduced pressure and the
residue was extracted with n-pentane (4ꢇ5 mLmmolÀ1).
The combined solutions were concentrated to one fourth of
the original volume and filtered. The crude product was pu-
rified by column chromatography over silica.
General Procedure B (GPB), Synthesis of
Organogold Compounds
Under nitrogen, the allenoate (1.1 equiv.) was dissolved in
dry DCM (10 mLmmolÀ1 Ph3PAuCl). Ph3PAuCl (1.0 equiv.)
and AgOTf (1.0 equiv.) were added and the mixture was
stirred at room temperature for 2 h. Water (5 mLmmolÀ1
Ph3PAuCl) was added and the mixture was stirred vigorous-
ly at room temperature for 1.5 h. The mixture was filtered
and the layers were separated. The aqueous layer was ex-
tracted with DCM (3ꢇ10 mLmmolÀ1 Ph3PAuCl). The com-
bined organic layers were dried over MgSO4, filtered and
the crude product was purified by column chromatography
over silica.
General Procedure C (GPC), Cross-Coupling
Reactions
General Procedure A (GPA), Synthesis of Allenoates
Under nitrogen, the phosphorus ylide (1.00 equiv.) was dis-
solved in dry DCM (5 mLmmolÀ1), triethylamine
(1.05 equiv.) was added and the mixture was stirred for
To the organogold compound (150 mmol) under nitrogen
was added [PdCl2ACTHNGUTERNNU(G dppf)] (1 mM in dry and degassed MeCN,
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Scope and Limitations of Palladium-Catalyzed Cross-Coupling Reactions
1.5 mL) and the halide (225 mmol). The mixture was stirred
at 608C for the specified time. The mixture was filtered and
the residue was washed with MeCN (2ꢇ1 mL). The crude
product was purified by column chromatography over silica.
Jimꢀnez-Nfflnez, A. M. Echavarren, Chem. Rev. 2008,
108, 3326–3350; f) Z. G. Li, C. Brouwer, C. He, Chem.
Rev. 2008, 108, 3239–3265.
[8] For selected examples, see: a) A. Buzas, F. Gagosz,
Org. Lett. 2006, 8, 515–518; b) M. Yu, G. Zhang, L.
Zhang, Org. Lett. 2007, 9, 2147–2150; c) A. S. K.
Hashmi, T. D. Ramamurthi, F. Rominger, J. Organo-
met. Chem. 2009, 694, 592–597; d) B. Gockel, N.
Krause, Eur. J. Org. Chem. 2010, 311–316.
Acknowledgements
This work was supported by the Chinesisch-Deutsches Zen-
trum [GZ 419 (362/3)] and the Deutsche Forschungsgemein-
schaft (HA 1932/11-1); we thank Umicore AG & Co. KG for
the generous donation of noble metal salts. C.L. is grateful
for financial support by the Studienstiftung des dt. Volkes e.V.
[9] a) L.-P. Liu, B. Xu, M. S. Mashuta, G. B. Hammond, J.
Am. Chem. Soc. 2008, 130, 17642–17643; b) L.-P. Liu,
G. B. Hammond, Chem. Asian J. 2009, 4, 1230–1236.
For the previous invention of the catalytic allenoate to
butenolide cyclization with tert-butyl esters, see: c) J.-E.
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[10] D. Weber, M. A. Tarselli, M. R. Gagnꢀ, Angew. Chem.
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