Ligand-Assisted Gold-Catalyzed Cross-Coupling with Aryldiazonium Salts: Redox Gold Catalysis without an External Oxidant

Article abstract of DOI:10.1002/anie.201503546

Gold-catalyzed C(sp)-C(sp²) and C(sp²)-C(sp²) cross-coupling reactions are accomplished with aryldiazonium salts as the coupling partner. With the assistance of bpy ligand, gold(I) species were oxidized to gold(III) by diazonium without any external oxidants. Monitoring the reaction with NMR and ESI-MS provided strong evidence for the nitrogen extrusion followed by Au^III reductive elimination as the key step.

Full text of DOI:10.1002/anie.201503546

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International Edition: DOI: 10.1002/anie.201503546
German Edition: DOI: 10.1002/ange.201503546
Redox Gold Catalysis
Hot Paper
Ligand-Assisted Gold-Catalyzed Cross-Coupling with Aryldiazonium
Salts: Redox Gold Catalysis without an External Oxidant**
Rong Cai, Mei Lu, Ellen Y. Aguilera, Yumeng Xi, Novruz G. Akhmedov, Jeffrey L. Petersen,
Hao Chen,* and Xiaodong Shi*
Abstract: Gold-catalyzed C(sp)–C(sp2) and C(sp2)–C(sp2)
cross-coupling reactions are accomplished with aryldiazonium
salts as the coupling partner. With the assistance of bpy ligand,
gold(I) species were oxidized to gold(III) by diazonium
without any external oxidants. Monitoring the reaction with
NMR and ESI-MS provided strong evidence for the nitrogen
extrusion followed by AuIII reductive elimination as the key
step.
Encouraged by this success and inspired by recent works from
the groups of Glorius and Toste (Scheme 1b), we were
intrigued by the possibility of C(sp)–C(sp2) cross-coupling
using aryldiazonium salts as the coupling partner under mild
photocatalytic conditions.[8]
To test this hypothesis, the alkyne 1a and diazonium salt
2a were selected as the model substrates. As shown in Table 1
(entry 1), the reaction of 1a and 2a in the presence of both
a photocatalyst and gold catalyst gave none of the desired
coupling products (low conversion of 1a). Considering that
the formation of a gold acetylide could be critical, a base
(Na2CO3) was added to assist the alkyne deprotonation. As
expected, the cross-coupling product 3aa was formed, though
in modest yield (entry 2). In the control experiments
(entries 3–5), almost identical kinetics and yields were
observed under either standard or dark conditions in the
absence of the photocatalyst, and thus negated the involve-
ment of a photocatalytic process. This result was exciting since
it implied that the diazonium salt alone, in this reaction, might
be fully capable of oxidizing the active gold(I) species into
gold(III). To optimize the reaction conditions, various addi-
tives and catalysts were tested (see the Supporting Informa-
tion for details). Through extensive screening, a conversion as
high as 74% could be achieved by simply increasing the
reaction concentration (entry 7). Eventually, using an organic
H
omogeneous gold catalysis has grown rapidly during the
past decade. Within this enduring field, the p-activation mode
has gained far more attention than redox gold catalysis.[1] This
reluctance is likely due to concerns regarding the high redox
potential between AuI and AuIII.[2] The “seal of hesitation”
was clearly broken in the past few years with some break-
through examples.[3]
For example, Zhang reported the propargyl ester rear-
rangement and sequential coupling with boronic acid using
Selectfluor as the oxidant.[4] Russell also developed the arene
cross-coupling with PhI(OAc)2 (PIDA) as the oxidant (Sche-
me 1a).[5] In these representative cases, strong oxidants were
required to facilitate the AuI/AuIII catalytic cycle. More
recently, the groups of Glorius and Toste demonstrated that
the AuI/AuIII catalytic cycle could be alternatively accessed
through the combination of a photocatalyst and a radical
precursor, such as aryldiazonium salts (Scheme 1b).[6] As
proposed by the authors, the oxidation of the gold catalyst was
achieved through the initial combination of an aryl radical
and a gold(I) species, and sequential single-electron transfer
from the resulting gold(II) intermediate to the photocatalyst.
Our group recently disclosed a gold-catalyzed cross-
coupling between aromatic and aliphatic terminal alkynes,
and it was achieved through the selective formation of gold
acetylide.[7] With the aid of a strong oxidant (PIDA) and
phenanthroline ligand, rapid reductive elimination of the
AuIII intermediate led to facile and selective diyne formation.
[*] R. Cai, E. Y. Aguilera, Y. Xi, Dr. N. G. Akhmedov, Dr. J. L. Petersen,
Prof. Dr. X. Shi
C. Eugene Bennett Department of Chemistry
West Virginia University, Morgantown, WV 26506 (USA)
E-mail: Xiaodong.Shi@mail.wvu.edu
M. Lu, Prof. Dr. H. Chen
Department of Chemistry and Biochemistry
Ohio University, Athens, OH 45701 (USA)
[**] We acknowledge the NSF (CHE-1362057, CHE-1228336, CHE-
1149367, CHE-1336071, EPS-1003907), and NSFC (21228204) for
financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503546.
Scheme 1. Redox gold catalysis. CSA=camphorsulfonic acid, Tf =tri-
fluoromethanesulfonyl, Ts =4-toluenesulfonyl.
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Table 1: Optimization of reaction conditions.[a,b]
Entry
Catalyst (%)
Additive (equiv)
t [h]
Conc. [m]
Conv. [%][c]
Yield [%]
3aa[d]
4a[c]
1
2
3
4
5
6
7
8
[Ru(bpy)3](PF6)2 (2.5), [Ph3PAuNTf2] (5)
[Ru(bpy)3](PF6)2 (2.5), [Ph3PAuNTf2] (5)
[Ru(bpy)3](PF6)2 (2.5)
[Ph3PAuNTf2] (5)
–
10
10
10
10
10
10
10
8
0.2
0.2
0.2
0.2
0.2
0.2
0.5
0.5
0.5
0.5
0.5
0.5
9
46
0
44
40
0
74
100
100
53
0
0
41
0
41
36
0
62
83
94
40
0
0
0
0
0
0
0
0
8
2
Na2CO3 (2)
Na2CO3 (2)
Na2CO3 (2)
Na2CO3 (2)
Na2CO3 (2)
Na2CO3 (2)
bpy (2)
Na2CO3 (2), bpy (0.2)
Na2CO3 (2), bpy (0.2)
bpy (2)
[Ph3PAuNTf2] (5)[e]
–
[Ph3PAuNTf2] (5)
[Ph3PAuNTf2] (5)
[Ph3PAuNTf2] (5)
[Ph3PAuNTf2] (2)
PdCl2 (5)
PdCl2 (5), [Ph3PAuNTf2] (5)
9
4
10
11
12
10
10
10
2
0
13
bpy (2)
86
56
[a] Reaction conditions: 1 (0.1 mmol), 2 (0.12 mmol), catalysts, and additives in acetonitrile at RT for 10 h. [b] Less than 5% of the homocoupling
product 4,4’-difluorobiphenyl (5a) was detected. [c] Determined by 1H NMR spectroscopy using p-xylene as the internal standard. [d] Determined by
19F NMR spectroscopy using benzotrifluoride as the internal standard. [e] Under dark conditions.
base, 2,2’-bipyridine (bpy), resulted in complete consumption
of 1a (3aa in 83% yield; entry 8).[9] We reasoned that the bpy
served both as a base and an ancillary ligand.[10] Finally, the
combination of Na2CO3 as the base and bpy as the ligand gave
the best result with 3aa obtained in 94% yield (entry 9).
Reducing the catalyst loading led to lower conversion, and is
likely due to gold decomposition. It has been reported that
trace amounts of PdII in gold salts might contribute to
Sonogashira-type coupling.[11] To rule out this possibility,
PdCl2 was tested as the catalyst either by itself or combined
with gold (entries 12 and 13). The results confirmed that this
reaction proceeds through a gold-catalyzed process.[12]
With the optimized conditions in hand, we explored the
reaction scope (Table 2). An electron-deficient diazonium
(see product 3aa; 4 h) reacted faster than a non-electron-
deficient diazonium (see product 3ab; 6 h). On the contrary,
an electron-rich diazonium (product 3ac) reacted more
slowly (8 h). Ortho-substituted phenyldiazonium salts gave
lower conversion and yield (< 30%), which hinted at the
steric sensitivity of this reaction. Various terminal alkynes
(both aliphatic and aromatic) with different functional groups
such as hydroxy (3a), ester (3e), heterocycles (3g, 3h), and
estrone derivative (3o) were well tolerated under these mild
and efficient reaction conditions.[13] In general, aliphatic
alkynes reacted faster than aromatic alkynes, thus suggesting
that the gold oxidation likely occurred after the formation of
gold acetylide.
After complete conversion of the gold, 2a was no longer
consumed, thus refuting the possibility of M being an active
species.
It is known that cationic AuI complexes can react rapidly
with terminal alkynes to form gold acetylides, especially
under basic conditions.[15] To prove that the gold acetylide was
the reaction intermediate, the complex 6a was prepared and
treated with the diazonium 2b (1.1 equiv; Figure 1). As shown
Table 2: Substrates scope screening.[a,b]
To study the reaction mechanism, experiments were
monitored by NMR spectroscopy. First, no decomposition
of 2a and Ph3PAuNTf2 occurred without bpy, even after 18 h
[Figure 1A(a,b)]. In another NMR tube bpy was added, and
the [Ph3PAu(bpy)]+ complex (31P NMR: d31.6 ppm) was
formed instantly [Figure 1A(c)]. Interestingly, this gold
[a] Reaction conditions: 1 (0.4 mmol), 2 (0.48 mmol), [Ph3PAuNTf2]
(0.02 mmol), bpy (0.08 mmol), and Na2CO3 (0.8 mmol) in 0.8 mL
acetonitrile at RT. [b] Yield of the isolated product. [c] 10% of
[Ph3PAuNTf2] was used. [d] 3 equiv of 2a was used.
+
complex decomposed during the reaction with ArN2 , thus
forming the phosphonium M overtime [Figure 1A(d,e)].[14]
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Figure 2. Direct reactions of gold complexes and diazonium salts.
and bpy in CD3CN. Similar to the C(sp)–C(sp2) coupling,
addition of bpy was critical for the reaction progress (gold
mirror was observed within 10 min if bpy was absent). A
complex reaction mixture was observed with the alkyl gold
complex [PPh3AuMe] under the same reaction conditions
even with the addition of bpy.
According to literature, aryl boronic acids can react with
AuI under basic conditions to form the corresponding aryl
gold complexes.[17] Thus, it is reasonable to envision a catalytic
C(sp2)–C(sp2) cross-coupling between aryl boronic acids and
diazonium salts. Under previously optimized reaction con-
ditions, the desired cross-coupling product 8 was successfully
obtained (around 40% yield), presumably because of the
faster gold catalyst decomposition. Applying the more stable
triazole gold complex [Ph3PAu(TA)OTf], developed by our
group,[18] higher yields were obtained (up to 86%, see the
Supporting Information for details). Under these new reac-
tion conditions, various substituted aryl boronic acids and
diazonium salts were tested. Overall, good to excellent yields
were obtained for most of the tested substrates (Table 3).
Figure 1. Exploring the reaction mechanism with 31P NMR spectrosco-
py.
in Figure 1B, without bpy, the gold acetylide 6a decomposed
overtime with only 22% of the cross-coupling product 3r
forming (no further conversion after 4 h). The formation of
a gold mirror and [(Ph3P)2Au]+ (31P NMR: d 44.9 ppm) was
observed within 1 hour. Impressively, with the addition of bpy,
3r was formed instantly and the reaction was completed in
a nearly quantitative yield (Figure 1C). After reaction
completion, the [Ph3PAu(bpy)]+ complex was confirmed as
the final gold species.[16]
À
Considering the importance of C C bond formation in
organic synthesis, this new approach is of great interest
since it offers an alternative strategy for cross-coupling.[19]
To verify whether the reaction proceeded through a rad-
ical mechanism, the diazonium salt 2c was prepared. As
shown in Figure 3, the reaction of 2c with 1a was quite
sluggish with only 25% of the desired coupling product 9a
formed. The radical trapping product 3-methyldihydrobenzo-
furan was not detected, and thus confirmed that the aryl
radical formation, if applicable, was not the key step in this
cross-coupling.[20]
These NMR experiments revealed important mechanistic
insights. Firstly, the Ph3PAuNTf2 catalyst alone could not
promote diazonium decomposition. The addition of bpy is
crucial in assisting the nitrogen extrusion. However, the gold
acetylide itself could react with the diazonium without bpy,
thus generating the cross-coupling products, albeit in low
yield. In the presence of bpy, a significantly faster reaction
rate was observed (Figure 1C). Considering the fact that M
([Ph3P-Ar]+) was formed after reaction completion, it is clear
that the oxidation of AuI occurs after the formation of gold
acetylide. This discovery was rather exciting since it suggested
that [Ph3P-Au-R] complexes were more feasible than cationic
gold in diazonium-promoted oxidation. By forming the [Ph3P-
Au-R] intermediate in situ, similar cross-coupling reactions
could be achieved under the proper conditions. To test this
hypothesis, reactions of aryl and alkyl gold complexes with
diazonium were performed.
Table 3: Substrate screening of the C(sp2)–C(sp2) coupling.[a,b]
Compared to the C(sp)–C(sp2) coupling (Figure 2a), the
C(sp2)–C(sp2) coupling with aryl gold was faster (Figure 2b).
Rapid gas evolution was observed almost simultaneously
after the addition of the diazonium salt into the mixture of 6b
[a] Reaction conditions: 7 (0.4 mmol), 2 (0.48 mmol), [Ph3PAu(TA)OTf]
(0.04 mmol), bpy (0.08 mmol), and Na2CO3 (0.8 mmol) in 0.8 mL
acetonitrile at RT. [b] Yields determined by NMR spectroscopy.
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Figure 3. Radical-trapping experiment.
Scheme 2. Proposed reaction mechanism.
To further shed light on the reaction mechanism, we
utilized the electrospray ionization mass spectrometry (ESI-
MS) studies to detect possible intermediates.[21] As shown in
Figure 4a, aryldiazenido gold complexes (10) were observed
diazonium to form A and understand the exact function of
bpy. Comprehensive mechanistic studies are currently ongo-
ing in our lab and will be reported in due course.
In summary, we report a novel gold-catalyzed cross-
coupling strategy using aryldiazonium salts as the oxidant. By
forming an alkynyl or aryl gold complex in situ, oxidation of
AuI to AuIII was achieved without strong oxidants or photo-
catalysts. Preliminary mechanistic studies revealed the impor-
tance of the bpy ligand in assisting nitrogen extrusion, and
appears to be a key step in this transformation. This strategy
offers a new effective cross-coupling approach together with
inspiring insight about gold redox catalysis.
Keywords: aryldiazonium salt · cross-coupling · gold ·
redox catalysis
How to cite: Angew. Chem. Int. Ed. 2015, 54, 8772–8776
Angew. Chem. 2015, 127, 8896–8900
Figure 4. Exploring the reaction intermediates with ESI-MS.
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at the beginning of all tested reactions (see the Supporting
Information).[22] To trap plausible intermediates, the com-
pound 2d (Figure 4b) was prepared. Under the standard
reaction conditions, a small amount of the coupling product
9c was detected by NMR spectroscopy and ESI-MS. After 10
minutes of reaction, ESI-MS analysis revealed the formation
of a major ion: m/z 744.1. With the help from MS/MS
fragment analysis (see the Supporting Information), the
structure 11 was proposed. It was highly likely that the
potential coordination of pyridyl with gold restricted the
reductive elimination of 11. However, without the presence of
bpy, a dominant gold cation was detected with a peak at m/
z 772.2, which was assigned as the aryldiazenido gold complex
10d based on the MS/MS analysis. Although the exact reason
is uncertain at this moment, the presence of bpy clearly
assisted the nitrogen extrusion, which was crucial for the
overall transformation. Based on these experimental results,
a simplified mechanism was proposed as shown in Scheme 2.
Both 31P NMR and ESI-MS confirmed the formation of
[Ph3PAu(bpy)]+ by the addition of bpy. Based on current
evidence, the diazonium oxidation occurred preferentially on
either aryl gold or gold acetylide, thus forming the inter-
mediate A (Scheme 2). The bpy-assisted nitrogen extrusion
followed by rapid AuIII reductive elimination gave the desired
coupling product and regenerated the AuI catalyst. Certainly,
it is interesting to uncover how L-Au-R reacts with the
[2] E0 (AuI/AuIII) =+ 1.40 V. See: CRC Handbook of Chemistry and
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best of our knowledge, there is no literature precedence
regarding the diazonium salt cross-coupling purely catalyzed
by gold.
[13] The high reactivity of p-methoxyphenyl acetylene (3l) resulted
in a messy reaction with the desired product isolated in only
36% yield (16% alkyne homocoupling product detected by
NMR spectroscopy).
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[14] The [Ph3P-Ar]+ was formed through reductive elimination of the
corresponding AuIII complex: see Ref. [6b].
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[16] After the reaction was completed, [Ph3P-Ar]+ (M) started to
form because of the reaction between excess 2b and [Ph3PAu-
(bpy)]+.
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[19] Much lower yield was observed using p-iodide diazonium salts.
This lower yield is likely caused by the potential oxidative
addition of aryl iodide by gold(I): J. Guenther, S. Mallet-Ladeira,
L. Estevez, K. Miqueu, A. Amgoune, D. Bourissou, J. Am.
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[20] The only byproduct identified was the compound 9b, formed
from the dimerization of 2c. (see the Supporting Information for
characterization details).
[21] For reviews of ESI-MS in mechanistic studies, see: a) L. S.
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[9] The alkyne homocoupling product 4a was observed when bpy
was present (Table1, entry 8), and it might be formed from
transmetallation of the AuIII intermediate with the gold acetylide
followed by reductive elimination. In most of the cases,
fluorobenzene was also observed (generally < 10%).
[10] The crystal structure of [Ph3PAu(bpy)]+ has been reported
previously: W. Clegg, Acta Crystallogr. Sect. B 1976, 32, 2712 –
2714. The bpy ligand possesses a pseudo-three-coordination with
gold. For the coordination of bpy with AuIII, see: A. P. Shaw,
M. K. Ghosh, K. W. Tçrnroos, D. S. Wragg, M. Tilset, O. Swang,
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[12] There are two precedent examples regarding the cross-coupling
of aryldiazonium salts and alkynes: a) G. Fabrizi, A. Goggia-
mani, A. Sferrazza, S. Cacchi, Angew. Chem. Int. Ed. 2010, 49,
4067 – 4070; Angew. Chem. 2010, 122, 4161 – 4164; b) B. Panda,
T. K. Sarkar, Chem. Commun. 2010, 46, 3131 – 3133. In both
cases, the reductive elimination occurred at the Pd center. To the
Received: April 18, 2015
Published online: June 5, 2015
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