Arenediazonium salts as electrophiles for the oxidative addition of gold(i)

Article abstract of DOI:10.1039/c6cc03105f

Arenediazonium salts generated in situ from anilines have been found for the first time to efficiently oxidize [AuCl(L)] (L = SMe₂, PPh₃) complexes in DMSO as a solvent, under thermal conditions. The structure of the [AuArCl₂(L)] complexes formed has been confirmed by X-ray diffraction analyses. These complexes have been used as intermediates, in a one pot cross-coupling reaction of anilines with silver acetylides.

Full text of DOI:10.1039/c6cc03105f

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Arenediazonium salts as electrophiles for the
oxidative addition of gold(I)†
Cite this:DOI: 10.1039/c6cc03105f
Eric Omar Asomoza-Solıs, Jonathan Rojas-Ocampo, Ruben Alfredo Toscano and
Susana Porcel*
´
´
Received 13th April 2016,
Accepted 29th April 2016
DOI: 10.1039/c6cc03105f
www.rsc.org/chemcomm
Arenediazonium salts generated in situ from anilines have been found for perform the oxidative addition of Au(I) to Au(III).9 To date some
the first time to efficiently oxidize [AuCl(L)] (L = SMe2, PPh3) complexes in radical transformations involving diazonium salts and Au(I)
DMSO as a solvent, under thermal conditions. The structure of the complexes have been described in the bibliography. In these
[AuArCl2(L)] complexes formed has been confirmed by X-ray diffraction processes the gold complexes act in combination with a photo-
analyses. These complexes have been used as intermediates, in a one pot sensitizer in a dual catalytic fashion.10 On the other hand Chen
cross-coupling reaction of anilines with silver acetylides.
and Shi reported gold catalyzed Csp–Csp2 and Csp2–Csp2 cross
coupling reactions using arenediazonium salts, which take
In recent years the development of new processes involving the place via initial gold(I) acetylide formation, and subsequent
oxidative addition of Au(I) has attracted significant interest. Due to oxidative addition of the arenediazonium salt assisted by a bpy
the high redox potential of the Au(I)/Au(III) couple (E0 = +1.41 V),1 ligand.11 By contrast, herein we report that [AuCl(L)] (L = SMe2,
the oxidation of Au(I) in most cases has been achieved by adding PPh3) complexes undergo the direct oxidative addition of
external oxidants such as fluorinating or hypervalent iodine arenediazonium salts, generated in situ from anilines, in DMSO
reagents.2 In this regard many efforts have been made in order as a solvent. In constrast to our results, the group of Hashmi
to understand and circumvent the high energy associated with this has very recently reported that [AuCl(L)] complexes can also
process.3 The group of Toste successfully achieved the oxidative undergo the direct oxidative addition of arenediazonium salts
addition of allylbromide to a bimetallic Au(I) complex, via the under irradiation with a blue light LED.12
´
formation of a Au(II)–Au(II) species.4 Fernandez and Bickelhaupt
We began our study analyzing the reaction of 4-benzoyl-
studied theoretically the factors governing the oxidative addition of benzenediazonium chloride (1) with [AuCl(SMe2)] in DMSO as a
aryl halides to Au(I),5 and in line with their conclusions, Amgoune model system, with the aim of stabilizing the desired arylAu(III)
and Bourissou described the first direct experimental evidence of complex by electronic and steric factors. 4-Benzoyl-benzenedi-
the oxidative addition of a Csp2–X bond to a single gold center, by azonium chloride was synthesized in situ from 4-aminobenzo-
using haloarenes with pendant phosphines, and diphosphino– phenone in THF,13 after removal of the solvent and addition of
carborane Au(I) complexes.6 Later on, independently Toste, [AuCl(SMe2)] in DMSO-d6, the reaction was monitored by
Amgoune and Bourissou found that it is possible to perform the 1H NMR spectroscopy at r.t. It could be observed that after
oxidative addition of a strained C–C bond to cationic Au(I) complexes 4.5 h of reaction, along with the set of signals corresponding
bearing NHCs or diphosphino–carborane ligands.7 More recently, to 4-benzoyl-benzenediazonium chloride [8.92 (d, 2H), 8.21
Ribas and coworkers have provided the first examples of external (d, 2H), and 7.94–7.39 (m, 5H) ppm], two new signals appeared
oxidant-free Au(I) catalysed carbon–heteroatom cross-coupling at a higher field [7.40 (d), and 7.26 (d) ppm], in a ratio 1 : 0.60.
reactions, employing aryl halide macrocycles or aryl halides Upon increasing the temperature to 50 1C, the reaction became
with a nitrogen chelating site.8
faster and 4-benzoyl-benzenediazonium chloride was totally
As an alternative to the aforementioned methods, we got consumed within 3 h (Scheme 1). The chemical shift of the new
interested in studying the use of arenediazonium salts to signals was congruent with additional electronic density coming
from the coordination to a gold atom, nonetheless attempts to
isolate the new compound were unsuccessful. Emulating the work
´
´
´
of Parkin14 we tried to stabilize and facilitate the isolation of 3a by
adding one equivalent of PPh3. After the addition of the phosphine,
a single signal at 32.7 ppm was detected by 31P NMR, pointing
a phosphorous–gold coordination and the formation of 3b.
Instituto de Quımica, Universidad Nacional Autonoma de Mexico, Circuito Exterior
´
´
s/n, Ciudad Universitaria, 04510 Mexico D.F., Mexico. E-mail: sporcel@unam.mx
† Electronic supplementary information (ESI) available. CCDC 1442893–1442894.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c6cc03105f
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Table 1 Optimization of the coupling reaction of 4-aminobenzophenone
with silver phenylacetylide
Anilinea
(equiv.)
Oxid. addition
conditions
AgRPh
(equiv.)
Yield
(%)
[AuI]
Scheme 1 Oxidative addition of [AuCl(SMe2)] and [AuCl(PPh3)] with
ArN2Cl (Ar = 4-PhCOC6H5, 4-NO2C6H5).
1
2
3
4
5
6
7
1
1
1
1
2
1
1
[Au(SMe2)Cl]
[Au(PPh3)Cl]
[Au(SMe2)Cl]
[Au(SMe2)Cl]
[Au(SMe2)Cl]
[Au(SMe2)Cl]
[Au(SMe2)Cl]
DMSO, 4 h
DMSO, 2 h
DMSO, 4 h
DMSO, 4 h
DMSO, 4 h
THF, 5 h
1
1
1.5
2
1.5
1
58
56
75
79
59
26
66
Contrary to the reports where it is shown that phosphines
decompose diazonium salts via single electron transfer,15 3b
could also be directly obtained from the reaction of 4-benzoyl-
benzenediazonium chloride with [AuCl(PPh3)]. In this case the
reaction time was reduced to 2 h, which is in accord with a more
electron-rich Au(I) precursor. Fortunately, single crystals of 3b could
be obtained by slow diffusion of Et2O into a solution of 3b in DCM
(37% yield), and its molecular structure was confirmed by X-ray
diffraction analysis. Employing a similar strategy, we also succeeded
in isolating (41% yield) and characterizing complex 4 obtained
by the reaction of 4-nitro-benzenediazonium chloride (2) with
[AuCl(PPh3)]. Again, single crystals of 4 could be obtained by
slow diffusion of Et2O into a solution of 4 in DCM, and its
structure was confirmed by X-ray diffraction analysis.
Both compounds are stable in air, but with time tend to
decompose to [AuCl(PPh3)] by reductive elimination. This was
confirmed by the appearance of a second signal in the 31P NMR
spectrum at 33.7 ppm and by mass spectrometry. The solid-
state molecular structures of 3b and 4 are depicted in Fig. 1.
The geometry around the gold center is essentially square-planar in
both complexes, and the distance Au–Cl1 is longer than Au–Cl2 due
to the higher trans influence of the aryl groups in both complexes:
[Cl1–Au–C5 176.5(2) and Cl2–Au–P 175.73(8)1 for 3b, Cl1–Au–C1
176.4(5) and Cl2–Au–P 177.59(5)1 for 3], and [Au–Cl1 2.369(2) Å
compared to Au–Cl2 2.323(2) Å for 3b, Au–Cl1 2.365(1) Å compared
to Au–Cl2 2.327(2) Å for 4].
CH3CN, 4 h
1.5
a
Aniline : tBuONO : HCl (1 : 1.2 : 2), [AuI] = 0.034 mmol mlꢀ1
.
that the high affinity of silver for halogens would facilitate
the transmetallation process.17 Satisfactorily, when 1 equiv. of
silver phenylacetylide was added at r.t. to the complex obtained
by the oxidative addition of [AuCl(SMe2)] with 1, the expected
coupling product 5 was formed in 58% yield (Table 1, entry 1).
Upon employing [AuCl(PPh3)] as a Au(I) source, 5 was obtained
in a similar yield (56%, entry 2), hence to study the coupling
reaction we preferred [AuCl(SMe2)] over [AuCl(PPh3)], since the
former is a precursor of the latter. Increasing the amount of
silver phenylacetylide up to 1.5 equiv. increased the yield to
75%. Comparatively, when 2 equiv. of silver phenylacetylide
was added the yield did not increase significantly (entries 3
and 4). We also examined the feasibility of the coupling
reaction in THF and CH3CN instead of in DMSO. In the first
case the yield decreased to 26% while in the second it was 66%
(entries 5 and 6). Lastly, and in order to corroborate that the
real intermediate of the coupling reaction is an arylAu(III)
complex, we took isolated complex 3b (1 equiv.) and subjected
it to the reaction with silver phenylacetylide (1.5 equiv.). As
expected, after stirring in DMSO for 1.5 h at rt, 5 was formed in
75% yield, thus confirming that the coupling is done via an
arylAu(III) complex.18
Once it was confirmed that it is possible to obtain arylAu(III)
complexes by oxidative addition of [AuCl(SMe2)] and [AuCl(PPh3)]
with arenediazonium salts, we turned our attention to study the
possibility of performing a one-pot coupling reaction with nucleo-
philes. At first instance we examined the coupling between
4-aminobenzophenone and silver phenylacetylide,16 hoping
Concerning the mechanism19 of the coupling reaction, as far
as we know, the reductive elimination step in Au(III) complexes
to form a Csp–Csp2 bond has not been studied before. None-
theless, there are reports dealing with reductive elimination
from Au(III) complexes to form Csp3–Csp3 and Csp2–Csp2 bonds. In
these studies it has been established that reductive elimination
to form Csp3–Csp3 bonds is slow and takes place directly from a
tetrasubstituted Au(III) species, whereas reductive elimination
to form Csp2–Csp2 bonds is very fast and takes place from a three
coordinated Au(III) species.20 In order to gain more insights into
the reductive elimination step, we examined the effect of
adding an excess of phosphine. Thus, we observed that the
addition of 6 and 10 equiv. of PPh3 reduces the yield of 5 to 45%
and 20% respectively (Scheme 2). On the other hand, upon
employing [AuCl(IPr)] as the Au(I) source, the formation of 5
was not observed. Instead, upon analysing the crude mixture by
mass spectrometry (DART, [M + H]*) two peaks were identified,
Fig. 1 ORTEP diagrams of 3b (left) and 4 (right). Hydrogen atoms are
omitted for clarity.
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32% (entry 7). The presence of an m-Cl substituent also resulted
in the acceleration of the oxidative addition step, so that the
arenediazonium salt was consumed after 1.25 h. In this case, the
yield of the coupling with silver phenylacetylide was also diminished
(45%, entry 6). The decrease in the yield for anilines bearing
p-CO2Me and m-Cl substituents was attributed to a visible
minor stability of the corresponding arenediazonium salts.
Regarding anilines bearing electron-donating groups ( p-Me,
m-Me and p-OMe), the oxidative addition step was remarkably
slow for 4-aminoanisole (36 h, entries 11 and 12). As a common
feature, it was observed that for these substituents the coupling
yields with silver phenylacetylide improved when the amount of
aniline was increased up to 2 equiv. (entries 8–12). In addition,
we got interested in examining the effect of adding alkyl
Scheme 2 Examination of the coupling step in the presence of an excess
of phosphine and an IPr ligand.
one corresponding to benzophenone (m/z = 183) and the second
one corresponding to 4-chlorobenzophenone (m/z = 217).21 This
result suggests that for the reductive coupling to take place, the
dissociation of a ligand is necessary.
substituents on the silver acetylide (entries 14–16, R2
=
(CH2)3CH3, CH2Ph, OCH2Ph).23 Using 4-aminobenzophenone
as a model aniline for the coupling, a decrease in the yields as
the size of the alkyl chain increased could be observed. Thus, a
lower yield was obtained when R2 = (CH2)3CH3 (26%, entry 15).
This effect can be attributed to the tendency of silver acetylide
molecules to associate, forming supramolecular frameworks,
which reduce their reactivity.23b
Aiming at extending the coupling reaction further, we studied
its scope over a variety of anilines with different electronic
properties, as well as with other silver acetylides containing alkyl
1
substituents (Table 2). In all cases we used H NMR to monitor
the oxidative addition, and as soon as the arenediazonium salt
was consumed,22 the silver acetylide was added. Upon employing
aniline and silver phenylacetylide as coupling partners, the
oxidative addition proceeded cleanly in only 1.5 h, however the
coupling product was obtained in 19% yield (entry 1). Anilines
containing electron-withdrawing groups at the para position
( p-CN, p-NO2, p-Br) behaved similar to 4-aminobenzophenone
with oxidative addition times of 4–5 h, and yields ranging from
49–70% for the coupling with silver phenylacetylide (entries 2–5).
An exception was the p-CO2Me substituent, which notably
increased the reactivity of the corresponding arenediazonium
salt. In this case the oxidative addition took 3 h at r.t., and the
yield of the coupling with silver phenylacetylide decreased to
Comparatively, the coupling with phenylacetylene as a
nucleophile was less effective. Thus starting from 4-aminobenzo-
phenone, and employing 2,6-lutidine as a base,24 the coupling
product was obtained in 38% when 1 equiv. of phenylacetylene
was added and in 46% yield when 1.5 equiv. of phenylacetylene was
added (Scheme 4).
Finally, and in order to study the effect of applying the optimal
conditions found by Chen and Shi for the gold catalyzed cross
coupling of arenediazonium salts with alkynes to our coupling,11
we studied the coupling of 10 with silver phenylacetylide
(Scheme 3). However, using a [AuCl(PPh3)]/AgNTF2 mixture (1 : 1.1
equiv.) and Bipy (2 equiv.) as the Au(I) source, 5 was isolated in 12%
when the oxidative addition was performed at r.t., and not observed
when the temperature was increased to 50 1ꢀC. This result suggests
that the Au(III) complex formed with a BF4 counteranion is not
stable enough and decomposes before reacting with silver phenyl-
acetylide. Similarly, when we applied our coupling conditions to 10,
5 was not observed confirming that 10 with a noncoordinating
anion is not an adequate precursor to form a stable arylAu(III)
complex.
Table 2 Scope of the coupling reaction of anilines with silver acetylides
R1 anilinea
(equiv.)
Oxid. addition
conditions
R2 acetyl.
(equiv.)
Yield
(%)
This work was supported by CONACyT (153523), DGAPA
´
´
(IA201615) and I. Quımica. Asomoza-Solıs thanks CONACyT
(21516) and DGAPA for undergraduate fellowships. Rojas-Ocampo
thanks CONACyT for a predoctoral fellowship (574767). The authors
1
2
3
4
5
6
7
8
H (1)
50 1C, 1.5 h
50 1C, 4 h
50 1C, 4 h
50 1C, 4.5 h
50 1C, 5 h
50 1C, 1. 25 h
r.t., 3 h
50 1C, 4 h
50 1C, 4 h
50 1C, 4 h
50 1C, 36 h
50 1C, 36 h
50 1C, 4 h
50 1C, 4 h
50 1C, 4 h
Ph (1.5)
Ph (1.5)
Ph (1.5)
Ph (1.5)
Ph (1.5)
Ph (1.5)
Ph (1.5)
Ph (1.5)
Ph (1.5)
Ph (1.5)
Ph (1.5)
Ph (1.5)
CH2Ph (1.5)
CH2OPh (1.5)
(CH2)3CH3 (1.5)
19
49
61
70
58
45
32
35
55
57
48
64
47
43
26
p-CN (1)
p-NO2 (1)
p-Br (1)
p-Cl (1)
m-Cl (1)
p-CO2Me (1)
p-Me (1)
p-Me (2)
m-Me (2)
p-OMe (1)
p-OMe (2)
p-PhCO (1)
p-PhCO (1)
p-PhCO (1)
˜
´
would like to thank M. A. Pena-Gonzalez, E. Huerta-Salazar,
B. Quiroz-Garcıa, I. Chavez-Uribe, H. Rıos-Olivares, M. R.
´
´
´
9
10
11
12
13
14
15
a
Aniline : tBuONO : HCl (1 : 1.2 : 2), [AuI] = 0.034 mmol mlꢀ1
.
Scheme 3 Coupling with phenylacetylene.
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