Gold-catalysed cross-coupling between aryldiazonium salts and arylboronic acids: Probing the usefulness of photoredox conditions

Article abstract of DOI:10.1039/c6cc04239b

The synthesis of biaryl compounds from aryldiazonium salts and arylboronic acids was achieved using PPh₃AuCl as catalyst, CsF as base and acetonitrile as solvent. Combined to photosensitizers, irradiation by blue LEDs allowed accelerating the reaction and expanding its scope. Various functional groups were compatible including bromoaryls, iodoaryls, aldehydes and alcohols. A 2-iodobenzyl alcohol moiety was smoothly introduced by this method, enabling its consecutive isotopic labelling by a Pd-catalysed alkoxycarbonylation.

Full text of DOI:10.1039/c6cc04239b

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Gold-catalysed cross-coupling between
aryldiazonium salts and arylboronic acids: probing
Cite this:DOI: 10.1039/c6cc04239b
the usefulness of photoredox conditions†
Received 19th May 2016,
Accepted 19th July 2016
Thomas Cornilleau, Philippe Hermange* and Eric Fouquet*
DOI: 10.1039/c6cc04239b
www.rsc.org/chemcomm
The synthesis of biaryl compounds from aryldiazonium salts and
arylboronic acids was achieved using PPh3AuCl as catalyst, CsF as base
and acetonitrile as solvent. Combined to photosensitizers, irradiation by
blue LEDs allowed accelerating the reaction and expanding its scope.
Various functional groups were compatible including bromoaryls,
iodoaryls, aldehydes and alcohols. A 2-iodobenzyl alcohol moiety was
smoothly introduced by this method, enabling its consecutive isotopic
labelling by a Pd-catalysed alkoxycarbonylation.
The field of homogeneous gold catalysis has been intensively
explored in the last few decades and is now a well-established
area.1 First employed for pi-activation and cascade reactions,2
many efforts have focused on catalytic cycles requiring changes
in the Au oxidation state.3 This led to the development of novel,
successful strategies for coupling reactions implying the formation
of gold(III) intermediates, using for example external oxidants4 or
Scheme 1 Gold-catalysed couplings with aryldiazonium salts under photo-
redox conditions.
radical partners. The latter approach has been recently extremely and others.7e–g Both phosphorus- and carbon-based partners
fruitful, especially when merged with radical generation through (sp3, sp or sp2) were described but surprisingly, no biaryl
visible light photoredox catalysis.5e Being prone to single-electron formation was reported under photoredox conditions.9
reduction,5 aryldiazonium salts proved to be ideal reactants for
However, it should be noted that the line between photoredox
gold catalysed couplings under photoredox conditions6 and have catalysis and photoredox activation may be thin.10 When light only
already been combined to various substrates (Scheme 1).7 plays a role in radical initiation, other methods of activation can
Indeed, under visible light and in the presence of a photosensitizer become competitive, as demonstrated by a few reports describing
[Ru(bpy)32+, fluorescein,...], they give in situ the desired aryl-radicals, gold-catalysed coupling of diazonium salts without visible light and
allowing consecutive oxidative addition onto Au(I) species. After PS.7d,11 For example, Shin et al. proved that simple thermal activation
another one-electron oxidation, the reductive elimination from the of diazonium salts at 60 1C was sufficient to induce the cross
resulting Au(III) complexes produces the desired compounds coupling of vinyl golds in the dark (which was previously developed
while regenerating the gold(I) catalyst.8 After the seminal work of at room temperature under blue LED irradiation).7d In a different
Glorius et al. exploiting the intramolecular cyclization of alkenes,7a
a
manner, Shi et al. recently published an elegant method employing
wide range of reactions were also found to be suitable for this cationic AuI catalysts which were able to promote direct N2 extrusion
strategy including methanol addition to alkenes,7b ring expansion,7c of diazonium salts under the assistance of 2,20-bipyridine.11
allenoate cyclization,7d Meyer–Schuster rearrangements7h–j Under this set of conditions, the couplings of terminal alkynes or
arylboronic acids with the aryl moiety of diazonium species were
achieved in good yields. Unfortunately, these conditions led to much
´
lower yields when using p-iodide diazonium salts (Scheme 2).12
Within this context, we were looking for methods allowing
the synthesis of biaryl compounds from boronic acids and
aryldiazonium salts that would be compatible with free benzyl
Univ. Bordeaux, Institut des Sciences Moleculaires, UMR-CNRS 5255,
´
351, Cours de la Liberation, 33405 Talence Cedex, France.
E-mail: philippe.hermange@u-bordeaux.fr, eric.fouquet@u-bordeaux.fr
† Electronic supplementary information (ESI) available: General experimental proce-
dures and spectroscopic data (1H NMR and 13C NMR). See DOI: 10.1039/c6cc04239b
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for further experiments. In combination with slightly increased
quantities of diazonium 1a (1.5 equivalent) and acridinium 6
(4 mol%), the desired biaryl 3a was isolated in 55% yield (entry 9).
Interestingly, exchanging the organic photosensitizer by 2 mol%
of Ru(bpy)3(PF6)2 allowed access to 3a in 63% yield (entry 10).
Finally, control experiments were performed in the absence of any
photosensitizer either under blue light irradiation or in the dark
Scheme 2 Gold-catalysed coupling of aryldiazonium salts with arylboronic
acids without photoredox conditions (Shi et al., ref. 11).
alcohols and iodoaryls. Considering the scarce reports of (entries 11 and 12). Both experiments led to almost identical results
Pd-catalysed Suzuki–Miyaura reactions potentially meeting (i.e. lower conversion and limited formation of 3a), indicating that
these criteria13 and the limited results obtained in our case,14 no direct effect of the visible light could be detected in this case.19
the possibilities offered by gold-catalysis were re-examined.
In contrast, the substitution on the aryl ring of the diazonium
Hypothesizing a potential detrimental effect of the cationic appeared to be crucial. Indeed, the 4-nitrobenzene-diazonium
gold source towards iodoaryls, the cross-coupling of 4-methoxy- tetrafluoroborate 1b was able to react with phenylboronic acid
benzene-diazonium tetrafluoroborate 1a with phenylboronic without any photoredox conditions, producing the cross-coupling
acid 2 was investigated with PPh3AuCl. This one was less active,15 product 3b in 81% yield within 16 h (entry 13). The course of this
and only small amounts of the desired product 3a could be obtained reaction was monitored when performed in the dark or under
without photoredox conditions. However, when combined with blue irradiation with blue LEDs in the presence of a photosensitizer
LED irradiation and a catalytic amount of 9-mesityl-10-acridinium (Fig. 1, see the ESI† for details).
tetrafluoroborate (6),7d,e,16 more interesting results were observed.
At 30 1C in the dark, pure thermal initiation was sufficient to
Indeed, the use of 10 mol% of PPh3AuCl and 2 equivalents of ensure smooth formation of 3b (Fig. 1, orange line). Indeed, the
Na2CO3 in CH3CN at 30 1C under photoredox conditions led to a easiness of the 4-nitro-substituted diazonium salt to undergo
57% conversion of the diazonium 1a. The desired coupling product one electron reduction20 allowed thermal initiation and innate
3a was produced in this case concomitantly with anisole 5a in a radical chains to operate efficiently.21 This mechanism difference
1
2/1 ratio, as determined by H NMR analysis of the crude mixture between the two substrates was confirmed by quantum yield
(Table 1, entry 1). KOH17 instead of Na2CO3 slightly improved this measurement, which were determined to be 1.4 and 9.6 when
result (entry 2) but CsF was demonstrated to be the best base,18 employing 1a (4-MeO–) or 1b (4-NO2–), respectively (see the
allowing full conversion of 1a to a 3a/5a mixture in a 7 : 3 ratio ESI† for details). However, in the latter, the use of photoredox
(entry 3). A control experiment in the dark with bpy as an conditions allowed completion of the reaction within 1 h 30
additive (20 mol%) gave only 40% conversion, demonstrating (Fig. 1, blue line) whereas only 50% of 3b was formed at the
the importance of the photoredox process under this set of same time in the dark (orange line). Thus, the benefits of the
conditions (entry 4). The use of (IPr)AuCl instead of PPh3AuCl photoredox initiation process10c were demonstrated in this case
was investigated but no biaryl products were formed in this case by a strong acceleration of the reaction.
(entry 5). An excess or catalytic amount of CsF was detrimental
Then, the scope was investigated with the 9-mesityl-10-
(entries 6–8), and the quantity of base was set to 1.05 equivalents acridinium 6 (conditions A) or with Ru(bpy)3(PF6)2 (conditions R)
Table 1 Optimization of the reaction conditionsa
Ar (x equiv.)
Photosensitizerb (y mol%)
Base (z equiv.)
1/3/4/5 ratioc (yield, %)d
1
2
1a (1 equiv.)
1a (1 equiv.)
1a (1 equiv.)
1a (1 equiv.)
1a (1 equiv.)
1a (1 equiv.)
1a (1 equiv.)
1a (1 equiv.)
1a (1.5 equiv.)
1a (1.5 equiv.)
1a (1.5 equiv.)
1a (1.5 equiv.)
1b (1.5 equiv.)
Acrid (2 mol%)
Acrid (2 mol%)
Acrid (2 mol%)
bpy (20 mol%)
Acrid (2 mol%)
Acrid (2 mol%)
Acrid (2 mol%)
Acrid (2 mol%)
Acrid (4 mol%)
Ru (2 mol%)
None
Na2CO3 (2 equiv.)
KOH (1 equiv.)
CsF (1 equiv.)
CsF (1 equiv.)
CsF (1 equiv.)
CsF (1.5 equiv.)
CsF (2.0 equiv.)
CsF (0.5 equiv.)
CsF (1.05 equiv.)
CsF (1.05 equiv.)
CsF (1.05 equiv.)
CsF (1.05 equiv.)
CsF (1.05 equiv.)
43/35/0/22
24/46/10/20
0/70/0/30
60/11/10/19
30/0/0/60
0/33/13/54
0/25/11/64
48/32/3/17
13/60/5/22 (55%)
0/73/2/25 (63%)
60/15/5/20
3
4e
5f
6
7
8
9
10
11
12e
13e
None
None
62/15/5/18
33/57/0/10 (81%)
a
b
c
Reactions run on a 0.25 mmol scale. Acrid = 9-Mesityl-10-acridinium tetrafluoroborate 6, bpy = 2,20-bipyridine, Ru = Ru(bpy)3(PF6)2. Ratio
determined by 1H NMR analysis of the crude mixture. Isolated yields of 3 after purification by column chromatography. Reaction performed at
30 1C in the dark. (IPr)AuCl was used instead of PPh3AuCl.
d
e
f
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Fig. 1 Evolution in time of the 1H NMR yield of 3b in the dark (orange line)
or with Ru(bpy)3(PF6)2 (2 mol%) under blue LED irradiation (blue line).
as photosensitizers. Under both conditions, the mixture was
stirred for 16 h under blue light irradiation to ensure the full
completion of the reaction.
Interestingly, both conditions gave good results for 4-nitro-
benzenediazonium 1b with phenylboronic 2, 2-methyl-phenyl
and 2-bromophenylboronic acids as partners. Compound 3b
was isolated in 75–76% yields, and products 3c and 3d were
obtained in 90–92% and 73–80% yields, respectively. The reaction
was also compatible with highly hindered nucleophiles, and product
3e was synthesized from 9-anthraceneboronic acid in yields up
to 55% (conditions A). When switching to 4-methoxybenzene-
diazonium tetrafluoroborate 1a, the Ru(bpy)32+-based conditions
gave the best yield (63% of 3a) and were employed with other
arylboronic acids. Compound 3f was isolated in a 55% yield
whereas 2-methyl- and 2-nitrophenylboronic led to the cross-
coupling products 3g and 3h with excellent results (respectively,
90% and 92% isolated yields). Similarly to 3b, identical yields of
3i were obtained from 4-(methoxycarbonyl)-benzenediazonium
with the two sets of conditions (65%). With this diazonium,
conditions A and R were compatible with aldehydes (50% of 3k)
and iodoaryls (up to 55% of 3l), demonstrating the mildness of
this method. 2-Nitro- and 4-iodobenzene-diazonium were also
suitable, providing 3m, 3n and 3o with moderate to good yields.
Finally, the most noticeable achievements were the syntheses of
the desired model compounds 3p, with a yield of 50% with both
photosensitizers, and 3q, in a 47% yield under conditions R.22
Indeed, introduction of the 2-iodobenzyl alcohol moiety occurred
smoothly, allowing its further isotopic labelling by a palladium-
catalysed alkoxycarbonylation (Scheme 3, 90% isolated yield of 7,
based on a limiting amount of 13CO).23
Scheme 3 Substrate screening and carbonylation of 3p. Isolated yields.
Conditions: Arylboronic acid (0.25 mmol), aryldiazonium tetrafluoroborate
(1.5 equiv.), PPh3AuCl (10 mol%), CsF (1.05 equiv.), CH3CN (2 mL) at 30 1C for
16 h. aConditions A: 9-mesityl-10-acridinium tetrafluoroborate (4 mol%).
bConditions R: Ru(bpy)3(PF6)2 (2 mol%). cConditions from ref. 13b: Pd/BaCO3
(0.5 mol%), MeOH, rt, 12 h, see the ESI† for details. dReaction time: 8 h.
eLimiting amount of 13CO, see the ESI† for details.
the aryl radical. The latter is consecutively regenerated by a one-
electron oxidation of the gold(II) intermediates. Then, the reductive
elimination from the resulting Au(III) complexes produces the biaryl
compound25 and regenerates the gold(I) catalyst. It should be noted
that the radical initiation step can occur through thermal processes,
but these are highly dependent on the substitution onto the
diazonium salts. In any case, the photoredox cycle allows an
acceleration of the reaction, thus expanding its scope to less
active substrates.
In summary, we described the gold-catalysed coupling of
aryldiazonium salts with arylboronic acids under photoredox
conditions. Employing the convenient PPh3AuCl as the gold
source, the conditions proved to be extremely mild (30 1C) and
compatible with a wide range of functional groups such as
To investigate the reaction mechanism, 31P and 1H NMR spectro-
scopies were realised on stoichiometric amounts of PPh3AuPh in
CD3CN. Acridinium 6 and 1a or 1b were added to the mixture, which
was then irradiated with blue LEDs for 3 h. Under these conditions,
the gold complex was able to produce the desired 3a and 3b as
the main products (see the ESI† for details). Based on these
results, and on the previous reports of gold-catalysed cross-
couplings with diazonium salts,7,8,11 the mechanism in Scheme 4
was proposed. After transmetalation with the arylboronic acids,24 the
triphenylphosphine-aryl-gold species undergo oxidative addition by
Scheme 4 Proposed mechanism of the reaction.
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10 For interesting discussions on radical propagation with aryldiazonium
salts, see: (a) M. Majek, F. Filace and A. Jacobi von Wangelin, Beilstein
J. Org. Chem., 2014, 10, 981; (b) M. A. Cismesia and T. Yoon, Chem. Sci.,
2015, 6, 5426. For a general discussion on catalysis in radical reactions,
see: (c) A. Studer and A. S. Curran, Angew. Chem., Int. Ed., 2016, 55, 58.
11 R. Cai, M. Lu, E. Y. Aguilera, Y. Xi, N. G. Akhmedov, J. L. Petersen,
H. Chen and X. Shi, Angew. Chem., Int. Ed., 2015, 54, 8772.
12 The yields for iodoaryl substrates were described as ‘‘low’’ in note (19)
of ref. 11. In our hands, 3n and 3o were isolated, respectively; in 14%
yield and 10% yield with Shi’s conditions. See the ESI† for details.
13 For Pd-catalysed Suzuki–Miyaura reactions with diazonium salts with
conditions compatibles with iodoaryls, see: (a) S. Sengupta and
S. K. Sadhukhan, Tetrahedron Lett., 1998, 39, 715; (b) F.-X. Felpin
and E. Fouquet, Adv. Synth. Catal., 2008, 350, 863. For a review on
Pd-catalysed Suzuki–Miyaura reactions with diazonium salts, see:
(c) H. Bonin, E. Fouquet and F.-X. Felpin, Adv. Synth. Catal., 2011,
353, 3063.
14 See Scheme 3 for results obtained for 3p and 3q using the best
conditions for p-iodoaryldiazonium tetrafluoroborate [Pd/BaCO3
(0.5 mol%), MeOH, rt, 12 h, ref. 13b]. See the ESI† for details.
15 In their cross-coupling of vinyl golds with diazonium salts under
photoredox conditions, Shin et al. measured quantum yields of 0.31
and 1.21 employing, respectively, PPh3AuCl or a PPh3AuCl/AgOTf
mixture as catalyst. This indicates that radical chain processes are
probably favoured by cationic gold species. See ref. 7d.
bromoaryls, iodoaryls, aldehydes and alcohols. The utility of
this method was demonstrated by performing the synthesis of a
biaryl with a 2-iodobenzyl alcohol moiety, which could not be
efficiently obtained using the methods described previously in
the literature. The resulting compound 3o was efficiently
labelled by 13C–carbon monoxide, confirming the orthogonality
with Pd-catalysed reactions. Aiming toward the synthesis of new
potential biotracers,23a gold-catalysed couplings of this 3-(hydroxy-
methyl)-4-iodobenzenediazonium salt 1o with biomolecule-based
boronic acids are being currently explored.
This study was supported by the MESR (doctoral fellowship),
and by a public grant from the French Agence Nationale de la
Recherche within the context of the Investments for the Future
Program, referenced ANR-10-LABX-57 and named TRAIL.
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