Photosensitizer-Free, Gold-Catalyzed C–C Cross-Coupling of Boronic Acids and Diazonium Salts Enabled by Visible Light

Article abstract of DOI:10.1002/adsc.201700121

The first photosensitizer-free visible light-driven, gold-catalyzed C–C cross-couplings of arylboronic acids and aryldiazonium salts are reported. The reactions can be conducted under very mild conditions, using a catalytic amount of tris(4-trifluoromethyl)phosphinegold(I) chloride [(4-CF₃-C₆H₄)₃PAuCl] with methanol as the solvent allowing an alternative access to a variety of substituted biaryls in moderate to excellent yields with broad functional group tolerance. (Figure presented.).

Full text of DOI:10.1002/adsc.201700121

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DOI: 10.1002/adsc.201700121
Photosensitizer-Free, Gold-Catalyzed C–C Cross-Coupling of
Boronic Acids and Diazonium Salts Enabled by Visible Light
Sina Witzel,a Jin Xie,a,* Matthias Rudolph,a and A. Stephen K. Hashmia,b,*
a
Organisch-Chemisches Institut, Universitꢀt Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax : (+49)-6221-54-4205; e-mail: xj850626@163.com or hashmi@hashmi.de (homepage: http://www.hashmi.de)
Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
b
Received: January 31, 2017; Revised: February 24, 2017; Published online: && &&, 0000
Supporting information for this article can be found under http://dx.doi.org/10.1002/adsc.201700121.
Abstract: The first photosensitizer-free visible light-
driven, gold-catalyzed C–C cross-couplings of aryl-
boronic acids and aryldiazonium salts are reported.
The reactions can be conducted under very mild
conditions, using a catalytic amount of tris(4-tri-
fluoromethyl)phosphinegold(I) chloride [(4-CF3-
C6H4)3PAuCl] with methanol as the solvent allowing
an alternative access to a variety of substituted biar-
yls in moderate to excellent yields with broad func-
tional group tolerance.
um or diaryliodonium salts) combined with visible
light irradiation.[6]
Our group could show that an additional photosen-
sitizer is not even needed, aryldiazonium salts in visi-
ble light-catalyzed reactions can undergo formal oxi-
dative addition to simple coordinatively saturated
gold(I) compounds to produce coordinatively unsatu-
rated gold(III) species which then initiate the typical
gold-catalyzed nucleophilic addition reactions to C–C
multiple bonds. In the final step instead of the proto-
deauration of the organogold intermediate typically
observed with gold(I), a new C–C bond is formed by
reductive elimination from the diorganogold(III) in-
termediate and the coordinatively saturated gold(I)
compound is regenerated. This new reactivity trend
has been tentatively applied in stoichiometric organo-
metallic chemistry[7] as well as catalytic C(sp2)–C(sp)
Keywords: aryldiazonium salts; boronic acids; C–C
coupling; gold catalysis; visible light
Since 2000 homogenous gold catalysis has received bond formation reactions.[8]
significant attention. Due to the excellent carbophilic
We have now investigated if the transmetallation of
p-acidity, both gold(I) and gold(III) serve as powerful organometallic reagents to such coordinatively unsa-
tools to activate unsaturated C–C bonds towards nu- turated gold(III) intermediates is possible.
cleophilic attack without a change in oxidation state
Although visible light-mediated gold-catalyzed
of gold during the catalytic cycle.[1] Recently, there C(sp2)–C(sp2) cross-couplings using dual gold/photo-
has been incredible interest in the exploration of oxi- redox catalysts have very recently been reported,[9]
dative additions of organic moieties to mononuclear there are, to the best of our knowledge, no examples
and polynuclear gold(I) complexes.[2] The aim of ex- of visible light-mediated, gold-catalyzed C(sp2)–C(sp2)
panding the application of gold-mediated processes cross-couplings without photosensitizers or an exter-
by combining these with coupling reactions,[3] mimick- nal oxidant (Scheme 1). As a first example we envi-
ing the classical Mn/Mn+2 redox cycles of other late sioned a gold-catalyzed Suzuki-type coupling.
transition metals.[4] Nonetheless, different from the
We initiated our studies using 4-methoxycarbonyl-
palladium(0)/palladium(II) cycle, the high redox po- phenylboronic acid (1a) and phenyldiazonium tetra-
tential of the gold(I)/gold(III) redox couple requires fluoroborate (2a) in the presence of 10 mol%
strong external oxidants such as hypervalent iodine Ph3PAuCl at room temperature under irradiation with
reagents or F+ donors in stoichiometric amounts.[5] blue LEDs in acetonitrile. Fortunately, the desired
These conditions diminish one of the attractive fea- cross-coupling product 3a was indeed formed, but the
tures of gold catalysis, mild reaction conditions and yield after 16 h was low (Table 1, entry 1). Several ob-
excellent functional group tolerance. In order to cir- vious by-products resulting from the solvent and the
cumvent these harsh conditions, the groups of Toste aryldiazonium salts were also obtained. Then differ-
and Glorius, respectively, reported the elegant use of ent gold(I) catalysts bearing various ligands were
photosensitizers and aryl radical sources (aryldiazoni- evaluated (entries 2–9). Use of the cationic catalyst
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Table 1. Optimization of the reaction conditions.[a]
Entry Catalyst (10 mol%)
Solvent T [8C] Yield [%]
1
2
3
4
5
6
7
8
Ph3PAuCl
MeCN r.t.
MeCN r.t.
MeCN r.t.
MeCN r.t.
MeCN r.t.
trace[c]
31
–
20
Ph3PAuNtf2
(RO)3PAuCl[b]
(4-Me-C6H4)3PAuCl
Cy3PAuCl
31
Ph2(8-quinolyl)PAuCl MeCN r.t.
(4-F-C6H4)3PAuCl
(4-CF3-C6H4)3PAuCl
(4-CF3-C6H4)3PAuNTf2 MeCN r.t.
trace[c]
MeCN r.t.
MeCN r.t.
trace[c]
51
22
–
85
–
–
21
–
–
–
–
–
Scheme 1. Strategies for the cross-coupling of arylboronic
acids and aryldiazonium salts.
9
10
11
12
13
14[d]
15
16[e]
17[f]
18[e]
19[e]
20[g]
(4-CF3-C6H4)3PAuCl
(4-CF3-C6H4)3PAuCl
(4-CF3-C6H4)3PAuCl
(4-CF3-C6H4)3PAuCl
(4-CF3-C6H4)3PAuCl
–
(4-CF3-C6H4)3PAuCl
(4-CF3-C6H4)3PAuCl
(4-CF3-C6H4)3PAuCl
(4-CF3-C6H4)3PAuCl
(4-CF3-C6H4)3PAuCl
(4-CF3-C6H4)3PAuCl
DMF
r.t.
MeOH r.t.
THF
DCM
r.t.
r.t.
Ph3PAuNTf2 significantly improved the yield to 31%
(entry 7). Interestingly, with a gold(I) chloride com-
plex possessing a strong electron-poor phosphine
ligand the reaction did not work at all (entry 3). On
the other hand, with relatively weak electron-donat-
ing phosphines, the desired product 3a was obtained,
but in lower yields (entries 4 and 5). The relatively
electron-poor phosphine (4-CF3-C6H4)3P resulted in
MeCN r.t.
MeOH r.t.
MeOH r.t.
MeOH 70
MeOH 70
MeOH 50
MeOH r.t.
MeOH r.t.
61
75
an increased yield of 51%, as expected use of (4-CF3- 21[h]
C6H4)3PAuNTf2 led to a dramatic drop of the yield
[a]
Reaction conditions: 4-methoxycarbonylphenylboronic
acid (1, 0.1 mmol), phenyldiazonium tetrafluoroborate
(2, 0.4 mmol) and gold catalyst (10 mol%) were reacted
with different light sources, temperatures and solvents.
R=1,3-di-tert-butylbenzene.
Determined using GC-MS.
2 equiv. H2O were added.
In the dark.
Compact fluorescent lamp (CFL).
With UVA light.
(entries 8 and 9). Finally, MeOH was identified as the
optimum solvent for this transformation, furnishing
the desired product 3a in 85% yield (entry 11). Con-
trol experiments confirmed that the cross-coupling
could not occur without gold(I) catalyst (entry 15).
Further experiments showed that light was essential
for the success of the cross-coupling, also simple heat-
ing of the reaction did not lead the desired product
(entries 16–19). Next, various light sources were ex-
[b]
[c]
[d]
[e]
[f]
[g]
[h]
With l=420 nm.
amined, and blue LEDs turned out to be the most ef-
ficient light source (entries 20 and 21). Furthermore,
this catalytic system does not require the addition of Also a thiophene-based heteroaromatic boronic acid
an external ligand and base. was tolerated (3f). Further ortho-, meta- and para-
With the optimized reaction conditions in hand substituted arylboronic acids can be applied to give
(Table 1, entry 11), we next turned our focus on the compounds 3a, 3k, 3l, in acceptable to good yields.
expansion of the substrate scope by first investigating Among the tested starting materials, (4-hydroxyphe-
a range of arylboronic acids. In general, all of the nyl)boronic acid, (3,5-dimethoxyphenyl)boronic acid
tested boronic acids can proceed the C–C-coupling and [3-(dimethylamino)phenyl]boronic acid failed to
readily affording the valuable biaryls 3a–l in 22–84% afford the desired cross-coupled biaryl products. We
yield (Table 2). Interestingly, the boronic acids bear- speculated that the boronic acids bearing strong elec-
ing electron-withdrawing substituents on the phenyl tron-donating groups underwent a slower transmetal-
ring appear to react more efficiently (3a–i, 3k and 3l) lation process.
than those with electron-donating substituents (3j). 4-
We next examined the substrate scope with respect
Haloboronic acids can be directly used to afford biar- to aryldiazonium salts. Both alkyl-substituted and
yls 3g–3i in 48–80% yields, which further underlines electron-poor aryldiazonium salts afforded their cor-
the synthetic potential of this methodology, if one responding biaryls in 70–90% yields (3q, 3m, 3s, 3u).
considers the fundamental organic transformations Also, the relatively steric hindered substrates can be
subsequently possible with these aromatic halides. transformed (3q). In contrast, 3r bearing an electron-
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Table 2. Reaction scope with respect to boronic acids 1 and aryldiazonium salts 2.[a]
[a]
All reactions were carried out using 1 (0.3 mmol), 2 (4 equiv., 1.2 mmol) in the presence of (4-CF3-C6H4)3PAuCl
(10 mol%) in MeOH (1.5 mL) under the optimized reaction conditions.
rich substituent afforded only 30% yield. Further- dous effect on the transformation (Table 3, entries 1
more, a methylthiophene-2-carboxylate-based diazo- and 2). This prompted us to further investigate the
nium salt can be cross-coupled to deliver 3t in accept- role of the anion of the aryldiazonium salt on the re-
able yield. With 3-fluorophenyldiazonium and 2-fluo- action mechanism. On that account, we added an ex-
rophenyldiazonium tetrafluoroborates, the cross-cou-
pled products 3v and 3w were obtained in 83% and
66%, respectively. Remarkably, this method exhibits
a good functional group tolerance on varying the sub-
stituents on both arylboronic acids and aryldiazonium
salts, and useful functional groups such as cyanides,
halides, ethers, ketones, aldehydes and methylsulfon-
yls were well compatible.
Notably, besides boronic acids, arylboronic pinacol
ester 4 is also a competent coupling partner and it can
give the biaryls 3a and 3h in excellent yields
(Scheme 2). Compound 3j bearing an electron-donat-
ing group was obtained in low yield.
Scheme 2. Reaction of arylboronic pinacol ester with 4-
chlorobenzenediazonium under the optimized reaction con-
During the examination of the substrate scope, we
observed that changing the counterion has a tremen- ditions.
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Table 3. Variation of the counterion of the aryldiazonium
by light irradiation, elimination of N2 can afford aryl
gold(III) species C. The next step is a transmetallation
of the in situ activated arylboronic acid complex to C
to give diaryl gold(III) species D. Finally, fast reduc-
tive elimination from the Ar1–Au(III)–Ar2 intermedi-
ate[11] delivers biaryls and completes the catalytic
cycle. Our first experiments on a possible reaction of
the Àcatalyst with the boronic acid, with or without
salt.[a]
Entry
Counterion XÀ
Additives
Yield [%] BF4 and with or without irradiation, indicated that
no transmetallation to an L–Au–R species occurs
under such conditions (see control experiment C in
À
1
2
3
BF4
–
–
95
–
À
NTf2À
the Supporting Information).
36
NTf2
CsF (2 equiv.)
In conclusion, we have developed the first applica-
[a]
Reaction conditions: 4-methoxycarbonylphenylboronic
acid (1, 0.1 mmol), 4-bromophenyldiazonium salt (x2,
0.4 mmol) and gold catalyst (10 mol%) were reacted in
methanol at room temperature under irradiation with
blue LEDs.
tion of photosensitizer- and external oxidant-free pho-
toredox gold catalysis for C–C cross-couplings of aryl-
boronic acids or arylboronic pinacol esters and aryl-
diazonium salts. It provides various biaryls, stemming
from aryldiazonium salts and arylboronic acids, in
moderate to excellent yields under very mild reaction
ternal fluoride source, which afforded 3o in 36% yield conditions. The additional functionalization of the
(entry 3). We hypothesize that a fluoride source addi- biaryl products opens up the possibility for further
tionally activates the boronic acid in situ and thus fa- functionalization, for instance, with the common pal-
vuors the transmetallation, since tetrafluoroborates ladium chemistry. The principles of these reactions
prefer to provide a fluoride compared to bis[(trifluor- can probably be transferred to a whole range of other
omethyl)sulfonyl]amines. In addition, a radical chain transmetallation reagents and thus significantly
pathway can be ruled out from the measured quan- extend the scope of gold catalysis. Further investiga-
tum yield (F=0.302).
tions of the reaction mechanism are ongoing in our
Based on this experimental evidence in combina- group.
tion with literature reports,[7,8,10] a possible mechanism
for the cross-coupling is shown in Scheme 3.
First, the reaction is possibly initiated by gold-in- Experimental Section
duced single electron transfer to the aryldiazonium
salt, generating an aryldiazo radical and gold(II) spe- General Procedure for Visible Light-Mediated Gold-
cies A; then the Ar2 radical combines with the Catalyzed C(sp2)–C(sp2) Coupling
gold(II) center, which oxidizes to gold(III) species B.
Alternatively, a direct addition of the diazonium salt
to the gold(I) complex is also conceivable. Assisted
In
a dried Pyrex screw-top reaction tube (4-CF3-
C6H4)3PAuCl (0.03 mmol, 10 mol%) and the corresponding
boronic acid (0.3 mmol, 1.0 equiv.) were dissolved in 1.5 mL
MeOH. After adding the corresponding diazonium salt
(1.2 mmol, 4.0 equiv.) the reaction mixture was degassed
under argon by sparging for 5–10 min. The tubes were irra-
diated at room temperature with 29W blue LEDs for 15–17
hours. The solvent was removed under reduced pressure and
the resulting crude product was purified by column chroma-
tography on SiO2.
Synthesis of Methyl [1,1’-Biphenyl]-4-carboxylate
Scheme 3. Proposed reaction mechanism.
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According to the general procedure, [4-(methoxycarbo-
nyl)phenyl]boronic acid (0.3 mmol, 54.0 mg, 1.0 equiv.) and
(4-CF3-C6H4)3PAuCl (0.03 mmol, 21.0 mg, 10 mol%) were
dissolved in 1.5 mL MeOH. After adding benzenediazonium
tetrafluoroborate (1.2 mmol, 230 mg, 4.0 equiv.) the reaction
mixture was degassed under argon by sparging for 5–10 min.
The tubes were irradiated at room temperature with blue
LEDs for 17 h and the crude product was purified by flash
column chromatography (SiO2, PE/EA, 300:1) to give 3a as
a pale yellow solid; yield: 53.2 mg (0.25 mmol, 84%).
1H NMR (300 MHz, CDCl3): d=3.90 (s, 3H), 7.33–7.46 (m,
3H), 7.58–7.70 (m, 4H) ppm, 8.05–8.09 (m, 2H).
argon by sparging for 5–10 min. The tubes were irradiated at
room temperature with blue LEDs for 15 h and the crude
product was purified by flash column chromatography
(SiO2, PE/EA, 250:1) to give 3n as a white solid; yield:
68.8 mg (0.25 mmol, 82%); mp 121–1228C. 1H NMR
(400 MHz, CDCl3): d=3.95 (s, 3H), 7.65–7.68 (m, 2H), 7.72
(s, 4H), 8.12–8.15 (m, 2H); 13C NMR (101 MHz, CDCl3):
d=52.2 (q), 125.9 (s, q: JC,F =3.8 Hz), 127.2 (d), 127.6 (d),
129.9 (s), 130.3 (d), 143.6 (s), 144.1 (s), 166.7 (s); 19F NMR
˜
(283 MHz, CDCl3): d=À62.5 (s, 3F); IR (ATR): n=2954,
1943, 1712, 1609, 1584, 1437, 1398, 1373, 1334, 1287, 1182,
1158, 1143, 1111, 1075, 1023, 1008, 956, 869, 842, 833, 774,
739, 700, 667 cmÀ1; HR-MS [EI(+)]: m/z=280.0695, calcd. for
[C15H11O2F3]+: 280.0706.
Synthesis of 4-Bromo-1,1’-biphenyl
Synthesis of Methyl 4’-(Methanesulfonyl)-[1,1’-
biphenyl]-4-carboxylate
According to the general procedure, (4-bromophenyl)bor-
onic acid (0.3 mmol, 60.2 mg, 1.0 equiv.) and (4-CF3-
C6H4)3PAuCl (0.03 mmol, 21.0 mg, 10 mol%) were dissolved
in 1.5 mL MeOH. After adding benzenediazonium tetra-
fluoroborate (1.2 mmol, 230 mg, 4.0 equiv.) the reaction mix-
ture was degassed under argon by sparging for 5–10 min.
The tubes were irradiated at room temperature with blue
LEDs for 18 h and the crude product was purified by flash
column chromatography (SiO2, 100% PE) to give 3i as an
According to the general procedure, [4-(methoxycarbo-
nyl)phenyl]boronic acid (0.3 mmol, 54.0 mg, 1.0 equiv.) and
(4-CF3-C6H4)3PAuCl (0.03 mmol, 21.0 mg, 10 mol%) were
dissolved in 1.5 mL MeOH. After adding 4-(methylsulfo-
nyl)benzenediazonium tetrafluoroborate (1.2 mmol, 324 mg,
4.0 equiv.) the reaction mixture was degassed under argon
by sparging for 5–10 min. The tubes were irradiated at room
temperature with blue LEDs for 17 h and the crude product
was purified by flash column chromatography (SiO2, 100%
DCM) to give 3t as an off-white solid; yield: 50.6 mg
(0.17 mmol, 58%); mp 196–1978C. 1H NMR (300 MHz,
CDCl3): d=3.10 (s, 3H), 3.96 (s, 3H), 7.68 (d, J=8.8 Hz,
2H), 7.80 (d, J=8.8 Hz, 2H), 8.04 (d, J=8.8 Hz, 2H), 8.16
(d, J=8.3 Hz, 2H); 13C NMR (75 MHz, CDCl3): d=44.5
(q), 52.2 (q), 127.3 (d), 128.0 (d), 128.1 (d), 130.2 (s), 130.3
1
off-white solid; yield: 55.8 mg (0.26 mmol, 80%). H NMR
(300 MHz, CDCl3): d=7.34–7.48 (m, 5H), 7.54–7.58 (m,
4H).
Synthesis of 4-Iodo-1,1’-biphenyl
˜
(d), 139.9 (s), 143.3 (s), 145.4 (s), 166.5 (s); IR (ATR): n=
3073, 3019, 2961, 2933 1946, 1925, 1715, 1608, 1580, 1561,
1456, 1440, 1396, 1311, 1294, 1273, 1214, 1196, 1181, 1150,
1117, 1096, 1021, 1005, 970, 867, 833, 784, 869, 751, 714, 699,
615 cm1; HR-MS [EI(+)]: m/z=290.0599, calcd. for
[C15H14O4S]+: 290.0607.
The reaction was carried out according to the general pro-
cedure, using 74.3 mg of (4-iodophenyl)boronic acid
(0.3 mmol, 1.0 equiv.), 21.0 mg of (4-CF3-C6H4)3PAuCl
(0.03 mmol, 10 mol%), 230 mg of benzenediazonium tetra-
fluoroborate (1.2 mmol, 4.0 equiv.) and 1.5 mL of MeOH.
After flash column chromatography (SiO2, 100% n-hep-
tane), 3j was isolated as a white solid; yield: 68.0 mg
(0.24 mmol, 81%). 1H NMR (300 MHz, CDCl3): d=7.33–
7.39 (m, 3H), 7.44–7.47 (m, 2H), 7.55–7.57 (m, 1H), 7.60–
7.62 (m, 1H), 7.76–7.79 (m, 2H).
Synthesis of Methyl 3’-Fluoro-[1,1’-biphenyl]-4-
carboxylate
Synthesis of Methyl 4’-(Trifluoromethyl)-[1,1’-
biphenyl]-4-carboxylate
According to the general procedure, [4-(methoxycarbo-
nyl)phenyl]boronic acid (0.3 mmol, 54.0 mg, 1.0 equiv.) and
(4-CF3-C6H4)3PAuCl (0.03 mmol, 21.0 mg, 10 mol%) were
dissolved in 1.5 mL MeOH. After adding 3-fluorobenzene-
diazonium tetrafluoroborate (1.2 mmol, 252 mg, 4.0 equiv.)
the reaction mixture was degassed under argon by sparging
for 5–10 min. The tubes were irradiated at room tempera-
ture with blue LEDs for 16 h and the crude product was pu-
rified by flash column chromatography (SiO2, PE/EA,
150:1) to give 3w as an off-white solid; yield: 57.0 mg
(0.25 mmol, 83%); mp 59–608C. 1H NMR (600 MHz,
According to the general procedure, [4-(methoxycarbo-
nyl)phenyl]boronic acid (0.3 mmol, 54.0 mg, 1.0 equiv.) and
(4-CF3-C6H4)3PAuCl (0.03 mmol, 21.0 mg, 10 mol%) were
dissolved in 1.5 mL MeOH. After adding 4-(trifluorome-
thyl)benzenediazonium
312 mg, 4.0 equiv.) the reaction mixture was degassed under
tetrafluoroborate
(1.2 mmol,
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CDCl3): d=3.94 (s, 3H), 7.07–7.10 (m, 1H), 7.32 (dt, J=
1.9 Hz, J=9.9 Hz, 1H), 7.39–7.45 (m, 2H), 7.63–7.65 (m,
2H), 8.10–8.12 (m, 2H); 13C NMR (151 MHz, CDCl3): d=
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Wolf, F. D. Toste, J. Am. Chem. Soc. 2014, 136, 7777–
7782; g) M. Joost, L. Estevez, K. Miqueu, A. Amgoune,
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Angew. Chem. Int. Ed. 2015, 54, 5236–5240; h) M.
Joost, A. Zeineddine, L. Estevez, S. Mallet-Ladeira, K.
Miqueu, A. Amgoune, D. Bourissou, J. Am. Chem.
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Chem. Soc. 2014, 136, 1778–1781.
52.5 (q), 114.5 (d, d: JC,F =23.4 Hz), 115.3 (d, d: JC,F
=
21.0 Hz), 123.2 (d, d: JC,F =3.0 Hz), 127.4 (d), 129.8 (s), 130.5
(d), 130.7 (d, d: JC,F =8.5 Hz), 142.5 (s, d: JC,F =7.4 Hz),
144.5 (s, d: JC,F =2.3 Hz), 163.5 (s, d: JC,F =245.3 Hz), 167.2
(s); 19F NMR (283 MHz, CDCl3): d=À112.7 (s, 1F); IR
˜
(ATR): n=3075, 3008, 2957, 2852, 1937, 1719, 1611, 1589,
1569, 1486, 1475, 1439, 1399, 1279, 1189, 1166, 1114, 1037,
1016, 1000, 961, 903, 881, 854, 828, 797, 770, 726, 700, 685,
648 cmÀ1
;
HR-MS [EI(+)]: m/z=230.0740, calcd. for
[C14H11O2F]+: 230.0743.
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These are not the final page numbers!

COMMUNICATIONS
8 Photosensitizer-Free, Gold-Catalyzed C–C Cross-Coupling
of Boronic Acids and Diazonium Salts Enabled by Visible
Light
Adv. Synth. Catal. 2017, 359, 1 – 8
Sina Witzel, Jin Xie,* Matthias Rudolph,
A. Stephen K. Hashmi*
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These are not the final page numbers!