Light-Induced Gold-Catalyzed Hiyama Arylation: A Coupling Access to Biarylboronates

Article abstract of DOI:10.1002/anie.201806427

Organoboron compounds are versatile synthetic building blocks. We herein report a new strategy, a photochemical gold-catalyzed chemo-selective Hiyama arylation of B,Si bifunctionalized reagents with diazonium salts, which is orthogonal to common strategies and therefore a unique tool for synthesis of valuable biarylboronates. With this new methodology a wide array of diversely functionalized sp²- and sp³-hybridized biarylboronates were obtained. Notably, the synergism of gold catalysis with copper catalysis or palladium catalysis, allows for one-pot iterative C?X (heteroatom) and C?C couplings for the rapid assembly of several simple fragments to relatively complex molecules. Mechanistic studies indicated that photosensitizer-free conditions were superior to gold/Ru(bpy)₃Cl₂ dual catalysis.

Full text of DOI:10.1002/anie.201806427

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Accepted Article
Title: Light-Induced Gold-Catalyzed Hiyama Arylation: A Coupling
Access to Biarylboronates
Authors: Jin Xie, Kohei Sekine, Sina Witzel, Petra Krämer, Matthias
Rudolph, Frank Rominger, and A. Stephen K. Hashmi
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10.1002/anie.201806427
Angewandte Chemie International Edition
Light-Induced Gold-Catalyzed Hiyama Arylation: A Coupling Access to Biarylboronates
Jin Xie,*,a Kohei Sekine,b Sina Witzel,b Petra Krämer,b Matthias Rudolph,b Frank Rominger,b A.
Stephen K. Hashmi*,b,c
aState Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic
Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P.
R. China
b Organisch-Chemisches Institut, Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
c Chemistry Department, Faculty of Science, King Abdulaziz University
Jeddah 21589 (Saudi Arabia)
E-mail: xie@nju.edu.cn; hashmi@hashmi.de
Abstract: Organoboron compounds are versatile synthetic building blocks. We herein report a new
strategy, a photochemical gold-catalyzed chemo-selective Hiyama arylation of B,Si bimetallic
reagents with diazonium salts, which is orthogonal to common strategies and therefore a unique tool
for synthesis of valuable biarylboronates. With this new methodology a wide array of diversely
functionalized sp2- and sp3-hybridized biarylboronates were obtained. Notably, the synergism of
gold catalysis with copper catalysis or palladium catalysis, allows for one-pot iterative C-X
(heteroatom) and C-C couplings for the rapid assembly of several simple fragments to relatively
complex molecules. Mechanistic studies indicated that photosensitizer-free conditions were
superior to gold/Ru(bpy)3Cl2 dual catalysis.
Keywords: gold catalysis; Hiyama coupling; diazonium salts; photochemistry; bimetallic reagents
Biaryl units represent a fundamental and important scaffold for the generation of molecular
complexity.[1] They are indispensable structural linkers for a great number of pharmacophores,
natural products, agrochemicals and molecular machines. So far, their synthesis mainly relies on
classical Pd-catalyzed cross-coupling between two aromatic subunits, one electrophilic (Ar1-X) and
one nucleophilic (Ar2-M) building block.[2] Recently, directed C-H arylation[3] and oxidative cross-
dehydrogenative coupling of two arenes[4] become an alternative, despite of the selectivity issue
these methods are associated with. Still, an entirely different synthetic disconnection involving
preformed boron-containing biaryl units which are applied as a versatile class of building blocks for
diversification, would be desirable. Although post-borylation of biaryl bromides (Suzuki coupling
products) can afford biarylboronates by additional steps, still this new synthetic strategy allows a
convergent route for a functionalization at a relatively late stage, which could be enormously
beneficial in lead identification in pharmaceutical/medicinal chemistry.[5] Due to the inherent poor
compatibility of palladium catalysts with boronic acid derivatives,[6] the synthesis of biarylboronates
by catalytic coupling represents an important aim in the field of catalytic C-C coupling; however,
practical strategies are still rare. For example, by exploiting the different reactivities of two types of
boronic acid derivatives, Burke[7,8] and Suginome[9,10] splendidly introduced Pd-catalyzed boron-
masking Suzuki-Miyaura couplings (Scheme 1a). Although powerful, these methods require
additional steps for the synthesis of the substrates and need special conditions such as additional
bases to prevent the masked organoborons from hydrolysis. Moreover, bifunctional R2B-Ar1-Ar2-X
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(X = I, Br, OTf etc.) products, which would represent the most interesting building blocks for
modular synthesis, are accessible by long synthetic routes only. A comparison of the key
intermediate Ar-Pd(II)L2X and the isoelectronic Ar-Au(III)L2X species indicates that the latter
potentially is more reactive. We envisioned that due to this feature an aryl activation/ipso
substitution[11] of an Ar-SiR3 moiety should be preponderant over the direct transmetalation with
arylboronic acid derivatives[12] (Scheme 1b). This would enable a simple site-selective arylation of
aryl compounds bearing both silyl and boronic ester derivatives giving rise to privileged
biarylboronate building blocks in an orthogonal manner to common methodologies. In palladium
catalysis substrates carrying both, boron and silicon substituents undergo a transmetalation of the
boronic ester,[13,14] thus needing at least two more steps for the conversion of the resulting arylsilane
products into arylboronates.
Scheme 1. Strategies for the synthesis of biarylboronates.
Generally, gold redox catalysis requires the use of sacrificial external oxidants to complete the
Au(I)/Au(III) catalytic cycle.[15-20] In this context, by using I(III)/F+ as oxidants, seminal works from
Lloyd-Jones’ and Russell’s,[11,21-23] Nevado’s[12,24] and You´s[25] groups have demonstrated that both
Ar-SiR3 and Ar-B(OR)2 (R = H or alkyl) were vital coupling partners for gold-catalyzed oxidative
biaryl syntheses with electronically/or directing group-controlled specific C(Ar)-H bonds.
Alternatively, gold-catalyzed oxidative cross-couplings between electron-rich and electron-poor
arenes have been recently achieved by Larrosa’s group.[26] Despite the significant progress, redox-
neutral gold-catalyzed site-selective arylations of bimetallic R3Si-Ar-B(OR)2 reagents are a
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10.1002/anie.201806427
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fundamentally new challenge. Recently, Glorius and Toste, among others, have contributed an facile
route to reactive Ar-Au(III) intermediates in the presence of Ru-based photosensitizer, not needing
external strong oxidants.,[27-36] In 2016, we developed the first photosensitizer-free, photochemical
generation of Ar-Au(III) species from diazonium salts and gold(I)-complexes, including catalytic
transformations.[37-39] Herein we report a visible-light-induced gold-catalyzed redox-neutral
arylation of mechanistically discriminative B,Si bimetallic coupling reagents (B-Ar-Si) with
diazonium salts (Scheme 1c); mechanistic studies demonstrate that photosensitizer-free conditions
are superior to gold/Ru(bpy)3Cl2 dual catalysis.
Table 1: Selected reaction parameter optimization.[a]
Entry Catalyst
Yield[b]
74%
68%
Nd
3a/4a[c]
>99:1
>99:1
Nd
1
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuNTf2
2[d]
3[e]
4
78%
>99:1
[a] Reaction conditions: metal catalyst (10 mol%), 1a (1 equiv, 0.2 M), Ar-N2BF4 2a (1.2 equiv),
MeCN, blue LEDs, N2, rt, 6-10 hours. [b] 1H NMR yield with 1,3,5-trimethoxybenzene as internal
standard; the isolated yield is given in brackets. [c] ratio determined by 1H NMR. [d] Pivalonitrile
instead of MeCN. [e] Other solvents (MeOH, DMF, DCE, acetone, dioxane) instead of MeCN.
The commercially available B,Si-bimetallic reagent 1a and 4-bromobenzenedizonium salt 2a were
selected for the test reaction to optimize the conditions (Table 1 and also Table S1 in Supporting
Information). When Ph3PAuCl was used as the catalyst under the irradiation of blue LEDs, the
desired bifunctionalized halobiarylboronate product 3a was exclusively formed in acetonitrile and
pivalonitrile (entries 1 and 2). Other solvents (MeOH, DCE, DMF, acetone, dioxane) gave no
product (entry 3). The important role of nitrile-based solvents might be rationalized by the effect of
ligand stabilization avoiding a rapid reductive elimination from the generated Ar-Au(III)-PPh3
intermediate to form [Ar-PPh3]+ salts.[40] The use of cationic Ph3PAuNTf2 can improve the yield to
78% (entry 4). Other transition-metal catalysts examined (Fe, Cu, Ni, Ir) failed to initiate this
transformation and Pd(Ph3P)4 led to 14% yield along with the undesired byproduct 4a at room
temperature (see Supporting Information). Further studies indicated that the use of Pd-based
catalysts leads to the normal Suzuki coupling products for –Bpin and –Bnep bimetallic substrates.
Shortening the reaction time from 10 hours to 5 hours even increased the yield from 78% to 82%
(entry 11). A lower yield was obtained when the catalyst loading was decreased to 5 mol% (entry
12). Control experiments demonstrated that both light and gold catalyst were essential for the
reaction to proceed (entries 13 and 14).
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Scheme 2. Scope with respect to the boronic acid derivative and the diazonium salt.[a]
[a] 200 μmol scale; yield after recrystallization or chromatography. [b] 1000 μmol scale.
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Under the optimized reaction conditions from Table 1 a wide array of arenediazonium salts were
investigated (Scheme 2). With 1.2-2.0 equivalents of arenediazonium salt, both the less reactive –
BMIDA and the reactive –Bpin/–Bnep containing B,Si-bimetallic reagents successfully furnished
the desired biarylboronates 3a-v in 40-86% yields. Especially, the excellent compatibility with
activated –Bpin and –Bnep makes this new technique very attractive for subsequent “classical” Pd-
catalyzed C-C couplings.[2] For BMIDA-type bimetallic reagents, the reactions are clean and the
desired products 3a-i can conveniently be obtained by recrystallization without chromatography.
Diazonium salts bearing electron-rich, as well as electron-poor functional groups in the o-, m-, and
p-positions mostly reacted well. A series of synthetically valuable functional groups (-CF3, -F, -
COOMe, -COMe, -NO2, -CN, -SO2Me etc.) were compatible. In addition, the successful coupling
to bifunctionalyzed halobiarylboronate motifs (-I, -Cl, -Br; 3a, 3c, 3g, 3j, 3k, 3p, 3t) allows post-
modifications by Pd-catalyzed C-C cross coupling. With 4-methoxylbenzenediazonium salts
containing strong electron-donating groups the reactions were sluggish. The identity of the product
3b was unambiguously confirmed by an X-ray single crystal structure analysis.[41]
The synthesis of structurally diverse Ar-CF3 units for agrochemicals and pharmaceuticals has gained
great momentum in the past ten years.[42] Valuable trifluoromethylated arylbonorates were now
obtained by using 4-CF3-substituted aryldiazonium salts, several representative B,Si-bimetallic
reagents were used (Scheme 3). Besides TMS, other trialkylsilyl-substituted arylboronates 1d-e
could also undergo the site-selective arylation. Both electron-withdrawing and electron-donating
groups at the aryl core of the B,Si-bimetallic reagents were efficient (1h-j, 1l-n), and –CHO and –
Br as versatile groups for post modification also were tolerated (3y and 3z). With an alkynyl moiety
(1k), only a low yield of desired coupling product was obtained due a competing 1,2-
difunctionalization of the alkyne.[37] The concise synthesis of congested biarylboronates (1l-n) is
also possible. The heteroaryl-containing B,Si-bimetallic reagent 1p affords polyarene-tethered
pyridin-3-ol 3bg in moderate yield via tandem selective arylation and oxidation.
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10.1002/anie.201806427
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Scheme 3. Scope with regard to B,Si-bimetallic reagents.[a]
[a] 200 μmol scale; yield after chromatography. [b] Successive oxidation of arylboronates with 5
equiv NaOH and 5 equiv H2O2 at 0 oC. [c] 2 equiv diazonium salts were used.
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Scheme 4. Gold-catalyzed selective arylation of bifunctionalized arenesilanes.
Both –I and –OTf are very important moieties for transition metal-catalyzed C-C couplings.
Therefore, it is still a great challenge to keep these relatively weak chemical bonds intact in any
cross-coupling scenario. By exploiting our new method, bifunctionalized haloarylsilane 5a and
pseudohaloarylsilane 5b both furnished the desired products 6a and 6b in satisfactory yields
(Scheme 4), which further complements existing Pd-catalysis.
Scheme 5. Iterative modular synthetic methodology. The final isolated yields are shown and the
average yield for each step is given in brackets.
To illustrate the synthetic possibilities, one-pot iterative C-C and C-X coupling strategies are
presented in Scheme 5. Through the synergism of gold catalysis with another catalysis system, it
was possible to synthesize interesting molecules in a one-pot iterative fashion. For example, the
gold-catalyzed site-selective arylation of B,Si-bimetallic reagent 1f and subsequent oxidation with
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H2O2 afforded trifluoromethylated biphenol 7 in 76% yield. Similarly, arylation and subsequent
amination was achieved in moderate yield (8). The combination of gold catalysis and palladium
catalysis demonstrates the versatility for the rapid construction of complex target molecules for
medicinal chemistry or molecular materials (9-11) from easily available substrates.
Several control experiments were conducted (Scheme 6). We found that both light and gold-catalyst
were crucial for a successful conversion. Without light, even by heating to 60 oC, no product was
formed. To our surprise, the addition of Ru(bpy)3Cl2 as photosensitizer significantly decreased the
yield from 97% to 37%; this is caused by a rapid and unselective decomposition of the
arenediazonium salts (Scheme 6a). The screening with different light sources revealed that UVA or
a fluorescent lamp (FL) instead of blue LEDs slightly lowers the yield (Scheme 6b). However, if
UVC light (λ= 254 nm) was used, only 12% yield were obtained. This may indicate that the direct
photolysis of diazomium salts with UV light to give aryl radical is less likely. It was also found that
the diazonium salt’s counter anion was important, OTf- and Cl- anions instead of BF4 stopped the
-
reaction (Scheme 6c).
BMIDA
SiMe3
1a
1 equiv
conditions
[Eq. 2]
CF3
BMIDA
F3C
N2BF4
3b
2b
1.5 equiv
(a) Control experiemnts
T/ oC
(NMR yield)
3b
Ph3PAuNTf2 Ru(bpy)3Cl2 BLue LEDs
+
+
-
+
+
-
-
-
+
-
+
-
+
+
-
25
25
25
25
60
97%
NR
ND
37%
ND
(b) The effect of light source
(c) The effect of ArN2X counteranion
Light
3b (NMR yield)
4-CF3C6H4N2X
3b (NMR yield)
Blue LEDs
97%
X = BF4
X = Cl
X = OTf
97%
ND
ND
UVC ( = 254 nm)
UVA ( = 365 nm)
FL ( = 420 nm)
12%
77%
87%
Scheme 6. Mechanistic experiments.
The kinetic profiles were examined as well and the initial reaction rates are presented in Figure 1.
The initial reaction rate for the arylation of 1a and 2b is first-order dependent on [1a] and
[Ph3PAuNTf2]. In contrast, the reaction rate is independent on the arenediazonium salts [2b]; a zero
order rate was observed. The first order dependence on gold catalyst suggests that the really reactive
catalytic species should be mononuclear gold complex. Moreover, these kinetic results indicate that
the turnover limiting step of gold-catalyzed Hiyama arylation may be the aryl ipso substitution step.
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1a
2b
Me3Si
BMIDA
F3C
N2BF4
Catalyst
Ph3PAuNTf2
Figure 1. Kinetic studies of the gold-catalyzed selective arylation by 1H NMR.
While Lee and co-workers recently presented the transmetalation of Ph3PAuNTf2 with boronic
acids,[33] no such activation was observed for arylsilane by mixing 1 equiv. of 1a and 1 equiv. of
Ph3PAuNTf2 for 6 hours. This indicates a completely different reactivity pattern of arylsilanes and
arylboronic acids. Monitoring the reaction in Eq. 2 by 31P NMR suggested the generation of [Ph3P+-
Ar]X, which should arise from the reductive elimination from the organogold(III) intermediate
40]
12.[ Stoichiometric treatment of Ph3PAuAr and 1a in the presence of the external oxidant
Selectfluor was carried out at room temperature for 2 hours without irradiation (Scheme 7).
Selectfluor enables the facile oxidation of 11 to Au(III) 12. Pleasingly, the desired products 3b and
14 can be formed in 75% and 19% yields. The better efficiency for the electron-withdrawing aryl
group can be rationalized by the higher reactivity towards an arylsilane ipso substitution of
intermediate 12. This would also explain the low yield for strongly electron-donating
arenediazonium salts in this transformation. At present, it seems that the highly electrophilic
organogold(III)-mediated π-activation and subsequent ipso substitution aryl fragment of 1a to
generate 13 is highly likely.
Scheme 7. Stoichiometric mechanistic study.
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Ar1
B
Ar2
L1AuIX
Ar2-N2BF4
r.e.
X
2Ar
hv
L1 = Ph3P
L2 =Solvent
AuIII
L1
X
L1
18
Ar1
AuIII
Ar2-PPh3
BF4
L2
Ar2 15
B
Observed by
HRMS-ESI
TMSF BF3
2Ar
L1
AuIII
L1
SiMe3
X
2Ar
AuIII
X
B
Ar1
17
F
BF3
BF4
Si
Ar1
Me3
Ar1
B
SiMe3
Aryl fragment/
Ipso substitution
B
1
-activation
16
Scheme 8. Proposed mechanism.
A possible mechanism is shown in Scheme 8. Under the irradiation of blue LEDs, gold-activated
diazonium salts are able to undergo oxidative addition to gold(I) [37-39] or an aryl radical addition
and subsequent single electron transfer to the intermediate gold(II)-complex[30,43] to generate
transient intermediate 15. The resulting highly electrophilic 15 coordinates to 1a to initiate the aryl
ipso substitution assisted by the BF4 anion.[11,44] Finally, reductive elimination from 18 gave the
desired boron-containing product. Alternatively, reductive elimination from intermediate 15 give
rise to byproduct Ar2-PPh3X, which could be detected by HRMS-ESI analysis. In addition, the
reaction needs continuous irradiation, and no conversion was observed without light. Thus, the
arenediazonium salts-triggered radical chain propagation pathway is less likely.[45-47]
In conclusion, a gold-catalyzed selective Hiyama arylation of B,Si-bimetallic reagents was
presented. The resulting biarylboronates are versatile building blocks for the robust assembly of
several small molecules via one-pot iterative orthogonal C-C and C-X couplings. This new protocol
complements the common Pd-catalyzed C-C coupling strategies from the view of chemoselectivity
and functional group compatibility. We anticipate that this redox-neutral arylation technology will
streamline pharmaceutical lead discovery, agrochemical and organic material synthesis.
Acknowledgements
Jin Xie gratefully acknowledges National Natural Science Foundation of China (21702098), “1000-
Youth Talents Plan” as well as the Fundamental Research Funds for the Central Universities (No.
020514380158) and Sina Witzel acknowledge the support by the Landesgraduiertenförderung of
the state of Baden-Württemberg, Germany.
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Angewandte Chemie International Edition
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