Denitrogenative Suzuki and carbonylative Suzuki coupling reactions of benzotriazoles with boronic acids

Article abstract of DOI:10.1039/c7sc00367f

Unprecedented palladium-catalyzed denitrogenative Suzuki and carbonylative Suzuki coupling reactions of benzotriazoles with boronic acids have been realized, which afforded structurally diverse ortho-amino-substituted biaryl and biaryl ketone derivatives. The key to this success is due to the development of a rationally designed strategy to achieve the ring opening of benzotriazoles with a synergistic activating-stabilizing effect, which enables the in situ generation of the corresponding ortho-amino-arenediazonium species. The present work opens up a new avenue to utilize benzotriazoles as synthetic equivalents of ortho-amino-arenediazoniums, which otherwise could not be directly accessed by existing synthetic methods.

Full text of DOI:10.1039/c7sc00367f

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Denitrogenative Suzuki and Carbonylative Suzuki Coupling
Reactions of Benzotriazoles with Boronic Acids†
Yuanhao Wang,a Yunfei Wu,b Yuanhe Lia and Yefeng Tang*ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The unprecedented palladium-catalyzed denitrogenative Suzuki and carbonylative Suzuki coupling reactions of
benzotriazoles with boronic acids have been realized, which afforded structurally diverse ortho-amino-substituted biaryl
and biaryl ketone derivatives. Key to the success relies on a rationally designed strategy to achieve the ring opening of
benzotriazoles with a synergistic activating-stabilizing effect, which enables in situ generation of the corresponding ortho-
amino-arenediazonium species. The present work opens a new avenue to utilize benzotriazoles as synthetic equivalent of
ortho-amino-arenediazoniums, which otherwise could not be directly accessed by existing synthetic methods.
www.rsc.org/
generated through the ring opening of N-aroylbenzotriazole.
Introduction
This seminal discovery shed
1,2,3-Triazoles are among the most important structural
elements in modern chemical, biological and material
sciences.1 As one of their unique chemical properties, they
could undergo ring-chain isomerization to form the
corresponding diazonium or diazo species via Dimorth-type
equilibrium (Scheme 1A).2 Considerable efforts have been
devoted to the development of novel transformations based
on this unique reactivity. As a paradigm, recently significant
advances have been made on the applications of 1-sulfonyl-
1,2,3-triazoles as synthetic equivalents of diazoimines in a
broad range of intriguing reactions (Scheme 1B).3 In contrast,
the ring-opening chemistry of benzotriazoles, a subset of 1,2,3-
triazoles well known for their versatile reactivity,4 has
remained underdeveloped, mainly due to their high stability
and innate reluctance to the ring-opening process.5
Historically, it was reported that benzotriazoles could undergo
ring opening followed by denitrogenative cyclization upon
photolysis6 or pyrolysis,7 however, those transformations
suffer from forcing conditions, narrow substrate scope and
moderate efficiency, and thus have rarely found applications in
organic synthesis.
Scheme 1 Ring-opening chemistry of 1,2,3-triazoles and benzotriazoles.
In 2009, Nakamura and co-workers reported a novel
palladium-catalyzed
denitrogenative
formal
[3+2]
light on the feasibility of implementing the ring opening of
benzotriazoles and transition-metal-catalyzed denitrogenative
transformations in one pot. However, such potential has been
overlooked by synthetic community over the past several
years, probably because that both the demanding reaction
conditions and moderate efficiency of the reaction make it less
cycloaddition of N-aroylbenzotriazoles with internal alkynes
(Scheme 1C).8 It was assumed that the reaction proceeded via
an ortho-amino-arenediazonium intermediate in situ
a.School of Pharmaceutical Sciences, Tsinghua University, Beijing 100084, China. E-
mail: yefengtang@tsinghua.edu.cn
attractive from
a practical point of view. Thus, the
b.Collaborative Innovation Center for Biotherapy, State Key Laboratory of
Biotherapy and Cancer Center, West China Medical School, Sichuan University,
Chengdu 610041, China.
development of a more general, efficient and robust method
to achieve the ring-opening chemistry of benzotriazoles
remains an unmet challenge.
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Our laboratory has been working on the development of result, the ring-opening/ring-closing equilibrium shifted to the
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novel reactions based on the ring-opening chemistry of 1,2,3- desired direction. More convincing eviDdOeIn: c10e.10w3a9/sC7oSbCt0a0i3n6e7dF
triazoles.9 Given that such type of chemistry remained from the extensive spectroscopic study performed by us. As
underdeveloped for benzotriazoles, we initiated a program to shown in Scheme 2C, the chemical shifts of 1c (toluene-d8, 25
confront this challenge. Notably, when our manuscript was in oC) notably move to downfield in the presence of equimolar
preparation, Glorius disclosed an elegant work on the subject, AgBF4, implying that there exists a strong coordination effect
wherein the first visible-light-promoted denitrogenative between AgBF4
functionalization of benzotriazoles via aryl radical
intermediates was realized.10 We herein report a different
strategy to achieve the ring opening of benzotriazoles with a
synergistic activating-stabilizing effect, which enables in situ
generation of an ortho-amino-arenediazonium species. As
proof-of-concept
cases,
the
palladium-catalyzed
denitrogenative Suzuki and carbonylative Suzuki coupling
reactions of benzotriazoles with boronic acids have been
developed, giving rise to diverse ortho-amino-substituted
biaryl and biaryl ketone derivatives. The present work opens a
new avenue to utilize benzotriazoles as synthetic equivalent of
ortho-amino-arenediazoniums, which otherwise could not be
accessed by existing synthetic methods.11
B. Computational study
Results and discussion
At the outset of our study, we were cognizant of two
challenges associated with the project: 1) how to effect the
ring opening of benzotriazoles under mild conditions; 2) how
to combine the ring-opening process with other synthetically
useful transformations? In terms of the first question, it has
been known that an electron-withdrawing N1-substituent
could facilitate the ring opening of benzotriazoles.12 For
examples, Ziegler reported that 1-nonafluorobutanesulfonyl-
benzotriazole12c and l-nitrobenzotriazole12f could undergo ring
opening upon treatment with strong nucleophiles such as
amines and deprotonated phenols. Nakamura8 and Glorius10
employed 1-aroylbenzotriazoles as effective substrates in their
studies. To get deeper insight into the role of the N1-
substituent, we conducted a computational study with three
C. 1H NMR evidence
representative benzotriazoles (1a-c) as model substrates
(Scheme 2B). It revealed that the electron-withdrawing N1-
substituent exerts a beneficial effect on the ring opening of
benzotriazoles by lowering both the activation free energy
(∆G≠) and Gibbs free energy difference (∆G). Not surprisingly,
such activating effect positively correlates to the electron-
withdrawing capability of N1-substituents (Tf > Ts > Bz).
Nevertheless, in all cases, the ring opening of benzotriazoles is
a thermodynamically unfavorable process, and thus the
resulting zwitterionic diazoniums readily return to their ring-
closing forms. To overcome this problem, we planned to use
an additive such as AgBF4 to further promote the ring-opening
process of benzotriazole by 1) activating the N1-N2 bond
through formation of a complex of benzotriazole-AgBF4 (I-1),
and 2) stabilizing the ring-opening product through formation
Scheme 2 A) Working hypothesis; B) Computational study; C) NMR evidence.
and 1c
.
Moreover, the variable temperature 1H NMR
experiments showed that a new group of broad signals
appeared when the 1H NMR was recorded at -50 oC, which
might be correlated to the putative ring-opening species I-2c
.
Of note, similar phenomenon was also observed in the variable
temperature 19F NMR experiments (for details, see Supporting
Information).
As to the second question, we envisaged that since
arenediazonium tetrafluoroborates have proved to be
versatile precursors in organic synthesis,13 it was feasible to
combine the ring-opening chemistry of benzotriazoles with
various other synthetically useful transformations, such as the
palladium-catalyzed denitrogenative Suzuki coupling reaction.
of an arenediazonium tetrafluoroborate species
(I-2).
Encouragingly, the synergistic activating-stabilizing effect of
AgBF4 was supported by the calculation result with 1c as
model substrate. Indeed, both ∆G≠ and ∆G of the ring-opening
process notably decreased in the presence of AgBF4. As a
2 | J. Name., 2012, 00, 1-3
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Indeed, Bohle et al. proved that the ring-opening forms of was obtained when the reaction was performed with reduced
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DOI: 10.1039/C7SC00367F
benzotriazoles could be trapped by coordination to a suitable equivalents of or in the absence of AgBF4 (entries 14-15).
organometallic complex.14 In the current scenario, the in situ
Having secured the optimal conditions, we next investigated
the substrate scope of the reaction. First, a variety of
formed ortho-amino-arenediazonium tetrafluoroborate (I-2
)
would undergo oxidative addition with Pd(0) to give an substituted benzotriazoles were evaluated with 2a as reaction
organopalladium complex (I-3), which, after transmetalation partner. Pleasingly, all of the examined benzotriazoles bearing
with boronic acid (
to the final product (
2
) and reductive elimination, could advance either electron-donating or -withdrawing substituents were
) (Scheme 2A). proved to be suitable substrates by affording the
corresponding products (3b 3i, Table 2) in good to excellent
3
-
Table 1 Condition optimizationa,b
yields. Generally, the 5- and 6-substituted benzotriazoles gave
a slightly higher yields than the 4-substituted one (e.g. 3b and
3c vs 3i). Moreover, the naphtha[2,3-d]-1,2,3-triazole was also
amenable to the reaction, indicating that it could be extended
to other aromatic ring-fused 1,2,3-triazole derivatives. Next,
the scope of boronic acids was evaluated with 1c as reaction
partner. As shown, an array of substituted phenylboronic acids
worked well to give the corresponding products in good
efficiency, regardless of the steric or electronic bias imposed
by the substrates. Of note, the functional groups of ester,
amide and cyano remained unchanged during the reactions,
demonstrating its good functionality tolerance. Some other
aryl boronic acids were also amenable to the reactions, as
witnessed by the cases leading to 3w and 3x. Not surprisingly,
the transformation could be applied to the synthesis of biaryl
entry
triazole
solvent
additive
T (oC)
Yield of 3
1
2
3
4
5
6
7
8
1a
1b
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
toluene
toluene
toluene
toluene
toluene
toluene
CH3CN
AgBF4
AgBF4
AgBF4
LiBF4
AgSbF6
AgOTf
AgBF4
100
100
100
100
100
100
100
100
100
100
100
100
80
n.r.
n.r.
58%
n.r.
18%
27%
17%
25%
n.r.
92%
39%
68%
94%
22%
13%
1,4-dioxane AgBF4
DMF
9
AgBF4
10
11
12c
13
14d
15
toluene
toluene
toluene
toluene
toluene
toluene
AgBF4/PPh3
AgBF4/dppf
AgBF4/PPh3
AgBF4/PPh3
AgBF4/PPh3
PPh3
80
80
Table 2 Substrate scope of Suzuki reactiona,b
aReaction conditions: 1a (0.30 mmol), 2a (0.45 mmol), Pd(OAc)2 (0.015 mmol),
PPh3 (0.09 mmol) and AgBF4 (0.75 mmol) in the solvent (3.0 mL). b Isolated yield.
cPd(PPh3)4 (5 mol%) was used. 1.0 equiv. of AgBF4 (0.30 mmol) was used. n.r. =
d
no reaction. Bz = benzoyl, Ts = p-toluenesulfonyl, Tf = trifluoromethanesulfonyl,
dppf = 1,1’-bis(diphenylphosphino)ferrocene.
We
commenced
the
study
by
treating
1-
benzoylbenzotriazole (1a) and phenylboronic acid (2a) with
AgBF4 (2.5 equiv.) and Pd(OAc)2 (0.05 equiv.) in toluene at 100
oC for 12 h. To our disappointment, no reaction occurred
under the conditions (entry 1, Table 1). We further evaluated
benzotriazoles 1b and 1c in the reaction. Gratifyingly, while 1b
also failed to give promising result (entry 2), 1c displayed
superior reactivity by affording the desired product 3a in 58%
isolated yield (entry 3). Encouraged by this outcome, we
moved to optimize the reaction with 1c as substrate. First of
all, several other additives including LiBF4, AgSbF6 and AgOTf
were employed in the reactions, however, none of them gave
improved results than AgBF4 (entries 4-6), which indicated that
both of the counter ions of AgBF4 played a crucial role for
promoting the reaction. Next, a simple evaluation of the
solvent effect was conducted, however, it failed to give
promising outcomes (entries 7-9). Gratifyingly, we found that
the usage of PPh3 as additive could notably improve the
reaction by affording 3a in an excellent yield (entry 10).
Comparably, the other commonly used phosphine ligand dppf
afforded only moderate yield (entry 11). Interestingly,
although Pd(PPh3)4 was proved to be effective catalyst for the
transformation, it only gave a decreased yield (entry 12). Of
o
note, lowering the reaction temperature to 80 C had no side-
effect on the reaction (entry 6). However, a poor yield of 3a
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a Condition A: 1 (0.30 mmol), 2 (0.45 mmol), Pd(OAc)2 (0.015 mmol), PPh3 (0.09
mmol) and AgBF4 (0.75 mmol) in toluene (3.0 mL). b Isolated yield.
drugs. For examples, the Suzuki coupling product 3y could undergo
DOI: 10.1039/C7SC00367F
deprotection (Red-Al, toluene, reflux, 82%) followed by an
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intramolecular Ir-catalyzed annulation15 to give the carbazole
alkaloid glycozoline,16 which exhibits antibiotic and antifungal
properties (eq. 1, Scheme 3). Similarly, 3q could undergo sequential
deprotection and condensation with 2-chloronicotinoyl chloride to
provide boscalid,17 a fungicide marketed by BASF company (eq. 2).
For carbonylative Suzuki coupling product 4l, it could be converted
to 7 in three steps (methylation, SmI2 mediated reductive
deprotection and Dess-Martin oxidation), which then underwent a
Cu-catalyzed intramolecular C-H bond activation/C-N bond
formation18 to give the alkaloid 2,3-methylenedioxy-10-methyl-9-
acridanone (eq. 3).19 The same protocol was also employed to
convert 4d to 8, which, after condensation with glycine methyl
ester, gave diazepam, a well-known drug for treating anxiety and
epilepsy (eq. 4).20
derivatives containing two substituted aromatic rings (e.g. 3y-ab).
Last but not least, vinylboronic acids also proved to be effective
substrates for the reactions (3ac-af), thus expending its synthetic
application to ortho-amino-substituted styrene derivatives.
To further showcase of the synthetic utility of the ring-opening
chemistry, we successfully developed an intriguing carbonylative
Suzuki coupling reaction, which provided a new method to access
ortho-amino-substituted biaryl ketone derivatives. As shown in
Table 3, a slightly different catalytic system (AgBF4, Pd(PPh3)2Cl2,
toluene, CO, 80 oC) was employed in the carbonylative Suzuki
coupling reactions (Table 3). Generally, the transformations
exhibited fair substrate scopes with regard to both benzotriazoles
and aryl boronic acid reaction partners. Most of the reactions
proceeded smoothly to furnish the corresponding products in good
to excellent yields, regardless of the electronic properties and
substituent patterns of the substrates. However, different from the
above-mentioned Suzuki coupling reactions, the vinyl boronic acids
failed to give the desired products (e.g. 4s) under the examined
conditions.
Table 3 Scope of carbonylative Suzuki coupling reactiona,b
Scheme 3 Applications in natural product and drug synthesis.
Conclusions
In summary, we developed a new strategy to achieve the ring
opening of benzotriazole with
a synergistic activating-
stabilizing effect, which enables the facile generation of a
versatile ortho-amino-arenediazonium species. As proof-of-
concept cases, palladium-catalyzed denitrogenative Suzuki and
carbonylative Suzuki coupling reactions of benzotriazoles with
boronic acids have been achieved. The great synthetic
potential of the developed chemistry was demonstrated by its
application in the synthesis of bioactive natural products and
drugs. We anticipated that the novel concept presented in this
work may inspire the development of more mechanistically
interesting and synthetically useful transformations. Related
studies on this subject are currently underway in our
laboratory and the progress will be communicated in due
course.
a Condition A: 1 (0.30 mmol), 2 (0.45 mmol), Pd(PPh3)2Cl2 (0.015 mmol), PPh3
(0.09 mmol) and AgBF4 (0.75 mmol) in toluene (3.0 mL) with a CO balloon (1 atm).
b Isolated yield.
To exemplify the power of the above reactions, we converted
some of the resulting products into bioactive natural products and
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(d) S. J. Barker and R. C. Storr, J. Chem. Soc., Perkin Trans. 1,
Acknowledgements
We gratefully acknowledge the financial supports from the
National Science Foundation of China (21572112) and Beijing
Natural Science Foundation (2172026).
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