Cross-Dehydrogenative N-N Coupling of Aromatic and Aliphatic Methoxyamides with Benzotriazoles

Article abstract of DOI:10.1021/acs.orglett.1c01034

Nitrogen-nitrogen bond containing motifs are ubiquitous in bioactive compounds and organic materials. However, intermolecular hetero-selective N-H/N-H oxidative coupling reactions remain very challenging and largely unexplored. Here, we report an unprecedented, simple and hetero-selective cross-dehydrogenative N-N coupling of amides and benzotriazoles, utilizing only a hypervalent iodine species as the terminal oxidant. The scope and mechanistic investigations are discussed.

Full text of DOI:10.1021/acs.orglett.1c01034

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Letter
Cross-Dehydrogenative N−N Coupling of Aromatic and Aliphatic
Methoxyamides with Benzotriazoles
Pooja Y. Vemuri and Frederic W. Patureau*
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ABSTRACT: Nitrogen−nitrogen bond containing motifs are
ubiquitous in bioactive compounds and organic materials. However,
intermolecular hetero-selective N−H/N−H oxidative coupling
reactions remain very challenging and largely unexplored. Here, we
report an unprecedented, simple and hetero-selective cross-
dehydrogenative N−N coupling of amides and benzotriazoles,
utilizing only a hypervalent iodine species as the terminal oxidant.
The scope and mechanistic investigations are discussed.
itrogen−nitrogen bonds represent an important func-
Scheme 1. N−N Bond Forming CDCs
N
tional group in numerous pharmaceuticals,1 natural
products,2 organic materials3 and dyes4 (Figure 1). However,
Figure 1. Important N−N bond containing compounds.
their construction typically relies on diazotization methods or
the use of electrophilic nitrogen sources (e.g., oximes,
hydrazones, nitriles, azides, and nitroso or other N-function-
alized reagents).5 Although these methods are effective, the
development of direct dehydrogenative N−N bond forming
strategies remains very scarce.
Meanwhile, cross-dehydrogenative coupling (CDC) reac-
tions have become an increasingly popular strategy due to their
step and atom economic nature (Scheme 1).6 Furthermore,
they have been extensively utilized to generate unique C−C,6
C−N,7 C−O,8 C−P,9 C−S,10 and hetero SA−SB bonds11 with
high selectivity, while surpassing the need of preactivated
substrates. Yet, intermolecular cross-dehydrogenative N−N
coupling reactions remain heavily unexplored.
Received: March 25, 2021
Published: May 11, 2021
In the past, dehydrogenative N−N coupling reactions have
been typically utilized for the construction of intramolecular
© 2021 The Authors. Published by
American Chemical Society
https://doi.org/10.1021/acs.orglett.1c01034
Org. Lett. 2021, 23, 3902−3907
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N−N bonds12 or N−N homocoupled products.13 However,
intermolecular dehydrogenative hetero N−N coupling reac-
tions remain elusive. To the best of our knowledge, the only
methods that demonstrate some intermolecular hetero-cross-
dehydrogenative N−N coupling were reported by Stahl and
independently by Jin (2018 and 2019, Scheme 1).14,15 These
methods however are strictly limited to carbazoles and
diarylamines, thereby leaving the N−N dehydrogenative
bond forming toolbox almost empty. Hence, there remains a
great demand for novel dehydrogenative reactions that allow
hetero N−H/N−H coupling of other ubiquitous nitrogen
motifs, besides carbazoles and diarylamines. We present here
the first highly selective and efficient dehydrogenative cross-
coupling of structurally and biologically relevant amides16 and
benzotriazoles,17 utilizing very simple and mild reaction
conditions.
Indeed, in none of these reactions (Table 1, Schemes 2, 3 and
4) were any homo-coupling products ever detected. If they are
Scheme 2. Cross-Dehydrogenative N−N Coupling of
Methoxyamides with Benzotriazoles, Isolated Yields
Our investigation began with N-methoxybenzamide 1a and
benzotriazole 2a as model substrates. Previous literature
conditions14,15 utilizing either Cu/O2 or KI/KIO4 did not
deliver the desired product 3aa (Table 1, entries 1 and 2).
a
Table 1. Reaction Optimization
a
Oxidant Equiv (1a/2a/Ox.) Solvent (Temp, °C) Yield (%)
b
1
2
3
4
5
6
7
8
O2
1:1.5:−
1:1:1.5
1:1:1
1:2:1
1:2:1
1:1:2
1:1.5:2
1:3:2
1:2:1
1:2:1
1:2:1
1:2:1
1:2:1
1:2:1
1:2:1
1:2:1
1:2:1
1:2:1
DMF (100)
HFIP (40)
HFIP (40)
HFIP (40)
HFIP (40)
HFIP (40)
HFIP (40)
HFIP (40)
HFIP (40)
HFIP (40)
HFIP (40)
HFIP (40)
TFE (40)
0
0
c
KIO4
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIFA
IBX
HTIB
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
51
49
62
45
65
65
73
41
0
54
58
54
50
60
58
68
b
c
9
10
11
12
13
14
15
16
17
18
TCE (40)
CH3CN (40)
HFIP (50)
HFIP (60)
d
HFIP (60)
a
b
c
Isolated yields. Reaction with CuBr(DMS).14 Reaction with KI.
d
Reaction for 6 h.
Instead, the starting materials remained completely unreacted,
showing no conversion. Next, hypervalent iodine oxidants were
tested. To our delight, a highly selective N−N cross-coupling
was observed immediately with (diacetoxyiodo)benzene18
(benzamide/triazole/PIDA = 1:1:1, entry 3), affording the
N−N hetero-coupling product in 51% yield. In the latter
reaction, the unreacted benzotriazole starting material was
recovered in 42% yield, indicating that no appreciable side
products (<7%) were formed during the process from the
triazole.
present, it must therefore be in very small amounts. Increasing
the ratio of benzotriazole to 2 equiv (1:2:1, entry 9) resulted in
73% of N−N cross coupled product 3aa (entry 9). Other
hypervalent iodine oxidants either rendered the product in
lower yields (entries 10 and 12) or not at all (entry 11). Out of
a few selected solvents (entries 8, 13−15), hexafluoroisopro-
In contrast, only a trace of benzamide starting material could
be detected, suggesting a competing oxidative decomposition
pathway which is moreover probably not homo-coupling.
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significantly enhancing the scope of the herein described
method.
Scheme 3. Competition Experiments
We then carried out some key mechanistic experiments.
Because it was observed that electron-poor benzotriazoles (e.g.,
dihalides and tetrahalides) resulted in lower conversion to the
desired heterocoupling product, such as for 3ac, we ran a
couple of competition experiments in order to probe the
relative philicity of each coupling partner in this reaction
(Scheme 3). From the latter it can be concluded that both
amides and benzotriazoles convert faster to the desired N−N
coupling product if they are more electron rich. There thus
does not exist any clear electrophile−nucleophile relationship
between the two coupling partners during the rate determining
step(s). Next, the reaction was also attempted by replacing N-
methoxy benzamide with N-methyl benzamide or N-phenyl
benzamide. No conversion occurred however in such cases,
suggesting that the N-methoxy group is essential and plays an
enabling role in this reaction. Furthermore, the fact that only
hypervalent iodine compounds competently operate as
oxidants in this reaction, in contrast to K2S2O8, Ag2O,
chloramine-T, DTBP, or O2, suggests the structural involve-
ment of the oxidant.
In order to elucidate the sequence of events that lead to the
N−N hetero-coupling product, we then performed sequential
addition experiments (Scheme 4). We thus initiated the
reaction while omitting either the benzotriazole (Experiment
A) or alternatively the amide (Experiment B). After 16 h, the
second substrate was added. Remarkably, no N−N hetero-
coupling product could be detected in the first scenario
(Experiment A), while a good yield of product 3ba was
obtained in the second scenario (Experiment B, Scheme 4). It
can therefore be concluded that the amide substrate
irreversibly decays in the presence of PIDA, while the
benzotriazole survives. The oxidative decay byproducts could
not be identified at this stage. This nevertheless suggests a
competing N−H activation scenario, wherein only the
activation of the benzotriazole would be reversible under
reaction conditions, justifying the need for an excess. These
mechanistic elements are summarized in Scheme 5.
Scheme 4. Sequential Addition Experiments
Next, the reaction could be scaled up with only minor
adjustments (reaction temperature from 40 to 60 °C, Scheme
Scheme 5. Proposed Reaction Mechanism
panol (HFIP) provided the best results. With the optimized
conditions in hand, the substrate scope of the reaction was
then investigated. Several N-methoxyamides and benzotria-
zoles were tested, which showed remarkable functional group
tolerance (Scheme 2). For example, alkyl, halide, CF3, cyano,
and carboxyl ester groups on both the amide and triazole gave
their corresponding N−N cross-coupled products in good
yields. When monosubstituted benzotriazoles are engaged,
isomeric mixtures of products are typically obtained due to
similarly reacting and no longer symmetrical N-centers. For
example, benzotriazole-6-carbonitrile led to an encouraging 1
to 2.2 mixture (3ak and 3ak′), whereas 7-methyl-benzotriazole
led to a 1 to 2.5 mixture of regioisomers (3aj and 3aj′).
Unfortunately, none of the regioisomers could be assigned at
this stage due to inconclusive NOESY characterization.
Importantly, however, ubiquitous aliphatic amides were well
accommodated in the reaction (3sa, 3sb, 3ta, 3ua), thus
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6) to afford an encouraging yield of 60%, which illustrates the
synthetic utility of this novel transformation.
AUTHOR INFORMATION
Corresponding Author
■
Frederic W. Patureau − Institute of Organic Chemistry,
RWTH Aachen University, 52074 Aachen, Germany;
orcid.org/0000-0002-4693-7240;
Scheme 6. Scale-up Experiment
Email: Frederic.Patureau@rwth-aachen.de
Author
Pooja Y. Vemuri − Institute of Organic Chemistry, RWTH
Aachen University, 52074 Aachen, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01034
Finally, in order to illustrate some potential future
application of this completely novel O−N−N−N−N func-
tional group, we investigated its directing group ability in a
simple RhCp* catalyzed C−H bond activation coupling
experiment. Thus, under typical Rh catalyzed C−H bond
activation conditions,19 we coupled 3aa to diphenylacetylene
in good yield (product 4, Scheme 7). To our surprise, however,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
ERC project 716136: “2O2ACTIVATION,” is acknowledged
for financial support.
■
REFERENCES
Scheme 7. Directing Group Ability in a C−H Bond
Activation Coupling Reaction
■
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ASSOCIATED CONTENT
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Experimental procedures, characterization and NMR
spectra of new compounds (PDF)
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