One-pot synthesis of 3,4,5-trisubstituted 1,2,4-triazoles via the addition of hydrazides to activated secondary amides

Article abstract of DOI:10.1021/acs.orglett.5b00128

A general approach has been developed for the one-pot synthesis of 3,4,5-trisubstituted 1,2,4-triazoles from secondary amides and hydrazides via triflic anhydride activation followed by microwave-induced cyclodehydration. In addition, the 1,2,4-triazole moiety is shown to be a useful directing group for Ru-catalyzed C-H arylation. Access to 1,2,4-triazolophenanthridine can be achieved from the reaction products using a Pd-catalyzed intramolecular C-H functionalization reaction.

Full text of DOI:10.1021/acs.orglett.5b00128

Letter
pubs.acs.org/OrgLett
One-Pot Synthesis of 3,4,5-Trisubstituted 1,2,4-Triazoles via the
Addition of Hydrazides to Activated Secondary Amides
William S. Bechara, Inna S. Khazhieva,† Elsa Rodriguez, and Andre B. Charette*
́
́ ́
Centre in Green Chemistry and Catalysis, Faculty of Arts and Sciences, Department of Chemistry, Universite de Montreal, P.O. Box
6128, Station Downtown, Montreal, Quebec Canada H3C 3J7
́ ́
S
* Supporting Information
ABSTRACT: A general approach has been developed for the one-pot
synthesis of 3,4,5-trisubstituted 1,2,4-triazoles from secondary amides and
hydrazides via triflic anhydride activation followed by microwave-induced
cyclodehydration. In addition, the 1,2,4-triazole moiety is shown to be a
useful directing group for Ru-catalyzed C−H arylation. Access to 1,2,4-
triazolophenanthridine can be achieved from the reaction products using
a Pd-catalyzed intramolecular C−H functionalization reaction.
he 1,2,4-triazole motif has attracted considerable interest
T
in the fields of medicinal and coordination chemistry as
well as in materials science.1 For example, this five-membered
ring scaffold is found in numerous biologically active
compounds2 and pharmaceuticals, including maraviroc,3
triazolam,4 and sitagliptin.5 Notably, this heterocycle is used
as an amide cis-bond isostere for both peptide mimicry and
drug design, where it can improve the pharmacological
properties of the corresponding lead compound.6 1,2,4-
Triazoles have also been extensively studied as ligands for
mononuclear and oligonuclear metal coordination, exhibiting
interesting physical properties.7 Recent applications illustrate
that the incorporation of the 1,2,4-triazole ligand into
functional heteroleptic Ir(III) complexes affords a sky-blue
emission with attractive quantum yields for potential use in
organic light emitting diodes (OLEDs).8
Owing to its broad spectrum of functions, a number of
synthetic methods have been developed for the synthesis of
substituted 1,2,4-triazoles 3 (Figure 1).1 The most commonly
investigated pathways involve cyclodehydration of N-acylami-
drazone derivatives 2, which can be formed from various
precursors, such as amides 1,1 amidrazones 4,9 N′-acetyl-N,N-
dimethylhydrazonamides 5,10 oxadiazoles 6,11 and N-acylhy-
drazides 7.12 Substituted 1,2,4-triazole derivatives are also
synthesized from dichloroaldazines 8 by treatment with anilines
at 170 °C.13 Unfortunately, most of the methods require
multistep synthetic procedures as well as the use of nonreadily
available starting materials and/or are limited to a methyl
substituent at the C-5 position (R3 = Me).10
Considering the ubiquity of substituted 1,2,4-triazoles in
synthetic and applied chemistry, from both step-economy and
medicinal chemistry perspectives, a process that allows the
direct synthesis of 1,2,4-triazoles from secondary amides is of
high interest. The formation of such a heterocycle via the
addition of hydrazides to activated amide derivatives have been
reported by different research groups (Figure 2). These
methods include the use of chloromethylene amides14 9a,
imidates 9b,15 thioamides 9c,6a,16 thioimidates 9d,17 and
Figure 1. Different synthetic pathways leading to 3,4,5-trisubstituted
1,2,4-triazoles.
imidoylbenzotriazoles 9e.18 However, these approaches suffer
from various limitations, such as the use of excess activating
reagent (which oftentimes must be removed before the
cyclodehydration step), the use of toxic metals, the isolation
of the activated reactant, long reaction times, sensitivity to
steric hindrance of starting materials, and an overall narrow
scope in terms of substitution diversity at the C-3, N-4, and C-5
positions. Therefore, developing general and practical one-flask
procedures to access multiple 3,4,5-trisubstituted 1,2,4-triazole
derivatives from readily available secondary amides is desirable.
To address these previous issues, we decided to pursue an
operationally simple trifluoromethanesulfonic anhydride
(Tf2O) mediated activation strategy toward 3 while employing
Received: January 14, 2015
Published: February 20, 2015
© 2015 American Chemical Society
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DOI: 10.1021/acs.orglett.5b00128
Org. Lett. 2015, 17, 1184−1187

Organic Letters
Letter
Scheme 2. Novel Metal-Catalyzed C−H Functionalization of
3,4,5-Trisubstituted 1,2,4-Triazoles
Figure 2. Synthesis of 3,4,5-trisubstituted 1,2,4-triazoles via activated
amides as intermediates.
Table 1. Optimization for the Activation/Cyclodehydration
readily available secondary amides 1 and hydrazides 10 (Figure
2).
b
solvent (0.3
M)
μW temp
(°C)
time
(h)
yield 2a
a
entry
base
(%)
Various methods based on the chemoselective electrophilic
activation of secondary amides with Tf2O in the presence of 2-
halopyridine and a suitable nucleophile have been disclosed by
our group19 and others.20 These processes allowed the
expedient and efficient synthesis of valuable functional groups,
including aldehydes, imimes, amines, ketone, ketimines, and
imidazopyridines.19 Inspired by these previous achievements,
we envisioned that the versatility and the electrophilicity of
imidoyl triflate 9f would allow it to be a suitable candidate for
the rapid nucleophilic addition of hydrazides at a low
1
2
3
4
5
none
DCE
DCE
DCE
DCE
DCM
110
110
140
140
140
1
1
1
2
2
27
52
68
85
81
2-FPyr
2-FPyr
2-FPyr
2-FPyr
a
Conditions: 1a (1.0 mmol), solvent (0.3 M), 2-FPyr (1.1 equiv),
Tf2O (1.1 equiv) at 0 °C for 10 min, then 10a (1.1 equiv) and μW
heating for the given time. Yields determined by H NMR analysis
using Ph3CH as an internal standard.
b
1
a
Scheme 1. Tf2O-Mediated One-Pot Synthesis of 3,4,5-Trisubstituted 1,2,4-Triazoles
a
b
c
Isolated yields. Activation performed with DCM from −78 to 0 °C. Cyclodehydration was performed for 4 h.
1185
DOI: 10.1021/acs.orglett.5b00128
Org. Lett. 2015, 17, 1184−1187

Organic Letters
Letter
temperature, combined with a sequential cyclodehydration of
the N-acylamidrazone intermediate 2. We first investigated
previously reported conditions19 by performing the activation
of amide 1a at 0 °C in DCE in the presence of 2-fluoropyridine
(2-FPyr) as a base. The in situ addition of hydrazide 10a to the
activated species afforded a reasonable 52% yield for triazole 3a,
following cyclodehydration at 110 °C for 1 h under microwave
(μW) irradiation (Table 1, entry 2). By performing the
cyclodehydration at 140 °C for 2 h, the corresponding triazole
was obtained in 85% yield (entry 4). The reaction could also be
performed successfully when employing DCM as solvent (entry
5).
To expand the scope of the one-pot activation/nucleophilic
addition/cyclodehydration sequence, various amides and
hydrazides were evaluated under our optimized reaction
conditions (Scheme 1). To our delight, the triazole synthesis
is applicable to a wide variety of substitution patterns at the C-
3, N-4, and C-5 positions. Triaryl triazoles 3b−d are obtained
in moderate to good yields, as observed with other methods. N-
Alkyl-substituted triazoles 3e−l bearing different functional
groups are well tolerated in the methodology. The combination
of N-alkyl with other C-aryl/alkyl substituents 3a and 3m−t is
also possible, in which different amides and hydrazides can be
successfully employed. The procedure can also be extended to
the synthesis of trialkyl-substituted triazole 3u in 54% yield.
Even substrates with a steric bias, which are difficult to obtain
by previous methods (such as 3g, 3n, and 3o), are currently
produced in moderate to good yields under our optimized
conditions. Moreover, the described method tolerates the
presence of some heterocycles, such as thiophene-yl (3h, 3i, 3t)
and benzothiophene-yl groups (3l).
Given the plethora of methodologies that exploit nitrogen-
based chelating groups for transition-metal-catalyzed C−H
functionalization,21 we sought to take advantage of the two
contiguous nitrogen atoms at N-1 and N-2 of the 1,2,4-triazole
to selectively perform a Ru-catalyzed C−H arylation at the
ortho position of the C-3 aryl substituent (Scheme 2).22
Nonoptimized conditions using the [RuCl2(p-cymene)]2
complex as the catalyst were applied to triazole 3v in the
presence of bromoacetophenone as the coupling partner. The
monoaryl product 3w was obtained in moderate yield (59%),
demonstrating the efficiency of the 1,2,4-triazole motif as a
directing group for C−H arylation.
To further expand the elaboration of 1,2,4-triazoles toward
the synthesis of more complex heterocycles, a Pd-catalyzed
intramolecular C−H activation reaction was carried out
(Scheme 2).23 Starting from triazoles 3b or 3c, 1,2,4-
triazolophenanthridine 3x was isolated in 45% and 28% yield
correspondingly. Although the phenanthridine scaffold is found
in bioactive natural alkaloids and medicinally relevant
compounds,24 there are only scarce examples of processes
allowing the synthesis of 1,2,4-triazolophenanthridines.25
In summary, we successfully developed a general one-pot
synthesis of 3,4,5-trisubstituted 1,2,4-triazoles from readily
available secondary amides and hydrazides via triflic anhydride
activation followed by microwave-induced cyclodehydration.
The method is effective at relatively short reaction times while
using minimal amounts of activating reagent. The conditions
were applied to the synthesis of a variety of 3,4,5-trisubstituted
1,2,4-triazoles with different alkyl/aryl substitution patterns.
Moreover, trisubstituted triazoles were proved to be useful
handles for both intramolecular Pd-catalyzed and intermolec-
ular Ru-catalyzed C−H functionalization reactions toward the
synthesis of triazolophenanthridine and highly substituted
triazole. Current efforts in our group are directed toward the
expansion of the scope of such C−H functionalization
reactions, and the results will be reported in due course.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures, NMR spectra, and X-ray and
characterization data. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: andre.charette@umontreal.ca.
Present Address
†Department of Organic Synthesis Technology, Ural Federal
University, Yekaterinburg, 620002, Russia.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Science and
Engineering Research Council of Canada (NSERC), the
Canada Foundation for Innovation, the Canada Research
Chair Program, the FRQNT Centre in Green Chemistry and
Catalysis (CGCC) and Universite
grateful to NSERC, FQRNT, and Universite
́
de Montrea
́
l. W.S.B. is
l for
́
de Montrea
́
postgraduate scholarships. I.S.K. is grateful to the Ministry of
Education and Science of the Russian Federation for Russian
Federation President’s scholarship for studying abroad. E.R. is
grateful to Universidad de Valencia for a postgraduate
scholarship for study abroad.
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