Rhodium-Catalyzed Denitrogenative Diazole-Triazole Coupling toward Aza-Bridged Structures and Imidazole-Based Chelating Ligands

Article abstract of DOI:10.1021/acs.orglett.1c01092

1,4,8-Triazaocta-1,3,5,7-tetraenes, generated in situ by Rh2(Piv)4-catalyzed denitrogenative coupling of pyrazoles with 1-sulfonyl-1,2,3-triazoles, smoothly form 2,6,8-triazabicyclo[3.2.1]octa-3,6-dienes via intramolecular aza-Diels-Alder cycloaddition. T

Full text of DOI:10.1021/acs.orglett.1c01092

pubs.acs.org/OrgLett
Letter
Rhodium-Catalyzed Denitrogenative Diazole−Triazole Coupling
toward Aza-Bridged Structures and Imidazole-Based Chelating
Ligands
Julia O. Strelnikova, Alexander N. Koronatov, Nikolai V. Rostovskii, Alexander F. Khlebnikov,
Olesya V. Khoroshilova, Mariya A. Kryukova, and Mikhail S. Novikov*
Cite This: Org. Lett. 2021, 23, 4173−4178
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ABSTRACT: 1,4,8-Triazaocta-1,3,5,7-tetraenes, generated in situ by
Rh2(Piv)4-catalyzed denitrogenative coupling of pyrazoles with 1-
sulfonyl-1,2,3-triazoles, smoothly form 2,6,8-triazabicyclo[3.2.1]octa-
3,6-dienes via intramolecular aza-Diels−Alder cycloaddition. This
domino reaction, combined with the subsequent thermal or acid-
catalyzed rearrangement of the cycloadducts, provides direct and flexible
access to N-sulfonylated (Z)-2-(2-aminovinyl)imidazoles.
zabuta-1,3-dienes are valuable intermediates in the
synthesis of various heterocycles, functionalized carbo-
Scheme 1. IMADA Reactions of 1-Azabuta-1,3-dienes
A
cycles, and nitrogen-containing chiral scaffolds.1 The unique
feature of azapolyene building blocks is in their extremely wide
reactivity, including various pericyclic and pseudopericyclic
transformations as well as ionic addition reactions. These
reactions underlie a great deal of highly atom-economical and
regio- and stereoselective syntheses of four-,2 five-,3 six-,4
seven-,5 eight-,6 nine-,7 and ten-membered8 nitrogen hetero-
cycles. This methodology can also be applied to polycyclic
systems, but their scope is restricted exclusively to ortho-fused
heterocycles. The intramolecular aza-Diels−Alder (IMADA)
reaction of 1-azabuta-1,3-dienes bearing tethered dienophile
units is one effective approach to these compounds.9 It allows
the formation of two rings in a highly stereoselective fashion in
one synthetic operation. In particular, this approach was used
for the preparation of several pyrido- and pyrimido-fused
systems by intramolecular trapping of 1-azabuta-1,3-dienes
with a carbon−carbon π bond or cyano group, respectively
(Scheme 1a).10−12 The dienophile can be attached to the N1
or C4 atom of the 1-azabutadiene by a three- or four-atom
linker, at least one atom of which is sp3-hybridized.
To the best of our knowledge, there are no examples in the
literature of the intramolecular trapping of azabutadienes with
a dienophile tethered with a two-atom linker. In addition, there
are still no answers to the following questions: (a) whether the
IMADA reaction can lead to the formation of bridged products
and (b) whether a fully conjugated azapolyene is capable of
undergoing the IMADA reaction.
Received: April 1, 2021
Published: May 17, 2021
We recently found that rhodium α-carbonyl carbenes
derived from α-diazo esters were able to cleave the pyrazole
ring to form unstable 1,5-diazahexa-1,3,5-trienes, which
smoothly cyclized to give 1,2-dihydropyrimidine derivatives.13
https://doi.org/10.1021/acs.orglett.1c01092
© 2021 American Chemical Society
Org. Lett. 2021, 23, 4173−4178
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This result motivated us to examine the reactivity of rhodium
azavinylcarbenes14 toward pyrazoles in the hope of finding a
route to hitherto unknown 1,4,8-triazaocta-1,3,5,7-tetraenes A
(Scheme 1b). It was expected that the additional CN bond
in these compounds could dramatically change the mode of
heterocyclization and, in particular, makes possible the
formation of bridged structures with a high degree of
unsaturation.
Herein we describe an unprecedented Rh(II)-catalyzed
denitrogenative coupling reaction of 1,2,3-triazoles with
pyrazoles that proceeds via IMADA stepwise cycloaddition of
the two-atom-linked azadiene−azadienophile intermediates
1,4,8-triazaocta-1,3,5,7-tetraenes A (Scheme 1b). The reaction
was applied for the preparation of bridged compounds of the
2,6,8-triazabicyclo[3.2.1]octa-3,6-diene series, which turned
out to be excellent precursors of new (aminovinyl)imidazoles.
The mechanistic insight into the reaction has been revealed by
DFT calculations.
We initiated our studies with the test reaction of pyrazole 1a
with 1,2,3-triazole 2a, used as a source of the rhodium
azavinylcarbene, in the presence of Rh2(Piv)4 (1 mol %) in
toluene at 110 °C. To our delight, the reaction proceeded
smoothly to afford 2,6,8-triazabicyclo[3.2.1]octa-3,6-diene 3a
in 97% yield (Scheme 2). Rh2(OAc)4 also catalyzed the
reaction, but the yield of 3a did not exceed 90% even at a
catalyst loading of 5 mol %. Bicycle 3a is a formal adduct of the
intramolecular aza-Diels−Alder reaction between the 1-
azabuta-1,3-diene and electron-deficient aldimine azadieno-
phile.
Since it was the first example of the IMADA reaction of a
fully conjugated azapolyene, we further studied the scope of
the reaction using a wide range of pyrazoles 1 and 1,2,3-
triazoles 2 (Scheme 2). Varying the Ar and R1-sulfonyl
substituents in the 1,2,3-triazole ring had no noticeable effect
on the yield of products 3b−j which was nearly quantitative in
all cases. Pyrazoles 1c−p bearing both electron-donating and
electron-withdrawing aryl substituents at the N1 and C4
positions smoothly reacted with 2a to afford triazabicycloocta-
dienes 3k−x in high yields.
Remarkably, pyrazole 1p containing an isoxazolyl substitu-
ent, which can react with rhodium azavinylcarbenes,15
underwent the transformation of the pyrazole ring exclusively
to afford triazabicyclooctadiene 3x in 92% yield. Benzo-fused
pyrazole substrates, N-aryl- and N-methyl-1H-indazoles 1q and
1r, also proved to be suitable in this protocol and gave bridged
fused-ring compounds 3y and 3z in high yields. The reaction
of 4,5-disubstituted pyrazole 1s with 2a gave adduct 3za in
good yield as well. These examples provide a good illustration
of the low sensitivity of the product yield to the nature of the
C4 and C5 substituents in the pyrazole.
Further exploration of the scope of the reaction using N-
alkylpyrazoles revealed rather unexpected reactivity of
triazabicyclooctadienes 3. The reaction of 2a with 1t (1 mol
% Rh2(Piv)4, toluene, 110 °C, 30 min) gave an inseparable
mixture of the corresponding adduct 3 and (Z)-2-(2-
aminovinyl)imidazole 4a in comparable amounts (Scheme
3). However, further heating for 3 h resulted in the formation
of imidazole 4a (57%) as the only product. 2-(2-Aminovinyl)-
imidazoles are scarcely known, likely because of the lack of
reliable and convenient methods for their synthesis.16 This fact
prompted us to study this reaction in more detail. It was found
that an increase in the catalyst loading to 5 mol % led to an
increase in the yield of imidazole 4a up to 87%.
a
Scheme 2. Scope of Triazabicyclooctadienes 3
The scope of the imidazole synthesis was then investigated
using a wide range of N-alkylpyrazoles (Scheme 3). The
reaction smoothly proceeded with pyrazoles bearing various
aryls, 3-indolyl, and benzoyl at C4 (imidazoles 4a−i). It is
gratifying that the reaction is almost insensitive to the nature of
the N1-alkyl substituent in the pyrazole (imidazoles 4j−n, 4q).
The synthetic applicability of this method was demonstrated
by the gram-scale synthesis of imidazole 4l (93%, 1.41 g).
Next, we returned to N-arylpyrazoles as attractive starting
materials for the preparation of 1-aryl-substituted imidazoles 4.
Initially, N-phenylpyrazole 1a, triazole 2a, and 5 mol %
Rh2(Piv)4 were heated for 5 min to form bicycle 3a (see
Scheme 2). When this reaction mixture was further heated at
110 °C for 16 h, 3a isomerized to imidazole 4o in 89% yield
(Scheme 3). Under the same conditions, N-o-tolylpyrazole 1b
gave imidazole 4p within 14 h in 90% yield. The addition of
silica gel to the reaction mixture significantly increased the
reaction rate and slightly increased the yield of 4p to 96%.
Imidazole 4p can also be obtained directly from bicycle 3h by
heating in toluene at 120 °C for 2 h in the presence of silica gel
(see Table S1 in the Supporting Information).
N-Sulfonyltriazoles are known to react with nitriles under
rhodium catalysis to yield N-sulfonylimidazoles.14e Expectedly,
the Rh2(Piv)4-catalyzed reaction of N-(2-cyanobenzyl)-sub-
stituted pyrazole 1zi with a 3-fold excess of triazole 2a afforded
diimidazole derivative 4x in high yield.
a
Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), Rh2(Piv)4 (1 mol
b
%), toluene (1 mL), 110 °C, 2−5 min, in a sealed tube. The yield of
3a was 90% according to 1H NMR spectroscopy when 5 mol %
c
d
Rh2(OAc)4 was used. PhCl (2 mL), 131 °C. Rh2(Piv)4 (5 mol %),
0.5 h.
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Letter
a
The proposed mechanism for the formation of compounds
3zb and 4r from pyrazole 1t and triazole 2b under Rh2(OAc)4
catalysis is presented in Figure 1. It involves an initial attack of
rhodium azavinylcarbene 5, derived from triazole 2, onto N2 of
pyrazole 1 to give pyrazolium ylide complex 6. The possible
routes for the transformation of complex 6 to 3zb and then to
4r were simulated by the DFT method (B3LYP/6-31+G(d,p)/
Stuttgart RSC 1997 ECP) (see section 9 in the Supporting
Information). The calculations revealed that the formation of
1,4,8-triazaocta-1,3,5,7-tetraene 8 from complex 6 does not
occur directly but rather involves two steps: the formation of
metal-free pyrazolium ylide 7 (TS1) followed by ring opening
(TS2). The IMADA cycloaddition of triazatetraene 8 to
bridged adduct 3zb proceeds in a stepwise fashion featuring
extremely low barriers for both steps (TS4 and TS5).
According to the experimental data, the formation of
imidazoles 4 from adducts 3 is catalyzed by acids. The role
of the acid catalyst (proton was used in the calculations) is to
accelerate the exchange by N-Me and N-Ms moieties in the
largest and smallest bridges of the 2,6,8-triazabicyclo[3.2.1]-
octa-3,6-diene framework (TS6−TS9) (see Figure S7 in the
Supporting Information). The presence of a strong electron-
withdrawing substituent (RSO2) on the nitrogen atom of the
three-atom bridge greatly weakens its bond with the bridge-
head atom. As a result, the transformation of bicyclic isomer
syn-12 into zwitterion 13 has a very low barrier (TS10, ΔG⧧ =
9.0 kcal mol−1). The rotation around the C−C single bond
(TS11) followed by a low-barrier 1,4-prototropic shift (TS12)
affords imidazole 4r. The extremely low value of this barrier
resulted from the formation of the aromatic imidazole ring.
Scheme 3. Scope of Imidazoles 4
a
Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), Rh2(Piv)4 (5 mol
%), toluene (1 mL), 110 °C, 3 h, in a sealed tube. 1.5 h. 16 h. 14 h.
Silica gel, 120 °C, 1.5 h. PhCl (2 mL), 131 °C. 2a (0.6 mmol).
b
c
d
e
f
g
Figure 1. Energy profile (Gibbs free energies in kcal/mol) for the transformation of complex 6 into imidazole 4r.
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According to this mechanism, the pyrazoles with an
electron-withdrawing substituent at N1 should produce
primary bridged adducts like 3zb that can be directly cleaved
across the three-atom bridge, bypassing the catalytic isomer-
ization 3 → 12. Ultimately, this should result in the formation
of 2-(2-aminovinyl)imidazoles with the sulfonyl substituent in
the imidazole ring, not in the side chain. Indeed, the reaction
of 2,4-dinitrophenyl-substituted pyrazoles 1l and 1m with
triazole 2a afforded 1-tosylimidazoles 4y and 4z in good yields
(Scheme 4a). Thereby, the ring−ring tautomerism in bridged
Imidazole 4m was transformed into imidazo[1,2-d][1,4]-
diazepine 16 in high yield via intramolecular nucleophilic
substitution. The aminovinyl substituent of imidazole 4l can be
effectively reduced with hydrogen on Pd/C to afford imidazole
17. Imidazole 4l was used for the preparation of zinc complex
18 in 87% yield. Difluoroboron complexes 19a−c were
synthesized from imidazoles 4l, 4p, and 4t in nearly
quantitative yield.
In conclusion, we have demonstrated the high-yield
synthesis of 2,6,8-triazabicyclo[3.2.1]octa-3,6-dienes by the
Rh2(Piv)4-catalyzed domino reaction of pyrazoles with 1-
sulfonyl-1,2,3-triazoles. The reaction proceeds via stepwise
IMADA cycloaddition of the fully conjugated triazatetraene
intermediates. (Z)-2-(2-Aminovinyl)imidazoles, the products
of thermal or acid-catalyzed isomerization of the cycloadducts,
were used for the preparation of boron and zinc complexes.
Scheme 4. Synthesis of Compounds 4y, 4z, 4za, 4zb, and 15
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01092.
Experimental procedures; characterization data; calcu-
lation details; X-ray data for compounds 3j, 4q, 4u, 4za,
1
and 15; and H and 13C NMR spectra (PDF)
Accession Codes
CCDC 1935704−1935706, 2058052, and 2058053 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44
1223 336033.
compounds 3 makes it possible to manage the location of the
sulfonyl substituent in the final imidazoles 4 through the
electronic effect of the N1 substituent in the pyrazole.
The synthesis of imidazoles 4za and 4zb from pyrazoles 1zj
and 1zk bearing CO2Et or CONMe2 substituents at C5
demonstrated that the last stage of the domino reaction is not
limited to prototropy but can also occur as the 1,4-shift of a
carbonyl-containing group (Scheme 4b).
AUTHOR INFORMATION
■
Corresponding Author
In contrast, the MeO substituent at C5 completely blocks
the formation of imidazoles 4. Thus, the reaction of 5-
methoxypyrazole 1zl with 2a stopped at the formation of
primary cycloadduct 3zc, which was transformed into imidazo-
line 15 during chromatographic purification (Scheme 4c).
To demonstrate the utility of imidazoles 4 for the design of
new heterocyclic systems and metal and boron complexes,
some additional experiments were carried out (Scheme 5).
Mikhail S. Novikov − Institute of Chemistry, St. Petersburg
State University, St. Petersburg 199034, Russia;
orcid.org/0000-0001-5106-4723; Email: m.novikov@
spbu.ru
Authors
Julia O. Strelnikova − Institute of Chemistry, St. Petersburg
State University, St. Petersburg 199034, Russia
Alexander N. Koronatov − Institute of Chemistry, St.
Petersburg State University, St. Petersburg 199034, Russia
Nikolai V. Rostovskii − Institute of Chemistry, St. Petersburg
State University, St. Petersburg 199034, Russia;
orcid.org/0000-0002-8925-794X
Scheme 5. Postsynthetic Transformations of Imidazoles 4
Alexander F. Khlebnikov − Institute of Chemistry, St.
Petersburg State University, St. Petersburg 199034, Russia;
orcid.org/0000-0002-6100-0309
Olesya V. Khoroshilova − Institute of Chemistry, St.
Petersburg State University, St. Petersburg 199034, Russia
Mariya A. Kryukova − Institute of Chemistry, St. Petersburg
State University, St. Petersburg 199034, Russia
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01092
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support of the Russian
Science Foundation (17-13-01078). This research used
resources of the Magnetic Resonance Research Centre,
Chemical Analysis and Materials Research Centre, Computing
Centre, Centre for X-ray Diffraction Studies, and Chemistry
Educational Centre of the Research Park of St. Petersburg
State University.
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