Organocatalytic Amination of Pyrazolones with Azodicarboxylates: Scope and Limitations

Article abstract of DOI:10.1002/ejoc.202100167

The organocatalytic amination of pyrazol-5-ones with azodicarboxylates (catalyzed by quinine) is reported. This asymmetric process furnishes enantiomerically enriched hydrazine adducts containing quaternary stereocenters in high yields (74–96 %) and enant

Full text of DOI:10.1002/ejoc.202100167

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Organocatalytic Amination of Pyrazolones with
Azodicarboxylates: Scope and Limitations
Bedřich Formánek,[a] Vít Šeferna,[a] Marta Meazza,[b] Ramon Rios,*[b] Mahendra Patil,*[c] and
Jan Veselý*[a]
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The organocatalytic amination of pyrazol-5-ones with azodicar-
boxylates (catalyzed by quinine) is reported. This asymmetric
process furnishes enantiomerically enriched hydrazine adducts
containing quaternary stereocenters in high yields (74–96%)
and enantioselectivities (up to ee 97%). Theoretical calculations
allow us to propose the relations between quinine catalyst and
reactants leading to observed stereochemical outcome and
trends in the effectivity of the reaction.
Introduction
Pyrazolones[1] are nitrogen-containing five-membered hetero-
cyclic compounds with widespread applications as pharmaceut-
ical and agrochemical agents, synthetic scaffolds in combinato-
rial and medicinal chemistry, dyes, and chelating agents.[2,3]
Noteworthy, numerous pyrazolone derivatives were approved
by FDA, such as antipyrine (A) firstly synthesized by Knorr,[4]
aminophenazone (B) with antipyretic and anti-inflammatory
activities or eltrombopag (C) used for the treatment of low
blood platelet counts in adults with idiopathic chronic immune
thrombocytopenia (Figure 1).[5]
Hence, new synthetic pathways for this class of hetero-
cycles, especially in an asymmetric fashion, have been exten-
sively explored. The pyrazolone structural motif, containing
lactam bearing two adjacent nitrogen atoms, features many
reactive sites, which can be utilized to get valuable derivatives.
Interestingly, until 2008, no organocatalytic methods were
developed for the enantioselective synthesis of functionalized
pyrazolones.[6] For this reason, we started a vigorous research
program developing organocatalytic strategies leading to the
enantioselective synthesis of these privileged structures.[7]
The first example of enantioselective organocatalyzed
reactions using pyrazolone derivatives was reported by Zhao in
2009.[8] They described a cupreine-catalyzed domino-Michael/
Thorpe-Ziegler process between pyrazolones and benzylidene
Figure 1. Selected examples of biologically relevant compounds.
malononitriles affording 6-amino-5-cyanodihydropyrano[2,3-c]-
pyrazoles with excellent enantioselectivity. Soon after, in 2010,
we published our first example of an enantioselective method-
ology for the synthesis of pyrazolones,[10,7b] few months before
Yuan and coworkers reported the first enantioselective pyrazo-
lone addition to nitrostyrenes using thiourea catalysts.[9] Later,
inspired by the previous work of Feng using gadolinium
catalysts,[11] we studied the amination of pyrazolones using
cinchona alkaloids as catalysts.[12] The reaction between pyrazo-
lones and azodicarboxylates renders the final products in good
yields and reasonable to excellent enantioselectivities.
In recent years, other groups devoted their efforts to the
development of new asymmetric methodologies using pyrazo-
lones; Enders, Wang, Lattanzi, Lu, Xu, and many others achieved
high levels of stereocontrol and high reactivity.[13]
Since the pioneering works on the α-amination of alde-
hydes reported by List and Jorgensen in 2002,[14] α-amination of
carbonyls with azodicarboxylates has been one of the most
common strategies for the enantioselective CÀ N bond
formation.[15] In this full paper, we describe the scope and
limitations of the amination of pyrazolones using azodicarbox-
ylates as a nitrogen source.
[a] B. Formánek, V. Šeferna, Dr. J. Veselý
Department of Organic Chemistry
Faculty of Science, Charles University
Hlavova 2030, 128 43 Prague 2, Czech Republic
E-mail: jan.vesely@natur.cuni.cz
[b] Dr. M. Meazza, Dr. R. Rios
School of Chemistry
University of Southampton
Highfield Campus, SO17 1BJ Southampton, UK
E-mail: rrt1f11@soton.ac.uk
Results and Discussion
[c] Dr. M. Patil
In our previous short report about organocatalyzed α-amination
of pyrazolones,[12] several 4-aryl and 4-alkyl substituted pyrazo-
lone derivatives were subjected to the reaction with azodicar-
boxylates to afford the corresponding enantiomerically en-
riched adducts. After optimization steps, quinine was identified
School UM-DAE Centre for Excellence in Basic Sciences “Nalanda”
University of Mumbai, Vidyanagari, Opp Nano Sciences Building
400098 Mumbai, India
E-mail: mahendra.patil@cbs.ac.in
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100167
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as a suitable catalyst in the reaction conditions indicated below.
the reactions took place smoothly with excellent levels of
enantioselectivity (ee 88–97%) and good yields (83-86%). In
addition, 4-substituted pyrazolones carrying a heteroaromatic
ring (3ia–3ka) also provide satisfactory results. Slightly higher
yields and optical purity of the adducts were observed in the
case of more electron-rich thiophene-bearing derivatives (3ja–
ka).
Then, we explored the effects of various groups at other
positions located on the pyrazolone ring (Table 2). Interestingly,
when switching the 3-methyl to 3-ethyl group of pyrazolone,
the reaction furnished product 3la in excellent yield and very
high enantioselectivity (ee 93%, entry 2). Hence, a similar
substrate, 4-allyl-3-ethyl pyrazolone, was also tested in the
reaction. Disappointingly, the corresponding adduct 3ma was
obtained only with moderate yield and stereoselectivity. Next,
the introduction of the larger phenyl groups at both 3- and 4-
positions on the pyrazolone ring led to almost complete
disruption of an asymmetric induction of the studied trans-
formation. Nevertheless, product 3na was isolated in high yield
(89%, entry 4). When the same sterically demanding substrate
1n was subjected to the reaction with less bulky diethyl
azodicarboxylate (DEAD), a remarkable increase of stereo-
induction of the process was observed (ee 24% vs. 90%,
entries 4 and 5).
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However, the lack of a deeper study on the effects of the
interactions between the organocatalyst and the reactants does
not allow us to appoint some general trends in relation to the
stereoselective outcome and yield of the reaction.
In the beginning, we set out experiments to examine the
reactivity of various 4-benzyl pyrazolones. The results are
summarized in Table 1. Both electron-withdrawing (EWG) and
electron-donating groups (EDG) on the benzyl substituent were
tolerated affording corresponding adducts 3 in very high yields
and good to excellent enantioselectivities. First, we ran the
reaction with 4-nitrobenzyl derivative to prove the reproduci-
bility of the method. As expected, 4-(4-nitrobenzyl) pyrazolone
afforded the adduct 3ba in 74% yield and good enantioselec-
tivity (ee 72%, entry 2), which is in agreement with the previous
report (yield 72%, ee 70%).[12] When using 4-substituted 4-
benzyl pyrazolones carrying EDG, a similar reaction efficiency
was observed as in the case of unsubstituted 4-benzyl
pyrazolone (3aa). For example, 4-methylbenzyl and 4-meth-
oxybenzyl derivatives produced the corresponding adducts 3ea
and 3ha in slightly higher yields with increased enantioselectiv-
ity (entry 5, 8), respectively. The substituted benzyl group at 3-
position, as exemplified with 3-methylbenzyl pyrazolone,
showed similar efficiency and level of enantioselectivity (ee
85%, entry 6). Conversely, the presence of 3-nitrobenzyl group
led to a large drop in the enantioselectivity of the reaction (ee
58%, entry 3). This observed deterioration of enantiocontrol
could be caused by preferential H-bonding between nitro
group and the organocatalyst. Luckily, changing the organo-
catalyst to cinchonidine rendered 3ba and 3ca with much
better enantioselectivity. Gratifyingly, when 4-benzyl pyrazo-
lones with the substituents attached at 2-position were used,
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After that, we turn our attention to bicyclic pyrazolone
derivatives 1o and 1p. The amination reaction of 1o with DIAD
gave the desired product in nearly quantitative yield 95% with
excellent enantioselectivity (ee 95%, entry 6). However, a similar
substrate carrying the tetraline fragment (1p) showed only
modest reactivity in terms of yield and enantioselectivity (78%,
ee 72%). A significant decrease of enantioselectivity was also
observed when the electron-withdrawing 2,4-dinitrophenyl
group was introduced on amide nitrogen of pyrazolone ring.
Table 1. Screening of various 4-substituted pyrazolone derivatives in
organocatalytic reaction.
Table 2. Screening of variously substituted pyrazolone derivatives in
organocatalytic reaction.
[a] Isolated yield after column chromatography. [b] Ee determined by HPLC
analysis. [c] For values in brackets, cinchonidine was used as a catalyst.
[a] Isolated yield after column chromatography. [b] Ee determined by HPLC
analysis.
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On the other hand, electron-rich substituents, such as OCH3,
using 2nd generation Hoveyda-Grubbs catalyst, and the corre-
sponding product 5ma was obtained as a pure E-isomer in high
yield 77% without change of optical purity (Scheme 1).
An absolute configuration of pyrazolone derivatives 3 was
determined as R by comparing the data with known com-
pounds from the literature.[11,12]
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had a positive impact on both yield and enantiomeric excess
(entry 8 vs. 9). In addition, we tested an effect of sterically
demanding tert-butyl group present on pyrazolone substrate;
unfortunately, the amination process proceeded with low
enantiocontrol.
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The inexpensive starting materials and catalyst used
predetermine this amination procedure for scale-up prepara-
tions. As shown in Scheme 1, the gram scale experiment was
performed successfully, albeit with slight erosion of reaction
efficiency to the experiments reported previously (92%, ee
84%,[12] vs. 72%, ee 74%). Besides the known deprotection
procedure of hydrazine derivatives 3,[12] we tested another
transformation valuable for enhanced structural diversity of the
enantiomerically enriched pyrazolones prepared. 4-Allyl deriva-
tive 3ma was employed in a metathesis reaction with styrene
To understand the origin of the stereoselectivity of this
reaction, we have investigated the stereoselectivity-determining
step of the reaction using the density functional theory (DFT)
methods.[16] The quinine catalyzed amination of pyrazolones
may follow the catalytic cycle shown in Scheme 2. It involves
deprotonation of pyrazolone, the addition of pyrazolone to
azodicarboxylate (CÀ N bond formation step), and proton trans-
fer from the catalyst to the adduct. Since chirality induction or
inversion is not possible in the proton abstraction and transfer
step i.e. in the first and last step of the reaction, the addition of
pyrazolone to azodicarboxylate (CÀ N bond formation step) is
expected to be the stereoselectivity-determining step of the
reaction. Transition states for the addition of pyrazolone to
azodicarboxylate were located at the M062X/6-31G* level of
theory.[17] The optimized geometries of key transition states (TS-
(R) and TS-(S)) are shown in Figure 2. The attack of Re-face of
pyrazolone to azodicarboxylate via TS-(R) leads to the ‘R’
product whereas the addition of Si-face of pyrazolone to
azodicarboxylate via TS-(S) provides the ‘S’ product. Relative to
separated reactants and catalyst, the free energies of transition
states TS-(R) and TS-(S) are 4.7 and 7.8 kcal/mol, respectively.
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Scheme 1. Further transformations of pyrazolone adducts.
Figure 2. Optimized transition states of TS-(R) and TS-(S). Select bond
distances are given in Å and corresponding electron densities at the bond
critical path is given in the parentheses. Atom colors: H=white, C=Silver,
N=blue, O=red.
Scheme 2. Catalytic cycle of the pyrazolone amination.
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The difference in free energies corresponds to enantiomeric
Furthermore, a computational model has shown that the
different degrees of hydrogen bonding interactions offered by
the catalyst in the competing transition states determine the
stereochemical outcome of the reaction.
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excess (ee) of 99% in favor of the R-product, which is in
reasonable agreement with the experimentally observed stereo-
chemical outcome of the reaction.
Having established the stereo-controlling transition states,
we sought to identify the factor that creates the stereo-
differentiation between TS-(R) and TS-(S). It is noticed that both Experimental Section
these transition states (TS-(R) and TS-(S)) are stabilized via
multiple non-covalent interactions. Three major interactions,
which involve hydrogen bonding interactions between (cata-
lyst)NH····N(azodicarboxylate), (catalyst)NH····O(pyrazolone), and
(catalyst)OH····O(azodicarboxylate) at the transition states, are
identified. Besides these interactions, the pyrazolone and
azodicarboxylate scaffolds enjoy additional stabilizing hydrogen
bonding interactions in the TS-(R) (Figure 2).
To corroborate the presence of these hydrogen-bonding
interactions, we have analyzed the topology features of electron
density distribution at transition states using multiwfn
program.[18] The distances of hydrogen bonding interactions
and corresponding electron densities at the BCP are provided in
Figure 2. The electron densities of bond critical points (BCPs) for
hydrogen bonding interactions are found to be in the range of
0.011 to 0.038 au which typically indicates moderate non-
covalent interactions. The strength of hydrogen bonding
General amination procedure of pyrazolones 3
Substituted pyrazol-5-one 1 (0.1 mmol, 1.0 equiv.) and quinine
(0.01 mmol, 0.1 equiv.) were dissolved in toluene (2 mL) and stirred
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°
for 10 min at room temperature. The solution was cooled to À 40 C
then azodicarboxylate 2 (0.2 mmol, 2.0 equiv.) was added. The
°
reaction mixture was stirred at À 40 C until reaching full conversion
(monitored by TLC, 2 d). Then the reaction mixture was directly
loaded on a silica gel column. Column chromatography (hexanes/
EtOAc: 7/1!3/1) furnished corresponding product 3. The ee value
of product 3 was determined by HPLC analysis on a chiral stationary
phase.
Supporting Information (see footnote on the first page of this
article): Spectral data for all prepared compounds with copies of
the 1H NMR, 13C NMR, and HPLC chromatograms are available.
interactions as suggested by electron densities of bond critical Acknowledgements
points for these interactions at the TS-(R) are slightly weaker
compared to the hydrogen bonding interactions found in TS-
(S). However, the network of hydrogen bonding interactions at
the TS-(R) can effectively stabilize the developing charges in
both the substrates (pyrazolone as well as azodicarboxylate)
rather than a couple of strong hydrogen bonding interactions
at the TS-(S). The difference in the stabilizing interactions at the
transition states is also reflected in the catalyst–substrates
Jan Veselý gratefully acknowledges the Czech Science Foundation
(No 20-29336S) for the financial support. Bedřich Formánek thanks
the Charles University Grant Agency (No 393615) for the financial
support. We also thank Dr. Simona Petrželová for the NMR service
provided and Dr. Martin Štícha for his MS analysis.
interaction energies (Eint) of TS-(R) and TS-(S).[19] The Eint Conflict of Interest
between catalyst and substrates at the TS-(R) is found to be
11.5 kcal/mol higher than the TS-(S). Hence, we reasoned that
subtle differences in the hydrogen bonding interactions offered
by the catalyst at the diastereomeric transition states might be
responsible for the energy difference between TS-(R) and TS-
(S).
The authors declare no conflict of interest.
Keywords: Amination
·
Organocatalysis
·
Nitrogen
heterocycles · Synthetic methods
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Conclusion
In summary, we successfully presented the enantioselective
amination of pyrazol-5-ones with azodicarboxylates catalyzed
by Cinchona alkaloids. The reaction showed a wide substrate
scope for variously decorated pyrazol-5-ones. The method
provided optically active compounds containing quaternary
stereocenter with very high yields and enantioselectivities using
readily available materials with no additional requirements.
The computational investigations of the quinine catalyzed
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catalyst interacts with both the substrates via multiple hydro-
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Manuscript received: February 10, 2021
Revised manuscript received: February 15, 2021
Accepted manuscript online: February 16, 2021
Eur. J. Org. Chem. 2021, 2362–2366
www.eurjoc.org
2366
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