[3+3]-cycloaddition of donor-acceptor cyclopropanes with nitrile imines generated in situ: Access to tetrahydropyridazines

Article abstract of DOI:10.1021/acs.orglett.5b03598

Donor-acceptor cyclopropanes are reacted under the influence of a Lewis acid with hydrazonyl chlorides to afford tetrahydropyridazines. Formally, this transformation can be regarded as a [3 + 3]-cycloaddition of three-membered rings and nitrile imines generated in situ. This efficient method provides fast access to a variety of structurally diverse pyridazine derivatives. The structure of a typical product was confirmed by X-ray crystallography.

Full text of DOI:10.1021/acs.orglett.5b03598

Letter
pubs.acs.org/OrgLett
[3 + 3]-Cycloaddition of Donor−Acceptor Cyclopropanes with Nitrile
Imines Generated in Situ: Access to Tetrahydropyridazines
Lennart K. B. Garve, Martin Petzold, Peter G. Jones,‡ and Daniel B. Werz*
Institut fur Organische Chemie, Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Donor−acceptor cyclopropanes are reacted
under the influence of a Lewis acid with hydrazonyl chlorides
to afford tetrahydropyridazines. Formally, this transformation
can be regarded as a [3 + 3]-cycloaddition of three-membered
rings and nitrile imines generated in situ. This efficient method
provides fast access to a variety of structurally diverse pyridazine derivatives. The structure of a typical product was confirmed by
X-ray crystallography.
onor−acceptor (D−A) cyclopropanes1 have been success-
Scheme 1. (a) Inverse Electron-Demand Hetero-Diels−Alder
Reactions of Diazadienes with Olefins and (b) Our Approach
for the Synthesis of Pyridazine Derivatives
D
fully employed in heterocycle synthesis by using ring-
opening,2 rearrangement,3 and formal cycloaddition reactions.4
Their special reactivity is explained by the high ring strain of
cyclopropanes (about 27.5 kcal/mol) and the vicinal arrange-
ment of donor and acceptor moieties that is able to stabilize a 1,3-
zwitterionic relationship.5
In the past decade convenient syntheses of nitrogen-
containing heterocyclic compounds, which represent versatile
structural motifs in natural products, pharmaceutical, and
material science, have been extensively sought. D−A cyclo-
propanes have proven to be an excellent synthetic tool for the
insertion of two-, three-, or four-atom components leading to
five-, six-, or seven-membered ring systems.
Thus, the one-step synthesis of numerous five-membered
nitrogen-containing heterocycles was achieved by the use of
imines,6 nitriles,7 and diazenes.8 The zinc-catalyzed reaction of
propargyl amines and D−A cyclopropanes led to functionalized
six-membered piperidines.9 Moreover, aromatic azomethine
imines deliver dihydroquinoline derivatives in a [3 + 3]-
cycloaddition reaction.10 Recently, Xu and co-workers published
an inspiring synthesis of functionalized triazinines,11 whereby the
TiCl4-mediated ring-opening of the cyclopropane led to an
attack by azides; the resulting six-membered triazinines were
converted to four-membered azetidines by simple thermolysis.
So far, no cyclopropane-based preparation of pyridazine or
tetrahydropyridazine derivatives, bearing two contiguous nitro-
gen atoms, has been described.12 Pyridazine and tetrahydropyr-
idazine derivatives form the framework for several natural
products and biologically active compounds.13 Thus, tetrahy-
dropyridazine structure motifs are found in pharmaceutically
active compounds such as neurotransmitter or influenza
neuraminidase inhibitors.14 The groups of Lou and of Xiao
developed a [4 + 2]-cycloaddition of in situ-generated 1,2-diaza-
1,3-butadienes 2 with olefins 3 leading to tetrahydropyridazines
(Scheme 1).15 The drawback of the reaction is that it requires a
large excess of the alkene (at least 7.5 equiv). Herein, we report
an efficient synthesis of tetrahydropyridazine derivatives of type
8 by a [3 + 3]-cycloaddition of nitrile imines of type 6, formed in
situ, with cyclopropane 1,1-diesters 7. Nitrile imines are generally
prepared by the treatment of hydrazonyl halides with
stoichiometric amounts of base.16
At the outset of our studies, we chose cyclopropane diester 7a
and hydrazonyl chloride 5a as model substrates to optimize the
conditions for the [3 + 3]-cycloaddition reaction. In the
literature, iPr2NEt is the base of choice to generate nitrile imines
from hydrazonyl chlorides.16 As starting point the reaction was
carried out in CH2Cl2 at 45 °C. Furthermore, a Lewis acid was
added in catalytic amounts to trigger the opening of the three-
membered ring by coordination to the ester groups. Unfortu-
nately, Sc(OTf)3, MgI2, and TiF4 (Table 1, entries 1−3) did not
lead to any conversion of the cyclopropane diester 7a. With AlCl3
an unexpected ring-opened product 9 was found and isolated in
61% yield (entry 4). Surprisingly, no nitrile imine formation with
concomitant liberation of HCl was observed. Instead, the
benzene ring of the hydrazonyl chloride attacked the cyclo-
propane, forming a carbon−carbon bond between the 4-position
of the benzene and the carbon next to the donor of the
cyclopropane.
Received: December 18, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03598
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Organic Letters
Letter
Table 1. Optimization of the [3 + 3]-Cycloaddition Reaction
to Yield Pyridazine Derivatives of Type 8aa
Scheme 2. [3 + 3]-Cycloaddition Reaction with Aryl
Substituted D−A Cyclopropanes
a
a
Lewis acid (20
mol %)
8aa yield
(%)
9 yield
(%)
entry
solvent
base
1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
NEt3
Sc(OTf)3
MgI2
0
0
2
0
0
3
TiF4
0
0
4
AlCl3
TiCl4
TiCl4
TiCl4
TiCl4
TiCl4
TiCl4
TiCl4
TiCl4
0
61
0
5
42
6
0
0
7
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
0
0
8
8
0
9
Cs2CO3
pyrazole
imidazole
imidazole
mixture
10
11
0
41
0
80
86
b
12
0
a
Reaction conditions: 7a (0.15 mmol), 5a (0.10 mmol), base (0.15
mmol), Lewis acid (20 mol %), solvent (2 mL), 45 °C, 16 h; yields
b
represent isolated products. 100 mol % of TiCl4 was used.
The use of TiCl4 as Lewis acid proved to be much more
effective. In the presence of iPr2NEt in CH2Cl2 the desired
tetrahydropyridazine ring was obtained in 42% yield (entry 5). A
change of the solvent to THF or toluene was ineffective (entries 6
and 7). Use of triethylamine instead of iPr2NEt decreased the
yield of the six-membered ring to 8% (entry 8) whereas the
reaction with Cs2CO3 yielded an inseparable mixture of desired
and undesired products (entry 9). Pyrazole delivered the
undesired product 9 in 41% yield (entry 10). The adoption of
imidazole as base brought a significant improvement, affording
the tetrahydropyridazine ring in 80% yield (entry 11). By using
stoichiometric amount of TiCl4 the yield was further increased to
a 86%. Nevertheless the following reactions were performed
catalytically with 20 mol % of TiCl4.
With the optimal conditions in hand, a wide range of different
cyclopropanes were explored for the [3 + 3]-cycloaddition
reaction. Methyl substituents in 3- or 4-position yielded the
corresponding six-membered rings 8ba and 8ca in 76% and 78%
yield, respectively (Scheme 2). Furthermore, electron-with-
drawing groups in 3- and 4-position of the phenyl unit do not
impact unfavorably on the ring-opening reaction. Surprisingly,
the cyano group in 4-position accomplished the highest yield
with 92% (8da). Even nitro-substituted arene rings are tolerated
and afforded the pyridazine congener 8ea in 67% yield. As we
cannot explain the marked difference in yield of 8da and 8ea we
omit any speculations. 4-(Trifluoromethyl)benzene-substituted
cyclopropane yielded the desired product 8fa in 75% yield. Nor
did halogens in 4-position of the benzene ring affect the
reactivity; the respective tetrahydropyridazine derivatives were
successfully obtained in yields between 74% and 79% (8ga−8ia).
Subsequently, the substrate scope with respect to the
hydrazonyl chloride was examined. Initially, the residue bound
to the carbon atom of the hydrazonyl chloride was varied. The
sterically demanding naphthyl substituent was installed at the six-
a
Reaction conditions: 7 (0.15 mmol), 5a (0.10 mmol), imidazole
(0.15 mmol), TiCl4 (20 mol %), CH2Cl2 (2 mL), 45 °C, 16 h; yields
represent isolated products.
membered ring (Scheme 3). Attached at 1-position of naphthyl,
the yield of the product 8ab was slightly worse (48%) than
attached at 2-position (8ac, 59%). In contrast, the smaller benzyl
group led to the desired product 8ad in 71% yield. Moreover, the
electron-donating methoxy group afforded congener 8ae in 72%
yield. However, a substitution by halogens had a serious negative
influence on the reactivity. Both bromine- and fluorine-
substituted congeners gave poor yields as the corresponding
pyridazine derivatives 8af and 8ag were only obtained in 38% and
16% yields, respectively. In contrast, compound 8ah illustrated
that even heterocycles bearing a thiophene ring could be
assembled (54% yield). The [3 + 3]-cycloaddition reaction also
proceeded smoothly with aliphatic groups at the hydrazonyl
chloride. A methyl- and even a cyclopropyl-substituted product
(8ai−8aj) were obtained in good yields of 64% and 66%,
respectively. In contrast, variation at the nitrogen atom of the
nitrile imine precursor proved to be successful only with
electron-donating groups. Methoxy- and methyl-substituted
phenyl rings yielded the tetrahydropyridazines 8ak and 8al in
78% and 80% yield, respectively. However, electron-withdrawing
substituents such as a trifluoromethyl group afforded product
8am in only 11% yield. In the work of Lou and Xiao the
installation of an acyl group at the nitrogen was a prerequisite for
a successful hetero-Diels−Alder reaction. Our attempts to attach
such an electron-withdrawing group at the nitrogen failed since
the acyl group decreases the nucleophilicity of the intermediate
nitrile imine, which is crucial for the nucleophilic attack on the
B
DOI: 10.1021/acs.orglett.5b03598
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Letter
Scheme 3. [3 + 3]-Cycloaddition Reaction with a Variety of
a
Different Hydrazonyl Chlorides
Figure 1. Molecular structure (50% ellipsoid probability) of 8aa in the
solid state. Oxygen atoms are shown in red, nitrogen atoms in blue.
Hydrogen atoms are omitted for clarity.16
Scheme 4. Proposed Reaction Mechanism
In conclusion, we have developed the first [3 + 3]-
cycloaddition reaction of D−A cyclopropanes with nitrile imines.
The nitrile imines were generated in situ from hydrazonyl
chlorides by treatment with imidazole. Various three-membered
rings, activated by a catalytic amount of TiCl4, were converted to
tetrahydropyridazine derivatives. Moreover, substituents on both
sides of the nitrile imines were varied. The transformations
proceeded smoothly, and the desired six-membered rings were
obtained in yields up to 92%. Further investigations to extend
these studies to other nitrogen-containing 1,3-dipoles are
ongoing.
a
Reaction conditions: 7a (0.15 mmol), 5 (0.10 mmol), imidazole
(0.15 mmol), TiCl4 (20 mol %), CH2Cl2 (2 mL), 45 °C, 16 h; yields
represent isolated products. 100 mol % of TiCl4 was used.
b
cyclopropane. Thus, our approach nicely complements the
already existing method regarding possible substitution patterns.
To examine the viability of this methodology on a larger
preparative scale, the [3 + 3]-cycloaddition reaction leading to
8aa′ was also performed on a 5.0 mmol (1.17 g) scale. The
reaction with the cyclopropane dimethyl ester also proceeded
very well and delivered 8aa′ in 82% yield.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03598.
Detailed experimental procedures and analytical data for
all new compounds (PDF)
Crystal data for 8aa (CIF)
To prove the formation of the six-membered ring and its
regioselectivity unambiguously, we were able to grow single
crystals of 8aa. The X-ray crystallographic analysis17,18 confirmed
the anticipated structure, which is depicted in Figure 1.
These observations and literature evidence led us to propose
the following mechanism, shown in Scheme 4.11,19 The strong
Lewis acid (LA) TiCl4, acting as catalyst, first affords the ring-
opened intermediate 10, a 1,3-zwitterionic species. Deprotona-
tion of the hydrazonyl chloride and the concomitant loss of HCl
leads to nitrile imine 6 with a 1,3-dipolar structure. Subsequently,
a nucleophilic attack, originating from the external nitrogen of
the nitrile imine, forms the zwitterionic intermediate 11. Finally,
the second nucleophilic attack results in the tetrahydropyridazine
derivative 8.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: d.werz@tu-braunschweig.de
Present Address
‡Technische Universitat Braunschweig, Institut fur Anorganische
̈
̈
and Analytische Chemie, Hagenring 30, 38106 Braunschweig,
Germany.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.5b03598
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Organic Letters
Letter
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ACKNOWLEDGMENTS
■
This research was supported by the German Research
Foundation (DFG) and by the Fonds der Chemischen Industrie
(Dozentenstipendium to D.B.W.). L.K.B.G. thanks the Studien-
stiftung des deutschen Volkes for a Ph.D. Fellowship.
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