TfOH promoted reactions of vinyl gem-dichlorocyclopropanes with arenes: access to aryl gem-dichloropentenes

Article abstract of DOI:10.1016/j.tetlet.2016.08.008

The reaction of 1,1-dichloro-2-vinylcyclopropane with arenes under the action of the superacid TfOH afforded 3-aryl-1,1-dichloropent-1-enes (25–43% yield), as the products of cyclopropane ring opening. The reaction of 1,1-dichloro-2-methyl-2-vinylcyclopro

Full text of DOI:10.1016/j.tetlet.2016.08.008

Accepted Manuscript
TfOH promoted reactions of vinyl gem-dichlorocyclopropanes with arenes: ac-
cess to aryl gem-dichloropentenes
Anna A. Kazakova, Anna A. Bogomazova, Roman O. Iakovenko, Simon S.
Zlotsky, Aleksander V. Vasilyev
PII:
S0040-4039(16)30995-9
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.08.008
TETL 47979
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
4 July 2016
26 July 2016
3 August 2016
Please cite this article as: Kazakova, A.A., Bogomazova, A.A., Iakovenko, R.O., Zlotsky, S.S., Vasilyev, A.V.,
TfOH promoted reactions of vinyl gem-dichlorocyclopropanes with arenes: access to aryl gem-dichloropentenes,
Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.08.008
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TfOH promoted reactions of vinyl gem-dichlorocyclopropanes with arenes:
access to aryl gem-dichloropentenes
Anna A. Kazakova,a Anna A. Bogomazova,b Roman O. Iakovenko,a Simon S. Zlotsky,c
Aleksander V. Vasilyev*a,d
a
Department of Organic Chemistry, Institute of Chemistry, Saint Petersburg State University,
Universitetskaya nab., 7/9, Saint Petersburg, 199034, Russia
b
Department of Chemistry, Sterlitamak Branch of Bashkir State University, Lenin ave. 29,
Sterlitamak, 453103, Russia.
c Department of Chemistry, Ufa State Oil Technical University, ul. Kosmonavtov 1, Ufa, 450062,
Russia.
d Department of Chemistry, Saint Petersburg State Forest Technical University, Institutsky per., 5,
Saint Petersburg, 194021, Russia
*Corresponding author: A.V. Vasilyev; e-mail: aleksvasil@mail.ru, a.vasilyev@spbu.ru
Keywords: 1,1-dichlorocyclopropanes, 1,1-dichloroalkenes, triflic acid, Friedel-Crafts reaction,
carbocations
Graphical Abstract
Abstract
The reaction of 1,1-dichloro-2-vinylcyclopropane with arenes under the action of the superacid
TfOH afforded 3-aryl-1,1-dichloropent-1-enes (25-43% yield), as the products of cyclopropane ring
opening. The reaction of 1,1-dichloro-2-methyl-2-vinylcyclopropane was realized as a two-step
procedure, including initial in situ generation of the corresponding triflate, 5,5-dichloro-4-
methylpent-4-en-2-yl trifluoromethanesulfonate, from the cyclopropane and TfOH, followed by
reaction with arenes and TfOH to give 1,1-dichloro-2-methyl-4-arylpent-1-enes (30-68% yield).
1

1,3-Butadiene and isoprene are important by-products1 of petrochemical synthesis which are
used for the production of synthetic rubbers, thermoplasts, and other materials.2 These dienes easily
react with dichlorocarbene to form vinyl substituted gem-dichlorocyclopropanes,3 which are
important intermediates in organic synthesis.4 Electrophilic activation of gem-dihalocyclopropanes
represents one of the methods for carbon-carbon bond formation involving these compounds.5,6
Previously, we have shown that the AlCl3-catalyzed reactions of chloromethyl- and 2-bromo-2-
phenyl- gem-dichlorocyclopropanes with arenes led to 1,1-dichloroalkenes.5 Due to the
functionalization of the CCl2-group, these gem-dichloroalkenes participate in the formation of
chloroalkenes, acetylenes,7,8 allenes,9 and heterocycles.10
The main goal of this work was to study the reactions of vinyl substituted gem-
dichlorocyclopropanes with arenes in the presence of Brønsted acids using two vinyl gem-
dichlorocyclopropanes 1a and 1b, obtained from butadiene-1,3 and isoprene, respectively (Fig. 1).
Figure 1. Examined vinyl gem-dichlorocyclopropanes 1a and 1b.
Table 1. Brønsted acid-promoted reaction of 1a with benzene.
Entry
Reaction conditions
Ratio of
Yield 2a (%)
Acid
Temperature (°C) Time (h)
1a : PhH : acid
1 : 3 : 50
1 : 3 : 50
1 : 3 : 1
1
2
3
4
5
6
7
8
TfOH
TfOHa
TfOHa
TfOHa
H2SO4
CF3CO2H
FeCl3
20
-35
-35
-35
20
1
1
oligomers
oligomers
13
1
1 : 3 : 1
3
25
1 : 50 : 5
1 : 50 : 5
1 : 50 : 1
1 : 50 : 1
9
15
20
24
1
no reaction
oligomers
oligomers
20
AlCl3
20
1
aSolvent СН2Cl2.
First, we explored the reactions of cyclopropane 1a with benzene in the presence of various
Brønsted or Lewis acids (Table 1). It was found that the main reaction product, 1,1-dichloro-3-
phenylpent-1-ene (gem-dichloropentene) 2a, was formed as a result of cyclopropane ring opening.
2

Among the acids examined, the best results were obtained using TfOH (1 equiv.) in CH2Cl2 at -35
°С for 3 h, to give 2a in 25% yield (Entry 4). Increasing the reaction time at -35 °С did not lead to a
higher yield of 2a. The use of larger amounts of TfOH gave rise to oligomeric compounds (Entries
1, 2), due to cationic oligomerization of the initially formed 2a. Other Brønsted acids H2SO4,
CF3CO2H (Entries 5, 6) or Lewis acids FeCl3, AlCl3 (Entries 7, 8) did not give satisfactory results
in this reaction.
Having the optimal conditions in hand (1 equiv. TfOH, CH2Cl2, -35 °С, 3 h), we synthesized
gem-dichloropentenes 2b-d from 1a and isomeric xylenes in moderate yields of 33-43% (Scheme
1). Under these conditions, the reactions of 1a with more electron-rich arenes, pseudocumene,
mesitylene, anisole and veratrole yielded oligomers.
Scheme 1. TfOH promoted reactions of 1a with arenes leading to gem-dichloropentenes 2b-d.
We then studied the transformations of the second vinyl gem-dichlorocyclopropane 1b in
TfOH. The reaction of 1b with benzene took 1 h at -35 °С and gave the product of cyclopropane
ring opening 3a (Scheme 2).
Scheme 2. TfOH promoted reaction of 1b with benzene.
The addition of 1b to a solution of more electron-rich arenes (isomeric xylenes,
pseudocumene) and TfOH in CH2Cl2 led to oligomers. We then decided to examine the reaction of
1b using TfOH (1 equiv.) in CH2Cl2 in the absence of the arene. It was found that, under these
conditions at -35 °С, after just 10 min compound 1b was transformed into triflate 4 in 76% yield
(Scheme 3). The reaction of the isolated compound 4 with benzene in CH2Cl2 in the presence of
3

TfOH (1 equiv.) at -35 °С for 1 h furnished 3a (Scheme 3). Compound 4 is unstable and
decomposed at room temperature within a few hours. It should be noted, that the reaction of 4 with
benzene and other arenes (vide infra) did not proceed without TfOH.
Scheme 3. Transformation of 1b in TfOH leading to triflate 4, followed by the reaction with
benzene resulting in 3a.
The obtained data revealed that triflate 4 may be the initial reaction product, which is further
transformed into 3a in the reaction with benzene. To check this hypothesis we carried out reactions
of 1b with various arenes via the in situ generation of intermediate 4 (Scheme 4). Indeed, using this
two-step procedure, initial preparation of triflate 4 followed by addition of the arene and a second
equivalent of TfOH (ESI, and Scheme 4), resulted in the formation of gem-dichloropentenes 3b-n
in 30-57% yield. These compounds were not obtained directly by the addition of 1b to a solution of
arene and TfOH in CH2Cl2, as mentioned above. In the cases of toluene, o-xylene, pseudocumene
and naphthalene, mixtures of regioisomers 3b,c, 3d,e, 3h,i and 3k,l, respectively, were obtained.
The reaction with durene gave rise to two isomers 3m and 3n, with the latter possibly formed as a
result of electrophilic attack onto the Cipso-(Me) carbon of the arene ring, followed by the shift of
methyl groups on the aromatic ring.
Unfortunately, we failed to obtain a similar triflate in the reaction of 1a with TfOH; therefore
utilization of this two-step procedure could not be used to give the reaction products from the
electron-rich arenes, pseudocumene, mesitylene, anisole and veratrole (vide supra).
Concerning the reaction mechanism for 1a,b, there are several literature examples of gem-
dichlorocyclopropane ring opening reactions under the action of Brønsted and Lewis acids.5,11 Most
probably, the reactions start from protonation of the double bond, giving rise to -cyclopropyl-alkyl
cation A (Scheme 5). According to the literature12 the structure of these cation may be represented
as non-classical bicyclobutonium ion B. In general, there is an equilibrium between several -
cyclopropyl-alkyl cations, such as A, C, and ion B. Finally, ring opening of species C leads to
homo-allyl cations D, which may react in two different ways. The first reaction pathway is realized
for 1b (R = Me) and includes a reaction with the triflate-ion, leading to compound 4. Upon being
isolated and dissolved in TfOH, triflate 4 is transformed to cation D. Then the latter reacts with
4

arenes, yielding compounds 3. The second reaction pathway (R = H) for species D is isomerization
into the more stable allyl cation E, which reacts with arenes, resulting in compounds 2.
Scheme 4. Reaction of 1b with arenes leading to gem-dichloropentenes 3b-n.
5

Scheme 5. Plausible mechanism for the TfOH-promoted reaction of 1a,b with arenes.
Of course, this reaction mechanism for 1a,b is debatable. Despite moderate yields of the
target compounds 2 and 3, the investigated reactions raise interesting questions regarding the
detailed mechanisms of these carbocationic transformations and we are currently working on the
reactions of other vinyl substituted gem-dihalocyclopropanes to better understand these reactions.
In conclusion, the reactions of vinyl substituted gem-dichlorocyclopropanes with arenes under
the action of the Brønsted superacid TfOH were studied for the first time. The reactions proceed
with cyclopropane ring opening and lead to aryl substituted gem-dichloropentenes.
Acknowledgements
This work was supported by Russian Foundation for Basic Research (RFBR, grant no. 16-33-
50045) and by Saint Petersburg State University, Saint Petersburg, Russia (grant no.
12.38.195.2014). Spectral studies were performed at the Center for Magnetic Resonance and the
Center for Chemical Analysis and Materials Research of Saint Petersburg State University, Saint
Petersburg, Russia.
Supplementary data
1
This data contains experimental procedures, characterization of compounds, H, 13C, 19F
NMR spectra of compounds.
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7

Highlights
-
-
-
Acid-promoted transformations of 1,1-dichloro-2-vinylcyclopropane
Synthesis of 1,1-dichloropent-1-enes;
Reaction mechanisms.
8