Lewis acid-catalyzed isomerization of 2-arylcyclopropane-1,1- dicarboxylates: A new efficient route to 2-styrylmalonates

Article abstract of DOI:10.1002/adsc.201000636

A facile efficient approach to the 2-styrylmalonates via the Lewis acid-catalyzed isomerization of 2-arylcyclopropane-1,1-dicarboxylates has been developed. The efficiency of this method was demonstrated for a representative series of such cyclopropanes.

Full text of DOI:10.1002/adsc.201000636

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DOI: 10.1002/adsc.201000636
Lewis Acid-Catalyzed Isomerization of 2-Arylcyclopropane-1,1-
dicarboxylates: A New Efficient Route to 2-Styrylmalonates
Alexey O. Chagarovskiy,a Olga A. Ivanova,a Eduard R. Rakhmankulov,a
Ekaterina M. Budynina,a,b Igor V. Trushkov,a,b and Mikhail Ya. Melnikova,*
a
Department of Chemistry, Moscow State University, Leninskie Gory 1/3, Moscow, 119991, Russia
Fax : (+7)-495-939-1814; e-mail: melnikov@excite.chem.msu.ru or melnikov46@mail.ru
Laboratory of Chemical Synthesis, Federal Research Center of Pediatric Hematology, Oncology and Immunology,
b
Leninskii av. 117/2, Moscow 105062, Russia
Received: August 13, 2010; Revised: October 5, 2010; Published online: December 7, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000636.
Abstract: A facile efficient approach to the 2-styryl-
malonates via the Lewis acid-catalyzed isomeriza-
tion of 2-arylcyclopropane-1,1-dicarboxylates has
been developed. The efficiency of this method was
demonstrated for a representative series of such cy-
clopropanes. The isomerization proceeds chemo-,
regio- and stereoselectively to afford E-styrylmalo-
nates in good yields.
tivity is often low, since the three-membered ring
opening is accompanied by the interaction of cyclo-
propane atom(s) with this functional group, instead of
the transformation of cyclopropane into propene.[7]
For example, this is the case in the vinylcyclopropane-
cyclopentene rearrangement[8] and its hetero ana-
logues,[9] the Cope-like transformation of 1,2-divinyl-
cyclopropanes into cycloheptadienes,[10] etc. Secondly,
monosubstituted cyclopropanes can form two regio-
ꢀ
Keywords: alkenes; C C activation; isomerization;
ACHTUNGTRENiNGNU someric propenes, both as an E/Z mixture. In princi-
ple, up to twelve isomeric alkenes can be formed
from 1,2-disubstituted cyclopropanes.
Lewis acids; small ring systems
We hypothesized that the problem of regioselectivi-
ty can be solved by using cyclopropanes containing
donor and acceptor substituents at the vicinal carbon
Due to a significant strain energy, cyclopropanes are atoms. The presence of both the donor and the ac-
generally less stable molecules than the corresponding ceptor connected to the neighbouring atoms will se-
isomers without a three-membered ring.[1] This small- lectively activate the C C bond between them. This
ꢀ
cycle peculiarity opens a simple and convenient route then allows the efficient transformation of donor-ac-
to various alkene derivatives via cyclopropane-to-pro- ceptor cyclopropanes into various products via elec-
pene isomerization if the starting cyclopropanes are trophilic, nucleophilic or radical ring opening as well
more easily available than the isomeric propenes. as by cycloaddition processes.[11,12] Among these trans-
However, the thermal isomerization of cyclopropanes formations, there are few examples of donor-acceptor
into propenes is a process which requires a very high cyclopropane-propene isomerization.[13] These pro-
activation energy, for both the parent unsubstituted cesses, involving cyclopropanes containing alkoxy-
cyclopropane[2] and cyclopropanes containing either and alkylsulfanyl groups, did not have a significant
acceptor[3] or donor[4] substituents. An analogous pho- practical usefulness, as the formed propenes were un-
tochemical isomerization is a low-selective process.[5] stable.
Therefore, as the most promising alternative, the cata-
Nowadays, one of the most studied type of the
lytic versions of this transformation can be used donor-acceptor cyclopropane is based on 2-arylcyclo-
which usually proceed under relatively mild reaction propane-1,1-dicarboxylates. Nevertheless, their iso-
conditions.[6]
merization into the corresponding propenes was not
Independently of the method applied, there is a reported yet, possibly, because the selectivity control
problem of chemo-, regio-, and stereoselectivity of was not achieved hitherto: cyclopropane esters are
the cyclopropane-to-propene isomerization. Firstly, readily transformed into g-butyrolactone derivatives
when the cyclopropane incorporates a side chain with and other products instead of propenes. We set out to
reactive functional group(s), the reaction chemoselec- fill this gap, and here we report an efficient and prep-
Adv. Synth. Catal. 2010, 352, 3179 – 3184
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3179

Alexey O. Chagarovskiy et al.
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We screened reaction conditions for Lewis acid-cat-
alyzed transformations of a model cyclopropane 1a.
The results obtained are summarized in Table 1. Cy-
clopropane 1a was found to be inert even under pro-
longed heating without catalysts or in the presence of
weakly or moderately activating Lewis acids, such as
ZnCl2, YbACHTNUGRTNE(NNGU OTf)3, NdAHCNUTRTEGG(NNUN OTf)3, CeCATHGUNTREN(NNGU OTf)3, CuOTf. On
the contrary, the application of stronger Lewis acids,
in particular, TiCl4, SnCl4, AlCl3, BF3·Et2O, TMSOTf,
can lead to three types of products (2a, 3a, 4) which
do not contain the three-membered ring. The first
product is g-phenyl-g-butyrolactone (2a). It was
formed easily under various reaction conditions ex-
amined. The best yields of 2a were achieved when 1a
was treated with BF3·Et2O in chlorobenzene, SnCl4 in
Figure 1. Examples of physiologically active 4-arylbutenic, 4-
arylbutanoic acids and 4-arylbutylamine derivatives.
dichloromethane, and Sn
ACHTUNGRTEN(NUNG OTf)2 in nitromethane (en-
tries 1, 2 and 7). The transformation of cyclopropane
arative procedure for the Lewis acid-catalyzed iso- 1a into the isomeric propene 3a was the most efficient
merization of 2-arylcyclopropane-1,1-dicarboxylates under heating of 1a in chlorobenzene in the presence
into the corresponding 2-styrylmalonates. These com- of TMSOTf (1.2 equiv.) (entry 9). When cyclopropane
pounds have both self-utility, i.e., as monomers for 1a was stirred with TiCl4 (1.2 equiv.), chloride 4 was
polymeric films useful for optical information stor- formed as a single product in high yield (entry 3). In
age,[14] or can be further functionally modified. For ex- this case a molecule of TiCl4 was the source of chlo-
ample, simple transformations of styrylmalonates can ride ion.[18]
be used for synthesis of close structural analogues of
We propose that a single intermediate for products
such medicines as Prozacꢁ, terfenadine, etc. 2–4 is zwitterion A (Scheme 1), which arises due to
(Figure 1).[15] Moreover, styrylmalonates are the coordination of the strong Lewis acid by the ester
useful building blocks for construction of more com- group. Whereupon lactone 2 is formed via the intra-
plex molecules.[16] Thus, the data reported here de- molecular attack of the ester oxygen onto the benzylic
scribe a new type of reaction as well as open up new carbon atom bearing a positive charge (path a) fol-
routes to a range of useful synthetic products which lowed by hydrolysis of the dihydrofuran B. In this
previously were difficult to access.
process the oxygen atom reacts as the nucleophile.
The starting cyclopropanes are readily available Styrylmalonate 3 is formed when the ester oxygen
from the corresponding aromatic aldehydes via the reacts as a base (path b), resulting in dienol C, which
synthetic sequence: Knoevenagel condensation/ tautomerizes into 3. The regioselective formation of
Corey–Chaykovsky reaction.[17]
alkene 3 only, where C=C bond is connected to an
Table 1. Screening of conditions for Lewis acid-induced transformations of cyclopropane 1a.[a]
Entry
Lewis Acid (LA)
mol%
Solvent
T [8C]
t [h]
Yield [%][b]
2a
3a
4
1
2
3
4
5
6
7
8
9
BF3·Et2O
SnCl4
TiCl4
AlCl3
AlCl3
120
120
120
120
120
10/20
10
PhCl
reflux
ꢀ78!r.t.
ꢀ20!r.t.
r.t.
50
reflux
r.t.
3
3
20
2
3
6
40
1
78
81
–
–
–
23
77
71
<5
–
–
–
–
31
56
–
10
67
–
–
76
34
–
–
–
–
–
CH2Cl2
CH2Cl2
PhCl
CH3NO2
C6H6
CH3NO2
CH3NO2
PhCl
Sc
A
Sn
A
TMSOTf
TMSOTf
200
120
reflux
reflux
3
[a]
Concentration of 1a in the corresponding solvent is 0.05–0.1M.
Isolated yield.
[b]
3180
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Adv. Synth. Catal. 2010, 352, 3179 – 3184

Lewis Acid-Catalyzed Isomerization of 2-Arylcyclopropane-1,1-dicarboxylates
Scheme 1. The possible pathways to products 2–4.
aryl substituent, seems to be a result of its higher sta- Table 2. Isomerization of dimethyl 2-arylcyclopropane-1,1-
dicarboxylates 1a–h into E-styrylmalonates 3a–h.
bility in contrast to the isomer with a C=C bond bear-
ing two esters groups. To prove this supposition, we
treated 3a with sodium methoxide in methanol. The
red coloration of reaction mixture shows clearly the
formation of highly conjugated anion of 3a. However,
the quenching of reaction mixture with H2O led to
the starting 3a only. Finally, the intermolecular nucle-
ophilic attack of chloride ion onto the benzylic carbo-
cation (path c) followed by hydrolysis of the formed
adduct gives rise to chloropropane 4.
Therefore, the chemoselectivity of this process can
be tuned by the appropriate choice of a catalyst. We
have succeeded in finding the set of suitable catalytic
conditions for the cyclopropane-to-propene isomeri-
zation. According to the results listed in Table 1,
TMSOTf in low-polar high-boiling solvent is the supe-
rior catalyst for the cyclopropane-to-propene isomeri-
zation.
1, 3
Ar
t [h]
Yield of 3 [%][a]
a
b
c
d
e
f
C6H5
4-BrC6H4
4-FC6H4
4-MeC6H4
2-MeO-5-(O2N)C6H3
a-naphthyl
b-naphthyl
biphenyl-4-yl
1
3
1
1
1
1
1
2
67
80
63
56
59
78
86
73
g
h
[a]
Isolated yield.
In an effort to extend the versatility of this synthet-
ic route towards propenes like 3, we examined a ment were ca. 16 Hz. Lactones 2 were additionally
series of cyclopropanes 1b–h bearing moderately nu- formed in trace amounts in several cases.
cleophilic aryl substituents in the TMSOTf-catalyzed
Cyclopropane 1i bearing a bulky 2,4,6-trimethyl-
transformation under the conditions selected above. phenyl substituent showed a different behavior than
The desired propenes 3b–h were obtained in moder- substrates 1a–h. In the presence of all Lewis acids ap-
ate to good yields (Table 2).
plied (TMSOTf, TiCl4, or SnCl4), alkene 3i was
All the reactions were performed on a ca. 1-mmol formed in a low yield only, whereas the major product
scale. Furthermore, we have successfully carried out of isomerization of 1i was propene 5, wherein the
the isomerization using 3 g of 1a and obtained styrene double bond is conjugated to the CO2R groups
3a in 77% yield. Thus the procedure can be easily (Scheme 2). The predominant formation of 5 can be
scaled up without the loss of efficiency.
reasoned by its higher stability in comparison with
According to the NMR data, the isomerization pro- isomer 3i where the steric repulsion between ortho-
ceeds regio- and stereoselectively. The NMR parame- methyl groups and vinyl hydrogens prevents the effi-
ters clearly reveal the formation of styrylmalonates cient conjugation between the double bond and the
3a–h as E-isomers exclusively: the vicinal proton- aryl group. Oppositely, the conjugation between the
proton coupling constants for the C(Ar)H=CH- frag- C=C bond and the ester groups is approximately the
Adv. Synth. Catal. 2010, 352, 3179 – 3184
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3181

Alexey O. Chagarovskiy et al.
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more polar solvent. Using the SnACTHNUGRTNE(UNG OTf)2/CH3NO2
system, the corresponding propenes 3l and m were
obtained in high yields (Table 3, entries 9 and 10). At
first thought, the isomerization of 1m into styrylmalo-
nate 3m is unexpected since another cyclopropane
with substituents in the both ortho-positions of phenyl
group (1i) yields 2-(2-arylethylidene)malonate 5.
However, in view of the lower steric demands of a
methoxy group in comparison to those of a methyl
Scheme 2. The formation of 2-(2-arylethylidene)malonate 5
via the strong Lewis acid-induced isomerization of 1i.
same in all compounds of type 5, independent of the group, we can presume that the repulsions between
aryl group. the ortho-methoxy groups and vinyl hydrogens in 3m
We found that the same reaction conditions are not do not significantly influence the conjugation between
suitable for the transformation of the cyclopropanes the aryl group and the double bond.
1j–n into the corresponding propenes 3j–n, due to
Cyclopropanes 1j, k, n were also found to be iso-
considerable polymerization of 1j–n, which bear merized into the corresponding propenes 3j, k, n
highly nucleophilic aromatic and heterocyclic sub- under Lewis acid-free vacuum thermolysis (Table 3,
stituents.
entries 5, 8, 11). Earlier, the intermediate formation
Polymerization was also observed even when less of propene 3k was mentioned in a study of the ther-
active Lewis acids, such as SnACTHNUTRGENUN(G OTf)2, in polar solvents mal rearrangement of cyclopropane 1k into naphtha-
(i.e., CH3NO2) were used for isomerization of 1j lene derivatives.[19] However, we found that in case of
(Table 3, entry 2). Fortunately, when CH3NO2 was re- cyclopropanes 1j–n this method of isomerization is
placed by low-polar CH2Cl2 or non-polar C6H6, the preparative only for indolyl-substituted cyclopropane
desired propene 3j was obtained in good yields 1n which was easily transformed into isomeric alkene
(Table 3, entries 3 and 4).
3n under mild conditions (1508C) in good yield. The
Sn(OTf)2 in CH2Cl2 was also efficient for isomeriza- isomerization of cyclopropanes 1j and k can be
ACHTUNGTRENNUNG
tion of cyclopropane 1k into 3k in high yield (Table 3, achieved under more harsh conditions (2408C) only.
entry 6), although other common Lewis acids, namely Their heating for 1 h proceeded with the maximum
BF3·Et2O, Yb
ACHTUNGTRENNUNG(OTf)3, GaCl3, failed to give the target conversion of 25%, whereas prolonged heating initiat-
propene derivative. A lower yield of 3k was achieved ed polymerization of 1j and k.
with the use of MgI2 (Table 3, entry 7).
In conclusion, we have developed a new valuable
The isomerizations of 1l and m, containing aryl methodology for the efficient Lewis acid-catalyzed
groups which are the most electron-enriched ones in isomerization of donor-acceptor cyclopropanes into
the series of studied substrates, appeared to require a the corresponding propenes. We have demonstrated
Table 3. Isomerization of cyclopropanes 1j–n bearing electron-enriched aromatic and heterocyclic substituents into propenes
3j–n.
Entry
1, 3
Ar
R
LA
mol%
Conditions
Yield of 3 [%][a]
Solvent
T [8C]
P [Torr]
t [h]
[b]
[b]
[b]
[b]
[c]
1
2
3
4
5
6
7
8
j
j
j
j
2-Th
2-Th
2-Th
2-Th
Me
Me
Me
Me
Me
Me
Me
Me
Et
TMSOTf
120
10
10
10
–
10
10
–
PhCl
reflux
20
reflux
reflux
220
reflux
50
240
0.5
24
4
3
1
2
3
1
24
4
–
–
[c]
Sn
Sn
Sn
–
N
CH3NO2
CH2Cl2
C6H6
65
75
26
80
68
25
78
88
62
j
2-Th
–
30
[b]
k
k
k
l
m
n
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-Me2NC6H4
2,4,6-(MeO)3C6H2
5-Br-1-Me-Ind
Sn
N
CH2Cl2
CH3NO2
–
CH3NO2
CH3NO2
–
[b]
MgI2
–
30
[b]
9
10
11
Sn
Sn
–
U
10
10
–
20
20
150
[b]
Et
Me
30
4
[a]
Isolated yield.
Atmospheric pressure.
Polymeric products.
[b]
[c]
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Adv. Synth. Catal. 2010, 352, 3179 – 3184

Lewis Acid-Catalyzed Isomerization of 2-Arylcyclopropane-1,1-dicarboxylates
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Acknowledgements
We thank the Russian Foundation of Basic Research (Project
09-03-00244-a) for financial support of this work.
Adv. Synth. Catal. 2010, 352, 3179 – 3184
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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