Novel formation of phenylcyclopropanes from the reaction of β-cyanostyrenes and related compounds with 2-methoxyfuran: Experimental and theoretical studies

Article abstract of DOI:10.1039/b814841d

As a diene, 2-methoxyfuran fails to generate a Diels-Alder adduct when it reacts with β-cyanostyrenes (cinnamonitriles). However, in the reaction with β-cyanostyrenes possessing additional electron-withdrawing groups (CN, CO₂Et and SO₂Ph), it yielded two new phenylcyclopropanes. We have used computational methods to investigate the mechanism and to probe the regioselectivity observed in the rearrangement reactions. We used a B3LYP/6-31G* level density functional calculation to locate the transition states (TS), and to account for the selectivity observed in these reactions, we examined the global electronic index involved. An analysis of the results for the reaction pathways of two adducts (the endo and exo isomers) shows that the reaction takes place via a polar stepwise mechanism. The first step involves a side-on nucleophilic attack by the α-carbon atom of the furan ring on the vinyl carbon of the cyanostyrene to give a zwitterionic intermediate. An intramolecular substitution within the intermediate (IN) gives the cyclopropane ring, together with fission of the furan ring. The potential energy barriers for these reactions had the following values: at the nucleophilic addition step, 14.6 and 13.7 kcal mol^-1 for 2b, and 18.4 and 17.8 kcal mol ^-1 for 2c; at the rearrangement step, 22.3 and 22.8 kcal mol ^-1 for 2b, and 25.1 and 23.1 kcal mol^-1 for 2c. Solvent effects in chloroform stabilized the rearrangement steps by 5-6 kcal mol ^-1, but the nucleophilic addition step appeared to be slightly affected. From the theoretical results, in the case of 2b, the energy of TS2-endo is lower than that of TS2-exo, so more trans-cyclopropane product from the endo form is formed under kinetic conditions. Conversely, in the case of 2c, the energy of TS2-exo is lower and more of the cis-product is formed. Density functional theory analysis of these reactions is in complete agreement with the experimental results.

Full text of DOI:10.1039/b814841d

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Novel formation of phenylcyclopropanes from the reaction of
b-cyanostyrenes and related compounds with 2-methoxyfuran:
experimental and theoretical studies
Kuniaki Itoh* and Shigehisa Kishimoto
Received (in Montpellier, France) 26th August 2008, Accepted 5th February 2009
First published as an Advance Article on the web 17th March 2009
DOI: 10.1039/b814841d
As a diene, 2-methoxyfuran fails to generate a Diels–Alder adduct when it reacts with
b-cyanostyrenes (cinnamonitriles). However, in the reaction with b-cyanostyrenes possessing
additional electron-withdrawing groups (CN, CO2Et and SO2Ph), it yielded two new
phenylcyclopropanes. We have used computational methods to investigate the mechanism and to
probe the regioselectivity observed in the rearrangement reactions. We used a B3LYP/6-31G*
level density functional calculation to locate the transition states (TS), and to account for the
selectivity observed in these reactions, we examined the global electronic index involved. An
analysis of the results for the reaction pathways of two adducts (the endo and exo isomers) shows
that the reaction takes place via a polar stepwise mechanism. The first step involves a side-on
nucleophilic attack by the a-carbon atom of the furan ring on the vinyl carbon of the
cyanostyrene to give a zwitterionic intermediate. An intramolecular substitution within the
intermediate (IN) gives the cyclopropane ring, together with fission of the furan ring. The
potential energy barriers forꢀt1hese reactions had the following values: at the nucleophilic addition
step, 14.6 and 13.7 kcal mol for 2b, and 18.4 and 17.8 kcal molꢀ1 for 2c; at the rearrangement
step, 22.3 and 22.8 kcal molꢀ1 for 2b, and 25.1 and 23.1 kcal molꢀ1 for 2c. Solvent effects in
chloroform stabilized the rearrangement steps by 5–6 kcal molꢀ1, but the nucleophilic addition
step appeared to be slightly affected. From the theoretical results, in the case of 2b, the energy of
TS2-endo is lower than that of TS2-exo, so more trans-cyclopropane product from the endo form
is formed under kinetic conditions. Conversely, in the case of 2c, the energy of TS2-exo is lower
and more of the cis-product is formed. Density functional theory analysis of these reactions is in
complete agreement with the experimental results.
azabicyclo compounds.7 The use of cyanostyrene derivatives
Introduction
in combination with benzaldehyde affords pyrrole derivatives.8
Recently, we reported some interesting novel reactions for
the formation of phenylcyclopropane from the reaction of
b-cyanostyrenes with 2-methoxyfurans (see Scheme 1).2e In
this paper, our aim is to contribute to a better understanding
and interpretation of the mechanistic features of these processes,
including of the addition and rearrangement reactions (see
Scheme 2), especially by locating and characterizing all of the
stationary points involved along the reaction coordinate in
this type of reaction, using a computational approach.
Over the last two decades, Diels–Alder cycloaddition reactions
using furans as the diene have been extensively studied by
many research groups as one method for the construction of
valuable synthetic intermediates,1 and we have also been
studying the Diels–Alder reactions of furans with some
electron-deficient dienophiles.2 Recently, we reported that
2-methoxyfuran reacts with b-nitrostyrenes to give not only
the Michael adducts, but also the unexpected new isoxazoline
N-oxide,2d,g without the formation of a Diels–Alder adduct.
Furans are known to be rather unreactive dienes3 and hence,
reactions of furans require stronger dienophiles to make them
proceed.1b,3a,4 Cyano-olefins are powerful and versatile reagents,
which have found extensive applications in organic synthesis.5
Due to the strong electron-withdrawing properties of the cyano
group, acrylnitriles are excellent Michael and Diels–Alder
acceptors, and the cyano group can be further transformed
into a wide range of functionalities.5,6 In Diels–Alder reactions,
they react with dienes to give not only bicyclonitriles, but also
It is well known that the furan ring opening in Diels–Alder
reactions occurs in the case of donor-substituted furans with
reactive olefins possessing an electron-withdrawing group,
which undergo rapid smooth cycloaddition with the furan.1b
The resulting adducts are easily transformed into cyclohexanols
and phenols via furan ring opening in the presence of Lewis
acids.9,10 An abnormal Diels–Alder reaction of 2-methoxyfuran
with benzopyranylidene resulted in the formation of a three-
membered ring product.11 Furthermore, Huisgen et al.12
have reported that the reaction between 2-methoxyfuran and
ethylene substituted with very strongly electrophilic groups
(two trifluoromethylene and two cyano groups) also leads to
the formation of a cyclopropane ring. The high yield of the
Department of Chemistry, Faculty of Science, Kobe University,
Nada-ku, Kobe, 657-8501, Japan. E-mail: kuitoh@kobe-u.ac.jp;
Fax: +81 78-803-5770; Tel: +81 6-6436-4865
ꢁc
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Scheme 1 The reaction of b-cyanostyrene (2) with 2-methoxyfuran (1).
Scheme 2 Formation mechanism of phenylcyclopropane.
isolated product, coupled with the disappearance of the strong
color associated with charge-transfer in a few seconds at low
temperatures, indicates that these strongly electrophilic groups
may stabilize the initially formed intermediate.
diene system, and not only electron-withdrawing cyano and
phenylsulfonyl groups in the dienophile system, but also an
electron-donating phenyl group. In the reaction with 2a
(CO2Et) and 2b (CN), the trans-product is favored, but with
2c (SO2Ph), the cis-product is more favored. Furthermore, the
presence of a phenyl group allows for effective stabilization of
This reaction presents some interesting points, that is to say,
it features an electron-donating group on the furan in the
ꢁc
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the developing electric charge, which favors a stepwise mechanism
via a zwitterionic intermediate (see Scheme 2).13 In addition,
after the formation of the zwitterionic intermediate, the
rearrangement reaction proceeds simultaneously with
a
stepwise process of fission of the carbon–oxygen bond of the
furan ring, similar to the reaction of 1-methylpyrrole with
dimethyl acetylenedicarboxylate reported by Domingo et al.5b
and the reaction of 2-methoxyfuran with b-nitrostyrene by us.2d,g
Results and discussion
Experimental results
No reaction between cyanostyrenes and furan or 2-methylfuran
took place; therefore, in the present study we used
2-methoxyfuran as a more reactive diene.1b,14
Fig. 1 The structure (ORTEP) of phenylcyclopropane 3c.
A typical experimental procedure was as follows: to a stirred
solution of 2-methoxyfuran (1; 2.5 mmol) in chloroform
(3 mL) was added ethyl 1-cyano-cinnamate (2a; 1.2 mmol)
at room temperature under a nitrogen atmosphere. After being
stirred for 4 d, the resulting mixture was concentrated under
reduced pressure to give an oil, which was separated by
column chromatography on silica gel (eluent solvent: ethyl
acetate–hexane, 1 : 4).
correlated with that of a vinyl group proton (d = 6.24, dd,
J = 9.3, 11.7 Hz). On the other hand, the stereochemistry of
the minor adduct was found to have a cis (syn) configuration
(based on prominent NOE signals from protons with d = 3.48,
d, J = 9.7 Hz and 4.17, dd, J = 9.7, 10.3 Hz). Finally,
the elemental structure of the adduct was determined by
single-crystal X-ray diffraction using the major adduct
obtained from the reaction of 2c to be a vinylcyclopropane
derivative (Fig. 1). A similar stereoselectivity was also
observed in the reaction of 1 with 2b. However, in case of
compound 2c, which has a bulky phenylsulfonyl group, the
amounts of the cis-cyclopropanes were greater than those of
the trans products, in spite of the similar configuration to 2a.
Also, we are currently conducting further investigations of
rearrangements utilizing styrenes with non-cyano groups
(e.g. Table 1, runs 7–10) and the reaction in run 7 indicates
their possible applications in the cases where the styrenes have
powerful electron-withdrawing groups (b,ß0-diacetylstyrene; 2g).
The results of the reactions of 1 with various b-cyanostyrenes
(2) are given in Table 1. In general, the b-cyanostyrenes with
relatively weak electron-withdrawing groups (2d–2f) failed to
produce an adduct with 1, even after a reaction time of
one week (Table 1, runs 4–6). However, we expected that
b-cyanostyrene derivatives with stronger electron-withdrawing
groups (2a–2c) might react with 1 to give the adducts, as
was the case in previous reaction with electron deficient
dienophiles.
Ethyl 1-cyanocinnamate (2a) produced a mixture of the
adducts (trans-3a (major) and cis-3a (minor): 46% yield, and
4a: 5% yield; Table 1, run 1), but gave no Diels–Alder
adducts. The NMR and IR spectra of 3a showed no evidence
Geometries and energies
for the presence of a furan ring (d = 6.2 and 7.4; n = 1260 cmꢀ1
)
1
or a methoxy group (d = 3.5; n = 1200 cmꢀ1). The H and
Figs. 2, 3, 4 and 5 show the geometries of the calculated
transition structures (TS2) including selected bond lengths,
Fig. 6 shows the free energy levels on the reaction coordinate,
and Fig. 7 shows the local function, while Tables 2 and 5 show
the relative energies of the stationary points and thermodynamic
parameters. In Tables 3 and 4, the corresponding geometries
obtained from a B3LYP/6-31G* calculation are reported
for the intermediates and TSs. In Table 6, the geometric
parameters from X-ray analysis are reported with the
13C NMR spectra of the major adduct indicate the presence of
a methyl ester group (CO2CH3: dH = 3.73, s, 3H; dC = 63.3
and 165.1) and a vinyl group (CHQCH, cis (syn) configuration:
d = 6.00, dd, J = 1.0, 11.7 Hz and 6.24, dd, J = 9.3,
11.7 Hz), and the cis configuration was confirmed by an
NOE experiment. These signals indicate that scission of the
furan ring had occurred. Furthermore, the proton signal at
higher field (d = 4.44, ddd, 1H, J = 1.0, 8.3, 9.3 Hz) is
Table 1 The reactions of b-cyanostyrenes and related compounds (2) with 2-methoxyfuran (1)
Run
Cyanostyrene
Cyclopropane
Yield (%)
trans : cis
Michael adduct
Yield (%)
1
2
3
4
5
6
7
8
9
10
2a
2b
2c
2d
2e
2f
2g
2h
2i
3a
3b
3c
—
—
—
3g
—
—
—
46
84
77
3.5 : 1.0
4.5 : 1.0
1.0 : 1.5
—
—
—
3.3 : 1.0
—
—
4a
4b
4c
—
—
—
4g
—
—
—
5
8
4
No reaction
No reaction
No reaction
36
—
No reaction
No reaction
—
—
—
51
Trace
—
—
2j
—
ꢁc
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Fig. 5 The transition structure (TS2-ex) of 1 and 2c. O(1)–C(4)–C(6)
distances in A.
calculation values, and in Table 7, the parameters of the global
properties are reported.
Scheme 2 illustrates that these reactions take place via a
stepwise mechanistic pathway. The reaction is initiated by
nucleophilic attack of the unsubstituted C4 carbon atom of
1 on the C5 carbon atom (the carbon atom attached to the
cyano group) of b-cyanostyrene 2 to give a zwitterionic
intermediate. The attack of 1 on 2 can take place via two
stereoisomers formed from the endo and exo approach of the
2-methoxyfuran ring to the phenyl group belonging to the
styrene. Thus, these regioisomers are two erythro isomers
(IN-en: gauche conformation between H4 of the furan and
H5 of the styrene, and IN-ex: synclinal conformation between
H4 and H5). The reactive carbanion of the cyanostyrene
moiety attacks the nucleophilic C4 position of the methoxyfuran
moiety in the intermediate accompanied by the carbon–oxygen
bond fission of the furan ring to yield the cyclopropane
product, 3.
Fig. 2 The transition structure (TS2-en) of 1 and 2b. O(1)–C(4)–C(6)
distances in A.
Structures and energetics of gas-phase calculation
At the first two transition states from the nucleophilic attack
of 1 on 2 (see Scheme 2), the potential energy barriers
associated with TS1-en and TS1-ex are 14.6 and 13.7 kcal molꢀ1
for 2b, and 18.4 and 17.8 kcal molꢀ1 for 2c. An analysis
of the relative energies indicates that TS1-ex is slightly less
favorable than TS1-en. In the structure of TS1, the lengths of
the C4–C5 bond being formed in the two stereoisomers are
both 1.86 A for 2b, and 1.88 and 1.83 A for 2c, whereas the
distances between C1 and C6 are about 3.0 A for both
compounds. This large difference might be due to steric
hindrance between the methoxy group and the cyano or
sulfonyl group. The values of the unique imaginary frequency
of TS1 are 284i and 288i for 2b, and 308i and 324i for 2c.
These similar vibrational frequencies indicate that these TSs
are mainly associated with the motion of the C4 and C5
carbon atoms along the C–C bond formation direction. A
measure of the extent of bond formation or bond breaking
along a reaction pathway is provided by the bond order (BO)
of the Weinberg index, derived from the natural bond orbital
(NBO) basis.15 The values of the C4–C5 BOs are 0.61 for 2b
and 0.58 for 2c, whereas the values of the C1–C6 BOs are 0.08
for 2b and 0.07 for 2c. Thus, the high values for the C4–C5
bond indicated that these C–C single bonds are forming, while
Fig. 3 The transition structure (TS2-ex) of 1 and 2b. O(1)–C(4)–C(6)
distances in A.
Fig. 4 The transition structure (TS2-en) of 1 and 2c. O(1)–C(4)–C(6)
distances in A.
ꢁc
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Fig. 6 The Gibbs free energy for the reaction between furan 1, and styrenes 2b and 2c.
atom centers have a planar arrangement, in agreement with
sp2 hybridization. This situation is similar to that of the
Diels–Alder adduct product. The values of the C1–C6 BOs
for the intermediates are 0.86 for 2b and 0.89 for 2c, whereas
the values of the C4–C5 BOs for the intermediates are
0.94–0.95, which indicates that a single bond is forming. The
BO values for the C1–O1 and C1–C2 bonds (0.985–0.996 and
0.98–0.99, respectively) at the furan site, point to an allyl
structure for the C2–C1–O1 framework that allows a favorable
stabilization of the positive charge.
It is also possible that the intermediates may occur via the
scission of a [4 + 2] cycloadduct (Diels–Alder adduct).9,10,16
The potential energy barriers of a concerted [4 + 2] transition
state for the cycloaddition reactions are 14.5 and 13.8 kcal molꢀ1
for 2b, and 18.3 and 18.8 kcal molꢀ1 for 2c, respectively, values
which are similar to that of TS1. Furthermore, the energy
barriers for the C1–C6 bond scission of a cycloadduct to the
intermediates are 13.8 and 10.7 kcal molꢀ1 for 2b, and 15.3
and 17.0 kcal molꢀ1 for 2c. The forming bond (C1–C6 and
C4–C5) distances of the cycloadduct are similar to those of
the intermediate; however, it differs in that generally, the
concerted pathway is favored over a stepwise mechanism by
Fig. 7 Fukui functions for furan 1, styrenes 2b and 2c, and
intermediates IN2b and IN2c.
on the other hand, the very low C1–C6 values indicate that C1
and C6 atoms are becoming bonded. Furthermore, the values
of the C5–C6 BOs in TS1 (about 1.2) show that the double
bond present in styrene is approximately a single bond in the
transition state.
a few kcal molꢀ1 17
Nevertheless, the concerted [4 + 2] path-
.
way can compete with the nucleophilic attack pathway, for
both TS11-ex and TS21-ex are lower than TS1-ex in free
energy. In addition, the valid trans/cis selectivity of the
product may be required in this reaction.
The formed zwitterionic intermediates are two configura-
tionally favorable stereoisomers (IN-en and IN-ex). The
potential energy barrier for the intermediate of IN-en and
Once formed, the intermediates can undergo further trans-
formations. The final step of these reactions corresponds to an
intramolecular nucleophilic substitution in the zwitterionic
intermediate to give the final cyclopropane, 3. This process
can take place by the nucleophilic attack of the carbanion (C6)
attached to the cyano group on the C4 carbon atom of the
furan ring, with resulting cyclopropyl ring formation, together
with a fission process in the plane of the C4–O1 bond of the
furan ring, to produce the carbonyl group. In both TS2 of 2b,
similar potential energy barriers exist (22.3 and 22.8 kcal molꢀ1).
In the transition structure of 2b-en, (see Fig. 2) the center
tetrahedral carbon C4 (C3–C4–C5: 118.71, C3–C4–H4: 116.91,
C5–C4–H4: 111.61, O1–C4–C5: 113.71) is located between
the C6 and O1 atoms of the furan. The O1–C4–C6 bond angle
is 154.31, that is, the O1 atom, the C4 atom and the C6 atom
IN-ex for 2b indicates similar values (6.4 and 7.6 kcal molꢀ1
respectively), but for 2c, IN1-en is 4.6 kcal molꢀ1 more
favorable than that of IN-ex (5.8 and 10.4 kcal molꢀ1
,
,
respectively). This result is due to the Coulombic interactions
that appear between the positively charged donor 2-methoxyfuran
and the negatively charged sulfonyl groups of the acceptor
styrene at the endo form. In the structures of all the inter-
mediates, the lengths of the C4–C5 bond are about 1.6 A,
which are similar to that of TS1. However, the lengths of
the C1–C6 bond are 1.60–1.66 A, which indicates a large
difference (about 1.4 A) from that of TS1 (non bonding
structure). In these intermediates, both the C4 and C5 carbon
atoms are completely pyramidalized, while the C1 and C6
ꢁc
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Table 2 The total energies (E, au), relative energies (DE, kcal molꢀ1), entropies (DS, cal Kꢀ1 molꢀ1), enthalpies (DH, kcal molꢀ1) and free energies
(DG, kcal/mol) at 298.15 K for the stationary points corresponding to the reaction between 2-methoxyfuran (1), and b-cyanostyrenes (2b and 2c)
2b-en
2b-ex
E
DE
0.00
14.61
6.39
22.31
DS
0.00
ꢀ49.45
ꢀ54.27
ꢀ46.24
ꢀ40.68
DH
0.00
15.36
7.46
22.88
DG
0.00
30.10
23.64
36.67
ꢀ2.63
E
DE
0.00
13.71
7.64
22.77
DS
0.00
ꢀ49.51
ꢀ53.98
ꢀ52.23
ꢀ40.56
DH
0.00
14.50
8.77
22.23
DG
0.00
29.26
24.86
37.80
ꢀ0.08
1 + 2
TS1
IN1
TS2
3
ꢀ838.670495
ꢀ838.647212
ꢀ838.660313
ꢀ838.634947
ꢀ838.695841
ꢀ838.670495
ꢀ838.648605
ꢀ838.658323
ꢀ838.634215
ꢀ838.692652
ꢀ15.90
ꢀ14.76
ꢀ13.90
ꢀ12.17
TS11
IN11
TS12
ꢀ838.647396
ꢀ838.660484
ꢀ838.648549
14.49
6.28
13.77
ꢀ58.04
ꢀ54.27
ꢀ49.45
15.89
8.08
14.72
33.19
24.26
29.46
ꢀ838.648471
ꢀ838.658464
ꢀ838.653380
13.82
7.55
10.74
ꢀ56.61
ꢀ53.98
ꢀ49.51
10.86
9.39
13.88
27.74
25.48
28.63
2c-en
2c-ex
E
DE
DS
DH
DG
E
DE
DS
DH
DG
1 + 2
TS1
IN1
TS2
3
ꢀ1526.052704
ꢀ1526.023353
ꢀ1526.043432
ꢀ1526.012725
ꢀ1526.077564
0.00
18.42
5.82
25.09
ꢀ15.60
0.00
ꢀ49.26
ꢀ62.70
ꢀ50.09
ꢀ42.98
0.00
19.14
6.87
25.04
ꢀ13.94
0.00
33.82
25.56
39.98
ꢀ1.12
ꢀ1526.052704
ꢀ1526.024306
ꢀ1526.036062
ꢀ1526.015943
ꢀ1526.081185
0.00
17.82
10.44
23.07
ꢀ17.87
0.00
ꢀ50.24
ꢀ55.47
ꢀ47.99
ꢀ43.99
0.00
18.56
12.06
23.11
ꢀ16.04
0.00
33.54
28.60
37.42
ꢀ2.93
TS11
IN12
TS12
ꢀ1526.023478
ꢀ1526.043584
ꢀ1526.028250
18.34
5.73
15.34
ꢀ52.04
ꢀ63.02
ꢀ47.78
16.47
6.23
20.03
31.98
25.02
34.87
ꢀ1526.017937
ꢀ1526.036299
ꢀ1526.025601
18.81
10.29
17.01
ꢀ55.04
ꢀ53.59
ꢀ50.78
18.06
12.61
19.66
34.47
28.59
34.80
Table 3 Selected bond lengths (A), bond angles (1) and dihedral
angles (1) for the TSs and intermediate of the reactions between
2-methoxyfuran (1) and b-cyanostyrenes (2b and 2c)
Table 4 Weinberg bond indices for the TSs and intermediate of the
reactions between 2-methoxyfuran (1) and b-cyanostyrenes (2b and 2c)
2b-en
2b-ex
2c-en
2b-ex
2b-en
2b-ex
2c-en
2c-ex
TS1
TS1
C1–C6
C4–C5
C5–C6
C1–C4–C5–C6
C1–C6
C4–C5
C5–C6
C4-O1
C1–O1
0.081
0.611
1.194
0.893
1.094
0.072
0.603
1.203
0.888
1.099
0.078
0.581
1.239
0.893
1.098
0.072
0.568
1.250
0.900
1.103
2.963
1.862
1.449
ꢀ9.98
3.047
3.000
1.883
1.437
ꢀ9.13
2.994
1.862
1.446
17.78
1.832
1.442
11.62
IN1
C1–C6
C4–C5
C4–C6
C5–C6
C1–O1
C6–C4–O1
C1–C4–C5–C6
H4–C4–C5–H5
IN1
1.661
1.575
2.421
1.599
1.419
69.46
0.03
1.620
1.578
2.427
1.602
1.429
68.99
2.80
1.598
1.582
2.459
1.588
1.438
67.59
4.96
1.614
1.614
2.454
1.584
1.425
69.66
2.18
C1–C2
C1–C6
C4–C5
C4–C6
C5–C6
C1–O1
0.987
0.861
0.941
0.005
0.912
0.996
0.980
0.857
0.939
0.004
0.917
0.996
0.982
0.892
0.939
0.006
0.939
0.985
0.991
0.895
0.954
0.005
0.951
0.986
ꢀ91.42
49.56
95.48
47.9
TS2
C4–C6
C4–O1
C5–C6
C1–O1
C1–C6
C4–C5
0.109
0.413
0.850
1.088
0.006
0.847
0.108
0.398
0.862
1.090
0.006
0.851
0.223
0.518
0.981
1.368
0.015
0.962
0.221
0.517
0.983
1.369
0.016
0.965
TS2
C4–C6
C4–O1
C5–C6
C1–O1
2.284
1.736
1.535
2.297
1.754
1.535
2.237
1.757
1.531
2.259
1.760
1.510
1.273
1.270
1.268
1.267
C3–C4–C5
C3–C4–H4
C5–C4–H4
C5–C4–O1
C6–C4–O1
C6–C4–C3
H4–C4–C5–H5
118.65
116.86
111.63
113.68
154.30
101.05
ꢀ179.83
120.28
116.71
111.41
111.46
152.98
102.29
41.06
119.69
116.58
114.32
114.32
154.36
104.43
ꢀ173.49
122.04
116.65
109.59
111.55
153.08
101.45
38.59
2b-en is consistent with formation of the trans-form of the
product (3b). Also, in the transition structure of 2b-ex (see
Fig. 3), the O1–C4–C6 bond angle of the center carbon C4 is
153.01; again, these atoms are arranged on almost a straight
line, similar to the case of 2b-en. However, the value of the
H4–C4–C5–H5 dihedral angle is 41.061, which translates to
the cis-form of the product.
are arranged nearly on a straight line. The value of this bond
angle indicates that the C4 center has sp hybridization, in which
the charge located at C4 occupies a p atomic orbital. The
length of the C4–C6 bond being formed in TS2 is 2.28 A and
the length of the C4–O1 bond being cleaved is 1.74 A. The
value of the H4–C4–C5–H5 dihedral angle (ꢀ179.831) for
On the other hand, the potential energy gaps of the
transformation process (TS2) of 2c are 25.1 kcal molꢀ1 for
the endo form and 23.1 kcal molꢀ1 for the exo form. The latter
is 2.0 kcal molꢀ1 more favorable than TS2-en, due to a
ꢁc
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Table 5 The total energies (E, au), relative energies (DE, kcal molꢀ1) at 298.15 K and dipole moments (D) for the stationary points corresponding
to the reaction between 2-methoxyfuran (1) and b-cyanostyrenes (2b and 2c) including the solvent effects
2b-en
2b-ex
E
DE
0.00
13.08
6.17
17.40
Dipole moment
E
DE
0.00
12.74
7.40
18.14
Dipole moment
1 + 2
TS1
IN1
TS2
3
ꢀ838.688090
ꢀ838.6672399
ꢀ838.678262
ꢀ838.6603686
ꢀ838.713436
—
ꢀ838.688090
ꢀ838.667795
ꢀ838.676302
ꢀ839.659186
ꢀ838.711247
—
7.93
5.19
12.47
5.68
7.88
6.09
12.87
5.71
ꢀ15.91
ꢀ14.51
2c-en
2c-ex
E
DE
Dipole moment
E
DE
Dipole moment
1 + 2
TS1
IN1
TS2
3
ꢀ1526.069299
ꢀ1526.042149
ꢀ1526.061002
ꢀ1526.039437
ꢀ1526.095294
0.00
17.04
5.26
18.74
ꢀ16.31
—
ꢀ1526.069299
ꢀ1526.042748
ꢀ1526.053650
ꢀ1526.041919
ꢀ1526.097909
0.00
16.66
9.82
17.18
ꢀ17.95
—
8.13
3.41
9.74
4.02
8.03
5.03
10.76
5.14
Table 6 Selected bond lengths (A), bond angles (1) and dihedral
angles (1) for 3c from X-ray analysis and calculation data
structure of 2c-ex (see Fig. 5), the O1–C4–C6 bond angle of
the center carbon C4 is 153.11, these atoms also being almost
arranged on
a straight line, while the value of the
Parameters
X-ray
Calc.
1.516
H4–C4–C5–H5 dihedral angle is 38.591, all of which translates
to the cis-form of the product. These comparisons for TS2
clearly indicate that the carbonyl group forms via scission of
the C4–O1 bond in the furan ring and imply that the exo form
is the more favorable transition structure. Interestingly, the
secondary orbital interaction determining the exo preference
in the 2c-exo form is due to the contribution from the
phenylsulfonyl group. Thus, the dihedral angle between the
SQO moiety and the C4–C6 group being formed,
+C–C–SQO, is 25.221, which may be attributed to electron
flow from the SQO moiety to the C4–C6 group.
C4–C5
C5–C6
C4–C6
C3–C5
C2–C3
O5–C4–C6
C4–C6–C5
C4–C5–C6
H4–C4–C6–H6
C1–C2–C3–C4
1.489(3)
1.514(3)
1.537(3)
1.473(3)
1.319(3)
59.79(13)
58.87(13)
61.34(13)
105.82(2)
106.6(1)
1.520
1.543
1.476
1.345
59.58
59.33
61.09
106.12
108.04
Table 7 The global properties of the masked furan (1) and styrenes
(2b, 2c, IN2b, IN2c, 2a and 2g), and yields (2a, 2b, 2c and 2g)a
The BO values of the forming C1–O1 double bonds are 1.09
for 2b and 1.37 for 2c, and for the C4–C6 forming single bonds
are 0.11 for 2b and 0.22 for 2c. These low BO values for the
forming bonds and the large BO values for the C4–O1 cleaving
bond (0.4 and 0.5) indicate that the TSs for the translation
processes are early. Moreover, the slightly larger BO value of
2c for these bonds (C4–C6, C5–C6, C4–C1 and C4–O1) in
comparison with those of 2b will account for a favorable
stabilization due to delocalization of the sulfur lone pairs of
the phenylsulfonyl group. The values of the unique imaginary
frequency of TS2 are 427i for 2b and 420i for 2c. Analysis of
the atomic motion related to these vibrational frequencies
indicates that these TSs are mainly associated with the motion
of the forming C4–C6 bond and the dissociating C4–O1 bond
accompanied by a push and pull motion of the SN2
substitution type.
Yield
(%)
Molecule HOMO LUMO
m/au
Z/au
o/eV
1
2b
2c
IN2b-en
IN2b-ex
IN2c-en
IN2c-ex
2a
ꢀ0.1939
0.0353 ꢀ0.0793 0.2292 0.3731
—
ꢀ0.2597 ꢀ0.1067 ꢀ0.1832 0.1530 2.9833 84
ꢀ0.2536 ꢀ0.0925 ꢀ0.1731 0.1611 2.5295 77
ꢀ0.2530 ꢀ0.0303 ꢀ0.1417 0.2227 1.2262
ꢀ0.2560 ꢀ0.0368 ꢀ0.1464 0.2192 1.3298
ꢀ0.2414 ꢀ0.0438 ꢀ0.1426 0.1976 1.3996
ꢀ0.2529 ꢀ0.0483 ꢀ0.1506 0.2046 1.5076
—
—
—
—
ꢀ0.2487 ꢀ0.0909 ꢀ0.1698 0.1578 2.4855 46
2g
ꢀ0.2394 ꢀ0.0772 ꢀ0.1583 0.1621 2.1022 36
a
The electronic chemical potential, m, and the chemical hardness, Z,
are given in atomic units; the electrophilicity power, o, is in given
electron volts.
We have also computed the relative enthalpies, entropies
and free energies at 25 1C for the different stationary points
along the reaction coordinate (Table 2). At TS1, the large
negative entropy change (ꢀ49 cal Kꢀ1 molꢀ1 for the endo
form, ꢀ50 cal Kꢀ1 molꢀ1 for the exo form) due to the
restricted geometry associated with the intermolecular nature
of the process is similar to that seen in the Diels–Alder
reaction due to the bimolecular nature of that process. This
entropy change, similar to that created during intermolecular
cycloadditions, is responsible for the increase of the activation
favorable arrangement. In the transition structure for this
process, the O1–C4–C6 bond angle (the C6 atom attacked
the cyano group and O1 atom of the furan) is 153–1541, which
makes this transformation likely. The length of the C4–C6
bond being formed for TS2-en (see Fig. 4) is 2.24 A and the
length of the C4–O1 bond being cleaved is 1.76 A. The value of
the H4–C4–C5–H5 dihedral angle is ꢀ173.51. These angles are
consistent with the formation of the trans-form of product 3,
in accord with the experimental results. Also, in the transition
ꢁc
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free energy of the rate-determining step to 29–34 kcal molꢀ1 18
.
the strong asynchronicity. Here, as described in the previous
section, the transition structures (TS1) for the nucleophilic
reaction are asynchronous structures, which have a high dipole
moment. The fact that 2c is more stable seems to indicate that
the values of the dipole moments at TS1 are somewhat lower
than those of TS2. It should be noted that for the dipole
moments, the values of the transition state are higher than
those of the intermediate. This contrasts with the situation
encountered in the intermediates, for which the values of the
dipole moments are almost same, both in the gas phase and in
solution. Thus, the interaction of the dipoles of the transition
state with the medium should be different with each one of the
intermediates. These results clearly suggest that the solvent
effect should be significant in those cases where the transition
structure is highly polarized. The energy barrier difference
obtained for the endo and exo forms in solution is in very good
agreement with the observed experimental stereoselectivity, in
contrast to the poor agreement with gas phase calculations.
Thus, the formation of cyclopropane 3b via the endo form is
kinetically more favorable than formation via the exo form,
so the potential energy differences at TS1 are the same
(gap: 0.34 kcal molꢀ1), but that of the endo form at TS2 is
more favorable than the exo form by 0.74 kcal molꢀ1. Thus,
the amount of the trans-cyclopropane produced from the endo
form will be greater than that of the cis compound resulting
from the exo form. On the other hand, with respect to the
formation of cyclopropane 3c, the potential energy difference
at TS1 is not observed (gap: 0.38 kcal molꢀ1), but the value
However, for the second step, the activation entropy
and the activation free energy change is smaller (less than
12.6 cal Kꢀ1 molꢀ1 and 14.4 kcal molꢀ1, respectively), due to
the smooth intramolecular nature of the rearrangement
process. This fact, together with stabilization of TS2 by the
inclusion of a solvent effect, emphasizes the stepwise nature of
the transformation process.
Interestingly, the formation of cyclopropane 3b is exother-
mic, ꢀ15.9 kcal molꢀ1 for the endo form and ꢀ13.9 kcal molꢀ1
for the exo form. Thus, the formation of the cyclopropane via
the endo form is thermodynamically more favorable than
formation via the exo form, but is kinetically almost the same
at TS1 and TS2 in the gas phase. However, at TS2, the lower
activation free energy and lower negative entropy of the endo
form compared with the exo form leads to the favorable
formation of trans product via the endo reaction channel. On
the other hand, 3c-en is kinetically favored by the potential
energy gaps at TS1, but disfavored at TS2 relative to the exo
form. Furthermore, a comparison of the free energy and
entropy at TS2 indicates that the exo form is slightly favored.
Therefore, the selectivity of both forms is not amenable to a
kinetic explanation with gas phase calculations.
Finally the geometries of the calculated structure are
compared with the structure obtained from the X-ray analysis
(see Table 6). The value of the H4–C4–C6–H6 dihedral angle
in 3c-ex from the B3LYP/6-31G* calculation is 106.121, which
is in reasonable agreement with that of 105.82(2)1 from the
X-ray analysis. The other geometrical parameters obtained for
cyclopropane 3c-ex from the calculation are in agreement with
the experimental analysis of the structure. The bond distances
in the cyclopropyl ring for the calculated structure are slightly
longer than those found by experiment (0.026 A is the highest
difference for the C2–C3 and C4–C5 bonds).
at TS2 is favorable for the exo form by 1.5 kcal molꢀ1
.
Therefore, the amount of the cis-cyclopropane is predicted
to be greater than that of the trans form, as found by
experiment.
Selectivity of trans/cis product
At a first glance, it will be see that the potential energy
difference from the intermediate, that is, the relative TS2
energy, may determine the selectivity of trans/cis product.
However, the barriers starting from the intermediate are
much lower, so the whole process is reversible and may shift to
the reagents. In order to obtain more definitive evidence, we
represent the different reaction pathways calculated in Fig. 6
graphically using the Gibbs free energy (DG) on the ordinate
axis. The dotted lines correspond to the reaction with 2b, and
dashed lines correspond to the reaction with 2c. It is also
evident that the back reaction from the intermediate to
substrates 1 and 2b has a lower barrier than the forward
reaction from the intermediate to product 3b, and that both
barriers are relatively low. Under these conditions, the steady
state approach for the intermediate can be used in the kinetic
analysis.
Solvent effects
Solvent effects on cycloaddition are well known19 and have
received considerable attention. Sustmann et al. reported that
such effects favor the transition structure that exhibits the
higher dipolar moment.19a Table 5 shows the energies at the
stationary points corresponding to the reaction in chloroform,
which was used as the experimental solvent. The energy
differences of the transition state at TS2 in solvent are
stabilized by 5–6 kcal molꢀ1 over those in the gas phase.
The transition structure involving nucleophilic attack is
preferentially stabilized relative to the reactants, due to the
large charge transfer that is developed along this polar transfer
path. The intensity of the solvent effect for 2c is slightly larger
than that for 2b. This result is due to the charge delocalization
of the phenyl group compensating for the high dipole moment
arising from the strong electron-withdrawing group—the
sulfonyl group of 2c. Thus, inclusion of the solvent effect
may accelerate this polar reaction, thanks to a greater
stabilization of the transition state corresponding to
nucleophilic attack. Also, the solvent effect for TS1 shows a
slight difference (1–2 kcal molꢀ1) in the gas phase and in
solution. According to the reactions studied by Sustmann
et al., the high polarity of the transition structure came from
We have calculated the stereoselectivity (trans/cis ratio) of
products
3 using terms of relative reaction rates and
equilibrium. The ratio may be combined into eqn (1) as a
basis of calculation:
trans Keq;endo k2;endo
¼
ð1Þ
cis
Keq;exo k2;exo
ꢁc
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where Keq is the equilibrium constant and k2 is the kinetic rate
constant of the second step.
at the non-substituted C4 position (f ꢀ = 0.225(ꢀ)), hence,
this is the more reactive site for electrophilic attack. On the
other hand, for the styrene systems 2b and 2c, there are two
electrophilic sites (at the C5 and C6 carbon atoms). Analysis
of the electrophilic Fukui functions for 2b and 2c indicates
that the C5 position has the lower electrophilic activity
(f+ = 0.158(+) 4 0.125(+) for 2b; 0.145(+) 4 0.103(+)
for 2c). Thus, the most favorable interaction will take place
between the C4 carbon atom of 1 and the C5 carbon atom of 2,
in agreement with the regioselectivity observed.
Conversion of eqn (1) using the energy term results in
eqn (2):
a
endo
RT
ꢀDG
ꢀDDG
endoꢀexo e
endoꢀexo
a
ꢀDDG
¼ e
RT
trans
cis
e
RT
¼
ð2Þ
a
ꢀDG
exo
RT
e
Using the free energies given in Table 2 and Table 5, the values
of the ratios are obtained as follows:
The second step in the reaction is the rearrangement reaction
of zwitterionic intermediate IN-en or IN-ex to give cyclopropane
products 3. This process involves the nucleophilic attack of the
rest (C6) of styrene 2 to the electron-poor furan residue (C4).
As a consequence, for IN-en and IN-ex, the site analysis must
be carried out at the electrophilic positions (C4) of the furan
residues, the C4 center (f+ = 0.052(+) and 0.043(+) for
IN-2b, and f+ = 0.018(+) and 0.028(+) for IN-2c) and
the nucleophilic position of the styrene, the C6 center
(f ꢀ = 0.326(ꢀ) and 0.211(ꢀ) for IN-2b, and f ꢀ = 0.077(ꢀ)
and 0.115(ꢀ) for IN-2c; see Fig. 7). These local Fukui
functions show that the more favorable interaction along
the addition reaction process corresponds to attack of the
nucleophilic C4 center on the electrophilic C6 one, and leads
to formation of the cyclopropane. Moreover, comparison of
the nucleophilic function for both isomers of 2b and 2c
indicate a large activity at C6, but quite a large difference is
found between the isomers, the value exhibiting an opposite
tendency. The function has a larger value for the endo form of
2b (0.326 4 0.211), but on the other hand, shows a larger
value (0.077 o 0.115) for the exo form of 2c, in spite of the
moderate stabilities of the exo structure in both cases. These
contrasting results allow us to explain the different ratios of
the isomer products, the more reactive nucleophilic attack for
the endo form at 2b, and the more reactive attack for the exo
form at 2c. This local analysis offers a reasonable explanation
for the experimental results.
ꢀDDGeando–exo
Gas phase
Solution
2b
2c
1.13
1.00
ꢀ2.56
ꢀ2.10
The analysis results indicate that in the case of 2b, the
trans-cyclopropane coming from the endo path way is favored;
on the other hand, in the case of 2c, the cis-cyclopropane
coming from the exo path way is favored. Also, solvent
influence is not required to obtain the correct interpretation.
This situation corresponds to pseudo-Curtin–Hammett
behavior,20 so that the energy differences in Table 2 and
Table 5 correspond to the absolute Gibbs free energy difference
of TS2-en and TS2-ex.
Global and local electrophilicity/nucleophilicity analysis
The reactivity and regioselectivity of these addition and
rearrangement reactions have been analyzed using global
and local indices.21 In Table 7, the static global parameters,
electronic chemical potential m, chemical hardness Z, and
global electrophilicity o, for 2-methoxyfuran (1), cyanostyrenes
2b and 2c, and intermediates IN2b-en, IN2b-ex, IN2c-en and
IN2c-ex are displayed. Calculations carried out by Domingo’s
group on the addition reaction with polar character have
shown that these indices are powerful tools to study both
reactivity and regioselectivity.21b,22
Summary
The value of m of cyanostyrenes 2b and 2c (m = ꢀ0.1832 and
ꢀ0.1731 au) is less than that of furan 1 (m = ꢀ0.0793 au),
therefore indicating that the net charge transfer will take place
from 2b and 2c towards 1, in agreement with the charge
transfer analysis (see the previous section). Styrenes 2b and
2c have a large electrophilicity power (o = 2.983 and
2.529 eV), whereas on the other hand, 1 has a low electro-
philicity (o = 0.373 eV). Thus, the global electrophilicity
difference between both reactants (Do = 2.610 and 2.156 eV)
indicates the normal electron demand reaction in which
electron-poor dienophile 2b or 2c reacts with the electron-rich
diene 1. Furthermore, the value of the difference may be
corrected with the reactivity (the product yield) including the
case of 2a and 2g.
This study has shown that the mechanism of the reaction of
2-methoxyfuran with b-cyanostyrenes possessing strong
electron-withdrawing groups has been characterized using
quantum mechanical calculations. Experimental data showed
that for the reaction with 2b (CN), more trans-cyclopropane is
obtained (a ratio 4.5 : 1 over the cis-cyclopropane). With 2c
(SO2Ph), more cis-product is obtained (a ratio 1.5 : 1 over the
trans-product). We have shown that: (i) an analysis of the
results shows that the reaction takes place along a polar
stepwise mechanism. The first step (TS1) corresponds to the
nucleophilic attack of the furan ring on the styrene or the
concerted addition-fission reaction to give two zwitterionic
intermediates (IN-endo and IN-exo). For the second step
(TS2), an intramolecular substitution in the intermediate give
the formation of the cyclopropane ring together with fission of
the furan ring. (ii) Trans-cyclopropanes come from the
endo pathway of the cyanostyrene to the furan ring, whereas
cis-cyclopropanes come from the exo pathway. (iii) From the
theoretical results, the selectivity of the reaction is determined
Recent studies of cycloaddition reactions with polar
character have used analysis of the electrophilic and nucleo-
philic Fukui functions to explain the observed experimental
regioselectivities.23 The local functions for our case are
summarized in Fig. 7. 1 has the largest nucleophilic activation
ꢁc
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by the difference between TS2-en and TS2-ex energies (so a
pseudo-Curtin–Hammett behavior is assumed). (iv) As a
consequence, in the case of 2b, the energy of TS2-en is lower
than that of TS2-ex, so more trans-cyclopropane product from
the endo form is formed under kinetic conditions. Conversely,
in the case of 2c, TS2-ex is lower in energy than TS2-en, so the
cis-cyclopropane is formed. (v) The qualitative agreement with
and without including solvent effects is therefore complete.
(vi) This also agrees with the conclusions drafted from gas
phase calculations of the local Fukui functions. (vii) The
calculation analysis offers a reasonable explanation for the
experimental results.
Yamamura,30 2-cyanoethylcinnamate,31 2-diacetylstyrene,32
2-dicyanostyrene33 and 2-cyano-2-phenylsulfonylstyrene.34
Methyl 3-((3S,1R,2R)-2-cyano-2-ethoxycarbonyl-3-phenyl-
cyclopropyl)-(2Z)-prop-2-enoate (3a). Major (trans form): Mp
= 65.5–66.5 1C. MS: m/z = 299 (M+), 267 (M+ ꢀ MeOH),
253, 221 (M+ ꢀ Ph), 195, 194, 167, 166, 139, 128, 115, 98. IR:
n = 3028, 2946, 1702, 1618, 1584, 1260, 1198 cmꢀ1. 1H NMR
(CDCl3): d = 1.28 (3H, t, J = 7.3 Hz), 3.36 (1H, d, J = 8.3
Hz), 3.73 (3H, s), 4.24 (2H, q, J = 7.3 Hz), 4.44 (1H, ddd,
J = 1.0, 8.3, 9.3 Hz), 6.00 (1H, dd, J = 1.0, 11.7 Hz), 6.24
(1H, dd, J = 9.3, 11.7 Hz), 7.24–7.45 (5H, m). 13C NMR
(CDCl3): d = 14.1, 30.2, 34.4, 40.1, 51.7, 63.3, 115.6, 124.2,
128.4, 128.7, 128.9, 130.1, 132.2, 140.0, 165.1, 166.2. Found: C,
68.06; H, 5.63; N: 4.44; calc. for C17H17NO4: C, 68.22; H, 5.72;
N, 4.68%.
Experimental
Computational details. The density functional method
computations for the gas phase calculations were performed
with the Spartan ’04 for Windows package 24 and the Gaussian
03 program.25 The reactant, product and transition state (TS)
geometries for the above reactions were optimized by DFT
calculations at the B3LYP/6-31G* level.26 After the optimization
of all transitional structures, we carried out vibrational
analyses in order to check the nature of the stationary points.
The transition states are defined by exactly one imaginary
frequency. Unless otherwise indicated, all energy changes in
the B3LYP/6-31G* optimizations reported in this paper
include zero-point energies without scaling. To calculate the
free energies of solvation needed to understand the effect of the
solvent on the stereoselectivity,27 we used the Gaussian 03
program.25 To evaluate the solvent effect, we carried out a
B3LYP/6-31G* single point calculation, using the method
based on the polarized continuum model (PCM).28 The
solvent used in the experimental work was chloroform; therefore,
we employed the dielectric constant, e = 4.81 and the solute
radius, a = 3.92. To reduce the time of the calculation, we
used b,ß0-dicyanostyrene (2b) and b-cyano-ß0-phenylsulfonyl-
styrene (2c) in the calculations to represent the actual styrenes
used in the experiments.
Methyl 3-((2S,1R,3R)-2-cyano-2-ethoxycarbonyl-3-phenyl-
cyclopropyl)-(2Z)-prop-2-enoate (3a0). Minor (cis form): Mp
= 69.8–70.8 1C. MS: m/z = 299, 253, 221, 194, 166, 140, 129,
115, 98. IR: n = 3068, 2957, 1744, 1641, 1443, 1316, 1251,
1146 cmꢀ1. 1H NMR (CDCl3): d = 1.32 (3H, t, J = 7.3 Hz),
3.48 (1H, d, J = 9.7 Hz), 3.72 (3H, s), 4.17 (1H, dd, J = 9.7,
10.3 Hz), 4.28 (2H, q, J = 7.3 Hz), 5.76 (1H, dd, J = 10.3,
11.7 Hz), 6.04 (1H, dd, J = 1.0, 11.7 Hz), 7.24–7.45 (5H, m).
13C NMR (CDCl3): d = 14.1, 28.1, 33.2, 38.4, 51.7, 63.4,
114.9, 124.4, 128.4, 128.7, 128.9, 129.3, 130.5, 139.9,
166.2, 166.5.
Ethyl
2-cyano-3-(5-methoxy(2-furyl))-3-phenylpropanoate
1
(4a). Oil, H NMR (CDCl3): d = 1.30 (3H, t, J = 3.3 Hz),
4.25 (2H, q, J = 7.3 Hz), 4.38 (1H, d, J = 7.6 Hz), 4.49 (1H,
dd, J = 0.4, 7.6 Hz), 5.12 (1H, d, J = 3.3 Hz), 6.21 (1H, dd,
J = 0.4, 3.3 Hz), 7.39–7.45 (5H, m) ppm.
Methyl 3-((3S,1R)-2,2-dicyano-3-phenylcyclopropyl)-(2Z)-
prop-2-enolate (3b). Trans form: Mp = 53.0–54.0 1C. MS:
m/z = 252 (M+), 220 (M+ ꢀ MeOH), 192, 165, 155, 138, 128,
115. IR: n = 3068, 3028, 2956, 2248, 1722, 1652, 1444, 1216,
1182 cmꢀ1. 1H NMR (CDCl3): d = 3.21 (1H, d, J = 8.3 Hz),
3.83 (3H, s), 4.57 (1H, ddd, J = 0.6, 8.4, 9.2 Hz), 6.00 (1H, dd,
J = 9.2, 11.3 Hz), 6.25 (1H, dd, J = 0.6, 11.3 Hz), 7.32–7.43
(5H, m). 13C NMR (CDCl3): d = 14.9, 32.0, 41.3, 51.9, 112.1,
113.1, 126.1, 128.4, 129.2, 129.5, 138.7, 165.7. Found: C, 71.38;
H, 4.79; N, 11.00; calc. for C15H12N2O2: C, 71.42; H, 4.79; N,
11.10%.
Global electronic indices were calculated by a very simple
operational formula in terms of the one-electron energies of
the HOMO and LUMO frontier molecular orbitals.18
Electrophilic and nucleophilic Fukui functions condensed to
atoms were evaluated from single point calculations performed
on the ground state of molecules at the same level of theory.18
This method evaluated Fukui functions using the gross
charges of the natural population analysis (NPA).
Methyl 3-((1R,3R)-2,2-dicyano-3-phenylcyclopropyl)-(2Z)-
prop-2-enolate (3b0). Cis form: Mp = 49.0–50.0 1C. MS: m/z
= 252, 220, 192, 165, 155, 128, 117. IR: n = 3136, 2960, 2208,
Apparatus. The IR spectra were recorded with KBr tablets
and an NaCl sandwich on a Hitachi I-2000 spectrometer. The
1H and 13C NMR spectra were recorded on Bruker AC-250
and GX-400 spectrometers in deuterated-chloroform.
Tetramethylsilane was used as the internal reference for the
proton spectra. The mass spectra were recorded on a
GCMS-QP2000A spectrometer. Melting points were
determined on a Yanaco micro hot-plate apparatus and are
uncorrected.
1
1734, 1432, 1052, 756. H NMR (CDCl3): d = 3.59 (1H, d, J
= 10.0 Hz), 3.84 (1H, d, J = 10.0 Hz), 3.82 (3H, s), 4.47 (1H,
ddd, J = 0.6, 9.9, 10.0 Hz ), 5.72 (1H, dd, J = 9.9, 11.3 Hz),
6.24 (1H, dd, J = 0.6, 11.3 Hz) 7.35–7.50 (5H, m).
[(5-Methoxy(2-furyl))phenylmethyl]-methane-1,1-dicarbo-
nitrile (4b). Oil, MS: m/z = 252 (M+), 187 (M+ ꢀ CH(CN)2),
1
155, 115. IR: n = 2912, 2256, 1260 cmꢀ1. H NMR (CDCl3):
Materials. b-Cyanostyrene derivatives (2a–2i) were
prepared either according to the Knoevenagel condensation
of benzaldehyde with cyanoalkenes reported by Dornow,29
d = 3.82 (3H, s), 4.38 (1H, d, J = 7.6 Hz), 4.49 (1H, dd,
J = 0.4, 7.6 Hz), 5.12 (1H, d, J = 3.3 Hz), 6.21 (1H, dd,
J = 0.4, 3.3 Hz), 7.39–7.45 (5H, m). 13C NMR (CDCl3): d = 28.8,
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1136 | New J. Chem., 2009, 33, 1127–1138

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46.6, 57.9, 80.5, 111.4, 111.5, 128.3, 128.5, 129.3, 129.4, 134.7,
138.9, 162.1. Found: C, 71.06; H, 4.86; N, 10.87; calc. for
C15H12N2O2: C, 71.42; H, 4.79; N, 11.10%.
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=
156–157 1C. IR: n = 3062, 2833, 2250, 1608, 1573, 1493,
1
1360, 1193, 1102 cmꢀ1. H NMR (CDCl3): d = 3.70 (1H, d,
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10.7 Hz), 6.65 (1H, J = 9.2, 10.7 Hz), 7.8–7.35 (5H, m),
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a
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1138 | New J. Chem., 2009, 33, 1127–1138