1,3-dipolar cycloaddition of 4-chlorobenzonitrile oxide with some dipolarophiles: Theoretical analysis of regioselectivity

Article abstract of DOI:10.1002/jhet.822

Reaction of 4-chlorobenzonitrile oxide () which was generated in situ with acrylo nitrile (3), vinyl acetate () and allyl bromide () as dipolarphile afforded the new cycloadducts, and compounds, respectively. Reactivity and regiochemistry of these reactio

Full text of DOI:10.1002/jhet.822

188
1,3-Dipolar Cycloaddition of 4-Chlorobenzonitrile Oxide with Some
Dipolarophiles: Theoretical Analysis of Regioselectivity
Vol 50
a
b
a
*
Mehdi Bakavoli, Farid Moeinpour, Arezoo Sardashti-Birjandi,
and Abolghasem Davoodniaa
aDepartment of Chemistry, School of Sciences, Islamic Azad University, Mashhad Branch,
Mashhad, Iran
bDepartment of Chemistry, School of Sciences, Islamic Azad University, Bandar Abbas Branch,
Bandar Abbas, Iran
*
E-mail: mbakavoli@yahoo.com
Received November 7, 2010
DOI 10.1002/jhet.822
Published online 7 March 2013 in Wiley Online Library (wileyonlinelibrary.com).
Reaction of 4-chlorobenzonitrile oxide (2) which was generated in situ with acrylo nitrile (3), vinyl acetate
(4) and allyl bromide (5) as dipolarphile afforded the new cycloadducts 6a, 7a, and 8a compounds,
respectively. Reactivity and regiochemistry of these reactions were investigated using activation energy
calculations and density functional theory-based reactivity indexes. The theoretical 13C NMR chemical shifts
of the cycloadducts which were obtained by gauge-invariant atomic orbital method were comparable with
the observed values.
J. Heterocyclic Chem., 50, 188 (2013).
INTRODUCTION
and reactivity differences. Although transition state (TS)
theory remains the most widely used and the most rigorous
approach for the study of the mechanism and the
regiochemistry of these reactions, the localization of TSs
is not always easier.
Furthermore, transition-state calculations are often very
time consuming when bulky substitutents are present in
reactive systems.
Recently, reactivity descriptors based on the density
functional theory (DFT), such as Fukui indexes, local
softnesses and local electrophilicity, have been extensively
used for the prediction of the regioselectivity. For instance,
several treatments of 1,3-DC reactions of nitrile oxides
with various dipolarophiles can be found in the literature
[7–9]. In this context, we became interested in the reactiv-
ity of nitrile oxide (2) as dipole towards acrylonitrile (3),
vinyl acetate (4) and allyl bromide (5) as dipolarophiles to
synthesize the new 3-(4-chlorophenyl)-4,5-dihydroisoxazole-5-
carbonitrile (6a), 3-(4-chlorophenyl)-4,5-dihydroisoxazol-4-yl
The cycloaddition of 1,3-dipolar species to an alkene for
the synthesis of five-membered rings is a classic and
important reaction in organic chemistry [1]. These 1,3-
dipolar cycloaddition (1,3-DC) reactions are used for both
academia and industrial purposes [2]. These reactions are
one of the most important processes with both synthetic
and mechanistic interest in organic chemistry. Current
understanding of the underlying principles in the Diels–
Alder reactions and the 1,3-DC has grown from a fruitful
interplay between theory and experiment [2–4]. The stereo-
chemistry of these reactions may be controlled either by
choosing the appropriate substrates or by controlling the
reaction using a metal complex acting as catalyst [5].
The 1,3-DC reactions possess several interesting charac-
teristics, in particular, regioselectivity. Fleming [6] demon-
strated that the Frontier molecular orbital model (FMO)
seems to be able to explain the observed regioselectivity
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1,3-Dipolar Cycloaddition of 4-Chlorobenzonitrile Oxide with Some Dipolarophiles:
Theoretical Analysis of Regioselectivity
acetate (7a) and 5-(bromomethyl)-3-(4-chlorophenyl)-
4,5-dihydroisoxazole (8a) (Fig. 1). In addition, we found
it worthwhile to analyze the regioselectivity of these 1,
3-DC reactions by several theoretical approaches, namely,
activation energy calculations and DFT-based reactivity
indexes. Finally, the gauge-invariant atomic orbital
(GIAO) method [10] was used to calculate NMR chemical
shifts, to help the experimental cycloadduct determination,
because it has shown to yield data comparable to those of
the experiment [11].
method [15]. The TSs for the 1,3-DC reactions have been
localized at the B3LYP/6-31G (d) level of theory. The sta-
tionary points were characterized by frequency calcula-
tions to verify that the TSs had one and only one
imaginary frequency.
The global electrophilicity for dipole and dipolarophile
was evaluated using Eq. 1 [16]:
μ2
2η
ω ¼
(1)
Regioselectivity criteria for two-center reactions.
A
In Eq. 1 μ and η are the electronic chemical potential and
the chemical hardness of the ground state of atoms and
molecules, respectively.
The electronic chemical potential μ and chemical hard-
ness η were evaluated in terms of the one electron energies
of the FMOs, HOMO, and LUMO, using Eqs. 2 and 3, re-
spectively [17,18]:
useful simplification for the study of the regioselectivity
in reactions may be obtained by looking at those
processes having a markedly polar character, where the
transition structure associated with the rate-determining
step mostly involves the formation of one single bond
between the most electrophilic and other nucleophilic
sites. According to the model recently proposed by
Domingo [12,13], during an electrophile-nucleophile
interaction process, The preferred interaction will be
through the most electrophilic site of the former and the
most nucleophilic site of the latter. Local philicity
indexes (for definition, see computational details) are
therefore expected to be useful descriptor of regional
electrophilicity/nucleophilicity patterns that may account
for the observed regioselectivity in two-center reactions
with a significant polar character.
ðεH þ εLÞ
μ≈
(2)
(3)
2
η≈ðεL−εHÞ
As usual, local indexes are computed in atomic con-
densed form [19]. The well-known Fukui function [20]
for electrophilic (f) and nucleophilic attack (f) have been
evaluated from single point calculations performed at
the ground state of molecules at the same level of the-
ory, using a method described elsewhere [21]. This
method evaluates the Fukui functions using the coeffi-
cients of the FMOs involved in the reaction and the
overlap matrix.
COMPUTATIONAL METHODS
The local electrophilicity index, ωk, condensed to atom k
is easily obtained by projecting the global quantity onto
any atomic center k in the molecule by using the electro-
philic Fukui function (i.e., the Fukui function for nucleo-
philic attack, f [22])
All calculations were performed with the Gaussian98
program suite [14]. For DFT calculations, the B3LYP/6-
31G (d) level of theory was employed. The optimizations
of equilibrium geometries of reactants and products were
performed using the Berny analytical gradient optimization
Figure 1. The regioisomeric pathways for 1,3‐DC of benzonitrile oxide (2) and dipolarophiles (3–5).
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M. Bakavoli, F. Moeinpour, A. Sardashti-Birjandi, and A. Davoodnia
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ωk ¼ ωfkþ
(4)
Table 1
The comparison of theoretical 13C NMR chemical shifts data (δ, ppm) of
C1 and C2 of each pair of regioisomers with those obtained from the
experimental 13C NMR spectroscopy.
Recently, Domingo et al. has introduced an empirical
(relative) nucleophilicity index [23], N, based on the
HOMO energies obtained within the Kohn–Sham scheme,
[18] and defined as EHOMO(Nu) − EHOMO (tetracyanoethy-
lene). This nucleophilicity scale is referred to tetracya-
noethylene taken as a reference.
Atom
number
Calculated
chemical shift
Experimental
chemical shift
Compound
6a
6b
7a
7b
8a
8b
C1
C2
C1
C2
C1
C2
C1
C2
C1
C2
C1
C2
41.3
70.9
56.4
21.3
42.3
98.1
60.8
69.4
39.4
77.6
35.1
75.7
41.0
66.8
Local nucleophilicity [24] index, Nk, was calculated
using the following equation:
41.2
96.0
Nk ¼ Nfk−
(5)
where f is the Fukui function for an electrophilic attack [20].
The 13C NMR chemical shifts were calculated by means
of the GIAO method [10], using the tetramethylsilane as
13C reference, at the B3LYP/6-31G (d) level of theory
(reference value of 13C 184 5307 ppm).
39.5
79.9
correspond to an asynchronous bond formation
processes. The extent of bond formation along a
reaction pathway is provided by the concept of bond
order (BO) [28]. The BO (Wiberg indexes) values of
the O-C and C-C forming bonds at TSs are shown in
brackets in Figure 2. These values are within the range
of 0.16 to 0.32. Therefore, it may be suggested that
these TSs correspond to early processes. In general,
the asynchronicity shown by the geometrical data is
RESULTS AND DISCUSSION
Nitrile oxide 2 was generated in situ from the oxime 1 in
a mixture of aqueous NaClO in THF. 1,3-DC of nitrile ox-
ide 2 with the dipolarophiles acrylonitrile 3, vinyl acetate 4
and allyl bromide 5 proceeded smoothly in a selective
manner to give a single regioisomer of each pair 6a-b,
7a-b, and 8a-b in very good yields (Fig. 1). It should be
noted that the absence of the nitrile absorption band in
the IR spectrum of the cycloadduct 6a, is the reminiscent
of the previously reported observations with the aliphatic
nitriles which are activated by a nitrogen or an oxygen
atom in their β-position [25–27]. The assignment of the
regiochemistry of these products was based upon (i) com-
paring the theoretical 13C NMR spectral data obtained by
GIAO method with the observed values for both regioi-
somers; (ii) activation energy calculations; and (iii) DFT-
based reactivity indexes.
Table 2
Energies of reactants, transition states and cycloadducts 6, 7, and 8, E
(hartree), relative activation energies ΔEa (kcal mol−1) and relative ener-
gies between products and reactants, ΔEr (kcal mol−1).
a
E
ΔEa
ΔEra
Reaction b
Nitrile oxide 2
Acrylonitrile 3
TS 6a
TS 6b
6a
−859.23462
−170.83155
−1030.04639
−1030.04560
−1030.12341
−1030.12172
i For further cycloadduct characterization, we obtained
the theoretical 13C chemical shifts values for the pro-
ducts through the GIAO method and compared it with
the observed values. As it can be seen in Table 1, the
observed values for C1 and C2 in each of the isolated
products (41.0, 66.8; 41.2, 96.0, and 39.5, 79.9 ppm
in compounds 6, 7, and 8, respectively) are in close
proximity to the theoretical values for compounds 6a,
7a, and 8a. It seems likely that the isolated regioi-
somers are structurally similar to 6a, 7a and 8a. Further
proofs came from activation energy and DFT studies
as followings:
ii Activation energy calculations: The TSs have been lo-
calized for both cyclization modes. The corresponding
activation energies and structures are given in Table 2
and Figure 2, respectively. An analysis of the geome-
tries at the TSs given in Figure 2 indicates that they
12.4
12.9
−35.9
−34.8
6b
Reaction b
Nitrile oxide 2
Vinyl acetate 4
TS 7a
TS 7b
7a
−859.23462
−306.47987
−1165.68492
−1165.67990
−1165.77693
−1165.76849
18.6
21.7
−39.2
−33.9
7b
Reaction c
Nitrile oxide 2
Allyl bromide 5
TS 8a
TS 8b
8a
−859.23462
−2689.00537
−3548.22689
−3548.22220
−3548.31542
−3548.30911
8.2
11.2
−47.3
−43.4
8b
aTo reactants; ΔEa = ETS – (Enitrile oxide + Edipolarophile); ΔEr = Eproduct
(Enitrile oxide + Edipolarophile
–
)
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1,3-Dipolar Cycloaddition of 4-Chlorobenzonitrile Oxide with Some Dipolarophiles:
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191
Figure 2. Optimized geometries for transition state structures at the B3LYP/6‐31G (d) level of theory. Hydrogen atoms have been omitted for clarity.
Distances of forming bonds are given in angstroms. The bond orders are given in brackets.
accounted for by the BO values. A qualitative
reactivity can be estimated by applying Hammond’s
postulate [29]. All of the reactions proceeded exother-
mically with large ΔEr (relative energies between
products and reactants) energy values. According to
Hammond’s postulate, the TSs should then be closer
to the reactants. The activation energy values, ΔEa,
also favor the formation of the cycloadducts 6a, 7a,
and 8a against their regioisomers 6b, 7b, and 8b,
respectively.
As it can be seen in Table 3, the electronic chemical
potential of dipolarophiles vinyl acetate and allyl bromide
is greater than that of nitrile oxide (dipole), which indicates
the charge transfer is taking place from dipolarophiles to
nitrile oxide. The electronic chemical potential of dipolaro-
phile acrylonitrile is smaller than that of nitrile oxide which
indicates the charge transfer is taking place from nitrile
oxide to dipolarophile. Consequently, the nitrile oxide
can act as electerophile in reactions a and c and in reaction
b acts as nucleophile. In Figure 3, we have reported the
values of local electrophilicities ωk for atoms O1 and C3
of the dipole (nitrile oxide) and the local nucleophilicities
Nk for atoms C4 and C5 of dipolarophiles in 7 and 8. For
6 the values of Nk for atoms O1 and C3 of dipole (nitrile
oxide) and ωk for atom C4 and C5 of dipolarophile (acry-
lonitrile) were reported. As it can be seen in Figure 3,
according to the Domingo’s model [23,24], in the reaction
between nitrile oxide 2 and acrylonitrile 3, the most favor-
able two-center interaction takes place between C3 of acry-
lonitrile and O1 of the nitrile oxide leading to the formation
iii DFT-based reactivity indexes: The chemical hardnesses
η, global electrophilicity ω and global nucleo-
philicity N of the nitrile oxide and dipolarophiles
are given in Table 3. The Fukui indexes for the
atoms O1 and C3 of the dipole (nitrile oxide) and
for the atoms C4 and C5 of the dipolarophiles (acry-
lonitrile, methyl methacrylate and allyl bromide),
local electrophilicity ωk and local nucleophilicity
Nk values are given in Table 4 (see Fig. 3 for atom
numbering).
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Table 3
HOMO, LUMO energies in a.u., electronic chemical potential (μ in a.u.), chemical hardness (η, in a.u.), global electrophilicity (ω, in eV) and global
nucleophilicity (N, in eV) for dipole and dipolarophile systems.
HOMO
LUMO
μ
η
ω
Na
Dipole(nitrile oxide)
−0.238
−0.289
−0.249
−0.264
−0.059
−0.056
−0.010
−0.004
−0.148
−0.173
−0.130
−0.134
0.178
0.233
0.239
0.260
1.68
1.74
0.96
0.93
2.65
1.25
2.33
1.94
Dipolarophil(acrylonitrile)
Dipolarophile (vinyl acetate)
Dipolarophile (allyl bromide)
aThe HOMO energy of tetracyanoethylene is −0.3351 a.u. at the same level of theory.
with theoretical data. Oxime 1 the precursor of nitrile oxide 2 is
a known compound was prepared according to generally used
methods [30].
of the cycloadduct 6a. It is noteworthy that during the reac-
tion of nitrile oxide 2 with vinyl acetate 4 and allyl bromide
5, the most favorable interactions take place between O1 of
the nitrile oxide and C4 of the vinyl acetate and C5 of the
allyl bromide, respectively. These results clearly indicate
that the two-center polar model, based on electrostatic
charges, predict correctly the experimental regioselectivity.
In conclusion, the regioselectivity for the 1,3-DC reac-
tions of nitrile oxide 2 with acrylonitrile 3, vinyl acetate
4 and allyl bromide 5 has been investigated using experi-
mental and theoretical 13CNMR studies together with the
activation energy calculations and the DFT-based reactiv-
ity indexes at the B3LYP/6-31G(d) level of theory. The
results obtained in this work clearly predict the regiochem-
istry of the isolated cycloadducts.
General procedure for the synthesis of cycloadducts. To a
mixture of 1 mmol of oxime and 1.5 mmol of dipolarophile in
tetrahydrofuran (THF) (25 mL) was added aqueous NaOCl
(11% Cl2 content, 1.62 mL, 2.50 mmol) at 5°C. The reaction
was stirred for 10 min, then at r.t. for 24 h. After that, H2O (30
mL) was added, the organic layer separated and remaining
product extracted from the aqueous layer using CHCl3 (15 mL).
The combined organic layers were washed with H2O (20 mL),
dried (Na2SO4) and the solvent was removed under reduced
pressure. The crude product was purified by precipitation with
cold Et2O to afford cycloadducts as white powders.
3-(4-Chlorophenyl)-4,5-dihydroisoxazole-5-carbonitrile (6a). This
compound was obtained as white powder (80%). m.p. 126-128°C. IR
(KBr) (νmax/cm−1): 1598. MS (EI, 70 eV) m/z: 206 (M+), 208 (M+ + 2).
1H NMR (400 MHz, CDCl3): δH 3.71(1H, dd, CHAHB, J = 6.40 Hz,
J = 10.40 Hz), 3.78 (1H, dd, CHAHB, J = 6.00 Hz, J = 10.80 Hz),
5.41 (1H, dd, CH, J = 6.40 Hz, J = 4.40 Hz), 7.43 (2H, d, CH-Ar, J
= 8.40 Hz), 7.61 (2H, d, CH-Ar, J = 8.80 Hz). 13C NMR (CDCl3),
δC: 41.0, 66.8, 117.0, 125.9, 128.3, 129.4, 137.4, 155.5. Anal. Calcd
for C10H7ClN2: C, 58.13; H, 3.41; N, 13.56%. Found: C, 58.23; H,
3.46; N, 13.37%.
EXPERIMENTAL
The melting points were recorded on an Electrothermal type
1
9100 melting point apparatus. The H NMR (400 MHz) spectra
were recorded on a Bruker AC 400 spectrometer. 13C NMR spec-
tra were determined using the Bruker AM-400 instrument operat-
ing at 100 MHz. NMR spectra were obtained on solutions in
CDCl3 using tetramethylsilane as internal standard. IR spectra
were determined as KBr pellets on a Bruker model 470 spectro-
photometer. The mass spectra were scanned on a Varian Mat
CH-7 instrument at 70 eV. Elemental analyses were carried out
on Eager CHNS-O 300 apparatus and were in good accordance
3-(4-Chlorophenyl)-4,5-dihydroisoxazol-4-yl acetate (7a). This
compound was obtained as white powder (85%). m.p. 90-91°C. IR
(KBr) (νmax/cm−1): 1750, 1598. MS (EI, 70 eV) m/z: 239 (M+), 241
Table 4
Fukui indexes for the O1 and C3 atoms of the nitrile oxide, for atoms C4
and C5 of the dipolarophiles, local electrophilicities, ωk, (eV) and local
nucleophilicities, Nk, (eV).
Atom number fkþ
fkꢀ
ωk
Nk
Nitrile oxide
Acrylonitrile
Vinyl acetate
Allyl bromide
O1
C3
C4
C5
C4
C5
C4
C5
0.077 0.325 1.176 0.861
0.056 0.139 0.582 0.370
0.469
0.265
0.816
0.461
0.470
0.281
0.006
0.025
1.095
0.655
0.012
0.049
Figure 3. Illustration of the favorable interactions using local nucleophi-
licities, Nk, for dipolaropile centers (in 7 and 8; for 6 local electrophilici-
ties ωk) and local electrophilicities, ωk, for the nitrile oxide centers (for six
local nucleophilicities, Nk).
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193
(M+ + 2). 1H NMR (400 MHz, CDCl3): δH 2.10 (3H, s, CH3), 3.34
(1H, dd, CHAHB, J = 1.60 Hz, J = 16.40 Hz), 3.61 (1H, dd,
CHAHB, J = 6.80 Hz, J = 10.80 Hz), 6.85 (1H, dd, CH, J = 1.60
Hz, J = 5.20 Hz), 7.42 (2H, d, CH-Ar, J = 8.40 Hz), 7.66 (2H, d,
CH-Ar, J = 8.40 Hz). 13C NMR (CDCl3), δC: 21.0, 41.2, 96.0,
126.8, 128.3, 129.2, 136.9, 156.1, 169.7. Anal. Calcd for
C11H10ClNO3: C, 55.13; H, 4.21; N, 5.84%. Found: C, 54.73; H,
4.27; N, 6.73%.
5-(Bromomethyl)-3-(4-chlorophenyl)-4,5-dihydroisoxazole
(8a). This compound was obtained as white powder (83%). m.
p. 97-98°C. IR (KBr) (νmax/cm−1): 1600. MS (EI, 70 eV) m/z:
273 (M+), 275 (M+ + 2). 1H NMR (400 MHz, CDCl3): δH
3.31(1H, dd, CHAHB, J = 6.40 Hz, J = 10.40 Hz), 3.44 (1H,
dd, CHAHB, J = 8.40 Hz, J = 2.40 Hz), 3.51 (1H, dd, CHAHB,
J = 10.40 Hz, J = 6.80 Hz), 3.60 (1H, dd, CHAHB, J = 4.40
Hz, J = 6.00 Hz), 5.04 (1H, m, CH), 7.40 (2H, d, CH-Ar,
J = 8.80 Hz), 7.62 (2H, d, CH-Ar, J = 8.80 Hz). 13C NMR
(CDCl3), δC: 33.4, 39.5, 79.9, 127.5, 128.0, 129.1, 136.4,
155.2. Anal. Calcd for C10H9BrClNO: C, 43.75; H, 3.30; N,
5.10%. Found: C, 43.72; H, 3.30; N, 5.22%.
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