Synthesis and structural characterization of new cu(I) complexes with the antithyroid drug 6-n-propyl-thiouracil. Study of the cu(I)-catalyzed intermolecular cycloaddition of iodonium ylides toward benzo[b]furans with pharmaceutical implementations

Article abstract of DOI:10.1021/ic3014255

The reaction of copper(I) iodide with 6-n-propylthiouracil (ptu) in the presence or absence of the triphenylphosphine (tpp) or tri(p-tolyl)phosphine (tptp) in a 1:1:2 molar ratio forms the mixed ligand Cu(I) complex with formula [CuI(ptu)₂](tol

Full text of DOI:10.1021/ic3014255

Article
pubs.acs.org/IC
Synthesis and Structural Characterization of New Cu(I) Complexes
with the Antithyroid Drug 6‑n‑Propyl-thiouracil. Study of the
Cu(I)-Catalyzed Intermolecular Cycloaddition of Iodonium Ylides
toward Benzo[b]furans with Pharmaceutical Implementations
K. Paizanos,† D. Charalampou,† N. Kourkoumelis,‡ D. Kalpogiannaki,§ L. Hadjiarapoglou,§
A. Spanopoulou,† K. Lazarou,† M. J. Manos,⊥ A. J. Tasiopoulos,⊥ M. Kubicki,∥ and S. K. Hadjikakou*,†
†Section of Inorganic and Analytical Chemistry, Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece
‡Medical Physics Laboratory, Medical School, University of Ioannina, Greece
§Section of Organic Chemistry, Department of Chemistry, University of Ioannina, Greece
⊥Department of Chemistry, University of Cyprus, 1678 Nicosia, Cyprus
∥Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
S
* Supporting Information
ABSTRACT: The reaction of copper(I) iodide with 6-n-propylthiouracil (ptu) in the presence or absence of the
triphenylphosphine (tpp) or tri(p-tolyl)phosphine (tptp) in a 1:1:2 molar ratio forms the mixed ligand Cu(I) complex with
formula [CuI(ptu)2](toluene) (1), [CuI(tpp)2(ptu)] (2), and [CuI(tptp)2(ptu)] (3). The complexes have been characterized by
1
FT-IR, H NMR, UV−vis, spectroscopic techniques, and single crystal X-ray crystallography. Two sulfur atoms from two ptu
ligands and one iodide form a trigonal geometry around the metal center in 1. Intramolecular interactions through hydrogen
bonds lead to a bend ribbon polymeric supramolecular architecture with zigzag conformation. Two phosphorus atoms from two
arylphosphines, one sulfur atom, and one iodide anion form a tetrahedron around the copper ion in case of 2 and 3.
Intramolecular hydrogen bonding interactions lead to dimerization. Complexes 1−3 and the already known ones with formulas,
[(tpSb)2Cu(μ2-I)2Cu(tpSb)2] (4) (tbSb = triphenylstibine), [(tpp)Cu(μ2-I)2Cu(tpp)2] (5), [(tpp)Cu(μ2-Cl)2Cu(tpp)2] (6),
[CuCl(tpp)3·(CH3CN)] (7), and [AuCl(tpp)] (8), were used to study their catalytic activity on the intermolecular cycloaddition
of iodonium ylides toward benzo[b]furans formation. The results show that both the metal and the ligand type affect the catalytic
continued...
Received: July 5, 2012
Published: October 30, 2012
© 2012 American Chemical Society
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affinity of the complexes. The highest yield of benzo[b]furan was derived when complexes 2, 3, and 4 were used as catalysts. The
mechanism of the Cu(I)-catalyzed and uncatalyzed intramolecular cycloaddition of iodonium ylide has been also thoroughly
explored by means of ab initio electronic structure calculation methods, and the results are compared with the experimental ones.
INTRODUCTION
and [CuI(tptp)2(ptu)] (3) (ptu = 6-n-propylthiouracil, tpp =
■
triphenylphosphine, and tptp = tri(p-tolyl)phosphine (Scheme 2)).
Benzofurans and their derivatives have demonstrated a wide
range of applications in modern pharmaceutical research.1 Benzo-
furans are used as either pharmaceutical agents or basic precursors
in the synthesis of important drugs such as antiallergic, anti-
microbial, or anticancer drugs.2−4 Recently, they have been studied
as anti-HIV3,4 agents and β-amyloid aggregation inhibitors.1a,5
Therefore, the development of novel synthetic routes for
benzofurans is of great interest. Toward this direction, the reaction
of phenyliodonium dimedonate with unsaturated reagents for the
formation of heterocyclic benzofurans (containing 5-member
ring) (Scheme 1) was investigated since the late 80s.6 The product
(benzofurans) formation was found to depend upon irradiation,
heating, and the presence of copper acetate (Scheme 1; reactions 1
and 2).6 Products were obtained in relatively high yield.6
With no catalyst present an intramolecular rearrangement in
phenyliodonium ylides might be also occurred leading to the
formation of iodoether (Scheme 1; reaction 3). This was
investigated theoretically in the case of intramolecular rearrange-
ment in phenyliodonium ylides of hydroxyquinones.7 It has been
proposed that phenyliodonium ylides of hydroxyquinones easily
undergo intramolecular rearrangements associated with phenyl-
group migration or ketene formation.7 Phenyl- group migration
is favorable thermodynamically by heating the reaction mixture.
The influence of the catalyst upon the product formation is still a
matter of investigation since the development of new catalyst for
the synthesis of new benzofuran derivatives is of great impor-
tance. Thus, several catalysts and catalytic systems such as
[Rh(CO)2acac],8a [Rh2(OAc)4],8b [Pd(OAc)2]/[CuI],8c
[Pd(PPh3)4],8d [Cu(OTf)2],8e [Cu(phen)(PPh3)]NO3,8e
[Ph3PAuNTf2],8f [MePosAuCl]/AgNTf2,8f etc. have been
used with various degrees of success in yield and time and cost
effectiveness. Copper based catalysts efficiently indulge these
requirements.8e
1
The complexes have been characterized by FT-IR, H NMR,
UV−vis, spectroscopic techniques, and single crystal X-ray cry-
stallography. Complexes 1−3 and the already known, [(tpSb)2-
Cu(μ2-I)2Cu(tpSb)2] (4) (tbSb = triphenylstibine), [(tpp)Cu-
(μ2-I)2Cu(tpp)2] (5), [(tpp)Cu(μ2-Cl)2Cu(tpp)2] (6), [CuCl-
(tpp)3·(CH3CN)] (7), and [AuCl(tpp)] (8),13,14 were used to
study their catalytic activity on the intermolecular cycloaddition
of iodonium ylides toward benzo[b]furans formation. The cata-
lytic activity on the intermolecular cycloaddition of iodonium
ylides toward benzo[b]furans formation caused by copper(II)
ions [Cu(acac)2]2+ is already reported.6 In order to ascertain
whether this catalytic activity is a redox process or not, copper(I)
iodide complexes 1−3 were designed and developed. Thus,
copper(I) iodide compounds of ptu with and without tpp or tptp
were used to ensure the Cu(I) oxidation state of the complex.
Copper(I) complexes of thioamides with and without
phosphines are known to exhibit structural diversity and often
form oligomers and polymers with various coordination network
types9 allowing the structure−activity relationship (SAR) study
of these compounds. Thereby, new, more efficient catalysts can
be developed for this process.
RESULTS AND DISCUSSION
■
General Aspects. Complexes 1−3 have been prepared by
reacting copper(I) iodide with 6-n-propylthiouracil (ptu) in the
presence or absence of the triphenylphosphine (tpp) or tri(p-
tolyl)phosphine (tptp) in 1:1:2 molar ratio in methanole/
acetonitrile 1:1 solution (Scheme 3). Crystals of complexes 2−3
have been prepared by slow evaporation of the solutions which
remain after the filtration of the reaction solutions. In the case of
1, crystals of the name compound were grown after recrystalliza-
tion from toluene.
Copper(I) ion, on the other hand, is an important metal ion
which has a strong tendency to form covalent bonds with ligands
containing S or P donor atoms.9 Copper compounds often show
short Cu−Cu contacts (known as cuprophilicity)9 which are less
than twice the van der Waals radius of Cu (2.00−2.27 Å).10 This
results in the formation of oligomers and polymers with various
coordination network types, leading to a new type of aromaticity,
due to a cyclical delocalization of d as well as (d−p) π-type orbital
electron density instead of the usual p orbitals on metal−ligand
rings,11 which may introduce greater stability to the higher-order
structures. The ligand 6-n-propyl-2-thiouracil (ptu) is among the
most commonly employed antithyroidal drugs in use, for the
treatment of hyper-thyroidism (Graves′ disease).12a It is found to
inhibit the formation of 3,5,3′-triiodothyronine (T3) and
3,5,3′,5′-tetraiodothyronine (T4) hormones, by blocking the
metabolism of iodine.12b The use of the thioamide ptu as ligand
in the synthesis of the Cu(I) catalysts 1−3 was chosen because
(i) ptu covers the chemical demands of the soft acid, copper(I)
ion, for soft ligands (sulfur donor atoms), (ii) it stabilizes the
copper(I) oxidation state, and (iii) it allows the use of the
product as catalyst for pharmaceutical processes since ptu is
already a drug in use.
The crystals of complexes are air stable when they were kept in
darkness at room temperature. The formulas of the complexes
were first deduced from their m.p., their spectroscopic data, and
single-crystal X-ray crystallography at ambient conditions.
Compounds 4−8 were prepared by reacting copper(I) halide
with tpp, tptp, or trriphenylstibine (tpSb) in proper molar ratios
(1:2 (4), 2:3 (5,6), 1:3 (7), and 1:1 (8)) in methanole/
acetonitrile 1:1 solution. Crystals of complexes 4−8 were
analyzed by X-ray diffraction crystallography, and they were
found to be identical with those already known.13,14 Small
differences in the unit cell parameters were only observed in case
4 where a = 24.4463(6), b = 13.9088(3), c = 20.2168(5), and β =
111.241(3)° in contrast to those already reported in ref 13a (a =
24.620(9), b = 14.090(4), c = 20.382(9), and β = 110.65(3)°).
Scheme 4 shows the molecular formulas of complexes 4−8.
Crystal and Molecular Structures of [CuI(ptu)2]-
(toluene) (1), [CuI(tpp)2(ptu)] (2), and [CuI(tptp)2(ptu)]
(3) Complexes. The structures of complexes 1−3 were
determined by X-ray diffraction at 100(2) K. Molecular diagrams
of complexes 1−3 are shown in Figures 1−3, while selected bond
distances and angles are given in Table 1.
Complexes 1−3 are monomers. Two sulfur atoms from two
ptu ligands and one iodide ion are coordinated to Cu(I) cation
forming a planar trigonal arrangement in case of 1 (Figure 1A).
Here, we report the synthesis of new copper(I) complexes
with formulas: [CuI(ptu)2](toluene)] (1), [CuI(tpp)2(ptu)] (2),
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Scheme 1
Scheme 2
Scheme 3
Two phosphorus from two tpp (2) or tptp (3) ligands, one
iodide anion, and one sulfur atom from a ptu ligand are
coordinated to the metal center in case of 2, 3 leading to a
tetrahedral arrangement around the copper(I) ion (Figures 2
and 3).
Structures ofcopper(I)iodide complexes with thioamides having
trigonal geometry are rare. These include [Cu(totp)(tzdtH)I]
(tzdtH = 1,3-thiazolidine-2-thione. totp = triortho-tolylphosphi-
ne),15a [Cu(dmimtH)2I] (dmimtH = 1,3-dimethylimidazole-2-
thione),15b [Cu(ettu)2I] (ettu = N,N′-ethylenethiourea-S),15c
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Scheme 4
Figure 1. (A) Molecular diagram of compound 1 together with the atomic numbering scheme. (B) Intramolecular hydrogen bonting interactions lead to
a bending 1D ribbon polymeric supra-molecular architecture with zigzag conformation.
[Cu(tohmim)2I] (tohmim = 2-thioxohexamethyleneimine),15d
[Cu(L)2I] (L = 3-propylimidazolidine-2-thione),15e and
[Cu(L)2I] (L = 3N-propylimidazolidine-2-thione).15f Exam-
ples of tetrahedral copper(I) iodide complexes with phos-
phines and thioamides on the other hand, include [CuI-
(PPh3)2(bzimtH2)] (bzimtH2 = benz-1,3-imidazole-2-thione,
PPh3 = triphenyphosphine),9b [CuI(PPh3)2(bztztH)]
(bztztH = benz-1,3-thiazole-2-thione) (2.3660(6) Å),9b
[Cu(PPh3)2(pymtH)I] (pymtH = pyrimidine-2-thione),16a
[CuI(PPh3)2(mmi)] (mmi = 1-methylimidazole-2-thio-
ne),16b and [CuI(PPh3)2(imtH)] (imtH = imidazoline-2-
thione).16c
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Figure 2. (A) Molecular diagram of compound 2 together with the atomic numbering scheme. (B) Intramolecular hydrogen bonding interactions lead
to a dimerization.
The two Cu−S bond distances, in complex 1, are 2.237(2) and
2.238(2) Å, respectively, which are in accordance to the
corresponding ones found in copper(I)-thioamide complexes
with trigonal geometry an in [Cu(totp)(tzdtH)I] (2.254(2) Å),15a
in [Cu(dmimtH)2I] (2.234 Å),15b [Cu(ettu)2I] (2.239 Å),15c
[Cu(tohmim)2I] (2.238 and 2.248 Å),15d and [Cu(L)2I] (2.227
and 2.240 Å).15e However, these Cu−S bond lengths are shorter
than the corresponding ones found in mixed ligand copper(I)
iodide complexes with tetrahedral geometry around the metal
center as in [CuI(PPh3)2(bzimtH2)] (2.3692(9) Å),9b in [CuI-
(PPh3)2(bztztH)] (2.3660(6) Å),9b in [Cu(PPh3)2(pymtH)I]
(2.338(4) Å),16a in [CuI(PPh3)2(mmi)] (2.369 Å),16b and
[CuI(PPh3)2(imtH)] (2.344 Å).16c
in [Cu(dmimtH)2I] (2.574 Å) [15b], [Cu(ettu)2I] (2.555 Å),15c
[Cu(tohmim)2I] (2.590 Å),15d and [Cu(L)2I] (2.531 Å),15e
but shorter than those found in the complexes with tetrahedral
geometry: [CuI(PPh3)2(bzimtH2)] (2.6901(6),9b [CuI-
(PPh3)2(bztztH)] (2.6807(3) Å),9b [Cu(PPh3)2(pymtH)I]
(2.674(2) Å),16a in [CuI(PPh3)2(mmi)] (2.681 Å),16b and [CuI-
(PPh3)2(imtH)] (2.673 Å).16c
The bond angles around the metal center (I1−Cu1−S1 =
124.19(6), I1−Cu1−S2 = 124.83(6), S1−Cu1−S2 =
110.98(8)°) (Figure 1A) varied from the ideal value of 120°
due to different valence shell electron pair repulsions (VSEPR).
This also explains why the I1−Cu1−S bond angles are higher
compared to the S1−Cu1−S2 since the sulfur electronegativity is
higher to the one of iodine.
The Cu−I bond distance in 1 is 2.5681(11) Å which is close
with the Cu−I bond distance found in [Cu(totp)(tzdtH)I]
complex (2.563(l) Å) of trigonal geometry around copper(I) ion15a
Strong intramolecular hydrogen bonding interactions
N1[H22]••O2 (2.748(8) Å) and N3[H24]••O1 (2.828(8) Å)
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Figure 3. (A) Molecular diagram of compound 3 together with the atomic numbering scheme. (B) Intra- and intermolecular hydrogen bonds form a
dimer with almost planar core.
lead to a bending 1D ribbon polymeric supramolecular architec-
ture with zigzag conformation (Figure 1B).
(2.6468(4) Å (2), 2.6660(12) Å (3)) are in accordance with the
corresponding ones found in [CuI(PPh3)2(bzimtH2)]
(2.6901(6),9b [CuI(PPh3)2(bztztH)] (2.6807(3) Å),9b and
[Cu(PPh3)2(pymtH)I] (2.674(2) Å).15b The Cu−P bonds,
2.2917(8) and 2.2840(7) Å (2) and 2.292(2) and 2.263(2) Å (3), are
close to those found in [CuI(PPh3)2(bzimtH2)] (2.2839(10) and
2.2796(9) Å),9b [CuI(PPh3)2(bztztH)] (2.2822(6) and 2.2796(6)
Å),9b [Cu(PPh3)2(pymtH)I] (pymtH = pyrimidine-2-thione)
The Cu−S bond distances in complexes 2−3 (2.3754(8) Å
(2), 2.368(2) Å (3)) are in the range of the distances found
in other copper(I) iodide complexes: 2.3692(9) Å, in [CuI-
(PPh3)2(bztztH)] (bztztH = benz-1,3-thiazole-2-thione)
(2.3660(6) Å),9b and in [Cu(PPh3)2(pymtH)I] (pymtH =
pyrimidine-2-thione) (2.338(4) Å).15b The Cu−I bond lengths
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spectra of ylide and iodoether, respectively, are attributed to the
proton of the methyl groups which are shifted at 1.16 ppm in
the spectrum of benzo[b]furnan. The presence of both signals at
1.16 and 1.06 ppm in the reaction mixture is an indication of
benzo[b]furnan product and iodoether as well.
Table 1. Selected Bond Lengths (Å) and Angles (deg) of
Complexes 1−3
complex 1
complex 2
complex 3
(a) bond lengths
(a) bond lengths
(a) bond lengths
I1−Cu1
2.5681(11)
I1−Cu1
2.6468(4)
2.3754(8)
2.2917(8)
2.2840(7)
1.680(3)
I1−Cu1
2.6660(12)
The results show that copper(I) complexes (1−7) exhibit
higher catalytic activity than the corresponding one of gold(I)
(8). Among copper(I) complexes, the dimer 4 and the mono-
mers 2−3 lead to the formation of benzo[b]furan in higher
than 30% yield (2 26−33%; 3 22−33%; and 4 30−41%).
Furthermore, both monomeric or dimeric copper(I) complexes
with tetrahedral geometry around the metal center exhibit strong
catalytic activity. Complexes 1−5 which contains iodo anion
show better activity than the corresponding ones with chloride
(6−8). Moreover, the kind of the ligand, affects on the catalytic
activity of the complexes. Therefore, the synergistic effect of
triphenystibine and iodide or thiarylphosphine and ptu ligands
increases the catalytic ability of the complexes.
Although the high toxicity of antimony(III) compounds pro-
hibits their usage as catalysts for pharmaceutical processes,
complex 4 is studied for its catalytic activity in order to draw
conclusion about the effect of the chemical composition of the
ligands. Since complex 4 showed the higher catalytic activity it
was used for detailed studies. Table 3 summarized the results.
Although the amount of banzo[b]furan yielded remains un-
changed when the amounts of catalyst and ylide are constant and
the amount of styrene is decreased, the amount of iodoether
derived is significantly lower. By increasing the amount of catalyst
the amount of benzo[b]furan also increases as expected.
Cu1−S1
Cu1−S2
S1−C1
S2−C8
O1−C2
O2−C9
2.237(2)
2.238(2)
1.680(7)
1.690(7)
1.236(8)
1.199(9)
Cu1−S1
Cu1−P1
Cu1−P2
S1 -C37
Cu1−S1
Cu1−P1
Cu1−P2
S1−C43
2.368(2)
2.292(2)
2.263(2)
1.672(9)
(b) angles
I1−Cu1−S1 124.19(6) I1−Cu1−S1 111.95(2) I1−Cu1−S1 111.79(7)
I1−Cu1−S2 124.83(6) I1−Cu1−P1 111.30(2) I1−Cu1−P1 99.64(7)
(b) angles
(b) angles
S1−Cu1−S2 110.98(8) I1−Cu1−P2 107.97(2) I1−Cu1−P2 111.78(7)
S1−Cu1−P1 107.00(3) S1−Cu1−P1 102.04(9)
S1−Cu1−P2 103.17(3) S1−Cu1−P2 103.01(8)
P1−Cu1−P2 115.24(3) P1−Cu1−P2 128.11(9)
hydrogen bonding interactions
N1[H22]••O2 2.748(8) N2[H44]••O1 2.794(3) N2[H51]••O1 2.818(9)
N3[H24]••O1 2.828(8)
(2.296(4) and 2.303(4) Å),15a and [Cu(totp)(tzdtH)I] (with
trigonal geometry) where the Cu−P bond distance is 2.269(2) Å.15a
Strong intramolecular hydrogen bonding interactions
(N2[H2]••O1 = 2.794(3) Å (2) and N2[H51]••O1 =
2.818(9) Å (3)) lead to dimerization (Figures 2B and 3B).
Catalysis. Complexes 1−8 were tested for their catalytic
activity upon the reaction of phenyliodonium dimedonate, with
phenyl-ethylene (styrene), for the formation of heterocyclic
benzo[b]furans. Styrene was also the solvent media. The reaction
mixture was refluxed at 114−118 °C under aerobic conditions for
3.5−4 min. The benzo[b]furan yielded was detected by the mean
of HPLC (Table 2). The progress of the reaction is also
Computational Studies. The diverse behavior of phenyl-
iodonium dimedonate in respect to its thermal transformation
yielding either benzofuran (reaction 1) or iodoether (reaction 3)
has motivated us to use theoretical calculations to understand
the thermodynamics involved in these mechanisms. The poten-
tial energy reaction profiles for reaction 1, at the HF/3-21G*
level, are depicted in Figure 5. The first reaction step involves
the dissociation of iodobenzene (PhI) molecule from the
(4,4-dimethyl-2,6-dioxocyclohexyl)(phenyl)iodonium via the
1
1
monitored by H NMR spectroscopy. Figure 4 shows the H
NMR spectra of ylide, styrene, iodoether, benzo[b]furan, and the
reaction mixture of ylide with styrene catalyzed by complex 6.
The resonance signals at 1.06 and 1.05 ppm observed in the
Table 2. Catalytic Activity of 1−8 upon the Reaction of Phenyliodonium Dimedonate, with Styrene, for the Formation of
Heterocyclic Benzo[b]furans and Iodoether
a
b
amount of
catalyst (g)
reaction time
(min)
styrene catalyst/ylide/styrene molar benzo[b]furan (B)
iodoether (I)
(area, %)
catalyst
MW
ylide (g)
(g)
ratio
(area, %)
ratio (B/I)
1
2
2
2
3
3
3
4
4
4
5
5
6
6
7
7
8
622.91
884.68
884.68
884.68
968.68
968.68
968.68
1793.18
1793.18
1793.18
1166.9
1166.9
984
0.00069
0.00232
0.00200
0.00088
0.00381
0.00197
0.00119
0.00371
0.00108
0.00452
0.00154
0.00424
0.00484
0.00341
0.00395
0.00253
0.00160
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
0.3099
0.3042
0.2965
0.3029
0.3451
0.3146
0.3118
0.3324
0.3326
0.3329
0.2980
0.2921
0.3120
0.3311
0.2663
0.3536
0.3249
0.5064
0.5039
0.5261
0.5000
0.5144
0.5160
0.5188
0.5198
0.5381
0.5090
0.4972
0.4986
0.5346
0.5534
0.5442
0.5135
0.5302
0.00111/0.90614/4.86923
0.00262/0.88947/4.84519
0.00226/0.86696/5.05865
0.00099/0.88567/4.80769
0.00393/1.00906/4.94615
0.00203/0.91988/4.96154
0.00123/0.91170/4.98846
0.00207/0.97193/4.99808
0.00060/0.97251/5.17404
0.00252/0.97339/4.89423
0.00132/0.87135/4.78077
0.00363/0.85409/4.79423
0.00492/0.91228/5.14038
0.00347/0.96813/5.32115
0.00446/0.77865/5.23269
0.00286/1.03392/4.93750
0.00324/0.95000/5.09808
27.3
25.7
32.7
27
20.2
24.9
12.8
10.3
26.0
19.3
21
1.35
1.03
2.55
2.62
0.85
1.31
1.57
1.89
3.3 (4)
5.6 (6)
0.3
22.0
25.2
33
41.3
36.5
30.4
13.3
26.6
21.7
25.6
18.8
26.2
8.9
21.8
11.1 (9)
5.4 (5.0)
45.7
31.5
30.2
32.0
37.1
32.1
85
0.8
0.7
984
0.8
885
0.5
885
0.8
494.42
0.10
a
b
Retention time is 10.4 0.4 min at λmax = 275 3 nm. Retention time is 16.7 0.5 min at λmax = 276 3 nm.
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Figure 4. 1H NMR spectra of ylide, styrene, iodoether, benzo[b]furan, and the reaction mixture of ylide with styrene catalyzed by complex 6.
Table 3. Catalytic Activity of 4 upon the Reaction of Phenyliodonium Dimedonate, with Styrene, for the Formation of Heterocyclic
Benzo[b]furans and Iodoether
amount of catalyst (mg) ilidio/styrene (mg) benzofuran retention time (min) benzofuran area % iodoether retention time (min) iodoether area % ratio B/I
3.56
3.59
3.10
3.35
3.41
7.58
7.10
6.65
6.89
6.81
1.08
2.33
10.95
338.4/668.4
332.0/545.6
330.0/405.4
331.0/279.6
337.6/168.6
338.4/668.4
337.2/521.2
329.8/387.6
329.3/274.6
335.0/168.8
168.5/268.2
171.6/296.0
331.7/527.2
10.495
10.538
10.575
10.618
10.632
10.646
10.649
10.641
10.620
10.587
10.479
10.409
10.484
35.72
32.41
34.27
32.97
34.11
37.70
33.83
31.05
37.10
30.46
36.76
36.19
33.42
16.166
16.237
16.302
16.377
16.399
16.399
16.406
16.396
16.360
16.310
16.120
16.023
16.104
17.83
16.86
15.50
10.87
4.97
2.00
1.92
2.21
3.03
6.86
1.69
1.93
2.52
3.93
6.84
2.41
2.11
1.42
22.29
17.57
12.34
9.44
4.45
15.26
17.17
23.52
TS1A transition state (Figure 5). The calculated bond lengths
of C−I and I−Ph were 2.083 and 2.121 Å, respectively. The
required activation energy was estimated to 55.8 kcal mol−1.
A single imaginary frequency corresponding to the reaction
coordinate of TS1A was calculated at 121i cm−1 involving -
the C···I bond motion, as expected. The reaction involves
a second transition state (TS1B) where styrene attacks the
cyclohexyl ion. The activation barrier for this step was
estimated to 44.7 kcal mol−1, and the single corresponding
imaginary frequency was at 302i cm−1. Significant intramole-
cular HOMO−LUMO interactions are evident (Figure 6)
where the donor MOs are mainly π-type and localized on
the ethenyl double bond while the acceptor MOs are mainly
localized on the electrophilic carbon atom of the cyclohexyl
ion. The rate determining step is the dissociation process
of PhI, and this is probably the reason why extra energy
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Figure 5. Potential energy (kcal mol−1) reaction profiles for reaction 1.
Figure 6. Frontier molecular orbitals of TS1B.
(from irradiation) is needed to proceed with the formation
of benzofuran. The whole process is strongly exergonic
(the change in the Gibbs free energy is negative, indicating
a spontaneous reaction), and the calculated energy was
−67.2 kcal mol−1. The uncatalyzed reaction toward 2-iodo-
5,5-dimethyl-3-phenoxycyclohex-2-en-1-one (reaction 3) is
a phenyl group migration similar to the one studied before7a
with higher level calculations (B3LYP/6-311+G(d,p) ∪ SDD(I)).
Our lower level calculations however, were also able to
accurately reproduce the reaction scheme (Figure 7). The
phenyl group migrates to quinonic oxygen via a single
transition state (TS3) requiring 37.3 kcal mol−1 with only one
imaginary frequency at 581i cm−1. Therefore, gentle heating is
required for this thermal transformation. The reaction is
thermodynamically feasible since the calculated exergonicity
was −45.9 kcal mol−1.
Figure 7. Potential energy (kcal mol−1) reaction profiles for reaction 3
(phenyl group migration).
CONCLUSIONS
■
Since benzofurans and their derivatives are either pharmaceutical
agents or basic compounds precursors in the synthesis of
important drugs, the development of novel synthetic routes for
benzofurans such the reaction of phenyliodonium dimedonate
with styrene, Scheme 1, was investigated since the late 1980s.6
It has been reported that iodoether is isolated when the reac-
tion is carryied out under reflux (Scheme 1 reaction 3), while
benzofurnas are readily derived when the reaction is carrying out
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open tubes with a STUART scientific apparatus and are uncorrected.
Infrared spectra in the region of 4000−370 cm−1 were obtain in KBr
discs while far-infrared spectra in the region of 400−50 cm−1 were obtain
in polyethylene discs, with a Perkin - Elmer Spectrum GX FT-IR
spectrometer. A Jasco UV/vis/NIR V 570 series spectrophotometer was
used to obtain the electronic absorption spectra.
either under irradiation or in the presence of copper(II) acetate
(Scheme 1 reactions 1, 2).
Our computational studies have shown that during the
benzo[b]furan formation from the reaction of phenyliodonium
dimedonate with styrene a dissociation of IPh is first occurring
with simultaneous attack of styrene to the cyclohexyl ion via a
Friedel−Crafts mechanism (Scheme 1; reactions 1, 2). Intra-
molecular HOMO−LUMO interactions are evident (Figure 6)
where the donor MOs are mainly π-type and localized on the
ethenyl double bond while the acceptor MOs are mainly
localized on the electrophilic carbon atom of the cyclohexyl ion.
The activation barrier for this step is estimated to 44.7 kcal. The
rate determining step is the dissociation process of PhI, and this
is probably the reason why extra energy from irradiation or
lowering of the barrier from a catalyst, is needed to proceed with
the formation of benzofuran. It is already reported that the
conversion of iodonium ylides into other ylides, upon heating of
their solution in the presence of various copper complexes or
salts, is proceeded through the formation of carbenes,17a while
the dissociation of PhI from phenyliodonium dimedonate to the
corresponding carbene is also observed during their reaction of
the iodonium ylide with iron(II)- or iron(III)-porphyrins,17b
supporting further the mechanism concluded through this work.
The present study aims in the elucidation of the role of the
catalyst and into the development of new complexes with higher
catalytic activity for the reaction of phenyliodonium dimedonate
with styrene toward benzofuran. Copper(I) chloride was
previously used as catalyst for the intramolecular cyclization of
aryl substituted iodonium ylides and for the regioselective cyclo-
propanation reactions using iodonium ylides for the synthesis of
prostaglandin precursors with high success.17c,d The antithyroid
drug 6-n-propylthiouracil (ptu) reacts with copper(I) iodide in
the absence or presence of arylphosphines to form a complexes
with planar trigonal or tetrahedral geometry around the metal
ions. Copper iodide complexes often forms oligomers and poly-
mers where the building blocks are catenated through iodo
bridges. Structures of monomer copper(I) iodide complexes with
thioamides having either trigonal or tetrahedral geometries are
rare, and only seven trigonal15 or five tetrahedral9,16 such struc-
tures are known up to now.
It has been demonstrated here that copper(I) complexes 1−7
catalyze the formation of benzo[b]furan as well as copper(II)
acetate,6 indicating that the mechanism might not be a redox
process. Also, copper(I) complexes (1−7) exhibit higher cata-
lytic activity than the corresponding one of gold(I) (8). More-
over, both monomeric or dimeric copper(I) complexes with
trigonal or tetrahedral geometry around the metal center exhibit
strong catalytic activity. Iodo containing complexes 1−5 show
better activity than the ones with chlorine (6−8). A synergistic
effect of triphenystibine and iodide or thiarylphosphine and ptu
ligands increases the catalytic ability of the complexes. Moreover,
the amount of iodoether derived is significantly lower, while the
amount of benzo[b]furan yielded remains unchanged by
decreasing of the amount of styrene, with simultaneous main-
taining constant amount of catalyst and ylide. Our work aims in
the design and development of new efficient catalyst for benzo-
furans formation.
Synthesis and Crystallization of [CuI(ptu)2](toluene) (1),
[CuI(tpp)2(ptu)] (2), [CuI(tptp)2(ptu)] (3), [(tpSb)2Cu(μ2-I)2Cu-
(tpSb)2] (4), [(tpp)Cu(μ2-I)2Cu(tpp)2] (5), [(tpp)Cu(μ2-Cl)2Cu-
(tpp)2] (6), [CuCl(tpp)3·(CH3CN)] (7), and [AuCl(tpp)] (8)
Complexes. A suspension of 0.5 mmol CuI (0.085 gr) and 1 mmol
6-n-propylthiouracil (ptu) (0.130 gr) in toluene were stirred and headed
under reflux. A clear solution was finally formed. The solution was then
filter off, and the clear solution was kept in the darkness at room
temperature (rt). After few days, a pale yellow crystal of complex 1
suitable for single crystal analysis by X-ray crystallography were grown,
and they were collected. Complexes 2 and 3 were prepared as follows:
1 mmol triarylphosphine (0.262 g triphenylphosphine (2) and 0.304 g
tri(p-tolyl)phosphine (3)), 0.5 mmol (0.065 g) 6-n-propylthiouracil
(ptu) were suspended to 20 cm3 methanol/acetonitrile solution (1:1)
which contains 0.5 mmol of copper(I) iodide (0.085 g). The mixture
was stirred and heated at 50 °C until a clear solution is formed. The clear
solution was then filtered off and kept in the darkness at rt. After 24 h
colorless crystals, of complexes (2, 3), suitable for single crystal analysis
by X-ray crystallography were filter off.
1. Pale yellow crystal, yield: 12%, decomposition point: 239−245 °C;
IR (cm−1), (KBr): 1641vs, 1552vs, 1438s, 1158s, 836m, 566s. UV−vis
(DMSO) λmax (log ε): 276 nm (4.47). UV−vis (CHCl3) λmax (log ε):
278 nm (4.32) (Supporting Information Figures S1, S6).
2. Pale yellow crystal, yield: 76%, decomposition point: 220−230 °C;
IR (cm−1), (KBr): 1675vs, 1541vs, 1434vs, 1158m, 741s, 695vs, 517s.
1H NMR-(DMSO-d6) (ppm): 12.3 (br H(N) and 12.2 (br, H(N)),
7.4−7.3 (m, Haromatic), 1.59−1.50 (q, i-pr- group), 1.24 (br, i-pr- group),
0.91−0.85 (t, i-pr- group). UV−vis (DMSO) λmax (log ε): 276 nm
(4.47) (Supporting Information Figures S2, S4, S7).
3. Pale yellow crystal, yield: 56%, decomposition point: 175−180 °C;
IR (cm−1), (KBr): 1668vs, 1541vs, 1497s, 1442s, 1187vs, 705vs, 518vs
1H NMR -(DMSO-d6) (ppm): 12.3 (br H(N) and 12.2 (br, H(N)),
7.2−7.1 (m, Haromatic), 2.3 (s, Methyl- group), 1.6−1.5 (q, i-pr- group),
1.24 (br, i-pr- group), 0.91−0.85 (t, i-pr- group). UV−vis (DMSO) λmax
(log ε): 276 nm (4.46) (Supporting Information Figures S3, S5, S8).
The synthesis and characterization of complexes 4−8 are already
reported.13,14 However, compounds 4 and 5 were prepared also here by
a modified procedure as follows: a suspension of 1 mmol (0.353 g)
triphenylantimony(III) (4) or 0.75 mmol (0.197 g) triphenylphosphine
(5) and 0.5 mmol copper(I) iodide (0.085 g) in 20 cm3 methanol/
acetonitrile solution (1:1) was heated at 50 °C until the formation
of a clear solution. The clear solution was then filtered off and kept in
the darkness at rt. After 24 h, colorless crystals, of complexes 4 and
5, suitable for single crystal analysis by X-ray crystallography were
filtered off.
Complexes 6−8 were prepared according to the methods described
in refs 13d and 14e. Their analytical and spectroscopic data were found
to be identical to that already reported.
X-ray Structure Determination. Intensity data for the crystals of
1−3 and 5 were collected on an Oxford Diffraction CCD instrument
while those of 1 were collected on a KUMA KM4CCD four-circle
diffractometer, using graphite monochromated Mo radiation (λ =
0.71073 Å). Cell parameters were determined by least-squares refine-
ment of the diffraction data.17a
Data of 1−3 and 5 were corrected for Lorentz-polarization effects and
absorption.18 The structures were solved with direct methods with
SHELXS9719 and refined by full-matrix least-squares procedures on F2
with SHELXL97.19 All non-hydrogen atoms were refined anisotropi-
cally, hydrogen atoms were located at calculated positions and refined
via the “riding model” with isotropic thermal parameters fixed at 1.2 (1.3
for CH3 groups) times the Ueq value of the appropriate carrier atom.
Significant crystal data are given in Table 4.
EXPERIMENTAL SECTION
■
Materials and Instruments. All solvents used were reagent grade.
Copper(I) iodide and chloride (Riedel-deHaen), triphenylphosphine,
and tri(p-tolyl)phosphine (Merk) ligand (Aldrich-Merk) were used
with no other purification prior to use. Melting points were measured in
Supplementary dataforcomplexes1−5 areavailablefromthe CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K. (E-mail: deposit@ccdc.cam.ac.uk),
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Table 4. Structure Refinement Details for the Complexes 1−5
1
2
3
4
5
empirical formula
formula weight
T (K)
C14H20CuIN4O2S2, C7H8
623.06
C43H40CuIN2OP2S
885.23
C49H52CuIN2OP2S
969.39
C72H60Cu2I2Sb4
1793.14
C54H45Cu2I2P3
1167.71
100
100
100
293
100
cryst syst
space group
a (Å)
monoclinic
P21/c
triclinic
triclinic
monoclinic
P21/c
monoclinic
P21
P1
̅
P1
̅
13.1237(6)
22.6268(8)
8.3711(3)
90
10.8524(4)
12.7124(5)
15.3797(5)
79.442(3)
81.971(3)
69.267(4)
1944.31(13)
2
11.5843(8)
14.6751(10)
15.0047(9)
80.494(5)
82.354(5)
72.311(6)
2387.5(3)
2
24.4463(6)
13.9088(3)
20.2168(5)
90
10.3675(4)
20.5572(6)
11.7116(4)
90
b (Å)
c (Å)
α (deg)
β(deg)
97.503(4)
90
111.241(3)
90
105.640(4)
90
γ(deg)
V (Å3)
2464.49(17)
4
6407.1(3)
4
2403.64(15)
2
Z
ρcalcd (g/cm3)
1.679
1.512
1.349
1.859
1.613
μ (mm−1
)
2.3
1.5
1.3
3.3
2.3
θmin < 2θ < θmax
3.6 < 2θ < 25.0
32191, 4320
2729, (0.150)
0.0622, 0.1565, 0.97
3.8 < 2θ < 25.0
26214, 6817
5827 (0.037)
0.0272, 0.0631, 1.04
2.9 < 2θ < 25.0
19368, 8388
6524 (0.048)
0.0816, 0.2437, 1.07
2.9 < 2θ < 26.5
53912, 12437
10239 (0.028)
0.0220, 0.0480, 1.05
2.8 < 2θ < 26.5
22356, 9541
7625 (0.071)
0.0488, 0.0723, 0.97
data collected, uniq. data
independent reflections (Rint)
R1, wR2 [I > 2σ(I)], S
on request, quoting the deposition nos. CCDC-888395 (1), 888396 (2),
888397 (3), 889071 (4), and 888398 (5), respectively.
ACKNOWLEDGMENTS
■
This research was carried out in partial fulfilment of the
requirements for the Master thesis of K.P. within the graduate
program of Bioinorganic Chemistry, at the University of
Ioannina, Greece, under the supervision of S.K.H.
Catalysis. In an Erlenmeyer spherical flask the proper amount of
ylide, styrene, and catalyst are added. The reaction mixture is refluxed
under continuous stirring at 114−118 °C. A few seconds later, the solid
ylide turned into a clear dark liquid and the refluxing is continued for
4 min more. The benzo[b]furan yielded was detected by the means of
HPLC.
HPLC Assay. The sample constituents were isocratically separated
using acetonitrile/water (55/45) with a flow rate of 1 mL/min, using a
chromatographic system comprised of a Shimadzu liquid chromato-
graph equipped with two LC-8A solvent delivery pumps coupled to a
communication bus module (CBM-20A) which was used to control
sample injection (SIL-10AP autosampler). The peaks representing the
sample constituents were recognized both by the retention time and
their spectrum pattern recorded on a Shimadzu SPD-M20A Diode
Array detector working under LC Solution v.1.2.3 chromatography
software.
Computational Details. All calculations regarding reactants,
transition states, and products were carried out using SPARTAN
′08.20 The equilibrium and transition gas-phase structures were fully
optimized, without any geometry constraints, using ab initio calculations
performed at the HF/3-21G* level. To characterize the nature of the
stationary points, harmonic vibrational frequencies were calculated at
the same level of theory. Only one negative Hessian element was found
for each transition state while no negative eigenvalues were computed
for reactants and products.
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ASSOCIATED CONTENT
■
S
* Supporting Information
́
Figures S1−S8 and crystallographic information files. This
material is available free of charge via the Internet at http://pubs.
acs.org.
́
AUTHOR INFORMATION
■
(9) (a) Hadjikakou, S. K.; Antoniadis, C. D.; Aslanidis, P.; Cox, P. J.;
Tsipis, A. C. Eur. J. Inorg. Chem. 2005, 1442. (b) Aslanidis, P.; Cox, P. J.;
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Corresponding Author
*E-mail: shadjika@uoi.gr. Tel.: xx30-26510-08374. Fax: xx30-
26510-08786.
Notes
The authors declare no competing financial interest.
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