Isolation of an inclusion complex of naphthol and its benzoate as an intermediate in the solvent-free benzoylation reaction of naphthol

Article abstract of DOI:10.1039/b303060c

A study was performed on isolation of an inclusion complex of naphthol and its benzoate as an intermediate in the solvent-free benzoylation reaction of naphthol. Solvent-free benzoylation reactions were carried out by heating a stirred mixture of phenols or naphthols and benzoyl chloride. The structure of the 2 : 1 inclusion complex of 2,3 naphthalenediol and its p-methylbenzoate was studied by X-ray analysis.

Full text of DOI:10.1039/b303060c

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Isolation of an inclusion complex of naphthol and its benzoate
as an intermediate in the solvent-free benzoylation reaction of
naphthol
Seiken Nakamatsu,a Kazuhiro Yoshizawa,a Sinji Toyota,a Fumio Toda*a and Ivanka Matijasic*b
a Department of Chemistry, Faculty of Science, Okayama University of Science, Ridai-cho 1-1,
Okayama 700-0005, Japan. E-mail: toda@chem.ous.ac.jp
b Department of Chemistry, Faculty of Science, Faculty of Science, University of Zagreb,
Strossmayerov trg 14,10000 Zagreb, Croatia. E-mail: matijas@rudjer.irb.hr
Received 17th March 2003, Accepted 29th May 2003
First published as an Advance Article on the web 9th June 2003
Solvent-free benzoylation of naphthol was found to proceed
via an inclusion complex intermediate of the naphthol and
its benzoate by IR spectral monitoring.
Introduction
From the viewpoint of green and sustainable chemistry,
solvent-free organic reactions represent a very important
synthetic procedure. Recently, we have been developing various
new solvent-free organic reactions.1 We have also developed a
simple, but very useful, method of analyzing organic reaction
mechanisms by using continuous IR spectral measurement of
the solvent-free reaction. For example, the Thorpe reaction
mechanism in the solid state was demonstrated to proceed via
an imine intermediate by monitoring the reaction by IR spec-
troscopy.2 The Rap–Stoermer reaction mechanism was also
almost fully clarified by using this technique.3 Recently, we have
found that benzoylation of phenols and naphthols with benzoyl
chloride proceeds efficiently under solvent-free conditions,
and that the reaction of naphthols involves the formation
of an inclusion complex of the naphthol with its benzoate by
using IR monitoring. These inclusion complexes have been
isolated and shown to be genuine intermediates. The structure
of the 2 : 1 inclusion complex of 2,3-naphthalenediol and its
p-methylbenzoate has also been studied by X-ray analysis.
Results and discussion
Solvent-free benzoylation reactions were carried out by heating
a stirred mixture of phenols (1, 2, see Scheme 1) or naphthols
(3–6) and benzoyl chloride (7). The HCl gas evolved during the
reaction was trapped by absorption in water. The reaction
mixture was washed with aqueous Na2CO3 and water, and air
dried to give the corresponding pure benzoates (8–13) as white
powders. For example, a mixture of 2-naphthol (3) (2.01 g, 14.4
mmol) and p-methylbenzoyl chloride (7b) (2.15 g, 13.9 mmol)
was heated at 60 ЊC for 2 h under magnetic stirring. When the
mixture started to solidify, it was occasionally stirred using
a spatula. The reaction mixture was washed with aqueous
Na2CO3 and water, and then air dried to give pure 2-naphthyl
p-methylbenzoate (10b) as a white powder (3.36 g, 97% yield,
mp 140–141 ЊC). Benzoylations of all phenols and naphthols
with benzoyl (7a), p-methylbenzoyl (7b) and p-bromobenzoyl
Scheme 1
chloride (7c) were carried out under the same solvent-free
reaction conditions (Table 1). In all reactions, pure products
were obtained in good yields just by washing the reaction
mixture with aqueous Na2CO3 and water.
as Nujol mulls (Figs. 1 and 2). First, the reaction of 3 with 7b at
80 ЊC was studied. Initially, on mixing of 3 and 7b, no signifi-
In order to clarify the reaction mechanism of the solvent-free
benzoylation process, the reactions of 3 or 4 with 7b at 80–100
ЊC were monitored by continuous measurements of IR spectra
cant new νOH absorption appeared, and only two νC᎐O
᎐
absorptions of 7b appeared at 1775 and 1740 cmϪ1 (I in Fig. 1).
After 10 min of mixing, new νOH and νC᎐O absorptions
᎐
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 2 3 1 – 2 2 3 4
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Table 1 Benzoylation of phenol and naphthol derivatives under solvent-free conditions
Reaction conditions
Phenol or naphthol
Benzoyl chloride
T /ЊC
t/min
Product
Mp/ЊC
Yield (%)
1
2
2
2
3
3
3
4
5
6
7b
7a
7b
7c
7a
7b
7c
7b
7b
7b
60
60
60
60
60
60
60
120
120
120
90
120
120
120
120
120
120
15
8b
9a
75–76
106–107
105–106
115
106–107
140–141
133
145–146
219–222
283–284
80
94
98
87
97
98
71
99
92
78
9b
9c
10a
10b
10c
11b
12b
13b
30
15
The appearance of the νOH and νC᎐O absorptions at relatively
᎐
low frequencies 3450 and 1720 cmϪ1, respectively, during the
reaction process suggests the formation of a complex of 3 and
10b. Hydrogen bond formation between the OH hydrogen of
3 and the C᎐O oxygen of 10b would shift their absorptions to
᎐
lower wavenumber. Recrystallization of 3 and 10b from diethyl
ether gave a 1 : 1 inclusion complex (14) as colorless crystals
showing νOH and νC᎐O absorptions at 3450 and 1710 cmϪ1
,
᎐
respectively. However, 14 did not form crystals appropriate for
X-ray analysis.
Secondly, the solvent-free benzoylation reaction of 4 with 7b
at 100 ЊC was also monitored by IR spectral measurements
(Fig. 2). Initially, on mixing 4 and 7b, the νOH of 4 and νC᎐O
᎐
of 7b appeared at 3500 (A) and 3450 (AЈ), and 1775 and 1740
cmϪ1, respectively (I in Fig. 2). As the benzoylation reaction
proceeded, new νOH absorptions appeared at 3487 (B), 3338
(C) and 3335 cmϪ1 (BЈ), and the original νOH absorptions of 4
at 3500 (A) and 3450 cmϪ1 (AЈ) disappeared after 6 min (II–III
in Fig. 2). At the same time, a new νC᎐O absorption appeared
᎐
at 1730 cmϪ1 and gradually increased in intensity (II–IV in
Fig. 2). After 20 min, only the 3338 cmϪ1 (C) absorption
remained as the sole νOH absorption, and the 1740 cmϪ1
Fig. 1 Monitoring of the solvent-free benzoylation of 3 with 7b at 80
ЊC by continuous IR spectral measurements for 40 min in Nujol mulls.
absorption remained as the strongest νC᎐O absorption (VI in
᎐
Fig. 2). The 3388 cmϪ1 absorption (C) is assigned to the νC᎐O
᎐
of the monoester (2-hydroxy-3-naphthyl p-methylbenzoate) by
comparison of the spectrum with that of an authentic sample.
When the reaction mixture was further heated at 100 ЊC for
90 min, the νOH absorption disappeared and only the νC᎐O
᎐
absorption at 1740 cmϪ1 remained (VIII in Fig. 2). The VIII
spectrum in Fig. 2 is identical to that of 11b. The appearance of
the new νOH and νC᎐O absorptions at 3487 (B) and 3335 (BЈ)
᎐
and 1730 cmϪ1, respectively, during the course of the reaction
(II–VII in Fig. 2) suggests the formation of a complex of 4 with
11b. In order to confirm the formation of this complex, both
compounds were recrystallized together from diethyl ether to
give the 2 : 1 complex (15) of 4 with 11b as colorless plates
(mp 140–141 ЊC, calc. for C46H36O8: C, 77.08; H, 5.06. Found:
C, 76.91; H, 4.87%). Since complex 15 showed the 3487, 3335
and 1730 cmϪ1 absorptions and since heating of 15 together
with 7b gave 11b, 15 was identified as being an intermediate in
the solvent-free benzoylation reaction of 4 with 7b. However,
inclusion complex of 1, 2, 5 and 6 with their corresponding
benzoates were not obtained in separate inclusion complexation
experiments.
In order to understand why 15 is so stabilized against
the benzoylation reaction with 7b at room temperature, the
structure of 15 was studied by X-ray analysis.4 The crystal
structure of 15 is shown in Fig. 3. The C–C bond lengths of the
naphthalene-2,3-diol moiety in the host and guest molecule do
not differ significantly from those of naphthalene itself,5 or
from those of naphthalene-2,3-diol.6 Besides, similar values
were reported in the complex with a cyclophane derivative7 and
in the molecular complexes of naphthalene-2,3-diol with fla-
vins.8–10 The diol molecule (4) (including hydroxyl O atoms) is
Fig. 2 Monitoring of the solvent-free benzoylation of 4 with 7b at 100
ЊC by continuous IR spectral measurements for 90 min in Nujol mulls.
appeared at 3450 and 1710 cmϪ1, respectively (II in Fig. 1). As
the reaction proceeds, a νC᎐O absorption appeared at 1720
᎐
cmϪ1, in addition to the νC᎐O absorption at 1710 cmϪ1 (II–IV in
᎐
Fig. 1). When the reaction mixture was further heated at 80 ЊC
for 10 min, the νOH absorption at 3450 cmϪ1 and νC᎐O absorp-
᎐
tions at 1775, 1740 and 1710 cmϪ1 disappeared and only the
νC᎐O absorption at 1720 cmϪ1 remained (VI in Fig. 1). The
᎐
spectrum of VI is identical to that of the final product 10b.
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Table 2 Hydrogen bonding geometry in complex 15 (Å, Њ)
D–Hؒ ؒ ؒA
D–H
Hؒ ؒ ؒA
Dؒ ؒ ؒA
D–Hؒ ؒ ؒA
O2–Hؒ ؒ ؒO1
O2–Hؒ ؒ ؒO1i
O1–Hؒ ؒ ؒO4ii
C18–Hؒ ؒ ؒO2iii
0.96(2)
0.96(2)
0.92(2)
0.93
2.15(2)
1.98(2)
1.75(2)
2.50
2.665(2)
2.765(2)
2.672(2)
3.360(2)
112(2)
138(2)
176(2)
153
Symmetry codes: (i) 1 Ϫ x, Ϫy, 1 Ϫ z; (ii) 1 Ϫ x, ½ Ϫ y, z; (iii) x, ½ ϩ y,
1 Ϫ z.
Fig. 4 A basic part of the hydrogen bonding network. Hydrogen
atoms on carbon are omitted for clarity.
Fig. 3 X-Ray crystal structure of 15 with atom labeling. Thermal
ellipsoids are shown at the 50% probability level.
planar, likewise the naphthalene moiety in the diester molecule.
In the latter moiety we found the O3 atom out of plane by
0.093(2) Å and the O3–C11–C11Ј–O3Ј torsion angle of 9.2(2)Њ,
due to the bulky substituents in the ortho-position. The dihedral
angle between the least-squares planes through the two
naphthalene rings of host and guest molecule is 57.33(9)Њ with
the closest contact C5ؒ ؒ ؒH14 being 2.81 Å. The conformation
of the guest molecule can be described by the dihedral angle
between the least-squares plane through the naphthalene ring
and the plane through the phenyl ring; the value is 59.5(1)Њ. Two
bulky substituents on the naphthalene ring influence not only
the conformation of the molecule, but also the crystal packing.
The naphthalene-2,3-diol molecule (4) shows the usual
intramolecular hydrogen bond present in ortho-dihydroxy-
aromatic compounds with a O2–Hؒ ؒ ؒO1 distance of 2.665(2)
Å, a O2–Hؒ ؒ ؒO1 angle of 112(2)Њ and Hؒ ؒ ؒO1 distance of
2.15(2) Å (Table 2, Fig. 2). There is also an intermolecular
O2–Hؒ ؒ ؒO1 hydrogen bond with a symmetry related diol mole-
cule resulting in a dimer. The distance between the least-squares
planes of these two related 4 molecules is about 0.095 Å. The
O1 atom, which is the acceptor in these two hydrogen bonds,
acts as a donor in intermolecular hydrogen bonding to the
carbonyl O atom of the guest molecule 11b. The distances and
the angle are indicative of strong hydrogen bonding (Table 2).
The core part of the chains present in the hydrogen bonding
network is shown in Fig. 4.
The one unusual, but consistent, deviation in bond angles
from 120Њ which occurs in 4 involves the oxygen atom which is
an acceptor in the internal hydrogen bond. In our structure the
O1 atom bends about 5.5Њ toward the donor hydroxyl group.
Similar characteristics have already been observed in structures
found in the CSD: in 46 [114.7(2) and 115.3(2) Å]; in the cyclo-
phane complex7 [113.4(4) Å]; and in three different complexes
with flavin derivatives8–10 [115.1(3) and 115.2(3); 112(1) and
113(1); 114.8(8) and 115.4(8), respectively].
The crystal packing of two entities (Fig. 5) is a result between
different intermolecular forces. The O–Hؒ ؒ ؒO hydrogen bond-
ing pattern is already described. Other possible attractive
interactions are parallel stacking of entities with π electrons
Fig. 5 Molecular packing of 15. Hydrogen bonds are denoted as
dashed lines.
and CHؒ ؒ ؒO hydrogen bonds. Our structure consists of both
interactions. A face-to-face π–π alignment is a rare phenom-
enon, while the usual π–π interaction is an offset or slipped
stacking, i.e. the rings are parallel displaced. The methylated
phenyl rings and naphthalene diol molecules from two
neighbouring hydrogen bonded chains are almost parallel with
an interplanar angle of 5.1Њ and a mean separation of 3.57 Å in
the offset stacking. The next aromatic moiety in the same
stacked column is the second p-methylphenyl ring of the diester
molecule. The intermolecular distance of two symmetry related
phenyl rings is rather large, 3.86 Å, with an interplanar angle
of 10.6Њ. This value is just slightly above the limit of 3.8 Å
suggested as approximately the maximum contact for which
π–π interactions are accepted.11
An additional factor that appears to influence the packing is
a weak C–Hؒ ؒ ؒO hydrogen bonding between guest and host
molecules: the C18–Hؒ ؒ ؒO2 distance of 3.359(2) Å and C18–
Hؒ ؒ ؒO2 angle of 153Њ complete an interesting ribbon motif.
Several close herringbone or edge-to-face interactions of the
C–Hؒ ؒ ؒπ type12 were observed for the H12 and H14 atoms
ranging from 2.81 to 3.05 Å.
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The remaining C–H hydrogen atoms were introduced in calculated
positions and allowed to ride on their parent atoms. At convergence,
the peaks and troughs of the difference density map did not exceed
0.21 and Ϫ0.19 e ÅϪ3, respectively. The figures were prepared using
PLATON98.15 CCDC reference number 207265. See http://
www.rsc.org/suppdata/ob/b3/b303060c/ for crystallographic data in
CIF or other electronic format.
5 D. W. J. Cruickshank, Acta Crystallogr., 1957, 10, 504; D. W. J.
Cruickshank, Tetrahedron, 1962, 17, 155.
6 J. A. Herbert and M. R. Truter, J. Chem. Soc., Perkin Trans. 2, 1980,
1253.
7 V. K. Bel’skii, E. V. Karchenko, A. N. Sobolev, V. E. Zavodnik, N. A.
Kolomiets, G. S. Prober and L. P. Oleksenko, Zh. Strukt. Khim.,
1990, 31, 116.
Acknowledgements
The financial support of this work from the Ministry of Science
and Technology, Croatia, is gratefully acknowledged.
Note and references
1 For reviews see for example: (a) F. Toda, Synlett, 1993, 302; (b)
F. Toda, Acc. Chem. Res., 1995, 28, 480; (c) F. Toda, Supramol. Sci.,
1996, 3, 139; (d ) F. Toda, Compr. Supramol. Chem., 1996, 6, 465; (e)
K. Tanaka and F. Toda, Chem. Rev., 2000, 100, 1025; ( f ) F. Toda,
Organic Solid State Reactions, Kluwer Academic Publishers,
Dordrecht, 2002.
2 K. Yoshizawa, S. Toyota and F. Toda, Green Chem., 2002, 4, 68.
3 K. Yoshizawa, S. Toyota, F. Toda and I. Csöregh, Green Chem.,
2003, 5, 353.
8 J. L. Wells, B. L. Trus, R. M. Johnston, R. E. Marsh and
C. J. Fritchie, Jr, Acta Crystallogr., Sect. B, 1974, 30, 1127.
9 M. C. Kuo, J. B. R. Dunn and C. J. Fritchie, Jr., Acta Crystallogr.,
Sect. B, 1974, 30, 1766.
10 C. H. Fritshie, Jr and R. M. Johnston, Acta Crystallogr., Sect. B,
1975, 31, 454.
11 C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885.
12 I. Dance, in The Crystal as a Supramolecular Entity, ed. G. R.
Desiraju, John Wiley & Sons, Chichester, 1996, p. 168.
13 G. M. Sheldrick, SHELXS97, Program for the Refinement of
Crystal Structures, University of Göttingen, Germany, 1997.
14 G. M. Sheldrick, SHELXL97, Program for the Refinement
of Crystal Structures, University of Göttingen, Germany, 1997.
15 A. L. Speck, PLATON98, Program for the Display and Analysis
of Crystal and Molecular Structures, University of Utrecht, The
Netherlands, 1998.
4 Crystal data for 15. 2C10H8O2ؒC26H20O4, M = 716.75, orthorhombic,
space group Pcnb (no. 60), a = 7.448(1), b = 15.243(1), c = 31.849(1)
Å, V = 3615.8(6) Å3, Z = 4, T = 293(1) K, F(000) = 1504, GooF =
1.027, Dc = 1.317 g cmϪ3, µ(Cu-Kα) = 0.729 mmϪ1, Rint = 0.0270.
5596 reflections were measured, 3062 unique were used in all least-
squares calculations, R1(F) = 0.0434 for 2383 reflections with Fo >
4σ(Fo), wR2 (F 2) = 0.1272 for all unique reflections and 255 param-
eters. Intensity data were collected on a Nonius Kappa CCD dif-
fractometer up to 2θmax = 130.20Њ using graphite-monochromated
Cu-Kα radiation (λ = 1.54178 Å). The structure was solved by the
direct methods (SHELXS97)13 and refined by full-matrix least
squares based on F 2 using SHELXL97.14 The O–H hydrogen atoms
were found from a difference Fourier syntheses and refined freely.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 2 3 1 – 2 2 3 4
2234