The dianion cyclization strategy for the synthesis of dibenzoheterocycles

Article abstract of DOI:10.1080/10426500701690863

A useful sequence of reaction for the synthesis of a variety of dibenzoheterocycles is described in one-pot. This synthesis involves the initial formation of dianion via deprotonation of Schiff base followed by nucleophilic attack on the suitable dielectr

Full text of DOI:10.1080/10426500701690863

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Phosphorus, Sulfur, and Silicon
and the Related Elements
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The Dianion Cyclization
Strategy for the Synthesis of
Dibenzoheterocycles
M. S. Singh a & Shweta Singh a
a Department of Chemistry, Faculty of Science ,
Banaras Hindu University , Varanasi, India
Published online: 04 Jun 2008.
To cite this article: M. S. Singh & Shweta Singh (2008) The Dianion Cyclization
Strategy for the Synthesis of Dibenzoheterocycles, Phosphorus, Sulfur, and Silicon and
the Related Elements, 183:6, 1510-1517, DOI: 10.1080/10426500701690863
To link to this article: http://dx.doi.org/10.1080/10426500701690863
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Phosphorus, Sulfur, and Silicon, 183:1510–1517, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701690863
The Dianion Cyclization Strategy for the Synthesis
of Dibenzoheterocycles
M. S. Singh and Shweta Singh
Department of Chemistry, Faculty of Science, Banaras Hindu
University, Varanasi, India
A useful sequence of reaction for the synthesis of a variety of dibenzoheterocycles
is described in one-pot. This synthesis involves the initial formation of dianion via
deprotonation of Schiff base followed by nucleophilic attack on the suitable dielec-
trophiles. Products (1–9) were characterized by spectroscopic data and elemental
analyses. A plausible mechanistic logic and concept utilized in the synthesis have
been discussed in detail.
Keywords dianion; dibenzoheterocycles; dielectrophiles; nucleophilic cyclization; Schiff
base
INTRODUCTION
Increasing interest has been paid for several years in the field of syn-
thetic organic chemistry due to its creative power and unlimited scope.
Schiff base ligands are considered “Privileged ligands”1 because they
are easily prepared by the condensation of aldehydes/ ketones and
amines. The more general term salen type2 is used in the literature
to describe the class of [O,N,N,O] tetradentate bis Schiff base ligands.
Stereogenic centers or other elements of chirality (planes, axes) can be
introduced in the synthetic design. In fact, these ligands are able to sta-
bilize many different metals in various oxidation states, controlling the
performance of metals in a variety of useful catalytic transformations.
From a practical point of view, the aspects involved in the prepara-
tion of Schiff base metal complexes are spread out in the literature.3−7
Vanadium and Ruthenium complexes of this ligand have already been
reported.8,9 Schiff bases are valuable synthons for the preparation
Received 8 August 2007; accepted 9 August 2007.
This work was supported by a research grant from the Department of Science and
Technology (DST), New Delhi.
Address correspondence to M. S. Singh Department of Chemistry, Faculty of Science,
Banaras Hindu University, Varanasi-221005, India. E-mail: mssinghbhu@yahoo.co.in
1510

Cyclization Strategy for Dibenzoheterocycles
1511
of macroheterocyclic molecules.10,11 The vigorous development in the
chemistry of macroheterocyclic compounds has been observed in recent
years.12,13 The peculiar behavior of silicon, tin and its complexes14,15 are
of immense importance in chemical and biological processes, and are
thought provoking in the reaction of coordination chemistry.16 Phospho-
rus heterocycles are found in the usual broad variety of ring size and
ring multiplicity familiar to the entire field of heterocyclic chemistry.
Phosphorus heterocycles containing another heteroatom in the ring was
not included till 1976.17 Since phosphorus is a well-established atom for
stabilization of low oxidation state transition metal complexes,18 crown
ethers incorporating phosphorus atoms in different valences have be-
come a important area of chemistry.19 Thus, in view of the synthetic and
catalytic importance of the ligand and in continuation of our work20−26
on the chemistry of heterocycles, we herein report the synthesis and
characterization of some new organosilicon, organotin, and organophos-
phorus macroheterocycles via dianion-mediated cyclizations.
RESULTS AND DISCUSSION
Considering the importance of benzoheterocycles in organic synthesis,
we explored the possibility for the preparation of previously unknown
dibenzoheterocyclic rings. Dibasic Schiff base ligand was prepared by
SCHEME 1 (a) Cl2SiPh2; (b) Cl2SiMe2; (c) Cl2SiEt2; (d) Cl2SiMePh; (e)
Cl2SiMeVinyl; (f) Cl2SnMe2; (g) Cl2SnBu2; (h) Cl2POPh; (h) Cl2PO2Me

1512
M. S. Singh and S. Singh
condensation of ethylene diamine and o-vanillin in 1:2 molar ratios.
This synthesis involves the initial formation of dianion (II) from de-
protonation of Schiff base (I) followed by nucleophilic attack on the
suitable dielectrophiles leading to the formation of macroheterocycles
1–9 (Scheme 1). In general, sodium chloride formed during the course
of the reaction is filtered using vacuum pump. Products 1–9 (Scheme 1)
were characterized by spectroscopic data and elemental analyses. All
these compounds were fully or partially soluble in common organic and
coordinating solvents like CHCl3, CCl4, C6H6, DMF, DMSO, etc.
The broad band present at 3450 cm−1 in the infrared spectrum of
the Schiff base N,N’-bis (o-vanillinidene)ethylenediamine is due to ν
(OH),27 which is found to be absent in all its compounds 1–9, suggesting
deprotonation of the proton from both OH groups. Comparison of IR
spectrum of the Schiff base with those of the heterocycles shows that the
band in the region 1631–1633 cm−1 due to ν(C N) is observed almost
at the same position in all compounds, indicating that the C N groups
are part of the ring. The disappearance of absorption band and signals
1
corresponding to OH groups in both IR and H NMR spectra of the
compounds suggested formation of the metal–oxygen bond. This fact
is further supported by the appearance of new bands in region 870–
892 cm−1 and 488–478 cm−1, which may be assigned to Si-O28 and
Sn-O29 bands, respectively. The stretching frequency for P O group in
compound 8 & 9 appears at 1275 and 1251 cm−1, respectively.
Proton magnetic resonance spectral data of these complexes have
been recorded in CDCl3 using TMS as an internal standard. A singlet
at δ 13.6 ppm due to OH groups of the ligand disappears completely in
the compounds 1–9 suggesting deprotonation of both the OH groups
leading to dianion (II). The protons of methyl and butyl groups appear
in their respective fields. 13C NMR spectra of the compounds have also
been recorded in CDCl3. The signal in the region 166 ppm is due to
azomethine carbon, which is quite distinguishable from other signals.
Signals in the region 118–152 ppm were due to aromatic carbons. Sig-
nals at 59.3 and 56.02 ppm show the presence of (N CH2) and ( OCH3)
groups, respectively. The methyl, butyl, and vinyl carbons attached to
the metal appear in their respective regions. The 29Si NMR data sup-
port that silicon is tetra-coordinated30 in all compounds 1–5. Since R
group is bound directly to 29Si Nucleus; it is not surprising that the
value of δ depends primarily on the nature of this R group. When R is
Phenyl, the complexes give much more negative values of δ than when
R is alkyl, because the delocalized π system in the phenyl compound
allows for dπ-pπ interaction, which dominate the overall shielding of
the 29Si nucleus.

Cyclization Strategy for Dibenzoheterocycles
1513
EXPERIMENTAL
Chemicals were obtained from Sigma-Aldrich, Merck, Fluka, and Lan-
caster, and were used as such without further purification. All solvents
(AR or extra pure grade) used for spectroscopic and other physical
studies were further purified as described in literature.31 All opera-
tions were performed under nitrogen atmosphere using standard glass
wares. Infrared spectra were recorded as KBr discs on JASCO FT/IR-
5300 spectrophotometers. Melting points were determined using a cali-
brated thermometer by Remi Digital Melting Point apparatus and were
uncorrected. Elemental analyses were performed by Central Drug Re-
search Institute, Lucknow. NMR (1H, 13C, 31P, 119Sn and 29Si) spectra
were recorded on a JEOL AL 300 instrument. All chemical shifts were
reported in parts per million relative to TMS as an internal standard
in CDCl3. Mass spectra were recorded at 70 eV ionizing voltage on a
JEOL-D300 MS instrument. Column chromatography was performed
on Merck Silica gel. Thin layer chromatography was carried out on
Merck silica gel (layer thickness 0.2 mm) and components were located
by observation under iodine/ UV chamber.
Preparation of Ditopic Ligand (I)
An ethanolic solution (30 mL) of o-vanillin (7.60 g, 50.0 mmol) was added
to the ethanolic solution (20 mL) of ethylenediamine (1.50 g, 25.0 mmol).
The reaction mixture was stirred at reflux for 4 h. A yellow colored solid
mass separated out on cooling. It was then filtered by suction, washed
with ethanol, diethyl ether, and subsequently dried over anhydrous
CaCl2 in a desiccator. The purity of the ligand was checked by TLC.
I
Yield, 6.20 g, (76%), m.p. 145◦C; IR (KBr, cm−1): 3450.96, 1631.93; 1H
NMR (300MHz, CDCl3, δ): 13.6 (s, 2H, OH, D2O exchangeable), 8.3 (s,
2H, CH=N), 7.00–6.74 (m, 6H, ArH), 3.96 (s, 6H, OCH3), 3.89–3.93 (m,
4H, CH2); 13C NMR (75 MHz, CDCl3, δ ppm): 166.79, 151.65, 148.43,
123.32, 118.51, 118.175, 114.32, 59.49, 56.19. MS: m/z (M+) 328.15;
anal. calcd. for C18H20N2O4: C, 66.21; H, 6.05. Found: C, 65.85; H, 6.14.
Preparation of Tricyclic Dibenzoheterocycles [1]
0.046 g (2.0 mmol) of sodium pieces and 20 mL of ethanol were taken
in a two-way round bottom flask, fitted with a cooled water con-
denser. It was refluxed in an inert atmosphere for about half an
hour until a clear solution of sodium ethoxide was obtained. After

1514

1515

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M. S. Singh and S. Singh
cooling, 20 mL of toluene containing 0.33 g (1.0 mmol) of N, Nꢀ-bis(o-
vanillinidene)ethylenediamine was added slowly at room temperature.
The mixture was stirred under reflux for 4 h. The solution became light
brown, which was allowed to attain room temperature and then 0.25 g
(1.0 mmol) of dichlorodiphenylsilane was added slowly. The solution
became yellow and the reaction mixture was further refluxed for 2
h to ensure the completion of the reaction. The precipitated sodium
chloride was removed by filtration. A novel product was obtained fol-
lowing removal of solvent under reduced pressure, which was further
washed with water and brine, and then dried over anhydrous MgSO4.
The solvent was evaporated in vacuo and the residue was column chro-
matographed on silica gel (hexane/ethyl acetate 5:1) to purify the prod-
uct 1. All other dibenzoheterocycles 2–9 were also synthesized analo-
gously as described above in the desired molar ratios. The analytical
and spectral data for the compounds 1 to 9 are listed in Tables I and II,
respectively.
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