Multicomponent reactions (MCRs) leading to silaheterocycles via dianion cyclization

Article abstract of DOI:10.1080/10426500601061991

Nucleophilic asymmetrical Schiff base that incorporating aromatic rings to induce rigidity to the system was prepared by the condensation of o-hydroxyacetophenone and salicylaldehyde hydrazone in a 1:1 molar ratio. A useful sequence of reactions for the s

Full text of DOI:10.1080/10426500601061991

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Phosphorus, Sulfur, and Silicon
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Multicomponent Reactions
(MCRs) Leading to
Silaheterocycles via Dianion
Cyclization
M. S. Singh a & Pratibha Singh a
a Department of Chemistry , Banaras Hindu
University , Varanasi, India
Published online: 24 Feb 2007.
To cite this article: M. S. Singh & Pratibha Singh (2007) Multicomponent Reactions
(MCRs) Leading to Silaheterocycles via Dianion Cyclization, Phosphorus, Sulfur, and
Silicon and the Related Elements, 182:4, 835-843, DOI: 10.1080/10426500601061991
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Phosphorus, Sulfur, and Silicon, 182:835–843, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500601061991
Multicomponent Reactions (MCRs) Leading
to Silaheterocycles via Dianion Cyclization
M. S. Singh
Pratibha Singh
Department of Chemistry, Banaras Hindu University, Varanasi, India
Nucleophilic asymmetrical Schiff base that incorporating aromatic rings to induce
rigidity to the system was prepared by the condensation of o-hydroxyacetophenone
and salicylaldehyde hydrazone in a 1:1 molar ratio. A useful sequence of reactions
for the synthesis of a variety of previously unknown silaheterocycles is described.
The reaction of salicylidene o-hydroxyacetophenone with sodium hydride in diox-
ane produces a 1,10-remote dianion. The cyclization of this dianion with diorgan-
odichlorosilanes affords dibenzodioxadiazasilaundecines in good yields. The prod-
1
ucts were characterized by satisfactory elemental analyses and spectral (IR, H,
13 29
C, Si NMR, and mass) studies.
Keywords dianion; dibenzodioxadiazasilaundecines; diorganodichlorosilanes; elec-
trophilic cyclizations; o-hydroxyacetophenone; salicylaldehyde hydrazone
INTRODUCTION
Schiff bases have been used as precursors and intermediates for the syn-
thesis of a variety of organic compounds useful in analytical, medicinal,
polymer, and liquid crystalline materials1 or as ligands for complexa-
tion. Monocyclic medium-ring heterocycles are an extremely important
class of compounds, which occur in a range of natural and unnatural
products. Cyclocondensation reactions are recognized as worthwhile
synthetic methods for preparing heterocyclic compounds.2 Increasing
interest has been paid for several years to the chemistry of hetero-
cycles containing tin and silicon due to their unique properties and
remarkable potential biological activity.3,4 Monocyclic eight-membered
silicon-containing5 heterocycles have been prepared by condensation
Received June 26, 2006; accepted September 11, 2006.
The authors are grateful for financial support of this work from the Council of Scientific
and Industrial Research (CSIR), New Delhi, India.
Address correspondence to M. S. Singh, Department of Chemistry, Faculty of Science,
Banaras Hindu University, Varanasi 221005, India. E-mail: mssinghbhu@yahoo.co.in
835

836
M. S. Singh and P. Singh
reactions.6 Chlorosilanes are synthetic intermediates in the prepara-
tion of all classes of organosilicon compounds.7,8 A variety of silahete-
rocycles have been obtained by intramolecular carbene or carbenoid
pathways from silyldiazoacetic esters.9 Organosilicon complexes with
O and N donor ligands have been reported10,11and are widely used
in medicinal and pharmaceutical chemistry.12 Certain organosilicon
compounds have been extensively used in chemical vapor deposition
and deoxygenation reactions in organic transformations.13 Strategies
involving dianion reactions14−16 have become powerful and versatile
tools in organic synthesis and have developed as a powerful method
for preparing various types of carbocyclic and heterocyclic compounds
via carbon-carbon and carbon-heteroatom bond-forming processes. To
the best of our knowledge, there are no reports on the synthesis of
eleven-membered silaheterocycles containing five heteroatoms (N, O,
and Si). Our aim is to broaden the range of useful silaheterocycles,
which may provide an easy access to synthetic intermediates and ther-
apeutic agents. With this background, the goal of the present article is
to provide cyclization reactions of remote dianion via cyclosilylation. A
remarkable feature of this reaction is the construction of oxygen-silicon
bonds via a tandem process. In our continuing studies on the synthesis
of new heterocyclic systems17−20 using efficient intermolecular cycliza-
tion reactions via a dianion intermediate, we herein describe the prepa-
ration of novel, previously unknown dibenzodioxadiazasilaundecines in
good yields.
RESULTS AND DISCUSSION
The reaction of equimolar amounts of salicylaldehyde hydrazone and
o-hydroxyacetophenone in methanol under reflux for 2 h according to
the literature method21,22 afforded the corresponding salicylidene-o-
hydroxyacetophenone (I) (Scheme 1). The structure of ligand (I) was
established from its spectral and analytical data. The IR spectrum
showed peaks at 3435 cm−1 for the hydroxyl group and at 1633 and
1
1689 cm−1 for C=N groups. In its H NMR spectrum two singlets at
δ 13.13 and δ 11.05 ppm exhibited for two unsymmetrical OH groups
SCHEME 1

MCRs Leading to Silaheterocycles
837
that are D2O exchangeable, and the downfield signal of −OH group
suggests that there is intramolecular hydrogen bonding between this
OH and one of the azomethine nitrogen. The azomethine proton of the
CH=N group manifested at δ 8.94 ppm as a sharp singlet. A multiplet
in the range δ 7.00–7.91 for aromatic protons and a sharp singlet at δ
2.73 for methyl protons attached to C=N has been observed.
The introduction of the diorganosilylene group is normally achieved
by the reaction of diorganodichlorosilane with a proper substrate in
the presence of a base to prepare heterocyclic compounds containing
a silicon atom. Our synthesis involves the initial formation of dianion
II from sequential deprotonations of the phenolic OH groups of ligand
(I) by sodium hydride in dry dioxane. The 1,10-dianion thus generated
attacks to a variety of diorganodichlorosilanes leading to the formation
of compounds 1–8 (Scheme 2). The beauty of the reaction procedure
resides in in situ formation of remote dianion and further cyclization
so that the multistep reaction sequence is synchronized in a simple
one pot. TLC of all compounds confirmed their purity. The compounds
dibenzodioxadiazasilaundecines are characterized on the basis of sat-
1
isfactory elemental analyses (Table I) and spectral (IR, H, 13C, 29Si
NMR, and mass) studies (Table II).
The disappearance of absorption band and signals corresponding to
–OH group in both IR and 1H NMR spectra and the appearance of a new
band in the region 1048–1023 cm−1 may be assigned to a Si-O bond,23
suggesting the cyclic structure of prepared compounds. The IR spectra
exhibited C=N absorptions in the range 1600–1625 cm−1 and Si-C ab-
sorptions in the range 1265–1275 cm−1. The lowering of ν(C=N)24 by
33–55 cm−1 is a further indication of the coordination of C=N to the
silicon atom exhibiting hexa-coordinated state, which is further sup-
1
ported by 29Si NMR chemical shifts. H NMR spectra displayed a sin-
glet at δ 8.71 ppm for the azomethine proton, a multiplet in the range
δ 6.83–7.74 for aromatic protons, a singlet around δ 2.62 for the methyl
proton, and singlets of silicon methyl in the range δ 0.11–0.70. The
signals for the vinyl and n-propyl protons were visible in the expected
regions. The alkyl groups attached to silicon displayed single resonance
for chemically equivalent protons and carbons. In addition, 13C NMR
spectra supported the assigned structures. The extremely high-field
shifts of the 29Si NMR signals of the compounds indicate the presence
of hexacoordinate25 silicon in all the compounds. Since the R group is
bound directly to the 29Si nucleus, it is not surprising that the value of δ
depends primarily on the nature of this R group. When R = phenyl, the
chemical shift is consistently more negative (by about 25 ppm) than
R = alkyl. Although the alkyl moiety has a greater electron pushing
capacity (σ donation) than the aryl, the delocalized π system in the

838
M. S. Singh and P. Singh
SCHEME 2

839

840
M. S. Singh and P. Singh
TABLE II Spectral Data of Compounds 1–8
NMR
Compound
no.
IR
(KBr, cm−1
)
1H
13C
29Si
1
1612, 1273,
964, 565
8.71 (s, 1H, CH=N),
6.90–7.74 (m, 8H,
ArH), 2.62 (s, 3H, CH3),
0.11 (s, 6H, CH3)
170.27, 164.62, 163.36,
160.42, 159.62, 133.25,
132.75, 132.66, 132.59,
119.54, 119.25, 118.15,
117.19, 117.07, 29.41,
10.12
−138.63
2
1620, 1271,
895, 535
8.71 (s, 1H, CH=N),
6.93–7.74 (m, 8H,
170.17, 164.72, 163.16,
160.62, 159.82, 133.45,
132.95, 132.56, 132.49,
119.74, 119.00, 118.05,
117.29, 117.17, 29.71,
6.55, 1.02
170.14, 168.07, 164.68,
163.17, 159.73, 134.42,
133.42, 132.55, 130.19,
128.94, 127.76, 118.81,
117.13, 14.76
170.37, 164.42, 163.76,
160.22, 159.32, 133.85,
132.65, 132.86, 132.79,
119.56, 119.12, 118.17,
117.19, 29.21, 8.55
164.17, 163.16, 159.82,
155.23, 132.94, 128.94,
119.74, 117.23, 42.68,
29.69, 14.78, 10.27
−142.42
ArH), 2.62 (s, 3H, CH3),
0.98 (t, J = 4.0 Hz, 6H,
CH3), 0.58 (quartet,
J = 2.0 Hz, 4H, CH2)
8.71 (s, 1H, CH=N),
6.83–7.74 (m, 18H,
3
4
5
1620, 1271,
895, 522
−162.27
−140.58
−146.07
ArH), 2.62 (s, 3H, CH3)
1621, 1275,
980, 562
8.70 (s, 1H, CH=N),
6.93–7.65 (m, 8H,
ArH), 2.60 (s, 3H, CH3),
4.72 (s, 1H, Si-H), 0.11
(s, 3H, CH3)
1622, 1275,
894, 524
8.70 (s, 1H, CH=N),
6.93–7.74 (m, 8H,
ArH), 2.58 (s, 3H, CH3),
0.94 (t, J = 4.0 Hz, 3H,
CH3), 0.54 (quartet,
J = 3.0 Hz, 2H, CH2);
0.11 (s, 3H, CH3)
6
1619, 1270,
967, 563
8.70 (s, 1H, CH=N),
170.15, 168.06, 164.70,
159.78, 136.18, 133.40,
132.56, 129.08, 119.72,
118.83, 117.28, 29.71,
10.46, 1.03, 0.57
170.17, 164.72, 160.60,
133.42, 132.49, 129.08,
127.80, 119.74, 117.04,
11.86
170.31, 168.10, 162.72,
160.80, 159.57, 158.55,
136.27, 133.24, 132.66,
129.69, 119.60, 118.71,
117.28, 116.35, 27.62,
15.56, 14.57, 10.63
−154.63
6.93–7.64 (m, 8H,
ArH), 5.97 (m, 3H,
CH=CH2), 2.59 (s, 3H,
CH3), 0.19 (s, 3H, CH3)
8.71 (s, 1H, CH=N),
6.93–7.64 (m, 13H,
ArH), 2.62 (s, 3H, CH3),
0.40 (m, 3H, CH3)
7
8
1612, 1269,
837, 520
−163.23
−147.63
1614, 1269,
839, 549
8.71 (s, 1H, CH=N),
6.93–7.67 (m, 8H,
ArH), 2.62 (s, 3H, CH3),
1.40 (t, J = 4.0 Hz, 3H,
CH3), 1.01 (m, 2H,
CH2), 0.57 (t, J = 4.0
Hz, 2H, CH2), 0.12 (s,
3H, CH3)

MCRs Leading to Silaheterocycles
841
phenyl-substituted compound allows for dπ-pπ interaction to dominate
the overall shielding of the 29Si nucleus.26 Mass spectroscopic data of the
compounds established their monomeric nature. The newly synthesized
silaheterocyclic rings gained rigidity due to the presence of two benzene
rings in their skeleton.
In conclusion, a simple work-up, low consumption of the solvent, fast
reaction rates, a mild reaction condition, good yields, and selectivity of
the reaction make this method a very appealing and useful contribution
to the preparation of a rare class of silaheterocycles.
EXPERIMENTAL
Chemicals were obtained from Sigma-Aldrich (St. Louis, USA), Merck
(Darmstadt, Germany), Fluka (Buchs, Switzerland), and Lancaster
(Lancanshire, England), and are used as such without further purifica-
tion. All solvents (Analytical Reagents (AR) or extra pure grade) used
for spectroscopic and other physical studies were further purified by
literature methods.27 All operations were performed under a nitrogen
atmosphere using standard glasswares. Infrared spectra were recorded
as KBr discs on an FT-IR Perkin-Elmer model RX-I and on JASCO
FT/IR-5300 spectrophoto-meters. Melting points were determined us-
ing a calibrated thermometer by Remi Digital Melting Point apparatus
and are uncorrected. Elemental analyses were performed by Central
Drug Research Institute, Lucknow. NMR (1H, 13C, and 29Si) spectra
were recorded on a JEOL AL 300 instrument (Japan). All chemical
shifts were reported in parts per million relative to TMS as an inter-
nal standard in CDCl3. Mass spectra were recorded at 70 eV ionizing
voltage on a JEOL-D300 MS instrument.
Synthesis of Salicylidene-o-hydroxyacetophenone (I)
The Schiff base I was prepared by simple condensation of salicylalde-
hyde hydrazone (4.76 g, 35 mmoles) and o-hydroxyacetophenone (4.76 g,
35 mmoles) in dry methanol (150 mL). The reaction mixture was stirred
at reflux for 2 h and cooled to r.t. The yellow needles of salicylidene-o-
hydroxyacetophenone thus obtained were washed with methanol, dried,
and recrystallized from ethanol.
I: Yield, 6.13 g, (70%), m.p. 159◦C; IR (KBr, cm−1): 3435, 1633, 1689;
1H NMR (300 MHz, CDCl3, δ): 13.13 (s, 1H, OH, D2O exchangeable),
11.05 (s, 1H, OH, D2O exchangeable), 8.94 (s, 1H, CH=N), 7.00–7.91
(m, 8H, ArH), 2.73 (s, 3H, CH3); 13C NMR (75 MHz, CDCl3, δ ppm):
170.14, 168.05, 164.67, 160.60, 159.77, 133.41, 132.92, 132.55, 129.07,
119.74, 118.96, 118.02, 117.62, 117.4, 14.78. MS: m/z (M+) 254.27; anal.

842
M. S. Singh and P. Singh
calcd. for C15H14N2O2: C, 70.85; H, 5.55; N, 11.02. Found: C, 70.43; H,
5.22; N, 10.81.
Synthesis of Dibenzodioxadiazasilaundecine [1]
To a stirred suspension of NaH (96 mg, 4 mmoles) in dry dioxane (5 mL)
was added dioxane solution (55 mL) of ligand I (508 mg, 2 mmoles) drop-
wise with constant stirring and, it was refluxed for 4 h in an inert atmo-
sphere. Thus the yellowish brown solution of dianion generated in situ
was cooled to r.t. dichlorodimethylsilane (258 mg, 2 mmoles) was added
dropwise with constant stirring, and the reaction mixture was refluxed
for an additional 5 h. Completion of the reaction was checked by TLC.
The reaction mixture was evaporated with the help of a rotary evapora-
tor, and the residue obtained was subjected to solumn chromatography
(n-hexane/ethyl acetate, 8:1) to give 1. All other silaheterocycles (2–8)
were also synthesized analogously as previously described in the de-
sired molar ratios. The analytical and spectral data for compounds 1 to
8 are listed in Tables I and II, respectively.
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