Facile solvent-free synthesis and structural elucidation of styrylcyclohex-2-enone derivatives

Article abstract of DOI:10.1007/s00706-007-0847-3

A remarkably efficient procedure for the synthesis of styrylcyclohex-2- enone derivatives at room temperature is described using a mild reaction medium consisting of lithium perchlorate and N-(trimethylsilyl)diethylamine. Several compounds of this class a

Full text of DOI:10.1007/s00706-007-0847-3

Monatsh Chem 139, 917–921 (2008)
DOI 10.1007/s00706-007-0847-3
Printed in The Netherlands
Facile solvent-free synthesis and structural elucidation of styrylcyclohex-
2-enone derivatives
Mohammad M. Mojtahedi1, Mohammad Saeed Abaee1, Mohammad Mehdi Zahedi1,
Mohammad R. Jalali1, A. Wahid Mesbah1, Werner Massa2, Roholah Yaghoubi3, Mehdi Forouzani3
1
Organic Chemistry Laboratory, Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
2
Fachbereich Chemie der Philipps-Universitaet Marburg, Marburg, Germany
3
Sari Center, Payam Noor University, Sari, Iran
Received 8 September 2007; Accepted 1 November 2007; Published online 26 June 2008
# Springer-Verlag 2008
Abstract A remarkably efficient procedure for the
synthesis of styrylcyclohex-2-enone derivatives at
room temperature is described using a mild reaction
medium consisting of lithium perchlorate and N-(tri-
methylsilyl)diethylamine. Several compounds of this
class are synthesized conveniently and rapidly. Spec-
troscopic and X-ray diffraction experiments confirm
the proposed structures.
been made to widen the synthetic scope of this con-
densation reaction [4]. However, Claisen-Schmidt
condensation of aldehydes with conjugated cyclic
enones at their allylic position to produce pentadie-
none systems has not been comprehensively studied
and in this regard only scattered data are reported
in literature [5, 6]. In many of these reports, either
no yields are provided or low quantities of products
are obtained under strong basic conditions at ele-
vated temperatures after long reaction times [5].
Thus, there is a demand for the investigation on syn-
thetic methodology and structural elucidation of
these products.
Keywords Claisen-Schmidt condensation; ꢀ,ꢁ-Unsaturated
carbonyls; Solvent-free; Lithium perchlorate.
Introduction
In recent years lithium perchlorate (LiClO4) has
emerged as a powerful Lewis acid to conveniently
catalyze different organic transformations [7]. As a
result, a dramatic improvement in many synthetic
procedures is observed by the use of LiClO4 in terms
of rate acceleration, selectivity enhancement, and
yield increase [8]. In continuation of our previous
efforts on the use of LiClO4 in synthetic organic
chemistry [9, 10] and in the framework of our inves-
tigations on Claisen-Schmidt condensation of vari-
ous cyclic ketones [11], we would like to introduce
a facile procedure for the solvent-free condensation
of 1 with various aldehydes under LiClO4 and N-
(trimethylsilyl)diethylamine (TMSNEt2) catalysis at
room-temperature (Scheme 1). Consequently, a rapid
ꢀ,ꢁ-Unsaturated carbonyl compounds are important
intermediates in synthetic organic chemistry [1] and
many methods for their synthesis have been reported
so far [2]. The mixed aldol reaction, known as the
Claisen-Schmidt condensation [3], is the most useful
method extensively employed for the synthesis of
ꢀ,ꢁ-unsaturated carbonyl systems via the reaction
of aromatic aldehydes with ꢀ-acidic carbonyl moi-
eties. In recent years several improvements have
Correspondence: Mohammad M. Mojtahedi and Mohammad
Saeed Abaee, Organic Chemistry Laboratory, Chemistry and
Chemical Engineering Research Center of Iran, P.O. Box
14335-186, Tehran, Iran. E-mails: mojtahedi@ccerci.ac.ir;
abaee@ccerci.ac.ir

918
M. M. Mojtahedi et al.
Scheme 1
access for the synthesis of the title compounds is
offered along with full spectral characterization of
the products and X-ray diffraction analysis of the
phenyl derivative.
Fig. 1 Structure of 2a at 193 K in the crystal. Displacement
ellipsoids at 50% probability level
Results and discussion
A variety of aldehydes were subjected to condense
with isophorone 1 at room-temperature using LiClO4
and TMSNEt2 in the presence of no additional sol-
vent. The results are summarized in Table 1. When
enone 1 was treated with benzaldehyde under these
conditions, TLC monitoring of the reaction showed
complete disappearance of the aldehyde and 85% for-
mation of 2a after 4 h (entry 1). Control experiments
shed light on the role of the medium. Conduction
of the reaction in the absence of LiClO4 led to no
formation of any product even after 24h illustrating
the promoting effect of the catalyst. Presence of
TMSNEt2 was also shown to be crucial for the reac-
tion to proceed. When TMSNEt2 was omitted from
the mixture, less than 10% of 2a formed after several
hours. The use of alternative amines such as Et3N and
HNEt2 prolonged the reaction time and the process
did not reach complete conversion.
drogen atoms with a coupling constant of about
16 Hz was a crucial indication for the condensation
occurring at the allylic methyl group. In order to ver-
ify this structure, a single crystal of 2a was prepared
and subjected to an X-ray diffraction experiments.
The outcome, as depicted in Fig. 1, clearly supports
the proposed structure.
Similar products were prepared in the same
manner by subjecting various aldehydes to the same-
conditions. Therefore, when different aldehydes
bearing various groups (entries 2–6) were mixed
with 1, TMSNEt2, and LiClO4, 2b–2f were obtained
in 83–93% within 3–4 h. Similarly, thiophene-2-car-
baldehyde (entry 7) and furfural (entry 8) yielded 2g
and 2h, respectively, within the same time period.
All reactions proceeded efficiently and rapidly and
formation of typical side products, which was re-
ported previously [5], was not observed. Structures
of 2b–2h were similarly assigned based on their
Structure of 2a was confirmed with spectroscopic
methods. Presence of a pair of vicinal olefinic hy-
1
spectroscopic analysis by H NMR, 13C NMR, IR,
and mass spectroscopy and their purity was con-
firmed by elemental analysis.
Table 1 LiClO4 mediated condensation of 1 with various
aromatic aldehydes
Based on these results, a mechanistic pathway can
be offered for the reactions as depicted in Fig. 2.
Reaction of 1 with TMSNEt2 affords formation of
1a and HNEt2. In situ formation of the iminium salt
3, as a result of the reaction of aldehydes with
HNEt2 in the presence of LiClO4, has been postulat-
ed previously and is attributed to the Lewis acidity of
the lithium ion and the high polarity of the reaction
medium [10, 12]. Nucleophilic addition of 1a to 3
followed by elimination of HNEt2 yields products
2a–2h.
Entry Aldehyde
Product
2=Ar
Yield Ref.
=%a
1
2
3
4
Benzaldehyde
4-Methylbenzaldehyde
4-Methoxybenzaldehyde 2c=4-MeOPh 88
2a=Ph
2b=4-MePh 83
85
[5c]
–
[5c]
–
2,4,6-
Trimethoxybenzaldehyde (MeO)3Ph
2d=2,4,6-
89
5
6
7
4-Chlorobenzaldehyde
4-Bromobenzaldehyde
Thiophene-2-
2e=4-ClPh
2f=4-BrPh
2g=2-thienyl 87
92
93
[5c]
–
–
carbaldehyde
Furfural
In conclusion, we introduced a very simple proce-
dure for the synthesis of the title compounds at room
temperature and without the presence of additional
8
2h=2-furyl 89
–
a
Isolated yields

Facile solvent-free synthesis
919
Fig. 2 Suggested mechanistic pathway for the reaction
(E)-5,5-Dimethyl-3-styrylcyclohex-2-enone (2a, C16H18O)
solvent. In contrast to the few related existing reports
[5] the medium used in our procedure is mild, reac-
tions complete rapidly, formation of side products is
not observed, and full spectroscopic characterization
of the products is provided. The products are cur-
rently under investigation for the synthesis of deca-
line systems via Diels-Alder reactions.
Yellow crystals were obtained in 85% yield; mp 77–78ꢀC
(Ref. [5c] 76–78ꢀC); IR (KBr): ꢃꢀ¼ 3034, 1639, 1288 cmꢁ1; 1H
NMR (500MHz, CDCl3): ꢂ ¼ 7.53 (d, 2H, J ¼ 7.3 Hz), 7.42–
7.35 (m, 3H), 7.04 (d, 1H, J ¼ 16.2Hz), 6.95 (d, 1H, J ¼
16.2Hz), 6.12 (s, 1H), 2.52 (s, 2H), 2.35 (s, 2H), 1.15 (s, 6H)
ppm; 13C NMR (125MHz, CDCl3): ꢂ ¼ 200.6, 155.2, 136.4,
135.4, 130.0, 129.5, 129.3, 127.7, 127.6, 51.9, 39.5, 33.8,
28.9ppm; MS (70 eV): m=z ¼ 226 (Mþ), 193, 170, 141, 115.
Experimental
(E)-5,5-Dimethyl-3-(2-p-tolylvinyl)cyclohex-2-enone
(2b, C17H20O)
Reactions were monitored by TLC using silica gel coated
plates and ethyl acetate=n-hexane solutions as mobile phase.
Melting points were determined by a Bu¨chi 530 melting
point apparatus. FT-IR spectra were recorded using KBr
disks on a Bruker Vector-22 infrared spectrometer and ab-
sorptions are reported as wave numbers. NMR spectra were
obtained on a FT-NMR Bruker Ultra ShieldTM (500 MHz)
instrument from CDCl3 solutions and the chemical shifts
are expressed as ꢂ units with Me4Si as the internal stan-
dard. Mass spectra were obtained on a Finnigan Mat 8430
apparatus at an ionization potential of 70 eV. Elemental
analyses were performed using a Thermo Finnigan Flash
EA 1112 instrument and results agreed favourably with
calculated values. All reagents were purchased from com-
mercial sources and were used after being purified by stan-
dard procedures.
Yellow crystals were obtained in 83% yield; mp 102–103ꢀC;
1
IR (KBr): ꢃꢀ¼ 3026, 1643, 1256 cmꢁ1; H NMR (500 MHz,
CDCl3): ꢂ ¼ 7.43 (d, 2H, J ¼ 8 Hz), 7.22 (d, 2H, J ¼ 8 Hz),
7.00 (d, 1H, J ¼ 16.2Hz), 6.91 (d, 1H, J ¼ 16.2Hz), 6.10
(s, 1H), 2.51 (s, 2H), 2.41 (s, 3H), 2.35 (s, 2H), 1.15 (s, 6H)
ppm; 13C NMR (125 MHz, CDCl3): ꢂ ¼ 200.6, 155.4,
139.8, 135.4, 133.7, 130.0, 129.0, 127.6, 127.1, 51.9, 39.5,
33.8, 28.9, 21.8ppm; MS (70 eV): m=z ¼ 240 (Mþ), 225,
169, 141, 115, 82.
(E)-3-[2-(4-Methoxyphenyl)vinyl]-5,5-dimethylcyclohex-2-
enone (2c, C17H20O2)
Yellow crystals were obtained in 88% yield; mp 61–62ꢀC
(Ref. [5c] 63–64ꢀC); IR (KBr): ꢃꢀ¼ 3025, 1644, 1254cmꢁ1
;
1H NMR (500 MHz, CDCl3): ꢂ ¼ 7.47 (d, 2H, J ¼ 8.7 Hz),
6.98 (d, 1H, J ¼ 16.2Hz), 6.92 (d, 2H, J ¼ 8.7 Hz), 6.81 (d,
1H, J ¼ 16.2Hz), 6.06 (s, 1H), 3.89 (s, 3H), 2.48 (s, 2H), 2.33
(s, 2H), 1.13 (s, 6H) ppm; 13C NMR (125MHz, CDCl3):
ꢂ ¼ 200.6, 160.9, 155.6, 135.1, 129.2, 129.1, 127.8, 126.7,
114.8, 55.8, 51.9, 39.5, 33.7, 28.9ppm; MS (70 eV):
m=z ¼ 256 (Mþ), 223, 172, 115.
General procedure
A solvent-free mixture of LiClO4 (425mg, 4 mmol) and
TMSNEt2 (852 mm3, 4.5mmol) was stirred at room tempera-
ture under inert atmosphere for 10 min. To this mixture was
added the aldehyde (4mmol) and stirring was continued for
another 10min. Enone 1 (580 mg, 4.2 mmol) was added to the
reaction vessel. The course of the reaction was monitored until
TLC and GC experiments showed complete disappearance of
the starting aldehyde. The mixture was diluted by diethyl ether
(10 cm3), washed with 5% HCl (15 cm3) solution, the organic
phase was dried over Na2SO4, and the solvent was removed in
vacuo by a rotary evaporator. The residue was recrystallized
from hexane and the product was analyzed by physical and
spectroscopic methods.
(E)-5,5-Dimethyl-3-[2-(2,4,6-trimethoxyphenyl)vinyl]-
cyclohex-2-enone (2d, C19H24O4)
Dark yellow crystals were obtained in 89% yield; mp 151–
152ꢀC; IR (KBr): ꢃꢀ¼ 3020, 1640, 1582 cmꢁ1
;
1H NMR
(500MHz, CDCl3): ꢂ ¼ 7.38 (d, 1H, J ¼ 16.5 Hz), 7.33 (d,
1H, J ¼ 16.5Hz), 6.17 (s, 2H), 6.06 (s, 1H), 3.92 (s, 6H),
3.88 (s, 3H), 2.53 (s, 2H), 2.33 (s, 2H), 1.14 (s, 6H) ppm;
13C NMR (125MHz, CDCl3): ꢂ ¼ 200.6, 162.1, 160.7, 158.1,

920
M. M. Mojtahedi et al.
130.6, 126.9, 125.7, 107.5, 91.1, 56.1, 55.7, 51.9, 39.2, 33.7,
29.0ppm; MS (70 eV): m=z ¼ 316 (Mþ), 301, 285, 231,
207, 115.
structure factors) for the structure reported in this paper have
been deposited with the Cambridge Crystallographic Data
Center as supplementary publication no. CCDC-629605.
Copies of the data can be obtained free of charge on applica-
tion to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
[Fax: int. Code þ44(1223)336-033; E-mail: deposit@ccdc.
cam.ac.uk or via www.ccdc.cam.ac.uk=conts=retrieving.html].
(E)-3-[2-(4-Chlorophenyl)vinyl]-5,5-dimethyl-cyclohex-2-
enone (2e, C16H17ClO)
Yellow crystals were obtained in 92% yield; mp 107–108ꢀC
1
(Ref. [5c] 111ꢀC); IR (KBr): ꢃꢀ¼ 3038, 1646, 1379 cmꢁ1; H
NMR (500 MHz, CDCl3): ꢂ ¼ 7.47 (d, 2H, J ¼ 8.5 Hz), 7.38
(d, 2H, J ¼ 8.5 Hz), 6.99 (d, 1H, J ¼ 16 Hz), 6.90 (d, 1H, J ¼
16Hz), 6.12 (s, 1H), 2.51 (s, 2H), 2.36 (s, 2H), 1.15 (s, 6H)
ppm; 13C NMR (125 MHz, CDCl3): ꢂ ¼ 200.5, 154.7, 135.2,
135.0, 133.9, 130.6, 129.5, 128.8, 127.9, 51.8, 39.5, 33.8,
28.9ppm; MS (70 eV): m=z ¼ 260 (Mþ), 227, 169, 141, 115.
Acknowledgement
Partial financial support of this work by the Ministry of Science,
Research, and Technology of Iran is gratefully appreciated.
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1264 cmꢁ1
;
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˚
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3
ꢀ
˚
˚
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˚

Facile solvent-free synthesis
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