A facile synthesis of 1,4-diacylbenzenes

Article abstract of DOI:10.1081/SCC-120023419

An efficient method for the synthesis of 1,4-diacylbenzenes has been developed employing bis-tetrabutylammonium dichromate as an oxidant. By this methodology, a series of aldehydes and ketones have been synthesised under mild reaction conditions in moderate yields.

Full text of DOI:10.1081/SCC-120023419

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A Facile Synthesis of 1,4-Diacylbenzenes
Harindran Suhana a & Panyencheri C. Srinivasan a
a Department of Organic Chemistry , University of Madras , Guindy Campus, Chennai, Tamil
Nadu, India
Published online: 17 Aug 2006.
To cite this article: Harindran Suhana & Panyencheri C. Srinivasan (2003) A Facile Synthesis of 1,4-Diacylbenzenes, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 33:18, 3097-3102, DOI:
10.1081/SCC-120023419
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 18, pp. 3097–3102, 2003
A Facile Synthesis of 1,4-Diacylbenzenes
*
Harindran Suhana and Panyencheri C. Srinivasan
Department of Organic Chemistry, University of
Madras, Guindy Campus, Chennai, Tamil Nadu, India
ABSTRACT
An efficient method for the synthesis of 1,4-diacylbenzenes has been
developed employing bis-tetrabutylammonium dichromate as an
oxidant. By this methodology, a series of aldehydes and ketones
have been synthesised under mild reaction conditions in moderate
yields.
Key Words: Benzylic bromides; Diacylbenzenes; Indole-2-alde-
hydes; Tetrabutyl ammonium dichromate; Benzophenones.
*Correspondence: Panyencheri C. Srinivasan, Department of Organic Chemistry,
University of Madras, Guindy Campus, Chennai 600025, Tamil Nadu, India;
E-mail: pancol2000@yahoo.co.in.
3097
DOI: 10.1081/SCC-120023419
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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3098
Suhana and Srinivasan
There is much interest in the design and synthesis of molecular systems
comprised of electron donors and acceptors that mimic the electron
transfer reactions in nature.[1] In the course of a proposed synthesis of
indoles with a benzophenone unit, we needed gram quantities of 4-acetyl-
benzophenone. A literature survey revealed that 4-acetylbenzophenone
had been prepared by the reaction of dimethylcadmium with p-benzoyl-
benzoylchloride[2] and by palladium catalyzed carbonylative cross
coupling reaction[3]of arylboronic acids with aryl electrophiles. There-
fore, it was of interest to develop a new and simple synthetic method
for 1,4-diacylbenzenes[4] using nontoxic and inexpensive reactants.
A study of the literature indicated that benzyl bromides could be
converted to the corresponding carbonyl compounds by bis-tetrabutyl-
ammonium dichromate.[5] Activated alkyl halides can be converted to
the corresponding aldehydes or ketones by chromate ion supported on
anion exchange resins[6] or in hexamethylphosphoramide as solvent in
the presence of stoichiometric amounts of crown ethers.[7] Our initial
attempts to oxidize 4-acetyldiphenylmethane to 4-acetylbenzophenone
using CrO3 in acetic acid or pyridine failed. So the alkyl benzenes
(C6H5R, R ¼ CH3, CH2CH3, CH2CH2CH3, CH2C6H5) were subjected
to Friedal Craft’s benzoylation followed by side chain bromination
with N-bromosuccinimide. The crude benzyl bromides were oxidized by
bis-tetrabutylammonium dichromate to the corresponding carbonyl com-
pounds in 60–90% yield. The reaction conditions employed and the yield
of the various products obtained are shown in Table 1.
(The compounds 2b,c,e and 3b,c,e were obtained as thick syrupy
liquids.)
This procedure was extended further to the synthesis of indole
2-aldehydes[8a,b] 4f and 4g as shown below. The known N-phenyl-
sulphonyl-2-bromomethylindoles were prepared by published methods[9]
and the results are summarized in Table 2.
CH2Cl2
AlCl3
C
O
Cl
R'
CH2R
+
R'
CH2R
C
O
1
2
[ (C4H9)4N ]2Cr2O7
CHCl3
NBS/ CCl4
R
C
O
R'
CHR
Br
C
R'
C
O
Benzoyl peroxide
O
4
3
Scheme 1.

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1,4-Diacylbenzenes
3099
Table 1. Synthesis of 1,4-diacylbenzenes using bis-tetrabutylammonium
dichromate.
Productyield (%)
Substrate 1a–e
R
R0
Solvent
Toluene
2a–e
3a–e
4a–e
Toluene
H
CH3
H
80
78
75
68
70
88
82
85
65
68
65
72
99
60
75
Ethyl benzene
Propyl benzene
Toluene
H
H
Dichloromethane
Dichloromethane
CH2CH3
H
CH3 Toluene
Dichloromethane
Diphenylmethane Ph
H
Table 2. Synthesis of indole-2-aldehydes 4f and 4g.
ProductYield (%)
M.p. (ꢀC)
2f
81
80
90
80
75
55
140
70
2g
3f
136
115
210
98
3g
4f
4g
R
R
NBS/ CCl4
Benzoyl peroxide
N
CH3
N
H
CH3
SO2Ph
1
2
f) R= CH3
g) R= SPh
R
R
[ (C4H9)4N ]2Cr2O7
CHCl3
CH2Br
N
CHO
N
SO2Ph
SO2Ph
4
3
Scheme 2.

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3100
Suhana and Srinivasan
The melting points, IR and NMR data of the above indole-
2-aldehydes match with those of the authentic samples reported from
this laboratory by other methods.[8b]
EXPERIMENTAL
Representative Procedure for the Synthesis of
Alkyl Benzenes (2a–e)
To a solution of the alkyl benzenes 1(a–e) (100 mmol) in dichloro-
methane was added the aroyl chloride (110 mmol) at 0ꢀC. Powdered
anhydrous AlCl3 (110 mmol) was added in small portions to this solution
with vigorous stirring for 3 h at 0ꢀC. It was subsequently allowed to warm
to 25–30ꢀC and then stirred for 1 h. The reaction mixture was poured
over crushed ice, extracted with dichloromethane and dried (Na2SO4).
The solvent was distilled off to give the crude product, which was purified
by chromatography over silica gel. The yields and melting points of the
ketones 2a and 2d are listed below.
(1) 4-Methylbenzophenone[10] (2a): m.p. 56ꢀC (Lit.[10] m.p.
56.5–57ꢀC). Yield 80%.
(2) 4,40-Dimethylbenzophenone (2d): m.p. 92ꢀC (Lit [1.1] m.p.
91–93ꢀC). Yield 68%.
The IR and NMR spectra of all these benzophenones were in accor-
dance with their structures. The ketones 2b[12], 2c[13] and 2e were obtained
as thick viscous liquids.
General Procedure for the Synthesis
of Benzyl Bromides (3a–e)
A mixture of the 4-benzoylalkylbenzenes (2a–e) (25 mmol) and finely
powdered N-bromosuccinimide (25 mmol) in dry CCl4 (150 mL) was
refluxed over a 100 W bulb until the succinimide floated at the top. The
resulting solution was cooled and the succinimide was filtered off. The
solution was concentrated to 50 mL and hexane was added to this at
room temperature to give the bromo compounds 3a[14] and 3d as crystal-
line solids. The remaining bromo compounds were obtained only as thick
liquids on removal of the solvent.

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1,4-Diacylbenzenes
3101
General Procedure for the Synthesis of 1,4-Diacylbenzenes
(4a–e) and Indole-2-aldehydes (4f, g)
A homogenous solution of the benzyl halides (0.1 mol) in chloroform
and bis-tetrabutyl ammonium dichromate (0.06 mol) in chloroform
(20 mL) was heated under reflux for 3 h. The mixture was filtered through
silica gel (20 g) to remove all the inorganic products. The silica was then
washed with diethyl ether (100 mL). Removal of the combined organic
filtrates and purification of the residue by chromatography on silica gel
afforded the pure carbonyl compounds in yields ranging from 60–90%.
The yield, melting points, IR and NMR data of all the benzophenones
are listed below.
(1) 4-Benzoyl benzaldeyde (4a): m.p. 58–60ꢀC. (Lit.
m.p.
[15]
62–65ꢀC) 65%. IR (KBr pellet, cmꢁ1) 1669, 1655; L H NMR
(CDCl3/TMS, ppm) ꢀ 10.04 (s, 1H,CHO), 7.2–7.9 (m, 9H, Ar).
(2) 4-Acetylbenzophenone (4b): m.p. 80–84ꢀC (Lit.[2] m.p. 83–84ꢀC)
72% IR (KBr pellet, cmꢁ1) 1689, 1656; 1H NMR (CDCl3/TMS,
ppm): ꢀ 2.56 (s, 3H, CH3) 7.2–8 (m, 9H, Ar).
1
(3) 4-Propionylbenzophenone (4c): m.p. 86ꢀC, 99% IR (KBr pellet,
1
cmꢁ1) 1678, 1601; H NMR (CDCl3/TMS, ppm): ꢀ 1.4 (t, 3H,
CH3), 3.1 (q, 2H, CH2), 7.1–8.1 (m, 9H, Ar). HRMS; Calcd. for
C16H14O2:238.2846, Found 238.1977.
(4) 4-p-Toluoylbenzaldehyde[3] (4d): m.p. 84–86ꢀC, 60%. IR (KBr
1
pellet, cmꢁ1) 1703, 1651; H NMR (CDCl3/TMS, ppm): ꢀ 2.3
(s, 3H, CH3), 7.2–7.9 (m, 8H, Ar), 10.05 (s, 1H, CHO). The
IR and NMR spectra matched with those of an authentic sample.
(5) 1,4-Dibenzoylbenzene (4e): m.p. 162–164ꢀC (Lit.[16] m.p.
160–164ꢀC) 75%. IR (KBr pelletcm ꢁ1) 1661, 1593.
The IR spectrum matched with that of an authentic sample.
We found that this is a safe and convenient method for the synthesis
of this class of important compounds because benzophenone is not only a
good photosensitizing moiety but also serves as a good spacer unit if it
has a carbonyl function at 4 or 4,40-positions.
ACKNOWLEDGMENT
The authors acknowledge the financial support for this work
from DST and UGC-SAP for instrument grants to the department.

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3102
Suhana and Srinivasan
The authors thank Prof. T. Ishiyama of Hokkaido University, Japan for
providing the IR and NMR spectra of compound 4d.
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Received in the USA October 15, 2002