N-benzyl DABCO tribromide - Promoted oxidative coupling of benzyl cyanides: A convenient procedure for the synthesis of α, α′- dicyanostilbenes

Article abstract of DOI:10.1080/00397910801982241

A convenient and efficient procedure was developed for preparing α,α′-icyanostilbenes through the oxidative coupling reaction of benzyl cyanide derivatives using N-benzyl DABCO tribromide as the oxidative bromination reagent in the presence of K2CO3 as a base. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910801982241

This article was downloaded by: [Stony Brook University]
On: 30 October 2014, At: 17:13
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office:
Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Synthetic Communications: An International
Journal for Rapid Communication of
Synthetic Organic Chemistry
Publication details, including instructions for authors and subscription
information:
http://www.tandfonline.com/loi/lsyc20
N‐Benzyl DABCO Tribromide–Promoted
Oxidative Coupling of Benzyl Cyanides: A
Convenient Procedure for the Synthesis of α,
α′‐Dicyanostilbenes
Firouz Matloubi Moghaddam a , Dordaneh Zargarani a & Hassan Zali Boeini a
a Department of Chemistry , Sharif University of Technology , Tehran, Iran
Published online: 09 May 2008.
To cite this article: Firouz Matloubi Moghaddam , Dordaneh Zargarani & Hassan Zali Boeini (2008) N‐Benzyl
DABCO Tribromide–Promoted Oxidative Coupling of Benzyl Cyanides: A Convenient Procedure for the Synthesis
of α, α′‐Dicyanostilbenes, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 38:11, 1694-1702, DOI: 10.1080/00397910801982241
To link to this article: http://dx.doi.org/10.1080/00397910801982241
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”)
contained in the publications on our platform. However, Taylor & Francis, our agents, and our
licensors make no representations or warranties whatsoever as to the accuracy, completeness, or
suitability for any purpose of the Content. Any opinions and views expressed in this publication
are the opinions and views of the authors, and are not the views of or endorsed by Taylor &
Francis. The accuracy of the Content should not be relied upon and should be independently
verified with primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities
whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or
arising out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any substantial
or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or
distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use
can be found at http://www.tandfonline.com/page/terms-and-conditions

Synthetic Communicationsw, 38: 1694–1702, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910801982241
N-Benzyl DABCO Tribromide–Promoted
Oxidative Coupling of Benzyl Cyanides: A
Convenient Procedure for the Synthesis of
a, a0-Dicyanostilbenes
Firouz Matloubi Moghaddam, Dordaneh Zargarani,
and Hassan Zali Boeini
Department of Chemistry, Sharif University of Technology, Tehran, Iran
Abstract: A convenient and efficient procedure was developed for preparing a,a0-icya-
nostilbenes through the oxidative coupling reaction of benzyl cyanide derivatives using
N-benzyl DABCO tribromide as the oxidative bromination reagent in the presence of
K2CO3 as a base.
Keywords: Dicyanostilbene, fumaronitriles, organic ammonium tribromide (OATB),
oxidative coupling
INTRODUCTION
Dicyanostilbenes are valuable intermediates that have been used in the
pharmacological synthesis[1,2] and preparation of N-methyl-3,4-bis(4-(N-(1-
naphthyl)phenylamino)phenyl)-maleimide (NPAMLMe), as an unusual,
nondoped, host emitter for red organic light-emitting diods (OLEDs).[3]
They are also suitable for the preparation of porphyrazines.[4] Symmetrical
and unsymmetrical cyanostillbenes and various substituted cyano- and
dihydrocyanostilbenes have been prepared, characterized, and tested for estro-
genic activity.[5] The method of preparation of these compounds has been
known to chemists and can be traced back to the late 19th century.[4,6]
Received August 23, 2007
Address correspondence to Firouz Matloubi Moghaddam, Department of
Chemistry, Sharif University of Technology, P. O. Box 11155-9516, Tehran, Iran.
E-mail: matloubi@sharif.edu
1694

Promoted Oxidative Coupling of Benzyl Cyanides
1695
These methods consisted of self-condensation of the respective benzyl
cyanides by means of iodine in the presence of sodium methoxide or
sodium amide.[7,8] The a,a0-dicyanostilbenes could also result from the dehy-
drohalogenation of benzyl cyanide derivatives.[9,10] Surveying the literature
reveals that the geometrical form of dicyanostillbene in this reaction is the
trans-form (E-isomer).[10]
RESULTS AND DISCUSSION
As an alternative reagent to liquid bromine, organic ammonium tribromide
(OATB), such as benzyltrimethyl ammonium tribromide and tetrabutylammo-
nium tri-bromide, which are high-molecular-weight, stable, crystalline solids,
are capable of delivering a stoichiometric amount of bromine where small
amount of bromine are necessary for microscale reactions.[11] Recently we
have developed N-benzyl diaza bicyclo octane (DABCO) tribromide, as one
of OATBs for efficient oxidative deprotection of dithioacetals and oxidative
cyclysation of benzothiazoles.[12,13] Even though the chemistry of OATBs
has been extensively studied, their use as a mild brominating reagent for
oxidative coupling reactions has been neglected.
In this article we report our attempt to extend the use of N-benzyl
DABCO tribromide as an electrophilic bromine source and one of the
OATBs for one-step oxidative coupling procedure of benzyl cyanides into
symmetrical a,a0-dicyanostilbenes. To the best of our knowledge this
reaction has not been reported previously (Scheme 1).
To find the optimum conditions for this reaction, we examined the stoichi-
ometry, temperature, and the effects of different OATBs and various solvents.
During these examinations we encountered better results by performing the
reaction at room temperature and applying 1 stoichiometric equiv. of
N-benzyl DABCO tribromide and 3 stoichiometric equiv. of K2CO3. For
investigating the effect of solvent the model compound 1 was selected.
Compound 1 was reacted with 1 molar equivalent of N-benzyl DABCO tribro-
mide (Table 1, entry 1) in dimethylsulfoxide at room temperature for 1 h to
Scheme 1. Oxidative coupling of benzyl cyanide derivatives using N-benzyl
DABCO tribromide.

1696
F. M. Moghaddam, D. Zargarani, and H. Z. Boeini
Table 1. Variation of solvent in the oxidative coupling of benzylcyanide with
N-benzyl DABCO tribromide
Entry
Solvent
Reaction conditions
Yield (%)
1
2
3
4
5
DMSO
MeOH
CH2Cl2
rt/1 h
rt/3 h
rt/3 h
rt/5 h
rt/5 h
88
63
45
CH2Cl2-CCl4
CCl4
,10
,10
give the desired product 2 in 88% yield. Dichloromethane and methanol were
also found effective, but the yields were low (Table 1, entries 2, 3). Other
nonpolar solvents such as CCl4 and its mixture with CH2Cl2 gave little
product (,10%) (Table 1, entries 4, 5)
.
By examining the effect of two different OATBs in the reaction course, it
was found that N-benzyl DABCO tribromide gave higher yield and the
reaction proceeded in a shorter time (88%, 1 h versus 80%, 1.5 h) (Table 2).
The generality of the method was confirmed by the successful conversion
of various substrates to the corresponding a,a0-dicyanostilbene compounds,
and the results are summarized in Table 3. The best yield (90%) was
obtained for 4-nitro benzyl cyanide (Table 3, entry 1), and the least yield
(57%) is reported for a-naphthyl acetonitrile (Table 3, entry 6). The
reaction proceeds easily and cleanly and no bromination takes place on the
aromatic ring.
Table 2. Variation of OATBs in the oxidative coupling of benzylcyanides
Entry
OATB
Reaction conditions
Yield (%)
1
2
PhCH2NEt3Br3
rt/1.5 h
rt/1 h
80
80

Table 3. Result of oxidative coupling reaction of benzyl cyanide leading to derivatives of a,a0-dicyanostilbenes
Entry
1
Arycyanide
Time (h)
1
Dicyanostilbenea
Mp found (8C) (mp lit.)
Yield (%)b
90
188–190 (190)[15, 16]
2
1
166–168 (168)[14]
88
3
1.5
212–214 (212–214)[7a]
82
(continued)

Table 3. Continued
Entry
4
Arycyanide
Time (h)
1.5
Dicyanostilbenea
Mp found (8C) (mp lit.)
Yield (%)b
73
174–176
5
2.5
161–163(161–163)[17]
60
57
6
3
167–169
a1H-NMR reveals that only the E-isomer is produced in the reaction course.
bAll yields refer to the isolated products.

Promoted Oxidative Coupling of Benzyl Cyanides
MECHANISM OF THE REACTION
1699
The condensation of two molecules of a-halobenzylcyanides with bases and
related reactions were studied by numerous investigators, and several mechan-
isms were postulated.[3,6,7b,18–22] On the basis of the scattered evidence found in
the literature, the proposed mechanism involves a-proton abstraction from
benzyl cyanide, followed by a rapid halogenation and subsequent nucleophilic
attack of the formed carbanion on a-bromobenzyl cyanide to form 2-bromo-
2,3-diaryl stillbene, and finally elimination of hydrogen bromide to give the
desired compound.[3,6] On the basis of these studies, we propose a mechanism
that indicates a logical explanation for the reaction pathway (Scheme 2).
In conclusion, an efficient method has been developed for the preparation
of a,a0-dicyanostilbenes. The reagent used in this methodology is safe, easily
available, and nontoxic as compared to the reported procedures.
EXPERIMENTAL SECTION
The compounds were characterized by their spectroscopic data (1H NMR and
13C NMR spectra and compared with authentic samples.[4,7a,9a,17] A Bruker
(DRX-500 Avance) NMR instrument was used to record the spectra, and all
of spectra were determined in CDCl3 at ambient temperature. N-Benzyl-
DABCO-tribromide was prepared as reported in reference [12].
General Procedure for the Preparation of Dicyanostillbnes
N-Benzyl-DABCO-tribromide (1 mmol) in one portion was added to a stirred
mixture of benzylcyanide (1 mmol) and K2CO3 (3 mmol) in DMSO (10 ml).
The stirring was continued for the time noted in Table 3, at room temperature.
By adding a portion of water (10 ml) the a,a0-dicyanostillbene was slowly
Scheme 2. Mechanism proposed for oxidative coupling of benzylcyanide by oxi-
dative brominative reagents:[3,6] a) base: K2CO3, and b) OATB: N-benzyl-DABCO-
tribromide.

1700
F. M. Moghaddam, D. Zargarani, and H. Z. Boeini
precipitated from the solution. The solid was separated by filtration and
washed with portions of water. Pure compound (E-isomer) was easily
obtained by recrystallization of the solid from dilute methanol and/or by
column chromatography for some derivatives.
Data
Bis-(3-chlorophenyl)fumaronitrile (Table 3, Entry 4). Lemon-yellow solid.
1
Yield 73%. Mp: 174–176 8C. H NMR (500 MHz, CDCl3): d (ppm) 7.85
(s, 2H), 7.77 (d, 2H, J ¼ 7.71 Hz), 7.54–7.61 (m, 4H). 13C NMR
(125 MHz, CDCl3): d (ppm) 135.9, 133.5, 132.5, 131, 129.1, 127.2, 125.5,
116.2.
Dinaphtylfumaronitril (Table 3, Entry 6). White solid. Yield 57%.
Mp: 167–169 8C. 1H NMR (500 MHz, CDCl3): d (ppm) 9.30 (d, 2H,
J ¼ 8.5 Hz), 8.13 (d, 2H, J ¼ 8.2 Hz), 8.03 (d, 2H, J ¼ 7.5 Hz), 7.95
(d, 2H, J¼ 7.5 Hz), 7.74 (t, 2H, J ¼ 7.3 Hz), 7.62–7.69 (m, 4H). 13C NMR
(125 MHz, CDCl3): d (ppm) 133.2, 132.5, 130.1, 127.9, 127.4, 125.4,
125.1, 124.9, 123.2, 122.2, 121, 116.8.
Bis-(4-bromophenyl)fumaronitrile (Table 3, Entry 3). Off-white solid.
1
Yield 82%. Mp: 212–214 8C. H NMR (500 MHz, CDCl3): d (ppm) 7.72
(d, 4H, J ¼ 8.8 Hz), 7.76 (d, 4H, J ¼ 8.8 Hz). 13C NMR (125 MHz,
CDCl3): d (ppm) 133.1, 131, 130.5, 127.2, 125, 116.5.
Diphenylfumaronitrile (Table 3 Entry 5). White solid. Yield 60%. Mp:
161–163 8C. 1HNMR (500 MHz, CDCl3): d (ppm) 7.89–7.91 (m, 4H),
7.48–7.52 (m, 6H). 13CNMR (125 MHz, CDCl3): d (ppm) 131, 129.5,
128.1, 126.2, 124.1, 116.9.
Bis-(4-chlorophenyl)fumaronitrile (Table 3, Entry 4). White solid. Yield
1
88%. Mp:166–168 8C. H NMR (500 MHz, CDCl3): d (ppm) 7.81 (d, 4H,
J ¼ 8.6 Hz), 7.56 (d, 4H, J ¼ 8.6 Hz). 13C NMR (125 MHz, CDCl3): d
(ppm) 133.5, 131.2, 130.7, 127.3, 125.6, 116.5.
Bis-(4-nitrophenyl)fumaronitrile (Table 3, Entry 1). Off-yellow solid.
1
Yield 90%. Mp: 188–190 8C. H NMR (500 MHz, DMSO d-6): d (ppm)
8.16 (d, 4H, J ¼ 8.4 Hz), 8.48 (d, 4H, J ¼ 8.4 Hz). 13C NMR (125 MHz,
DMSO d-6): d (ppm) 142.1, 132, 131.4, 125.2, 121, 116.9.
REFERENCES
1. Cheng Fu, S.; Sah, P. P. T. Synthesis of 4,40-dicyanostilbene. J. Amer. Chem. Soc.
1942, 64, 1482.

Promoted Oxidative Coupling of Benzyl Cyanides
1701
2. Irie, M.; Mohri, M. Thermally irreversible photochromic system: Reversible
photocyclization of diarylethene derivatives. J. Org. Chem. 1988, 53, 803–808.
3. (a) Yeh, H. C.; Wu, W. C.; Wen, Y.-S.; Dai, D. C.; Wang, J. K.; Chen, C. T.
Derivatives of a,b-dicyanostilbene: Convenient precursor for the synthesis of
diphenylmaleimide compounds, E-Z isomerization, crystal structure, and solid-
state florescence. J. Org. Chem. 2004, 69, 6455–6462; (b) Yeh, H. C.;
Chan, L. H.; Wu, W. C.; Chen, C. T. Non-doped red organic light-emitting
diodes. J. Mater. Chem. 2004, 14, 1293–1298.
4. Cook, A. H.; Listead, R. P. Phthalocyanines, part XI: The preparartion of octaphe-
nylporphyrazines from diphenylmaleinitrile. J. Chem. Soc. 1937, 929–933.
5. Niederl, J. B.; Ziering, A. Symmetrical cyanostilbens. J. Amer. Chem. Soc. 1942,
2486–2487.
6. Ogata, Y.; Nagura, K. Kinetics of the oxidative coupling of benzyl cyanides by
halogen or hypohalite. J. Org. Chem. 1974, 39, 394–340.
7. (a) Weizmann, M.; Patai, S. 4,40-Dibromo-a,a0-dicyanostilbene. J. Amer. Chem.
Soc. 1949, 2587; (b) Boyer, J. H.; Selvarajan, R. a,b-Dicyanostilbene from pheny-
lethynyl azide and from phenylethynyl isocyanate. J. Am. Chem. Soc. 1969,
6122–6126.
8. Davies, W.; Maclaren, J. A. The reaction of a-bromobaenzyl cyanide with ethyl
xanthamidate (thioncarbamate). J. Chem. Soc. 1951, 1434–1437.
9. (a) Taylor, E. C.; Berchtols, G. A.; Goeckner, N. A.; Stroehmann, F. G. Some
further reaction of a-cyanobenzyl benzenesulfonate. J. Org. Chem. 1961, 26,
2715–2718; (b) Minami, T.; Takimoto, F.; Agawa, T. Reaction of N-sulfinylary-
lamines with carbonyl compounds and a nitrile in the presence of copper. J. Org.
Chem. 1976, 41, 3811–3813.
10. Williams, A. J.; Le Fever, J. W. The configuration of the so-called “diphenylma-
leinitrile”. J. Chem. Soc. 1957, 2425–2426.
11. Jordan, A. D.; Luo, C.; Reitz, A. B. Efficient conversion of substituted aryl
thioureas to 2-aminobenzothiazoles using benzyltrimethylammonium tribromide.
J. Org. Chem. 2003, 68, 8693–8696.
12. Matloubi Moghaddam, F.; Zali Boeini, H. Oxidative cyclization of thiobenzani-
lides to benzothiazoles using N-benzyl-DABCO tribromide under mild conditions.
Synlett. 2005, 1612–1614.
13. Matloubi Moghaddam, F.; Zali Boeini, H.; Zargarani, D.; Rezanejad Bardjee, G.
N-Benzyl-DABCO tribromide as an efficient and mild reagent for deprotection
of dithioacetals. Synth. Commun. 2005, 36, 1093–1096.
14. Anbar, M.; Dostrovsky, I.; Samuel, D.; Yoffe, A. D. Esters of inorganic oxyacids,
part I: The mechanism of hydrolysis of alkyl esters, 18O being used as tracer, and
its relation to other reactions in alkalin media. J. Chem. Soc. 1954, 3603–3611.
15. Hanna, S. B.; Iskander, Y.; Raid, Y. The influence of the nitro-group upon side-
chain reactivity, part I: The reaction between 4-nitrobenzyl cyanide and alkali.
J. Chem. Soc. 1961, 217–220.
16. Torston, J. T.; Shriner, R. L. Salts of nitro compounds, I: Preparation, alkylation
and acylation of salt of phenylnitro acetonitrile. J. Org. Chem. 1937, 2, 183–194.
17. Dodson, T.; Turner. a-Cyanoalkyl sulfonates, I: The reactions of a-cyanobenzyl
benzenesulfonate. J. Am. Chem. Soc. 1951, 73, 4517–4519.
18. Hassner, A.; Isbister, R. J. Stereochemistry, XLV: Additions to bromopheny-
lethyne. J. Amer. Chem Soc. 1969, 6126–6128.
19. Swain, C. G.; Thornton, E. R. Mechanism of a-elimination by hydroxide ion on
p-nitrobenzylsulfonium ion in aqueous solution. J. Amer. Chem. Soc 1961, 83,
4033–4034.

1702
F. M. Moghaddam, D. Zargarani, and H. Z. Boeini
20. Coe, D. G.; Gale, M. M.; Linstead, R. P.; Timmons, C. J. The cis and trans-a,b-
dicyanostilbenes, part I: The geometrical configuration and cyclisation of the
trans-isomer, the previously alleged diphenylmaleidinitrile. J. Chem. Soc. 1957,
123–130.
21. Haueser, C. R.; Brasen, W. R.; Skell, P. S.; Kantor, S. W.; Brodhag, A. E. Dimeric
olefins versus amines from a-aryl alkyl halides with alkali amides in liquid
ammonia. Intermediate dimeric halides and their dehydrohalogenation: Imines
from amines. J. Am. Chem. Soc. 1956, 78, 1653–1658.
22. Shipp, K. G.; Kaplan, L. A. Reaction of a-substituted polynitrotoluenes, II: The
generation and reactions of 2, 4, 6-trinitrobenzyl anion. J. Org. Chem. 1966, 31,
857–861.