The new alkoxy-substituted 4-piperidinobenzylidene malononitrile (PDCST) nonlinear optical chromophores were synthesized for investigation into substitutional effect on electrooptic properties and response time. The concise four-step procedure afforded PDCST skeleton with various alkoxy-substituted groups such as methoxy, ethoxy and isobutoxy. Structures were confirmed by H, C-NMR and EI-MS spectroscopic analysis. Keywords. Nonlinear optical chromophores, 4-piperidinobenzylidene malononitrile (PDCST), bromination; formylation; Buchwald-Hartwing, Knoevenagel condensation

Nakjoong, Kim

Phuong Thao, Tran Thi

Sung, Tran Van

Tuan, Nguyen Le

Vuong, Nguyen Quoc

Vietnam Academy of Science and Technology: Journals Online

  
Vietnam Journal of Chemistry, International Edition, 55(2): 163-166, 2017 
DOI: 10.15625/2525-2321.2017-00437 
163 
 
 
 
Synthesis of new alkoxy substituted 4-piperidinobenzylidene 
malononitrile (PDCST) nonlinear optical chromophores 
Nguyen Quoc Vuong
1*
, Kim Nakjoong
2
, Tran Thi Phuong Thao
3
, Nguyen Le Tuan
4
, Tran Van Sung
3
 
1
Institute of Marine Biochemistry, Vietnam Academy of Science and Technology 
2
Department of Chemistry, Hanyang University, Seoul, Korea 
3
Institute of Chemistry, Vietnam Academy of Science and Technology 
4
Department of Chemistry, Quy Nhon University 
Received 19 December 2016; Accepted for publication 11 April 2017 
 
Abstract 
The new alkoxy-substituted 4-piperidinobenzylidene malononitrile (PDCST) nonlinear optical chromophores were 
synthesized for investigation into substitutional effect on electrooptic properties and response time. The concise four-
step procedure afforded PDCST skeleton with various alkoxy-substituted groups such as methoxy, ethoxy and 
isobutoxy. Structures were confirmed by H, C-NMR and EI-MS spectroscopic analysis. 
Keywords. Nonlinear optical chromophores, 4-piperidinobenzylidene malononitrile (PDCST), bromination; 
formylation; Buchwald-Hartwing, Knoevenagel condensation. 
  
1. INTRODUCTION  
 
During the past decades, organic photorefractive 
(PR) materials have received considerable attention 
because of their potential application in 
communication, data storage and image processing 
devices [1-5]. To exhibit photorefractive property, 
PR materials have to possess both photoconductivity 
and optical nonlinearity simultaneously so that 
spatially oscillating space-charge field is formed 
under the illumination of non-uniform light 
generated by the interference of two coherent laser 
beams via an electro-optic effect. One of the 
categories of this material is photoconductive 
polymers doped with second order nonlinear optical 
(NLO) chromophores. The relationship between 
structure and NLO property of chromophores has 
been investigated, along with synthesis of new NLO 
chromophores and their characterization. The alkoxy 
chains attached to NLO chromophores are expected 
to provide free volume to each chromophore, which 
will result in the fast orientation behavior under 
applied electric field [6-9]. Continuing our work on 
synthesis and application of new PR materials [10-
14], we report herein the synthesis of methoxy, 
ethoxy and isobutoxy substituted 4-piperidino-
benzylidene-malononitrle (PDCST) derivatives for 
investigation into substitutional effect on electro-
optic properties and response time. 
 
2. EXPERIMENTAL 
 
2.1. Materials and instruments    
 
All reagents were purchased from Aldrich 
Chem. Co. Products were purified by 
chromatography on silica gel 60 (Merck). Melting 
point of products was measured on Electrothermal 
9100, 
1
H, 
13
C-MNR; spectra were recorded on 
Varian Mecury 400MHz NMR spectrometer using 
CDCl3 as solvent. EI-MS was measured on JMS-700 
(Jeol, Japan). All the reactions were carried out 
under nitrogen or argon atmosphere. 
 
2.2. General synthetic methods   
 
Synthesis of 1,4-dibromo-2,5-dialkoxybenzene (2a, 
2b, 2c)  
To a solution of 1(a-c) (50 mmol) dissolved in 
AcOH (150 ml), ZnCl2 (15 g, 110 mmol) was added 
at once and the mixture was stirred until complete 
dissolution. Under rigorous exclusion of light, Br2 (7 
ml, ~130 mmol) in AcOH (10 ml) was added 
dropwise into the above solution for over 30 
minutes. The reaction mixture was stirred at room 
 VJC, 55(2), 2017  Nguyen Quoc Vuong et al. 
164 
temperature (rt) for 5 hrs, then quenched with dilute 
aqueous NaOH solution. The white precipitate was 
filtered, washed with water and cooled MeOH (0 
o
C). The MeOH filtrate was concentrated to 
appearance of white precipitate which was filterred, 
washed to get 2
nd
 crop. The combined crude 
products were recrystallized from mixture of 
EtOH/CH2Cl2 (4/1, v/v) to give crystals of 2a in 75 
% yield, 2b in 68 % yield, 2c in 57 % yield. 
Compound 2a: 1,4-dibromo-2,5-
dimethoxybenzene; mp 145 
o
C; 
1
H-NMR δ 7.12 
(2H, s, H-3, H-6 of ArH), 3.90 (6H, s, 2 × OCH3). 
Compound 2b: 1,4-dibromo-2,5-diethoxybenzene; 
mp 132
o
C; 
1
H-NMR δ 7.02 (2H, s, H-3, H-6 of 
ArH), 4.02 (4H, q, 2 × OCH2-), 1.45 (6H, t, 2 × 
CH3). Compound 2c: 1,4-Dibromo-2,5-
diisobutoxybenzene; mp 54
o
C; 
1
H-NMR δ 7.06 (2H, 
s, H-3, H-6 of ArH), 3.72 (4H, d, 2 × OCH2), 2.13 
(2H, septet, 2 × CH (CH3)2), 1.07 (12H, d, 4 × CH3). 
 
Synthesis of 1’-(4-bromo-2,5-dialkoxy-
phenyl)piperidine (3a, 3b, 3c) 
A solution of BINAP (0.3 g, 0.4 mmol) and 
Pd2(dba)3 (0.3 g, 0.3 mmol) in toluene (150 ml) was 
stirred at rt for 30 minutes and 1,4-dibromo-2,5-
dialkoxybenzene (2a-c) (30 mmol) was added, then 
the mixture was stirred at rt for 30 minutes before 
NaO
 
t-Bu (3.4 g, 35 mmol) was added. The reaction 
mixture was heated to 96-100 
o
C and piperidine (3.0 
g, 35 mmol) was added dropwise for one hour, then 
the stirring was continued at 95-100 
o
C for 5 hrs. 
After that, the reaction mixture was cooled to rt, 
water (100 mL) was added and the mixture was 
extracted with diethyl ether for three times. The 
ethereal layers were combined and washed with 
water, dried over anhydrous MgSO4 then evaporated 
in vacuo and the product was purified by column 
chromatography on silica gel using n-hexane/EtOAc 
(9/1) as eluent to afford pure compounds 3a in 62 % 
yield, 3b in 58 % yield, 3c in 47 % yield.  
Compound 3a: 1’-(4-bromo-2,5-
dimethoxyphenyl)piperidine; mp 79 
o
C; 
1
H-NMR δ 
6.96 and 6.56 (2H, 2×s, Ar-H), 3.85 and 3.81 (6H, 
2×s, OCH3), 2.97 (4H, t, N-CH2 of piperidine ring), 
1.75-1.58 (6H, m, -CH2-CH2-CH2- of piperidine 
ring). Compound 3b: 1-(4-bromo-2,5-
diethoxyphenyl)piperidine; mp 78 
o
C; 
1
H-NMR δ 
6.96 and 6.54 (2H, 2×s, Ar-H), 4.04 and 3.99 (4H, 
2×q, OCH2-), 2.98 (4H, t, N-CH2 of piperidine ring), 
1.73-1.57 (6H, m, -CH2-CH2-CH2- of piperidine 
ring), 1.43 (6H, t, 2×CH3). Compound 3c: 1-(4-
bromo-2,5-diisobutoxyphenyl)piperidine; mp 69 
o
C; 
1
H-NMR δ 6.96 and 6.52 (2H, 2×s, Ar-H), 3.73 and 
3.69 (4H, 2×d, OCH2-), 2.98 (4H, t, N-CH2 of 
piperidine ring), 2.12 (2H, 2×septet, CH (CH3)2), 
1.62-1.58 (6H, m, -CH2-CH2-CH2- of piperidine 
ring), 1.08 and 1.06 (12H, 2×d, CH3). 
 
Synthesis of 2,5-dialkoxy-4-piperidin-1-yl-
benzaldehyde (4a, 4b, 4c) 
A solution of 1.6 M n-BuLi/n-hexane (5 ml, 8 
mmol) was added dropwise to a  solution of 1-(4-
bromo-2,5-dialkoxy-phenyl)piperidine 3(a-c) (5 
mmol) in dry THF (50 ml) at -78 
o
C, then the 
mixture was stirred continuously at this temperature 
for 1 hour and 1-formyl piperidine (2.83 g, 25 
mmol) in anhydrous THF (10 ml) was slowly added 
for over 30 minutes. The reaction mixture was 
stirred at -78 
o
C for 30 minutes, then warmed to -5 
o
C and followed by quenching with saturate aqueous 
ammonium chloride solution. The resulted mixture 
was extracted with CHCl3 (3×25 ml). The extract 
was washed with water, dried over anhydrous 
MgSO4 then evaporated under reduced pressure. The 
crude product was purified by column 
chromatography on silica gel using n-hexane/EtOAc 
(8/2) as eluent to give 4a (85 %), 4b (78 %) and 4c 
(72 %). 
Compound 4a: 2,5-dimethoxy-4-piperidin-1-yl-
benzaldehyde; mp 92 
o
C; 
1
H-NMR δ 10.25 (1H, s, H 
-CHO), 7.22 and 6.39 (2H, 2×s, Ar-H), 3.87 and 
3.83 (6H, 2×s, OCH3), 3.10 (4H, t, N-CH2 of 
piperidine ring), 1.77-1.60 (6H, m, -CH2-CH2-CH2- 
of piperidine ring). Compound 4b: 2,5-diethoxy-4-
piperidin-1-yl-benzaldehyde; mp 99 
o
C; 
1
H-NMR δ 
10.27 (1H, s, H -CHO), 7.22 and 6.40 (2H, 2×s, Ar-
H), 4.13 and 4.05 (4H, 2×q, OCH2-), 3.17 (4H, t, N-
CH2 of piperidine ring), 1.75-1.62 (6H, m, -CH2-
CH2-CH2- of piperidine ring), 1.40 and 1.45 (6H, 
2×t, CH3). Compound 4c: 2,5-diisobutoxy-4-
piperidin-1-yl-benzaldehyde; mp 89 
o
C; 
1
H-NMR δ 
10.31 (1H, s, H-CHO), 7.22 and 6.47 (2H, 2×s, Ar-
H), 3.8 and 3.75 (4H, 2×d, OCH2-), 3.28 (4H, t, N-
CH2 of piperidine ring), 2.15 (2H, 2×septet, CH 
(CH3)2), 1.75-1.65 (6H, m, -CH2-CH2-CH2- of 
piperidine ring), 1.05 (12H, d, CH3). 
 
Synthesis of 2-(2,5-dialkoxy-4-piperidin-1-yl-
benzaldehyde)-malononitrile (5a, 5b, 5c) 
To a solution of 2,5-dialkoxy-4-piperidin-1-yl-
benzaldehyde 4(a-c) (6 mmol) in 30 ml of 
anhydrous EtOH, malononitrile (0.5 g, 8 mmol) was 
added at once and the mixture was stirred until 
turned into a homogeneous solution, few drops of 
piperidine were added. The reaction mixture was 
then stirred for 1 hour and refluxed for 30 minutes 
and cooled to room temperature. The orange 
precipitate was filtered, washed with a solution of 
EtOH/H2O (1/1, v/v) then recrystallized from 
CH2Cl2/EtOH (1/5, v/v). The recrystallization was 
 VJC, 55(2), 2017  Synthesis of new alkoxy substituted… 
165 
repeated 3 times to give pure compounds 5a in 82 % 
yield, 5b in 75 % yield, 5c in 74 % yield. 
Compound 5a: 2-(2,5-dimethoxy-4-piperidin-1-
yl-benzylidine)-malononitrile; mp 176 
o
C; 
1
H-NMR 
δ 8.12 (1H, s, –CH=C(CN)2), 7.76 and 6.3 (2H, 2×s, 
Ar-H), 3.86 (6H, s, OCH3), 3.3 (4H, t, N-CH2 of 
piperidine ring), 1.75 - 1.65 (6H, m, -CH2-CH2-CH2- 
of piperidine ring); 
13
C-NMR δ 157.5, 152.7, 153.0, 
146.4, 117.2, 116.3, 113.0, 110.5, 101.0, 73.4, 57.1, 
57.0, 52.1, 26.98 25.2; EI-MS(m/z): 297[M]
+
(100). 
Compound 5b: 2-(2,5-diethoxy-4-piperidin-1-yl-
benzylidine)-malononitrile; mp 174 
o
C; 
1
H-NMR δ 
8.15 (1 H, s, –CH=C(CN)2), 7.75 and 6.26  (2H, 2×s, 
Ar-H), 4.0-4.1 (4H, m, OCH2-), 3.31 (4H, t, N-CH2 
of piperidine ring), 1.74 - 1.67 (6H, m, -CH2-CH2-
CH2- of piperidine), 1.46 and 1.45 (6H, 2×t, CH3); 
13
C-NMR δ 156.7, 152.4, 151.8, 145.3, 117.1, 116.1, 
112.8, 111.0, 101.4, 72.5, 65.1, 65.1, 51.8, 26.5, 
24.9, 15.41, 15.39; EI-MS (m/z): 325[M]
+
(100). 
Compound 5c: 2-(2,5-diisobutoxy-4-piperidin-1-
yl-benzylidine)-malononitrile; mp 98
o
C; 
1
H-NMR δ 
8.09 (1H, s, –CH=C(CN)2), 7.74 and 6.25 (2H, 2 × s, 
Ar-H), 3.78 and 3.74 (4H, 2×d, OCH2-), 3.31 (4H, t, 
N-CH2 of piperidine ring), 2.16 (2H, septet, CH 
(CH3)2), 1.75-1.67 (6H, m, -CH2-CH2-CH2- of 
piperidine ring), 1.06 (12H, d, CH3); 
13
C-NMR δ 
156.8, 152.2, 151.9, 145.6, 117.1, 116.1, 112.8, 
110.1, 101.4, 76.1, 75.6, 72,6, 51.8, 29.1, 28.1, 24.9, 
20.1, 20.0; EI-MS(m/z): 381[M]
+
(100). 
 
3. RESULTS AND DISCUSSION  
 
Alkoxy substituted 4-piperidinobenzylidene 
malononitrile (PDCST) derivatives were synthesized 
in a four-step procedure as described in the      
scheme 1. 
 
                              
Scheme 1: Synthesis of alkoxy substituted 4-piperidinobenzylidene malononitrile (PDCST) chromophores 
 
In step 1, the alkoxy arenes 1(a-c) were 
brominated by reagent Br2 in acetic acid using ZnCl2 
catalyst at room temperature and under rigorous 
exclusion of light to avoid substitution on the side 
chain. The alkoxy groups is ortho/para directing, so 
the bromination of 1(a-c) afforded compounds 1,4-
dibromo-2,5-dialkoxybenzene 2(a-c). In the step 2, 
Buchwald-Hartwig reaction was carried out between 
dibromo arenes 2(a-c) and piperidine in toluene 
using Pd2(dba)3/BINAP catalyst and NaOt-Bu
 
base 
to form C-N bond of obtained compounds 3(a-c) in 
accepted yield (47-62 %). Next, the formylation of 
the aryl bromides 3(a-c) was carried out using n-
BuLi to metalate the aryl bromides 3(a-c) then using 
N-formylpiperidine as transfer reagent to afford 
benzaldehyde compounds 4(a-c) in high yield (72-
78 %). Finally, Knoevenagel condensation of 
obtained aldehydes 4(a-c) with malononitrile using 
catalytic piperidine gave crystalline products 5(a-c) 
in high yield (74-78 %). The structures of the 
products were confirmed by 
1
H, 
13
C-NMR and EI-
MS spectroscopic analysis. The 
1
H, 
13
C-NMR 
indicated signals of alkoxy groups such as the 
methoxy group (OCH3)  give signals at δH 3.8-3.9 
(s) and δc 57.1-57.0; the ethoxy group give signals 
at δH 4.01-4.05 (q) and δc 65.1-65.1 of 2×OCH2-), 
δH 1.40-1.45 (t) and δc 15.41, 15.39 of (2×CH3); the 
isobutoxy groups give signals at δH 3.69-3.78 (d) 
and δc 75.6- 72.6 of (2×OCH2-), δH 2.12-2.16 
(septet) and δc 29.1, 28.1 of 2×CH (CH3)2), δH 1.05-
 VJC, 55(2), 2017  Nguyen Quoc Vuong et al. 
166 
1.07 (m) and δc 20.1-20.0 of (4×CH3). The signals 
of piperizine ring at δH 2.97-3.7 (t) and δc 52.1-51.9 
of 2×N-CH2; δH 1.57-1.75 (m) and δc 26.9-20.1 of     
(-CH2-CH2-CH2-). The signals of aromatic aldehyde 
group (ArCHO) at δH 10.25 (4a), δH 10.27 (4b), δH 
10.31 (4c). The signals of the malononitrile moiety 
were indicated at δc 117.2, 116.3 (2×CN), 73.4  
(=C(CN)2) of 5a; δc 117.1, 116.1 (2×CN), 72.6 
(=C(CN)2) of 5b; δc 117.1, 116.1 (2×CN), 76.1 
(=C(CN)2) of 5c. 
 
4. CONCLUSION 
 
We have repoted the synthesis of three new 
alkoxy-substituted 4-piperidinobenzylidene 
malononitrile (PDCST) nonlinear optical 
chromophores 5(a-c) which will be used for further 
study on their non-linear electro-optic property 
which will be reported in due course.   
 
Acknowledgement. We are gratefull to Professor 
Kim Nakjoong who supported the finance for this 
research.   
 
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Corresponding author: Nguyen Quoc Vuong  
Institute of Marine Biochemistry 
Vietnam Academy of Science and Technology  
18 Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam   
E-mail: nguyenvuong@imbc.vast.vn; Telephone number:  0973972767.   
 


Synthesis of new alkoxy substituted 4-piperidinobenzylidene malononitrile (PDCST) nonlinear optical chromophores

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Synthesis of new alkoxy substituted 4-piperidinobenzylidene malononitrile (PDCST) nonlinear optical chromophores

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