Solvent-free syntheses of salicylaldimines assisted by microwave irradiation

Article abstract of DOI:10.1081/SCC-120006012

A microwave-assisted condensation of salicylaldehyde and aryl amines without solvent were efficiently performed to form a series of salicylaldimines in high yields, which were confirmed by IR, ¹H NMR, ¹³C NMR and elemental analyses. The microwave-assisted condensation provided a convenient environmental-friendship methodology for syntheses of Schiff-base in organic syntheses.

Full text of DOI:10.1081/SCC-120006012

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SOLVENT-FREE SYNTHESES OF SALICYLALDIMINES
ASSISTED BY MICROWAVE IRRADIATION
Haijian Yang a , Wen-Hua Sun b , Zilong Li a & Leyong Wang a
a State Key Laboratory of Engineering Plastics and The Center for Molecular Science,
Institute of Chemistry, The Chinese Academy of Sciences, Beijing, 100080, China
b State Key Laboratory of Engineering Plastics and The Center for Molecular Science,
Institute of Chemistry, The Chinese Academy of Sciences, Beijing, 100080, China
Published online: 16 Aug 2006.
To cite this article: Haijian Yang , Wen-Hua Sun , Zilong Li & Leyong Wang (2002): SOLVENT-FREE SYNTHESES OF
SALICYLALDIMINES ASSISTED BY MICROWAVE IRRADIATION, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 32:15, 2395-2402
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 15, pp. 2395–2402, 2002
SOLVENT-FREE SYNTHESES OF
SALICYLALDIMINES ASSISTED BY
MICROWAVE IRRADIATION
*
Haijian Yang, Wen-Hua Sun, Zilong Li,
and Leyong Wang
State Key Laboratory of Engineering Plastics and
The Center for Molecular Science,
Institute of Chemistry, The Chinese Academy of
Sciences, Beijing 100080, China
ABSTRACT
A microwave-assisted condensation of salicylaldehyde and
aryl amines without solvent were efficiently performed to
form a series of salicylaldimines in high yields, which were
confirmed by IR, 1H NMR, 13C NMR and elemental analyses.
The microwave-assisted condensation provided a convenient
environmental-friendship methodology for syntheses of
Schiff-base in organic syntheses.
Chemists meet the controversy of synthetic procedures developing and
economic to environment friendship, therefore solvent-free reactions have
played strategic roles in methodologies of organic syntheses.[1] Among the
most promising pathways, microwave-assisted technique has been popularly
*Corresponding author. Fax: þ86-10-62566383; E-mail: whsun@infoc3.icas.ac.cn
2395
DOI: 10.1081/SCC-120006012
Copyright & 2002 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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2396
YANG ET AL.
used since the earliest publication by Gedye and Majetich in 1986[2,3]
Solvent-free organic synthesis mediated by microwave irradiation (M.W.)
offers significant advantages, such as higher atom economy, environmental
friendship, simple work-up procedure and good-to-high yield along with
fairly mild conditions.[1,4] On the contrary, in classical organic syntheses,
it is common to meet the problem of removing solvents especially in the case
of aprotic dipolar solvent with high boiling point, or the isolation of reac-
tion products through liquid–liquid extraction. The absence of solvent
reduces the risk of hazardous explosions when the reaction takes place in
a closed vessel in a microwave oven.[5]
The chemistry of the carbon–nitrogen double bond has played a
vital role in the progresses of chemistry science.[6] By virtue of both the
presence of a lone-pair of electrons on the nitrogen atom and the general
electron-donating character of the double bond, compounds containing
the azomethine group (Schiff-base compounds) have been used as fine
chemicals and medical substrates, as well as ligands coordinating with
metal ions in the formation of complexes. Recently multi-dentate com-
plexes of iron and nickel showed high activities for ethylene oligomeriza-
tion and polymerization.[7] In our efforts for ligands of polymerization
catalysts, syntheses of Schiff-base through classical condensation of alde-
hydes (or ketones) and amines were pursued. However, the corresponding
products were formed in different yields along with the various reaction
times. Driven by industrial application of polymerization catalysts
considering the Schiff-base ligands, the screening of simple and economic
methods for preparation of Schiff-base is targeting in our current research
project. Herein, on the base of the previous successes in microwave-
assisted technique, the microwave-promoted solvent-free condensation
reaction of salicylaldehyde and aryl amines displayed the convenient
practicing way for forming a series of salicylaldimines in good yields
(Scheme 1).
The synthesis of Schiff-base is a classical reaction. It is often carried
out with acid-catalyzed and generally by refluxing the mixture of aldehyde
(or ketone) and amine.[8] Recently, stoichiometric solid–solid reaction was
successfully employed for Schiff-base formation.[9] However, the reaction
Scheme 1.

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SOLVENT-FREE SYNTHESES OF SALICYLALDIMINES
2397
time was relative longer. Improvingly, condensation imines formation was
carried out in water suspension.[10] To shorten the reaction time, micro-
wave-mediated synthesis of heterocyclic amines with aldehydes were effi-
ciently performed.[11] In present case, with the assistance of microwave
irradiation, it was found that general Schiff-base formation of salicylald-
imines proceeded fast and efficiently through the condensation reaction of
salicylaldehyde and various aryl amines (Table 1). It is noteworthy that
since the reaction vessel was an opened Erlenmeyer flask, the polar mol-
ecules of condensation water were immediately vaporized, which was a
driven force for the reaction as well as obviated the dangerous explosion.
The work-up procedure was simplified as re-crystallization of the products
in an appropriate solvent, such as ethanol, for purification and separation.
The products 3a–j were literature compounds which were formed through
thermal condensation of aldehyde with amines (see the corresponding
numbers in the reference),[12] while the products 3k–y were first reported
with the best of our knowledge. All compounds were confirmed by their IR,
1H NMR, 13C NMR spectra as well as elemental analyses. Further
investigation of condensation of ketones and various amines are currently
in progress.
General Consideration: Melting points obtained with an electrical
apparatus were uncorrected. IR spectra were recorded on a PERKIN
ELMER System 200 FT-IR spectrometer; the chemical shifts of NMR
spectra were measured with a Bruker BMX-300 MHz instrument and were
expressed in ppm using TMS as internal standard; elemental analyses were
performed by using HPMOD 1106 microanalyzer.
General Procedure for the Syntheses of Salicyaldimines 3a–y: The
microwave-assisted condensations of salicylaldehyde and aryl amines were
carried out in
a domestic oven, Midea PJ21B-A 800 W (21 L).
Salicylaldehyde 1 (3 mmol) and an equivalent aryl amines 2 were mixed
together at ambient temperature in an opened Erlenmeyer flask (25 mL).
The mixture was subjected to microwave for an optimized time on the
‘‘M-High’’ setting (616 W), except ‘‘High’’ setting (800 W) for compounds
3n and 3t (see Table 1). The crude products were re-crystallized with etha-
nol, while the products 3j, 3m and 3x were re-crystallized with EtOH–
CH2Cl2 (2 : 1), EtOH–CH2Cl2 (1 : 3) and diethyl ether, respectively.
2-[[(5-Methyl-1H-pyrazole-3-yl)imino]methyl]-phenol (3k): M.p. 145–
146.5ꢀC; IR (KBr pellet) 3430.6, 3204.7, 1614.4, 1576.6, 1499.3, 1470.2,
1
1433.6, 1277.9, 1026.9, 754.7 cmÀ1; H NMR (CDCl3) ꢀ 2.33 (s, 3H), 6.12
(s, 1H), 6.91–7.39 (m, 4H), 8.82 (s, 1H), 11.90 (s, 1H), 13.00 (s, 1H) ppm;
13C NMR (CDCl3) ꢀ 11.3, 95.2, 117.0, 118.6, 118.9, 132.2, 133.0, 141.7,
156.4, 160.8, 162.5 ppm; Anal calcd. for C11H11N3O: C, 65.66%; H,
5.51%; N, 20.88%; Found: C, 65.65%; H, 5.49%; N, 20.86%.

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2398
YANG ET AL.
Table 1. Condensation Products of Salicylaldehyde and Aryl Amines
Yield
(%)
Yield
Compounds (%)
Compounds
Time
2 min
Time
6 min
3a
95
94
3n
82
3b
3c
4 min
3o
3p
87
89
2 min
2 min
94
84
30 sec
4 min
3d
3e
3f
3q
3r
88
97
4 min
4 min
98
98
92
98
87
3 min
4 min
4 min
3 min
30 sec
3s
3t
76
77
4 min
4 min
3g
3h
3i
3u
90
4 min
3v
3w
3x
3y
89
65
96
78
3 min
2 min
4 min
4 min
3j
92
96
4 min
30 sec
3k
88
68
4 min
4 min
3l
3m

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SOLVENT-FREE SYNTHESES OF SALICYLALDIMINES
2-[[(3-Benzyloxypyridine-2-yl)imino]methyl]-phenol
2399
(3l):
M.p.
106–107ꢀC; IR (KBr pellet) 3436.8, 1605.3, 1580.3, 1555.6, 1453.9, 1434.8,
1382.4, 1287.8, 1212.8, 1119.5, 1021.1, 762.3 cmÀ1; 1H NMR (CDCl3) ꢀ 5.21
(s, 2H), 6.79–7.48 (m, 11H), 8.10 (d, 1H), 9.44 (s, 1H), 14.17 (s, 1H) ppm;
13C NMR (CDCl3) ꢀ 70.3, 117.4, 118.3, 118.7, 119.1, 121.1, 126.7, 127.9,
128.5, 133.0, 133.6, 135.9, 139.9, 147.4, 148.5, 162.4, 162.9 ppm; Anal. calcd.
for C19H16N2O2: C, 74.98%; H, 5.30%; N, 9.20%; Found: C, 74.97%;
H, 5.33%; N, 9.17%.
2-[[(Acridine-9-yl)imino]methyl]-phenol (3m): M.p. 231–233ꢀC; IR (KBr
pellet) 3442.7, 2925.8, 1622.3, 1554.0, 1516.6, 1462.1, 1277.5, 1065.9,
1
753.5 cmÀ1; H NMR (CDCl3) ꢀ 7.05–8.26 (m, 12H), 8.64 (s, 1H), 12.43
(s, 1H) ppm; 13C NMR (CDCl3) ꢀ 116.4, 116.7, 117.2, 118.4, 122.2, 124.5,
128.3, 129.2, 131.9, 133.4, 148.0, 150.4, 160.0, 167.9 ppm; Anal. calcd. for
C20H14N2O: C, 80.52%; H, 4.73%; N, 9.39%; Found: C, 80.46%; H,
4.69%; N, 9.42%.
2-[[(4-Methylbenzothiazole-2-yl)imino]methyl]-phenol
99.5–102ꢀC; IR (KBr pellet) 3440.2, 2971.9, 1617.5, 1597.9, 1566.5,
1473.1, 1280.7, 1148.9, 752.2 cmÀ1 1H NMR (CDCl3) ꢀ 2.75 (s, 3H),
(3n):
M.p.
;
7.04–7.70 (m, 7H), 9.27 (s, 1H), 12.29 (s, 1H) ppm; 13C NMR (CDCl3) ꢀ
18.4, 117.6, 118.3, 119.0, 119.6, 125.1, 127.1, 133.2, 133.9, 134.4, 135.1,
150.8, 161.8, 167.1, 167.7 ppm; Anal. calcd. for C15H12N2OS: C, 67.14%;
H, 4.51%; N, 10.44%; Found: C, 67.16%; H, 4.50%; N, 10.39%.
2-[[(6-Ethylpyridine-2-yl)imino]methyl]-phenol (3o): M.p. 38.5–40ꢀC; IR
(KBr pellet) 3436.9, 2971.6, 1612.8, 1554.8, 1496.5, 1458.8, 1281.5, 1188.9,
1
815.5, 754.9 cmÀ1; H NMR (CDCl3) ꢀ 1.35 (t, 3H), 2.85 (q, 2H), 6.86–7.71
(m, 7H), 9.46 (s, 1H), 13.62 (s, 1H) ppm; 13C NMR (CDCl3) ꢀ 13.6, 31.1,
117.0, 117.3, 118.9, 120.8, 133.2, 133.4, 138.4, 156.6, 161.7, 163.0, 164.1,
172.2 ppm; Anal. calcd. for C14H14N2O: C, 74.31%; H, 6.24%; N,
12.38%; Found: C, 74.36%; H, 6.22%; N, 12.34%.
2-[[(4-Ethylpyridine-2-yl)imino]methyl]-phenol (3p): M.p. 46–48ꢀC; IR
(KBr pellet) 3433.1, 2969.0, 1601.9, 1575.9, 1545.9, 1455.6, 1411.0, 1280.1,
1147.2, 758.1 cmÀ1; 1H NMR (CDCl3) ꢀ 1.29 (t, 3H), 2.68 (q, 2H), 6.93–7.51
(m, 6H), 8.38 (d, 1H), 9.43 (s, 1H), 13.52 (s, 1H) ppm; 13C NMR (CDCl3) ꢀ
14.1, 28.0, 117.4, 119.0, 119.8, 121.9, 122.3, 133.2, 133.5, 148.5, 155.7, 157.5,
161.7, 164.3 ppm; Anal. calcd. for C14H14N2O: C, 74.31%; H, 6.24%; N,
12.38%; Found: C, 74.35%; H, 6.21%; N, 12.39%.
2-[[(3,5-Dibromo-6-methylpyridine-2-yl)imino]methyl]-phenol (3q): M.p.
149–150.5ꢀC; IR (KBr pellet) 3442.8, 1607.1, 1574.8, 1533.7, 1448.9,
1419.7, 1280.3, 1184.7, 1054.2, 757.7 cmÀ1 1H NMR (CDCl3) ꢀ 2.63
;
(s, 3H), 6.93–7.56 (m, 4H), 8.09 (s, 1H), 9.43 (s, 1H), 13.43 (s, 1H)
ppm; 13C NMR (CDCl3) ꢀ 24.3, 114.8, 117.5, 118.7, 118.9, 119.2,
133.7, 134.4, 144.1, 152.5, 155.5, 162.1, 165.2 ppm; Anal. calcd. for

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2400
YANG ET AL.
C13H10Br2N2O: C, 42.20%; H, 2.72%; N, 7.57%; Found: C, 42.17%; H,
2.72%; N, 7.55%.
2-[[(3-Acetylbenzene-1-yl)imino]methyl]-phenol (3r): M.p. 90–92ꢀC; IR
(KBr pellet) 3436.4, 1676.4, 1618.0, 1572.4, 1498.9, 1436.0, 1271.0, 1222.0,
1
756.2 cmÀ1; H NMR (CDCl3) ꢀ 2.65 (s, 3H), 6.90–7.85 (m, 8H), 8.66 (s,
1H), 13.01 (s, 1H) ppm; 13C NMR (CDCl3) ꢀ 26.6, 117.1, 118.8, 119.1, 120.2,
126.0, 126.5, 129.5, 132.4, 133.4, 138.1, 148.7, 160.9, 163.6, 197.5 ppm; Anal.
calcd. for C15H13NO2: C, 75.30%; H, 5.48%; N, 5.85%; Found: C, 75.31%;
H, 5.42%; N, 5.83%.
2-{[(3-Chloro-5-(trifluoromethyl)pyridine-2-yl)imino]methyl}-phenol
(3s): M.p. 146.5–148ꢀC; IR (KBr pellet) 3447.9, 1614.7, 1599.1, 1559.8,
1451.9, 1323.1, 1288.7, 1224.5, 1166.8, 1125.8, 1091.2, 915.4, 762.9 cmÀ1
;
1H NMR (CDCl3) ꢀ 6.99–7.58 (m, 4H), 8.05 (s, 1H), 8.64 (s, 1H), 9.51 (s,
1H), 13.31 (s, 1H) ppm; 13C NMR (CDCl3) ꢀ 117.6, 118.6, 119.5, 120.8,
124.4, 127.5, 134.1, 135.3, 135.9, 143.7, 156.5, 162.5, 167.3 ppm; Anal
calcd. for C13H8ClF3N2O: C, 51.93%; H, 2.68%; N, 9.32%; Found: C,
51.91%; H, 2.65%; N, 9.28%.
2-{[(2-Carboxylbenzene-1-yl)imino]methyl}-phenol (3t): M.p. 199.5–
201.5ꢀC; IR (KBr pellet) 3439.7, 3070.6, 1683.3, 1620.0, 1488.6, 1457.5,
1366.8, 1244.8, 757.0 cmÀ1 1H NMR (d6-DMSO) ꢀ 6.41–7.68 (m, 8H),
;
8.84 (s, 1H), 10.25 (s, 1H), 10.70 (s, 1H) ppm; 13C NMR (d6-DMSO) ꢀ
116.5, 119.3, 119.7, 122.5, 129.4, 129.5, 130.7, 131.4, 134.0, 136.7, 151.7,
160.8, 169.8, 191.9 ppm; Anal. calcd. for C14H11NO3: C, 69.70%; H,
4.60%; N, 5.81%; Found: C, 69.62%; H, 4.67%; N, 5.80%.
2-{[(Isoquinoline-1-yl)imino]methyl}-phenol (3u): M.p. 117–118.5ꢀC; IR
(KBr pellet) 3452.1, 1614.0, 1579.2, 1551.4, 1494.7, 1392.4, 1330.3, 1280.0,
1
769.0 cmÀ1; H NMR (CDCl3) ꢀ 6.87–8.84 (m, 10H), 9.51 (s, 1H), 13.64 (s,
1H) ppm; 13C NMR (CDCl3) ꢀ 117.1, 119.0, 119.2, 120.3, 123.7, 124.8,
126.3, 127.6, 130.5, 133.6, 134.1, 137.6, 141.3, 156.0, 161.9, 165.8 ppm;
Anal. calcd. for C16H12N2O: C, 77.40%; H, 4.87%; N, 11.27%; Found:
C, 77.35%; H, 4.84%; N, 11.25%.
2-{[(5-Chlorobenzophenone-2-yl)imino]methyl}-phenol
(3v):
M.p.
141–143ꢀC; IR (KBr pellet) 3439.5, 1666.4, 1614.1, 1579.4, 1472.5, 1453.2,
1284.0, 1184.4, 756.4 cmÀ1; 1H NMR (CDCl3) ꢀ 6.81–7.82 (m, 12H), 8.49 (s,
1H), 11.61 (s, 1H) ppm; 13C NMR (CDCl3) ꢀ 117.1, 118.6, 119.0, 120.0,
128.2, 128.6, 128.7, 129.8, 131.3, 132.2, 132.5, 133.7, 135.6, 136.4, 145.5,
160.6, 164.0, 195.3 ppm; Anal. calcd. for C20H14ClNO2: C, 71.54%; H,
4.20%; N, 4.17%; Found: C, 71.56%; H, 4.15%; N, 4.10%.
2-{[(Isoquinoline-5-yl)imino]methyl}-phenol (3w): M.p. 88–89ꢀC; IR
(KBr pellet) 3436.9, 1616.0, 1577.3, 1485.7, 1459.6, 1279.6, 1211.8, 1154.7,
1
756.3 cmÀ1; H NMR (CDCl3) ꢀ 6.98–8.01 (m, 8H), 8.59 (d, 1H), 8.68 (s,
1H), 9.27 (s, 1H), 13.10 (s, 1H) ppm; 13C NMR (CDCl3) ꢀ 116.0, 117.4,

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SOLVENT-FREE SYNTHESES OF SALICYLALDIMINES
2401
117.9, 119.2, 119.4, 126.2, 127.4, 129.0, 131.0, 132.6, 133.9, 143.7, 145.1,
152.3, 161.2, 164.5 ppm; Anal. calcd. for C16H12N2O: C, 77.40%; H,
4.94%; N, 11.28%; Found: C, 77.37%; H, 4.94%; N, 11.35%.
2-{[(5-t-Butylisoxazole-3-yl)imino]methyl}-phenol (3x): M.p. 69.5–71ꢀC;
IR (KBr pellet) 3445.4, 2972.1, 1620.6, 1598.8, 1577.1, 1457.3, 1419.5,
1278.2, 757.9 cmÀ1 1H NMR (CDCl3) ꢀ 1.38 (s, 9H), 6.08 (s, 1H),
;
6.94–7.42 (m, 4H), 8.88 (s, 1H), 12.45 (s, 1H) ppm; 13C NMR (CDCl3) ꢀ
28.6, 33.0, 93.2, 117.4, 118.3, 119.3, 133.2, 134.5, 161.5, 166.9, 167.4,
182.9 ppm; Anal. calcd. for C14H16N2O2: C, 68.83%; H, 6.60%; N,
11.47%; Found: C, 68.80%; H, 6.61%; N, 11.47%.
2-{[(9-Fluorenone-1-yl)imino]methyl}-phenol (3y): M.p. 89.5–92ꢀC, IR
(KBr pellet) 3451.9, 1707.1, 1680.1, 1614.6, 1592.1, 1454.2, 1284.1, 1191.2,
1
756.1 cmÀ1; H NMR (CDCl3) ꢀ 6.80–7.62 (m, 11H), 8.78 (s, 1H), 13.24 (s,
1H) ppm; 13C NMR (CDCl3) ꢀ 109.5, 117.1, 117.6, 118.1, 118.8, 119.6,
120.2, 123.0, 124.1, 127.4, 129.2, 132.5, 133.3, 133.7, 134.2, 135.3, 136.0,
145.2, 161.5, 164.2 ppm; Anal. calcd. for C20H13NO2: C, 80.25%; H,
4.38%; N, 4.68%; Found: C, 80.17%; H, 4.35%; N, 4.65%.
ACKNOWLEDGMENTS
We are grateful for the financial supports from the Chinese Academy
of Sciences under Core Research for Engineering innovation KGCX203-2
and Fund of ‘‘One Hundred Talent’’ for WHSun.
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2402
YANG ET AL.
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Received in Japan June 4, 2001