An expedient phase transfer catalyst-mediated synthesis of 2-alkyl/aryl-1,8-phenanthrolines

Article abstract of DOI:10.1002/jhet.992

Several 2-alkyl and 2-aryl-1,8-phenanthrolines were synthesized efficiently from 5-aminoisoquinoline and seven,β-unsaturated aldehydes in aq. HCl-toluene mixture at 100 C using benzyltriethylammonium chloride as a phase transfer catalyst.

Full text of DOI:10.1002/jhet.992

November 2012
An Expedient Phase Transfer Catalyst‐Mediated Synthesis of
2‐Alkyl/Aryl‐1,8‐phenanthrolines
1443
*
Manas Chakrabarty and Ajanta Mukherji
Department of Chemistry, Bose Institute, Kolkata 700009, India
*
E-mail: chakmanas09@gmail.com
Received July 28, 2010
DOI 10.1002/jhet.992
View this article online at wileyonlinelibrary.com.
Several 2‐alkyl and 2‐aryl‐1,8‐phenanthrolines were synthesized efficiently from 5‐aminoisoquinoline
and seven α,β‐unsaturated aldehydes in aq. HCl–toluene mixture at 100°C using benzyltriethylammonium
chloride as a phase transfer catalyst.
J. Heterocyclic Chem., 49, 1443 (2012).
INTRODUCTION
by carrying out the reactions in a biphasic reaction medium
(6M HCl–toluene) containing a phase transfer catalyst
(PTC). As a result, a number of 2‐alkyl and 2‐aryl‐1,8‐
phenanthrolines were synthesized in moderate to very good
yields. The outcome of our successful venture is presented
below.
During the last decade, we have been engaged in
developing newer syntheses of condensed nitrogen
heterocycles [1]. Our ongoing interest drew our attention
to phenanthrolines [2] primarily, because many members
of the five out of 10 possible isomeric phenanthrolines
are bioactive. We were specially interested in 1,8‐
phenanthrolines, because, besides being bioactive, these mole-
cules appeared to be interesting from synthetic point of view.
Thus, regarding the bioactivities, 1,8‐phenanthroline
itself inhibits incorporation of proline into proteins, the 8‐
oxide is antibacterial [2], certain bromo derivatives [3]
and some N‐methyl‐1,8‐phenanthrolinium salts [4] are
fungicidal and bactericidal, several (N,N‐diethylamino)-
alkoxy derivatives are DNA‐binding agents [5], the benzo
[b] derivatives are amoebicides [6], and benzo[c] deriva-
tives are cytotoxic, DNA‐intercalators, and inhibitors of
topoisomerase I [7].
As regards their syntheses reported after the early syn-
thesis of 2,3‐diphenyl‐1,8‐phenanthroline by the Doebner
pyruvic acid reaction [8]8, nearly all the methods suffered
from cognizable shortcomings like low yields (e.g., 5%) of
the products [9] and the use of strong mineral acid
(e.g., conc. H2SO4; refs. 9 and 10) and toxic arsenic
compounds (e.g., H3AsO4; refs. [9] and [10]), and As2O5
[11]. A latter, somewhat improved but multistep synthesis
of 3,4‐disubstituted 1,8‐phenanthrolines [12] notwithstanding,
this situation called for the development of a short synthesis
which would be void of these difficulties. We have indeed
been able to achieve this goal by the application of Skraup/
Doebner–von Miller reaction [13] between 5‐amino‐
isoquinoline and a number of α,β‐unsaturated aldehydes but
RESULTS AND DISCUSSION
As the Skraup/Doebner–von Miller reaction requires the use
of a strong acid, we initially tried montmorillonite K10 clay, a
well‐known environmentally benign acidic substance [14] in
this reaction using microwave irradiation (MWI) as an alter-
nate source of energy [15] with a view to develop a green syn-
thesis. Accordingly, 5‐aminoisoquinoline 1 was prepared from
5‐nitroisoquinoline, procured commercially, by reduction with
palladized charcoal and hydrazine hydrate [16]. It was then
treated with an equimolar amount of crotonaldehyde 2a,
adsorbed on montmorillonite K10 clay, under MWI (70%
power; 560 W) for 12 min (6 × 2‐min pulses) in a solvent‐free
manner. But the amine still remained largely unconsumed. The
amine was not fully consumed even when two equivalents of
2a were used in a separate experiment (not described in
Experimental section). Thus, this approach failed.
Next, 1 was heated with two equivalents of 2a in HCl
(6M)‐toluene (4:1) at 100°C, until (2 h) the amine was fully
consumed. A single product was formed. It was isolated in
68% yield after a simple work‐up, followed by chromatog-
raphy over a column of silica gel, and identified as the
known 2‐methyl‐1,8‐phenanthroline 3a by comparison
1
(melting point [9] and supportive H and 13C NMR data).
The reaction is represented in Scheme 1.
© 2012 HeteroCorporation

1444
M. Chakrabarty and A. Mukherji
Vol 49
Scheme 1
of 1,8‐phenanthroline itself [17,18]. But such data on
2‐substituted 1,8‐phenanthrolines were, to the best of our
knowledge, lacking. We, therefore, determined the definitive
1H and 13C chemical shifts of two representative members,
namely, 3a and 3d by analyzing their heteronuclear multiple
quantum coherence (HMQC) and heteronuclear multiple
bond correlation (HMBC) spectra and deduced the same for
the rest by comparing their NMR data with those of 3a and
3d. Since there was a good agreement amongst the two sets
1
As the yield of 3a was only moderate, we tried to im-
prove upon the yield by carrying out the reaction separately
in the presence of 5 mol % of six different PTCs, namely,
benzyltriethylammonium chloride (BTEAC), Adogen‐464,
benzyldimethylhexadecylammonium chloride (BDHC),
cetylpyridinium bromide (CPB), cetyltrimethylammonium
bromide (CTAB), and tetra‐n‐butylammonium bromide
(TBAB). The results are shown in Table 1.
Clearly, the reaction was not only more expeditious but
also furnished 3a in a much higher yield (86%), when
BTEAC was used as the PTC. To find out the appropriate
catalytic and acidic concentrations, the reaction between
1 and 2a was repeated separately using (i) 10 mol % of
BTEAC in 6M HCl–PhMe (4:1) and (ii) 5 mol % of BTEAC
in 12M HCl–PhMe (4:1), both at 100°C. Both the reactions
were complete in 1 h but furnished 3a in 74 and 55% yields,
respectively. Obviously, 5 mol % of BTEAC in 6M
HCl–PhMe (4:1) was a better option in this reaction.
of values, the individual H and 13C NMR chemical shift
ranges for all the products were easily ascertained (see
Experimental section). These data unveiled certain NMR
characteristics, stated below, which are useful in identifying
and determining the individual NMR assignments of 2‐alkyl
and 2‐aryl‐1,8‐phenanthrolines.
Regarding the 1H NMR assignments, the chemical shifts
of ring C protons are totally unaffected by the substituents
at C‐2. Thus, H‐7, H‐10, and H‐9 appear at δ 9.25–9.45 (s),
9.0–9.2 (d, J = 5.5 Hz), and 8.8–8.9 (d, J = 5.5 Hz), respec-
tively. For 2‐alkylphenanthrolines, the protons of rings A
and B are distinguishable, as they appear at about δ 7.5
(H‐3), 7.75 (H‐5), 7.8 (H‐6), and 8.1 (H‐4) (d each,
J = 8–9 Hz). However, for 2‐arylphenanthrolines, these
protons cannot be distinguished, and they appear at
δ 7.8–8.7 (d each, 1H, J = 8–9 Hz).
As regards the 13C NMR data, C‐2, C‐10a, and C‐10b
appear at about δ 160 4, 136, and 145, respectively,
whereas C‐4a appear at about δ 127 and C‐6a at about δ
129. The difference between the chemical shifts of C‐4a
Consequently, these conditions were used in all subse-
quent reactions. Thus, 1 was separately treated with seven
other α,β‐unsaturated aldehydes 2b–h under the conditions
stated above. Only 2‐nitrocinnamaldehyde 2e did not react
even after 8 h. The rest of the substrates took 1–6 h for
completion and furnished a single product in each case.
These products were purified by column chromatography
Scheme 2
and identified spectroscopically (IR, H, and 13C NMR,
1
GC EI‐MS, elemental analysis) as the expected 2‐alkyl‐
1,8‐phenanthrolines 3b,c and the 2‐aryl‐1,8‐phenanthrolines
3d,f–h. The reactions, including that of 1 with 2a, are shown
in Scheme 2, and the results are listed in Table 2.
Till the time of our taking up this work, there existed a
1
few reports on the H and 13C NMR spectroscopic data
Table 2
Reaction of 1 with 2a–h using BTEAC (5 mol %) in 6M HCl—PhMe
at 100°C.
Producta
Yield (%)
Table 1
E‐RCH CHCHO Time (h)
Effect of different PTCs on the reaction of 1 with 2a in 6M HCl–PhMe
Entry
(2): R =
of reaction
(3)
of 3
at 100°C.
1
2
3
4
5
6
7
8
a: Me
b: Et
c: n‐Pr
1
2
2
5
8
6
3
3
a
b
c
d
N.R.
f
g
h
86
84
79
60
−
58
69
76
Catalyst
(5 mol %)
Time (h)
of reaction
Yield (%)
of 3a
Entry
d: Ph
1
BTEAC
Adogen‐464
BDHC
CPB
CTAB
1.0
1.0
1.0
2.0
4.0
2.0
86
59
72
54
34
48
e: 2‐NO2C6H4
f: 4‐NO2C6H4
g: 2‐MeOC6H4
h: 4‐MeOC6H4
2
3
4
5
6
N.R.: No reaction.
aExcept 3a, all were new compounds.
TBAB
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

November 2012
An Expedient Phase Transfer Catalyst‐Mediated Synthesis
of 2‐Alkyl/Aryl‐1,8‐phenanthrolines
1445
(C‐6a), 136.0 (C‐10a), 136.4 (CH‐4), 144.7 (C‐10b), 145.6
(CH‐9), 151.3 (CH‐7), 159.1 (C‐2); GC EI‐MS: m/z (%) 194
(M+, 100), 193 (40), 167 (26), 166 (13). Anal. Calcd. for
C13H10N2: C, 80.41; H, 5.15; N, 14.43. Found: C, 80.47; H,
5.18; N, 14.40.
and C‐6a, though marginal, is observed in all the
compounds. The chemical shifts of CH‐4 (δ 136
1),
CH‐5 (δ 127), CH‐7 (δ 151+), CH‐9 (δ 146), and CH‐10
(δ 118) are distinct and well separated. However, those of
CH‐3 (ca. δ 121–125) and CH‐6 (ca. δ 125) are close.
To our knowledge, the present synthesis constitutes the
first PTC‐mediated Skraup/Doebner–von Miller synthesis
of the 1,8‐phenanthroline skeleton—2‐alkyl and 2‐aryl
derivatives, to be precise. The reactions do not involve
any toxic substance, the reaction periods are not too long
(1–6 h), and the yields of the products are acceptable
(58–86%), which renders the present method useful.
2‐Ethyl‐1,8‐phenanthroline (3b). Light yellow liquid, 0.175
g (84%); IR: 1588, 1559, 1507, 1441, 1387, 1037, 854, 814 cm−1
;
1H NMR: δ 1.48 (t, 3H, J = 7.5 Hz), 3.11 (q, 2H, J = 7.5 Hz), 7.50
(d, 1H, J = 8.0 Hz), 7.75 and 7.81 (d, each, 1H, J = 8.5 Hz), 8.11
(d, 1H, J = 8.0 Hz), 8.81 and 9.06 (d, each, 1H, J = 5.5 Hz), 9.27
(s, 1H); 13C NMR: δ 14.0 (CH3), 32.4 (CH2), 117.8, 123.5, 124.8,
127.3, 136.4, 145.6, 151.3 (all Ar‐CH), 126.6, 128.8, 136.2,
144.6, 163.9 (all Ar‐C); GC EI‐MS: m/z (%) 208 (M+, 66), 207
(100). Anal. Calcd. for C14H12N2: C, 80.77; H, 5.77; N, 13.46.
Found: C, 80.82; H, 5.75; N, 13.49.
2‐n‐Propyl‐1,8‐phenanthroline (3c). Light yellow liquid,
0.176 g (79%); IR: 1586, 1559, 1507, 1441, 1386, 1036, 853,
EXPERIMENTAL
Melting points were determined in open capillaries and are un-
corrected. FT‐IR (KBr) spectra were recorded in a Perkin‐Elmer
RX 1 FT‐IR spectrophotometer, NMR (recorded in CDCl3, unless
stated otherwise; 1H, 500 MHz; 13C, 125 MHz; DEPT 135,
HMQC, and HMBC) in a Bruker DRX‐500 spectrometer, the
GC EI‐MS spectra in a Thermo Scientific Trace GC Ultra—PO-
LARIS Q 230LT mass spectrometer and elemental analyses in a
Perkin Elmer 2400 Series II C, H, N Analyzer. The analytical thin
layer chromatographies were carried out on silica gel G (Merck,
India) plates and column chromatographies on silica gel (60–120
mesh, Qualigens, India). PE refers to petroleum ether, bp 60–80°C,
and DCM stands for dichloromethane. MWI was carried out using
a domestic BPL SANYO microwave oven (800 W).
5‐Aminoisoquinoline (1). A solution of 5‐nitroisoquinoline
(0.35 g, 2 mmol) in EtOH (20 mL) containing NH2NH2·H2O
(2 mmol) and 10% Pd/C (10% w/w; 0.035 g) was refluxed on
steam‐bath for 30 min. The solution was filtered hot through a
bed of Celite®, washed with hot ethanol (2×10 mL), solvent
distilled from the pooled filtrates, and the resulting residue
crystallized from PE–DCM to furnish pure 1 as orange‐yellow
needles, 0.282 g (98%), mp 127–128°C (lit. mp [19] 128–129°C).
General procedure for preparation of 2‐substituted 1,8‐
phenanthrolines. The araldehyde 2a–h (2 mmol) was added
dropwise/portionwise with stirring to a warm solution of 1
(1 mmol) in 6M aq. HCl (6 mL)‐PhMe (1.5 mL) containing 5
mol % BTEAC and heated at 100°C until (1–6 h), the amine
was consumed. The solution was cooled to r.t. and diluted with
water (10 mL) and EtOAc (10 mL). In some cases, turbidity
appeared during dilution with water, when dil. HCl was added
until the turbidity disappeared. The EtOAc layer was separated
and washed with water (2× 5 mL). The pooled aqueous extracts
were basified (pH ca. 8–9) with 10% aq. NaOH in cold and
extracted with EtOAc (3× 10 mL). The organic layer was
washed with water, dried, and the solvent distilled off. The
resulting residue was purified by CC and eluted with 8–20%
PE‐EtOAc to furnish pure 3a–d,f–h. The solid products 3a,d,f,
g,h were further purified by crystallization.
814 cm−1 1H NMR: δ 1.06 (t, 3H, J = 7.5 Hz), 1.95 (sextet,
;
2H, J = 7.5 Hz,), 3.05 (t, 2H, J = 7.5 Hz), 7.47, 7.74, 7.80, and
8.09 (d, each, 1H, J = 8.5 Hz), 8.80 and 9.05 (d, each, 1H,
J = 5.5 Hz), 9.26 (s, 1H); 13C NMR: δ 14.3 (CH3), 23.2, 41.3
(both CH2), 117.8, 123.9, 124.7, 127.2, 136.3, 145.6, 151.3
(all Ar‐CH), 126.6, 128.8, 136.2, 144.7, 162.8 (all Ar‐C); GC
EI‐MS: m/z (%) 222 (M+, 4), 207 (21), 195 (11), 194 (100).
Anal. Calcd. for C15H14N2: C, 81.08; H, 6.30; N, 12.61. Found:
C, 81.03; H, 6.30; N, 12.65.
2‐Phenyl‐1,8‐phenanthroline (3d). Light brown needles
(PE‐EtOAc), 0.154 g (60%), mp 93–95°C; IR: 1625, 1601,
1
1588, 1555, 1439, 1382, 1216, 1037, 1022, 854, 757 cm−1; H
NMR: δ 7.50 (dt, 1H, H‐4′, J1 = 7.5 Hz, J2 = 1.0 Hz), 7.57
(t, 2H, H‐3′,5′, J = 7.5 Hz), 7.79 and 7.85 (d, each, 1H, H‐5,
and H‐6, respectively, J = 9.0 Hz), 8.09 and 8.25 (d, each, 1H,
H‐3, and H‐4, respectively, J = 8.5 Hz,), 8.32 (dd, 2H, H‐2′,6′,
J1 = 7.5 Hz, J2 = 1.5 Hz,), 8.85 and 9.16 (d, each, 1H, H‐9, and
H‐10, respectively, J = 5.5 Hz), 9.29 (s, 1H, H‐7); 13C NMR: δ
118.0 (CH‐10), 121.0 (CH‐3), 125.5 (CH‐6), 127.0 (CH‐5),
127.3 (C‐4a), 127.8 (CH‐2′,6′), 129.0 (C‐6a), 129.3 (CH‐3′,5′),
130.0 (CH‐4′), 136.5 (C‐10a), 137.1 (CH‐4), 139.4 (C‐1′),
145.0 (C‐10b), 145.9 (CH‐9), 151.4 (CH‐7), 156.7 (C‐2); GC
EI‐MS: m/z (%) 256 (M+, 100), 255 (20), 229 (19), 228 (14).
Anal. Calcd. for C18H12N2: C, 84.37; H, 4.68; N, 10.93. Found:
C, 84.30; H, 4.72; N, 10.98.
2‐(4′‐Nitrophenyl)‐1,8‐phenanthroline (3f). Yellow needles
(PE‐EtOAc), 0.174 g (58%), mp 255–256°C (dec.); IR: 1600,
1
1587, 1556, 1512, 1348, 1108, 1030, 850, 730 cm−1; H NMR
(DMSO‐d6): δ 8.10 and 8.15 (d, each, 1H, J = 9.0 Hz), 8.43 (d,
2H, J = 9.0 Hz), 8.58 and 8.69 (d, each, 1H, J = 8.5 Hz), 8.73
(d, 2H, J = 9.0 Hz), 8.89 and 9.10 (d, each, 1H, J = 5.5 Hz),
9.44 (s, 1H); 13C NMR: could not be recorded because of its
poor solubility in DMSO‐d6; GC EI‐MS: m/z (%) 301 (M+, 84),
299 (34), 271 (100), 255 (51), 243 (28), 229 (24), 227 (26).
Anal. Calcd. for C18H11N3O2: C, 71.76; H, 3.65; N, 13.95.
Found: C, 71.70; H, 3.63; N, 13.99.
2‐(2′‐Methoxyphenyl)‐1,8‐phenanthroline (3g). Cream‐
colored needles (PE–DCM), 0.197 g (69%), mp 117–119°C
(dec.); IR: 1600, 1586, 1553, 1491, 1439, 1258, 1119, 1016,
2‐Methyl‐1,8‐phenanthroline (3a). Light brown needles
(PE–DCM), 0.167 g (86%), mp 98–99°C (lit. mp [9] 97–99°C);
IR: 1585, 1507, 1440, 1383, 1030, 854, 812, 730 cm−1
;
1H
850, 738, 725 cm−1 1H NMR: δ 3.92 (s, 3H, OCH3), 7.07
;
NMR: δ 2.84 (s, 3H, CH3), 7.48, 7.75, 7.81, and 8.09 (d, each,
1H, H‐3, H‐5, H‐6, and H‐4, respectively, J = 8.5 Hz), 8.81 and
9.02 (d, each, 1H, H‐9, and H‐10, respectively, J = 5.5 Hz),
9.27 (s, 1H, H‐7); 13C NMR: δ 25.7 (CH3), 117.7 (CH‐10),
124.4 and 124.8 (CH‐3,6), 126.4 (C‐4a), 127.2 (CH‐5), 128.8
(d, 1H, J = 8.5 Hz), 7.19 (dt, 1H, J1 = 7.5 Hz, J2 = 1.0 Hz),
7.46 (dt, 1H, J1 = 7.5 Hz, J2 = 2.0 Hz), 7.82 and 7.86 (d,
each, 1H, J = 9.0 Hz), 8.16 (dd, 1H, J1 = 7.5 Hz, J2 = 2.0
Hz), 8.20 and 8.25 (d, each, 1H, J = 8.5 Hz,), 8.82 and 9.13
(d each, 1H, J = 5.5 Hz,), 9.29 (s, 1H); 13C NMR: δ 56.0
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

1446
M. Chakrabarty and A. Mukherji
Vol 49
(OCH3), 112.0, 118.0, 121.7, 125.4, 126.0, 127.18, 131.0,
132.3, 135.6, 145.7, 151.3 (all Ar‐CH), 127.14, 128.8, 129.4,
136.6, 145.0, 156.3, 157.9 (all Ar‐C); GC EI‐MS: m/z (%)
286 (M+, 100), 285 (80), 258 (14), 257 (58), 256 (30), 255
(32), 229 (23), 228 (24), 181 (81). Anal. Calcd. for
C19H14N2O: C, 79.72; H, 4.89; N, 9.79. Found: C, 79.77; H,
4.91; N, 9.76.
Y. Tetrahedron Lett 2005, 46, 2865; (d) Chakrabarty, M.; Kundu, T.;
Arima, S.; Harigaya, Y. Tetrahedron 2008, 64, 6711.
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Katritzky, A. R.; Boulton, A. J., Eds.; Academic Press Inc.: London, U.K.,
1978; Vol.22, pp 1–69.
[3] Mlochowski, J. Rocz Chem 1974, 48, 2145; Chem Abstr 1975,
83, 58682d.
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[6] Elslager, E. F.; Tendick, F. H. J Med Chem 1962, 5, 546.
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987; Chem Abstr 1977, 86, 29678p.
2‐(4′‐Methoxyphenyl)‐1,8‐phenanthroline (3h). Light brown
globules (PE–DCM), 0.216 g (76%), mp 100–101°C; IR: 1594,
1
1552, 1499, 1253, 1177, 1019, 837, 785 cm−1; H NMR: δ 3.83
(s, 3H, OCH3), 7.0 (dd, 2H, J1 = 7.0 Hz, J2 = 2.0 Hz,), 7.69, 7.74,
7.95 and 8.12 (d, each, 1H, J = 8.5 Hz), 8.2 (dd, 2H, J1 = 7.0 Hz, J2
= 2.0 Hz,), 8.76 (d, 1H, J = 5.0 Hz,), 9.06 (d, 1H, J = 5.5 Hz), 9.21
(s, 1H); 13C NMR: δ 55.8 (OCH3), 114.7 (2×), 118.0, 120.5, 125.1,
127.1, 129.2 (2×), 137.0, 145.5, 151.3 (all Ar‐CH), 132.0, 136.5,
144.9, 156.4, 161.5 (all Ar‐C); GC EI‐MS: m/z (%) 286 (M+, 100),
271 (30), 243 (38), 227 (17). Anal. Calcd. for C19H14N2O: C, 79.72;
H, 4.89; N, 9.79. Found: C, 79.69; H, 4.87; N, 9.81.
[12] Markees, D. G. Helv Chim Acta 1983, 66, 620.
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Acknowledgments. The authors express their sincere thanks to the
Director, Bose Institute for providing laboratory facilities, the C.S.I.
R., Govt. of India for providing a fellowship (to A.M.) and Mr. B.
Majumder, Mr. P. Dey, and Mr. S. Maji, all of Bose Institute, for
recording the NMR, IR, and GC EI‐MS spectra, respectively.
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet