A mild and practical procedure for synthesis of substituted 2-aminobenzophenones

Article abstract of DOI:10.1007/s11164-014-1751-1

Abstract A convenient three-step procedure has been developed for synthesis of substituted 2-aminobenzophenones from substituted anilines. The anilines are first protected as acetanilides, by reaction with acetic anhydride. These are then benzoylated with (trichloromethyl)benzene in the presence of aluminium-generated 2-acetamidobenzophenone. Finally, removal of the acetyl group from the amino group provides the substituted 2-aminobenzophenones in moderate to good yields.

Full text of DOI:10.1007/s11164-014-1751-1

Res Chem Intermed
DOI 10.1007/s11164-014-1751-1
A mild and practical procedure for synthesis
of substituted 2-aminobenzophenones
•
Er-Qian Ma Ping Wang Pei-He Li Li-Ping Mo
•
•
Received: 2 May 2014 / Accepted: 18 June 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract A convenient three-step procedure has been developed for synthesis of
substituted 2-aminobenzophenones from substituted anilines. The anilines are first
protected as acetanilides, by reaction with acetic anhydride. These are then ben-
zoylated with (trichloromethyl)benzene in the presence of aluminium-generated
2-acetamidobenzophenone. Finally, removal of the acetyl group from the amino
group provides the substituted 2-aminobenzophenones in moderate to good yields.
Keywords 2-Aminobenzophenones Á Anilines Á (Trichloromethyl)benzene
Introduction
2-Aminobenzophenones are valuable synthons in a variety of organic reactions for
synthesis of important heterocyclic compounds, for example quinolines [1–4],
tetrahydroquinolines [5], quinazolines [6], 1,2-dihydroquinazolines [7, 8], 2,3-
disubstituted indoles [9], 3,3-dimethyl-9-phenyl-3,4-dihydroacridin-1(2H)-one [10],
dibenzodiazepines [11], and benzodiazepin-2-one [12]. They are also used as
starting materials for preparation of the antidepressant drug tampramine [13] and
tetradentate Schiff-base ligands [14]. Their synthesis is difficult because they have
both amino and carbonyl active groups in the position ortho to the benzene ring.
Synthetic strategies reported for preparation of substituted 2-aminobenzophenones
include:
E.-Q. Ma Á P.-H. Li Á L.-P. Mo (&)
College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050024, China
e-mail: moliping@126.com
P. Wang
Department of Chemical and Environmental Engineering, Hebei Chemical and Pharmaceutical
Vocational Technology College, Shijiazhuang 050026, China
123

E.-Q. Ma et al.
(a) one-pot reaction of aryl bromides, anthranilic acid N-methoxy-N-methylamide,
and n-butyllithium [15];
(b) protection of o-aminobenzoic acid by tosylation, then Friedel–Crafts reaction,
and, finally, hydrolysis by use of concentrated H2SO4 [16];
(c) palladium(II) bipyridine-catalyzed addition of phenylboronic acid to 2-ami-
nobenzonitrile [17];
(d) carbonylative Suzuki–Miyaura coupling of 2-iodoaniline with phenylboronic
acid [18];
(e) Pd-catalyzed arylation/oxidation of benzylic C–H bonds leading to diaryl
ketones and subsequent cleavage of the amide by hydrolysis under basic
conditions [19];
(f) nucleophilic addition of aryl Grignard reagents to 2-aminobenzonitrile and
subsequent hydrolysis [20];
(g) reduction of 2-azidobenzophenone by use of FeSO4Á7H2O/NH3 [21];
(h) catalytic hydrogenation of 2-nitrobenzophenone [22]; and
(i) Pd-catalyzed addition of arylsulfinates to 2-aminobenzonitriles [23].
However, some of these methods have such disadvantages as limited substrate
scope, complex manipulations, inaccessible starting materials, or use of expensive
catalysts and ligands. Because of their extensive application, it is very desirable to
develop a practical and convenient approach for synthesis of substituted
2-aminobenzophenones.
As part of our continuing interest in developing novel synthetic methods [24–33],
we describe herein a convenient three-step procedure for synthesis of substituted
2-aminobenzophenones starting from substituted anilines (Scheme 1).
Experimental
Melting points were determined on an X-4 apparatus and are uncorrected. IR spectra
were acquired by use of a Bruker-Tensor 27 spectrometer. NMR spectra were
acquired by use of a Bruker DRX-500 spectrometer at 500 MHz (1H) and 125 MHz
(13C) with CDCl3 as solvent and TMS as internal standard. Mass spectra (MS) were
acquired by use of a ThermoFinnigan LCQ Advantage instrument with an ESI
source (4.5 keV).
Representative procedure for synthesis of acetanilides
CaO (0.2 mol) was added to a solution of a substituted aniline (0.1 mol) in
2-methyltetrahydrofuran (2-MeTHF, 270 mL) and the resulting suspension was
stirred at room temperature (rt) for 5 min. A solution of acetyl chloride (0.11 mol)
in 2-MeTHF (100 mL) was then added dropwise over 10 min. The mixture was
stirred at rt and the progress of the reaction was monitored by TLC. On completion,
the reaction mixture was filtered and the filtrate was washed with aqueous saturated
NaHCO3. The two resulting phases were separated and the organic phase (2-
123

Procedure for synthesis of substituted 2-aminobenzophenones
O
O
O
NH2
NH
R2
R1
CCl
R2
R1
3 /AlCl3
i) HCl, MeOH-H2O
ii) NaOH
X
CH3COCl
X
ClCH2CH2Cl
0-25 °C
NH
R1
CaO, 2-MeTHF, rt
R1
NH2
X
R2
R2
O
4
1
2
3
Scheme 1 Synthesis of 2-aminobenzophenones from substituted anilines
MeTHF) was dried over anhydrous MgSO4, filtered, and dried in vacuo to afford the
acetanilide.
Representative procedure for synthesis of 2-acetamidobenzophenones
(Trichloromethyl)benzene (0.11 mol) was added dropwise over 15 min, with
efficient stirring, to a cooled solution (0–5 °C) of anhydrous AlCl3 (0.3 mol) in 1,2-
dichloroethane (100 mL). Acetanilide, prepared as described above, was added over
15 min and the reaction mixture was stirred at rt for an appropriate time. On
completion (monitored by TLC), the mixture was poured into crushed ice (500 g)
and the resulting mixture was stirred at 70 °C for 0.5 h. After cooling, the organic
layer was separated, washed with distilled water, and dried over anhydrous Na2SO4.
The 2-acetamidobenzophenone was obtained after removal of the solvent under
reduced pressure.
Representative procedure for synthesis of substituted 2-aminobenzophenones
The 2-acetamidobenzophenone obtained as described above was heated under reflux
with 100 mL MeOH–HCl (MeOH:6 mol/L HCl = 1:1) in a round-bottomed flask
for 2 h. The hot mixture was poured into a beaker and cooled. The hydrochloride of
the 2-aminobenzophenone was isolated by suction filtration, washed with chilled
alcohol, then suspended in 800 mL water in a beaker with a mechanical stirrer. The
pH was adjusted to 8.0 by addition of a solution of 2 mol/L sodium hydroxide. The
mixture was extracted with ethyl acetate, and the extract was washed with water and
brine, dried over MgSO4, and concentrated. The crude product was purified by
recrystallization or by column chromatography on silica gel with petroleum ether–
ethyl acetate as eluent.
(2-Amino-4-methylphenyl)(phenyl)methanone (4a)
Yellow-red crystals; IR (KBr): 3454, 3330, 3066, 2918, 2320, 1959, 1627, 1573,
1541, 1446, 1379, 1319, 1245, 1178, 966, 900, 867, 790, 752, 696, 613, 570,
1
447 cm-1; H NMR (CDCl3, 500 MHz) d ppm 7.61 (d, J = 7.0 Hz, 2H), 7.51 (t,
J = 7.5 Hz, 1H), 7.44 (t, J = 7.2 Hz, 2H), 7.34 (d, J = 8.5 Hz, 1H), 6.55 (s, 1H),
6.42 (d, J = 8.0 Hz, 1H), 6.11 (s, 2H), 2.30 (s, 3H); 13C NMR (125 MHz, CDCl3) d
ppm 21.8, 115.8, 116.9, 117.3, 128.2, 129.1, 130.9, 134.7, 140.6, 145.4, 151.7,
198.6; ESI–MS: m/z = 212 (M ? 1)?.
123

E.-Q. Ma et al.
Table 1 Synthesis of 2-aminobenzophenones from substituted anilines
Entry
1
Amine
Product
Yield (%)
48
mp (°C)
66–67
(67–68) [23]
2
68
65–66
(64–66) [37]
3
4
81
75
91–92
69–70
(68–70) [38]
5
6
7
45
66
82
109–111
(106–108) [37]
49–50
(51–52) [37]
45–46
8
9
60
69
129–130
(127–128) [37]
105–106
(106–107) [37]
123

Procedure for synthesis of substituted 2-aminobenzophenones
Table 1 continued
Entry
10
Amine
Product
Yield (%)
70
mp (°C)
96–97
(95–97) [37]
(2-Amino-5-methylphenyl)(phenyl)methanone (4b)
Yellow solid; IR (KBr): 3473, 3350, 3057, 3022, 2920, 2858, 2736, 1907, 1633,
1579, 1558, 1485, 1444, 1301, 1249, 1163, 1136, 954, 821, 750, 702, 655,
1
532 cm-1; H NMR (CDCl3, 500 MHz) d ppm 7.64 (d, J = 7.0 Hz, 2H), 7.53 (t,
J = 7.5 Hz, 1H), 7.46 (t, J = 7.2 Hz, 2H), 7.23 (s, 1H), 7.13 (dd, J = 8.5, 1.5 Hz,
1H), 6.67 (d, J = 8.5 Hz, 1H), 5.91 (s, 2H), 2.17 (s, 3H); 13C NMR (125 MHz,
CDCl3) d ppm 20.4, 117.3, 118.2, 124.5, 128.2, 129.2, 131.1, 134.1, 135.5, 140.3,
149.0, 199.1; ESI–MS: m/z = 212 (M ? 1)?.
(2-Amino-4,5-dimethylphenyl)(phenyl)methanone (4c)
Yellow crystals; IR (KBr): 3438, 3327, 3195, 3020, 2943, 2856, 2343, 1631, 1593,
1533, 1490, 1442, 1361, 1317, 1257, 1176, 1097, 1024, 925, 896, 866, 758, 698,
605, 472 cm-1; 1H NMR (CDCl3, 500 MHZ) d ppm 7.62 (d, J = 7.0 Hz, 2H), 7.51
(t, J = 7.0 Hz, 1H), 7.45 (t, 2H, J = 7.5 Hz), 7.18 (s, 1H), 6.56 (s, 1H), 5.94 (s, 2H),
2.22 (s, 3H), 2.08 (s, 3H); 13C NMR (125 MHz, CDCl3) d ppm 18.8, 20.3, 116.3,
118.1, 123.8, 128.1, 129.1, 130.8, 134.7, 140.6, 144.5, 149.7, 198.6; Anal. Calcd for
C15H15NO: C, 79.97; H, 6.71; N, 6.22. Found: C, 80.23; H, 6.48; N, 6.01; ESI–MS:
m/z = 226 (M ? 1)?.
(2-Amino-3,5-dimethylphenyl)(phenyl)methanone (4d)
Yellow flaky crystals; IR (KBr): 3454, 3373, 3244, 2970, 2927, 2322, 1635, 1608,
1577, 1504, 1456, 1377, 1317, 1299, 1257, 1172, 916, 887, 825, 738, 678,
1
592 cm-1; H NMR (CDCl3, 500 MHz) d ppm 7.80 (d, J = 7.0 Hz, 2H), 7.56 (t,
J = 7.0 Hz, 1H), 7.44 (t, J = 7.5 Hz, 2H), 6.97 (s, 1H), 6.65 (s, 1H), 3.55 (s, 2H),
2.20 (s, 3H), 2.19 (s, 3H); 13C NMR (125 MHz, CDCl3) d ppm 17.3, 19.1, 115.4,
125.2, 126.2, 128.4, 130.1, 132.9, 133.1, 136.8, 138.2, 142.2, 198.9; ESI–MS: m/
z = 226 (M ? 1)?.
(2-Amino-4-methoxyphenyl)(phenyl)methanone (4e)
Colorless crystals; IR (KBr): 3475, 3344, 3014, 2343, 1612, 1537, 1444, 1371,
1226, 1124, 900, 856, 700, 607, 590, 536 cm-1; 1H NMR (CDCl3, 500 MHz) d ppm
7.58 (d, J = 6.5 Hz, 2H), 7.49 (t, J = 7.2 Hz, 1H), 7.44 (t, J = 7.2 Hz, 2H), 7.39 (d,
J = 9.5 Hz, 1H), 6.35 (s, 2H), 6.18 (d, J = 7.5 Hz, 1H), 6.16 (s, 1H), 3.82 (s, 3H);
123

E.-Q. Ma et al.
13C NMR (125 MHz, CDCl3) d ppm 55.3, 99.3, 104.1, 112.3, 128.1, 128.8, 130.5,
137.0, 140.7, 153.8, 164.5, 197.8; ESI–MS: m/z = 228 (M ? 1)?.
(2-Amino-5-methoxyphenyl)(phenyl)methanone (4f)
Orange crystals; IR (KBr): 2968, 2900, 1662, 1558, 1496, 1448, 1404, 1319, 1232,
1178, 1026, 856, 813, 700, 644, 545 cm-1; 1H NMR (CDCl3, 500 MHz) d ppm 7.82
(d, J = 7.5 Hz, 2H), 7.55 (t, J = 7.5 Hz, 1H), 7.43 (t, J = 7.8 Hz, 2H), 6.79–6.84
(m, 2H), 6.71 (d, J = 2.5 Hz, 1H), 3.63 (s, 3H); 13C NMR (125 MHz, CDCl3) d
ppm 56.3, 113.4, 116.0, 118.5, 128.3, 129.4, 129.8, 133.0, 137.7, 140.6, 150.0,
196.8; ESI–MS: m/z = 228 (M ? 1)?.
(2-Amino-5-(tert-butyl)phenyl)(phenyl)methanone (4g)
Yellow solid; IR (KBr): 3473, 3350, 2960, 1633, 1577, 1548, 1485, 1363, 1305,
1245, 1172, 956, 860, 825, 761, 711, 653, 590 cm-1; 1H NMR (CDCl3, 500 MHz) d
ppm 7.66 (d, J = 7.0 Hz, 2H), 7.53 (t, J = 7.5 Hz, 1H), 7.45–7.48 (m, 3H), 7.36
(dd, J = 8.5, 2.5 Hz, 1H), 6.71 (d, J = 8.5 Hz, 1H), 5.92 (s, 2H), 1.20 (s, 9H); 13
C
NMR (125 MHz, CDCl3) d ppm 31.4, 33.8, 117.2, 117.7, 128.1, 129.4, 130.6,
131.2, 132.0, 137.9, 140.4, 149.1, 199.1; Anal. Calcd for C17H19NO: C, 80.60; H,
7.56; N, 5.53. Found: C, 80.41; H, 7.75; N, 5.30; ESI–MS: m/z = 254 (M ? 1)?.
(2-Amino-5-hydroxyphenyl)(phenyl)methanone (4h)
Pale yellow powder; IR (KBr): 3741, 3448, 3357, 3251, 2358, 2327, 1647, 1558,
1
1515, 1336, 1294, 1228, 935, 835, 698, 621, 462 cm-1; H NMR (CDCl3, 500
MHZ) d ppm 12.83 (s, 1H), 7.61 (d, J = 7.0 Hz, 2H), 7.53 (t, J = 7.2 Hz, 1H), 7.47
(t, J = 7.5 Hz, 2H), 7.37 (d, J = 9.0 Hz, 1H), 6.21 (d, J = 2.5 Hz, 1H), 6.10 (dd,
J = 9.0, 2.5 Hz, 1H), 4.29 (s, 2H); 13C NMR (125 MHz, CDCl3) d ppm 100.7,
106.5, 111.1, 128.2, 128.7, 131.0, 136.0, 138.7, 154.4, 166.4, 199.1; ESI–MS: m/
z = 214 (M ? 1)?.
(2-Amino-5-methylphenyl)(2-chlorophenyl)methanone (4i)
Yellow solid; IR (KBr): 3454, 3330, 3271, 1660, 1611, 1498, 1454, 1317, 1233,
1055, 964, 914, 825, 528, 446 cm-1; 1H NMR (CDCl3, 500 MHz) d ppm 7.42–7.39
(m, 3H), 7.34–7.31 (m, 1H), 7.07 (d, J = 8.0 Hz, 2H), 6.76 (dd, J = 8.0, 2.5 Hz,
1H), 6.65 (d, J = 2.5 Hz, 1H), 3.58 (s, 2H), 2.34 (s, 3H); 13C NMR (125 MHz,
CDCl3) d ppm 20.1, 117.7, 119.0, 126.7, 129.1, 130.0, 130.3, 131.4, 131.9, 132.6,
137.6, 139.7, 144.0, 197.4; ESI–MS: m/z = 246 (M ? 1)?.
(2-Amino-5-methylphenyl)(4-chlorophenyl)methanone (4j)
Yellow solid; IR (KBr): 3458, 3333, 2922, 2359, 1662, 1583, 1454, 1319, 1240,
1085, 846 cm-1; 1H NMR (CDCl3, 500 MHz) d ppm 7.75 (d, J = 8.5 Hz, 2H), 7.42
(d, J = 8.5 Hz, 2H), 7.06 (d, J = 8.0 Hz, 1H), 6.74 (dd, J = 8.5, 2.5 Hz, 1H), 6.61
123

Procedure for synthesis of substituted 2-aminobenzophenones
(d, J = 2.5 Hz, 1H), 3.64 (s, 2H), 2.16 (s, 3H); 13C NMR (125 MHz, CDCl3) d ppm
18.9, 114.7, 117.3, 126.1, 128.8, 131.5, 131.9, 136.0, 138.9, 139.6, 143.8, 197.6;
ESI–MS: m/z = 246 (M ? 1)?.
6-Methyl-9-phenyl-1,2,3,4-tetrahydroacridine (6)
A mixture of (2-amino-4-methylphenyl)(phenyl)methanone (4a, 1.0 mmol) and
cyclohexanone (1.0 mmol) in 1.5 g L-(?)-tartaric acid–DMU (30:70) was stirred at
70 °C. The progress of reaction was monitored by TLC. After completion of the
reaction, the reaction mixture was cooled to rt and water was added. The solid
product which separated out was isolated by filtration, washed with water, and dried
to afford product 6 in good purity. Yellow solid; 75–77 °C; IR (KBr): 3412, 3385,
3051, 3024, 2922, 2854, 1728, 1575, 1458, 1352, 1286, 1118, 879, 810, 750, 702,
1
572 cm-1; H NMR (CDCl3, 500 MHz) d ppm 7.80 (s, 1H), 7.51 (t, J = 7.5 Hz,
2H), 7.50 (t, J = 7.5 Hz, 1H), 7.32–7.19 (m, 3H), 7.14 (dd, J = 8.5, 1.5 Hz, 1H),
3.18 (t, J = 6.5 Hz, 2H), 2.58 (t, J = 6.5 Hz, 2H), 2.51 (s, 3H), 1.98–1.93 (m, 2H),
1.80–1.76 (m, 2H); 13C NMR (125 MHz, CDCl3) d ppm 21.7, 22.9, 23.1, 27.9, 34.2,
124.7, 125.4, 127.4, 127.5, 127.6, 127.7, 128.6, 129.1, 137.3, 138.4, 146.3, 146.5,
158.9; Anal. Calcd for C20H19N: C, 87.87; H, 7.01; N, 5.12. Found: C, 87.90; H,
6.83; N, 5.07; ESI–MS: m/z = 274 (M ? 1)?.
Results and discussion
Synthesis was conducted as illustrated in Scheme 1. First, commercially available
substituted anilines (1) were acetylated with acetyl chloride. The reaction was
performed in 2-MeTHF by use of CaO as an innocuous acid scavenger [34]. The
corresponding acetanilides (2) were obtained in excellent yields without chromato-
graphic purification of the products. 4-Aminophenol underwent acylation exclu-
sively at the aminic site under these conditions.
It is reported that phenyldichlorocarbenium tetrachloroaluminate complex,
[PhCCl2]?AlCl4À; derived from (trichloromethyl)benzene and aluminium chloride,
is a highly stable, efficient electrophile [35, 36]. Subsequently, the acetanilides (2)
were benzoylated with (trichloromethyl)benzene in the presence of aluminium in
1,2-dichloroethane. A variety of 3 or 4-substituted acetanilides (2) were converted
to 2-acetamidobenzophenones (3). 3-Substituted acetanilides gave lower yields than
4-substituted acetanilides, because of partial formation of 4-acetamidobenzophe-
nones. Similarly, treatment of substituted (trichloromethyl)benzenes with acetan-
ilides resulted in the corresponding acetamido benzophenones in high yields
(Table 1, entries 9, 10). However, a complex mixture of polymeric products was
obtained when substrates bearing a free amino group were benzoylated with
(trichloromethyl)benzene. The aniline-protection step is, therefore, essential for
benzoylation.
Finally, N-deacetylation of 2-acetamidobenzophenones was conducted in meth-
anolic HCl, under reflux, to give the desired 2-aminobenzophenones (4). The
123

E.-Q. Ma et al.
Ph
O
O
L-(+)-Tartaric acid-N,N-dimethylurea
Ph
+
70 °C, 1h
86%
N
NH2
6
4a
5
¨
Scheme 2 Synthesis of quinoline via the Friedlander reaction
general trend with regard to yield seems to follow the pattern of substitution of the
anilines. The substrate with a methyl group in the three-position of the aniline gave
the lowest yield (Table 1, entry 1), that with 3,4-disubstituted methyl groups gave
the highest yield (Table 1, entry 3), and that with a 4-substituted methyl group gave
moderate yield (Table 1, entry 2).
The structures of the products were identified from their 1H and 13C NMR spectra
¨
and MS. The structure of compound 4a was also confirmed by Friedlander reaction
of 4a and cyclohexanone (Scheme 2) [1].
In conclusion, we have investigated an innovative and convenient synthetic
approach for preparation of substituted 2-aminobenzophenone derivatives, starting
from substituted anilines, by use of a three-step sequence. The advantages of this
approach include use of readily available anilines, mild reaction conditions, and a
simple procedure; a series of substituted 2-aminobenzophenones were obtained in
moderate to good yields.
Acknowledgments We gratefully acknowledge financial support from National Natural Science
Foundation of China (21272053).
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