Low-valent titanium promoted self-coupling of N-acylbenzotriazoles and their cross-coupling with diarylketones

Article abstract of DOI:10.1081/SCC-120021983

The self-coupling reaction of N-acylbenzotriazoles and their cross-coupling with diarylketones promoted by Sm/TiCl₄ system were investigated. Self-coupling reaction could afford α-diketones or benzoins in moderate yields, while the cross-coupli

Full text of DOI:10.1081/SCC-120021983

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Low-Valent Titanium Promoted Self-Coupling of N-
Acylbenzotriazoles and Their Cross-Coupling with
Diarylketones
a
a b
Xiaoxia Wang & Doctor Yongmin Zhang
a
Department of Chemistry , Zhejiang University (Campus Xixi) , Hangzhou, P.R. China
b
State Key Laboratory of Organometallic Chemistry , Shanghai Institute of Organic
Chemistry , Chinese Academy of Sciences , Shanghai, P.R. China
Published online: 17 Aug 2006.
To cite this article: Xiaoxia Wang & Doctor Yongmin Zhang (2003) Low-Valent Titanium Promoted Self-Coupling of N-
Acylbenzotriazoles and Their Cross-Coupling with Diarylketones, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 33:15, 2627-2634, DOI: 10.1081/SCC-120021983
To link to this article: http://dx.doi.org/10.1081/SCC-120021983
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Õ
SYNTHETIC COMMUNICATIONS
Vol. 33, No. 15, pp. 2627–2634, 2003
Low-Valent Titanium Promoted Self-Coupling
of N-Acylbenzotriazoles and Their
Cross-Coupling with Diarylketones
Xiaoxia Wang and Yongmin Zhang1,2,*
1
1
Department of Chemistry, Zhejiang University
(
Campus Xixi), Hangzhou, P.R. China
2
State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, Shanghai, P.R. China
ABSTRACT
The self-coupling reaction of N-acylbenzotriazoles and their cross-
coupling with diarylketones promoted by Sm/TiCl4 system were
investigated. Self-coupling reaction could afford a-diketones or
benzoins in moderate yields, while the cross-coupling reaction gave
1
,2,2-triaryl ethanones in good yields.
Key Words: N-Acylbenzotriazole; Self-coupling; Cross-coupling;
Diarylketone; 1,2,2-Triaryl ethanone; Low-valent titanium.
*Correspondence: Yongmin Zhang, Doctor, Department of Chemistry, Zhejiang
University (Campus Xixi), Tianmu-Shan Road, Hangzhou, 310028, P.R. China;
Fax: þ86-571-88807077; E-mail: yminzhang@mail.hz.zj.cn.
2
627
DOI: 10.1081/SCC-120021983
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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2
628
Wang and Zhang
Carbon–carbon bond formation is the essence of organic synthesis
and the reductive coupling of carbonyl derivatives is one of the most
[
1]
valuable methods forthe fomation of ca br on–ca br on bonds.
Low-
valent titanium has been extensively investigated as a versatile reagent
in organic synthesis, especially with its exceptional ability to promote
[
reductive coupling of many functional groups. Benzotriazole is a
2]
[3]
good leaving group and Li/TiCl3 low-valent titanium system was
[
reported to be an effective reagent for dehydroxybenzotriazolylation.
4]
We thus deduce that the benzotriazolyl group in N-acylbenzotriazoles
can probably be cleaved in the action of low-valent titanium, producing
the corresponding acyl radicals, which may dimerize. Or, it could be
expected that an active carbonyl group, when treated with low-valent
[5]
titanium, may attack the N-acylbenzotriazoles, thus cleaving the
benzotriazolyl group and yielding a cross-coupling product. We now
describe the results of our experiments based on the above consideration.
Both the self-coupling reaction of N-acylbenzotriazoles and their cross-
coupling reaction with diarylketones promoted by Sm/TiCl low-valent
4
titanium system were investigated.
Substrates 1a and 1 g, when subjected to the Sm/TiCl system at
4
room temperature for 24 h, resulted only in the recovery of the starting
materials, while substrates 1b and 1c could afford a-diketones 2b and 2c
in moderate yields under the same reaction conditions. When substrates
1a and 1 g were treated with Sm/TiCl system under re flux conditions, the
4
corresponding benzoins were obtained in moderate yields plus mixtures
of otherunidentified-p ro ducts (Sch. 1).
Further investigation revealed that the cross-coupling reaction
between N-acylbenzotriazoles and diarylketones were more successful
than the self-coupling one. When N-acylbenzotriazoles and diaryl-
ketones were treated with Sm/TiCl4 system under er flux conditions,
O
C
O
C
O
C
N
Sm/TiCl4
r.t.
N
+
N
R
R
R
N
H
N
N
1
b, R = m Cl
m
p
Cl
36%
2
b, R =
1
c, R = p Cl
2c,
R =
Cl 45%
O
O
C
OH
CH
N
Sm/TiCl4
reflux
C
N
+
N
R
R
R
N
N
N
H
1
a, R = H
2a, R = H
52%
2g, R =
p
3
CH O
46%
1
g, R = p CH O
3
Scheme 1.

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Self-Coupling Reaction of N-Acylbenzotriazoles
2629
O
C
O
C
O
C
2
R
2
3
Sm/TiCl4
reflux
CH
N
+
R
R
1
1
R
R
+
HN
N N
N
N
3
1
R
3
Scheme 2.
Table 1. The cross-coupling of N-acylbenzotriazoles and
diarylketones promoted by Sm/TiCl system.
4
Reaction Yield
time (h) Product (%)
1
2
3
Entry
R
R
R
a
1
2
3
4
5
6
7
8
9
H
p-CH
H
H
H
H
H
H
H
H
H
H
H
H
H
24
24
24
24
24
24
24
24
3a
3b
3c
3d
3e
3f
68
80
82
57
82
86
83
85
68
81
70
73
a
a
a
a
a
a
a
3
H
p-Et N
2
H
p-CH OC
H
p-Et N
3
6
H
4
H
Cl
Cl
Cl
2
p-CH OC H
3
3g
3h
3i
6
4
p-CH
Cl
3
OC
6
H
4
CH
H
3
b
b
b
a
48
48
48
10
11
12
Cl
Cl
CH3
Cl
3j
3k
3l
p-CH
3
Cl
24
a
The two substrates were mixed together and refluxed for the
reported time.
b
The reaction mixture was stirred at room temperature for 24 h
and then refluxed for further 24 h, or else, it was found that the
yield decreased greatly.
the corresponding 1,2,2-triaryl ethanones were obtained in good yields.
The results are shown in Sch. 2 and Table 1.
1
3
As shown in the table, whetherR and R are electron-donating
group or electron-withdrawing group, the yields are both satisfactory.
It should be pointed that, as foraliphatic N-acylbenzotriazoles, the
self-coupling reaction and the cross coupling reaction with diarylketones
both failed to give desirable products.
In conclusion, the benzotriazole group in the N-acylbenzotrizoles
can be cleaved efficiently when treated with Sm/TiCl system. The self-
4
coupling reaction of N-acylbenzotrizoles could afford a-diketones
or a-hydroxy ketones in moderate yields depending on the reaction

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2
630
Wang and Zhang
conditions. And their cross-coupling reaction with diarylketones could
proceed smoothly to give 1,2,2-triaryl ethanones. Due to the easy
availability of the starting materials, mild and neutral conditions,
moderate to good yields of the products, the above study may provide
a useful method in organic synthesis.
EXPERIMENTAL
Tetrahydrofuran was distilled from sodium-benzophenone ketyl
immediately prior to use. All reactions were conducted under a nitrogen
1
atomsphere. Melting points are uncorrected. H NMR spectra were
recorded on a Bruker 400 MHz instrument as CDCl orDMSO-d solu-
3
6
tions using TMS as internal standard. Chemical shifts (ꢀ) are reported in
ppm and coupling constants J is given in Hz. Infrared spectra were
recorded using KBr disks with a Bruker Vector-22 infrared spectrometer.
Elemental analyses were performed on a EA-1110 instrument. Metallic
samarium and all solvents were purchased from commercial sources,
without further purification before use. Substrates 1a–1m were synthe-
[
6]
sized according to reported procedure.
General Procedure for the Synthesis of Compounds 2
Under nitrogen atmosphere, samarium powder (0.33 g, 2.2 mmoL)
and TiCl (0.22 mL) in dry THF (20 mL) was refluxed for 2 h to obtain
4
black slurry, to which, after being cooled to room temperature, was added
a solution of N-acylbenzotriazole (1 mmoL), the mixture was then stirred
at room temperature or under reflux conditions till the disappearance of
the starting materials (detected by TLC). Dilute hydrochloric acid (2 M,
5
mL) was added and the resulting mixture extracted with diethylether
(
3 ꢀ 20 mL). The combined organic layer was washed with brine, dried
over anhydrous sodium sulphate, and concentrated under reduced
pressure. The residue was separated on preparative TLC on silica gel
with ethyl acetate and cyclohexane (1 : 3) as eluent to afford products 2.
ꢁ
[7]
ꢁ
Benzoin (2a). White crystals, m.p. 132–134 C (Lit. 133–135 C).
ꢀH: 7.91 (2H, d, J ¼ 8.52 Hz), 7.50–7.54 (1H, m), 7.27–7.41 (7H, m),
À1
5
.95 (1H, s), 4.54 (1H, s). IR (KBr): ꢁ
(cm ) 3415, 3285, 3028,
max
3060, 3028, 2830, 1678, 1595, 1449.
1
,2-Di(3-chlorophenyl)-1,2-ethanedione (2b). Yellow crystals, m.p.
ꢁ ꢁ
16–117 C (Lit. 115 C). ꢀ : 7.97 (2H, s), 7.83–7.85 (2H, m), 7.64–7.66
H
[7]
1

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Self-Coupling Reaction of N-Acylbenzotriazoles
2631
À1
(
2H, m), 7.46–7.49 (2H, s). IR (KBr): ꢁmax (cm ) 3076, 3059, 1673, 1588,
1
570, 1198.
,2-Di(4-chlorophenyl)-1,2-ethanedione (2c). Yellow crystals, m.p. 195–
1
ꢁ
[7]
ꢁ
1
97 C (Lit. 199 C). ꢀ : 7.91 (4H, d, J ¼ 8.4 Hz), 7.50 (4H, d, J ¼ 8.5 Hz).
H
À1
IR (KBr): ꢁ
4
(cm ) 3010, 1659, 1586, 1571, 1404, 1209.
,4 -Dimethoxybenzoin (2g). White crystals, m.p. 109–112 C (Lit.
max
0
ꢁ
[7]
ꢁ
1
6
10–112 C). ꢀ : 7.99 (2H, d, J ¼ 8.8 Hz), 7.25 (2H, d, J ¼ 9.5 Hz),
H
.83–6.87 (4H, m), 5.84 (1H, s), 4.49 (1H, s), 3.82 (1H, s), 3.76 (1H, s).
À1
IR (KBr): ꢁmax (cm ) 3465, 3077, 2982, 2921, 2843, 1668, 1597, 1514.
General Procedure for the Synthesis of Compounds 3
The low-valent system was prepared as above and after being cooled
to room temperature, a solution of N-acylbenzotriazole (1 mmoL) and
diarylketone (1 mmoL) dissolved in dry THF (5 mL) was added. The
resulting mixture was heated to reflux for adequate time and then
cooled to room temperature. Dilute hydrochloric acid (2 M, 5 mL) was
added and the resulting mixture extracted with diethyl ether (3 ꢀ 20 mL).
The combined organic layer was washed with brine, dried over
anhydrous sodium sulphate, and concentrated under reduced pressure.
The residue was separated on preparative TLC on silica gel with ethyl
acetate and cyclohexane (1:3) as eluent to afford products 4.
ꢁ
2
,2-Diphenyl-1-phenyl ethanone (3a). White crystals, m.p. 136–138 C
ꢁ
[8]
(
Lit. 138–138.5 C). ꢀ : 7.99 (2H, d, J ¼ 8.88 Hz), 7.24–7.33 (10H, m),
H
À1
6
.88 (2H, d, J ¼ 8.91Hz), 6.00 (1H, s), 3.83 (3H, s). IR (KBr): ꢁ
(cm
)
max
3086, 3059, 3028, 2972, 2935, 2844, 1674, 1595, 1508.
2
,2-Diphenyl-1-(4-methylphenyl) ethanone (3b). White crystals, m.p.
ꢁ
[9]
ꢁ
1
(
00–102 C (Lit. 100–101 C). ꢀ : 7.90 (2H, d, J ¼ 8.11 Hz), 7.19–7.31
H
À1
12H, m), 6.02 (1H, s), 2.37 (3H, s). IR (KBr): ꢁ
(cm ) 3084, 3062,
max
3004, 2920, 1674, 1603, 1494.
2
,2-Diphenyl-1-(4-diethylaminophenyl) ethanone (3c). White crystals,
ꢁ
m.p. 160–161 C, ꢀ : 7.93 (2H, d, J ¼ 8.87 Hz), 7.20–7.32 (10H, m),
H
6
1
2
0
.63 (2H, s), 5.98 (1H, s), 6.00 (1H, s), 3.39 (4H, q, J ¼ 7.06 Hz),
À1
.17 (6H, t, J ¼ 7.03 Hz). IR (KBr): ꢁ
(cm ) 3084, 3061, 3016,
975, 2920, 2868, 1656, 1589, 1548, 1530. m/z (%): 344 (M þ1,
max
þ
þ
.43), 343 (M , 1.10), 176 (100), 177 (13.06), 167 (4.14). Anal. calcd.
C H NO: C, 83.93; H, 7.34; N, 4.08. Found C, 80.89; H, 7.42; N, 3.96%.
2
5
25
2
,2-Diphenyl-1-(4-methoxyphenyl) ethanone (3d). White crystals,
ꢁ
ꢁ
[10]
m.p. 126–128 C (Lit.
128–130 C). ꢀ : 7.99 (2H, d, J ¼ 8.88 Hz),
H
7
(
.24–7.33 (10H, m), 6.88 (2H, d, J ¼ 8.91 Hz), 6.00 (1H, s), 3.83 (3H, s). IR
À1
KBr): ꢁ
(cm ) 3086, 3059, 3028, 2972, 2935, 2844, 1674, 1595, 1508.
max

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632
Wang and Zhang
2
-Phenyl-2-(4-chlorophenyl)-1-phenyl ethanone (3e). White crystals,
ꢁ
[5c]
ꢁ
m.p. 103–105 C (Lit.
718–7.52 (12H, m), 6.00 (1H, s). IR (KBr): ꢁ
105–106 C). ꢀ : 7.97 (2H, d, J ¼ 8.19 Hz),
H
À1
(cm ) 3086, 3059,
max
3028, 2972, 2853, 1679, 1595, 1579.
2
-Phenyl-2-(4-chlorophenyl)-1-(4-diethylaminophenyl) ethanone (3f ).
ꢁ
Orange crystals. m.p. 99–101 C. ꢀ : 7.90 (2H, d, J ¼ 8.87 Hz), 719–7.32
H
(
(
1
9H, m), 6.62 (2H, s), 6.00 (1H, s), 3.38 (4H, q, J ¼ 7.07 Hz), 1.17
À1
þ
6H, t, J ¼ 7.08 Hz). IR (KBr): ꢁ
(cm ) 3086, 3057, 3038, 2972, 2853,
max
þ
646, 1587, 1547, 1527. m/z (%): 378 (M þ 1, 0.32), 377 (M , 0.77), 201
(
2.08), 203 (0.64), 177 (13.16), 176 (100). Anal. calcd. C H ClNO: C,
2
4
24
76.28; H, 6.40; N, 3.71. Found C, 76.18; H, 6.45; N, 3.61%.
2-Phenyl-2-(4-chlorophenyl)-1-(4-methoxyphenyl) ethanone (3g). White
crystals, m.p. 91–94 C. ꢀ : 7.96–7.98 (2H, m), 7.18–7.34 (9H, m),
ꢁ
H
À1
6
3
3
7
.87–6.89 (2H, m), 5.96 (1H, s), 3.83 (3H, s). IR (KBr): ꢁ
max
þ
(cm
)
016, 2952, 2838, 1671, 1594, 1574, 1509. m/z (%): 337 (M þ 1, 0.08),
20 (0.10), 201 (1.71), 165 (12.51), 135 (100). Anal. calcd. C H ClO : C,
2
2
1
17
4.89; H, 5.09; Found C, 74.80; H, 5.15%.
2
-Phenyl-2-(4-methylphenyl)-1-(4-methoxyphenyl) ethanone (3h). White
ꢁ
crystals, m.p. 107–109 C. ꢀ : 7.99 (2H, d, J ¼ 8.87), 7.23–7.32 (5H, m),
H
7
(
.10–7.17 (4H, m), 6.87 (2H, d, J ¼ 8.86), 5.96 (1H, s), 3.83 (3H, s), 2.31
À1
þ
3H, s). IR (KBr): ꢁ
(cm ) 3028, 3016, 2966, 2934, 2838, 1670, 1600,
max
1575, 1509. m/z (%): 316 (M , 0.10), 181 (6.11), 165 (13.69), 135 (100).
Anal. calcd. C H O : C, 83.52; H, 6.37. Found C, 83.41; H, 6.45%.
2
2
20
2
2
,2-Diphenyl-1-(4-chlorophenyl) ethanone (3i). White crystals, m.p.
ꢁ
ꢁ
[11]
1
(
10–112 C (Lit.
108–109 C). ꢀ : 7.76–7.78 (2H, m), 7.08–7.22
H
À1
12H, m), 5.80 (1H, s). IR (KBr): ꢁmax (cm ) 3083, 3062, 3039, 3024,
3
003, 1678, 1588, 1571.
2-Phenyl-2-(4-mehtylphenyl)-1-(4-chlorophenyl) ethanone (3j). Oil, ꢀH:
.91–7.93 (2H, m), 7.77–7.79 (2H, m), 7.22–7.31 (5H, m), 7.13 (4H, s),
7
2
À1
.30 (3H, s). IR (KBr): ꢁmax (cm ) 3085, 3059, 3027, 2921, 2838,
þ
þ
1
686, 1587, 1511, 1398. m/z (%): 321 (M þ 1, 0.86), 320 (M , 0.33),
304 (0.55), 181 (100), 165 (28.66), 139 (27.64). Anal. calcd. C H ClO:
C, 78.62; H, 5.34. Found C, 78.60; H, 5.48%.
2
1
17
2-Phenyl-2-(4-chlorophenyl)-1-(4-chlorophenyl) ethanone (3k). Oil, ꢀH:
7
.90 (2H, d, J ¼ 8.50 Hz), 7.73–7.77 (2H, m), 7.43–7.49 (2H, m), 7.15–7.36
À1
(
7H, s), 5.93 (3H, s). IR (KBr): ꢁ
(cm ) 3085, 3062, 3028, 2924,
max
þ
þ
1
686, 1587, 1490. m/z (%): 341 (M þ 1, 3.08), 340 (M , 0.55), 324
(
1.27), 201 (28.12), 165 (41.86), 139 (100). Anal. calcd. C H Cl O: C,
20 14 2
70.40; H, 4.14. Found C, 70.45; H, 4.25%.
2
-Phenyl-2-(4-chlorophenyl)-1-(4-methylphenyl) ethanone (3l). White
ꢁ
crystals, m.p. 66–68 C, ꢀ : 7.88 (2H, d, J ¼ 8.24), 7.17–7.34 (11H, m),
H
À1
5
.98 (1H, s), 2.37 (3H, s). IR (KBr): ꢁ
(cm ) 3086, 3060, 3028, 3003,
max

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Self-Coupling Reaction of N-Acylbenzotriazoles
2633
þ
2
921, 1682, 1605, 1490. m/z (%): 321 (M þ1, 0.06), 304 (0.10), 165 (14.66),
119 (100). Anal. calcd. C H ClO: C, 78.62; H, 5.34. Found C, 78.59;
H, 5.46%.
21 17
ACKNOWLEDGMENT
We are grateful to the National Natural Science Foundation of
China (Project No. 20072033) and the NSF of Zhejiang Province,
China forfinancial suppo rt .
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1
2
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4
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Received in Japan September21, 2002