Friedel-crafts reaction of BIS(Trichloromethyl)carbonate for the preparation of benzophenones

Article abstract of DOI:10.1081/SCC-120006007

Benzophenones (RC₆H₄COC₆H₄R, R=H, Cl, CH₃ or (CH₃)₂N) have been prepared by the Friedel-Crafts reactions of aromatic compounds C₆H₅R with bis(trichloromethy

Full text of DOI:10.1081/SCC-120006007

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FRIEDEL-CRAFTS REACTION OF
BIS(TRICHLOROMETHYL)CARBONATE FOR THE
PREPARATION OF BENZOPHENONES
Xiaojun Peng a , Jingyun Wang a , Jingnan Cui a , Rong Zhang a & Ying Yan a
a State Key Laboratory of Fine Chemicals , Dalian University of Technology , Dalian, 116012,
P.R. China
Published online: 16 Aug 2006.
To cite this article: Xiaojun Peng , Jingyun Wang , Jingnan Cui , Rong Zhang & Ying Yan (2002) FRIEDEL-CRAFTS REACTION OF
BIS(TRICHLOROMETHYL)CARBONATE FOR THE PREPARATION OF BENZOPHENONES, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 32:15, 2361-2367, DOI: 10.1081/SCC-120006007
To link to this article: http://dx.doi.org/10.1081/SCC-120006007
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 15, pp. 2361–2367, 2002
FRIEDEL-CRAFTS REACTION OF
BIS(TRICHLOROMETHYL)CARBONATE
FOR THE PREPARATION OF
BENZOPHENONES
*
Xiaojun Peng, Jingyun Wang, Jingnan Cui,
Rong Zhang, and Ying Yan
State Key Laboratory of Fine Chemicals, Dalian
University of Technology, Dalian 116012, P.R. China
ABSTRACT
Benzophenones (RC6H4COC6H4R, R¼H, Cl, CH3 or
(CH3)2N) have been prepared by the Friedel-Crafts reactions
of aromatic compounds C6H5R with bis(trichloromethyl)car-
bonate (BTC) instead of toxic phosgene. ACl3, FeCl3 or
ZnCl2 were used as Lewis acids in the reaction. The prepara-
tion is safe, convenient and in higher yields than phosgene-
processes.
Key Words: Benzophenone; bis(Trichloromethyl)carbonate;
Triphosgene; Phosgene; Friedel-Crafts reactions
Benzophenones are a class of very useful fine chemicals such as UV
absorbents and dye intermediates. They are usually prepared by toxic phos-
*Corresponding author. E-mail: pxjun@dlut.edu.cn
2361
DOI: 10.1081/SCC-120006007
Copyright & 2002 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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2362
PENG ET AL.
Scheme 1.
gene-processes with poisoning danger and in low yields. Recently, bis(tri-
chloromethyl)carbonate (BTC) (1), a stable and safe crystalline is developed
into a new substitute of phosgene.[1,2] Since BTC is easy to be transported,
stored, and handled, it has been used in the convenient phosgenation of
various amines,[3] alcohols,[4,5] carboxylic acids,[6] and amino acids.[7]
However, no report is on the reaction of BTC with aromatic compounds
for preparation of benzophenones. In this paper, we disclose our first study
of a convenient method for the preparation of benzophenones (3) by
Friedel-Crafts reaction of BTC with related aromatic hydrocarbons (2)
using AlCl3, FeCl3, or ZnCl2 as Lewis acids (Scheme 1).
EXPERIMENTAL
Infra red spectra were recorded on a Nicolet FTIR-20 spectrophot-
1
ometer. The H-NMR spectra were obtained on a JEOL FX-90Q. GC and
GC/MS spectra were carried out using HP5890 and Finnigan MAT
312/SS200 combined system.
Anhydrous aluminum trichloride and anhydrous ferric trichloride
were from Dalian Dyestuff Company and purified by sublimation in
vacuum. Zinc chloride was from Shenyang Chemical Company (China).
Bis(trichloromethyl)carbonate (BTC) was prepared from the per-chlorina-
tion of dimethyl carbonate according to the method of Eckerr and Forster[1]
and purified by recrystallization from hexane.
Benzophenone (3a)
To a solution of anhydrous aluminum chloride (4.0 g, 30.0 mmol) in
benzene (10 mL), BTC (2.0 g, 6.74 mmol) in benzene (10 mL) was added
dropwise over 1 h under nitrogen atmosphere at 2–4ꢀC with stirring. The

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FRIEDEL-CRAFTS REACTION OF BTC
2363
reaction was maintained for 10 h. Then the reaction mixture was poured into
an ice—concentrated hydrochloride acid solution. The organic layer was
separated, washed successively with water, 5% sodium carbonate, and
water, and dried over anhydrous sodium sulfate. Evaporation of the
unreacted benzene from the solution and distillation of the residue in
water steam, afforded 3a as a white solid (2.84 g, 77% yield) with mp
47–48ꢀC (purity: 99.2% by GC). IR n (KBr, cmÀ1): 3030 (C–H),
1
1665(C¼O), 1596, 1576, 1492 (benzene ring). H-NMR ꢀ (ppm): 7.48 (t,
J ¼ 7.6 Hz, 4H), 7.59 (t, J ¼ 7.4 Hz, 2H), 7.81 (d, J ¼ 8.0 Hz, 4H). MS m/z:
182 (Mþ).
Dimethylbenzophenone (3b)
With the similar procedure as 3a, the reaction of toluene with BTC
gave dimethylbenzophenone, 3b, in 72% yield. GC (capillary column:
OV101) and GC/MS analyses showed that 3b was a mixture of two isomers
with a ratio of 1 : 3.86. By preparative TLC (silica as absorbent, benzene:
hexane: acetone 1 : 1 : 1 in volume as eluent), the isomers were separated.
The first isomer was confirmed to be 2,40-dimethylbenzophenone: oil, bp:
181–182ꢀC/2999.5Pa. IR n (KBr, cmÀ1): 3030 (Ar–H), 2920 (CH3), 1645
1
(C¼O), 1605 (benzene ring). H-NMR ꢀ (ppm): 2.30 (s, 3H), 2.37 (s, 3H),
7.2–7.7 (m, 8H). MS m/z: 210 (Mþ); the second isomer was 4,40-dimethyl-
benzophenone: mp 94–95ꢀC, IR n (KBr, cmÀ1): 3030 (Ar–H), 2919 (CH3),
1645 (C¼O), 1602 (benzene ring). 1H-NMR ꢀ (ppm): 2.40 (s, 6H), 7.22 (dd,
J ¼ 8.4, 1.7 Hz, 4H), 7.70 (dd, J ¼ 8.4, 1.7 Hz, 4H). MS m/z: 210 (Mþ).
Dichlorobenzophenone (3c)
Similarly, the reaction of chlorobenzene with BTC afforded dichloro-
benzophenone, 3c. The gas chromatography of the reaction mixture showed
it comprised 4 compounds (I–IV) whose retention times (and contents: were:
0.69 (7.6%); 3.15 (35.7%); 3.38 (7.5%); 3.56 (34.0%) respectively. GC/MS
of I gave an m/z of 174 [Mþ] and 139 [M-35]þ (base peak) and identified to
be chlorobenzoyl chloride. The next three compounds, II, III, IV, exhibited
similar mass spectra in GC/MS with the same m/z of [Mþ]: 250. They were
confirmed to be three isomers of dichlorobenzophenones. Separated by pre-
parative TLC (silica, benzene/hexane/acetone 1 : 1 : 1 in volume) as the case
in dimethylbenzophenone, II and IV were obtained. II: 2,40-dichlorobenzo-
phenone: m.p. 66–67ꢀC, IR n (KBr, cmÀ1): 3060 (Ar–H), 1662 (C¼O), 1580,
1482 (benzene ring). NMR ꢀH (ppm): 7.20–7.40 (m, 6H), 7.50–7.70 (m, 2H).

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2364
PENG ET AL.
MS m/z: 250 (MÀ). IV: 4,40-dichlorobenzophenone: m.p. 145–147ꢀC, IR n
(KBr, cmÀ1): 3030 (Ar–H), 1666 (C¼O), 1588, 1570, 1482 (benzene ring).
NMR ꢀH (ppm): 7.47 (dd, J ¼ 8.3, 1.1 Hz, 4H), 7.71 (dd, J ¼ 8.3, 1.1 Hz, 4H).
MS m/z: 250 (Mþ).
N,N,N0,N0-tetramethyl Michler’s Ketone (3d)
To a solution of N,N-dimethylaniline (2.4 g, 19.8 mmol) in ethylene
dichloride (20 mL), BTC (1.0 g, 3.38 mmol) in ethylene dichloride (10 mL)
was added dropwise over 1 h at 3–5ꢀC with stirring. The reaction was carried
out at 15–20ꢀC for 6 h without Lewis acid, then at 35–40ꢀC for 14 h with
anhydrous zinc chloride (1.5 g). The mixture was cooled, washed succes-
sively with diluted hydrochloric acid, water, 5% of sodium carbonate, and
water, dried over sodium sulfate. Evaporation of the solvent and recrystal-
lization of the residue solid from alcohol gave 1.7 g of bluish crystal of
N,N,N0,N0-tetramethyl Michler’s ketone in 64% yield based on dimethyl-
aniline. Mp: 175–177ꢀC. IR n (KBr, cmÀ1): 2901, 2820 (CH3), 1608 (C¼O).
NMR ꢀH (ppm): 3.01 (s, 12H), 6.67 (d, J ¼ 8.2 Hz, 4H), 7.75 (d, J ¼ 8.2 Hz,
4H), MS m/z: 268 (Mþ).
RESULTS AND DISCUSSION
Phosgene is considered as ‘‘gold block’’ in organic synthesis and related
with a large amount of chemicals. However, it is one of the most toxic gases.
BTC, a white crystal, has been developed as a safe substitute for phosgene.
For the Fridel-Crafts reaction, the conditions were very mild and convenient:
BTC in 2 was dropped into the cold solution of Lewis acid, benzophenones
were produced with no releasing of toxic phosgene. The reaction conditions
and yields of the products are listed in Table 1. Generally, in a phosgene
reaction, a part of phosgene releases from the reaction system when phosgene
is introduced. So 4 or 5 times excess of phosgene are always needed for the
phosgenation, and the yield base on phosgene is very low. For BTC, however,
theoretical amount or a very little excess is enough.
For the preparation of benzophenone, the reaction was monitored by
GC and GC/MS. In the case of AlCl3 as Lewis acid, an intermediate, ben-
zoyl chloride was detected by GC at the beginning of reaction, reached its
maximal amount in 1 h, and then gradually converted to benzophenone. The
reaction finished in 10 h. after steam distillation, 77% yield (based on the

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FRIEDEL-CRAFTS REACTION OF BTC
2365

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2366
PENG ET AL.
BTC) was obtained with purity of 99.2%. This yield is much higher than
with phosgene which gave only 38% yield based on phosgene in our con-
trasting experiments. When reaction temperature was above 15ꢀC, however,
some by-products might produced. When ZnCl2 or FeCl3 was used as Lewis
acid the reaction speed was very slow so that only 19% and 37% of theor-
etical amount of benzophenone was formed respectively after reactions
proceeded for 10 h.
Toluene reacted with BTC, producing two isomers of dimethyl-benzo-
phenone with total yield of 72%. After separated by preparative TLC, the
isomers were confirmed to be the corresponding 4,40-(m.p. 94–95ꢀC) and
2,40-isomer (oil, bp: 181–182ꢀC/2999.5 Pa). GC showed their mole ratio was
3.86 : 1.
The products of chlorobenzene were more complex. Chlorobenzoyl
chloride (compound I) and three isomers of dichlorobenzophenone (com-
pound II, III, IV) were detected as main products by GC and GC/MS. The
relative contents of the isomers are: II 46.2%, III 9.7% and IV 44.1%.
Separated by preparative TLC, II and IV were obtained and identified. II
was 2,40-dichlorobenzophenone; IV was 4,40-dichlorobenzophenone. The
identification of isomer III and other two by-products (<2% by GC) was
unsuccessful due to the little contents.
For the reaction of N,N-dimethylaniline with BTC, when CS2 used as
solvent and AlCl3 as Lewis acid, only a trace of aimed product was
obtained. The main product was a triarylmethane-type blue compound
which was formed from the condensation of the aimed product,
N,N,N0,N0-tetra alkyl Michler’s ketone with unreacted N,N-dialkylaniline.
This blue compound also appears in the commercial manufacturing pro-
cesses for the Michler’s ketone from phosgene. In order to improve the yield,
the reaction was carried out in ethylene dichloride and in two steps: firstly at
15–20ꢀC for 6 h without Lewis acid, and then at 40–45ꢀC for 14–19 h with
ZnCl2 as Lewis acid. In this way, with only a little excess (2%) of BTC, pure
Michler’s ketone were obtained in 64% yield based on N,N-dimethylaniline,
much higher than 46%, the yield of a commercial phosgene-process in which
400 to 500% excess of phosgene and much longer reaction time (2 to 3 days)
are required.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (NSFC) and the Foundation of Chinese Ministry of
Education.

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FRIEDEL-CRAFTS REACTION OF BTC
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Received in Japan March 26, 2001

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