Ytterbium triflate catalyzed Friedel-Crafts reaction: Facile synthesis of diaryl ketones

Article abstract of DOI:10.1081/SCC-200039332

Friedel-Crafts reaction of aromatic compounds (benzenes, thiophene, furan, pyrrole, naphthalene, and benzothiophene) with bis(trichloromethyl) carbonate [BTC] was efficiently catalyzed by ytterbium triflate [Yb(OTf)₃] to give diaryl ketones wit

Full text of DOI:10.1081/SCC-200039332

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Ytterbium Triflate Catalyzed
Friedel–Crafts Reaction: Facile
Synthesis of Diaryl Ketones
Weike Su a & Can Jin a
a College of Pharmaceutical Sciences , Zhejiang
University of Technology , Hangzhou, 310014, P.R.
China
Published online: 10 Jan 2011.
To cite this article: Weike Su & Can Jin (2004) Ytterbium Triflate Catalyzed
Friedel–Crafts Reaction: Facile Synthesis of Diaryl Ketones, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 34:23, 4249-4256, DOI: 10.1081/SCC-200039332
To link to this article: http://dx.doi.org/10.1081/SCC-200039332
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 23, pp. 4249–4256, 2004
Ytterbium Triflate Catalyzed
Friedel–Crafts Reaction: Facile Synthesis
of Diaryl Ketones
*
Weike Su and Can Jin
College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou, P.R. China
ABSTRACT
Friedel–Crafts reaction of aromatic compounds (benzenes, thiophene,
furan, pyrrole, naphthalene, and benzothiophene) with bis(trichloro-
methyl) carbonate [BTC] was efficiently catalyzed by ytterbium triflate
[Yb(OTf)3] to give diaryl ketones with moderate to good yields.
Key Words: Friedel–Crafts reaction; Ytterbium triflate; Bis(trichloro-
methyl) carbonate; diaryl ketones.
*
Correspondence: Weike Su, College of Pharmaceutical Sciences, Zhejiang University
of Technology, Hangzhou 310014, P.R. China; Fax: (0086)57188320752; E-mail:
suweike@zjut.edu.cn.
4249
DOI: 10.1081/SCC-200039332
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

4250
Su and Jin
INTRODUCTION
Aromatic ketones play very important roles in organic chemistry. The
usual method for preparation of these compounds are well known and are
named Friedel–Crafts reaction[1] in the presence of anhydrous AlCl3. This
method, which uses a stoichiometric or even excess amounts of conventional
Lewis acids, suffers from severe corrosion, waste, and safety problems. In
recent decades, as the consciousness of protecting the environment has been
strengthened in people, methods that are cheap or environmentally friendly
are always advocated. Thus, Lanthanide triflates [Ln(OTf)3], especially
Yb(OTf)3, as a new versatile Lewis acid have been widely used in organic
chemistry. The stability of Yb(OTf)3 in water and ease of recovery allows
its reuse and promotes its environmental benefits.[2] However, this method
can only obtain simple aromatic ketones. Substituted diaryl ketones are diffi-
cult to prepare by Friedel–Crafts acylation of aromatic compounds with aroyl
chlorides, because the substituted aroyl chloride is not always available (such
as thiophene-2-carbonyl chloride, N-methylpyrrole-2-carbonyl chloride,
naphthalene-1-carbonyl chloride).
Also, benzophenones are prepared by decarboxylation of o-benzoylben-
zoic acid in the presence of copper catalyst.[3] Aromatic ketones are also
available from organometallic compounds,[4] etc.[5] However, these methods
are limited by low yields, complex products, and extravagant cost. Thus, a
simple, efficient approach toward preparation of diaryl ketones is desirable.
BTC was widely used instead of phosgene[6] in synthetic chemistry.
These methods avoided toxic phosgene processes. Recently, Peng et al.
reported
a Friedel–Crafts reaction of benzenes with BTC using
AlCl3, FeCl3, ZnCl2 to form benzophenones with good yields.[7]
However, the stoichiometric or even excess amounts of catalysts were
involved.
Here we report that Yb(OTf)3 is a useful catalyst for Friedel–Crafts
reaction of aromatics with BTC without stoichiometric additive
(Scheme 1).
Scheme 1.

Facile Synthesis of Diaryl Ketones
RESULTS AND DISCUSSION
4251
In our experiment, the Friedel–Crafts reactions of aromatic compounds
with BTC proceeded smoothly in the presence of a catalytic amount of
Yb(OTf)3, and the results are summarized in Table 1.
When Yb(OTf)3 was used as the catalyst, the corresponding products (3)
are prepared in moderate to good yields. The liquid substrates are also used as
the solvents in the reactions (entries 1, 4, 5, 7, 8, 9). We also use dichloro-
methane and chloroform as the solvent, but lower yields are obtained. For
example, when the substrate is benzene (1a) only afford 50% and 55% yield
of benzophenone (3a), respectively, in dichloromethane and chloroform at
refluxing for 8 h (entry 2). Only 15% yield of the desired product (3a) is
detected at room temperature for 24 h (entry 1). So, the temperature obviously
affects the yields.
1-H Pyrrole is unsuitable for this reaction because of the direct acylation
on the nitro atom to form amide. It is interesting that when the N-methyl
pyrrole (1g) is the substrate, high yield (86%) of bis-(1-methyl-1H-pyrrole-
2-yl)-methanone (3g) is obtained after 5 h at 808C. Thiophene (1e) and
furan (1f) were excellent substrates in the same condition.
The products of toluene (1b) are complex. Another two isomers (di-o-
tolyl-methanone and o-tolyl-p-tolyl-methanone) are detected in the mixture,
and the ratio is 1 : 4.35 [determined by high-performance liquid chromato-
graphy (HPLC)]. But, mostly the product is 3b with isolated yield 60%.
The condition of chlorobenzene was similar to that of toluene but more
complex. When benzothiophene (1i) was the substrate, another product
[bis(benzothioph-3-yl)methanone] is isolated with 8% yield. However, N,N-
dimethyl benzene (1d), thiophenes (1e), furan (1f), pyrrole (1g), or naphtha-
lene (1h) only afford one product.
In addition, it was noteworthy that when furan and thiophene were
used for preparation of ketones in the presence of AlCl3 according to
Ref.,[7] we got very complex products, and the majority was the ring-
opening products.
Thus, Yb(OTf)3 is active for the preparation of diaryl ketones via
Friedel–Crafts reaction. The most important characteristic is that a cataly-
tic amount of ytterbium triflate was enough and was effective for the
reaction.
To summarize, in contrast to a vast majority of other approaches, this
work involves the use of common reagents and a simple operation.
Other advantages of this method are its good yields, ease of handing of
reagents, mild conditions, and the possibility of working on large scales.
The present work gives a facile and efficient way to prepare diaryl
ketones.

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Su and Jin
Table 1. Yb(OTf)3 (5% mol, based on BTC) catalyzed Friedel–Crafts reactions of
aromatic compounds with BTC.
Entry
1
ArH
C6H6 (1a)
Product
Yielda %
68 (15)b
2
3
4
5
C6H6 (1a)
C6H6 (1a)
C7H8 (1b)
C6H5Cl (1c)
50e (55)f
71g
60
40c
6
7
(CH3)2NC6H5 (1d)
C4H4S (1e)
58d
77
8
9
C4H4O (1f)
76
N-CH3C4H4N (1g)
86f
10
11
C10H8 (1h)
C8H6S (1i)
57d
70
aBased on 1/3 BTC.
bAt r.t for 24 h.
cTotal yield of 3c and 30c.
dRefluxing in chloroform for 8 h; the yield was based on aromatics.
eDichloromethane is used as the solvent; the yield was based on benzene.
fChloroform is used as the solvent; the yield was based on aromatics.
g50% mol (based on BTC) of Yb(OTf)3 was used.

Facile Synthesis of Diaryl Ketones
EXPERIMENTAL
4253
1H NMR and 13C NMR spectra were recorded on a Varian-400 MHz
instrument using CDCl3 as the solvent. Infrared (IR) spectra were recorded
on a AVATAR-370 infrared spectrophotometer. Melting points were deter-
mined on a digital melting point apparatus WRS-1B and were uncorrected.
Ytterbium triflate was prepared according to the literature.[8]
General Experimental Procedure
Preparation of Benzophenone Using Yb(OTf)3
To a solution of benzene (2 mL) containing bis(trichloromethyl) carbonate
[BTC] (2 mmol, 0.594 g), then Yb(OTf)3 (0.1 mmol, 0.062 g) was added at room
temperature. The mixture was stired for 1 h at room temperature, and it was
allowed slowly to reach 808C. After 5 h at 808C, when the mixture turned
dark red, the solvent was distilled off, then the residue was treated with 5 mL
water and 5 mL saturated NaHCO3 solution, subsequently. The corresponding
product 3a was obtained by TLC in 68% (0.74 g) yield. Mp: 46.58C (Lit.[7]
1
47–488C); H NMR d (ppm): 7.83 (d, J ¼ 8.0 Hz, 4H, ArH), 7.60 (t, J ¼ 7.4
Hz, 2H, ArH), 7.50 (t, J ¼ 7.8 Hz, 4H, ArH); IR (cm21): 3032, 1668 (CO).
Preparation of Di(naphthalen-1-yl)methanone Using Yb(OTf)3
in CHCl3
To a solution of chloroform (5 mL) containing bis(trichloromethyl) carbon-
ate [BTC] (2 mmol, 0.594 g) and naphthalene (12 mmol, 1.54 g), then Yb(OTf)3
(0.1 mmol, 0.06 g) was added at room temperature. The mixture was stired for
1 h at room temperature, and it was allowed slowly to reflux. After 5 h, when
the mixture turned dark red, the solvent was distilled off, and the residue was
treated with 5 mL water and 5 mL saturated NaHCO3 solution, subsequently.
The corresponding product 3 h was obtained by TLC in 52% (0.88 g) yield.
Mp: 1008C (Lit.[4b] 100–1018C); 1H NMR d (ppm): 8.53–8.58 (m, 2H,
ArH), 8.00–8.03 (m, 2H, ArH), 7.92–7.96 (m, 2H, ArH), 7.51–7.62 (m, 6H,
ArH), 7.39–7.44 (m, 2H, ArH); 13C NMR (100 MHz): 199.1, 137.1, 133.8,
133.1, 131.2, 130.5, 128.5, 127.8, 126.6, 125.8, 124.4; IR (cm21): 1645 (CO).
Other Reactions Were Performed as Above and the Data Were Followed
4,40-Dimethylbenzophenone (3b): Mp: 90.5–928C (Lit.[7] 94–958C);
1H NMR d (ppm): 7.72 (dd, J ¼ 8.4 Hz, J ¼ 1.8 Hz, 4H, ArH), 7.24

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Su and Jin
(dd, J ¼ 8.4 Hz, J ¼ 1.8 Hz, 4H, ArH), 2.38 (s, 6H, CH3); IR (cm21): 3034,
1647 (CO).
4,40-Dichlorobenzophenone (3c): Mp: 144.8–145.58C (Lit.[7] 145–1478C);
1H NMR d (ppm): 7.48 (d, J ¼ 8.3, 1.2 Hz, 4H, ArH), 7.69 (d, J ¼ 8.3, 1.2 Hz,
4H, ArH); IR (cm21): 3022, 1665 (CO).
4,40-Bis(dimethylamino)benzophenone (Michlers ketone) (3d): Mp: 177.58C
1
(Lit.[7] 175–1778C); H NMR d (ppm): 7.75 (d, J ¼ 8.2Hz, 4H, ArH), 6.68
(d, J ¼ 8.2 Hz, 4H, ArH), 3.04 (s, 12H, CH3); IR (cm21): 3033, 1615 (CO).
1
Di(thiophen-2-yl)methanone (3e): Mp: 85–868C (Lit.[4f] 86–888C); H
NMR d (ppm): 7.11 (dd, J ¼ 5.0, 4.0 Hz, 2H, ArH), 7.61 (dd, J ¼ 5.0,
1.0 Hz, 2H, ArH), 7.74 (dd, J ¼ 4.0, 1.0 Hz, 2H, ArH); IR (cm21): 1658 (CO).
Di(furan-2-yl)methanone (3f): Mp: 30.58C (Lit.[4f] 30–328C); 1H NMR d
(ppm): 7.54 (dd, J ¼ 1.7, 0.8 Hz, 1H, ArH), 7.25 (dd, J ¼ 3.7, 0.8 Hz, 1H,
ArH), 6.58 (ddd, J ¼ 3.7, 1.7, 0.8 Hz, 1H, ArH); IR (cm21): 1645 (CO).
Bis(N-methyl-1H-pyrrol-2-yl)methanone (3g): Mp: 238C (Lit.[4f] 24–
268C); 1H NMR d (ppm): 6.95–6.98 (m, 2H, ArH), 6.78–6.81 (m, 2H, ArH),
6.11–6.15 (m, 2H, ArH), 3.94 (s, 6H, CH3); IR (cm21): 1641 (CO).
Bis(benzothiophen-2-yl)methanone (3i): Mp: 165.58C (Lit.[4f] 167–
1
1698C); H NMR d (ppm): 8.01 (s, 2H, ArH), 7.05–7.86 (m, 8H, ArH); 13C
NMR (100MHz): 176.1, 151.8, 148.1, 143.6, 140.8, 134.1, 123.8, 122.9,
122.6; IR (cm21): 1639 (CO).
ACKNOWLEDGMENTS
We are grateful to the National Basic Research Program of China [973
Program: 2003CB114400], National Natural Science Foundation of China
[20276072], and National Natural Science Foundation of Zhejiang Province
[2001062 and 2002095] for financial help.
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Received in Poland June 1, 2004