Lanthanide bis(trifluoromethylsulfonyl)amides as effective reusable catalysts for catalytic Friedel-Crafts acylation

Article abstract of DOI:10.1039/a901606f

By using a catalytic amount of a lanthanide bis(trifluoromethylsulfonyl)amide, Ln(NFf₂)₃, the Friedel-Crafts acylation of substituted benzene proceeds smoothly under mild conditions and the catalysts are easily recovered from the reaction mixture.

Full text of DOI:10.1039/a901606f

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446 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 446^447y
Lanthanide Bis(trifluoromethylsulfonyl)amides
as Effective Reusable Catalysts for Catalytic
Friedel^Crafts Acylationy
Jin Nie,* Jun Xu and Guangyong Zhou
Department of Chemistry, Huazhong University of Science & Technology, Wuhan 430074,
Hubei, P.R. China
By using a catalytic amount of a lanthanide bis(trifluoromethylsulfonyl)amide, Ln(NFf2† , the Friedel^Crafts
acylation of substituted benzene proceeds smoothly under mild conditions and the catalysts3 are easily recovered
from the reaction mixture.
Some Lewis acid catalysts have been employed in the
Friedel^Crafts acylation in place of aluminum trichloride.1;2
Kobayashi and co-workers3 have especially developed the
lanthanide tri£uoromethanesulfonates [lanthanide tri£ates,
Ln(OTf)3] as e¡ective and reusable Lewis acid catalysts
for many organic reactions such as the aldol reactions,
Diels^Alder and Michael reactions. For the Friedel^Crafts
acylation the lanthanide tri£ates are also e¡ective Lewis
acid catalysts.4 We have found that the metal salts
lytic activity to a¡ord good yield at room temperature
for 4 h (Table 2, entry 6). Interestingly, when the acetylation
reaction of anisole was carried out in tetrahydrofuran for
10 min the reaction mixture gradually became sticky. Inves-
tigation of this result showed that the solvent, THF, had
polymerized in the presence of the catalyst and meant that
YbꢀNTf2† is a very strong Lewis acid catalyst.
3
To explore the scope of the above ytterbium(iii)
tris[bis(tri£uoromethylsulfonyl)amide] catalysed acylation,
the reaction was examined with several substituted benzenes
and acetic anhydride (Table 3). The acylation of benzene
was not detected and m-xylene was acetylated to the
dimethylacetophenone in moderate yield under very mild
conditions. The introduction of a methoxy group a¡orded
high yields of the acylation products. The reactivity of sev-
eral structurally diverse acid anhydrides was also checked
of bis(tri£uoromethylsulfonyl)amine ‰HNꢀSO2CF3†
or
HNTf25Š are e¡ective Lewis acid catalysts for2 the
Diels^Alder reaction of cyclopentadiene with methyl vinyl
ketone in non-polar organic solvents6;7 and the results
indicated that lanthanide bis(tri£uoromethylsulfonyl)amides
are much more e¡ective Lewis acid catalysts than
lanthanide tri£ates. Mikami et al.8 reported recently some
new metal bis(tri£uoromethanesulfonyl)amides as highly
e¤cient Lewis acid catalysts for acylation reactions. We
report here on the acylation reaction of substituted benzenes
with acetic anhydride by using a catalytic amount of the
Table 1 LnꢀNTf2)3 catalysed Friedel^Crafts acylation of anisole
a
in CH3NO2
lanthanide bis(tri£uoromethylsulfonyl)amides, LnꢀNTf2†
3
Ln(NTf2)3(20 mol%)
OMe
CH3CO
OMe
(CH3CO)2O
Results and discussion
The catalytic activities of various lanthanide bis(tri£uoro-
methylsulfonyl)amides as Lewis acid catalysts were ¢rst
examined in the acylation reaction of anisole with acetic
anhydride (Table 1). In accordance with the results for
Entry
Catalyst
T/ 8C
t/h
Yield(%)b
1
2
3
4
5
6
LaꢀNTf2†
25
25
25
25
25
50
4
10
4
8
4
55
60
85
90
99
99c
3
SmꢀNTf2†
YbꢀNTf2†
lanthanide tri£ates,4 YbꢀNTf2† was the most e¡ective cata-
3
3
lyst in this case and this is consistent with that what was
found by Mikami et al.8 The activities of LaꢀNTf2† were
relatively low (Table 1, entries 1^5), however much 3higher
than those of the corresponding lanthanide tri£ates.4 The
acetylation reaction of anisole catalysed by a catalytic
3
YbꢀOTf†
18
3
aReaction conditions: anisole 1.25 mmol, ꢀCH3CO†2O 2.5mmol,
CH3NO2 5 mL. bDetermined by HPLC. cSee ref. 4.
amount of LnꢀNTf2† proceeded smoothly under very mild
3
Table 2 Solvent effect on Friedel^Crafts acylation of anisole
conditions and the di¡erences among the catalytic activities
of the lanthanide cations were consistent with their Lewis
acidities, which was investigated by the halochromism of
2,4,40,6-tetramethoxyazobenzene in toluene.9
a
catalysed by YbꢀNFf2†
3
Yb(NTf2)3
OMe
CH3CO
OMe
(AcO)2O
Solvent e¡ects on the acetylation reaction of anisole
catalysed
by
ytterbium(iii)
tris[bis(tri£uoromethyl-
YbꢀNTf2†
(AcO2†2O
3
sulfonyl)amide] and optimization of the conditions in
Entry
(equiv.)
(equiv.)
t/h
Solvent
Yield(%)b
nitromethane are shown in Table 2. Although YbꢀNTf2†
3
1
2
3
4
5
6
7
8
9
10
0.2
0.2
0.2
0.2
0.2
0.01
0.05
0.1
0.2
0.2
2.0
2.0
2.0
excess
2.0
2.0
2.0
2.0
2.0
4
4
4
4
4
4
4
4
CH3CN
CCl4
Et2O
Ac2O
CH2Cl2
CH3NO2 66
CH3NO2 77
CH3NO2 85
CH3NO2 99
n.d.c
20
24
55
80
could be used as a homogeneous catalyst in acetonitrile,
diethyl ether, acetic anhydride, nitromethane, and
tetrahydrofuran under the same experimental conditions,
totally di¡erent results were obtained (Table 2, entries 1,
3^4, and 9^10. Nitromethane was the best solvent for
the acetylation reaction of anisole and, even if 1 mol%
of catalyst was used, YbꢀNTf2† also showed higher cata-
3
4
0.5
d
2.0
THF
* To receive any correspondence (e-mail: niejin@wuhan.cngb.com).
y This is a Short Paper as de¢ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is
therefore no corresponding material in J. Chem. Research (M).
aReaction conditions: anisole 1.0equiv., solvent 5mL, 25 8C.
bDetermined by HPLC. cNot detected. dThe solvent THF was
found to be polymerized in the presence of the catalyst.

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J. CHEM. RESEARCH (S), 1999 447
Table 3 YbꢀNTf2† (20 mol%) catalysed Friedel^Crafts acylation
Finally, the catalytic activities of recovered catalysts were
examined. As shown in Table 5, from the yields of
4-methoxyacetophenone in the second and third uses of
the catalyst it was obviously that the catalytic activities
of the lanthanide bis(tri£uoromethylsulfonyl)amides were
almost same as that in the ¢rst use. Furthermore, more than
90% of the catalysts could be easily recovered from the
aqueous layer by simple extraction.
In summary, lanthanide bis(tri£uoromethylsulfonyl)-
amides are e¡ective Lewis acid catalysts for Friedel^Crafts
acylation and are stable in water, easily recovered, and
reusable.
3
at 258Ca
Yb(NTf2)3
Ar
H
+
(CH3CO)2O
Ar COCH3
Yield(%)b
Ar^H
Product
t/h
24
n.d.c
10
COMe
Me
Me
9
4
Me
Me
COMe
COMe
MeO
MeO
99
aReaction conditions: ArH 1.25 mmol, ꢀCH3CO†2O 2.5 mmol,
CH3NO2 5 mL, 258C. bDetermined by HPLC. cNot detected.
Experimental
The lanthanide bis(tri£uoromethylsulfonyl)amides were prepared
from the reaction of the amine acid, HNTf2, with the corresponding
oxides Ln2O3 in water.6 A typical experimental procedure for the
acylation reaction of anisole with acetic anhydride catalysed by
ytterbium(iii) tris[bis(tri£uoromethylsulfonyl)amide], YbꢀNTf2† , is
described as follow. A mixture of anisole (670 lL, 6.2 mmol) 3and
acetic anhydride (1.16 mL, 12.4 mmol) was added to a solution of
YbꢀNTf2†3 (312.5 mg, 0.31 mmol) and dried nitromethane (5 mL) with
stirring at 25 8C and the reaction carried out for 4 h at room
temperature (25 8C). After dilution with water (10 mL), the reaction
mixture was extracted with chloroform (3 Â 10 mL). The acylation
product, 4-methoxyacetophenone, was dissolved in the chloroform
layer and the yield determined by HPLC (77%). The aqueous layer
was then concentrated to give a solid residue, which was ®nally
heated at 100 8C for 6.5 h under 1 mmHg to afford 281.3 mg of white
Table 4 YbꢀNTf2† catalysed Friedel^Crafts acylation of anisole
in CH3NO2 with s3tructurally diverse acid anhydrides
a
Yb(NTf2)3, 25 ˚C
OMe
RCO
OMe
(RCO)2O
Entry Anhydride
Catalyst/mol%
t/h
Yield(%)b
1
2
3
4
ꢀCH3CO†2O
10
10
20
20
2
2
2
86
87
99
ꢀCH3CH2CO† O
ꢀCH3ꢀCH2† C2O† O
(PHCO)2O6
24
^
2
c
aReaction conditions: anisole 1.25 mmol, ꢀCH3CO†2O 2.5 mmol,
CH3NO2 2.5 mL. bDetermined by 1HNMR. cProduct was not
detected at 1008C.
powder of YbꢀNTf2† (90%). The recovered catalyst was reused in
3
the next acylation reaction.
This work is supported by the National Education Com-
mittee of China, and Professor Takaaki Sonoda of the Insti-
tute of Advanced Material Study of Kyushu University of
Japan is thanked for his kind help. The LiNTf2 was supplied
by Mr Hiroaki Sakaguchi of Ube Research Center of
Central Glass Co. Ltd. of Japan.
Table 5 Recycling of LnꢀNTf2† in acylation of anisolea
3
Ln(NTf2)3
OCH3
CH3CO
OCH3
(CH3CO)2O
Entry Catalyst (mol%)
Yield(%)b
Recovery of catalyst(%)c
Received, 1st March 1999; Accepted, 1st April 1999
Paper E/9/01606F
1
2
3
4
5
6
7
8
9
LaꢀNTf2†3ꢀ20†
SmꢀNTf2†3ꢀ20†
YbꢀNTf2†3ꢀ20†
YbꢀNTf2†3ꢀ5†
58
56
80
81
99
99
77
76
77
95 (1st)
93 (2nd)
91 (1st)
90 (2nd)
92 (1st)
90 (2nd)
90 (1st)
92 (2nd)
90 (3rd)
References
1
2
3
4
5
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aReaction conditions: anisole 1.0 equiv. ꢀCH3CO†2O 2.0 equiv.,
CH3NO2 5 mL, 25 8C, 4 h. bDetermined in HPLC. cDetermined by
the ratio of the isolated amount of catalyst from the aqueous layer
based on initial amount of LnꢀNTf2†3.
6
7
H. Kobayashi, J. Nie and T. Sonoda, Chem. Lett., 1995, 307.
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(Table 4). For the aliphatic acid anhydrides, from acetic
anhydride to octanoic acid anhydride the reactivity of
acylations seemed to change little. However, the case of aro-
matic acid anhydrides for example benzoic anhydride the
acylation of anisole did not occur even at 100 8C for 24 h.
8
9