Functional selectivity in Friedel-Crafts alkylations with allylic halides promoted by solid composite lead fluoride reagent

Article abstract of DOI:10.1039/a704606e

The composite lead fluoride reagent prepared from PbF₂ and NaBr is a nonhygroscopic and efficient solid reagent for promoting Friedel-Crafts type reaction of aromatic compounds with allylic halides selectively to afford the monoallylated compounds.

Full text of DOI:10.1039/a704606e

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Functional selectivity in Friedel–Crafts alkylations with allylic halides
promoted by solid composite lead fluoride reagent
Junko Ichihara*
Institute of Scientific and Industrial Research, Osaka University, Mihogaoka, Ibaraki, Osaka 567, Japan
The composite lead fluoride reagent prepared from PbF2
and NaBr is a nonhygroscopic and efficient solid reagent for
promoting Friedel–Crafts type reaction of aromatic com-
pounds with allylic halides selectively to afford the
monoallylated compounds.
Using the solid reagent A, an electrophilic substitution of
p-xylene with 1-chlorobut-2-ene proceeded at room temp. to
give the corresponding monosubstituted products (ArR) in 86%
yield after 2 h (Table 1). The composite lead fluoride reagent B,
prepared from NaCl and PbF2, also showed high reactivity.
Under the same reaction conditions, PbF2 showed no reactivity,
but a mixture of NaBr and PbF2 promoted the reaction.
Friedel–Crafts type allylation of aromatic compounds using
allylic alcohols or halides and Lewis acid catalysts has proved
unsatisfactory, because the reaction occurs both at the double
bond and at the allylic carbon to yield polyaryl-substituted
alkanes.1 The reactions with allylic halides liberates hydrogen
halide, which further attacks the double bond of the products to
induce the second reaction. Recently Kodomari et al. have
reported successful selective monoallylation by using a combi-
nation of a solid acid and a solid base, ZnCl2–SiO2 and K2CO3–
Al2O3.2 The solid base operates as a scavenger of hydrogen
halide liberated during the reaction and does not influence the
catalytic activity of the solid Lewis acid because of the distance
between both solid supported reagents. Solid acids such as
acidic zeolites and clays has been applied to achieve function-
ally selective allylations with allylic alcohols.3,4
Table 2 Friedel–Crafts allylation using Pb3BrF5 solid reagenta
Allylic Halides
Aromatics
T/°C t/h
Products
Yield (%)
Br
94
Benzene
60
60
2
Br
p-Xylene
p-Xylene
2
91
91
Cl
60
10
We have studied the use of synergistic interactions between
diverse inorganic solid reagents for enhancing solid–liquid
heterogeneous reactions with organic compounds dissolved in
organic solvents. Recently we have observed that a combination
of lead fluoride and a small amount of sodium bromide
accelerates the fluorination of allylic and benzylic halides.5,6 It
was found that a new inorganic lead compound was formed
from the solid reagents during the fluorination of organic
halides, and that its formation process is associated with
activation of the alkyl halides. This prompted us to examine it
as a promoter for Friedel–Crafts alkylation reactions. Here we
report that the composite lead fluoride reagent is a mild and
efficient solid reagent for promotion of selective Friedel–Crafts
type monoallylation when used in stoichiometric amounts.7
A composite lead fluoride reagent A (Pb3BrF5) was easily
prepared by mixing aqueous NaBr and PbF2 powder (3 equiv.).†
room
temp.
Cl
Cl
45b
Benzene
Toluene
3
2
room
temp.
99c,d
room
temp.
Cl
p-Xylene
Toluene
p-Xylene
2
2
2
77
room
temp.
85d
77
Cl
Cl
room
temp.
Table 1 Friedel–Crafts allylation of p-xylene and 1-chlorobut-2-ene
room
temp.
87c
93c
room temp.
48
2
Cl
p-Xylene
p-Xylene
p-Xylene
Cl
+
room
temp.
GC Yield of ArR
Br
Run
Reagent
(%) (t/h)a
1
2
3
4
Reagent A (Pb3BrF5)
Reagent B (Pb3ClF5)
NaBr–PbF2
43 (1) 86 (2)
27 (1) 80 (2)
36 (1) 48 (2)
0 (1) 0 (2)
Br
room
temp.
70b,c
1
PbF2
5
6
TiCl4 (10 mol%) at 3–4 °C
ZnCl2–K10 (10 mol%)
65 (1)b
a
b
63 (0.5)b
Alkyl halide:aromatic Pb3BrF5
=
1:ca. 20:0.5.
Alkyl
halide:aromatic:Pb3BrF5 = 1:140:1. c Isolated yields. The others are GC
yields. opm-isomer ratio = 1:1:0.1, determined by H NMR spectros-
a
b
d
1
1-Chlorobut-2-ene:p-Xylene: Reagent = 1:ca. 20:0.6. By-products
were observed by GC.
copy.
Chem. Commun., 1997
1921

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Accordingly, in the latter case the reagent A or B, formed in the
course of the reaction, was considered to participate in the
reaction.
smoothly under mild reaction conditions and the corresponding
monoallylated compounds were obtained in good to excellent
yields. The allylation of p-xylene by the more reactive 1-chloro-
3-methylbut-2-ene was also performed without consecutive
cyclization at C–C double bond. Further, our allylation method
can be performed under the atmosphere, and requires simple
isolation procedures, since the solid reagent is nonhygroscopic
and easy to handle.
In conclusion, composite lead fluoride reagents were found to
promote Friedel–Crafts allylation selectively and efficiently,
thus demonstrating that they are a new and unique type of solid
reagent exhibiting mild Lewis acidity and halide capture
abilities.
Catalytic activities of TiCl4 and ZnCl2–Montmollironite
K-10 (ZnCl2–K-10), well-known as mild Lewis acid catalysts,
were compared with those of the lead reagents in the
reaction.8,9‡ These catalysts showed higher reactivities but gave
considerable amounts of by-products. In the presence of 10
mol% of ZnCl2–K-10, a 63% yield of the monoallylated product
ArR and a 24% yield of its hydrogen chloride adduct (ArRX)
were formed as major products, with a 2% yield of diaryl
substituted alkanes (Ar2R). Kodomari et al. have reported that
ArRX- and Ar2R-type products are mainly formed in the
reaction of benzene and allyl chloride with silica-supported
ZnCl2. On the contrary, in the case of our reagent A such by-
products were scarcely detected by GC, and the desired product
ArR was selectively formed and maintained intact in the
reaction mixture even after prelonged. Thus, the reagent was
found to have functional selectivity, in that it operates on the
allylic carbon rather than on the double bond. The selective
allylation occurs because the reagent behaves as both the Lewis
acidic catalyst and a trap for the liberated halide, via exchange
with fluoride.
Footnotes and References
* E-mail: ichihara@sanken.osaka-u.ac.jp
† Powder X-ray diffraction analysis of the reagent A showed not a mixture
of the known lead compounds PbF2, PbBrF or PbBr2, but the formation of
a new lead compound. It was determined by X-ray fluorescence analysis
that the reagent was composed of Pb, Br and F in 1.0:0.3:1.8 molar ratio.
Therefore, the reagent A has the formula Pb3BrF5.
‡ ZnCl2–K10 purchased from Fluka was used.
Cl
1 R. Koncos and B. S. Friedmann, Friedel–Crafts and Related Reactions,
ed. G. H. Olah, Interscience, New York, 1964, vol. II, part 1, pp. 289–
412.
Cl
2 M. Kodomari, S. Nawa and T. Miyoshi, J. Chem. Soc., Chem. Commun.,
1995, 1895.
3 P. H. Espeel, B. Janssens and P. A. Jacobs, J. Org. Chem., 1993, 58,
7688.
ArRX
4 K. Smith and G. M. Pollaud, J. Chem. Soc., Perkin Trans 1, 1994,
3519.
Ar
Ar
Ar
5 J. Ichihara, T. Hanafusa, Y. Takai and Y. Ito, Chem. Lett., 1992, 1161.
6 J. Ichihara, Y. Takai, T. Hanafusa and K. Tomioka, J. Fluorine Chem.,
1995, 71, 131.
Ar
Ar
Ar
Ar2R
7 J. Ichihara, T. Hanafusa, Y. Ito and K. Tomioka, Jpn. Kokai Tokkyo
Koho, Jap. Pat., 95-51 250, 1995.
8 P. Laszlo and A. Mathy, Helv. Chim. Acta, 1987, 70, 577.
9 J. H. Clark, A. P. Kybett, D. J. Macquarrie, S. J. Barlow and P. Landon,
J. Chem. Soc., Chem. Commun., 1989, 1353.
The reaction required at least stoichiometric amounts of the
reagent. It was shown by powder X-ray diffraction analysis that
the reagent A is converted to PbClF during the reaction.
Table 2 shows typical examples for some allylic halides. In
all cases the heterogeneous reaction of aromatic compounds and
allylic halides using the Pb3BrF5 solid reagent proceeded
Received in Cambridge, UK, 1st July 1997; 7/04606E
1922
Chem. Commun., 1997