Mack et al.
1263
are insoluble in water, and readily dissolve the metal triflates.
They also represent two major classes of ionic liquids, tetraal-
kylphosphonium and imidazolium salts. The reaction mixtures
were heated by microwave radiation. Because of the polar nature
of the ionic liquids,16 reaction temperatures of 120 °C may be
obtained within a few seconds and reactions can be completed in
Table 1. Product yields in the reaction of benzyl alcohol
with benzene utilizing various catalysts and ionic liquid
solvents.
[
thtdp][NTf ], yield of
[emim][NTf ], yield of
2
2
Catalyst
diphenylmethane (%)
diphenylmethane (%)
Sc(OTF)3
79
77
63
46
3
85
87
76
60
0
1.5 h, thus making the entire process energy efficient. Results
In(OTF)3
fromthevariousreactions(Table1)revealthatbothtetraalkylphos-
phonium- and imidazolium-based ionic liquids are effective sol-
vents for this alkylation reaction.
Yb(OTF)3
Sm(OTF)3
Nd(OTF)3
The imidazolium ionic liquid is the preferred solvent because of
the ease of isolating the product. The product is isolated by simple
extraction of the reaction mixture with pentane in which the
product is soluble and the ionic liquid is insoluble. The catalyst
remains in the ionic liquid and water formed as the by-product
can be easily removed. By contrast, isolation of the product from
the tetraalkylphosphonium ionic liquid is more difficult because
the ionic liquid is also soluble in common solvents such as pentane,
ethyl ether, and dichloromethane.
The catalyst and ionic liquid can be recycled easily. Following
the reaction of benzyl alcohol with benzene (Table 2, entry 1),
droplets of water were observed in the reaction mixture. Extrac-
tion with pentane removed the product and any unreacted ben-
zene, while the indium(III) triflate remained dissolved in the ionic
liquid. Water was removed by warming the catalyst solution un-
der reduced pressure. After the drying process was completed,
additional benzyl alcohol and benzene were added to the catalyst
solution and the reaction repeated. The system was cycled a total
of three times with yields of 85%, 87%, and 86% obtained. A mini-
mum of 95% of the mass of the catalyst solution was recovered
after each reaction cycle. Spectral analyses of the catalyst solution
before and after reaction indicated no decomposition of the ionic
liquid. In related experiments, Seddon and co-workers1 found
that insignificant amounts of indium(III) chloride were leached
from the catalyst solution during acylation reactions conducted in
[bmim][NTf2].
The catalytic activity of indium triflate was found to vary as the
anion in the ionic liquid changes (Table 2). Similar variations have
been reported for Friedel−Crafts acylation reactions carried out in
1
5e,17
ionic liquids.
Reactions in hydrophobic ionic liquids such as
[
emim][NTf ] proceed with high yields whereas those conducted
2
in water-miscible ionic liquids such as [emim][BF ] are largely in-
4
effective. Since the reaction by-product is water, its exclusion
from the reaction medium helps to drive the reaction toward
completion. At elevated temperatures, the presence of water in
ionic liquids leads to hydrolysis of the [BF ] and [PF ] counterions
5e
4
6
with liberation of HF.18 This does not completely explain the
lower activity, since HF is also known to catalyze the alkylation of
To gain a better understanding of the mechanism of the indi-
um(III) triflate catalyzed alkylation reaction, a variety of simple
primary, secondary, and tertiary alcohols were used as substrates
(Table 4). No alkylated product could be observed in the reaction
mixture when primary alcohols were used (entries 1 and 2). Cyclic
secondary alcohols gave high yields of the desired products (en-
tries 4 and 5). Interestingly, 2-butanol (entry 6) formed only trace
amounts of the alkylated product with the majority of the product
appearing to be polymeric hydrocarbons. When 2-methyl-2-propanol
(entry 9), a tertiary alcohol, was reacted with benzene, only a
small amount of tert-butylbenzene was detected. Recently, it has
aromatics by alcohols. By comparison, the [NTf ] counterion is a
2
stable, weakly coordinating anion with a high degree of charge
delocalization. This may allow for an increase in the effective
acidity of the In(III) ion, thus contributing to the overall efficiency
of the reaction. From these results, it can be seen that indium
triflate is an excellent catalyst for the Friedel−Crafts alkylation of
benzene with benzyl alcohol utilizing [emim][NTf ] as the solvent.
2
The effects of temperature and irradiation time on the rate of
product formation are shown in Fig. 1. As can be seen, the reaction
occurs faster at 120 °C than at 80 °C. Reaction rates at 100 °C are
virtually identical to those at 120 °C. Below 80 °C, the formation of
dibenzyl ether competes with the formation of diphenylmethane.
Benzyl alcohols are known to undergo dehydration with the for-
mation of dibenzyl ethers when heated in various phosphonium
ionic liquids in the absence of additional catalysts.19 In the current
system, no dibenzyl ether was detected at reaction temperatures
above 100 °C, even at shorter reaction times. When dibenzyl ether
was added to the reaction mixture at temperatures above 100 °C,
it was rapidly converted to diphenylmethane; therefore, it appears
that any ether formed is also converted to alkylated product.
The study was expanded to determine the effect that various
electron-withdrawing and -releasing groups attached to the
benzene ring would have on the reaction. Results are shown in
Table 3. As can be seen, benzyl alcohols possessing a variety of
electron-withdrawing groups give high product yields (entries 2–7).
Benzyl alcohols with an electron-releasing group at the meta-
position also give excellent yields (entries 9 and 11) whereas
electron-releasing groups attached at the para-position give signif-
icantly lower yields. These results are interesting because pre-
sumed reactive carbocation-like intermediates should be formed
readily from benzyl alcohols possessing electron-releasing groups.
To determine whether the high temperature employed in the
reaction was responsible for the lower yields, reactions were con-
ducted at lower temperatures (70 and 90 °C) with reduced micro-
wave power; however, yields were not improved. The majority of
the material formed in these reactions was insoluble in pentane
and appeared to be polymeric in nature.
2
0
been reported that substantial amounts of isobutene dimers
and trimers were formed during the attempted tert-butylation
of aromatic hydrocarbons with tert-butyl chloride catalyzed by
silica-supported indium salts. When more reactive aromatic hy-
drocarbons were substituted for benzene (entries 7, 8, 10, and 11),
monoalkylated products were obtained in satisfactory yields.
Elimination appears to be a competing reaction that is suppressed
when more reactive arenes are used. Allyl alcohol (entry 3) reacted
readily at lower temperatures (70 °C), but neither allylbenzene
nor 1-phenylpropene was detected by NMR or IR analysis of the
product mixture. Instead, a material with spectral properties
similar to substituted polystyrene was obtained. To determine
whether a radical mechanism might be involved, reactions were
conducted in the presence of a radical inhibitor. When hydroqui-
none was added to a reaction mixture of benzene and benzyl
alcohol, the reaction proceeded normally, suggesting that the al-
kylation reaction is not a radical process. Similarly, addition of
hydroquinone to a reaction mixture of allyl alcohol and benzene
did not prevent the formation of a polymeric product. Nonradical
polymerization initiated by a carbocation intermediate has been
proposed previously in indium alkylation reactions2 and several
transition metal bistriflimide salts are known to catalyze polym-
0
2
1
erization of styrene. These observations suggest that highly re-
active alcohols tend to form intermediates that polymerize unless
captured rapidly by activated aromatic compounds. By contrast,
even slightly deactivated aromatics such as chlorobenzene (en-
try 12) give acceptable yields with benzyl alcohol.
Published by NRC Research Press