reactions and synthetic approaches which involve superelec-
trophilic activation of reagents mediated by liquid superacids or by
excess of aluminium halides.1–3 Unfortunately, the procedures,
often attractive for production of fine chemicals and pharmaceuti-
cals, suffer from the significant disadvantage of using a large excess
of non regenerable strong acid. Obviously, the utility of solid acids
to provide these reactions is highly desirable from practical and
environmental standpoints.
reactivity in the presence of aluminium halides, which, in contrast
to Broensted superacids readily provokes the isomerization of
intermediate 4-phenyl-1-naphthol into 3-phenyl-1-naphthol.17
Analogously, the reaction of maleimide (7) initiated with HUSY
gives the same result as obtained upon action of aluminium halides,
whereas the use of exclusively protonic solid acid, Nafion does not
provide any reaction.
In summary, we have shown here that solid acids instead of
superacids may be successfully used to carry out some reactions
proceeding only through dicharged intermediates and we suggest
that similar procedures can be followed to carry out analogous
reactions. Our method seems promising for practical applications
because solid acids could be easily reused in contrast with
superacids.
Recent reviews show significant progress in the application of
solid acids in synthetic organic chemistry.4 However, the scope of
the reactions involves generally classical Friedel–Crafts or other
acid catalysed reactions which do not require superacidity. In the
present paper we report solid acid mediated reactions which occur
normally only under superacidic conditions or in the presence of
large excess of aluminium halide. As the acidity of solid acids such
as H-zeolites, sulfated zirconias or Nafion-H is generally con-
sidered5 to be well below that of superacids (212 < Ho < 224),6
the hypothesis of identical dicationic intermediates may be easily
discarded. However, the framework of a solid acid may provide
close proximity of acidic sites enabling the formation of diproto-
nated (coordinated) species. Moreover, an effective compensation
of comparatively low proton acidity of zeolites may be obtained
due to nucleophilic assistance of the lattice oxygens in the transition
state of a concerted mechanism resulting in an increase of the
protonation rate.5b,7 Similarly, the enzyme mediated protonation of
a heterocyclic carbocation8 provides an example showing that
structural factors of acids can have a profound influence in
electrophilic activation.
The reactions which we report here encompass Friedel–Crafts
alkylations and cyclo-alkylations of aromatics accompanied in
some cases by cationic rearrangements (Table 1). Typically,
reactions were performed in high pressure tubes under stirring.
Reaction mixtures, after cooling, were worked up with water and
extracted with ether to isolate crude products, which were purified
by silica gel chromatography or crystallisation. The solid acids
were recovered and reused without loss of activity. The comparison
between superacid or aluminium halide mediated reactions with
those observed on solid acids shows a strong analogy. It is
particularly interesting to note the reactivity of amide 3 towards o-
dichlorobenzene, a poor nucleophile, normally inert towards
monocationic C-electrophiles,19 supporting the strong electro-
philicity of key intermediates. It is also worth noting that successful
results were obtained only when an effective excess of acidic sites20
was used. Moreover, saturation of reaction mixtures with gaseous
HCl appeared to be beneficial in promoting some reactions, such as
reaction of 1-naphthol (5) with benzene. Likewise, the increased
reactivity of 6 can be explained by elimination of HCl upon the
reaction. These experimental data are in accord with the suggested
mechanisms, involving the intermediacy of adsorbed dicationic or
dicoordinated species, probably in equilibrium with their mono-
cationic precursors. It seems reasonable, that the generation of these
intermediates is a limiting step. The resulting superelectrophiles
should in no way be considered as free dications but rather as
intermediates stabilised by strong interaction with counterionic
surface of the solid acid. The ability of HUSY and sulfated
zirconias to initiate the reactions appeared to be comparable and
notably higher than that of Nafion, which was successfully used
only in the case of the most active compound 1. This may be in
relation with the contribution of Lewis acid sites in the activation
with former acids. In agreement with this suggestion, the HUSY
induced reactivity of 4-chloro-1-naphthol (6) is similar to its
Financial support of the work by the Loker Hydrocarbon
Institute, U.S.C., Los-Angeles, and the “CNRS” is gratefully
acknowledged.
Notes and references
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C h e m . C o m m u n . , 2 0 0 4 , 1 7 5 4 – 1 7 5 5
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