2
70
F. MUGNIER ET AL.
that benzoyl chloride probably crystallizes even more
quickly than benzene under high pressure. However, this
solvent being identical and the proportions being the
same for all of the experiments conducted, the conse-
quences of this crystallization are expected to be similar
for the three aromatic substrates. It remains clear from
Fig. 1 that the effect of high pressure is still fully
dependent on the substrate. Maximum efficiency is ob-
served with fluorobenzene, the benzoylation yield of
which doubles on going from 1 bar to 1 kbar (entries 9
and 10) and increases 10-fold between 1 bar and 5 kbar
(
entries 9 and 11). At higher pressure (10 kbar, entry 12),
the activation becomes less effective, leading to lower
conversions. Hence an ‘optimum pressure’ (OP) is asso-
ciated with an optimal effect. A similar evolution is
observed with benzene (entries 1–4), the best yield being
obtained at a lower OP (1 kbar, entry 2). By contrast, for
toluene the yield constantly decreases when the pressure
increases (entries 5–8). This result suggests that the
crystallization of the aromatic substrate cannot be held
directly responsible for the yield variations since toluene,
ꢀ
the melting-point of which increases from ꢁ95.1 C at
ꢀ
Scheme 3. Suggested mechanisms of Friedel–Crafts
reactions
1 bar to 30 C at 9.6 kbar, remains probably more fluid
than benzene with increasing pressure.
Such result is in complete accordance with Coillet
et al.’s work on the nitration of toluene, benzene and
chlorobenzene between 1 bar and 2.3 kbar in acetic acid
need any activation to reach TS1 or TS2, the limiting step
of the reaction would become HCl elimination, which is
increasingly disfavored on raising the pressure.
8a
or nitromethane.
activation volumes for all the substrates considered, the
The authors reported negative
In conclusion, we have reported new examples of
Friedel–Crafts acylations conducted under high pressure.
This activation affords better results with deactivated
benzenes. Further studies should involve other deacti-
vated aromatics such as chlorobenzene, nitrobenzene and
ꢀ,ꢀ,ꢀ-trifluorotoluene. Concerning the last substrate,
aluminum trichloride will have to be replaced as
this catalyst reacts with trifluorotoluene to give
6¼
|
ÁV | values being larger for deactivated aromatics,
suggesting that the latter would indeed be more sensitive
6¼
toluene
to the effect of the pressure (in AcOH: ÁV
¼
6¼
3
ꢁ1
6
3
ꢁ1
ꢁ
10 cm mol ; ÁV
¼ ꢁ22 cm mol ; ÁV
benzene
ꢁ1
3
¼
ꢁ 23.5 cm mol ).
chlorobenzene
13
DISCUSSION
trichlorotoluene.
The details of Friedel–Crafts acylation are not completely
understood, although two mechanisms are retained to
date (Scheme 3). For this reason, it is difficult to
propose a rationale for the results described above. The
following may nevertheless be considered.
Acknowledgments
1
2
We thank Rhodia-Recherche and the CNRS for a PhD
grant to F.M.
Whatever the mechanism, the reaction between ArH
and the putative 1:1 acyl chloride–AlCl complex (me-
3
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chanism 1) or the acylium (mechanism 2) should be
disfavored on deactivation of the aromatic. This step
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TS2. By contrast, the next step, which is irreversible and
which corresponds to HCl elimination, should be dis-
favored under high pressure (increased number of mole-
cules), whatever the aromatic substrate. The ‘optimum
pressures’ observed could therefore correspond to the
balance between these two opposite effects, which would
obviously depend on the substrate. As toluene would not
1
2
3
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