Chemistry Reference
In-Depth Information
Several Newman-Kwart rearrangements were performed by Kappe et al. in supercritical
1,2-dimethoxyethane (300
C, 80 bar) to yield products with high purity (
90%) and in
>
short reaction times (
8 minutes) [235].
<
4.3.5.2 Ionic Liquids
ILs are widely considered to be green due to their low toxicity, nonflammability and
nonvolatility. They can stand for the solvent, catalyst or catalyst support within the reaction
medium [236], as demonstrated for the polycondensation of 4-(3-hydroxynaphthalene)-
1,2,4-triazolidine-3,5-dione with aliphatic diacid chlorides, eliminating the need for low-
volatile solvent and catalyst [237]. In the Michael addition of N-heterocycles to
a
,
b
-unsaturated compounds, the IL 1-methyl-3-butylimidazolium hydroxide could be
reused for several cycles with consistent activity [238]. However, if recycle of the solvent is
not employed, the greenness of ionic liquids is more questionable, as shown by a more
holistic evaluation through life cycle assessment (LCA) [239].
A multiscaled flow system was developed and used to produce ethylmethylimidazole
ethylsulfate ([EMIM][EtSO
4
]) under solvent-free conditions via highly exothermic alkyl-
ation of methylimidazole with diethylsulfate [240]. Precise temperature and conversion
control through multistaged flow reactor assembly was required in order to achieve high-
quality products and avoid temperature runaway of the reactor [241]. Recovery and reuse
was achieved for the IL ([bmim]I and [bmim]PF
6
) in the selective alkylation of amino
groups within amine derivatives with a variety of alkyl halides [242].
Polycondensation of 4-(3-hydroxynaphthalene)-1,2,4-triazolidine-3,5-dione with ali-
phatic diacid chlorides, circumventing the need for an organic solvent and catalyst via
the use of ILs and tetrabutylammonium bromide, was realized [237]. A significant
reduction in reaction time from hours to seconds was achieved when 1-butyl-3-methyl-
imidazolium methylsulfate and InCl
3
3H
2
O were used in the synthesis of optically active
furan diol from d-glucal in quantitative yields (78%) [243].
4.3.5.3 Solvent-free or High-concentration Processing
Apart from using master solvents, the reduction of the load of 'normal' solvents is still an
industrially and enviromentally relevant issue (high-concentration (high-c) processing).
Solvents constitute 80 and 90% of mass utilization in a typical pharmaceutical/fine
chemicals batch chemical operation [244]. Applicable harsh conditions in microreactors
in terms of temperature and pressure suggest a similar boost is achievable under solventless
and solvent-free high-c conditions.
The impact of high-c operation was demontrated by Wu et al. in the peroxidation of
methyl ethyl ketone. The process cannot be speeded up by the use of high temperature, due
to the favourable decomposition of the peroxides. Higher H
2
O
2
and H
þ
concentration
increased the active oxygen content and led to a higher yield [245].
Loeb et al. studied the bromination of toluene, m-nitro toluene and thiophene under
solvent-free conditions. Aromatic compounds are pretty unreactive under normal condi-
tions, especially in absence of catalyst and photoinitiation. The operation conditions were
intensified by the use of undiluted elemental bromine and toluene mixed in an interdigital
triangular micromixer. In this way, fast reaction in flow could be achieved for each of the
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