Chemistry Reference
In-Depth Information
As outlined below, a major difference among the
respective methods is the presence or absence of
solvents. Solvent-free conditions can be employed
using neat reactants only: reactants adsorbed onto
solid supports or reactants in the presence of phase-
transfer catalysts [7,15]. Methods employing sol-
vents (including some that are environmentally
benign, such as water) use either pressurised systems
[9] or open vessels for superheating [37], reflux and
microwave oven reaction enhanced (MORE) [19]
chemistry.
solid and a liquid), reaction could occur by dissolu-
tion of the solid in the liquid or by adsorption of the
liquid onto the solid, and in either case a diluting
solvent would slow the reaction [7].
'Dry media' reactions
For 'dry media' reactions the organic reactants are
adsorbed onto acidic or basic supports such as
alumina, silica, bentonite, montmorillonite K10 or
KSF clays and zeolites and subjected to irradiation,
often in domestic microwave ovens without tem-
perature measurement [7,9,10,12,15]. The supports
also can be doped with inorganic reagents [7,15]. If
a strong base is required, KF on alumina can ionise
carbon-containing acids up to p
K
a
35, whereas clays
like montmorillonite K10 offer acidities comparable
with those of nitric or sulfuric acids [7]. Two exam-
ples are presented in Scheme 17.3. 'Dry media' seem
well suited to transformations involving a single
organic species, e.g. as in deprotection, rearrange-
ment, oxidation and dehydration. Condensations,
including alkylation of carboxylates and acetalisa-
tion, also have been reported [7,9,10,12,15].
5.1 Solvent-free methods
Applications involving solvent-free conditions have
been the subject of the greatest activity and have
been reviewed extensively [7,12,13,15]. Besides the
apparent potential benefits in minimising solvent
usage, reactions can be conducted conveniently and
rapidly without temperature measurement in dom-
estic microwave ovens. A dedicated commercial
reactor for applications at atmospheric pressure also
has been used [7].
Neat reactants in an open container
The simplest solvent-free method involves irradia-
tion of neat reactants in an open container. In the
absence of reagents or supports, the scope for such
processes appears to be limited to relatively straight-
forward condensations that can be conducted with-
out added catalysts, or to intramolecular thermolytic
processes such as rearrangement or elimination.
As with the thermolysis of starch, which afforded
1,6-anhydroglucose in yields of only about 2% [47],
outcomes can be highly dependent upon sample size.
Other examples include amidation of carboxylic
acids (see Scheme 17.2) [48], the reaction of
4-chloroazine derivatives with benzotriazoles [49]
and 1,3-dipolar cycloaddition of pyridinium ylides
[50] and nitrones to acrylates, methacrylates and
fluorinated dipolarophiles [7].
Loupy
et al
. have argued that if the mixture of neat
starting materials is heterogeneous (comprising a
Phase-transfer-catalysed reactions
Solid-liquid solvent-free phase-transfer catalysis
(PTC) is specific for anionic reactions, including base-
catalysed isomerisation as shown in Scheme 17.4
[7]. Usually, a catalyst (typically a tetraalkylammo-
nium salt or a cationic complexing agent) is added
to an equimolar mixture of an electrophile and a
nucleophile, one of which serves as both a reactant
and the organic phase.
In a recent extension, a support of polyethylene
glycol (MW = 3400) also acted as the organic phase
[51].
Advantages and disadvantages of solvent-free methods
The following environmental benefits have been
reported for microwave-heated reactions under
solvent-free conditions:
Scheme 17.2
Direct amidation [4
8
].
