Chemistry Reference
In-Depth Information
Scheme 17.3
Examples of 'dry media'
reactions [7].
It is not always acknowledged that solvents are
used for 'dry media' reactions, both to load the reac-
tants onto the support and to elute the products after
reaction. If the supports are polar materials such as
alumina or silica gel, which are commonly used
in liquid chromatography, substantial quantities of
solvent may be required to remove the organics. For
clean processing, recycling of the solvent and the
support would be essential. The latter has not been
demonstrated in many instances and may prove dif-
ficult if residual organic reactants and/or products
are retained strongly. The advent of generally applic-
able protocols for recycling spent supports would
minimise waste and significantly advance the scope
of the technique.
A potential hazard, particularly for scale-up, con-
cerns the toxic effects of mineral dusts [52]. Chem-
ical composition and active surface states are critical
determinants of biological response, so minerals
doped with inorganic or organic oxidants such as
MnO
2
, CrO
3
, iodobenzene diacetate and sodium peri-
odate, or reductants like NaBH
4
and catalysts includ-
ing KF and CsF, which have been employed as 'dry
media' [15], could have severe biological side effects
if inhaled. Strict safety precautions should be taken
when handling such materials.
Scheme 17.4
Isomerisation of eugenol [7].
(1)
Avoidance of large volumes of solvent reduces
emissions and the need for redistillation
(2)
Work-up is simple by extraction, distillation or
sublimation.
(3)
Recyclable solid supports can be used instead of
polluting mineral acids and oxidants.
(4)
The absence of solvent facilitates scale-up.
(5)
Safety is enhanced by reducing the risks of over-
pressure and explosions [7,17].
The technique could be improved by overcoming
deficiencies that include a low ratio of organic reac-
tants to solid support and the lack of facilities for
measuring reaction conditions. If the reaction tem-
perature is not known and/or not uniform through-
out the sample, reactions may not be reproducible
between microwave systems. Given the variability in
operation and performance of domestic microwave
ovens, perhaps it is not surprising that few, speci-
fic, literature syntheses have been reproduced by
others and occasionally such attempts have resulted
in alternative outcomes or failure [5,31,32]. Unfor-
tunately, these shortcomings demand technical im-
provements that may place the equipment beyond
the financial reach of a large group of users.
5.2 Methods with solvents
MORE chemistry
An approach termed 'microwave-induced organic
reaction enhancement' (MORE) chemistry was
developed by Bose
et al
. [19,53-55]. Polar, high-
boiling solvents are employed with open vessels in
unmodified domestic microwave ovens. The solvents
