Razzaq et al.
2
reactor. However, there is experimental evidence that certain
essentially no effect compared to conventional microwave
processing or even thermal heating at the same temperature.
9–11
chemical transformations when carried out at the same measured
reaction temperature using either microwave or conventional
heating lead to different results in terms of product distribution
As an alternative to the simultaneous cooling approach where
higher power levels can be applied, any method that would allow
a lower level of microwave power to be administered to the
reaction mixture under otherwise identical conditions (temper-
ature) would appear to be an additional useful probe for the
investigation of specific and nonthermal microwave effects. In
2006, we introduced silicon carbide (SiC) cylinders as passive
3
,4
(
selectivity) and yield. These difficult to rationalize effects
have been referred to as “specific” or “non-thermal” microwave
3
,5
effects. Specific microwave effects, although still the result
of a thermal phenomenon, cannot be duplicated by conventional
heating and result from the uniqueness of the microwave
dielectric heating phenomenon. In this category fall, for example,
1
2
heating elements (PHEs) for microwave chemistry. These
chemically inert heating inserts efficiently absorb microwave
energy and subsequently transfer the generated thermal energy
(
(
i) the superheating effect of solvents at atmospheric pressure,
ii) the selective heating of, e.g., strongly microwave absorbing
heterogeneous catalysts or reagents in a less polar reaction
medium, and (iii) the elimination of wall effects caused by
1
2
via conduction phenomena to the reaction mixture. The use
of these passive heating elements allows otherwise microwave
transparent or poorly absorbing reaction mixtures to be ef-
fectively heated under microwave conditions. Apart from this
useful practical feature, we have realized at the time that
chemical transformations using SiC cylinders often required only
a fraction of the microwave magnetron output power as
compared to experiments attempted without these passive
5
inverted temperature gradients. In contrast, nonthermal micro-
wave effects have been proposed to be the consequence of a
direct interaction of the electric field with specific molecules in
the reaction medium that is not related to a macroscopic
temperature effect. This interaction may lead to a decrease
in activation energy or an increase in the pre-exponential fac-
tor in the Arrhenius law due to orientation effects of polar
3
,5
1
2–15
heating elements.
While our previous investigations in this
3
12,14
species in the electromagnetic field. A similar effect may be
area were mainly concerned with the practical benefits
of
observed for polar reaction mechanisms, where the polarity is
increased going from the ground-state to the transition state,
resulting in an enhancement of reactivity by lowering of the
using such heating aids in microwave synthesis, we are now
focusing on the more fundamental role of applying these devices
in microwave chemistry. It has occurred to us that the use of
these strongly microwave absorbing additives represents an
experimentally very easy way to modulate the electric field
strength in a microwave-heated experiment performed at
constant temperature. Therefore, comparison studies of micro-
wave-heated reactions performed in the presence or absence of
a SiC heating element seem to be an ideal tool to probe the
existence of specific and nonthermal microwave effects. The
results of our investigations involving a representative selection
of six synthetic organic transformations are presented herein.
3
activation energy.
Both specific and nonthermal microwave effects will be
directly influenced by the electromagnetic (microwave) field
strength. The stronger the microwave field, the more pronounced
the observed microwave effect. These considerations have led
to the notion that simultaneous external cooling of the reaction
mixture (or maintaining subambient reaction temperatures) while
heating by microwave irradiation can lead, in some cases, to
6
an enhancement of the overall process. Under the “heating-
while-cooling” conditions, the reaction vessel is cooled from
the outside by compressed air or with the aid of a cooling fluid
while being irradiated by microwaves. This allows a higher level
of microwave power to be directly administered to the reaction
mixture, thereby potentially enhancing any specific or nonther-
mal microwave effects that are dependent on the electric field
Results and Discussion
Experimental/Technical Considerations. In our previous
work, we have employed sintered SiC cylinders as passive
heating elements for microwave-assisted reactions involving
6
12
strength. By monitoring internal reaction temperatures using
low-absorbing solvents. The cylindrical shape allows the use
fiber-optic probes and carefully adjusting the temperature and/
or flow of the external cooling gas or liquid, experiments can
be performed where at constant bulk reaction temperature
of these inserts in both single-mode and multimode microwave
reactors utilizing the appropriate dedicated microwave vials.
For the present study, we have reevaluated the heating efficiency
of various SiC materials using not only cylinders of two different
sizes (10 × 18 mm, 4.35 g; 10 × 8 mm, 1.94 g) but also SiC
powder and granules. Our comparison studies involved heating
12–15
7
–11
distinctly different microwave power levels can be applied.
In some cases, significant improvements from applying an
increased microwave power level have been reported using this
7
,8
16
technology, while for other chemistry examples there was
low microwave-absorbing toluene (tan δ 0.040) together with
varying amounts of SiC material in a standard 10 mL sealed
Pyrex microwave vial at constant microwave magnetron output
power using a single-mode microwave reactor. Based on
(
4) For some recent examples, see: (a) Dressen, M. H. C. L.; van de Kruijs,
B. H. P.; Meuldijk, J.; Vekemans, J. A. J. M.; Hulshof, L. A. Org. Process Res.
DeV. 2007, 11, 865. (b) Holtze, C.; Tauer, K. Macromol. Rapid Commun. 2007,
2
(
8, 428. (c) Young, D. D.; Deiters, A. Angew. Chem., Int. Ed. 2007, 46, 5187.
d) Conner, W. C.; Tompsett, G. A. J. Phys. Chem. B 2008, 112, 2110.
(9) (a) Leadbeater, N. E.; Pillsbury, S. J.; Shanahan, E.; Williams, V. A.
Tetrahedron 2005, 61, 3565. (b) Leadbeater, N. E.; Stencel, L. M.; Wood, E. C.
Org. Biomol. Chem. 2007, 1052. (c) Massi, A.; Nuzzi, A.; Dondoni, A. J. Org.
Chem. 2007, 72, 10279.
(
5) For a more detailed definition and examples for thermal, specific, and
nonthermal microwave effects, see: (a) Kappe, C. O.; Stadler, A. MicrowaVes
in Organic and Medicinal Chemistry; Wiley-VCH: Weinheim, Germany, 2005;
Chapter 2, pp 9-28. See also refs 2a and 3.
(10) Hosseini, M.; Stiasni, N.; Barbieri, V.; Kappe, C. O. J. Org. Chem.
2007, 72, 1417.
(
6) (a) Hayes, B. L.; Collins, M. J., Jr. World Patent 2004, WO 04002617.
(
b) Hayes, B. L. Aldrichim. Acta 2004, 37, 66.
(11) Herrero, M. A.; Kremsner, J. M.; Kappe, C. O. J. Org. Chem. 2008,
73, 36.
(
7) (a) Singh, B. K.; Appukkuttan, P.; Claerhout, S.; Parmar, V. S.; Van der
Eycken, E. Org. Lett. 2006, 8, 1863. (b) Appukkuttan, P.; Husain, M.; Gupta,
R. K.; Parmar, V. S.; Van der Eycken, E. Synlett 2006, 1491. (c) Singh, B.;
Mehta, V. P.; Parmar, V. S.; Van der Eycken, E. Org. Biomol. Chem. 2007, 5,
(12) Kremsner, J. M.; Kappe, C. O. J. Org. Chem. 2006, 71, 4651.
(13) Kremsner, J. M.; Stadler, A.; Kappe, C. O. J. Comb. Chem. 2007, 9,
285.
2
962.
(14) Geuens, J.; Kremsner, J. M.; Nebel, B. A.; Schober, S.; Dommisse, R. A.;
Mittelbach, M.; Tavernier, S.; Kappe, C. O.; Maes, B. U. W. Energy Fuels 2008,
22, 643.
(
8) (a) Vanier, G. S. Synlett 2007, 131. (b) Baxendale, I. R.; Griffiths-Jones,
C. M.; Ley, S. V.; Tranmer, G. T. Chem. Eur. J. 2006, 12, 4407. (c) Arvela,
R. K.; Leadbeater, N. E. Org. Lett. 2005, 7, 2101.
(15) Razzaq, T.; Kappe, C. O. ChemSusChem 2008, 1, 123.
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