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M. Bucos et al. / Tetrahedron 66 (2010) 2061–2065
of some type of intermediary such as microwave induced phonons,
which have been invoked by others to explain certain physico-
chemical phenomena.14,15
4.2. General classical heating experimental procedure
In a 25 mL round bottom flask equipped with a water-cooled
condenser and covered by aluminium foil for protection from light,
were added 0.39 g (3.0 mmol) of NCS and 3.0 mmol of salt. The
mixture was pre-heated to 102–104 ꢀC for 10 min with stirring
using an oil-bath on a stirrer hot-plate equipped with an electronic
temperature feedback system. Subsequently, 0.32 g (3.0 mmol) of
xylene was added and stirring was continued for 60 min. The
mixture was then rapidly cooled to room temperature and the same
work-up procedure followed as described above for the microwave
assisted experiments.
3. Conclusions
The microwave promoted chlorination reaction of simple
alkylaromatics with NCS, in the presence of an ionic species (salt,
metal oxide, etc.) and in the absence of water produced in-
teresting results. Total yields were increased up to ninefold when
compared with the reactions in the absence of these compounds.
In addition, chemoselectivities varied according to the additive
used and in some cases it was possible to obtain exclusively either
the ring or the side-chain chlorinated isomer. These additives play
little or no chemical role in these reactions but, as the main in-
stantaneous absorbers of microwave energy, they consequently
transfer part of this energy to the active reagents, thus increasing
their reactivity via a complex mechanism. The heating aid be-
haviour of these ionic species in our microwave assisted reactions
is similar to that observed with the use of ionic liquids. The solid
additives used in this study have the advantage that they are very
cheap and, since they are easily separated from the reaction
products, can also be used repeatedly. We therefore believe that
our microwave assisted method is of synthetic interest, since it
dramatically increases yields (up to 77%, nine times) and by
choosing the appropriate salt can afford almost exclusively the
desired isomer.
Acknowledgements
This investigation was funded by the General Secretariat of Re-
search and Technology of Greece and by the EU Marie Curie Early
Stage Research Training (EST) project ‘EURODESY: A European Re-
search Training Site for the Design and Synthesis of Novel Neuro-
protective and Hypoglycaemic Agents through a Multi-Disciplinary
Approach’. Contract No. MEST-CT-020575 (2006–2010).
Supplementary data
Supplementary data associated with this article can be found in
References and notes
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