First International Electronic Conference on
Synthetic Organic Chemistry (ECSOC-1), www.mdpi.org/ecsoc/, September 1-30, 1997
[A0020]
DSC Assisted Organic Thermolysis Reactions
Wolfgang Stadlbauer, Susanne Prattes and A. Elisabeth Täubl
Institute of Organic Chemistry, Karl-Franzens University Graz, Austria
Received: 20 July 1997 / Uploaded: 5 August 1997
General Aspects
Differential Scanning Calorimetry (DSC) has been shown to provide useful
information in organic synthesis in the stage of planning thermolytical
reactions [1]. This information can be obtained before the reaction itself is
performed and include the temperature range of the planned reaction, hints on
subsequent rearrangement and decomposition reactions, choice of suitable
solvents and safety precautions in exothermic processes.
Another point of interest is that also reactions with two reaction partners and
the influence of solvents at the reaction temperature can be studied.
In this contribution three applications of DSC [2] in organic synthesis are presented
Aromatic azides are known to give in some cases strong exothermic reactions. To overcome
these safety aspects we compared the reactions heat values of several azides obtained from
the DSC diagrams in order to get safety informations for the synthetic reactions.
We found that most of the ortho substituted azidohetarenes of our research field
showed values ranging between 200-300 mcal/mg; only in some cases (depending rather
on the structure of the basic hetarene than on the orthosubstituent) values up to
500-600 mcal/mg could be observed [3], which promped us to be cautious in these reactions
in order to avoid explosions.
We found many heterocyclic azides with ortho-ester groups to give on
thermolysis two exothermic reaction steps, which often could not be
separated without DSC hints. Experimentally it could be proved
that the first thermolytic reaction step gave, as a ring closure
product, the isoxazols with the alkoxy group in the isoxazole ring,
whereas at slightly higher
temperatures a rearrangement to isoxazolones took place.
As an example, ethyl 4-azido-coumarin-3-carboxylate (1) gave a DSC plot with well
separated signals:
- The reaction peak of the ring closure of the azide with the
ester group to the isoxazole 2 at 146oC,
- followed by the next reaction step, the migration of the ethoxy group from the
isoxazole ring to the pyrane ring with a reaction peak at 174oC
to give the isoxazolone 3.
- A decomposition area started at about 210oC
The synthetic experiments were performed by thermolysis of 1 in chlorobenzene (b.p. 132 oC)
in order to obtain 2 and in 1,2-dichlorobenzene.
(b.p. 180 oC)
to obtain 3, both in good yields. Moreover, the isoxazole 2 could
be rearranged to the isoxazolone 3 by thermolysis in 1,2-dichlorobenzene.
The methylester of the azidocoumarin did not show the
two reaction signals well separated, and also the synthetic findings
revealed that in this case the preparation of both isomers was much more
difficult, however, the DSC data gave valuable information to perform
the reaction in order to obtain both isomers in moderate yields.
Thermolysis of ortho-Nitroazides in the Solid State and in Solution
The influence of the solvent during a reaction caused by solvatation or
catalytic effects is an important factor which must not be neglected.
Therefore DSC measurements offer an additional help for planning and
performing of synthesis reaction.
For comparison we studied many thermolysis reactions in the solid state and
in different solvents in order to obtain hints on effects of the solvents which could
lower the reaction temperature to obtain milder conditions.
- As an example 4-azido-2-chloro-3-nitroquinoline 4 was thermolyzed in
the solid state and in diphenyl ether solution to study the reaction to the
furoxane 5.
- The thermolysis in the solid state showed an
onset temperature of about 140oC which contains the cyclization
to the furoxane; beginning from 250oC
a slow decomposition could be detected.
- In the second diagram the influence of the solvent is visible:
in solution the onset point for the degradation of the azide was lowered to
106oC (nearly 40oC) and the rearrangement or cyclization
area which followed the cyclization can be observed much better in solution
than in the solid sample; the decomposition peak was moved to 270 oC.
In synthetic experiments the reaction was performed in refluxing
toluene to obtain
4-chloro-1,2,5-oxadiazolo[3,4-cquinolin-3-oxide 5 in good yields.
It is obvious that
immediately and without long lasting and material intensive experiments it
is possible to determine the ideal solvent for the particular reaction.
Reaction of Furoxanes with Triphenylphosphane
Furoxanes (oxadiazolo-N-oxides) are reported to react with phosphanes and
phosphites to oxadiazoles by deoxygenation. Since the reported reaction
temperatures are varying within a broad temperature range, we wanted to
determine the suitable reaction temperature by DSC.
This query involves - in opposite to the intramolecular reactions of
ortho-substituted azides as shown above - the observation of the intermolecular
reaction of two reaction partners by DSC. We found that DSC can also give answers in these
intermolecular reactions with more reaction partners - an application which
we have not found till now in DSC experiments described in the literature.
A 1:1 mixture of 4-oxo-oxadiazolo[3,4-c]quinoline-3-oxide 6 and triphenylphosphane
showed in the DSC-diagram, that after the melting point of triphenylphosphane
(at 82 oC) a strong exothermic reaction started at about 120oC.
This reaction could be carried out in good yields in a preparative scale in chlorobenzene
as solvent to obtain 4-oxo-oxadiazolo[3,4-c]quinoline 7. In some other furoxanes
the temperature must be raised up to 200oC,
which can easily be determined by DSC.
Acknowledgement:
This work was supported by the
FWF (Österreichischer Fonds zur
Förderung der wissenschaftlichen Forschung) project No. P 10785-CHE .
[1] Th. Kappe, W. Stadlbauer, Molecules 1 (1996) 255-263
[2] DSC data were obtained on a Rheometric Scientific DSC-Plus instrument with the
DSC software V 5.42. The DSC plots were recorded between 25-500oC, the
heating rate was 2-10oC/min, using 1.5-3 mg of substance in sealed
aluminium crucibles.
[3] Dang V. T., W. Stadlbauer, Molecules 1 (1996), 201-206.
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