Institute of Chemistry, Organic Synthesis Group, Karl-Franzens-University of Graz Heinrichstrasse 28, A-8010 Graz (Austria) Received: 23 July 2001 / Uploaded: 6 August 2001 |
|
Fifth International Electronic Conference on Synthetic Organic Chemistry (ECSOC-5),
http://www.mdpi.net/ecsoc-5, September 1-30, 2001
[A0003] |
|
|
|
Institute of Chemistry, Organic Synthesis Group, Karl-Franzens-University of Graz Heinrichstrasse 28, A-8010 Graz (Austria) Received: 23 July 2001 / Uploaded: 6 August 2001 |
|
Abstract:
The reaction temperatures obtained from DSC measurements did in several
cases not agree with the experimentally obtained results in synthetic reactions.
In most cases it was found that lower temperatures were necessary. So we
studied the influence of solvents on the reaction or decomposition temperatures
of a number of azides (1-7) in order to obtain data which are comparable with
the real experimental results of decomposition reactions under synthetic
conditions.
A
further point of interest was to obtain information not only about different
solvents but also about catalysts. So we performed some thermal reactions
in DSC crucibles using different acid catalysts to study the effects on
the thermal behavior of the samples.
The
results obtained in this study show that solvent effects on the thermolysis
of azidohetarenes without reactive ortho substituents in diphenylether
have mainly the effect to give a uniform decomposition temperature of 140-160
oC. With acid catalysts, the reaction temperatures were up to
90 oC lower than without catalysts, which must be considered
in reactions with such catalysts. Azidohetarenes with reactive ortho substituents
such as 8 show in solution a reaction temperature which is about
60 oC lower than in the solid state. This result is important
in planning thermal strain of azides.
|
| Contents: |
1. General Aspects
|
|
Since several years we have studied the thermal properties of azido-hetarenes
by differential scanning calorimetry (DSC) because of our interest in this
class of compounds for cyclization reactions [1]. The reaction temperatures
obtained from DSC measurements did in several cases not agree with the
experimentally obtained results in synthetic reactions. In most cases it
was found that lower temperatures were necessary. So we studied the influence
of solvents on the reaction or decomposition temperatures of a number of
azides in order to obtain data which are comparable with the real experimental
results of decomposition reactions under synthetic conditions.
A further point of interest was to obtain information not only about different solvents but also about catalysts. So we performed some thermal reactions in DSC crucibles using different acid catalysts to study the effects on the thermal behavior of the samples. |
|
2. Influence of solvents on the thermal decomposition of azido-hetarenes
|
The choice of the solvent was important because it had to meet several reqirements:
Decomposition of azidohetarenes without reactive ortho-groupsThe first series of azido hetarenes which were studied in this work included azido compounds which had no reactive ortho group, because we intended only to study the solvent influence which should not be superimposed by neighbour group effects. Azides 1-4 showed as solids reaction temperatures between 150-170 oC (blue triangles in diagram 1), with melting points far from decomposition temperatures (red squares in diagram 1).In diphenylether solution, all reaction temperatures of azides 1-4 were found in a small temperature range between 140-156 oC (green squares in diagram 1). These findings are in agreement with the results from experimental data, which show that decomposition of azido groups occurs in boiling dimethylformamide (bp 156 oC); the second result is that there is no significant difference between data obtained from solids and from solvents.
2-Azidoquinolines 5-6 with a ring-N in ortho position are known to exist mainly in the tautomeric tetrazolo form. This is also shown by the rather high
decomposition temperatures (230-250 oC as solids, blue
triangles in diagram 1). In solution, the
high decomposition temperatures were lowered again to 150 oC
(green squares
in diagram 1).
Azidoquinolines such as 7 with the azidogroup at a sp3 carbon atom, show
a result similar to the azidohetarenes 1-4: As solid, the
reaction temperature was observed at 180 oC (blue
triangle in diagram 1); dissolved in diphenylether,
the decomposition took place at 140 oC (green
square in diagram 1).
The interesting result of this series of DSC measurements shows, that in solution
all reaction and decomposition temperatures of the azides 1-7
range within a small temperature area of 140-156 oC (green
squares in diagram 1), independent from their
structures and decomposition temperatures as solids.
Decomposition of 4-azido-3-nitroquinoline4-Azido-3-nitroquinoline 8 was studied as an example of an azidohetarene with a highly reactive ortho-substituent. The thermal decomposition gives in good yields as cyclization product the furoxane 9.
The DSC diagram of solid 8 (upper part in diagram 2) shows that immediately after the melting point of 153 oC a reaction starts with a calculated onset of 140 oC, which gives the furoxane 9 (red line in diagram 2). In diphenylether solution, the calculated onset is lowered to 106 oC (green line), but the reaction start is already observed at about 80 oC (blue line in diagram 2). These findings mean that the difference between the reaction start of 8 as solid and in solution is about 60 oC because of solvent effects; the practical important result is that in such a case the decomposition begins at a significant lower temperature, which is not only important for reactions, but also for stablilty and purification processes. Diagram2
|
|
3. Influence of catalysts on the thermal decomposition of azido-hetarenes
|
| Besides
solvent effects it was of interest to obtain evidence of the effects of
catalysts on the thermal decomposition of azidohetarenes. In the literature
there are some decomposition reactions described using strong acids such
as sulfuric acid or polyphosphoric acid, mainly explained in the formation
of nitrenium ions. For DSC measurements, only less volatile acids could
be used.
As example material for the decompositions, 4-azidocoumarin (1) was used. 4-azidocoumarin (1) gives as solid at 160 oC a melting point, followed immediately by a decomposition with an onset temperature of 155 oC (column 1 of diagram 3). Column 2 of diagram 3 shows the decomposition of 4-azidocoumarin (1) dissolved in diphenylether without catalyst. Column 3 of diagram 3 shows the decomposition of 4-azidocoumarin (1) dissolved in 1,2-dichlorobenzene without catalyst. Column 4 of diagram 3 shows the decomposition of 4-azidocoumarin (1) dissolved in diphenylether with 4-toluenesulfonic acid as catalyst. Column
5 of diagram 3 shows the decomposition of 4-azidocoumarin (1) in
methanesulfonic acid as suspension.
Column
6 shows the decomposition of 4-azidocoumarin (1) dissolved in methanesulfonic
acid.
The diagram reveals that 4-azidocoumarin (1) in the solid state (column 1) and in diphenylether (column 2) as the solvent have only a small difference both in onset and in maximum reaction temperature, as already shown in diagram 1. In 1,2-dichlorobenzene (column 3), the onset temperature is lowered to 130 oC, the maximum could not be determined because of superimposed effects of the boiling point of the solvent. This effect of 1,2-dichlorobenzene could be explained by impurities of e.g. hydrogen chloride and reveals, that solvent effects cannot be generalized. Toluenesulfonic acid as catalyst (column 4 of diagram 3) did not show a great influence on the decomposition temperature, which is explainable by solution difficulties. Experiments in methanesulfonic acid (columns 5 and 6 of diagram 3) showed large effects: the onset temperatures were lowered to 91 oC in suspension (column 5) and 65 oC in methanesulfonic acid solution (column 6), which means a temperature difference of up to 90 oC. This results shows again that reaction and stability questions must be checked for each system. |
|
4. Conclusion
|
| The results obtained in this study show that solvent effects on the thermolysis of azidohetarenes without reactive ortho substituents in diphenylether have mainly the effect to give a uniform decomposition temperature of 140-160 oC. With acid catalysts, the reaction temperatures were up to 90 oC lower than without catalysts, which must be considered in reactions with such catalysts. Azidohetarenes with reactive ortho substituents such as 8 show in solution a reaction temperature which is about 60 oC lower than in the solid state. This result is important in planning thermal strain of azides. |
|
5. Experimental
|
| General
preparation of the azides used in this work:
The preparations of the azidohetarenes are described in detail in ref. [1, 2]. A standard procedure starts from either the corresponding chloro- or tosyloxy compounds, which were reacted with sodium azide in dimethylformamide or N-methylpyrrolidone at temperatures between 0 - 80 oC. After the reaction, the reaction mixture was poured onto ice, and the solid which precipitated was collected by filtration. Measurements:
|
|
6. Acknowledgement
|
| This work was supported by the FWF (Österreichischer Fonds zur Förderung der wissenschaftlichen Forschung) project No. P 10785-CHE . |
|
7. References
|
