Ring opening reactions of tetrathiacyclopropenonophanes and the subsequent formation of a molecular box stabilised by hydrogen bonds

Sulfur atoms next to a cyclopropenone moiety change their reactivity completely. Tetrathiacyclopropenonophanes 6 and 10 were treated with protic solvents such as methanol and water yielding ring opened diesters 9a and 9b , and the diacid 11 , respectively. This observation is in contrast to alkyl and aryl substituted cyclopropenones which are stable in the presence of protic solvents. The molecular structures of the diester and the diacid could be elucidated by means of X-ray diffraction analyses. In the latter case a molecular box was formed in such a way that the two acid moieties dimerise via hydrogen bonding. The resulting hollow core is able to host one molecule of chloroform.


Results and Discussion
The reaction of tetrathiacyclopropenonophane 6 in a solution of dichloromethane with excess of methanol at room temperature yielded almost quantitatively a mixture of 9a and 9b in a 3:1 ratio (Scheme 1).The separation of the two regioisomers by column chromatography proved to be impossible.
The reaction itself is quite slow and was monitored in a separate experiment by NMR spectroscopy over a period of three days.Figure 1 shows the corresponding 13 C NMR spectra in the range of δ = 36-26 for different reaction times.This is the region where all carbon resonance signals of the CH 2 tethers appear.Before staring the reaction, the spectrum shows three signals being in line with the D 2h symmetry of the starting material 6.After t = 1.0 h twelve further signals appeared.These signals correspond to a compound where all carbon atoms in the tethers are different giving rise to a C s symmetric molecule.We ascribe this set of signals to compound 8 in which one cyclopropenone moiety has opened to from an ester unit.A potential acetal structure 7 with a mirror plane orthogonal to the plane of the large cycle would show only a set of six signals in this region.One can conclude that such a molecule that is presumably an intermediate before 8 is formed, is either too short-living compared to the NMR timescale or the concentration is too low.Thus, it cannot be observed by this method.
After a 12.0 h reaction period at least 28 signals can be discerned: still three signals of the starting material 6 are present, twelve of the monoester 8 and several other (new) signals giving rise to the final products 9a and 9b.The last measurement after t = 79.0 h shows two sets of six signals in the final ratio of 3:1.The reaction was also followed by means of 1 H NMR spectroscopy.Here, the most characteristic signals are the alkene protons of the three different alkenes 8, 9a and 9b in the range between 8.0 and 8.1 ppm.The reason why one of the two regioisomers is dramatically favored must be due to conformational preference and spatial requirements in the ring opening of the second hemiacetal formed from 8.
A similar reaction with water instead of methanol was carried out with the smaller homologue 10.In contrast to the former results, a ring opening did not take place, even experiments where the mixture was heated or the nucleophilicity of water was increased by addition of NaOH were in vain.Nevertheless, by adding a catalytic amount (three drops) of hydrochloric acid the ring opening yielding 11 succeeded (Scheme 2).However, many byproducts were formed as mass spectrometric investigations revealed.A chromatographic separation of the compounds proved to be unsuccessful, therefore a separation by means of crystallisation was performed.However, the amount of material obtained by this method was not sufficient for any NMR studies.Single crystals of 9a and 11 were obtained and investigated by X-ray diffraction studies.In Figure 2 we display the molecular structure of 9a.The unit cell shows a center of symmetry as the molecule also does.The alkyl tethers obtain a strain-free zig-zag conformation.The unit cell of the diacid 11 reveals two independent molecules.Each of them dimerises with a corresponding symmetry-equivalent molecule which is generated by rotation around a C 2 axis.In such a way a hollow core is formed that is able to include one molecule of chloroform.These guest molecules are located on the C 2 axis and are disordered in a 1:1 ratio.Such a molecular aggregate of 11 with included chloroform is shown in Figure 3.
The special feature of this hollow core is strongly determined by the push-pull-substituted C=C bond.As anticipated by simple resonance structures the SCH 2 moiety which is located trans to the ester moiety is in plane with the double bond, whereas the other SCH 2 unit which is situated geminal to the ester moiety adopts a perpendicular position.Thus, one side of the cavity shows a wider opening than the other.