CLASIFICACIÓN ÁREAS CONSOLIDADAS

Upon trimerisation with dicobalt octacarbonyl in refluxing dioxane, both 14 and 15 gave only the respective antisymmetric isomers 16 and 17 upon recrystallisation from methanol (Figure 2-21). 2 1 6 5 4 3 2 1 6 5 4 3 S S S S S S 16 17

Figure 2-21: The labelling of the central phenyl core for the asymmetric products 16 and 17. For 16 the 1H NMR spectrum shows three sets of 3-thiophene signals which have been fully assigned by long range NMR. The most deshielded thiophene is observed at δ 7.55,

7.47 and 7.45 and through a HMBC experiment has been shown to lie at a position on the central ring between two hydrogens, as such this thiophene must be at position 4 (Figure 2-22). Similar HMBC analysis shows that the thiophene with protons at δ 7.24, 7.12 and

6.84 ppm lies at phenyl position 1 and the thiophene at δ 7.25, 7.17 and 6.88 ppm lies at

position 2. The three phenyl protons are seen at δ 7.72 (position 3), 7.61 (position 5) and

3 4 5 6 1 2 b4 b₅ S _b 2 d₄ d5 S d2 a4 a5 S a2 3 5 6 d₅ d4 d_{2 b} 5 a5 b₂ a₂ b4 a4 CHCl3 3 4 5 6 1 2 b4 b₅ S _b 2 d₄ d5 S d2 a4 a5 S a2 3 5 6 6 d₅ d₅ d4 d4 d₂ d_{2 b} 5 b₅ a₅ b₂ a₂ bb44 a4 CHCl3

Figure 2-22: The 1_{H NMR spectrum of 16 (600 MHz, CDCl}

3, 25°C).

Of the three thiophenes seen in the 1H NMR spectrum of 16 the most deshielded has been assigned to the thiophene on the 4-position of the central ring with hydrogen substituents either side of it. This is because of the shielding effect of the ring current on the other thiophene rings which are ortho to it. (Figure 2-23). This shielding effect on thiophenes ortho to other aromatic substituents is well documented.15,20

S

S H

Figure 2-23: One conformation of two ortho thiophene rings to each other showing the hydrogen of one ring buried within the shielding region of the other thiophene ring.

For 17 again three sets of thiophene signals are observed in the 1_{H NMR spectrum, these}

have the expected splitting patterns for 2-substituted thiophenes. The furthest deshielded set of thiophene signals has been assigned via a HMBC experiment to be the thiophene ortho to two hydrogen atoms on the central phenyl ring, this is consistent with the assignment of 16. This is the thiophene at position 4 on the central ring and is observed at

7.30, 6.99 and 6.93 ppm, while the thiophene at the 2 position is seen at δ 7.33, 7.03 and

6.99 ppm. The three phenyl protons resonate at δ 7.77 (position 3), 7.63 (position 5) and

7.56 ppm (position 6). 3 4 5 6 1 2 b2 d2 b3 b4 b5 S d3 d4 d5 S a2 a3 a4 a5 S 3 5 6 d3 d₅+ b₅ a₅ CHCl3 d₄ b4 a₄+ b₃ a₃ 3 4 5 6 1 2 b2 d2 b3 b4 b5 S d3 d4 d5 S a2 a3 a4 a5 S 3 5 6 d3 d₅+ b₅ a₅ CHCl3 d₄ b4 a₄+ b₃ a₃

Figure 2-24: The 1_{H NMR spectrum of 17 (400 MHz, CDCl}

3, 25°C).

Single crystals of 16 and 17 suitable for X-ray crystallographic analysis were obtained from the slow evaporation of a saturated solution of the sample from a 1:1 mixture of methanol: dichloromethane. Both 16 and 17 crystallise in the same monoclinic space group P2(1)/c; however the packing of the two structures are different. In 16 there are four molecules in the unit cell.

Figure 2-25: Perspective view of the molecular structure of 16, with selected atomic labelling shown. Selected bond lengths (Å) and angles (°): C3-C5 1.464(4), C5-C6 1.399(4), C6-C7 1.377(4), C6-C16 1.470(4), C7-C8 1.395(4), C8-C9 1.382(4), C8-C12 1.452(4), C9-C10 1.353(5), C5-C10 1.400(4), C1-S1-

a) b)

Figure 2-26: Crystal packing along the a) a axis and b) c axis of 16.

For 17 the increased interaction of the sulfur atoms with the core of the molecule results in the structure packing in the solid state in such a way that a pair of molecules differing from each other by an 180° rotation of one of the three thiophenes is observed (S3 and S6, Figure 2-27).

Figure 2-27: Perspective view of two molecules of the molecular structure of 17, with atomic labelling shown. Selected bond lengths (Å) and angles (°): C21-C22 1.377(7), C22-C23 1.391(6), C23-C24 1.398(6),

C24-C25 1.396(6), C25-C26 1.377(7), C21-C26 1.394(7), C11-C21 1.468(7), C23-C41 1.497(6), C24-C31 1.478(6), C11-S1-C14 93.1(3), C31-S2-C33 95.9(3), C41-S3-C44 97.3(4).

As a result of this there are a total of eight molecules in the unit cell. Between adjacent molecules along the b-axis of the cell two of the three thiophenes remain at the same orientation while the third rotates by 180° (Figure 2-28).

Figure 2-28: The asymmetric unit of compound 17 viewed along the b axis of the cell.

Each of the six sulfur atoms in 17 show disorder, with each thiophene having two possible orientations. This is due to a possible 180°rotation of each thiophene ring which results in an occupancy for each 2-position sulfur at the carbon 5-position on the same thiophene ring (values are between 22 and 45%). As such the goodness of fit on F2 value for 17 is higher than normal (1.254).

Looking at the trimerisation yields and melting points of 16 and 17 some interesting observations can be made. The 3-thienyl 16 was obtained in 63% yield via trimerisation, whereas 2-thienyl 17 was only obtained in 23% under the same conditions of temperature and reaction time.

2.4 Conclusion

This chapter has shown that the cyclotrimerisations of thienyl containing acetylenes and ethylenes has been achieved. Six thiophene-phenylene acetylenes (1-6) were successfully synthesised using Sonagashira coupling and fully characterised by NMR, IR spectroscopy, Mass spectrometry and X-Ray crystallography.

These acetylenes were then trimerised using dicobalt octacarbonyl as catalyst to give six isomeric mixtures of phenylene systems (8-13). A series of complex NMR investigations

were carried out on the compounds obtained. Changing the substituents affects both the mechanism of the trimerisation reaction and the ratio of the symmetric and antisymmetric isomers formed. In the case of systems involving a 3-thienyl substituent the isomeric ratio of 1:3 expected from the norbornadiene like reaction intermediate is seen, while in the case of the 2-thienyl systems the effect of the greater sulfur interaction with the molecular core appears to drive the trimerisation to go via the metallacycloheptatriene intermediate resulting in an increase in the relative concentration of the symmetric isomer.

Two ethylenes 14 and 15 were trimerised to give 16 and 17 which were fully characterised by both NMR spectroscopy and X-Ray crystallography. In these cases as one of the acetylene substituents is a sterically undemanding hydrogen, trimerisation via the norbornadiene-like intermediate results in the exclusive formation of the asymmetric isomer.

2.5 References

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Part 3: Synthesis and oxidation of di-thienyl