Variable temperature 1 H NMR of 116

The iohexol protected products 116 (TsOH) and 116 (CuSO4) were difficult to analyse by 1H NMR as in addition to a number of diastereomers the spectra were complicated by the slow rotation around the amide bonds and the potential atropoisomeric nature of the C-N bond in the anilide (due to the large iodine substituents). Variable temperature NMR is often used for similar compounds, by changing the temperature peaks can shift, sharpen, broaden and coalesce.190 Variable temperature is often used to calculate the barrier to rotation about a bond. High barriers to rotation can lead to the existence of two separate rotamers, particularly at low temperatures. This can then lead to two sets of peaks in the 1H NMR. As you heat up the sample you observe coalescence of these peaks to give one set of signals at high temperature. Using line shape analysis the rate of coalescence of the two sets of peaks can be measured allowing the barrier to rotation to be calculated. However

76 if the barrier is too high it can’t be calculated by this method because coalescence is

not observed at temperatures accessible for 1H NMR experiments.

The two acetonide products 116 (CuSO4) and 116 (TsOH) from routes A and B were analysed using variable temperature 1H NMR in d6-DMSO in an attempt to simplify the spectra and aid interpretation. Heating up the samples from r.t. to 100 oC caused the amide NH peaks to broaden and coalesce, and the peaks at 3.25-3.50 ppm to broaden, but no simplification of the spectra was observed. Both products behaved in the same way (only the spectra for 116 (CuSO4) are shown Figure 2.27). Attempts to cool down the samples dissolved in CDCl3 (r.t. to -59 oC) also led to unhelpful broadening for both samples (Figure 2.28). No useful information on barriers to rotation or significant simplifying of the spectra was observed.

Figure 2.27: VT 1H NMR of 116 (CuSO4) from route A, 298 K – 373 K. The spectra for 116 (TsOH) were identical

298 K 333 K 373 K

77

Figure 2.28: VT 1H NMR of 116 (CuSO4) from route A, 214 K – 298 K. The spectra for 116 (TsOH) were identical

A recent study into the rotational barriers of iodixanol 58 calculates the rotation around the phenyl-N bond to be between 28-33 kcal mol-1.191 This is sufficiently high to give rise to non-interconvertible rotamers at r.t., as iodixanol 58 is the dimer form of iohexol 55, the same barrier to rotation would be expected, which is why VT 1

H NMR didn’t yield any useful information.

2.2.2.7 X-ray diffraction of 116 (CuSO4) and 116 (TsOH)

The two products 116 (CuSO4) and 116 (TSOH), while appearing identical (1H, 13C NMR, IR, M.S, m.p., elemental analysis, TGA and DSC) were made via different methods (A and B) and exhibit differential solubility in ethyl cyanoacrylate 37. The different preparation methods (including work-up), may lead to differences in the crystallisation patterns of the two solids. This could explain why two seemly

214 K 256 K 298 K

78 identical products have different solubility characteristics in the cyanoacrylate adhesive. 116 is a powder and therefore it was not possible to get a crystal structure of the two different products, for this reason both solids were analysed by X-ray diffraction (XRD). XRD is a powerful technique for studying powdered materials, as different materials produce distinctive diffraction patterns. By producing a diffraction pattern for each solid we the two samples could be compared, any significant differences could explain why they behave differently in the adhesive.

XRD works by the detector sitting at a certain position and counting the X-rays scattered from the sample in that position, before moving on to the next position and so on and so on. However no Bragg peaks were observed for either sample indicating that they both were amorphous solids. A Bragg peak is a pronounced peak on the Bragg curve which plots the energy loss of the X-ray as it travels through matter. This means that the sample is amorphous (effectively structureless or disordered) so all the molecules are packed together randomly, rather than in an ordered way. The lack of Bragg peaks lead to powder diffraction patterns for both samples which were mainly background noise (Figure 2.29 and 2.30. The only sharp peaks observed were at 39o and 45o, these are from the aluminium sample holder. Significant amounts of sample were analysed and so the low signal must be due to amorphous behaviour of both solids.

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Figure 2.29: Powder diffraction pattern from 116 (CuSO4) via Route A

Figure 2.30: Powder diffraction pattern from 116 (TsOH) via Route B

It is still entirely possible that the differences in the properties of the two materials are due to a difference in the way the molecules are packed, but unfortunately because they are not crystalline this can’t be observed by XRD. In summary after extensive analysis (1H, 13C NMR, mass spectroscopy, m.p., ICP-MS, IR, TGA, DSC,

Al holder Al holder

80 XRD) both samples 116 (CuSO4) and 116 (TsOH) appear identical apart from their solubility in ethyl cyanoacrylate 37. Despite inconclusive results from XRD it seems likely that the difference in solubility is due to different morphologies, although it wasn’t possible to prove this.