My PhD dissertation - Institut Fresnel

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To check the reproducibility of the isomerisation process and be sure that
decomposition of the dilithiated substrate occured, several other spectra were recorded while
decreasing back the temperature until –50 °C (Figure 8b)
T = 35 °C
T = 24 °C
T = 20 °C
T = 10 °C
T = 0 °C
T = - 10 °C
.
O
HA Li
HB Li
HB HA 21
O
T = 10 °C
T = 0 °C
T = - 10 °C
T = - 30 °C T = - 25 °C
T = - 50 °C T = - 50 °C
4.16 4.14 4.12 4.10 4.08 4.06 4.04 4.02
4.16 4.14 4.12 4.10 4.08 4.06 4.04 4.02
Figure 8. Temperature dependence of the 1 H-NMR (400 MHz) methylene
signals of 21 in perdeuterated tetrahydrofuran: (a) increase in temperature from
- 50 °C to 35 °C (left) and (b) reversible decrease to – 50 °C (right).
For the same purpose as previously reported, the concentration and the
stoichiometry of the lithiating reagent were varied and further 1 H-NMR spectra were
recorded. The coalescence temperature observed in all cases was once again unsensitive to
such modifications, suggesting a monomolecular process. These experimental
observations, as proposed in chapter 4.4.2, could possibly be explained by steric bulk and
charge delocalization arguments. Following the same trend as dilithiobiphenyl 47,
compound 49 showed, this time, an even larger activation barrier than dilithiobenzoxepin