My PhD dissertation - Institut Fresnel

57
This finally allows one to use the Eyring equation to calculate the energy barrier for
free rotation at the coalescence temperature. Making the appropriate substitution in the Eyring
equation, one obtains:
∆G
≠
coal
=
4.
57⋅10
−3
⎡ ⎛
⎢ ⎜ Tc
⋅Tc
9.
972 + log
⎢ ⎜ 2
⎣ ⎝ ∆ν
+ 6 J
2
AB
⎞⎤
⎟⎥
⎟
⎠
⎥
⎦
(kcal·mol -1 )
4.4.2 Coalescence Study of 2,2’-Dilithio-6,6’-bis(methoxymethoxy)
biphenyl
The variable-temperature 1 H-NMR method was first employed to evaluate the
rotational barrier of the 6,6'-dilithiobiphenyl-2,2'-diol motif. Since the presence of a
suitable diastereotopic probe is required for such thermodynamical investigation, the two
hydroxyl functions in 31 were protected by reaction with chloromethyl methyl ether to
form 34 (see § 4.1.3). Indeed, the 1 H-NMR spectrum of methoxymethoxy-protected
biphenol 34 showed two distinct doublets (coupled AB system) for the methylene
hydrogens at ambient temperature, revealing their magnetic non-equivalence and
anisochrony. Dibromobiphenyl 34 was finally treated with four equivalents of tert-
butyllithium in perdeuterated tetrahydrofuran at -75 °C to generate the reactive 2,2’-
dilithio-6,6’-bis(methoxymethoxy)biphenyl (47).
HB HA Li
O
O
Li
47
O
O
Several 1 H-NMR spectra of 19 were then recorded at various temperatures,
starting from -50 °C and increasing it gradually until the AB-system "quartet" collapsed
into a broad singlet. To prove the reproducibility of the rotational process observed and
check wether no decomposition occurred, several spectra were then recorded while
lowering back the temperature from the coalescence to -50 °C. In this way, the
coalescence temperature was precisely determined to be Tc = -11 ± 1 °C and thus, the
HB HA