My PhD dissertation - Institut Fresnel

60
each in the coplanar transition state. This steric constraint causes this configuration to be
energetically very high, thus contributing to a significant increase in activation energy
compared to unbridged dilithiobiphenyls. Contrary to all expectations, the torsional value
calculated for biphenyl 47 only varies from that of compound 48 by approximately 0.5
kcal·mol -1 . An explanation could be tentatively proposed with two arguments: first,
oxygen atoms, even if less bulky than isopropylidene groups, exert an electronic repulsion
thanks to their nonbonding electrons ("lone pairs"), thus increasing the coplanar transition
state energy. In addition, since the methoxymethoxy protective group is known to
efficiently coordinate lithium species
[ ] 217 , it could favor a twisted conformation and
induce some rigidity in the biphenyl motif by coordinating a lithium placed on the
adjacent aromatic ring.
Li
O
O
47
O
Li
O
4.4.3 Coalescence Study of 2,2’-Dilithio-6,6’-bis(methoxymethyl)biphenyl
Contrary to the previously investigated substrate, bis(hydromethyl) biphenyl 39 was
protected by conversion to its corresponding dimethyl ether 41 rather than with dimethoxymethoxy
group since the latter was partially prone to α-deprotonation and subsequent Wittig rearrangement
219 ]
upon treatment with organolithium reagents. 2,2’-Dilithio-6,6’-bis(methoxymethyl)biphenyl (49)
was generated by treatment of the corresponding dibromo derivative 41 with four equivalents of tert-
butyllithium in perdeuterated tetrahydrofuran at -75 °C.
The 1 H-NMR spectrum of dilithio- species 49 showed, once again, two distinct doublets
with a significant "roof effect", corresponding to the diastereotopic methylene hydrogens of the
hydromethyl- groups. The line shape variation of these signals was followed as a function of the
increasing temperature (Figure 8a) and displayed coalescence at Tc = 24 ± 1 °C, which eventually
≠
allowed to calculate an energy barrier to free rotation of G = 12.9 ± 0.2 kcal·mol
∆ coal
-1 .
[ 218,