The structure, modelling and dynamics of 2,7-diisopropoxy-1,8-diarylnaphthalenes

2,7-Diisopropoxy-1,8-dibromonaphthalene 5 was prepared in two steps from 2,7-dihydroxynaphthalene and was coupled under Suzuki cross-coupling conditions with boronic acids 9 and 10 to provide the corresponding 1,8-diarylnaphthalene systems 12a and 13a respectively. In contrast, attempted coupling of dibromide 5 with o-tolylboronic acid 11 proved unrewarding. Single crystal X-ray structure determination of compounds 12a and 13a showed that both structures possessed a high degree of structural deformation due to high internal steric repulsions between the 1,8-diaryl rings and their substituents. Dynamic 1H NMR experiments showed that these two systems possessed very slow phenyl-naphthalene bond rotation (ca. 2 s-1), corresponding to rotation barriers (ΔG*) of 16-18 kcal mol-1. Molecular modelling predicts that such systems have approximately similar rotation barriers and that in order to completely prevent phenyl-naphthalene bond rotation, an ortho-phenyl substituent is required, with a barrier to rotation of ca. 40 kcal mol-1.