CONFORMATION AND STEREODYNAMICS OF DECAETHYLBIPHENYL

The stereochemistry of decaethylbiphenyl (5) is analyzed. The potentia lly low-energy conformers of 5 were generated by formally linking two pentaethylphenyl subunits, and by assuming that no more that one syn i nteraction is present per ring. Molecular mechanics calculations (MM3 program) indicate that the forms ''f'', ''i'', ''j'', and ''m'' repres ent the lowest energy conformations. As previously observed for decaki s(bromomethyl)biphenyl, the ''a'' conformation (devoided of any syn ar rangement of two neighboring ethyl groups) is destabilized by the mutu al steric interactions of ortho ethyl groups at different rings. Decae thylbiphenyl (5) was synthesized by exhaustive ethylation of biphenyl. The compound exists in the crystal in three different conformations ( ''i'', ''j'', and ''m''). Low-temperature C-13 NMR data show that the compound exists in CDCl2F at 149 K in two conformations in a 4:1 ratio , and the major conformer was assigned to the ''m'' form. Dynamic NMR data indicate that ''m'' undergoes ethyl rotation with a barrier of De lta G(176)(double dagger) = 8.2 +/- 0.1 kcal mol(-1). The interconvers ion graph of 5 was analyzed, and on the basis of the MM calculations a nd NMR data, it is concluded that the rotational process followed by N MR involves the stepwise rotation of the meta and para ethyl groups of ''m''.