Conformation and stereodynamics of decaethylbiphenyl

The stereochemistry of decaethylbiphenyl (5) is analyzed. The potentially low-energy conformers of 5 were generated by formally linking two pentaethylphenyl subunits, and by assuming that no more that one synz interaction is present per ring. Molecular mechanics calculations (MM program) indicate that the forms 'f', 'i', 'j', and 'm' represent the lowest energy conformations. As previously observed for decakis(bromomethyl)biphenyl, the 'a' conformation (devoided of any syn arrangement of two neighboring ethyl groups) is destabilized by the mutual steric interactions of ortho ethyl groups at different rings. Decaethylbiphenyl (5) was synthesized by exhaustive ethylation of biphenyl. The compound exists in the crystal in three different conformations ('i', 'j', and 'm'). Low-temperature ¹³C 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 ΔG₇₆(t) = 8.2 ± 0.1 kcal mol^-1. The interconversion graph of 5 was analyzed, and on the basis of the MM calculations and NMR data, it is concluded that the rotational process followed by NMR involves the stepwise rotation of the meta and pura ethyl groups of 'm'.