Conformational Shift by Mutual Steric Interactions between Two Multiarmed Units

Decakis(bromomethyl)biphenyl (3) has been prepared by reaction of decamethylbiphenyl with Br₂/CCl₄ in the presence of tetrabutylammonium bromide as catalyst. The attempted synthesis of decakis(bromomethy)benzophenone resulted only in bromopentakis(bromomethyl)benzene (8). X-ray diffraction of a single crystal of 3 shows that the preferred conformation of 3 is not the alternate “up-down” conformation observed for the parent hexakis(bromomethyl)benzene, but a conformation in which the up-down alternation of the CH₂Br groups is disrupted at the meta-para positions of each ring. MM2(85) calculations on 3 indicate that some conformations with disrupted “up-down” alternation of the CH₂Br groups are favored over the all-alternate up-down form. It is suggested that this conformational shift is the result of the mutual steric interactions between arms of different moieties, and that syn interactions between m- and p-CH₂Br groups operate in order to avoid bromine contacts between ortho groups at different rings. The barrier for Ar-CH₂Br rotation was estimated from dynamic NMR data as 12.5 kcal mol^-1.