Synthesis of rotationally restricted and modular biphenyl building blocks

A series of modular biphenyl building blocks with stepwise adjusted torsion angles and terminally functionalized with leaving groups have been synthesized. The two phenyl rings of the biphenyl synthon are clamped by alkyl chains of various lengths. The desired building blocks 3 and 4 were obtained by copper-mediated C-C biaryl coupling reactions followed by the construction of interlinking alkyl bridges. The key intermediates 14 and 15 were transformed into the corresponding cycloheptadienones 16 and 17, which were reduced to the desired propyl-bridged biphenyls 3b and 3c. The butyl-bridged derivatives 4b and 4c were obtained from 14 and 15 by an allylation reaction followed by ring-closing metathesis (RCM) and hydrogenation. The pentyl chain in precursor 24 was obtained by two aldol C-C bond-forming reactions followed by a sequence of reduction steps. It was subsequently cyclized in an aryl-aryl coupling reaction to the pentyl-bridged biphenyl 26 along with the macrocyclic dimer 25. The UV absorption spectra of the acetylsulfanyl-functionalized series 1a-8a were recorded and analyzed: A linear correlation between the conjugation band in the UV absorption spectra and the cos2 Phi values (Phi is the interplanar torsion angle) is observed.