Synthesis of 4-Bromo-1,1′:4′,1″-terphenyl and 4-Methyl-1,1′:4′,1″-terphenyl

Abstract

Possible synthetic routes to 4-bromo-1,1′:4′,1″-terphenyl and 4-methyl-1,1′:4′,1″-terphenyl have been studied. Stevens rearrangement of quaternary ammonium salts containing 3-phenylprop-2-en-1-yl and 3-(4-bromo- or 4-methylphenyl)prop-2-yn-1-yl groups gave 1-(4-bromophenyl)-N,N-dimethyl-4-phenylhex-5-en-1-yn-3-amine, 1-(4-bromophenyl)-N,N-diethyl-4-phenylhex-5-en-1-yn-3-amine, 1-(4-bromophenyl)-4-phenyl-N,N-dipropylhex-5-en-1-yn-3-amine, 1-[1-(4-bromophenyl)-4-phenylhex-5-en-1-yn-3-yl]piperidine, 4-[1-(4-bromophenyl)-4-phenylhex-5-en-1-yn-3-yl]morpholine, 1-[1-(4-methylphenyl)-4-phenylhex-5-en-1-yn-3-yl]piperidine, and 4-[1-(4-methylphenyl)-4-phenylhex-5-en-1-yn-3-yl]morpholine. Vacuum distillation of the resulting amines, by analogy with structurally related compounds, was accompanied by deamination with the formation of 4-bromo-1,1′:4′,1″-terphenyl and 4-methyl-1,1′:4′,1″-terphenyl in high yields. This transformation is a domino reaction involving β-elimination of secondary amine to form conjugated dienyne, electrocyclization of the latter to cyclic allene intermediate, and fast 1,3- or 1,5-hydride shift.