Steric Effects on Atropisomerism in Tetraarylporphyrins

Abstract

The rate of aryl ring rotation in a series of 5,10,15,20-tetrakis(2-X-5-methoxyphenyl)porphyrins, where X = H, F, Cl, Br, and I, was studied by NMR spectroscopy and chromatography. Where X = Cl, Br, and I, the four atropisomers a(3aj3, aaft(3, aaa(3, and aaaa were isolated and were stable at 298 K. When X = H or F, facile interconversion of the atropisomers at 298 K prevented isolation of pure atropisomers, but for X = F samples of >80% isomeric purity were isolated at low temperatures. The approach of nonequilibrated mixtures of atropisomers to equilibrium at elevated temperatures was monitored by HPLC experiments for X = Cl, Br, and I, and19F NMR spectroscopy for X = F. Rate constants were obtained for the six processes by which the atropisomers can interconvert. The rotational barriers increased monotonically as the van der Waals radii of the substituent X. The rotational barriers were correlated with the “apparent overlap” of the ortho substituents, 2>*, a geometric parameter previously proposed to quantify steric interactions on the internal rotation in biaryls. Show more