Selective Convergence to Atropisomers of a Porphyrin Derivative Having Bulky Substituents at the Periphery.

Abstract

Four kinds of possible atropisomers of a porphyrin derivative (1), having mesityl groups at one of the o-positions of each meso-aryl group, can be selectively converged to targeted atropisomers among the four isomers (αααα, αααβ, αβαβ, and ααββ) under appropriate conditions for each atropisomer. For example, protonation and subsequent neutralization of a free base porphyrin (H2-1) induces a convergence reaction to the αβαβ atropisomer, H2-1-αβαβ, from an atropisomeric mixture. The αααα isomer, H2-1-αααα, was also obtained by heating a solution of H2-1 in CHCl3 in 60% isolated yield, probably owing to a template effect of the solvent molecule. Remarkably, when an atropisomeric mixture of its zinc complex, Zn-1, was heated at 70 °C in a ClCH2CH2Cl/MeOH mixed solvent, crystals composed of only Zn-1-αααα were formed. The hydrophobic space formed by the four mesityl groups in the αααα isomer can be used for repeatable molecular encapsulation of benzene, and the encapsulation structure was elucidated by powder X-ray diffraction analysis. Heating the solid of an atropisomeric mixture of Zn-1 to 400 °C afforded the ααββ isomer almost quantitatively. On the other hand, the solid of H2-1-αααα can be converted by heating, successively to H2-1-αααβ at 286 °C and then to H2-1-ααββ at 350 °C.