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

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.