Synthesis and Characterization of Chiral Porphyrin Derived from (–)‐α‐Phellandrene

Chiral pyrrole and porphyrin: A diastereomerically pure chiral pyrrole was synthesized in the reaction of ethyl isocyanoacetate and a bicyclic Diels–Alder adduct from naturally occurring (−)-α-phellandrene and 1,2-bis(phenylsulfonyl)ethene. The unidirectional chiral porphyrin was prepared from the corresponding chiral pyrrole by using InCl₃. The crystal structures of the pyrrole and the porphyrin were determined by X-ray diffraction analysis.

Abstract

The bicyclo[2.2.2]octadiene (BCOD) skeleton-fused chiral pyrrole 3a was successfully prepared by Diels–Alder reaction of (–)-α-phellandrene and disulfonyl ethylene, and seed-crystal precipitation method. The following InCl₃-catalyzed tetrameric cyclization and subsequent dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) oxidation afforded the chiral porphyrin like a unidirectionally oriented wind turbine without scrambling. The free-base form and its Zn-, and Pt complexes (4-H₂ , 4-Zn, and 4-Pt) were characterized by NMR spectra, absorption and emission spectra. Their chiral characters were investigated by circular dichroism (CD) spectra and their solid-state structures were determined by means of X-ray diffraction analysis. The retro Diels–Alder reaction of this chiral BCOD-fused porphyrin was performed at 200 °C, yielding the resulting π-conjugation expanded tetramethyltetrabenzoporphyrin quantitatively.

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