Low‐Valent Zirconocene‐mediated Synthesis of Porphyrin(2.1.2.1)s and Its Extension to Synthesis of a Porphyrin(2.1.2.1) Nanobarrel

Rosenthal’s-reagent-mediated intramolecular cyclometallation of a , a -dialkynyldipyrrin nickel(II) complex and subsequent acid treatment afforded a 1,3-butadiene-embedded porphyrin(2.1.2.1), 6 , which served as a reactive diene towards dienophiles such as dimethyl acetylenedicarboxylate (DMAD) and benzyne to give corresponding Diels-Alder adducts. Encouraged by the highly bent saddle-like structures of these adducts, we challenged the synthesis of a porphyrin(2.1.2.1) nanobarrel. Diels-Alder reaction of 6 and benzdiyne gave adducts 14 , 15a , and 15b along with a trace amount of porphyrin(2.1.2.1) barrel 13 . Stepwise routes using 14 or 15a/15b as a substrate allowed for the synthesis of 13 as a single stereoisomer in 12% or 9.5% yield. The nanobarrel structure for 13 was revealed by X-ray diffraction, where its cavity holds two chloroform molecules via C-H···p interaction. DFT calculations revealed that the electrostatic attraction is dominant with binding energy of 32.8 kcal/mol.