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 α,α-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. Diels-Alder reaction of 6 and benzdiyne gave adducts 14, 15 a, and 15 b along with a trace amount of porphyrin(2.1.2.1) barrel 13. Stepwise routes using 14 or 15 a/15 b as a substrate allowed for the synthesis of 13 as a single stereoisomer. The nanobarrel structure for 13 was revealed by X-ray diffraction, where its cavity held two chloroform molecules via C-H⋅⋅⋅π interaction. DFT calculations revealed that the electrostatic attraction was dominant with binding energy of 32.8 kcal mol-1 .