X‑Shaped Cyclobutane-Linked Tetraporphyrins through a Thermal [2+2] Cycloaddition of Etheno-Fused Diporphyrins

A nickel-mediated tandem double cyclization
of ethynylene-linked
dibromodiporphyrins afforded highly reactive etheno-fused diporphyrins,
which undergo a thermal [2+2] cycloaddition at the fused C–C
double bond to afford the cyclobutane moiety in X-shaped cyclobutane-linked
tetraporphyrins. During the reaction of a Zn­(II) complex precursor,
the initial double cyclization product was converted into the diketodiporphyrin,
which exhibits red chemiluminescence under ambient conditions.