The dual behaviour of β-vinylporphyrins in the presence of α,α′-dioxothiones

Abstract

α,α′-Dioxothiones are very reactive species and can participate in cycloaddition reactions with several compounds. Porphyrins bearing vinyl groups are interesting scaffolds for further functionalization namely by cycloaddition reactions; it is well known that porphyrins can react as either a 2π or 4π component in different cycloaddition approaches, such as hetero Diels–Alder reactions. This study reports for the first time the reactivity of a α,α′-dioxothione in the presence of 5,10,15,20-tetraphenylporphyrin bearing a vinyl group (2-VinylTPP) and of its Zn(II) complex (Zn-VinylTPP). The results revealed that the reactivity of α,α′-dioxothione as a heterodienophile or as a heterodiene is dependent on the absence or on the presence of Zn(II) in the porphyrin inner core. Thus, the free base 2-VinylTPP reacted as a diene affording two diastereomeric chlorins (3a and 3b) that are in a tautomeric equilibrium with porphyrin 4, and Zn-VinylTPP reacted as a dienophile affording porphyrin 5 and different oxidized products. All the cycloadducts obtained were revealed to be emissive in the red region and be able to produce ¹O₂.