Adjustable Regioselectivity for the Diels-Alder Reactions of Sulfolenodipyrrins upon Molecular Engineering on H-Bonds

The Diels-Alder reactions of sulfolenodipyrrins prove to be an efficient way to construct aromatic ring-fused dipyrrins. However, adjustable annulation is still hard to achieve. To address this, molecular engineering on the H-bond has been employed. The α-position aryl group-modified sulfolenodipyrrins have been synthesized to react with various dienophiles in Diels-Alder reactions, affording the monoannulation products with different regioselectivity in good yields (45-76%). The remaining sulfolenopyrrole in monoadducts can undergo further fusion in the presence of dienophiles and TEMPO, giving the bisadducts with a lactam subunit in an appropriate yield. According to the crystal structures and theoretical calculations, the intramolecular H-bonds between the α-substituent and the nearby pyrrole confine the conjugation pathway of the dipyrrin core. With respect to the normal NH-sulfolenopyrrole, the imino-type one features low aromaticity, from which SO₂ extrusion generates more stable dipyrrin-diene, achieving regioselectivity. In addition, aromatic ring fusion results in red-shifted absorption and emission spectra, and the annulation units regulate the emission intensity. This work shows the versatility of intramolecular H-bonds in regulating the reaction through confinement of the conjugation system.