One‐Pot Preparation of Non‐Symmetric meso‐Aryl‐BODIPYs: Functional Derivatives with Unusual Reactivity

Abstract

The formation of meso‐aryl‐BODIPYs (boron dipyrrins) through the acidic condensation of 3,4‐dialkylpyrroles with aromatic aldehydes, followed by 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) oxidation and BF2 complexation, has been reviewed. Surprisingly, it was found that the major products from these reactions were not the anticipated symmetric BODIPYs, but non‐symmetric derivatives carrying one benzyl substituent at the BODIPY α position. The best yields and simple purification conditions were be achieved if the oxidant was not employed in the one‐pot reaction sequence. Electron‐rich benzaldehydes provided the best results, whereas precursors with electron‐withdrawing substituents gave significant amounts of the symmetric BODIPYs as side‐products. This unexpected result can be rationalized mechanistically on the basis of two reaction pathways that diverge from a common intermediate at an early step in the condensation sequence. Preliminary reactivity investigations showed chlorination to give unexpected results, but a typical substitution and coupling chemistry.