Reinvestigation of the Room Temperature Photochemical Reaction between N-Methyl-1,2,4-triazoline-3,5-dione (MeTAD) and Benzene

The photochemical reaction of N-methyl-1,2,4-triazoline-3,5-dione (MeTAD) with benzene is known to lead to a Diels–Alder cycloaddition product when conducted at low temperatures (i.e., <−60 °C). This reactivity has been exploited recently for novel synthetic applications. It was previously reported that no reaction between MeTAD and benzene occurs at room temperature. However, it has now been discovered that MeTAD reacts effectively with benzene upon visible light irradiation over a several day period at room temperature. The major product is a para-substituted bisurazole adduct. Our studies indicate that the adduct is formed via sequential aromatic substitution reactions made possible by electron transfer from the aromatic ring to the highly electrophilic triplet state of photoactivated MeTAD.