Photocatalyzed [2+2]-cycloaddition of 3-(2-oxo-2-phenylethylidene)-2- oxindole by ruthenium complexes.

Photocatalysis has become a prominent theme in organic chemistry as a unique and effective tool for electron or energy transfer processes under mild reaction conditions.[1] Ruthenium(II) complexes have demonstrated useful properties as photocatalysts.[2] Although Ru(bpy)3 2+ has been extensively studied in photoredox systems, the ability of metal polypyridyl complexes to facilitate triplet sensitization via an energy transfer process has not been extensively investigated.[3] Therefore, in the present study we have investigated the ability of ruthenium complexes to photocatalyse a [2+2]-cycloaddition reaction. The catalytic test was based upon the synthesis reported by Wu et al.[4] A mixture of 1 (0.10 mmol) and catalyst (4 mol%) were dissolved in 20 mL of MeOH and stirred whilst being irradiated by a commercial 8 W household LED lamp. The catalysts used were: rose bengal (as standard), [Ru(bpy)3](PF6)2 (bpy = bipyridyl) and [Ru(bpy)2dppz](PF6)2. The reaction scheme, catalyst structure and yields are shown in table 1. The energy transfer mechanism was investigated by Stern-Volmer quenching experiments of the transient absorption and of the emission from the ruthenium (II) triplet complexes. The Stern-Volmer plots for the ruthenium complexes showed quenching of the emission at 620 nm and of triplet absorption at 370 nm, both with a quenching constant of 6-7x109 L mol-1 s-1. In conclusion, ruthenium complexes catalysed the intermolecular [2+2]-cycloaddition of a 3- ylideneoxindole, via a triplet sensitization energy transfer process, with good product yield (75%) when irradiated by visible light from a commercial 8 W household LED lamp.