SAB-15-Supported Rose Bengal as a Heterogeneous Photoredox Catalyst

Key words

photoredox reaction - aza-Henry reaction - mesoporous silica - rose bengal

Significance

A series of solid-supported dye composites consisting a rose bengal (RB) unit covalently linked to inorganic porous supports (e.g., silica or zeolites) were prepared for use as heterogeneous photoredox catalysts. An ordered mesoporous silica SAB-15-supported RB tethered to a 3-aminopropylsilane moiety (SAB-15-APTES-RB) catalyzed the aza-Henry reaction under green LED irradiation. For example, tetrahydroisoquinoline 1a reacted with nitromethane in the presence of 1 mol% of SAB-15-APTES-RB at room temperature under LED irradiation to give the aza-Henry adduct 2a in 91% yield. SAB-15-APTES-RB was also effective for reductive coupling of the aryl diazonium compound 3 with furan, showing a catalytic performance that was comparable to that of its homogeneous counterpart carried out with RB.

Comment

A robust and readily recyclable RB-based photocatalyst for organic synthesis has not been well studied so far, although a similar mesoporous silica-supported RB has been developed as a photosensitizer (C. Mendoza et al. ChemPhotoChem 2018, 2, 890). SAB-15-APTES-RB was characterized by means of IR, UV/Vis, ¹³C, and ²⁹Si NMR analyses. In the reaction of 1a with nitromethane, SAB-15-APTES-RB was recovered and reused for a total of eight runs without significant loss of its catalytic ­activity, though a slight decrease of activity was observed in the ninth run or later. Molecular oxygen served as the sole oxidant for the aza-Henry reaction, which did not proceed in darkness under otherwise similar conditions.