Direct C–N coupling of aniline to aminodiphenylamines with a platinum-loaded titanium oxide photocatalyst

Abstract

Aminodiphenylamines (ADPAs) and their derivatives have been known to be used as dye intermediates and antioxidants of rubber. Due to their high demand in industrial production, their synthesis methods have been widely developed. In the present study, a photocatalytic asymmetric homocoupling route was examined, i.e., the photocatalytic C–N coupling of aniline in an alkaline solution with a Pt-loaded titanium oxide (Pt/TiO₂) photocatalyst. A nearly 20% yield and 77% selectivity were achieved with the Pt/TiO₂ photocatalyst for 30 min. It was found that this C–N bond formation showed a high dependence on the pH of the solvent and irradiation light wavelength, i.e., the reaction can only proceed when pH > 11.79, and an incident light wavelength of 365 nm gave a high ADPA selectivity. The mechanistic studies by using isotopic compounds suggested that the rate-determining step (RDS) for ADPA production would be the formation of a transition complex with rehybridization to sp³ on the aromatic ring of aniline. The UV-vis spectral analysis and Arrhenius plots further revealed the presence of three formation routes of ADPAs in the present system: the first one begins with the hole-oxidation of aniline by the TiO₂ photocatalyst excited by UV light, the second one starts with one electron transfer from the N atom in aniline to Ti in TiO₂, which only occurred with an incident light wavelength of 365 nm, and the last one is the oxidation of aniline molecules by hydroxyl radicals as a minor route.