N ‐Arylphenothiazines and N , N ‐Diarylphenazines as Tailored Organophotoredox Catalysts for the Reductive Activation of Alkenes

A small library of N-arylphenothiazines, N-arylbenzo[b]phenothiazines and N,N-diarylphenazines has been synthesized as strongly reducing organophotocatalysts without the need for sacrificial reagents or electrochemistry. Their opto-electronic properties and their suitability as organophotoredox catalysts for the efficient activation of olefins was investigated using the addition of methanol to α-methylstyrene as a model reaction. The findings were supported by DFT calculations. Three photocatalysts were identified that allowed quantitative yields with 10 mol % catalyst loading and 20 h irradiation by a 365 nm LED. The catalyst loading was gradually reduced to 0.5 mol % and the irradiation time was reduced to only 20 min. These experiments revealed that N,N-diisobutyl-4′-(10H-phenothiazin-10-yl)-[1,1′-biphenyl]-4-amine is the most efficient photocatalyst among the N-arylphenothiazines. This chromophore combines high electron density introduced by the diisobutylamino phenyl group with high absorptivity at 365 nm caused by the phenylene bridge to the phenothiazine core. In an attempt to shift the excitation wavelength into the visible light range for selectivity and sustainability purposes (potential use of sunlight as a light source), N-arylbenzo[b]phenothiazines N,N-diarylphenazines were designed that exhibit a red-shifted and broad absorbance shoulder into the visible range, but only N,N-diarylphenazines allow photocatalysis and the one-electron activation of alkenes by visible light (450 nm LED).