Abstract
Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible-light photocatalysis. Here we show that the redox potential of a carbon nitride photocatalyst (CN-OA-m) can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials. This tuning was the key to realizing photo-chemo-enzymatic cascades that give either the (S)- or the (R)-enantiomer of phenylethanol. In combination with an unspecific peroxygenase from Agrocybe aegerita, green light irradiation of CN-OA-m led to the enantioselective hydroxylation of ethylbenzene to (R)-1-phenylethanol (99 % ee). In contrast, blue light irradiation triggered the photocatalytic oxidation of ethylbenzene to acetophenone, which in turn was enantioselectively reduced with an alcohol dehydrogenase from Rhodococcus ruber to form (S)-1-phenylethanol (93 % ee).
Highlights
Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible‐light photocatalysis
Catalytic reactions can be controlled by varying the catalyst/coordinated ligands, directing groups[2] or by tuning external parameters (Scheme 1, A).[1a, 3] The selectivity of photochemical reactions varies with different wavelengths,[4] but examples that use this for visible‐light photocatalysis are rare.[5]
Selective control between either a one‐ or two‐fold substitution of 1,3,5‐ tribromobenzene with N‐methylpyrrole using Rhodamin 6G (Rh‐6G) as photocatalyst was demonstrated (Scheme 1, B)[5a] This selectivity switch is explained by the chromoselective generation of two photocatalytic species that differ in their reduction potential
Summary
Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible‐light photocatalysis. We recently realized that the choice of the wavelength is crucial for high selectivities in metallophotocatalytic cross couplings using a heterogeneous carbon nitride material, which is made from urea and oxamide in molten salt (CN‐OA‐m).[5b, 5c, 6] While this can be rationalized by a purely kinetic effect, there is evidence that a wavelength‐controlled generation of excited species with different oxidation potentials could be responsible for this phenomenon.
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