Photoswitchable Catalysts: Correlating Structure and Conformational Dynamics with Reactivity by a Combined Experimental and Computational Approach

Photocontrol of a piperidine's Bronsted basicity was achieved by incorporation of a bulky azobenzene group and could be translated into pronounced reactivity differences between ON- and OFF-states in general base catalysis. This enabled successful photomodulation of the catalyst's activity in the nitroaldol reaction (Henry reaction). A modular synthetic route to the photoswitchable catalysts was developed and allowed for preparation and characterization of three azobenzene-derived bases as well as one stilbene-derived base. Solid-state structures obtained by X-ray crystal structure analysis confirmed efficient blocking of the active site in the E isomer representing the OFF-states, whereas a freely accessible active site was revealed for a representative Z isomer in the crystal. To correlate structure with reactivity of the catalysts, conformational dynamics were thoroughly studied in solution by NMR spectroscopy, taking advantage of residual dipolar couplings (RDCs), in combination with comprehensive DFT computational investigations of conformations and proton affinities.