Azo-dimethylaminopyridine-functionalized Ni(II)-porphyrin as a photoswitchable nucleophilic catalyst.

Jannis Ludwig,Hendrik Zipse,Rainer Herges,Julian Helberg

doi:10.3762/bjoc.16.179

Abstract

We present the synthesis and the photochemical and catalytic switching properties of an azopyridine as a photoswitchable ligand, covalently attached to a Ni(II)-porphyrin. Upon irradiation with 530 nm (green light), the azopyridine switches to the cis configuration and coordinates with the Ni2+ ion. Light of 435 nm (violet) isomerizes the ligand back to the trans configuration, which decoordinates for steric reasons. This so-called record player design has been used previously to switch the spin state of Ni2+ between singlet and triplet. We now use the coordination/decoordination process to switch the catalytic activity of the dimethylaminopyridine (DMAP) unit. DMAP is a known catalyst in the nitroaldol (Henry) reaction. Upon coordination to the Ni2+ ion, the basicity of the pyridine lone pair is attenuated and hence the catalytic activity is reduced. Decoordination restores the catalytic activity. The rate constants in the two switching states differ by a factor of 2.2, and the catalytic switching is reversible.

Highlights

Photoswitchable catalysis has been realized following several approaches using a variety of photochromic systems
We present the synthesis and the photochemical and catalytic switching properties of an azopyridine as a photoswitchable ligand, covalently attached to a Ni(II)-porphyrin
We present a photoswitchable catalyst whose basicity is controlled by coordination/decoordination to the Ni2+ ion in a Ni-porphyrin

Summary

Introduction

Photoswitchable catalysis has been realized following several approaches using a variety of photochromic systems. Feringa et al recently published a review including systems based on double bond isomerizations [1]. An earlier review from the same group summarized light and redox responsive catalytic systems including azobenzenes, diarylethenes, spiropyranes, and stilbenes [2]. Diarylethenes were reported in the context of photoswitchable catalysis as inhibitors of the Karstedts catalyst [3] and for pKa modulation in acid–base-controlled processes [4]. Interesting and close to our approach is an early work by Inoue et al who achieved control of the transformation of CO2 and 1,2-epoxypropane to propylene carbonate using an aluminum porphyrin and a photoresponsive ligand.

Methods

Results

Conclusion