We thank the Deutsche Forschungsgemeinschaft (DFG) for their financial support (EXC81, SFB623). We also acknowledge Stephen Hashmi (Heidelberg University) for fruitful discussions. Volker Huch is gratefully acknowledged for X-ray crystallography. Michael Schwering and Dominik Brox have continuously supported the project with their expertise in microscopy.Supporting information for this article, including details of reagents used, instruments, and analytical data, including spectroscopic characterization, is available on the WWW under http://dx.

Experimental and DFT-based computational results on the aziridination mechanism and the catalytic activity of (bispidine)copper(I) and -copper(II) complexes are reported and discussed (bispidine=tetra- or pentadentate 3,7-diazabicyclo[3.1.1]nonane derivative with two or three aromatic N donors in addition to the two tertiary amines). There is a correlation between the redox potential of the copper(II/I) couple and the activity of the catalyst. The most active catalyst studied, which has the most positive redox potential among all (bispidine)copper(II) complexes, performs 180 turnovers in 30 min. A detailed hybrid density functional theory (DFT) study provides insight into the structure, spin state, and stability of reactive intermediates and transition states, the oxidation state of the copper center, and the denticity of the nitrene source. Among the possible pathways for the formation of the aziridine product, the stepwise formation of the two N-C bonds is shown to be preferred, which also follows from experimental results. Although the triplet state of the catalytically active copper nitrene is lowest in energy, the two possible spin states of the radical intermediate are practically degenerate, and there is a spin crossover at this stage because the triplet energy barrier to the singlet product is exceedingly high.

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Here, we demonstrate a versatile screening platform for NHC ligand based catalysts by coating fused-silica micro capillaries with a bonded 1,3-bismesityl-2-imidazolidinylidene ligand. Such micro capillaries can be efficiently converted into (pre)-catalysts from various organometallic precursors by solid-phase chemistry techniques and can be quantitatively screened using on-column reaction chromatography.

An improved immobilised Grubbs 2 nd generation catalyst and its application in flowthrough devices, shown for on-column reaction gas chromatography (ocRGC), has been studied. The coupling of a reaction capillary and a separation column in GC/MS allows direct reaction monitoring and analysis of conversion as well as reaction kinetics. The presented permanently bonded N-heterocyclic carbene ligand shows a great stability and activity in ring closing metathesis reactions. A salt-free approach was used to generate the carbene ligand, which can be directly monitored by mass spectrometry. The very flexible design of the immobilised ligand system in reaction channels and capillaries of flow through systems allows the preparation of various catalysts using a broad variety of metal precursors. This strategy of immobilised catalytically active complexes offers a wide range of on-column reactions combinable with fast reaction screening by high throughput experimentation.