Redoxneutral Propargylic CH-Activation: from Mechanistic Investigations towards better Catalysts

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With this project the mechanistic investigations on rhodium catalyzed propargylic C-H-activation shall be continued. With the atom economic, redox neutral catalysis in question, allylic esters and enolesters can be synthesized starting form carbon acids and terminal alkynes or allenes. In the synthesis of branched allylic esters with DIOP as chiral ligand enantiomeric excesses up to 96% can be achieved. In the cooperation of B. Breit and D. Heller, the resting state of the catalytic cycle was identified. This key intermediate was characterized by means of NMR- and Raman spectroscopy, as well as by X-ray analysis and mass spectrometry. In addition, a qualitative rate law for the catalytic transformation was determined through in-situ IR- and NMR-spectroscopy. In these investigations a catalyst inhibition in reactions with the alkyne substrate was detected. The experiments were supported by DFT-calculations, which matched the findings. The results obtained in this collaboration were used to refine the proposed mechanism. The successful cooperation of B. Breit and D. Heller will be continued on basis of the project presented here. The main objectives of this project are further mechanistic investigations of the formation of esters as well as the transferability of the proposed mechanism to the analogous rhodium catalyzed formation of allylic amines with nitrogen-containing pronucleophiles. Within this project the effects of alkynes and allenes on the catalytic reaction, especially the origin of the inhibition of the catalyst, will be examined. In addition, an explanation for the observed chemoselectivity towards different ester products depending on the chosen ligand shall be elaborated and the relation of the product to the structure of the rhodium ligand complex shall be investigated. The mechanistic investigations of the rhodium catalyzed formation of allylic amines with anilines and benzotriazoles shall indicate whether the proposed mechanisms for the different pronucleophiles share a common base or if every pronucleophile has its own unique mechanism. During the investigations of the addition of the benzotriazole to terminal allenes, a reason for the observed regioselectivity towards the N1- and N2-product depending on the ligand will be saught after. The mechanistic investigations will consist of NMR-, IR-, UV/Vis-, and Raman-spectroscopic, crystallographic and mass spectrometric experiments and will be complemented with DFT calculations.