Copper-Catalyzed Asymmetric Cyclizations Initiated by Hydrocupration

The synthesis of cycloalkenes via hydrocupration chemistry demonstrates a new versatile possibility to create cyclized compounds. For five-membered cycles the reaction occurs to be faster even with a lower catalyst loading, while the six-membered cycles, comparatively, seem more difficult to achieve. Even though the reactions work only up to moderate yield, an excellent enantiomeric ratio up to 98.5:1.5 could be observed. For both the five- and the six-membered cycles smaller groups like methyl or ethyl on the stereogenic center led to the highest selectivity (98.5:1.5 e.r.), whereas bulkier aromatic groups afforded a maximum e.r. of 96:4, exemplary for a p-toluyl substituted system. With further optimization of the reaction conditions, the yield should be able to be increased without influencing the enantiomeric ratio. The possibility to use this reaction to also create heterocycles makes this work a fundamental achievement for synthetic chemistry.