Synthesis of New Functionalized Organocopper Reagents via a Halogen-Copper Exchange Reaction
Highly functionalized organometallic compounds are key intermediates for the preparation of polyfunctionalized molecules. Halogen-metal exchange reaction is a general and chemoselective method for preparing functionalized organometallics, such as organolithium or organomagnesium. However, more sensitive functionalities, such as ketones and aldehydes are usually not compatible with these reagents. To overcome these problems, we developed a halogen-copper exchange reaction by using two sterically hindered lithium dialkylcuprates, which allowed a practical preparation of functionalized lithium cuprate reagents. Various functionalized cuprate reagents were prepared by using this method like functionalized aryl, heteroaryl and alkenyl cuprate reagents. A wide range of functional groups were tolerated by this method, such as ketones, esters, cyanide, halides, methoxyl group, or even aldehyde. The resulting functionalized copper reagents reacted readily with various electrophiles leading to highly functionalized molecules. The application of functionalized aryl cuprates obtained via I/Cu-exchange reaction in stereoselective allylic substitution reaction was also described. Finally, a related new method was developed for the preparation of polyfunctional magnesium arylcuprates obtained by an I/Cu-exchange reaction. Especial interest was that the reaction proceeded in a very clean way with the only side-product cyclopentane. Various functionalized organocopper reagents were obtained by this new method.