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Abstract:
An alternative way for performing Suzuki reactions is presented. The necessary borate which is the actual nucleophile in these palladium catalyzed C-C-bond formations is prepared from 9-methoxy-9-bora-bicyclo[3.3.1]nonane (9-OMe-9-BBN) and a polar organometallic reagent RM, and not as usually from a borane and a base. This approach allows cross couplings of aryl halides with e.g. alkynyl-, methyl-, or TMSCH2-groups, which were beyond the scope of the conventional Suzuki reaction. The method is highly chemoselective and turned out to be compatible with aldehyde-, amide-, ketone-, ester- and cyano functions as well as with basic nitrogen atoms in the substrates. It was applied to the synthesis of the acetylenic natural products junipal (9a) and eutypine methyl ether (10). Since 11B NMR studies revealed that the 9-OMe-9-BBN only serves as a shuttle for delivering the RM reagent but remains unchanged during the course of the reaction, it has been possible to device the first Suzuki-type reaction sub-stoichiometric in boron. This “catalytic” protocol was used to prepare compound 8 which is highly valuable for its chemoluminescence properties.
