Addition reactions carbometallation, catalyst systems

To improve the poor reactivity of 95, a number of copper-catalyst systems for carbomagnesiations were screened in a subsequent work [27]. While CuBr was an ineffective catalyst for the carbometallation, the addition of PBUj in the presence of a substoichiometric amount of Cul improved the efficiency and led to the syn-adduct 97 in 87% yield at room temperature. The optimal ratio of PBUj/CuI was found to be 4 1 at 30 mol% of catalyst loading. A further increase in selectivity (>96%) for syn-97 was achieved when the reaction was carried out at —78°C, albeit with a lower conversion (80%) after 3 h. The optimized catalytic system was then applied to the synthesis of substituted cyclopropane derivatives containing quaternary all-carbon-substituted stereocenters (Scheme 10.30). 

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