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A Catalytically Generated Transition Metal Analog of the Simmons-Smith Reagent
Jacob Werth1, Kristen Berger1, Christopher Uyeda1
1Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States.
Abstract:
(PDI)CoBr2 complexes (PDI = pyridine-diimine) serve as precatalysts for reductive alkene cyclopropanations using CH2Br2 and Zn. A key question regarding the mechanism of this process is whether CH2 transfer occurs through a Co(CH2Br) species or through an isomeric carbene complex, similar to those formed in diazoalkane-based cyclopropanation reactions. Mechanistic studies are consistent with the intermediacy of a cationic (PDI)Co(CH2Br) species, which is proposed to function as a transition metal analog of the Simmons-Smith reagent. This model rationalizes the critical role of ZnBr2 Lewis acid in sequestering Br- from the reaction. Finally, the cobalt-catalyzed cyclopropanation exhibits a distinct selectivity profile from the Simmons-Smith reaction due to differences in the steric properties of (PDI)Co and Zn carbenoids.
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