Reductive eliminations – trends across the d block

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Reductive eliminations constitute the product-forming step in transition-metal catalyzed C−C, C−CF3, and C−F coupling reactions. The value and potential of these transformations for organic synthesis are truly outstanding. To optimize these reactions in a rational manner, detailed mechanistic insight into the operating catalytic cycles in general and the elementary step of the reductive elimination in particular is needed. Except for the case of palladium- and platinum-mediated reactions, such insight is largely missing. This lack of understanding results from the instability of most of the relevant transition-metal complexes, which has prevented their isolation and in-depth examination. The project aims at circumventing this problem by probing the unimolecular reactivity of ionic complexes isolated in the gas phase. In previous work, the applicant has demonstrated the suitability of electrospray ionization for transferring intact organometallic ions from solution into the gas phase, where mass selection ensures a well-defined population of reactant ions. Collisional activation of these ions and mass-spectrometric detection of the resulting fragment ions afford detailed information on the tendencies of the reactant ions to undergo reductive eliminations or competing processes and the relative barriers of these reactions. The project will include complexes of the medium and late 3d metals as well as selected late 4d and 5d metals and determine the trends that control their unimolecular reactivity. At the same time, it will systematically examine how the overall charge of the complexes, their aggregation state, the oxidation state of the metal center, the coordination of ligands as well as the electronic and steric properties of the metal-bound organyl substituents affect the reductive elimination. The experiments will be moreover complemented by quantum-chemical calculations to obtain additional energetic and structural information. Thus, the studies will greatly advance our fundamental understanding of reductive eliminations and promise to provide guidelines for improving practically important C−C, C−CF3, and C−F coupling reactions.