Ligand Conformational Flexibility Enables Enantioselective Tertiary C–B Bond Formation in the Phosphonate-Directed Catalytic Asymmetric Alkene Hydroboration
| Autor: | Peng Liu, James M. Takacs, Xiaotian Qi, Huiling Shao, Suman Chakrabarty |
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| Rok vydání: | 2021 |
| Předmět: |
Steric effects
chemistry.chemical_classification Ligand Stereochemistry Chemistry Alkene Migratory insertion Molecular Conformation Organophosphonates Enantioselective synthesis Stereoisomerism General Chemistry Alkenes Ligands 010402 general chemistry 01 natural sciences Biochemistry Article Catalysis Reductive elimination 0104 chemical sciences Hydroboration Colloid and Surface Chemistry Catalytic cycle Boranes |
| Zdroj: | J Am Chem Soc |
| ISSN: | 1520-5126 0002-7863 |
| Popis: | Conformationally flexible ancillary ligands have been widely used in transition metal catalysis. However, the benefits of using flexible ligands are often not well understood. We performed density functional theory (DFT) and experimental studies to elucidate the mechanisms and the roles of conformationally flexible α,α,α',α'-tetraaryldioxolane-4,5-dimethanol (TADDOL)-derived ligands on the reactivity and selectivity in the Rh-catalyzed asymmetric hydroboration (CAHB) of alkenes. DFT calculations and deuterium labeling studies both indicated that the most favorable reaction pathway involves an unusual tertiary C-B bond reductive elimination to give high levels of regio- and enantioselectivities. Here, the asymmetric construction of the fully substituted carbon center is promoted by the flexibility of the TADDOL backbone, which leads to two ligand conformations with distinct steric environments in different steps of the catalytic cycle. A pseudo-chair ligand conformation is preferred in the rate-determining tertiary benzylic C-B reductive elimination. The less hindered steric environment with this conformation allows the benzylic group to bind to the Rh center in an η3 fashion, which stabilizes the C-B reductive elimination transition state. On the other hand, a pseudo-boat ligand conformation is involved in the selectivity-determining alkene migratory insertion step, where the more anisotropic steric environment leads to greater ligand-substrate steric interactions to control the π-facial selectivity. Thus, using a conformationally flexible ligand is beneficial for enhancing both reactivity and enantioselectivity by controlling ligand-substrate interactions in two different elementary steps. |
| Databáze: | OpenAIRE |
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