Enantiospecific Synthesis of Planar Chiral Rhodium and Iridium Cyclopentadienyl Complexes: Enabling Streamlined and Computer-Guided Access to Highly Selective Catalysts for Asymmetric C–H Functionalizations
Chiral cyclopentadienyl (Cp<sup>X</sup>) metal complexes are frequently used in asymmetric catalysis by virtue of their high reactivity and selectivity. Planar-chiral-only rhodium and iridium cyclopentadienyl complexes are particularly promising due to unrestricted chemical space for Cp&...
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| المؤلف الرئيسي: | |
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| مؤلفون آخرون: | , , , , , , |
| منشور في: |
2024
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| الموضوعات: | |
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| الملخص: | Chiral cyclopentadienyl (Cp<sup>X</sup>) metal complexes are frequently used in asymmetric catalysis by virtue of their high reactivity and selectivity. Planar-chiral-only rhodium and iridium cyclopentadienyl complexes are particularly promising due to unrestricted chemical space for Cp<sup>X</sup> ligand design while retaining structural simplicity. However, they are currently still niche because of a lack of efficient synthetic strategies that avoid lengthy chiral auxiliary routes or chiral preparatory HPLC resolution of the complexes. To streamline access to such planar-chiral-only Cp<sup>X</sup>-metal complexes, we designed a straightforward, highly enantiospecific, point-to-planar chirality transfer complexation via facially selective concerted-metalation-deprotonation between metal-carboxylate precursor [M(olefin)<sub>2</sub>OAc]<sub>2</sub> and a chiral cyclopentadiene. This entirely avoids the typical stereoablative complexation of an achiral cyclopentadienyl anion that detrimentally yields a racemate. Exploiting the described enantiospecific complexation protocol and a simple divergent synthetic route to suitable chiral cyclopentadienes, we generated a structurally diverse library of new planar chiral Cp-Rh(I), Cp-Ir(I), Cp-Rh(III), and Cp-Ir(III) complexes. Moreover, the enantiospecific complexation step can be concatenated with a preceding Au-catalyzed cyclization in an efficient one-pot process that likely involves an elaborate point-to-axial-to-point-to-planar chirality transfer. Guided by computational selectivity predictions, the structure of a Cp<sup>X</sup>-Rh complex in our library was tuned to optimize reactivity and selectivity in the asymmetric C–H functionalization of a benzamide with various challenging alkenes. With an optimized Cp<sup>X</sup>-Rh complex in hand, we showcased its excellent catalytic performance and high selectivity for refractory alkene substrates that reacted in poor selectivity with previous Cp<sup>X</sup>-Rh catalysts. |
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