Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 27, Issue 33, Pages 8557-8563Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.202100703
Keywords
asymmetric catalysis; chiral-at-metal; hetero-Diels-Alder; iron; stereogenic metal
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Funding
- European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme [883212]
- Projekt DEAL
- European Research Council (ERC) [883212] Funding Source: European Research Council (ERC)
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This study introduces a new chiral-at-iron catalyst that can achieve high enantioselectivities in asymmetric reactions by increasing steric hindrance at the catalytic site. The catalyst was applied to the inverse electron demand hetero-Diels-Alder reaction and showed excellent diastereoselectivities and enantioselectivities. Other electron rich dienophiles, such as vinyl azide, are also suitable for this catalyst.
This study demonstrates that chiral-at-iron complexes, in which all coordinated ligands are achiral and the overall chirality the consequence of a stereogenic iron center, are capable of catalyzing asymmetric transformations with very high enantioselectivities. The catalyst is based on a previously reported design (J. Am. Chem. Soc. 2017, 139, 4322), in which iron(II) is surrounded by two configurationally inert achiral bidentate N-(2-pyridyl)-substituted N-heterocyclic carbenes in a C-2-symmetric fashion and complemented by two labile acetonitriles. By replacing mesityl with more bulky 2,6-diisopropylphenyl substituents at the NHC ligands, the steric hindrance at the catalytic site was increased, thereby providing a markedly improved asymmetric induction. The new chiral-at-iron catalyst was applied to the inverse electron demand hetero-Diels-Alder reaction between beta,gamma-unsaturated alpha-ketoester and enol ethers provide 3,4-dihydro-2H-pyrans in high yields with excellent diastereoselectivities (up to 99 : 1 dr) and excellent enantioselectivities (up to 98 % ee). Other electron rich dienophiles are also suitable as demonstrated for a reaction with a vinyl azide.
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