Chiral-at-Iron Catalyst for Highly Enantioselective and Diastereoselective Hetero-Diels-Alder Reaction.

Abstract

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 β,γ‐unsaturated α‐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.