Unsymmetric N-Aryl Substituent Effects on Chiral NHC-Cu: Enantioselectivity and Reactivity Enhancement by Ortho-H and Syn-Configuration

The N-aryl substituent effects on chiral C1 NHC's configuration and noncovalent interactions were studied by Cu-catalyzed borylation. An inconspicuous N-aryl ortho-H, which can be found in many chiral C1 NHC designs, is now explored as a nonclassical hydrogen bonding site to achieve higher chiral induction ability. According to density functional theory (DFT) calculations, it cooperates with other noncovalent interactions, resulting in an unusual dual maxima shape e.r.-Hammett plot. A 2,6-unsymmetric substituted N-aryl on the other side also allowed for better chiral induction ability (similar to 3-f old e.r. increase) and higher relative reactivity (increase to 20:1). A catalyst with a syn- configuration between the NHC core Ph and the bulkier N-aryl o-substituent, which is supported by the crystal structures, the DFT calculations, and nuclear magnetic resonance (NMR), was found to be more effective than the more regularly hypothesized catalyst with an anti-configuration. A well-defined chiral NHC-Cu catalyst was then developed for highly enantioselective B-substituted quaternary center synthesis, which is effective for both E-/Z-enones (up to 98% e.e.) and substitution patterns with limited steric differences at beta-positions.