Recent Advances in the Creation of Asymmetric Carbon Centre(s) by Generation of Carbon‐Heteroatom Bond(s) Using Metal‐Pybox Complexes

Abstract

Asymmetric synthesis is pivotal in modern-day organic synthesis in which metal-catalyzed protocols with compatible ligands play a key role. Many metal-chiral ligand combinations have been extensively reported in which "pybox" as the chiral ligand has occupied an indisputable place owing to its widespread use. This review focuses on the reactions of various metal-pybox combinations for the creation of asymmetric carbon centres by carbon-heteroatom bond manipulations. For brevity and simplicity, these reactions are classified based on the metal used and the literature from 2006-2018 is covered. Scheme 1. Stereoselective allylic oxidation of cyclic olefins using Cu-pybox catalyst. . Cu-Pybox catalyzed propargylic amination of propargylic acetates by aryl amines. lane was used in combination with L 18 (5 mol%) at low temperature (4°C).Use of various alkyl aryl ketones afforded the corresponding products with high ee value. For ketones with substituents at the meta-position of the aromatic ring, EWG's were found to result in higher yield than EDG's. In the case of parasubstituents, EWGs did not have much influence on the ee value. Again para-methoxyacetophenone was found to result in the formation of racemic alcohol. For ortho-substituents, the ee was found to reduce due to steric reasons. 2 3 4 5 6 7 8

Chiral Tridentate-Based Ligands

“…The first successful family of chiral tridentate ligands belong to the Pybox family (Pybox for pyridinebis(oxazoline), see Figure 5.1), 1,2,3 introduced in 1989 by Nishiyama et al, 4 which have been successfully used in various reactions such as the Mukaiyama aldol reaction, in particular with Cu(II) catalysts, with main contributions from Evans and coworkers, 5 the hydrosilylation of ketones mainly in combination with Rh(III), 4,6,7 the cyclopropanation with Ru(II), 8 the allylic oxidation with Cu(II), 9,10 the Dields-Alder reaction with Sc(III) 11,12,13 and several others. Pybox are now well established and privileged ligands, regularly used to develop new catalytic systems.…”

Synthesis, Structures, and Reactivities of Iron Complexes Bearing an Isoindoline‐Based, Polyprotic Pincer‐Type Pyrazole Ligand

“…The meridional pincer ligands have been prominent scaffolds for bond activation, homogeneous catalysis, and supramolecular chemistry [1,2] . Typical examples include 2,2′:6′,2“‐terpyridines [3] and 2,6‐bis(oxazolinyl)pyridines [4] as tri(nitrogen)‐donors. Unlike these electronically neutral NNN tridentate ligands, 1,3‐bis(2‐pyridylimino)isoindolines (BPI), readily accessible from phthalonitrile, bind to the metal in both neutral 1 H ‐isoindole and monoanionic isoindolin‐2‐yl forms owing to facile deprotonation of the central pyrrole NH group and tautomerization (Scheme 1).…”

Rational Design of Chiral Tridentate Ligands: Bifunctional Cobalt(II) Complex/Hydrogen Bond for Enantioselective Michael Reactions

“…The design and synthesis of highly efficient and selective chiral ligands is the most attractive and challenging goal in asymmetric catalysis . Since Nishiyama’s pioneering work, in which Pybox ligands were successfully developed, chiral tridentate nitrogen ligands containing a pyridine ring have gained considerable attention due to their deeper chiral concave pocket and rich coordination chemistry . Notable ligands including chiral terpyridine, Pybim, BPI, PyBidine, BIP, IPO, and PyBTHQ have been successively reported and widely applied in many catalytic asymmetric reactions.…”