Cobalt Alkyl Complexes of a New Family of Chiral 1,3-Bis(2-pyridylimino)isoindolates and Their Application in Asymmetric Hydrosilylation

Abstract

The synthesis of a new family of chiral tridentate monoanionic NNN-pincer ligands based on the 1,3-bis(2-pyridylimino)isoindoline (BPI) framework is reported. Ligands with substituents of varying steric demand were prepared starting from achiral and low priced materials. A kinetic enzymatic resolution was used as a key step for the preparation of enantiomerically pure ligands. In this way, both enantiomers of a given ligand could be produced enantioselectively (>99.5% ee). The corresponding cobalt alkyl complexes were obtained using a pyridine alkyl cobalt precursor complex and were applied in asymmetric hydrosilylation of several prochiral alkylaryl ketones with high yields (up to 100%) and enantioselectivity (up to 91% ee) to give the chiral alcohols after hydrolysis.

Isoindoline-derived ligands and applications

“…Phthalocyanines or REVIEW RSC Advances 2 | J. 13,[36][37][38] The preparation of BIIs by all known methods is a two-step procedure. 24 35 To obtain any optically active BII is only one step away.…”

“…13,37 The variety of asymmetric test reactions has been extended to the fluorination of oxindoles and β-ketoesters with Ni-chiBIIs. Usually, chiral pincer type ligands perform poorly in enantioselective catalysis but with careful design on the stereodirecting unit and with protection from backside attack to the metal center remarkable results have been obtained.…”

New Chemistry with Anionic NNN Pincer Ligands

“…Bis(2-pyridylimino)isoindole (BPI) ligands were first reported over 60 years ago [30][31][32] and are examples of tridentate pincer ligands which have found applications in catalysis and materials science [33]. The current preferred method of synthesis involves the reaction of commercially available phthalonitrile with 2-aminopyridines in a high boiling alcohol solvent in the presence of CaCl 2 (Scheme 5) [34][35][36][37][38].…”

“…The asymmetric hydrosilylation of ketones using metal-BPI (M¼Fe, Co) and Fe (OAc) 2 /BOPA catalytic systems is an effective method for the synthesis of chiral alcohols following hydrolysis (Scheme 21) [37,38,[102][103][104]. While the use of BPI-Co and BPI-Fe complexes resulted in hydride transfer to the Si-face to give the R-alcohol, the stereochemistry of the product with BOPA ligands was dependent upon the presence (or absence) of zinc powder.…”

Chromophores, Fluorophores and Robust Ancillary Ligands for Molecular Catalysts: 1,3‐Bis(2‐pyridylimino)isoindolines

“…Chiral pyridyl substituents appear to be more useful for preparing chiral ligands because of the close spatial proximity of the chiral centre to the metal atom. So far there have been only three reports on chiral BPI ligands 45,46. Two focus on their synthesis starting from readily accessible terpenoids out of the chiral pool to gain access to ligands with maximum synthetic variability, whereas the third one involves a more general approach to the preparation of BPI ligands in both enantiomeric forms through enantioselective functionalisations 46…”