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A Practical and Versatile Access to Dihydrosalen (Salalen) Ligands: Highly Enantioselective Titanium In Situ Catalysts for Asymmetric Epoxidation with Aqueous Hydrogen Peroxide
Author(s) -
Berkessel Albrecht,
Brandenburg Marc,
Leitterstorf Eva,
Frey Julia,
Lex Johann,
Schäfer Mathias
Publication year - 2007
Publication title -
advanced synthesis and catalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.541
H-Index - 155
eISSN - 1615-4169
pISSN - 1615-4150
DOI - 10.1002/adsc.200700221
Subject(s) - chemistry , enantioselective synthesis , catalysis , ligand (biochemistry) , aldehyde , combinatorial chemistry , imine , epoxide , hydrogen peroxide , kinetic resolution , titanium , denticity , chiral ligand , schiff base , polymer chemistry , organic chemistry , biochemistry , receptor , crystal structure
A modular synthesis for chiral and non‐symmetrical salalen ligands (i.e., mono‐reduced salens), carrying an imine and an amine functionality has been developed. The ligands can be assembled in situ by Schiff base formation of an N ‐(2‐hydroxybenzyl)diamine with a salicylic aldehyde, thus allowing rapid and easy variation/optimization of the ligand structure. Aiming at optimal activity and enantioselectivity in the titanium‐catalyzed asymmetric epoxidation of non‐functionalized olefins, a positional scanning of the two ligand halves was carried out. High epoxide yields were achieved, and up to 98 % ee . The composition of the titanium complex catalysts was determined by high resolution mass spectrometry and X‐ray crystallography for one selected example.