A Practical and Versatile Access to Dihydrosalen (Salalen) Ligands: Highly Enantioselective Titanium In Situ Catalysts for Asymmetric Epoxidation with Aqueous Hydrogen Peroxide
✍ Scribed by Albrecht Berkessel; Marc Brandenburg; Eva Leitterstorf; Julia Frey; Johann Lex; Mathias Schäfer
- Publisher
- John Wiley and Sons
- Year
- 2007
- Tongue
- English
- Weight
- 414 KB
- Volume
- 349
- Category
- Article
- ISSN
- 1615-4150
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✦ Synopsis
Abstract
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.