Construction of Pseudo‐Heterochiral and Homochiral Di‐μ‐oxotitanium(Schiff base) Dimers and Enantioselective Epoxidation Using Aqueous Hydrogen Peroxide

Abstract

Remarkable advances in asymmetric synthesis using optically active metal complexes as catalysts have been achieved in the last half century.[1] Monometallic complexes are used as the catalysts in most of the asymmetric reactions developed so far. Particularly in the last two decades, it has been revealed that chiral metallosalen complexes result in diverse and excellent asymmetric catalysis.[2] Chiral salen ligands can be synthesized in a single step from chiral diamine and chiral and/or appropriately substituted salicylaldehydes and form complexes with a variety of metal ions. Moreover, metallosalen complexes are conformationally flexible as a result of the presence of two methylene carbon atoms and they can adopt three different configurations (trans, cis-a, and cis-b). Metallosalen complexes usually adopt trans configurations, but they can be readily transformed in the presence of a bidentate ligand into the corresponding cis-b complexes. [2,3] Thus, various chiral metallosalen complexes have been synthesized and used as catalysts for a wide range of asymmetric reactions. Most of the metallosalen complexes used are monomeric, but for some reactions, depending on their mechanisms, dimeric or oligomeric metallosalen complexes have proven to be superior catalysts to the corresponding monomeric ones.[4] Nevertheless, the use of such dimeric or oligomeric metallosalen complexes as catalysts has been limited mainly because their synthesis involves laborious routes, except for metallosalen complexes prepared by selfassembly. Thus, di-m-oxotitanium(salen) complexes that can be prepared spontaneously by treatment of monomeric [Ti(salen)] complexes with water, [5] attracted our attention. [6] Furthermore, in contrast to the usual monomeric [Ti(salen)] complexes that adopt trans configurations, each {Ti(salen)} unit of the di-m-oxo complexes take a cis-b configuration. In contrast to the trans isomer, the cis-b isomer is chiral and exists in enantiomeric forms (D or L; Figure 1). Thus, there are six possible isomers for the di-m-oxotitanium(salen) dimer

Transition Metal‐Catalyzed Epoxidation of Alkenes

“…However, recently the groupof Katsuki reported on a significant breakthrough in the field of titanium-catalyzed asymmetric epoxidation of nonfunctionalized alkenes [11][12][13]. The successful catalysts used in this system are based on a combination of titanium and reduced salen-type ligands (also known as salenan and salan, respectively).…”

Asymmetric Oxidations and Related Reactions

“…Katsuki and coworkers identifi ed a chiral titanium complex as a catalyst for asymmetric olefi n epoxidation [37] . They found that di -μ -oxo Ti(salalen) complex 18 (salalen = salen/ salan hybrid ONNO -type tetradentate ligand), which is readily prepared from Ti(O i Pr) 4 and the corresponding salen ligand 19 via an intramolecular Meerwein -PonndorfVerley reduction, effi ciently promotes the epoxidation of unfunctionalized olefi ns in the presence of one equivalent of 30% hydrogen peroxide as the oxidant (Scheme 11.21 ).…”

Asymmetric Oxidation with Hydrogen Peroxide, an Effective and Versatile Oxidant

“…In the course of this study, another new (aR,R)-di-µ-oxo titanium complex was isolated and found to show unique epoxidation catalysis using a urea-hydrogen peroxide adduct [36]. X-ray analysis of the complex disclosed that it carries a salalen ligand that has one imino nitrogen atom and one amino nitrogen atom, and coordinates to a titanium ion in a cis-β configuration.…”