Bunnai Saito

Ti(salen)-catalyzed enantioselective sulfoxidation using hydrogen peroxide as a terminal oxidant

(R,R)-Di-μ-oxo Ti(salen) 4 was found to serve as an efficient catalyst for asymmetric oxidation of various sulfides with hydrogen peroxide. For example, oxidation of methyl phenyl sulfide by using 4 as the catalyst in the presence of a urea·H2O2 adduct showed high enantioselectivity of 94% ee. The high enantioselectivity of this reaction was considered to be related to the cis-β-structure of a monomeric Ti(salen) species generated from 4 in a methanol–hydrogen peroxide solution.

Highly Enantioselective Hydrophosphonylation of Aldehydes: Base-Enhanced Aluminum–salalen Catalysis†

a-Hydroxy phosphonates and a-hydroxy phosphonic acids are an important class of molecules that are widely used in biological applications. The asymmetric hydrophosphonylation of aldehydes with phosphonates is a powerful and direct method for synthesizing enantioenriched a-hydroxy phosphonates. Thus, intense research has been devoted to developing highly enantioselective catalysts for this reaction, and it is now becoming an emerging area in organic chemistry. A variety of chiral Lewis acid and heterobimetallic catalysts have been reported and high enantioselectivities have been achieved. However, most of these methods require relatively high catalyst loading and a longer reaction time to obtain the products in acceptable yields. Dialkyl phosphonates exist in equilibrium between their phosphite and phosphonate forms. The phosphite form is thought to be the active species; however, under neutral conditions the equilibrium lies predominantly toward the phosphonate form, which leads to sluggish reactivity. Consequently, the facilitation of phosphite–phosphonate tautomerization is essential for achieving hydrophosphonylation with low catalyst loading. For example, Abell and Yamamoto utilized the reactive reagent (CF3CH2)2PO(OH) to achieve a highly enantioselective hydrophosphonylation with only 1 mol% of catalyst. Ooi and co-workers applied chiral triaminoiminophosphoranes as organic base catalysts and achieved high yield and enantioselectivity with low catalyst loading at 98 8C. 8] These results further highlighted the importance of rapid phosphite-phosphonate tautomerization. A simple technique for accelerating the phosphite– phosphonate tautomerization is the deprotonation of phosphonates using a base. However, the hydrophosphonylation of aldehydes is a well-known base-mediated process, and the participation of the base-mediated pathway is a critical problem for the enantioselective reaction. Nevertheless, we believed that a judicial choice of base and catalyst would facilitate the Lewis acid catalyzed asymmetric hydrophosphonylation reaction without eroding the enantioselectivity, presuming that the trapping of the phosphite anion by the catalyst and release of the hydrophosphonylation product could proceed rapidly enough for the catalytic process to exclusively occur before the non-catalytic process (Scheme 1). Herein, we report that inorganic bases significantly enhance the rate of reaction of the Al(salalen)catalyzed asymmetric hydrophosphonylation of aldehydes, in which high enantioselectivities ranging from 93 to 98 % ee were achieved for the reactions of both conjugated and nonconjugated aldehydes.

Highly enantioselective oxidation of cyclic dithioacetals by using a Ti(salen) and urea.hydrogen peroxide system

Abstract Asymmetric oxidation of 2-substituted 1,3-dithianes using a Ti(salen) and urea·hydrogen peroxide system in methanol was found to proceed with high enantioselectivity to give the corresponding mono-sulfoxides.

Mechanistic consideration of Ti(salen)-catalyzed asymmetric sulfoxidation

Abstract Asymmetric sulfoxidation using di-μ-oxo Ti(salen) complex 2 as the catalyst was revealed to proceed through C 2 -symmetric Ti(salen) complex 6 and peroxo Ti(salen) complex 4 in sequence on the basis of MS and NMR studies of the complexes and the observation of positive non-linear effect.

Asymmetric catalysis of metal complexes with non-planar ONNO ligands: salen, salalen and salan

Chiral metal (M)-salen complexes are one of the most versatile asymmetric catalysts and the catalysis of trans-M(salen) complexes has been well cultivated. On the other hand, non-planar cis-beta M(salen) complexes were recently found to show unique asymmetric catalysis that cannot be attained by trans-M(salen) complexes. Moreover, related non-planar M(salalen) and M(salan) complexes were also found to exert unprecedented asymmetric catalysis. This Feature Article summarizes the seminal studies on asymmetric catalysis of non-planar M(ONNO) complexes, full utilization of which will provide marked improvement in asymmetric synthesis.