Iron(II) and Zinc(II) Complexes with Designed pybox Ligand for Asymmetric Aqueous Mukaiyama-Aldol Reactions

Abstract

An iron(II) complex with a hindered hydroxyethyl-pybox (he-pybox) ligand shows improved catalytic activity and enantioselectivity for asymmetric Mukaiyama-aldol reactions in aqueous media. This water-stable chiral Lewis acid promotes condensation of aromatic silyl enol ethers with a range of aldehydes with good yields, excellent syn-diastereoselectivity and up to 92% ee. The combination of the same ligand with ZnII salt is also demonstrated as a remarkably efficient and water-compatible chiral Lewis acid.

Aerobic oxidation of secondary alcohols using NHPI and iron salt as catalysts at room temperature

“…However, because oxygen molecule cannot be activated directly by NHPI in the absence of co-catalyst at room temperature, a co-catalyst is required for this catalytic reaction. Iron has a number of advantages over other transition metals for it's relatively non-toxic, cheap and environmentally friendly [27], and iron-based catalyst systems have been applied in a variety of organic transformations [28][29][30][31][32][33][34][35][36], especially in some oxidation reactions using oxygen as oxidant [37][38][39]. Therefore, we supposed iron salts have a potential as good co-catalysts in NHPI catalytic system and applied NHPI and iron salts as the catalytic system to the oxidation of alcohols.…”

Iron-Catalyzed Enantioselective Reactions Through the Use of Chiral Bipyridine-Containing Ligands

“…Ratio of cyclopropanes to coupling products. Aliphatic aldehydes, which often lead to decreased enantioselectivities with other chiral catalysts, [14][15][16][17] were found to undergo aldolization in high enantioselectivity (Table 4, entries [19][20][21]. Noteworthy that an aldehyde bearing a free alcohol can even be used without the necessity of a preliminary protection step ( A silyl enol ether derived from an aliphatic ketone was also used ( Table 5).…”

Asymmetric Hydrosilylation of Ketones Catalyzed by Zinc Acetate with Hindered Pybox Ligands

“…Enantioselective Hydrosilylation of Prochiral Ketones (Table 3) The catalyst was prepared by stirring the solution of TPShe-pybox [9,10] (5, 0.043 mmol, 33.2 mg) with ZnA C H T U N G T R E N N U N G (OAc) 2 (0.043 mmol, 7.6 mg) in 2 mL of THF under argon at room temperature. To the homogenous solution of the catalyst both ketone (1 mmol) and silane (2 mmol) were added with 15 min intervals, respectively.…”

“…ZnA C H T U N G T R E N N U N G (OTf) 2 -TPS-he-pybox has been previously used to catalyze asymmetric Mukaiyama-aldol reactions carried out in a water environment, but has never been applied for the asymmetric reduction of ketones. [9] Initial optimization of the reaction conditions was carried out for commercially available i-Pr-pybox (1). From among the tested zinc salts (acetate, chloride, bromide, triflate and trifluoroacetate) applied for the reduction of acetophenone, only zinc acetate complexes have shown the expected high activity in THF solution with diethoxymethylsilane as hydrogenating agent (Table 1).…”