Catalytic Asymmetric Baeyer–Villiger Oxidation in Water by Using Pt^II Catalysts and Hydrogen Peroxide: Supramolecular Control of Enantioselectivity

Abstract

The enantioselective Baeyer-Villiger oxidation of cyclic ketones is a challenging reaction, especially when using environmentally friendly oxidants. The reaction was carried out in water by using soft Lewis acid Pt(II) complexes that have chiral diphosphines as well as monophosphines. Addition of a surfactant is crucial, which leads to the formation of micelles that act as nanoreactors in which the substrate and catalyst encounter each other in an ordered medium that in several cases positively influences both the conversion and the selectivity of the reactions. This is due to the combination of the hydrophobic effect (which confines the components of the reaction in the micelles), together with supramolecular interactions between the partners within the ordered palisade provided by the alkyl chains of the surfactant. For the oxidation of meso-cyclobutanones, addition of surfactant allowed the reaction to proceed in high yields and the enantiometic excess (ee; 56%) was higher than in organic solvents. Subsequent extension to meso-cyclohexanones resulted in a general decrease in yields but an enhancement of enantioselectivity (ee up to 92%) moving from organic to water-surfactant media, regardless of the substrate or the catalyst employed. Different behaviour was observed with chiral cyclobutanones 7 and 10: with 7 the best catalyst was 1 g, whereas with the larger substrate, 10, complexes 1 a-b performed better in terms of enantioselectivity. Each combination of substrate, catalyst and surfactant is a new system and supramolecular reciprocal interactions together with the hydrophobic character of the counterparts play crucial roles. The asymmetric Baeyer-Villiger oxidation in water catalyzed by 1 a-h in the presence of micelles is a viable reaction that often benefits from the hydrophobic effect, leading to substantial increases in enantioselectivity.

Baeyer–Villiger oxidation under Payne epoxidation conditions

“…26 Metal-free methods also exist and make use of enzymes, 27 ionic liquids, 28 carboxylic acids, 29 phosphoric acids, 30,31 and bisflavins 32 to activate the peroxide. Of these methods, a few have been shown to be effective for enantioselective BV oxidations, which include metals such as Pt, Pd or Zr with chiral ligands, 21,24,26 pro-chiral carboxylic acids, 29 phosphoric acids, 30,31 and bisflavins. 32 While there are a variety of methods of activating hydrogen peroxide, it is still desirable to develop methods to broaden the transformation, specifically by defining new pathways to potential asymmetric oxidations.…”

“…3 Many activation methods involve the use of abundant Sn catalysts, the most efficient being Sn-zeolites. [10][11][12][13][14][15][16][17] Other metals used for the activation of hydrogen peroxide in the BV oxidation include Se, 18 Re, 19 Li and Ca borates, 20 Pt, [21][22][23] Pd, 24 Si polyoxometalates, 25 and Zr. 26 Metal-free methods also exist and make use of enzymes, 27 ionic liquids, 28 carboxylic acids, 29 phosphoric acids, 30,31 and bisflavins 32 to activate the peroxide.…”

Creating and mastering nano-objects to design advanced catalytic materials

“…An example is the enantioselective Baeyer-Villiger oxidation of cyclic ketones using environmentally friendly oxidants [93]. The reaction was carried out in water by using soft Lewis acid Pt(II) complexes that have chiral diphosphines as well as monophosphines.…”

Oxidation of Carbonyl Compounds

“…Recently, an efficient system for the oxidation of 4-substituted cyclohexanones (3) to lactones (4) was reported by Strukul and coworkers [55], in which they use a chiral enantiopure Pt(II) catalyst in water with added surfactant and hydrogen peroxide as the terminal oxidant (up to 92% ee), (Eq. (10.6)).…”