Catalytic asymmetric Baeyer-Villiger oxidation in water by using Pt(II) catalysts and hydrogen peroxide: supramolecular control of enantioselectivity.
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.
Cavarzan
Alessandra
A
Dipartimento di Chimica, Università Ca' Foscari di Venezia, Dorsoduro 2137, 30123 Venezia (Italy), Fax: (+39) 041-2348517.
Bianchini
Giulio
G
Dipartimento di Chimica, Università Ca' Foscari di Venezia, Dorsoduro 2137, 30123 Venezia (Italy), Fax: (+39) 041-2348517.
Sgarbossa
Paolo
P
Dipartimento di Processi Chimici dell'Ingegneria, Università degli Studi di Padova, Via F. Marzolo 9, 35131 Padova (Italy).
Lefort
Laurent
L
DSM Research, Innovative Synthesis and Catalysis, PO Box 18, 6160 MD Geleen (The Netherlands).
Gladiali
Serafino
S
Dipartimento di Chimica, Università di Sassari, Via Vienna 2, 07100 Sassari (Italy).
Scarso
Alessandro
A
Dipartimento di Chimica, Università Ca' Foscari di Venezia, Dorsoduro 2137, 30123 Venezia (Italy), Fax: (+39) 041-2348517.
Strukul
Giorgio
G
Dipartimento di Chimica, Università Ca' Foscari di Venezia, Dorsoduro 2137, 30123 Venezia (Italy), Fax: (+39) 041-2348517.