Cavarzan Alessandra, Bianchini Giulio, Sgarbossa Paolo, Lefort Laurent, Gladiali Serafino, Scarso Alessandro, Strukul Giorgio
Dipartimento di Chimica, Università Ca' Foscari di Venezia, Dorsoduro 2137, 30123 Venezia (Italy), Fax: (+39) 041-2348517.
Dipartimento di Processi Chimici dell'Ingegneria, Università degli Studi di Padova, Via F. Marzolo 9, 35131 Padova (Italy).
Chemistry. 2009 Aug 10;15(32):7930-7939. doi: 10.1002/chem.200900294.
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.
环状酮的对映选择性拜耳-维利格氧化反应是一个具有挑战性的反应，尤其是在使用环境友好型氧化剂时。该反应在水中进行，使用具有手性二膦以及单膦的软路易斯酸Pt(II)配合物。添加表面活性剂至关重要，这会导致形成胶束，其充当纳米反应器，底物和催化剂在有序介质中相互接触，在几种情况下，这种有序介质对反应的转化率和选择性都有积极影响。这是由于疏水效应（将反应组分限制在胶束中）与表面活性剂烷基链提供的有序栅栏内的伙伴之间的超分子相互作用共同作用的结果。对于内消旋环丁酮的氧化，添加表面活性剂使反应能够高产率进行，对映体过量值（ee；56%）高于在有机溶剂中的情况。随后将该反应扩展到内消旋环己酮，结果是产率普遍降低，但从有机介质转变为水-表面活性剂介质时，对映选择性提高（ee高达92%），无论使用何种底物或催化剂。对于手性环丁酮7和10观察到了不同的行为：对于7，最佳催化剂是1 g，而对于较大的底物10，配合物1 a - b在对映选择性方面表现更好。底物、催化剂和表面活性剂的每一种组合都是一个新系统，超分子相互作用以及对应物的疏水特性起着关键作用。在胶束存在下由1 a - h催化的水中不对称拜耳-维利格氧化反应是一个可行的反应，该反应常常受益于疏水效应，导致对映选择性大幅提高。
