1. "On Water" Direct Catalytic Vinylogous Aldol Reaction of Silyl Glyoxylates
Hong Pan, Man-Yi Han, Pinhua Li, Lei Wang J Org Chem. 2019 Nov 1;84(21):14281-14290. doi: 10.1021/acs.joc.9b01945. Epub 2019 Oct 16.
The unique reactivity of water in the direct catalytic vinylogous aldol reaction of silyl glyoxylates is reported. With the hydrogen-bonding networks from water, the unfavorable homogeneous reactions in organic solvents were severely suppressed, and the "on water" relay chemistry for the vinylogous aldol reaction was realized in heterogeneous conditions (water as solvent), providing the desired α-hydroxysilanes with a quaternary carbon center in high yields. Moreover, new insights on the role of water in biological systems were demonstrated.
2. Organocatalyzed highly enantioselective direct aldol reactions of aldehydes with hydroxyacetone and fluoroacetone in aqueous media: the use of water to control regioselectivity
Xiao-Hua Chen, Shi-Wei Luo, Zhuo Tang, Lin-Feng Cun, Ai-Qiao Mi, Yao-Zhong Jiang, Liu-Zhu Gong Chemistry. 2007;13(2):689-701. doi: 10.1002/chem.200600801.
An organocatalyst prepared from (2R,3R)-diethyl 2-amino-3-hydroxysuccinate and L-proline exhibited high regio- and enantioselectivities for the direct aldol reactions of hydroxyacetone and fluoroacetone with aldehydes in aqueous media. It was found that water could be used to control the regioselectivity. The presence of 20-30 mol% of the catalyst afforded the direct aldol reactions of a wide range of aldehydes with hydroxyacetone to give the otherwise disfavored products with excellent enantioselectivities, ranging from 91 to 99% ee, and high regioselectivities. Aldolizations of fluoroacetone with aldehydes mediated by 30 mol% of the organocatalyst in aqueous media preferentially occurred at the methyl group, yielding products with high enantioselectivities (up to 91% ee); however, these additions took place dominantly at the fluoromethyl group in THF. Optically active 3,5-disubstituted tetrahydrofurans and (2S,4R)-dihydroxy-4-biphenylbutyric acid were prepared by starting from the aldol reaction of hydroxyacetone. Theoretical studies on the role of water in controlling the regioselectivity revealed that the hydrogen bonds formed between the amide oxygen of proline amide, the hydroxy of hydroxyacetone, and water are responsible for the regioselectivity by microsolvation with explicit one water molecule as a hydrogen-bond donor and/or an acceptor.
3. Organocatalytic Asymmetric Vinylogous Aldol Reaction of Allyl Aryl Ketones to Silyl Glyoxylates
Man-Yi Han, Wen-Yu Luan, Pei-Lin Mai, Pinhua Li, Lei Wang J Org Chem. 2018 Feb 2;83(3):1518-1524. doi: 10.1021/acs.joc.7b02546. Epub 2018 Jan 16.
A direct organocatalytic asymmetric vinylogous aldol reaction of allyl aryl ketones to silyl glyoxylates has been developed through the bifunctional catalyst, giving the α-hydroxysilanes with excellent enantioselectivity (up to 95% ee) and in high yields (up to 96%). The success of this catalytic methodology offers an opportunity to tackle the problems in the nucleophilic addition to acylsilanes. To activate both allyl aryl ketones and acylsilanes, the utilized bifunctional catalyst was an ideal organocatalyst in this unprecedented transformation.