✍ Scribed by Tomoaki Hamada; Kei Manabe; Shū Kobayashi
No coin nor oath required. For personal study only.
Catalytic asymmetric Mannich‐type reactions of an α‐hydrazono ester with silicon enolates in aqueous media have been developed by using ZnF~2~ and chiral diamines as catalysts. In these reactions, both Zn^2+^ and a fluoride anion were necessary to achieve high yields and enantioselectivities, suggesting a double activation mechanism, in which Zn^2+^ activates the α‐hydrazono ester and the fluoride anion simultaneously activates the silicon enolate. When chiral diamine ligands bearing methoxy‐substituted aromatic rings were employed, the reactions in aqueous THF were markedly accelerated. Furthermore, the use of these diamines facilitated the asymmetric Mannich‐type reactions in water without any organic cosolvents. It is noteworthy that either syn or anti adducts were stereospecifically obtained from (E)‐ or (Z)‐silicon enolates, respectively. Interestingly, these reactions proceeded smoothly only in the presence of water. On the basis of several experimental results, it can be concluded that the reaction mechanism is likely to be a fluoride‐catalyzed one, in which the ZnF~2~ chiral diamine complex is regenerated from the Me~3~SiF formed during the reaction.
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