Asymmetric allylboration of aldehydes is an extremely important method for preparation of homoallylic alcohols, as evidenced in numerous complex natural product syntheses.1 Although a variety reagents have been developed to execute this reaction, the most broadly adopted have been Brown’s α-pinene-derived (–)- and (+)-β-allyldiisopinocampheylboranes (i.e., Ipc2B(allyl)).3 Typically the allylboration reagents are generated in situ and used immediately at –78 to –100 °C, since they are not stable for extended periods at ambient temperature (Scheme 1).
Scheme 1.Asymmetric Allylboration
New, salt-free, Ipc2B(allyl) reagents have been developed as stable solutions in pentane or dioxane. Under refrigerator storage, no appreciable decreases in selectivity are observed after several months. They exhibit excellent reactivity and reactions can be performed at 5 °C, even in water.
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