An unprecedented and general hydroboration of alkenes with BX3 (X=Br, Cl) as the boration reagent in the presence of i Pr2 NEt is reported. The addition of i Pr2 NEt not only suppresses alkene polymerization and haloboration side reactions but also provides an "H" source for hydroboration. More importantly, the site-fixed installation of a boryl group at the original position of the internal double bond is readily achieved in contrast to conventional transition-metal-catalyzed hydroboration processes. Further application to the synthesis of 1,n-diborylalkanes (n=3-10) is also demonstrated. Preliminary mechanistic studies reveal a major reaction pathway that involves radical species and operates through a frustrated Lewis pair type single-electron-transfer mechanism.
Keywords: alkenes; frustrated Lewis pairs; hydroboration; single-electron transfer.
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