Homoallenylboration of carbonyl compounds using inert 2-pinacolateboryl 1,3-butadienes via in situ generated borinic–TFA mixed anhydrides: efficient synthesis of homoallenyl alcohols

Abstract

A novel homoallenylboration reaction for the efficient and practical synthesis of valuable homoallenyl alcohols is presented. This reaction is initiated by the in situ activation of relatively inert 2-pinacolateboryl 1,3-butadienes via the sequential addition of 1.25 equiv. of ⁿBuLi and the requisite 2.0 equiv. of TFAA. Under the optimized reaction conditions, various 2-pinacolateboryl 1,3-butadienes react with different aldehydes to deliver the corresponding homoallenyl alcohols in moderate to high yields with excellent regioselectivity. Furthermore, the homoallenylboration of ketones can be extended to synthesize precious homoallenyl tertiary alcohols for the first time. Monitoring the reaction by ¹H/¹¹B NMR and high-resolution MS spectroscopy indicated that the reactive boron intermediate in the homoallenylboration is a borinic–TFA mixed anhydride, which is generated by a further borinic ester–TFAA exchange reaction of the in situ formed borinic ester with excess TFAA in the reaction.