Direct Synthesis of B-Allyl and B-Allenyldiisopinocampheylborane Reagents Using Allyl or Propargyl Halides and Indium Metal Under Barbier-Type Conditions

Abstract

We report the first one-pot process for the asymmetric addition of allyl, methallyl, and propargyl groups to aldehydes and ketones using B-chlorodiisopinocampheylborane ((d)DIP-Cl) and indium metal. Under Barbier-type conditions, indium metal was used to generate allyl- and allenylindium intermediates, and subsequent reaction with (d)DIP-Cl successfully promoted the transfer of these groups to boron forming the corresponding chiral borane reagents. The newly formed borane reagents were reacted with aldehydes and ketones to produce the corresponding alcohol products in high yields and up to excellent enantioselectivity (98% ee). This method produced excellent enantioenriched secondary homoallylic alcohols from the allylation and methallylation of benzaldehyde. Using this method, the methallylation and cinnamylation of ketones afforded the highest enantioselectivities, while the propargylation of both aldehydes and ketones provided low enantiomeric excesses. In addition, this procedure provided the first synthesis of B-allenyldiisopinocampheylborane, which was characterized by (1)H and (11)B NMR spectroscopy. This is the first example of the direct synthesis of allylboranes that contained substitutions from the corresponding allyl bromide and indium, thereby expanding the utility of the DIP-Cl reagent. Hence, a general and straightforward route to these chiral organoborane reagents in one-pot has been developed along with the asymmetric Barbier-type allylation and propargylation of aldehyde and ketone substrates using these chiral organoborane reagents in subsequent coupling reactions.