ORGANOBORANES FOR SYNTHESIS .15. B-ALLENYL-9-BBN - A HIGHLY REGIOSPECIFIC AND CHEMOSELECTIVE REAGENT FOR ALLENYLBORATION OF REPRESENTATIVE CARBONYL-COMPOUNDS, LEADING TO HOMOPROPARGYLIC ALCOHOLS AND AMINES

Reactions of B-allenyl-9-BBN (11) with representative carbonyl compounds, such as aldehydes, ketones, acid chlorides, carboxylic acid esters, and imines, proceed cleanly with the allenyl moiety undergoing transfer to the carbonyl or imine carbon, with allenic to propargylic rearrangement, and the boron moiety to the carbonyl oxygen or imine nitrogen. A simple oxidation with alkaline hydrogen peroxide results in the formation of the corresponding homopropargylic alcohols and amines in excellent yields. Aldehydes, ketones, and imines react in a 1:1 stoichiometry, while acid chlorides and carboxylic acid esters react with 2 equiv of the reagent. The relative reactivities of representative aldehydes, ketones, and esters toward 11 were also explored. Besides being highly regiospecific, the reagent 11 also possesses a remarkable chemoselectivity. B-Allenyl-9-BBN can distinguish between less and more sterically hindered aldehydes, ketones, and esters, making possible the clean and selective propargylboration of a desired carbonyl group in complex organic molecules containing less reactive functional groups.