Chiral synthesis via organoboranes .44. Racemic and diastereo- and enantioselective homoallenylboration using dialkyl 2,3-butadien-1-ylboronate reagents. Another novel application of the tandem homologation-allylboration strategy

A highly general and efficient racemic and diastereo- and enantioselective homoallenylboration has been achieved with a novel boron reagent, dialkyl 2,3-butadien-1-ylboronate (dialkyl homoallenylboronate). The starting diisopropyl 2,3-butadien-1-ylboronate is prepared from allenylmagnesium bromide and diisopropyl (halomethyl)boronate. This beta,gamma-unsaturated boronate reagent reacts readily with aldehydes via the usual allylic rearrangement to give the alkyl(1,3-butadien-2-yl)methanols in excellent yield. Among the solvents examined, toluene favored enhanced reaction rates. The reaction is relatively sensitive to steric effects, so that sterically hindered aldehydes react significantly slower. Generally, the reactivities are moderately lower than those of the corresponding simple allylboronates, possibly due to the reduced HOMO electron density of the internal double bond. Also, the reagent exhibits a unique anti diastereoselectivity in reaction with alpha-chiral aldehydes in contrast to the syn selectivity observed with a corresponding organosilicon reagent. However, this anti selectivity is similar to that observed for the allylboration reactions. We have successfully extended this reaction to the first general synthesis of optically active alkyl(1,3-butadien-2-yl)methanols using chiral tartrate boronate reagents with diisopropyl tartrate (DIPT) and bis(2,4-dimethyl-3-pentyl) tartrate (DMPT) as the chiral modifiers. These reagents react with aldehydes even at -78 degrees C, albeit slowly, and exhibit remarkable enantioselectivity with all classes of aldehydes examined with the exception of aromatic and alpha-alkoxy aldehydes. Again, while the selectivity parallels that of the corresponding allylboronate, the reactivities are lower. Also, the double asymmetric homoallenylboration of 2,3-O-isopropylidene-D-glyceraldehyde using the DMPT modified boronate reagent gives an excellent 98.5% anti selectivity in the matched case and a moderately lower value, 82% syn selectivity, in the mismatched case.