Allenylboronic esters, preparation

Homopropargylic alcohols are available by reaction of carbonyl compounds with allenyl silanes.105 Full details of this chemistry include preparations of variously substituted allenysilanes together with methods for their reaction using TiCl4. The same compounds are available in optically active form by use of chiral allenylboronic esters.106 This method is rather similar to the chemistry outlined in Scheme 27 for the preparation of homoallylic alcohols. A third method for the preparation of homoallylic... 

Another new method for the preparation of 3-acetylenic alcohols uses chiral allenylboronic esters, derived from dialkyl tartrate esters and alleneboronic acid,... 

Chiralallenylboronic esters.1 These reagents can react with aldehydes to provide /(-acetylenic alcohols enantioselectivity (60-95% ee). Thus, allenylboronic acid 1, prepared as shown, reacts with aldehydes in the presence of ( + )-DET or ( + )-DlPT to give (S)-alcohols (2), whereas (R)-alcohols (2) are obtained in the presence of (-)-DF.T or ( —) DIPT, both in optical yields of 85-95%. The selectivities are lower in reaction with aryl and z,/3-unsaturated aldehydes. 

PhCHO) being less effective substrates than their aliphatic counterparts. The relative complexity and low yields (40-60%) of the starting allenylboronic acid coupled with the requirement that the chiral boronic esters have to be freshly prepared immediately before their use in the allenylboration detract somewhat from the overall value of the process as do the long reaction times (20 h, -78 °C), the hydrolysis of the chiral boron ligation upon work-up and the relative inefficiency of chirality transfer for aromatic substrates. 

---