Nucleophilic Addition of Allylic Groups from Boron Compounds

Nucleophilic Addition of Allylic Groups from Boron Compounds 

Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl oxygen, which both increases the electrophilicity of the carbonyl group and weakens the C-B bond to the allyl group. The dipolar adduct then reacts through a cyclic TS. Bond formation takes place at the 7-carbon of the allyl group and the double bond shifts.36 After the reaction is complete, the carbinol product is liberated from the borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and ketones were usually above 90% for 9-allyl-9-BBN. 

Carbon-Carbon Bond-Forming Reactions of Compounds of Boron, Silicon, and Tin 

The cyclic mechanism predicts that the addition reaction will be stereospecific with respect to the geometry of the double bond in the allylic group, and this has been demonstrated to be the case. The E- and Z-2-butenyl cyclic boronates 1 and 2 were synthesized and allowed to react with aldehydes. The F-boronate gave the carbinol with anti stereochemistry, whereas the Z-boronate resulted in the syn product.37 

This stereochemistry is that predicted by a occupies an equatorial position. 

---