Alkenes borane derivatives, stereoselective

Highest regio- and stereoselectivity among all boranes is exhibited by 9-BBN. Terminal alkenes are transformed into the corresponding B-alkyl-9-BBN derivatives in purities approaching 100%. Unsymmetrically disubstituted alkynes and alkenes also react selectively . Addition to 3-methylcyclohexene and 3-methylcyclopentene gives no cis-1,2-isomer . 1,3-Dialkylcycloalkenes are transformed into single stereo isomers . 

The boron adds stereoselectively to the less sterically hindered face of the alkene. Reaction with norbornene, for example, gave 99.6% of the exo-borane (5) with only 0.4% of the endo- borane. l This is consistent with the normal addition of reagents to the exo face of norbornyl derivatives (sec. 1.5.E). 

Stereoselectivity in addition reactions of borane with bicyclic alkenes is similar to that observed for reduction of bicyclic ketones with hydride reducing agents. Boranes add to norbornene derivatives from the exo face (27 gave 85% exo attack upon reaction with diborane). In 27-30, all values are percentage yields of the organoborane products obtained by reaction from that face. Interestingly, only 80% exo attack was observed when 27 reacted with disiamylborane, and the size of the borane appears to have little effect on selectivity. 

Oxidative Heck reactions via Pd(II) C—H functionalization of terminal alkenes with pinacol boranes have been described for the preparation of styrenes and derivatives through electrophilic Pd(II) catalysis (Scheme 3.20). ° Treatment of a functionalized allylic precursor with the Pd(II) catalysts listed facilitated an allylic C—H activation. Subsequent transmetallation of the aryl boronic acid and reductive elimination afforded the desired olefin with excellent stereoselectivity. The scope of the transformation allows for a variety of activating and deactivating substituents on the aryl boronic acid as well as numerous functional groups on the starting alkene. A tandem allylic C—H oxidation/vinylic arylation protocol has also been reported. " ... 

---