Examples of Metal-Catalyzed Hydroboration

The examples in Equation 16.45 show that the regiochemistry of the metal-catalyzed hydroboration of alkenes contrasts with that of tlie metal-catalyzed hydroboration of vinylarenes. The predominant products from the metal-catalyzed hydroborations of 1-alkenes are terminal boranes. - These terminal boranes can also be formed from internal alkenes, as shown in Equation 16.46. Similar to hydrosilylation witli Speier s catalyst, the 

Early transition metal and lanthanide complexes also catalyze the hydroboration of alkenes. As shown in Equation 16.44, titanocene complexes catalyze the hydroboration of vinylarenes, and as shown in Equation 16.45, lanthanocene complexes catalyze the hydroboration of alkenes. Ligand-less lanthanides and zirconocene complexes have also been reported to catalyze these reactions, but the apparent catalysis of the hydroboration in these cases more likely occurs by catalytic generation of BH3 from catecholborane and uncatalyzed addition of BHj to the olefin. 

The hydroboration of 1,3-dienes has also been reported, and these reactions generate Z-allylic boronic ester products. These reactions have been reported with palladium catalysts and are thought to occur through Tr-allyl intermediates. Reactions with butadiene and isoprene, followed by addition of the product to benzaldehyde, are shown in Equation 16.47. This equation also shows the presumed mechanism that proceeds by generation of a palladium allyl from the combination of diene and palladium hydride formed by oxidative addition of the borane. 

The metal-catalyzed hydroboration of alkynes has also been studied, and this process provides a route to vinylboronate esters that are useful in palladium-catalyzed cross-coupling chemistry. In this case, the transition-metal-catalyst strongly affects the 

---