Boron as Lewis Acid Catalysts

Aryl boron compounds, Ar,B(OH)3 (n = 1-3), bearing electron-withdrawing aromatic groups such as triarylboranes, 

Aldol reactions of various silyl enol ethers or ketone silyl acetals with aldehydes and imines proceed smoothly in the presence of catalytic amount of 516 or 519 (Equations (242) and (243)).1034-1037 B(G6F5)3 516 catalyzed the 

The ring-opening reaction of trans-2,3-epoxy alcohols with NaN3 was mediated with PhB(OH)2 (Equation (247)).1041 

Chiral(acyloxy)boranes 523-525 were reported to be excellent catalysts for the enantioselective Mukaiyama condensation (Equation (248)) and allylation reaction of aldehydes leading to homoallylic alcohols (Equation (249)) 1042-1045 

Chiral alkyldihaloboranes are among the most powerful of chiral Lewis acids. The Diels-Alder reaction of cyclopentadiene with methyl acrylate proceeded smoothly at — 78 °C in the presence of 530 or 531 to give the endo-adduct with 99% ee (Equation (250)).1046-1052 

---