3.22 Chiral Ligation for Boron and Aluminum in Stoichiometric Asymmetric Synthesis

Abstract

The present work describes chiral organoborane reagents in asymmetric conversions with a few examples of analogous processes with their organoaluminum counterparts, particularly in asymmetric reduction and as chiral activators/directors in the Diels–Alder cycloaddition. The most commonly used asymmetric stationary chiral ligands for these organometallics are derived from or utilize optically active terpenes, amino alcohols, diols, or diamines in their construction. For organoboranes, the boronic esters and amides are generally much less reactive toward atmospheric oxygen than are their dialkylborane analogs, but they are also generally much less reactive. Electron-withdrawing groups in the ligands can be used to increase the reactivity for boron esters and amides as can acid catalysts. With the capability of two chiral ligands on the metal center, both reagents with C2 and S1 symmetry are known. Organized around the more extensive chemistry of organoboranes, common asymmetric processes such as hydroboration, reduction, allylboration, and related processes, allenylboration, propargylboration, Matteson chain extension and homologation, Michael additions, enolboration, and Diels–Alder cycloadditions are specifically highlighted. Wherever possible, the relative merits and drawbacks of the reagents with differing ligands are compared and contrasted. Catalytic asymmetric processes are considered only when they represent a direct extension of an analogous stoichiometric process.