The Use of Chiral Organoboranes in Organic Synthesis

Organoboranes have been the key to some of the most highly enantioselective syntheses known. Five major classes of chirally directed processes have been reported: (1) Hydroboration of olefins; (2) Allylic borane reactions with aldehydes; (3) Homologation of boronic esters; (4) Boron enolate condensations; (5) Reductions of carbonyl compounds with boranes or borohydrides. Only the first three of these topics are treated in this review. Chiral hydroboration was the first truly successful directed asymmetric synthesis, and new developments have increased the generality and extended the range of chiral products accessible from the boranes. Allylic borane reactions with aldehydes produce homoallylic alcohols, and good control of the simultaneous formation of two chiral centers has been achieved. Homologation of boronic esters with dichloromethyllithium provides 1-chloroalkyl boronic esters of high stereochemical purity, which can be converted stereospecifically to a variety of other boronic esters. The process can be repeated to introduce additional chiral centers. 1. Introduction 1.1. Boranes and Chirality 1.2. Properties of Boranes 2. Hydroboration 2.1. Isopinocampheylboranes 2.2. Other Terpenoid Boranes 2.3. 2,5-Dimethylborolanes 2.4. Substrate-Directed Chiral Hydroborations 3. Reaction of Allylic Boranes with Aldehydes 3.1. Introduction 3.2. Allylic Boronic Esters 3.3. Allylic Borinic Esters 3.4. B-Allylic-B,B-dialkylboranes 3.5. Allenylboronic Esters 4. Boronic Ester Homologation 4.1. Introduction 4.2. The Fundamental Process 4.3. Synthetic Applications 5. Conclusions