13 Boron “Ate” Complexes for Asymmetric Synthesis

Abstract Addition of a nucleophile to a boronic ester results in the generation of a tetravalent boronate “ate” complex. If there is a leaving group stationed on the carbon atom α to the boron atom, the boronate complex can undergo stereospecific 1,2-migration with simultaneous expulsion of the leaving group to form a homologated boronic ester. The enantioselectivity of the process is dictated by either incorporating a chiral substituent into the boronic ester component (substrate control), or by forming a boronate complex through the addition of an enantioenriched carbenoid species to a boronic ester (reagent control). Activation of a boronic ester with organolithium reagents generates a nucleophilic boronate complex that acts as a chiral organometallic-type reagent, reacting with a wide range of electrophiles with inversion of stereochemistry. This chapter discusses methodology available for the enantioselective homologation of boronic esters using both substrate- and reagent-controlled strategies, and the development of boronate complexes as chiral nucleophiles.