Boronate Esters

A prominent feature of boronic acids is their reversible formation of esters with diols in aqueous solution. Boronate esters are air- and chromatography-stable and suitable for spectroscopic study. The Suzuki-Miyaura cross-coupling reaction can be used with boronate esters. An underlying problem, however, involves reaction scheme incompatibilities between most synthetic reagents. An often employed solution to this incompatibility is to use boronic ester counterparts. These are more compatible with many synthetic schemes, although liberation of the boronic acid requires harsh conditions that interfere with the synthetic substrates.

N-Methyliminodiacetic acid (MIDA)-protected boronate esters are a new class of reagents that offer great promise in iterative Suzuki-Miyaura cross-coupling reactions. Compared to earlier reagents, MIDA esters are easily handled, unreactive under anhydrous cross-coupling conditions and are bench-top stable indefinitely under air, and can be deprotected easily using mild aqueous basic conditions. The success of this new class of reagents is tied to their unique molecular architectures. Compared to the simpler B-N containing molecules ammonia borane and trimethylamine borane, MIDA esters are much larger, and the sp³ hybridized boron atom is secured by two five membered rings, dramatically increasing the stability of the boronic acid and allowing for the synthesis of complex molecules to take place.

Chiral α-aminoboronate esters, compounds having a tremendous scope of applications in pharmacology, can be synthesized via metal-free nucleophilic boryl addition to tosylaldimines. Boronate esters have also been utilized in organic electronic devices.

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