Synthesis of Boronic Esters Derived From Boronic and Diboronic Acids

Boronic acid and their esters are highly valuable compounds which have found extensive applications in organic and medicinal chemistry. Tartaric acid derived boronic ester have been used in asymmetric reductions of ketones with very good to excellent enantiomeric excess. For that reason we are interested in the synthesis of new boronic ester tartrate derivatives, in particular, in those which derive from diboronic acids. Our aim is to compare the results between mono and diboronic ester tartrate derivates. The synthesis of boronic esters from boronic acids and alcohols is an equilibrium which can be driven to the boronate product removing the water produced in the reaction by distillation with a Dean-Stark apparatus or by using dehydrating agents like CaH₂, MgSO₄ or molecular sieves. In this work, the boronic esters were synthesized by heating the reactants with a Dean-Stark apparatus or by the addition of CaH₂ or molecular sieves as dehydrating agents. The solvents of choice were toluene and THF. We present here the synthesis of some boronic esters derived from phenyl boronic acid, 9-anthracene boronic acid, 1,4-phenyldiboronic acid, 1,4-naphtalene diboronic acid and 9,10-anthracene diboronic acid using tartaric acid, dimethyl tartrate and dibenzyl tartrate as diols. NMR spectroscopic data of ¹H, ¹³C and ¹¹B are informed.