Heteroarylboronic esters

Heteroarylboronic esters, useful as organometalUc partners in Suzuki reactions, have also been prepared under microwave irradiation by Fiirst-... 

Pd-catalyzed Miyaura boration Pd-catalyzed cross-coupling of aromatic and heteroaromatic halides or triflates with tetraalkoxydiboron compounds to give arylboronic and heteroarylboronic esters. 296... 

Relative to other aza-heterocycles, the borylation process was reported to exhibit poor regioselectivity, and a mixture of products was obtained. This was proposed to be due to electronic reasoning arising from the presence of a sulfur atom in the heterocyclic framework. Elaboration of the heteroarylboronate ester product by further methods (e.g., Suzuki coupling, trifluoromethylation, and oxidation) could certainly lead to a range of substituted derivatives in an efficient manner. 

Heteroarylboronic esters were also effective, although lower yields were observed compared with heteroarylboronic acids for the Boc-protected indole derivative (eq 70). 

PCy3 also proved to be the ligand of choice for a wide-ranging study of Suzuki couplings of heteroaryl chlorides and heteroarylboronic acids (Equation 2.18) where aryl boronate esters and aryl trifluoroborates could also be employed [33]. 

A further improvement was the development of ligands 5 and 6, the use of which allowed for the synthesis of tetra-ortho-substituted biaryls, as well as the use of heterocyclic aryl chlorides and heteroarylboronic acids and esters (Equation 2.22) [40]... 

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