Catalyst-Free Synthesis of Borylated Lactones from Esters via Electrophilic Oxyboration

A catalyst-free oxyboration reaction
of alkynes is developed. The
resulting borylated iso­coumarins and 2-pyrones are isolated
as boronic acids, pinacol­boronate esters, or potassium organo­trifluoro­borate
salts, providing a variety of bench-stable organo­boron building
blocks for downstream functionalization. This method has functional
group compatibility, is scalable, and proceeds with readily available
materials: B-chloro­catechol­borane and
methyl esters. Mechanistic studies indicate that the B-chloro­catechol­borane acts as a carbophilic Lewis acid
toward the alkyne, providing a mechanistically distinct pathway for
oxy­boration that avoids B–O σ bond formation and
enables this catalyst-free route.