Boron Recycling in the Metal-Free Transfer C–H Borylation of Terminal Alkynes and Heteroarenes

Transfer C–H
borylation is an isodesmic approach to the
borylation reaction using B–C-containing molecules as boron
sources. In this work, we report that 2-mercaptothiazole and other
analogues are active for the metal-free borylation of heterocycles
and terminal alkynes. Alkynes are challenging substrates to C–H
borylate because they undergo side reactions with most borylating
agents. The ability of these metal-free catalysts to activate B–C
bonds can also be translated to the activation of B–O bonds
in the products of the 1,4-conjugate addition of alkynylboranes to
chalcones. It is therefore possible to prepare β-alkynylketones
from the corresponding alkynes in a process where the boron source
is used in substoichiometric amounts. The mechanism and degradation
patterns of these catalytic transformations have been investigated.