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 beta-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.