To shed the light on the plausible reaction mechanism, few additional experiments were performed (Scheme 5). Thus, the reaction of substrates 1 and 2a at the typical optimized conditions developed in this work in the presence of the radical trapping agent TEMPO resulted in full suppression of the expected product formation; however, we could not detect any adducts with TEMPO. Generation of the reactive radical species from 1 by action of Na2S2O4 57 resulted in the complete conversion of the starting ester, while boronate 2a remained intact. Reaction of compound 2a with copper powder at the typical conditions in the absence of ester 1 led to the clean formation of protodeborylation product 12. These two results confirm importance of the copper catalyst for the overall process. Finally, measuring B NMR spectra of compound 1a without additive and in the presence of 100 mol.% NaBr in DMSO-d 6 revealed no considerable changes in the chemical shift of the Boron nucleus. This result show that no tetracoordinated organoboron species are likely formed in the reaction media in the presence of bromide anion. proposed a plausible mechanism for the title reaction (Scheme 6). The process starts with the classical Cu 0mediated halogen atom transfer (XAT) from bromodifluoroacetate 1 with generation of radical 12 and Cu(I) species 13. Intermediate 13 undergoes transmetalation with boronate 2 generating alkenylcopper compound 14, that, in turn, reacts with radical 12. The latter step can occur through the formation of organocopper(II) compound 15 or, more likely, as the radical addition to the double bond of 14 giving radical 16. The substrate reactivity and stereochemical outcome (i.e., the formation of ca. 1:1 mixtures of E/Z isomers) of most transformations shown in Scheme 3 are more consistent with the latter option, i.e., formation of intermediate 16. Still, the alternative mechanism (through intermediate 15) might be possible when the substrate stereochemistry was retained (i.e., for products 4m, 4n, and 4u). Finally, elimination of copper from either 15 or 16 provides target product 4 and regenerates Cu(0), thus completing the catalytic cycle. Indirect additional evidence of such metallic copper participation in the reaction is the formation of copper mirror observed at the stirring bar (see the photo in the Scheme 6). Scheme 6. Plausible mechanism of the studied reaction.