Mechanistic insights into the ligand-controlled regioselectivity in Cu-catalyzed terminal alkynes alkylboration

The recent Cu-catalyzed alkylboration of unactivated terminal alkynes shows intriguing ligand-controlled regioselectivity. The anti-Markovnikov alkylboration products were obtained with dppbz as ligand, while the Markovnikov products were obtained with DMAP as ligand. In the present study, the theoretical calculation was carried out to investigate the detailed mechanism of the alkylboration process and the mechanistic origin for the ligand-regulated regioselectivity. For both dppbz and DMAP, it was found that the alkylboration undergoes alkyne insertion and sigma-complex-assisted metathesis processes. In the concerted process, the carbon atom of the alkyl iodides directly bonded with the alkenyl carbon which is generated from alkyne insertion step. The anti-Markovnikov and Markovnikov alkyl-boration is, respectively, favorable for dppbz and DMAP ligand, which is consistent with the regioselectivity in experiment. What's more, the alkyne insertion step is the regioselectivity determining step for both of dppbz and DMAP ligands. The bulky dppbz-coordinated situation makes steric hindrance the main determinant factor of selectivity, therefore the less steric hindrance makes the anti-Markovnikov selectivity more favorable. For DMAP ligands, the electronic effect becomes the main factor due to the strong complexation of DMAP with Cu metal center. Therefore the better FMO interaction between Cu-B bond and alkyne favors the Markovnikov selectivity in the DMAP-coordinated situation. (C) 2018 Elsevier B.V. All rights reserved.