Theoretical Study On Copper-catalyzed Borocyanation Of 1,3-Butadienes

Transition-metal catalyzed borylation of unsaturated carbon-carbon bonds is one of the most efficient approaches for the synthesis of organoboron compounds.In this thesis,density functional theory calculations have been performed to investigate the detailed reaction mechanism of the copper-catalyzed borocyanation of2-aryl-substituted 1,3-butadienes.The computations show that the overall catalytic cycle consists of three major steps:（1）the borocupration of 1,3-butadiene to give the allyl copper intermediates;（2）the electrophilic cyanation via six-membered transition state;（3）the 1,2-elimination of copper tosylamide.The results show that the regioselectivity of the overall reaction is determined by the combination of the inherent regioselectivity of the borocupration and electrophilic cyanation steps.Theπ-conjugation effect of the 1,3-butadiene makes the terminal carbon atoms more electrophilic compared with the internal carbon atoms,which coupled with the steric effect,results in the 4,3-and 1,2-borocupration being intrinsically more favorable than the other possibilities.The steric repulsion around the breaking Cu-C bond was found to play a key role in determining the regioselectivity of the electrophilic cyanation.For the bulky Xant Phos ligand,the 4,3-borocupration was found to be more favorable than the 1,2-borocupration due to the steric repulsion around the forming Cu-C bond,resulting in the formation of the 4,3-borocyantion product.On the other hand,with the small monodentate phosphine ligand PCy₃,the regioselectivity of the borocupration was governed by the electronic effect,where theπ-electron-withdrawing aryl group at the C² atom makes the C¹ atom more electrophilic than the C⁴ atom,enabling the 1,2-borocupration to be more favorable than the 4,3-borocupration.The origins of the experimentally observed ligand-controlled regioselectivity were thus ascribed to both electronic and steric effects of the borocupration and electrophilic cyanation steps.