Regioselective control of the nickel-mediated coupling of acetylene and carbon dioxide - A DFT study

The nickel-mediated coupling of asymmetric alkynes with carbon dioxide is known to be highly regioselective with respect to the formation of nickelacycle intermediates and alpha,beta-unsaturated carboxylic acid products. Using density functional theory (DFT), we have investigated the effect that parameters such as acetylene-substituent, ancillary ligand and solvent have on the potential energy surface of the nickelacycle coupling reaction. 3-R-substituted nickelacycles are the thermodynamically preferred product in all cases surveyed, however, the transition structure characterised by the attack of CO2 on the alkyne carbon distal from the R-group is generally lower in energy, making the 2-R-substituted nickelacycle the kinetically favoured product. Ligating the zerovalent nickel species with the diazabicyclo[5.4.0] undec-7-ene (DBU) ancillary ligand in preference to 2,2'-bipyridine (BIPY) leads to lower activation energies for the coupling reaction and products that are less susceptible to steric bulk in the 2-position of the nickelacycle. Solvation with dimethylformamide (DMF) has the advantage of lowering the activation barrier for the coupling reaction when compared to tetrahydrofuran (THF). (c) 2008 Elsevier B. V. All rights reserved.