ortho -Fluoro or ortho Effect? Oxidative Addition of Zerovalent Nickel into the C-CN Bond of Substituted Benzonitriles.

A recent study of the oxidative addition of zerovalent Ni to the C-CN bond of F-substituted benzonitriles showed significantly increased stabilization of the C-CN oxidative addition products with o -F groups (-6.6 kcal/mol per o -F) compared to m -F groups (-1.8 kcal/mol per m -F). To answer the question of whether this is an o -F effect or an ortho effect, in this study the effect of CF 3 and CH 3 groups on the oxidative addition of the [Ni(dmpe)] fragment [dmpe = 1,2-bis(dimethylphosphino)ethane] to the C-CN bond of benzonitriles has been studied. A density functional theory study of the reaction pathway between η 2 -CN complexes and the C-CN oxidative addition products shows stabilization of the C-CN oxidative addition product with the electron-withdrawing CF 3 group and destabilization with the electron-donating CH 3 group in both tetrahydrofuran and toluene. There is a slightly larger ortho effect with CF 3 (-7.4 kcal/mol) than with F. However, due to steric crowding, two o- CF 3 groups did not show considerably more stabilization than one o -CF 3 group. There is a linear relationship between Δ G° and the number of meta groups (2.0 kcal/mol stabilization per m -CF 3 and 0.8 kcal/mol destabilization per m -CH 3 ). On the basis of natural population analysis, as the C-CN bond becomes more polarized, the stability of the C-CN oxidative addition products with respect to the η 2 -CN complexes increases.