Can iron mimic the behavior of palladium in cross-coupling reactions?
A recent paper by the TheoCheM group at the Vrije Universiteit Amsterdam shows that trends in the cross-coupling reaction of C−X bonds are very similar for an iron-based catalyst and a palladium catalyst.
The cross-coupling reaction of C(spn)−X bonds (n = 1−3; X = F, Cl, Br, I) mediated by the iron-d8 catalyst Fe(CO)4 with the model substrates H3C−CH2−X, H2C=CH−X, and HC≡C−X was studied. The barrier of the oxidative-addition step lowers as the C(spn)–X bond varies from X = F to Cl to Br to I and from C(sp3) to C(sp2) to C(sp). Activation strain and energy decomposition analyses reveals that the lowering of the reaction barrier from X = F to I is caused by (i) a weaker C(spn)–X bond that needs to be broken, (ii) enhanced HOMO–LUMO interactions, and (iii) a stronger electrostatic attraction between the catalyst and the substrate due to the more diffuse electron density and higher nuclear charge of the X atom when varying from X = F to I.
The iron-based catalyst demonstrates similar behavior to established palladium analogs in the bond-activation step of cross-coupling reactions, despite some characteristic differences. One difference is the strong σ-donation from substrates into the vacant d-orbitals of iron, a process not feasible for palladium catalysts due to their closed d10 shell. This might be a step forward in finding alternatives for palladium in cross-coupling reactions.
