Density Functional Study of the [2+2+2] Cyclotrimerization of Acetylene Catalyzed by Wilkinson's Catalyst, RhCl(PPh3)3

In this work we report density functional calculations at the B3LYP level of the [2+2+2] intermolecular cyclotrimerization of three acetylene molecules catalyzed by Wilkinson's catalyst. This process corresponds to the simplest [2+2+2] cyclotrimerization reaction. The results obtained show that this reaction is thermodynamically very favorable and that the rate-determining step is the initial oxidative coupling between two acetylene molecules with a relatively low Gibbs free energy barrier of 19.8 kcal.mol(-1). The energy profile derived from the real [RhCl(PPh3) Wilkinson's catalyst is compared with that obtained with a model of the catalyst in which the PPh3 ligands have been substituted by the smaller and computationally less expensive PH3 Molecules. Our results show that, at least for this reaction, this substitution has little influence on the thermodynamics obtained, while the barrier of the rate-determining step is somewhat increased (about 5 kcal.mol(-1)) in the model system. These results justify the use of this simplified model of the catalyst in theoretical studies of more complex cyclotrimerizations. Finally, we compare the results of the [2+2+2] intermolecular cyclotrimerization of three acetylene molecules catalyzed by [RhCl(PH3)(3)] with those of the [2+2+2] intramolecular cyclotrimerization in a 15-membered azamacrocyclic triyne recently reported (Chem.-Eur.J.2009, 15, 5289). This comparison shows that the entropic term changes the preference for the intramolecular cyclotrimerization at low temperature to the intramolecular one at high temperatures.