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

Abstract

In this work we report density functional calculations at the B3LYP level of the [2þ2þ2] intermolecular cyclotrimerization of three acetylene molecules catalyzed by Wilkinson's catalyst. This process corresponds to the simplest [2þ2þ2] 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 3 mol -1 . The energy profile derived from the real [RhCl(PPh 3 ) 3 ] Wilkinson's catalyst is compared with that obtained with a model of the catalyst in which the PPh 3 ligands have been substituted by the smaller and computationally less expensive PH 3 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 3 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 [2þ2þ2] intermolecular cyclotrimerization of three acetylene molecules catalyzed by [RhCl(PH 3 ) 3 ] with those of the [2þ2þ2] 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 intermolecular cyclotrimerization at low temperatures to the intramolecular one at high temperatures.

Intramolecular [2+2+2] Cycloaddition Reactions of Yne‐ene‐yne and Yne‐yne‐ene Enediynes Catalysed by RhI: Experimental and Theoretical Mechanistic Studies

“…The simplicity of the 1 H and 13 C NMR spectra proved a symmetrical structure due to the presence of a C 2 axis. The anti disposition of the new CH c À CH c' centres was established by taking into account the trans configuration of the original double bond in the starting compounds 1 and 2, respectively, and the NOE data.…”

“…Aquests endiins shan emprat com a substrats en processos de cicloaddició [2+2+2] catalitzats pel To reduce the computational effort required, the tosyl moieties present in the experimental enediynes and the three phenyl groups of the Wilkinson catalyst were substituted by hydrogen atoms. [13,14] A previous study [15] with macrocyclic systems revealed that the substitution of the tosyl groups by hydrogen atoms reduces the exothermicity of the [2+2+2] cycloaddition by about 10 %. Although this quantity is not negligible, we expect it to have only a slight effect on the different reaction mecha- nisms studied here and therefore the conclusions reached by our model systems will still be valid for experimental systems.…”

“…For terminal alkynes (H instead of CH 2 À NH 2 ), the barriers to the alkyne-alkyne coupling were 31.9 and 25.4 kcal mol ). In another earlier study [13] we compared the reaction mechanisms of the [2+2+2] cycloaddition reaction of three acetylene molecules catalysed by [RhCl-…”

“…[13,14,17] This result is not unexpect- www.chemeurj.org ed given the different electronic and steric effects exerted by the PPh 3 ligands as compared with PH 3 . Our previous study [13] led us to expect that the Gibbs energy profiles for the [2+2+2] cycloaddition reactions of the enediynes studied would be slightly lower if computed with the [RhClA C H T U N G T R E N N U N G (PPh 3 ) 3 ] catalyst.…”

“…[13,14,17] This result is not unexpect- www.chemeurj.org ed given the different electronic and steric effects exerted by the PPh 3 ligands as compared with PH 3 . Our previous study [13] led us to expect that the Gibbs energy profiles for the [2+2+2] cycloaddition reactions of the enediynes studied would be slightly lower if computed with the [RhClA C H T U N G T R E N N U N G (PPh 3 ) 3 ] catalyst. [13,14,17] If we study the 16-electron 2-C complex (arising from alkyne-alkyne oxidative addition with [RhClA C H T U N G T R E N N U N G (PH 3 )]), two possibilities emerge from this point ( Figures 5 and 6).…”