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.

A new series of 20- and 25-membered polyacetylenic azamacrocycles have been satisfactorily prepared and completely characterised by spectroscopic methods. Various [2+2+2] cyclotrimerisation processes catalysed by the Wilkinson's catalyst, [RhCl(PPh(3))(3)], were tested in the above-mentioned macrocycles. The 25-membered azamacrocycle (like the previously synthesised 15-membered azamacrocyle) led to the expected cyclotrimerised compound in contrast to the 20-membered macrocycle, which is characterised by its lack of reactivity. The difference in reactivity of the 15-, 20- and 25-membered macrocycles has been rationalised through density functional theory calculations.

A series of fused tetracycles with a benzene or cyclohexadiene core (2a-h) is satisfactorily prepared by intramolecular [2 + 2 + 2] cycloadditions of triynic and enediynic macrocycles (1a-h) under RhCl(PPh3)3 catalysis; the enantioselective cycloaddition of macrocycles 1b and 1e and gives chiral tetracycles with moderate enantiomeric excess.

A domino process is described combining an ene reaction between two alkynes and a Diels-Alder cycloaddition of the vinylallene formed. The process accounts for the thermally induced cycloisomerization of macrocyclic triynes and enediynes to give fused tetracycles in a stereoselective manner.

The mechanism of the Rh-catalysed [2+2+2] cycloaddition reaction of diynes with monoynes has been examined using ESI-MS and ESI-CID-MS analysis. The catalytic system used consisted of the combination of a cationic rhodium(I) complex with bisphosphine ligands, which generates highly active complexes that can be detected by ESI(+) experiments. ESI-MS on-line monitoring has allowed the detection for the first time of all of the intermediates in the catalytic cycle, supporting the mechanistic proposal based mainly on theoretical calculations. For all ESI-MS experiments, the structural assignments of ions are supported by tandem mass spectrometry analyses. Computer model studies based on density functional theory (DFT) support the structural proposal made for the monoyne insertion intermediate. The collective studies provide new insight into the reactivity of cationic rhodacyclopentadienes, which should facilitate the design of related rhodium-catalysed C-C couplings.