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The development of catalytic methodologies involving the formation of C−C bonds to enable the generation of cyclic systems constitutes a field of great relevance in synthetic organic chemistry. One paradigmatic process to accomplish this goal efficiently is the transition-metal-catalyzed [2 + 2 + 2] cycloaddition reaction, since it permits the formation of a wide range of highly functionalized 6-membered carbo-and heterocyclic molecules in a single step with high efficiency and perfect atom economy. A key feature of these transformations is the mechanistic pathway that they follow, since a deep knowledge of this mechanism may enable us to understand and improve the efficiency of the reaction. This review covers the mechanistic aspects, studied both from theoretical and experimental points of view, of the transition-metal-catalyzed [2 + 2 + 2] cycloaddition reaction involving all kinds of unsaturated substrates with metals such as Co, Ni, Ru, Rh, Ir, Pd, Zr, Ti, Ta, and Nb. A thorough overview is undertaken, from the seminal studies until the present day, of the key mechanistic aspects that influence the reactivity and selectivity of the reaction, comparing the involvement of different unsaturated substrates as well as the different transition metals used.

Three new Ru-aqua complexes containing a mixed carbene and pyridylic ligands with general formulas [Ru(CNC)(bpy)(H2O)](PF6)2 (1) (CNC is 2,6-bis(butylimidazol-2-ylidene)pyridine; bpy is 2,2'-bipyridine) and cis-/trans-[Ru(CNC)(nBu-CN)(H2O)](PF6)2 (cis-2 and trans-2) (nBu-CN is 2-(butylimidazol-2-ylidene)pyridine) have been prepared and structurally characterized both in the solid state (monocrystal X-ray diffraction analysis for 1 and for the related complex trans-[Ru(Br)(CNC)(nBu-CN)](PF6)) and in solution (for all of them) through NMR. The electrochemical properties of these three Ru-aqua complexes have been investigated by cyclic voltammetry, differential pulse voltammetry and Coulombimetric techniques. It is found that, for complex 1 at pH 7, the difference between the IV/III and the III/II redox couples (DeltaE1/2) is 50 mV, which is the smallest ever reported for this type of complex. On the other hand, for complexes cis-2 and trans-2, the oxidation state III is unstable with respect to disproportionation to II and IV. The reactivity of their Ru=O species has been tested toward cis-beta-methylstyrene oxidation, and it has been compared to [Ru(O)(trpy)(bpy)]2+. An inverse correlation between the degree of cis/trans-epoxide isomerization and DeltaE1/2 is found. In particular, for complexes cis-2 and trans-2, which have a DeltaE1/2 < 0, the epoxidation is highly stereoselective, yielding only cis-epoxide.

Several intermediates of the oxidative coupling of areneboronic acids to afford biaryls have been identified by electrospray ionization mass spectrometry. Knowledge has been gained about the steps occurring after the biaryl formation and leading to the recovery of the catalytic species.

The air- and moisture-stable phosphine-free palladium(0) complex 1 is a highly active and recoverable catalyst for Heck olefination of aryl diazonium tetrafluoroborates. The reactions were performed under aerobic conditions at room temperature to give the coupling products in excellent yields. [reaction: see text]

High enantioselection is obtained in Michael additions of cyclic beta-ketoesters in the presence of lanthanium triflates and (S,S)-ip-pybox. Intermediates based on simultaneous coordination of the lanthanide to both (S,S)-ip-box and beta-ketoester (in keto and enolate forms) are detected by means of ESI mass spectrometry and NMR experiments, and a possible mechanism is proposed through theoretical calculations.

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.