Cyclotrimerization Reactions Catalyzed by Rhodium(I) Half-Sandwich Complexes:  A Mechanistic Density Functional Study

Abstract

We propose and examine a comprehensive mechanism of the [(η 5 -C 5 H 5 )Rh]-catalyzed [2+2+2] cycloadditions of acetylene to benzene and of acetylene and acetonitrile to 2-methylpyridine, based on an extensive and detailed exploration of the potential energy surfaces using density functional theory. Both processes involve the formation of a coordinatively unsaturated 16-electron metallacycle, occurring after the replacement of the ancillary ligands L of the catalyst precursor of general formula [(η 5 -C 5 H 5 )-RhL 2 ] (typically L ) C 2 H 4 , CO, PH 3 or L 2 ) 1,5-cyclooctadiene) by two acetylene molecules. The facile coordination of a third acetylene molecule, and its subsequent addition to the π electron system of the rhodacycle, leads to the formation of an intermediate, which is characterized by a six-membered arene ring coordinated to the metal in η 4 fashion. The release of benzene occurs by stepwise addition of two acetylene molecules, which regenerates the catalyst. In the presence of acetonitrile, a nitrile molecule coordinates to the rhodacycle, and different stages are outlined for the process, leading to the eventual release of 2-methylpyridine. The steric and electronic effects of the π ligand coordinated to the metal are also included in our exploration by addressing the whole mechanism of the [(η 5 -C 9 H 7 )Rh]-catalyzed alkyne self-trimerization to benzene. The kinetic parameters, i.e., the energies in vacuum and in different solvents, and the geometries of the intermediates and of the transition states are analyzed in detail.

Rhodium(I)‐Catalysed Intramolecular [2+2+2] Cyclotrimerisations of 15‐, 20‐ and 25‐Membered Azamacrocycles: Experimental and Theoretical Mechanistic Studies

“…[14,15,17] On the other hand, similar barriers but lower exothermicities were reported for RhCp and CoCp-catalysed cyclotrimerisations. [10,17,18] As found previously in similar Co and Rh complexes, the 15-A3 complex exhibits p localisation with C a À C b and C b À C b bond lengths of 1.351 and 1.453 , respectively, despite the presence of metal lone pairs that could favour the formation of an aromatic ring. [10,18] The RhÀ C bond length of 2.04 is not far from the average crystallographic bond length of 2.01 .…”

“…The reaction mechanism for the alkyne cyclotrimerisation reaction catalysed by [CoCp(L) 2 ] (Cp= cyclopentadiene; L = CO, PR 3 , THF and olefin), [10][11][12][13] [RuCpCl] [14][15][16][17] complexes and the {RhCp} [18] and {RhInd} (Ind = indene) [18] fragments has been analysed in these DFT studies. The generally accepted reaction mechanism is drawn in Scheme 3.…”

“…[13] On the other hand, the cyclotrimerisation of nitriles and acetylenes to afford pyridine rings has been also discussed in a series of theoretical studies. [17,18,20,21] The rate-determining step of the overall catalytic cycle changes to become the addition of the nitrile molecule to the metallacyclopentadiene or metallacyclopentatriene intermediate. [17,18] One of the key aspects in the cyclotrimerisation of nitriles and acetylenes is the competition between arene and pyridine formation.…”

“…[17,18,20,21] The rate-determining step of the overall catalytic cycle changes to become the addition of the nitrile molecule to the metallacyclopentadiene or metallacyclopentatriene intermediate. [17,18] One of the key aspects in the cyclotrimerisation of nitriles and acetylenes is the competition between arene and pyridine formation. Relevant to our discussion are also several papers that address the co-cyclisation of two acetylene molecules with alkenes or CS 2 .…”

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

“…[14] A previous study found that solvent effects due to toluene and acetonitrile in [2+2+2] cycloadditions are minor, likely due to the absence of charged or polarised intermediates and transition states in the reaction mechanism. [28] Because the reactions studied were carried out in toluene, solvent effects have not been included in the present calculations. Finally, because there are no experimental data suggesting the presence of paramagnetic intermediates, our studies were limited to the singlet potential energy surfaces.…”