Reactions of Alkynes with [RuCl(cyclopentadienyl)] Complexes: The Important First Steps

Abstract

Cyclopentadienyl-ruthenium half-sandwich complexes with eta(2)-bound alkyne ligands have been suggested as catalytic intermediates in the early stages of Ru-catalyzed reactions with alkynes. We show that electronically unsaturated complexes of the formula [RuCl(Cp--)(eta(2)-RC[triple bond]CR')] can be stabilized and crystallized by using the sterically demanding cyclopentadienyl ligand Cp-- (Cp--=eta(5)-1-methoxy-2,4-tert-butyl-3-neopentyl-cyclopentadienyl). Furthermore we demonstrate that [RuCl(2)(Cp==)](2) is an active and regioselective catalyst for the [2+2+2] cyclotrimerization of alkynes. The first elementary steps of the reaction of mono(eta(2)-alkyne) complexes containing {RuCl(Cp*)} (Cp*=eta(5)-C(5)Me(5)) and {RuCl(Cp--)} fragments with alkynes were investigated by DFT calculations at the M06/6-31G* level in combination with a continuum solvent model. Theoretical results are able to rationalize and complement the experimental findings. The presence of the sterically demanding Cp-- ligand increases the activation energy required for the formation of the corresponding di(eta(2)-alkyne) complexes, enhancing the initial regioselectivity, but avoiding the evolution of the system towards the expected cyclotrimerization product when bulky substituents are present. Theoretical results also show that the electronic structure and stability of a metallacyclic intermediate is strongly dependent on the nature of the substituents present in the alkyne.

Ruthenium‐Catalyzed trans‐Selective Hydrostannation of Alkynes

“…[25] Comparison of entries 7 and 8 confirms that the largely improved regioselectivity is intimately related with the presence of an unprotected hydroxy group and not merely caused by dipolar interactions in the transition state. [27][28][29] An equally pronounced effect was recorded for acetylene carboxylate derivatives. Likewise, a propargylic sulfonamide also showed high preference for proximate stannylation when reacted in the presence of the complex 7 (entry 14).…”

“…The fate of 7 in the presence of Bu 3 SnH and a protic substrate and hence the nature of the active species responsible for this striking regioselectivity are presently under investigation. [27][28][29] An equally pronounced effect was recorded for acetylene carboxylate derivatives. Hydrostannations in the presence of 3, albeit highly trans selective, were regio-indiscriminative (Table 3, entries 15 and 17).…”

Applications and validations of the Minnesota density functionals

“…Dutta et al [172] employed M06 in combination with a continuum solvent model to investigate the first elementary steps of the reaction of mono(g 2 -alkyne) half-sandwich catalysts containing RuCl(Cp * ) and RuCl(Cp^) fragments with alkynes, where Cp * = g 5 -C 5 Me 5 and Cp^= g 5 -1-methoxy-2,4-tert-butyl-3-neopentylcyclopentadienyl. M06 combined with a continuum treatment of the solvent was able to rationalize and complement the experimental findings and was used to examine steric, substituent, and mechanistic effects.…”

Efficient and Rapid Synthesis of Chlorido‐Bridged Half‐Sandwich Complexes of Ruthenium, Rhodium, and Iridium by Microwave Heating

“…As a result of the bulky π‐ligand, Cp ∧ Ru complexes display a unique reactivity in stoichiometric and catalytic transformations 8. For example, it was possible to isolate electronically unsaturated Cp ∧ Ru complexes, which are not known for Cp*Ru 8b,8d …”