Rate Studies on the Release of Terminal Alkynes from Indenylruthenium(II) Vinylidene Complexes. Implications on the Mechanism of η¹-Vinylidene into η²-Alkyne Isomerization

Abstract

The indenylruthenium(II) vinylidene complexes [Ru{CC(H)R}(η5-C9H7)(PPh3)2][PF6] [R = Ph (1), 4-MeOC6H4 (2), 4-MeC6H4 (3), 4-PhC6H4 (4), 4-FC6H4 (5), 4-ClC6H4 (6), 4-IC6H4 (7), 4-MeCOC6H4 (8), 4-O2NC6H4 (9), n Bu (10), (η5-C5H4)Fe(η5-C5H5) (11, Fc)] have been synthesized from [RuCl(η5-C9H7)(PPh3)2] and RCCH, in a methanolic solution of NaPF6 at room temperature. The complexes, when dissolved in acetonitrile-d 3, release the vinylidene ligand upon thermal activation in the form of the corresponding terminal alkyne, with formation of the solvato complex [Ru(NCCD3)(η5-C9H7)(PPh3)2][PF6] (1a). The reactions, followed by 31P{1H} NMR spectroscopy, exhibit first-order behavior in the vinylidene substrates, and the activation parameters ΔH ⧧ = 24 ± 1 kcal mol−1 and ΔS ⧧ = −3 ± 2 cal mol−1 K−1 for complex 1 (36−54 °C). The Hammett plot derived from the aryl-substituted complexes yields the reaction parameter ρ = −1.5, indicating a rate enhancement effect by electron-donor substituents on the β-C atom. Formation of the vinylidene complex 1 from PhC13CH as well as release of the alkyne proceeds by an exclusive 1,2 shift of the H atom, as determined by the reactions of [RuCl(η5-C9H7)(PPh3)2] with the 13C-labeled alkyne and of the Cα-enriched vinylidene complex [Ru{13CC(H)Ph}(η5-C9H7)(PPh3)2][PF6]. The vinylidene moiety undergoes rapid H/D exchange with D2O/H2O at room temperature, while the presence of a nitrogen base transforms complex 4 into the corresponding neutral acetylide derivative. The reactions of complexes 4, 10, and 11, performed in solvent mixtures CD3CN/H(D)2O, exhibit primary kinetic isotopic effects in the range k H/k D = 1.17−1.88, in agreement with an intramolecular 1,2 hydrogen shift mechanism characterized by a nonlinear C−H(D)−C structure of the rate-determining transition state.

Recent Mechanistic and Synthetic Developments in the Chemistry of Transition‐Metal Vinylidene Complexes

“…In a recent study the effect of the alkyne on this process has provided important insight into the alkyne/vinylidene interconversion. [18] A series of vinylidene…”

Hydro(trispyrazolyl)borato‐Ruthenium(II) Diphosphinoamino Complex‐Catalyzed Addition of β‐Diketones to 1‐Alkynes and Anti‐Markovnikov Addition of Secondary Amines to Aromatic 1‐Alkynes

“…[18] TpRu[4-CF 3 C 6 H 4 NA C H T U N G T R E N N U N G (PPh 2 ) 2 ]OTf (I)-Catalyzed Addition of Secondary Amines to Aromatic 1-Alkynes…”

Heteroatoms moving protons: Synthetic and mechanistic studies of bifunctional organometallic catalysis

“…Although several mechanisms for alkyne to vinylidene tautomerization have been proposed [44,49], in the case of CpRu chemistry it was thought that the first step is movement of the terminal alkyne hydrogen from C1 to C2 [50][51][52][53][54]. However, in 2001 Tokunaga and Wakatsuki [42] showed that deuterium label at C1 was completely retained in the ultimate aldehyde product, inconsistent with simple movement of H1 to C2.…”