Mono- and bis-imidazolidinium ethynyl cations and the reduction of the latter to give an extended bis-1,4-([3]cumulene)-p-carbo-quinoid system

Sherpa Romeo yellow journal. This is the peer reviewed version. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived VersionsAn extended π-system containing two [3]cumulene fragments separated by a p-carbo-quinoid and stabilized by two capping N-heterocyclic carbenes (NHCs) has been prepared. Mono- and bis-imidazolidinium ethynyl cations have also been synthesized from the reaction of an NHC with phenylethynyl bromide or 1,4bis(bromoethynyl)benzene. Cyclic voltammetry coupled with synthetic and structural studies showed that the dication is readily reduced to a neutral, singlet bis-1,4-([3]cumulene)-p-carbo-quinoid due to the πaccepting properties of the capping NHCsYe

Organoaluminum(III) complexes of the bis-N,N[prime]-(2,6-diisopropylphenyl)imidazolin-2-imine ligand

Publisher's Version/PDFThe reaction of trimethylaluminum with the bis-N,N[prime] -(2,6-diisopropylphenyl)imidazolin-2-imine (L–H) ligand 1 afforded several new organometallic aluminium complexes. Reaction in a 1 : 1 stoichiometry at room temperature afforded a Lewis acid–base adduct, whereas thermolysis resulted in the loss of methane and the formation of a dimer complex, (L-AlMe[subscript 2])[subscript 2]. 3. When reacted in a 1 : 2 ratio at 110 [degreesl]C, the loss of two equivalents of methane yielded L[subscript 2]AlMe, 5, whereas when the reaction was performed at 60 [degrees]C, (L–H)AlMe[subscript 2](L), 4, was found as an intermediate in the reaction. Compound 2 is, to the best of our knowledge, the first example of a structurally characterized primary imine coordinated to a triorganoaluminum(III) center. Attempts to form a two coordinate aluminum cation by CH[subscript 3][superscript -] abstraction are documented. All products were characterized via normal spectroscopic techniques and single crystal X-ray diffraction

Donor-Induced Decomposition of the Grubbs Catalysts: An Intercepted Intermediate

The σ-alkyl species RuCl₂(CH₂PCy₃)­(py)₃ (<b>3a</b>) is intercepted on adding pyridine to the first-generation Grubbs catalyst <b>1a</b> during RCM or to the isolated resting-state species RuCl₂(PCy₃)₂(CH₂) (<b>2a</b>). Complex <b>3a</b> is formed by pyridine-induced displacement of PCy₃ and nucleophilic attack of the liberated PCy₃ on the methylidene carbon. The rapid, near-quantitative conversion of <b>2a</b> into <b>3a</b> indicates that nucleophilic attack by PCy₃ is the primary deactivating event. Once formed, <b>3a</b> decomposes more slowly via several competing pathways. One such pathway involves elimination of the σ-alkyl ligand as [CH₃PCy₃]Cl (<b>A</b>), following proton and chloride abstraction. Observation of nearly 80% <b>3a</b> during RCM by <b>1a</b> in the presence of pyridine confirms the relevance of this behavior to metathesis and implicates the resting-state methylidene <b>2a</b> as the vulnerable species, rather than the metallacyclobutane intermediate. Any donor capable of displacing PCy₃ and stabilizing a five-coordinate methylidene adduct is predicted to trigger the same deactivation sequence, steric factors permitting

Donor-Induced Decomposition of the Grubbs Catalysts: An Intercepted Intermediate

The σ-alkyl species RuCl₂(CH₂PCy₃)­(py)₃ (<b>3a</b>) is intercepted on adding pyridine to the first-generation Grubbs catalyst <b>1a</b> during RCM or to the isolated resting-state species RuCl₂(PCy₃)₂(CH₂) (<b>2a</b>). Complex <b>3a</b> is formed by pyridine-induced displacement of PCy₃ and nucleophilic attack of the liberated PCy₃ on the methylidene carbon. The rapid, near-quantitative conversion of <b>2a</b> into <b>3a</b> indicates that nucleophilic attack by PCy₃ is the primary deactivating event. Once formed, <b>3a</b> decomposes more slowly via several competing pathways. One such pathway involves elimination of the σ-alkyl ligand as [CH₃PCy₃]Cl (<b>A</b>), following proton and chloride abstraction. Observation of nearly 80% <b>3a</b> during RCM by <b>1a</b> in the presence of pyridine confirms the relevance of this behavior to metathesis and implicates the resting-state methylidene <b>2a</b> as the vulnerable species, rather than the metallacyclobutane intermediate. Any donor capable of displacing PCy₃ and stabilizing a five-coordinate methylidene adduct is predicted to trigger the same deactivation sequence, steric factors permitting

A General Decomposition Pathway for Phosphine-Stabilized Metathesis Catalysts: Lewis Donors Accelerate Methylidene Abstraction

Sterically accessible Lewis donors are shown to accelerate decomposition during catalysis, for a broad range of Grubbs-class metathesis catalysts. These include benzylidene derivatives RuCl₂(NHC)­(PCy₃)­(CHPh) (<b>Ru-2</b>: NHC = H₂IMes, <b>a</b>; IMes, <b>b</b>; H₂IPr, <b>c</b>; IPr, <b>d</b>; H₂ITol, <b>e</b>) and indenylidene complexes RuCl₂(NHC)­(PCy₃)­(C₁₅H₁₀) (NHC = H₂IMes, <b>Ru-2f</b>; IMes, <b>Ru-2g</b>). All of these precatalysts form methylidene complex RuCl₂(NHC)­(CH₂) <b>Ru-3</b> as the active species in metathesis of terminal olefins, and generate RuCl₂(NHC)­(PCy₃)­(CH₂) <b>Ru-4</b> as the catalyst resting state. On treatment with a 10-fold excess of pyridine, <b>Ru-4a</b> and <b>Ru-4b</b> decomposed within minutes in solution at RT, eliminating [MePCy₃]Cl <b>A</b> by net loss of three ligands (PCy₃, methylidene, and one chloride), and a mesityl proton. In comparison, loss of <b>A</b> from <b>Ru-4a</b> in the absence of a donor requires up to 3 days at 55 °C. The σ-alkyl intermediate RuCl₂(¹³CH₂PCy₃)­(NHC) (py)₂ resulting from nucleophilic attack of free PCy₃ on the methylidene ligand was undetectable for the H₂IMes system, but was spectroscopically observable for the IMes system. The relevance of this pathway to decomposition of catalysts <b>Ru-2a</b>–<b>g</b> was demonstrated by assessing the impact of pyridine on the in situ-generated methylidene species. Slow initiation (as observed for the indenylidene catalysts) did not protect against methylidene abstraction. Importantly, studies with <b>Ru-4a</b> and <b>Ru-4b</b> indicated that weaker donors (THF, MeCN, DMSO, MeOH, and even H₂O) likewise promote this pathway, at rates that increase with donor concentration, and severely degrade catalyst productivity in RCM, even for a readily cyclized substrate. In all cases, <b>A</b> was the sole or major ³¹P-containing decomposition product. For DMSO, a first-order dependence of decomposition rates on DMSO concentration was established. This behavior sends a warning about the use of phosphine-stabilized metathesis catalysts in donor solvents, or with substrates bearing readily accessible donor sites. Addition of pyridine to RuCl₂(H₂IMes)­(PCy₃)­(CHMe) did not result in ethylidene abstraction, indicating that this decomposition pathway can be inhibited by use of substrates in which the olefin bears a β-methyl group

Mono‐ and Bis(imidazolidinium ethynyl) Cations and Reduction of the Latter To Give an Extended Bis‐1,4‐([3]Cumulene)‐p-carboquinoid System

An extended π-system containing two [3]cumulene fragments separated by a p-carboquinoid and stabilized by two capping N-heterocyclic carbenes (NHCs) has been prepared. Mono- and bis(imidazolidinium ethynyl) cations have also been synthesized from the reaction of an NHC with phenylethynyl bromide or 1,4-bis(bromoethynyl)benzene. Cyclic voltammetry coupled with synthetic and structural studies showed that the dication is readily reduced to a neutral, singlet bis-1,4-([3]cumulene)-p-carboquinoid as a result of the π-accepting properties of the capping NHCs.peerReviewe