Sterically accessible Lewis donors are shown to accelerate decomposition during catalysis, for a broad range of Grubbs-class metathesis catalysts. These include benzylidene derivatives RuCl(NHC)(PCy)(═CHPh) (Ru-2: NHC = HIMes, a; IMes, b; HIPr, c; IPr, d; HITol, e) and indenylidene complexes RuCl(NHC)(PCy)(═CH) (NHC = HIMes, Ru-2f; IMes, Ru-2g). All of these precatalysts form methylidene complex RuCl(NHC)(═CH) Ru-3 as the active species in metathesis of terminal olefins, and generate RuCl(NHC)(PCy)(═CH) Ru-4 as the catalyst resting state. On treatment with a 10-fold excess of pyridine, Ru-4a and Ru-4b decomposed within minutes in solution at RT, eliminating [MePCy]Cl A by net loss of three ligands (PCy, methylidene, and one chloride), and a mesityl proton. In comparison, loss of A from Ru-4a in the absence of a donor requires up to 3 days at 55 °C. The σ-alkyl intermediate RuCl(CHPCy)(NHC) (py) resulting from nucleophilic attack of free PCy on the methylidene ligand was undetectable for the HIMes system, but was spectroscopically observable for the IMes system. The relevance of this pathway to decomposition of catalysts Ru-2a-g 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 Ru-4a and Ru-4b indicated that weaker donors (THF, MeCN, DMSO, MeOH, and even HO) 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, A was the sole or major P-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 RuCl(HIMes)(PCy)(═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.

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

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.

Trimethylaluminum reacts with bis-N,N′-(2,6-diisopropylphenyl)imidazolin-2-imine to form a number of new organoaluminum complexes.

Mono-and Bis(imidazolidinium ethynyl) Cations and Reduction of the Latter To Give an