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.

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 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.