ESI-MS Study on First-Generation Ruthenium Olefin Metathesis Catalysts in Solution: Direct Detection of the Catalytically Active 14-Electron Ruthenium Intermediate

Abstract

Electrospray ionization mass spectrometry (ESI-MS) and subsequent MS/MS analyses were used to study two first-generation ruthenium catalysts, 1 and 2, in solution. In situ exchange of neutral tricyclohexylphosphine in 1 and 2 by cationic phosphine 3 allowed directly to detect and to characterize mass spectrometrically the catalytically active 14-electron ruthenium intermediates present in equilibrium in solution. Moreover, the catalytic activity of these species was directly demonstrated by gas phase reaction with ethylene to give the 14-electron methylene ruthenium species. It could be confirmed that all monomolecular steps in the catalytic cycle of ring-closing metathesis of diallyl compounds 5, 6, and 7 following olefin coordination are fast, and no intermediate can be accumulated to become detectable by ESI-MS. 18-Electron ruthenium species containing three phosphine ligands were shown to be present at low concentration in the solution of catalyst 1 or 2 in presence of 1.2 equiv of phosphine 3. Finally, benzylidene trialkylphosphane 13 (in the presence of 3) and phosphonium cation 11 were easily detected as degradation products of catalyst 1 and phosphonium cation 12 as a degradation product of catalyst 2.