Catalyst Screening by Electrospray Ionization Tandem Mass Spectrometry: Hofmann Carbenes for Olefin Metathesis

Abstract

A new screening methodology, which combines in situ synthesis of complexes with an assay by electrospray ionization tandem mass spectrometry (ESI-MS), is introduced in order to investigate highly active, cationic ruthenium-carbene catalysts in ring-opening metathesis polymerization (ROMP). The parameter space, whic is defined by systematic variation of four structural features of the catalyst [[R2P(CH2),PR2-kappa2-P]XRu=CHR']+ (the halogen ligand, the diphosphane bite-angle, the steric bulk of the phosphane, and the carbene ligand) and the variation of the metathesis substrate, is mapped out. Chloride as the anionic ligand X, a small chelating angle (n = 1), and reduced steric demand of the substituents R (Cy versus tBu) lead to the most reactive complex in acyclic olefin metathesis, whereas variation of the carbene moiety CHR' has only a modest influence. The overall rate in the gas phase depends on the pi-complex preequilibrium and metallacyclobutane formation, which was found to be the rate-determining step. In ROMP reactions backbiting has a profound influence on the overall rate. Moreover, we were able to establish that the reactivity trends determined in the gas phase parallel solution-phase reactivity. The overall rate in solution is also determined by a favorable dimer/ monomer preequilibrium providing the active catalyst by facile dissociation of dicationic, dinuclear catalyst precursors.

On the ligand properties of the P- versus the N-heterocyclic carbene for a Grubbs catalyst in olefin metathesis

“…In principle, the basic mechanism for the olefin metathesis was predicted some time ago by Hérisson and Chauvin [15]. The details of the mechanism are now well substantiated by an intense effort of experimental [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] studies. We may note here that very effective ruthenium carbene catalysts have also been presented in cationic [25] and other systems [26][27][28][29].…”

Electrospray ionization mass spectrometric study of mercury complexes of N-heterocyclic carbenes derived from 1,2,4-triazolium salt precursors

“…A number of detailed ESI MS studies devoted to the Fisher carbene-complexes have been already published [15][16][17][18]. The activity of Grubbs type ruthenium-carbene catalysts in ring-opening metathesis polymerization has also been screened by the use of ESI [19]. On the other hand ESI MS (as well as other ionization methods) only occasionally have been used for molecular weight confirmation of N-heterocyclic carbene-metal complexes.…”

Dinitrosyl rhenium complexes for ring-opening metathesis polymerization (ROMP)

“…Therefore, the mechanistic details of the olefin metathesis reaction catalyzed by ruthenium carbene complexes have been the subject of intense experimental studies to explore the parameters with the strongest influence for the activity of the particular catalysts [33][34][35][36][37][38][39][40][41][42][43]. These studies included systematic kinetic measurements in solution [26][27][28][29][30][31][32][33] as well as electrospray ionization tandem mass spectrometry (ESI-MS/MS) [38][39][40][41] in the gas phase and characterizations of model complexes for catalytic intermediates by X-ray diffraction studies [30c, 42,43]. These investigations have resolved a number of mechanistic questions, and it turns out that the ligands of the Grubbs-type complexes have significant impact on initiation rates and on catalyst activities in reactions of different substrates in solution [37].…”

“…These investigations have resolved a number of mechanistic questions, and it turns out that the ligands of the Grubbs-type complexes have significant impact on initiation rates and on catalyst activities in reactions of different substrates in solution [37]. Although there is quite a number of efficient catalysts known, screening is still highly desirable with regard to the type of metal center involved [41,44,45].…”