Catalyst screening by electrospray ionization tandem mass spectrometry: Hofmann carbenes for olefin metathesis.

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.