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 pa rameter space, whic is defined by systematic variation of four structural f eatures of the catalyst [{R2P(CH2)(n)(PR2KP)-P-2}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 mappe d out. Chloride as the anionic ligand X, a small chelating angle (n = 1), a nd reduced steric demand of the substituents R (Cy versus tBu) lead to the most reactive complex in acyclic olefin metathesis, whereas variation of th e carbene moiety CHR ' has only a modest influence. The overall fate in the gas phase depends on the pi -complex preequilibrium and metallacyclobutane formation, which was found to be the rate-determining step. In ROUT reacti ons backbiting has a profound influence on the overall rate. Moreover, we w ere able to establish that the reactivity trends determined in the gas phas e parallel solution-phase reactivity. The overall rate in solution is also determined by a favorable dimer/ monomer preequilibrium providing the activ e catalyst by facile dissociation of dicationic, dinuclear catalyst precurs ors.