Mao. Volland et al., Catalyst screening by electrospray ionization tandem mass spectrometry: Hofmann carbenes for olefin metathesis, CHEM-EUR J, 7(21), 2001, pp. 4621-4632
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.