| الوصف: |
A screening methodol. combines in situ synthesis of complexes with an assay by electrospray ionization tandem mass spectrometry (ESI-MS) for evaluation of highly active, cationic ruthenium-carbene catalysts in ring-opening metathesis polymn. (ROMP). The parameter space, which is defined by systematic variation of four structural features of the catalyst [{R2P(CH2)nPR2-κ2P}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. The 180 reactions (repeated 10 times each) were carried out in two weeks to map out the parameter space. Chloride as the anionic ligand X, a small chelating angle (n = 1), and reduced steric demand of the substituents R (Cy vs. 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 reaction rate in the gas phase depends on the π-complex preequil. and metallacyclobutane formation, the rate-detg. step. In ROMP reactions backbiting has a profound influence on the overall rate. The reactivity trends detd. in the gas phase parallel soln.-phase reactivity. The overall rate in soln. was also detd. by a favorable dimer/monomer preequil. providing the active catalyst by facile dissocn. of dicationic, dinuclear catalyst precursors. |
