Initial Catalyst−Substrate Association Step in Enyne Metathesis Catalyzed by Grubbs Ruthenium Complex Probed by Time-Dependent Fluorescence Quenching
摘要:
This communication introduces an FRET-based simple and efficient method for monitoring kinetics and thermodynamics of organic reactions and describes its application to studies on the initial catalyst-substrate association step in the enyne metathesis catalyzed by a Grubbs Ru complex to probe the reaction initiation on the alkyne versus the alkene. The kinetic and thermodynamic parameters of alkene and alkyne with the first generation Grubbs Ru complex, determined by the time-dependent fluorescence quenching of the dye-conjugated substrate by the Ru catalyst, strongly support the dominance of the reaction initiation on alkene over alkyne in the reaction both kinetically and thermodynamically.
Correlation between Functionality Preference of Ru Carbenes and <i>exo</i>/<i>endo</i> Product Selectivity for Clarifying the Mechanism of Ring-Closing Enyne Metathesis
Functionality preferences of metathesis Ru carbenes to various alkenes and alkynes with electronic and steric diversity were determined by using time-dependent fluorescence quenching. The functionality preferences depend not only on the properties of multiple bonds but also on the ligands on Ru. There was a good correlation between functionality preference and the metathesis reaction outcome. The correlation between functionality preference and exo/endo product ratio offers a solution to resolve the mechanistic issue related with alkene- vs alkyne-initiated pathway in ring-closing enyne metathesis. The correlation indicates the preference is likely to dictate the reaction pathway and eventually the outcome of the reaction. The Ru catalyst favoring alkyne over alkene provides more endo product, indicating that the reaction mainly initiates at the alkyne. By changing the substitution pattern, the preference can be reversed to give an exclusive exo product.