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
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Abstract
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 <i>exo</i>/<i>endo</i> 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 <i>endo</i> product, indicating that the reaction mainly initiates
at the alkyne. By changing the substitution pattern, the preference
can be reversed to give an exclusive <i>exo</i> product