2 research outputs found
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 <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
Preference of Ruthenium-Based Metathesis Catalysts toward <i>Z</i>- and <i>E</i>‑Alkenes as a Guide for Selective Reactions to Alkene Stereoisomers
As a guide for selective
reactions toward either <i>Z</i>- or <i>E</i>-alkene
in a metathesis reaction, the relative
preference of metathesis Ru catalysts for each stereoisomer was determined
by a method using time-dependent fluorescence quenching. We found
that <b>Ru-1</b> prefers the <i>Z</i>-isomer over
the <i>E</i>-isomer, whereas <b>Ru-2</b> prefers the <i>E</i>-isomer over the <i>Z</i>-isomer. The <i>Z</i>/<i>E</i>-alkene preference of the catalysts
precisely predicted the <i>Z</i>/<i>E</i> isomeric
selectivity in the metathesis reactions of diene substrates possessing
combinations of <i>Z</i>/<i>E</i>-alkenes. For
the diene substrates, the rate order of the reactions using <b>Ru-1</b> was <i>Z,Z-</i>1,6-diene > <i>Z,E-</i>1,6-diene > <i>E,E-</i>1,6-diene, while the completely
opposite order of <i>E,E-</i>1,6-diene > <i>Z,E-</i>1,6-diene > <i>Z,Z-</i>1,6-diene was exhibited in the
case
of <b>Ru-2</b>