Dynamic π-Bonding of Imidazolyl Substituent in a Formally 16-Electron Cp*Ru(κ²-P,N)⁺ Catalyst Allows Dramatic Rate Increases in (E)-Selective Monoisomerization of Alkenes

Abstract

Alkene isomerization can be an atom-economical approach to generating a wide range of alkene intermediates for synthesis, but fully equilibrated mixtures of disubstituted internal alkenes typically contain significant amounts of the positional as well as geometric (E and Z) isomers. Most classical catalyst systems for alkene isomerization struggle to kinetically control either positional or E/Z isomerism. We report coordinatively unsaturated, formally 16-electron Cp*Ru catalyst 5, which facilitates simultaneous regio- and stereoselective isomerization of linear 1-alkenes to their internal analogues, providing consistent yields of (E)-2-alkenes greater than 95%. Because nitrile-free catalyst 5 is more than 400 times faster than previously published nitrile-containing analogues 2 + 2a, very reasonable 0.1–0.5 mol % loadings of 5 complete ambient-temperature reactions within 15 min to 4 h. UV–vis, NMR, and computational studies depict the imidazolyl fragment on the phosphine as a hemilabile, four-electron donor in κ2-P,N coordination. For the first time, we show direct experimental evidence that the PN ligand has accepted a proton from the substrate by characterizing the intermediate Cp*Ru­[η3-allyl]­[κ1-P)­P–N+H], which highlights the essential role of the bifunctional ligand in promoting rapid and selective alkene isomerizations. Moreover, kinetic studies and computations reveal the role of alkene binding in selectivity of unsaturated catalyst 5.

New Access Routes to Privileged and Chiral Ligands for Transition‐Metal Catalyzed Hydrogen Autotransfer (Borrowing Hydrogen), Dehydrogenative Condensation, and Alkene Isomerization Reactions

“…This was partially successful, in that crystals of BArF salt 64 suitable for X‐ray diffraction were obtained, even if the recrystallized bulk material was not uniform and may have contained another non‐identified species, likely a coordination/solvation isomer ( Scheme 9 ,b ; Figure 6). Related complexes with substituted cyclopentadienyl units have been structurally characterized by Grotjahn and Vidovic and coworkers, and it was shown that depending on the steric requirements of the cyclopentadienyl and the phosphine donor groups, either two, one or no acetonitriles bind to the ruthenium center [65,112] …”

“…Related complexes with substituted cyclopentadienyl units have been structurally characterized by Grotjahn and Vidovic and coworkers, and it was shown that depending on the steric requirements of the cyclopentadienyl and the phosphine donor groups, either two, one or no acetonitriles bind to the ruthenium center. [65,112]…”

“…Finally, cationic cyclopentadienyl‐ruthenium complex 8 with an imidazolyl‐phosphine ligand has been shown to be a highly active alkene isomerization catalyst by Grotjahn and coworkers, who named this bifunctional catalyst an alkene zipper by virtue of its ability to move a double‐bond across entire hydrocarbon chains [62–65] . When we planned some time ago to explore the synthetic utility of catalytic DHC, HAT/BH and isomerization reactions in the field of catalytic organic synthesis (which we collectively think of as hydrogen moving reactions ), 1 it became clear that the success of such studies would critically depend on access to key catalysts.…”

Selecting Double Bond Positions with a Single Cation-Responsive Iridium Olefin Isomerization Catalyst

“…Steric factors have proven essential in controlling the selectivity in olefin isomerization. Grotjahn and coworkers observe distinct selectivity based on the cyclopentadienyl substitution pattern in Ru catalysts, 7,59 for example, and Jiao, Liu, and coworkers saw large variations in selectivity as a function of steric parameter in pincer-ligated Co catalysts. 60 These observations suggest that the two catalyst states in a responsive system should be sterically distinct.…”

Selecting Double Bond Positions with a Single Cation- Responsive Iridium Olefin Isomerization Catalyst

“…Steric factors have proven essential in controlling the selectivity in olefin isomerization. Grotjahn and coworkers observe distinct selectivity based on the cyclopentadienyl substitution pattern in Ru catalysts, 7,59 for example, and Jiao, Liu, and coworkers saw large variations in selectivity as a function of steric parameter in pincer-ligated Co catalysts. 60 These observations suggest that the two catalyst states in a responsive system should be sterically distinct.…”