A Ferrocenyl‐Benzo‐Fused Imidazolylidene Complex of Ruthenium as Redox‐Switchable Catalyst for the Transfer Hydrogenation of Ketones and Imines

Abstract

A ferrocenyl‐benzo‐fused imidazolylidene complex of RuII was prepared and fully characterized. In the presence of acetylferrocenium tetrafluoroborate this complex can be oxidized to generate a complex with a cationic ligand. The neutral complex can be recovered by reducing the oxidized cationic compound with cobaltocene. The activity of the neutral and oxidized complexes was tested in the transfer hydrogenation of ketones and imines, using isopropyl alcohol as the hydrogen source. The neutral complex is very active in the reduction of all tested substrates, whereas the oxidized species shows low activity in the reduction of ketones. The rate of the reduction of hexaphenone could be modulated by addition of subsequent amounts of oxidant and reductant. The addition of acetylferrocenium tetrafluoroborate caused a decrease in the catalytic activity, whereas the addition of cobaltocene restored the activity. The catalytic activity shown by both catalysts in the reduction of N‐benzylideneaniline was similar.

Redox‐Switchable Transfer Hydrogenations with P‐Chiral Dendritic Ferrocenyl Phosphine Complexes

“…[4][5][6][7][8][9][10] Although RSC has been applied to a wide variety of catalytic reactions, such as ring-opening polymerizations, [11][12][13][14][15][16][17] ring-closing metatheses [18,19] and other reactions, [20][21][22][23][24][25][26][27] surprisingly little attention has been devoted to the redox-controlled formation of alcohols by transfer hydrogenation (TH). [28] This is particularly remarkable, given that TH is an attractive alternative to direct hydrogenation and has yielding an enantioenriched alcohol. By adding a chemical oxidant or reductant, the catalytic activity of the complexes was reversibly switched off and back on again over the course of the hydrogen transfer reaction.…”

“…This outstanding work laid the foundation for a spectacular development of RSC as an important area of homogeneous catalysis . Although RSC has been applied to a wide variety of catalytic reactions, such as ring‐opening polymerizations, ring‐closing metatheses, and other reactions, surprisingly little attention has been devoted to the redox‐controlled formation of alcohols by transfer hydrogenation (TH) . This is particularly remarkable, given that TH is an attractive alternative to direct hydrogenation and has recently become the center of research in hydrogenation science …”

Selectivity control in hydrogenation through adaptive catalysis using ruthenium nanoparticles on a CO2-responsive support

“…Known methods to 'switch' the reactivity of catalysts typically apply external stimuli, including temperature 25,26 , pH (ref. 27 ), solvent variation 24,28 , irradiation 24,28 or redox processes [29][30][31] . Although these methods are able to generate two different states of a catalyst that exhibit different selectivities, the underlying physical and chemical elementary processes are mostly irreversible or associated with the generation of additional components that accumulate in the reaction mixture.…”

Gold(ii) in redox-switchable gold(i) catalysis

“…[26][27][28] Indeed, RSC is a strongly expanding field in homogeneous catalysis. [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] The redox-switching event occurs either at a remote redox-active unit, e.g. a Fc unit located on the ligand [26][27][28]30,33,34,[36][37][38] or directly at the catalytic metal centre, e.g.…”