Hydrogenation and Transfer Hydrogenation Promoted by Tethered Ru−S Complexes: From Cooperative Dihydrogen Activation to Hydride Abstraction/Proton Release from Dihydrogen Surrogates

Abstract

Hydrogenation and transfer hydrogenation of imines with cyclohexa-1,4-dienes, as well as with a representative Hantzsch ester dihydrogen surrogate, are reported. Both processes are catalyzed by tethered Ru-S complexes but differ in the activation mode of the dihydrogen source: cooperative activation of the H-H bond at the Ru-S bond leads to the corresponding Ru-H complex and protonation of the sulfur atom, whereas the same cationic Ru-S catalyst abstracts a hydride from a donor-substituted cyclohexa-1,4-diene to form the neutral Ru-H complex and a low-energy Wheland intermediate. A sequence of proton and hydride transfers on the imine substrate then yields an amine. The reaction pathways are analyzed computationally, and the established mechanistic pictures are in agreement with the experimental observations.

A Tethered Ru–S Complex with an Axial Chiral Thiolate Ligand for Cooperative Si–H Bond Activation: Application to Enantioselective Imine Reduction

“…The thus‐generated diastereomeric chiral‐at‐ruthenium hydrosilane adducts were applied to catalytic imine hydrosilylation but enantioinduction (40 % ee ) was at best promising. One challenge could be that there is no bonding interaction between the neutral ruthenium hydride and the silyliminium ion in the enantioselectivity‐determining hydride transfer . This work is nevertheless proof of concept for asymmetric catalysis involving cooperative Si−H bond activation.…”

Electrophilic Aromatic Substitution with Silicon Electrophiles: Catalytic Friedel–Crafts C−H Silylation

“…[32] Heterolytic cleavage of the Si À Hb ond at the metal-sulfur bond of these complexes results in the formation of as ulfur-stabilized silicon cation and ar uthenium hydride (XVII!XVIII, Scheme 11,bottom). XX is known to be unstable at room temperature, [31] immediately releasing dihydrogen to reform XVII. Thes uccess of these steps for Friedel-Crafts silylation hinges on the Lewis basic sulfur atom in complex XIX,w hich acts as an internal base,a bstracting the proton from Wheland intermediate XII (XII!XIII and XIX!XX).…”

“…Afew years ago,Oestreich in collaboration with Ohki and Tatsumi found that cooperative Si À Hb ond activation at Lewis acidic metal centers is an entry into catalysis with electrophilic silicon reagents, [29,30] for example,f or Friedel-Crafts silylation. [29] Oestreich and co-workers used Ohki-Tatsumi complexes,c oordinatively unsaturated ruthenium thiolate complexes initially applied to dihydrogen activation, [31] for this purpose (38,S cheme 11, top). [32] Heterolytic cleavage of the Si À Hb ond at the metal-sulfur bond of these complexes results in the formation of as ulfur-stabilized silicon cation and ar uthenium hydride (XVII!XVIII, Scheme 11,bottom).…”

“…This deprotonation yields the C3-silylated heteroarene XIII as well as complex XX,the dihydrogen adduct of the starting catalyst XVII. XX is known to be unstable at room temperature, [31] immediately releasing dihydrogen to reform XVII.…”

Elektrophile aromatische Substitution mit Siliciumelektrophilen: die katalytische Friedel‐Crafts‐C‐H‐Silylierung

“…B. für Friedel-Crafts-Reaktionen, bietet. [29] Oestreich und Mitarbeiter nutzten zu diesem Zweck Ohki-Tatsumi-Komplexe, also koordinativ ungesättigte Rutheniumthiolatkomplexe,die ursprünglich zur Diwasserstoffaktivierung verwendet worden waren [31] (38,S chema 11, oben). [32] Die heterolytische Spaltung der Si-H-Bindung an der Metall-Schwefel-Bindung resultiert in der Bildung eines schwefelstabilisierten Siliciumkations und eines Rutheniumhydrids (XVII!XVIII,S chema 11, unten [31] und bildet unter sofortiger Diwasserstofffreisetzung wieder XVII.…”