Cooperative N‐Heterocyclic Carbene (NHC) and Ruthenium Redox Catalysis: Oxidative Esterification of Aldehydes with Air as the Terminal Oxidant

Abstract

The paper describes a cooperative NHC (N-heterocyclic carbene) and ruthenium-based redox catalysis for the mild aerobic oxidative esterification of various aromatic and heteroaromatic aldehydes. ) 2 (bpz = 2,2'-bipyrazine) as catalyst is shown to be compatible with free NHCs. The NHC is used in these cascade reactions for the umpolung of the aldehyde to form the corresponding Breslow intermediate which in turn gets oxidized to an acylazolium ion by the ruthenium redox catalyst. Air is used as a terminal oxidant for oxidation (regeneration) of the ruthenium catalyst. In addition, we will show that in the absence of the ruthenium redox catalyst and alcohol, NHC-catalyzed aerobic oxidation of aldehydes delivers the corresponding acids in good to excellent yields. Mechanistic studies and DFT calculations supporting our suggested mechanisms are provided.

Catalytic kinetic resolution of a dynamic racemate: highly stereoselective β-lactone formation by N-heterocyclic carbene catalysis

“…66,122–136 This study builds on these earlier reports and reveals not only an unusual method of stereocontrol, but also an unprecedented mechanism. The computed catalytic cycle and the reaction coordinate diagram are shown in Figure 1.…”

Aerobic Oxidation/Annulation Cascades through Synergistic Catalysis of RuCl3 and N‐Heterocyclic Carbenes

“…The homoenolate is generated from the enal by reaction with the NHC. RuCl 3 oxidizes the homoenolate by an SET process, in an manner analogous to that proposed by Rovis, Chi, Studer, and Sun . The resulting radical cation is deprotonated to yield a zwitterionic radical species.…”

“…Rovis used [Ru-(bpy) 3 ]Cl 2 (bpy = bipyridine) to tether ap hotocatalytic cycle with formation of an NHC-catalyzed Breslow intermediate (Scheme 1a). [8] Studerr eported single-electron oxidation of the Breslow intermediate by [Ru(bpz) 3 ][PF 6 ] 2 (bpz = 2,2-bipyrazine; Scheme 1b), [9] but low conversions wereo btained for the corresponding homoenolate intermediates. To the best of our knowledge,s ynergistic catalysisi nvolving an NHCa nd ar uthenium with labile ligands is unknown.The a,b-unsaturateda cylazolium speciesi sav ersatile reaction intermediate that allowsh ighly enantioselective annulation reactionsu sing various bisnucleophilic reagents.…”

A cooperative N-heterocyclic carbene/palladium catalysis system

“…1 Although there has been remarkable progress in this area, the combination of N -heterocyclic carbenes (NHCs) 2 with late transition metals remains underexplored, and more importantly, quite counterintuitive given the strong propensity for NHCs to bind to transition metals with high affinity. While cooperative catalysis with NHCs and Lewis or Brønsted acids has been shown to increase reactivity and afford products with unprecedented levels of selectivity 3 , the incorporation of NHCs with late transition metals (TMs) presents a considerable challenge due to the potential for the formation of stable NHC-TM complexes (Figure 1), 4 which do not possess the desired catalytic activity.…”