Chiral Rhodium(I) and Iridium(I) Amino−Olefin Complexes:  pK_a, N−H Bond Dissociation Energy, and Catalytic Transfer Hydrogenation

Abstract

The chiral tetrachelating amino−olefins (R,R)-N,N‘-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diaminocyclohexane ((R,R)-trop2dach) and (S,S)-N,N‘-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diphenyl-1,2-ethylenediamine ((S,S)-trop2dpen) were prepared and used as ligands in the complexes (R,R)-[Rh(trop2dach)]OTf and (S,S)-[Rh(trop2dpen)]OTf (OTf- = CF3SO3 -). Quasi-reversible reductions, d8-[RhI(trop2diamine)]+ + e- → d9-[Rh0(trop2diamine)] and d9-[Rh0(trop2diamine)] + e- → d10-[Rh-I(trop2diamine)]-, at rather negative potentials (trop2diamine = trop2dach, E 1/2 1 = −1.83 V, E 1/2 2 = −2.27 V; trop2diamine = trop2dpen, E 1/2 1 = −1.78 V, E 1/2 2 = −2.24 V; vs Fc+/Fc) indicate the donor capacity of the amine functions. One NH group in (R,R)-[Rh(trop2dach)]OTf (pK a = 15.7(2), NH bond dissociation energy (BDE) 317(2) kJ mol-1) is easily deprotonated to give the neutral amide (R,R)-[Rh(trop2dach-H)], which is quasi-reversibly oxidized at E° = −0.34 V (vs Fc/Fc+) to the radical cation (R,R)-[Rh(trop2dach-H)]•+. The pK a and BDE values of the NH group in these 16-electron complexes are lower than in related pentacoordinated 18-electron complexes. X-ray diffraction analyses show that the rhodium complexes have distorted-square-planar structures. The Rh−N bond shortens by about 7% upon deprotonation. The rhodium complexes are inactive as catalysts for transfer and direct hydrogenation of ketones. However, the distorted-trigonal-bipyramidal iridium complex (S,S)-[IrCl(CO)(trop2dpen)], where the amino−olefin ligand serves as a tridentate ligand, serves as a chiral precursor to an active phosphane-free catalyst in the transfer hydrogenation of acetophenone with 2-propanol, and the R isomer of 1-phenylethanol was obtained in 82% ee (>98% conversion when acetophenone:KOtBu:cat = 1:0.1:0.01, T = 80 °C, reaction time 1 h).

A Class of Benzene Backbone-Based Olefin–Sulfoxide Ligands for Rh-Catalyzed Enantioselective Addition of Arylboronic Acids to Enones

“…1À6 Pioneered by the groups of Hayashi and Carreira, chiral olefins as a new class of promising ligands have attracted great attention in recent years, 7À9 and many excellent olefin-based ligands such as diene ligands, 10À23 olefinÀphosphine ligands, 24À30 and olefinÀnitrogen ligands 31,32 have been successfully developed in the HayashiÀMiyaura asymmetric reactions. Meanwhile, except for a chiral auxiliary in numerous asymmetric transformations, the application of sulfoxides 33 as ligands, especially the chiral bis-sulfoxides, 34À38 has most recently been appealing in the HayashiÀMiyaura reaction.…”

Katalyse nach dem Vorbild von Metalloenzymen: selektive Oxidation von primären Alkoholen mit einem Iridium‐Aminylradikal‐Komplex

“…60 % der Spinpopulation des ungepaarten Elektrons am Aminylstickstoff bereitwillig HAtome abstrahieren. [18,19] Die Synthese des Iridiumkomplexes, den wir nun in den katalytischen Alkoholdehydrierungen einsetzen, gelingt in wenigen einfachen Schritten (Schema 2): Die Reaktion des vierzähnigen Liganden trop 2 dach (1) [20] (Abbildung 1 a). [21] Deutlich sind die Starrheit des trop 2 dach-Liganden und die sterische Abschirmung des Ir-und der N-Atome zu erkennen.…”

Synthesis of Chiral Olefin Ligands and their Application in Asymmetric Catalysis

“…Since Grützmacher and co-workers reported the first phosphine/ olefin hybrid ligand 13 with a 5H-dibenzoA C H T U N G T R E N N U N G [a,d]-cycloheptatriene backbone for Ir-catalyzed asymmetric hydrogenation of imines to give the reductive products with up to 86 % ee in 2004, [14] phosphorous/olefin ligands 14-20 have been successfully developed for transition-metal-catalyzed asymmetric additions, allylic alkylations or aminations, and intramolecular hydroacylations (Scheme 2). [15][16][17][18][19][20][21] Nitrogen atoms were also incorporated into hybrid olefin ligands by the groups of Grützmacher, [22] Glorius, [23] and FranzØn [24] (Scheme 3). Ligands 21-23 were used in Rh I -catalyzed asymmetric conjugated additions.…”