Asymmetric Transfer Hydrogenation of α,β‐Unsaturated Carbonyl Compounds to Saturated Alcohols as Catalyzed by Iridium Complexes of Tricyclic Bioxazoline‐Fused Imidazole‐Derived N‐Heterocyclic Carbene Ligands

Abstract

A series of chiral iridium(I) (1-3)e complexes of bioxazoline fused imidazole derived N-heterocyclic carbene (NHC) ligands successfully carried out asymmetric transfer hydrogenation of α, β-unsaturated ketones in good to excellent yields (ca. 36-91%) and in low enantioselectivites (ca. 5-31% ee) at 1 mol % of the catalyst (1-3)e loading in the presence of NaOH as a base in i-PrOH at 75°C in 3 hours of reaction time. The iridium(I) (1-3)e complexes were synthesized directly from the bioxazoline fused imidazole derived NHC precursors namely {(3R,7R)-3,7-di-R-2,3,7,8-tetrahydrodioxazolo[3,2-c:3',2'-e]imidazol-4-ium trifluoromethanesulfonate, R = s- butyl (1 d), i-butyl (2 d), i-propyl (3 d)} by the treatment with {(COD)IrCl} 2 (COD = 1,5-cyclooctadiene) in presence of t-BuOK as a base at room temperature. The chiral bioxazoline fused imidazole NHC ligand precursors (1-3)d were synthesized from the commercially available optically pure amino acids by a sequence of reactions without requiring any chiral resolution.

1,4‐Conjugate Addition of Aryl boronic Acids on Cyclohexenone as Catalyzed by Rhodium(I) Complexes of C2‐Symmetric Bioxazoline Fused N‐heterocyclic Carbenes

“…The rhodium(I) complexes of the type {(3 R ,7 R )‐3,7‐ di ‐R‐2,3,7,8‐tetrahydrodioxazolo[3,2‐c:3′,2′‐e]imidazol‐5‐ylidene}Rh(COD)Cl [R= s ‐butyl ( 1 ), i ‐butyl ( 2 ), i ‐propyl ( 3 ), COD=1, 5‐cyclooctadiene] were conveniently synthesized by the treatment of the chiral C 2 ‐symmetric tricyclic bioxazoline fused imidazole derived NHC precursors with {(COD)RhCl} 2 at room temperature in presence of the base t ‐BuOK (Scheme ). The tricyclic bioxazoline fused NHC ligand precursors {(3 R ,7 R )‐3,7‐ di ‐R‐2,3,7,8‐tetrahydrodioxazolo[3,2‐c:3′,2′‐e]imidazol‐4‐ium trifluoromethanesulfonate (R= s ‐butyl, i ‐butyl, i ‐propyl) were prepared from commercially available optically pure amino acids . More interestingly, the 1 H NMR and 13 C{ 1 H} NMR spectrum of rhodium(I) ( 1 – 3 ) complexes showed resonances consistent with the loss of C 2 ‐symmetry from that in the N‐heterocyclic carbene precursor .…”

“…1‐4% ( ee )]. The observation of low enantioselectivities in case of the rhodium(I) ( 1 ‐ 3 ) complexes may be attributed to the rotation of the NHC ligand along the Rh‐C carbene bond as has been reported in case of the related complexes namely, the {( R )‐2‐benzhydryl‐5‐phenyl‐6,7‐dihydro‐5H‐pyrrolo[1,2‐ c ]imidazol‐2‐ylidene}Ir(COD)Cl, {(3 R ,7 R )‐3,7‐di‐R‐2,3,7,8‐tetrahydrodioxazolo[3,2‐c:3′,2′‐e]imidazol‐5‐ylidene}Ir(COD)Cl, [R= s ‐butyl, i ‐butyl, i ‐propyl], {(3 S )‐6‐methyl‐7‐phenyl‐3‐R‐2,3‐dihydroimidazo[5,1‐b]‐oxazol‐5‐ylidene}Ir(COD)Cl (R= s ‐butyl, i ‐butyl, i ‐propyl) and {(3 S )‐6‐methyl‐7‐phenyl‐3‐R‐2,3‐dihydroimidazo[5,1‐b]‐oxazol‐5‐ylidene}Rh(COD)Cl (R= s ‐butyl, i ‐butyl, i ‐propyl) …”

Ru-Catalyzed Asymmetric Addition of Arylboronic Acids to Aliphatic Aldehydes via P-Chiral Monophosphorous Ligands

“…Thus, the invention of methods for the preparing of such chiral aryl alkyl alcohols features an important role in organic synthesis. Classic methods for constructing such chiral molecules include asymmetric hydrogenation of ketones [11][12][13], asymmetric hydrogen transfer reduction of ketones or unsaturated ketones [14][15][16][17][18][19][20][21], along with the asymmetric addition of organometallic reagents to aldehydes [22][23][24]. As an alternative, strategies via the asymmetric homologation reaction [25][26][27][28], tandem Michael-MPV reaction and subsequent reductive desulfurization [29], as well as the asymmetric addition of aldehydes with arylboronic acids were also established [30,31].…”

Design of Iridium N-Heterocyclic Carbene Amino Acid Catalysts for Asymmetric Transfer Hydrogenation of Aryl Ketones

“…Although noble metal NHC complexes generally display excellent activity for catalytic transfer hydrogenation, examples of highly enantioselective catalysts of this type remain relatively scarce [2,[40][41][42][43]. Iridium NHC complexes have been extensively studied for ATH of ketones; however, the majority of these catalysts employ synthetically challenging chiral NHC ligands that generate the alcohol products in only low to moderate enantiomeric excess (Figure 2) [44][45][46][47][48][49][50][51][52].…”