Chiral Oxazolidine‐Fused N‐Heterocyclic Carbene Complexes of Rhodium and Iridium and Their Utility in the Asymmetric Transfer Hydrogenation of Ketones

Abstract

The catalytic potential of new N-heterocyclic carbene ligands, derived from a chiral fused bicyclic ring scaffold with restricted rotation along the C-N bond bearing the chiral auxiliary, has been explored in the transition-metal-mediated asymmetric transfer hydrogenation reactions of ketones. In particular, the chiral oxazolidine-fused N-heterocyclic carbene precursors (3S)-3-R-6-methyl-7-phenyl-2,3-dihydroimidazo[5,1-b]oxazol-6-ium iodide [R = sec-butyl (1f ), i-butyl (2f ), isopropyl (3f )] were synthesized from commercially available optically pure amino acids in a multistep sequence that avoids tedious chiral resolution. The reactions of the chiral imidazolium iodide salts 1f-3f with Ag 2 O yielded the corresponding silver complexes [a]

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

“…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 . Notably, the characteristic Rh‐C carbene resonances of the rhodium(I) ( 1 ‐ 3 ) complexes appeared at 157.9 ppm ( 1 J Rh–C =50 Hz) for ( 1 ), 158.2 ppm ( 1 J Rh–C =48 Hz) for ( 2 ) and 158.7 ppm ( 1 J Rh–C =50 Hz) for ( 3 ), displaying the expected one‐bond 1 J Rh–C coupling as observed in related analogs, {(3 S )‐6‐Me‐7‐Ph‐3‐R‐2,3‐dihydroimidazo[5,1‐ b ]‐oxazol‐5‐ylidene}Rh(COD)Cl, [R= s ‐Bu (176.3 ppm, 1 J Rh‐C =48 Hz); i ‐Bu (171.7 ppm, 1 J Rh‐C =50 Hz) and i ‐Pr (171.9 ppm, 1 J Rh‐C =50 Hz)], {[N‐(CH 2 Ph)‐2‐(3‐ i ‐Pr‐4,5‐(Ph) 2 ‐2,3‐dihydro‐1H‐imidazol‐2‐ylidene)cyclohexanamine]Rh(COD)}BAr 4 [Ar=3,5‐(CF 3 ) 2 C 6 H 3 ] (176.5 ppm, 1 J Rh‐C =51 Hz), and (1‐Bu‐3‐Me‐imidazol‐2‐ylidene)Rh(COD)Cl (181.8 ppm, 1 J Rh‐C =50 Hz) …”

“…The molecular structure of the rhodium(I) ( 1 ‐ 3 ) complexes showed the rhodium center bound to the chiral C 2 ‐symmetric tricyclic bioxazoline fused imidazole derived NHC ligand along with a chloride atom and a η 4 ‐bound 1,5‐cyclooctadiene moiety in an inverted boat conformation, all attached in a tetrahedral geometry around the metal centre (Figure –). Of particular interest is the Rh‐C carbene bond lengths of 2.020(11) Å ( 1 ), 2.009(5) Å ( 2 ) and 2.036(4) Å ( 3 ) that are slightly shorter than the sum of the individual covalent radii of the Rh and C atoms [d(Rh–C)=2.182 Å] but are comparable to that observed in {(3 S )‐6‐Me‐7‐Ph‐3‐R‐2,3‐dihydroimidazo[5,1‐ b ]‐oxazol‐5‐ylidene}Rh(COD)Cl, {R= s ‐Bu [2.020(18) Å]; i ‐Bu [2.023(4) Å] and i ‐Pr [2.019(3) Å]}, {[N‐(CH 2 Ph)‐2‐(3‐ i ‐Pr‐4,5‐(Ph) 2 ‐2,3‐dihydro‐1H‐imidazol‐2‐ylidene)cyclohexanamine]Rh(COD)}BAr 4 [Ar=3,5‐(CF 3 ) 2 C 6 H 3 ] [2.025(8) Å] and (1‐Bu‐3‐Me‐imidazol‐2‐ylidene)Rh(COD)Cl [2.227(6) Å] . Similarly, the Rh–Cl bond length of 2.393(2) Å ( 1 ), 2.3714(13) Å ( 2 ) and 2.3875(13) Å ( 3 ) also closely resembles that in {(3 S )‐6‐Me‐7‐Ph‐3‐R‐2,3‐dihydroimidazo[5,1‐ b ]‐oxazol‐5‐ylidene}Rh(COD)Cl, {R= s ‐Bu [2.3823(5) Å]; i ‐Bu [2.376(10) Å] and i ‐Pr [2.3810(9) Å]} and (1‐Bu‐3‐Me‐imidazol‐2‐ylidene)Rh(COD)Cl [2.3866(18) Å] complexes.…”

“…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) …”

“…With our longstanding interest in exploring the utility of the transition metal complexes of the N‐heterocyclic carbene ligands in catalytic and biomedical applications, we became interested in exploring the catalytic potential of rhodium N‐heterocyclic carbene complexes, and have recently reported their application in asymmetric transfer hydrogenation reactions, and proceeding further along the line, we set out to explore their utility in the 1,4‐conjugate addition using aryl boronic acid and α , β ‐unsaturated carbonyl compounds. We have also reported the use of transition metal based N‐heterocyclic carbene complexes for C‐C bond forming reactions like the bifunctional catalysis of the base‐free Micheal addition reactions, C‐C cross coupling C‐N bond formation in β ‐enaminones, hydroamination of alkynes and alkenes, C‐B bond forming reactions, tandem reactions involving Heck alkynylation/cyclization reaction and in the Hiyama alkynylation/cyclization reactions, hydrosilylation, cyanosilylation and a transfer hydrogenation reactions …”

“…We have also reported the use of transition metal based Nheterocyclic carbene complexes for C-C bond forming reactions like the bifunctional catalysis of the base-free Micheal addition reactions, [18] C-C cross coupling [19] C-N bond formation in βenaminones, [20] hydroamination of alkynes [21] and alkenes, [22] C-B bond forming reactions, [23] tandem reactions involving Heck alkynylation/cyclization reaction [24] and in the Hiyama alkynylation/cyclization reactions, [25] hydrosilylation, [26] cyanosilylation [27] and a transfer hydrogenation reactions. [17] Here in this manuscript, we report rhodium(I) (1-3) complexes of chiral C 2 -symmetric tricyclic bioxazoline fused imidazole derived N-heterocyclic carbene ligands of the type {(3R,7R)-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] that effectively catalyzed the 1,4 conjugate addition of wide variety of aryl boronic acids with α,β-unsaturated carbonyl compounds (Figure 1).…”