Asymmetric Hydrosilylation Of Ketones Research Articles

Synthesis and Characterization of Air‐Stable Palladium(II) and Copper(I) Ph‐Garphos Complexes and their Applications to Asymmetric Catalysis

AbstractAir‐stable palladium(II) and copper(I) complexes containing the 4,4′,6,6′‐tetramethoxybiphenyl‐2,2′‐diyl)bis(diphenylphosphine) ligands, (R)‐Ph‐Garphos ((R)‐L1) and ((S)‐Ph‐Garphos ((S)‐L1), were synthesized and characterized by 1H‐, 13C‐, and 31P{1H} NMR spectroscopy and elemental analyses. The PdCl2((R)‐Ph‐Garphos) ((R)‐1) and PdCl2((S)‐Ph‐Garphos) ((S)‐1) complexes were assessed for the Suzuki‐Miyaura cross‐coupling of aryl halides with phenylboronic acid under ambient conditions, or naphthyl substrates under reflux. X‐ray crystal structures of the copper complexes revealed that [Cu((R)‐Ph‐Garphos)I] ((R)‐2) crystallized as a three‐coordinate monomer in the orthorhombic space group P212121 whereas [Cu((S)‐Ph‐Garphos)I] ((S)‐2) crystallized as a tetrahedral (μ‐I) dimer in the same crystal system. The synthesis of the analogous [Cu((R)‐Ph‐Garphos)OAc] ((R)‐3) is also described. Complexes (R)‐2 and (R)‐3 were evaluated for the asymmetric hydrosilylation of ketones wherein enantioselectivities of up to 83 % ee were achieved.

Enantioselective Carbonyl 1,2- or 1,4-Addition Reactions of Nucleophilic Silyl and Diazo Compounds Catalyzed by the Chiral Oxazaborolidinium Ion.

Boron Lewis acid catalysis has a long history and has become one of the most powerful methods for organic synthesis. In addition to achiral boron catalysts such as BX3 (X = F, Cl, Br) and B(C6F5)3, chiral boron catalysts are also significant synthetic tools used by organic chemists in academic laboratories and industry. Since first reported by Corey et al. in 2002( Corey et al. J. Am. Chem. Soc. 2002 , 124 , 3808 ), the chiral oxazaborolidinium ion (COBI), an activated form of proline-derived oxazaborolidine, has been used as a strong Lewis acid catalyst. Although the early examples of asymmetric synthesis through COBI-catalyzed nucleophilic 1,2- or 1,4-carbonyl additions were reported in 2004-2006, Diels-Alder and cycloaddition reactions of various carbonyl compounds were mostly developed over the next several years to afford enantioenriched cyclized products. The power of COBI in catalyzing carbonyl 1,2- or 1,4-addition reactions triggered our interest in developing asymmetric synthetic methodologies to generate versatile enantiomerically enriched compounds. In this Account, we summarize our recent studies on COBI-catalyzed asymmetric nucleophilic carbonyl addition and tandem reactions. Logical mechanistic explanations of asymmetric COBI catalysis are also discussed. The proton-activated COBI catalyst, which can activate various carbonyl compounds such as aldehydes, ketones, acroleins, and enones through Lewis acid-base interactions and synergistic hydrogen bonds, facilitates asymmetric 1,2- or 1,4-carbonyl additions of nucleophiles. Nucleophiles bearing trialkylsilyl groups successfully reacted with aromatic, aliphatic, and α,β-unsaturated aldehydes through 1,2-addition reactions resulting in chiral β-hydroxy esters. In addition, efficient asymmetric hydrosilylation of ketones was achieved with a TfOH-activated COBI catalyst. Optically active β-keto esters and all-carbon quaternary aldehydes were synthesized successfully through asymmetric 1,2-addition of diazo compounds and tandem H- or C-migration, respectively. In some cases, epoxide products were obtained as side products via the Darzens reaction pathway. Solvent and π-π interactions played important roles in favoring C-migration over H-migration. Nucleophilic 1,4-addition of diazo compounds and chemoselective ring-closure afforded an efficient approach to cyclopropanes, and their tandem rearrangements provided four- and seven-membered cyclic compounds with excellent stereoselectivity. After a Michael addition of diazo compounds, the selective β-hydride shift pathway afforded the β-substituted cyclic enones with high diastereo- and enantioselectivity. The presence of π-bond(s) in the substituents at the α-position of the diazo compound hindered the β-hydride shift pathway and, as a result, favored the cyclopropanation pathway. While there still remain challenges to be overcome, these results further understanding of COBI catalysis and open a window for future development of new asymmetric synthetic methods using carbonyl addition and tandem reactions.

Chiral Spiro Dienes Derived Boranes for Asymmetric Hydrosilylation of Ketones

Chiral Spiro Dienes Derived Boranes for Asymmetric Hydrosilylation of Ketones

Synthesis of BINAM‐Based Chiral Di‐1,2,3‐triazolylidene Complexes and Application of the Di‐NHC RhI Catalyst in Enantioselective Hydrosilylation

AbstractDespite the prolific use of (di‐)NHC complexes in homogeneous catalysis, there are relatively few reports on their successful application in asymmetric transformations. In this work the atropisomeric binaphthyl backbone was combined with readily obtainable 1,2,3‐triazolylidenes to develop a strongly electron‐donating C2‐symmetric ligand. The ligand was efficiently synthesized in a three‐step procedure in an overall yield of 91 % starting from commercially available materials. Strategies for the synthesis of the corresponding di‐NHC silver(I), palladium(II), rhodium(I), and iridium(I) complexes have been developed. The rhodium(I) complex was employed in the catalytic asymmetric hydrosilylation of ketones, providing good conversions at catalyst loadings as low as 0.2 mol‐% and giving chiral inductions of up to 51 % ee.

ChemInform Abstract: Enantioselective Hydrosilylation of Ketones Catalyzed by a Readily Accessible N‐Heterocyclic Carbene—Ir Complex at Room Temperature.

AbstractNovel types of air‐stable Ir NHC‐complexes containing a hydroxy‐amide functionalized azolium ligand are developed and used as catalyst for the asymmetric hydrosilylation of ketones in the presence of (EtO)2MeSiH under mild reaction conditions.

Iron-catalyzed asymmetric hydrosilylation of ketones.

A series of iron complexes of chiral iminopyridine-oxazoline (IPO) ligands have been synthesized. The most sterically hindered iron catalyst exhibits excellent activity (up to 99% yield) and high enantioselectivity (up to 93% ee) in asymmetric hydrosilylation of aryl ketones.

New chiral bis(oxazolinyl)bipyridine ligands and application in the iron catalyzed asymmetric hydrosilylation of ketones

C2 symmetrical 6,6′-bis(oxazolinyl)-2,2′-bipyridine (bipybox) chiral ligands have been synthesized from readily available 2,2′-bipyridine. Catalytic asymmetric hydrosilylation of ketones was studied using this family of ligands in the presence of iron(II) acetate.

ChemInform Abstract: Asymmetric Hydrosilylation of Ketones Catalyzed by Zinc Acetate with Hindered Pybox Ligands.

ChemInform Abstract: Asymmetric Hydrosilylation of Ketones Catalyzed by Zinc Acetate with Hindered Pybox Ligands.

Enantioselective Hydrosilylation of Ketones Catalyzed by a Readily Accessible N‐Heterocyclic Carbene–Ir Complex at Room Temperature

AbstractA series of functionalized azolium compounds were synthesized from chiral α‐amino acid derivatives such as β‐amino alcohols. Reaction of hydroxy‐amide‐functionalized azolium salts thus obtained with Ag2O afforded N‐heterocyclic carbene‐Ag (NHC–Ag) complexes. Subsequent treatment of the resulting silver compound with [IrCl(cod)]2 yielded monodentate IrCl(cod)(NHC), which was stable in air. The NHC–Ir complex facilitated efficient asymmetric hydrosilylation of ketones using (EtO)2MeSiH under ambient conditions. A chiral ligand containing an isobutyl stereodirecting group was found to be the best ligand for the functionalized NHC–Ir complexes that were examined. A linear relationship was found between the catalyst ee and the product ee. The hydroxy functional group on the NHC ligand side‐arm not only induced stereocontrol but also enhanced the reaction rate.

Asymmetric Hydrosilylation of Ketones Catalyzed by Zinc Acetate with Hindered Pybox Ligands

AbstractA highly efficient asymmetric hydrosilylation (AHS) of a wide variety of prochiral aryl ketones catalyzed by zinc acetate with TPS‐he‐pybox (tert‐butyldiphenylsilyl hydroxyethyl pybox) ligand has been successfully developed. Cheap and readily available chiral Lewis acids based on zinc salts have been used as promising catalyst for the reduction of aryl ketones under mild conditions at room temperature leading to chiral alcohols in excellent yields and good to high enantioselectivities (up to 85% ee).magnified image

Carbohydrate-based N-heterocyclic carbenes for enantioselective catalysis

Versatile syntheses of C2-linked and C2-symmetric carbohydrate-based imidazol(in)ium salts from functionalised amino-carbohydrate derivatives are reported. The novel NHCs were ligated to [Rh(COD)Cl]2 and evaluated in Rh-catalysed asymmetric hydrosilylation of ketones with good yields and promising enantioselectivities.

Mesoporous silica KIT-6 supported superparamagnetic CuFe2O4nanoparticles for catalytic asymmetric hydrosilylation of ketones in air

Asymmetric hydrosilylation of ketones catalyzed by magnetically recoverable superparamagnetic CuFe2O4nanoparticles supported on mesoporous silica KIT-6 proceeded in air with up to 97% ee.

Mechanism and Enantioselectivity of [Zinc(diamine)(diol)]‐Catalyzed Asymmetric Hydrosilylation of Ketones: DFT, NMR and ECD Studies

AbstractNMR and ECD measurements together with density functional theory computations were used to analyze the mechanism of [Zn(diamine)(diol)]‐catalyzed ketone hydrosilylation. Of the three possible pathways, the one that assumes formation of a Zn‐hydride species acting as an active catalyst appears energetically most favorable. This conclusion is in contrast to a previously proposed mechanism that assumed the reaction between the [Zn(diamine)(diol)] catalyst, the silane, and the substrate, took place through Zn‐activation of the carbonyl group. The absolute stereochemistry of the final product predicted on the basis of our proposal is in agreement with the available experimental data. It appears that the most important factor that controls stereochemistry of the whole process is preorganization of the reaction substrates by formation of a NH···O=C hydrogen bond between the catalyst and the substrate.

Asymmetric hydrosilylation of ketones catalyzed by complexes formed from trans-diaminocyclohexane-based diamines and diethylzinc.

Chiral acyclic and macrocyclic amines derived from trans-1,2-diaminocyclohexane in complexes with diethylzinc efficiently catalyze asymmetric hydrosilylation of aryl–alkyl and aryl–aryl ketones with enantiomeric excess of the product up to 86 %. A trianglamine ligand with a cyclic structure or the presence of an additional coordinating group increases the enantioselectivity of the reaction, in comparison with catalysis by a simple acyclic N,N′-dibenzyl-1,2-diaminocyclohexane ligand. In addition, the effect of the asymmetric activation of the catalyst by a variety of alcohols and diols is studied.Graphical Electronic supplementary materialThe online version of this article (doi:10.1007/s00706-012-0754-0) contains supplementary material, which is available to authorized users.

Progress in Asymmetric Hydrosilylation of Ketones by Non-precious Metal Catalyst

As important intermediates for the development of biologically active molecules and fragrance etc., chiral secon- dary alcohols are synthesized through asymmetric hydrogenation. Recently, the asymmetric hydrosilylation process has been attracted because of its mild reaction conditions as well as inexpensive and safer reagent system. In this paper, some informa- tion about the recent development of the catalysts of the non-precious metal (Zn, Cu and so on) for asymmetric hydrosilylation reaction of prochiral ketones is briefly reviewed. Keywords asymmetric hydrosilylation; non-precious metal catalyst; chiral alcohol; prochiral ketone; progress

Origins of enantioselectivity in the chiral diphosphine-ligated CuH-catalyzed asymmetric hydrosilylation of ketones

Computational investigations on the asymmetric hydrosilylation of acetophenone over ligated CuH catalysts were performed with the DFT method. The calculations predict that the catalytic reaction involves two steps: (1) CuH addition to the carbonyl group via a four-membered transition state (TS) with the formation of copper-alkoxide intermediates; (2) regeneration of the ligated CuH catalyst by an external SiH(4) through a metathesis process to yield the corresponding silyl ether. The calculations in the chiral diphosphine-ligated CuH systems suggest that the metathesis process is the rate-determining step (RDS). The CuH addition step is vital for the distribution of the racemic products and therefore represents the stereo-controlling step (SCT). In this step, the greater steric hindrance between the aromatic rings of the ligands and the substrate is identified as the major factor for enantioselectivity. The corresponding TS in the face-to-face mode, suffering less steric hindrance, is more stable than its analogue in the edge-to-face mode. The enantioselectivities are calculated to be related not only to the P-Cu-P bite angles in the stereo-controlling TSs, but also to the substituents at the P-aryl rings of the chiral ligands. In short, a larger P-Cu-P bite angle and suitably modified P-aryl rings together are necessary to achieve excellent ee values.

Highly efficient and convenient asymmetric hydrosilylation of ketones catalyzed with zinc Schiff base complexes

Several chiral Schiff base ligands derived from α-amino acids were prepared, and zinc complexes with these chiral Schiff base ligands prepared were tested for the catalytic asymmetric hydrosilylation of ketones, and the results showed that excellent ee values were obtained, which are the prominent examples of catalytic asymmetric hydrosilylation of ketones catalyzed with zinc complexes in the presence of readily available and inexpensive α-amino acids based Schiff base ligands.

A Novel 1‐Glycosyl‐1H‐triazole‐Based P,N Ligand for Rhodium‐Catalyzed Asymmetric Hydrosilylation of Ketones

AbstractA novel class of chiral glycosyl‐triazole‐based P,N ligands were synthesized by ‘click chemistry’ and were applied to the Rh‐catalyzed asymmetric hydrosilylation of a range of substituted acetophenones. Enantioselective hydrosilylation of acetophenone gave (S)‐1‐phenylethanol in moderate enantioselectivity (72% ee) and in good conversion (93%).

ChemInform Abstract: Rhodium‐Catalyzed Asymmetric Hydrosilylation of Ketones Employing a New Ligand Embodying the Bis(oxazolinyl)pyridine Moiety.

A general approach to new ligands embodying bis(oxazolinyl)pyridine has been developed, employing palladium-catalyzed Suzuki coupling and base-mediated cyclization as pivotal steps. A rhodium catalyst derived from the ligand 4-(4-ethylphenyl)-2,6-bis(4-isopropyl-4,5-dihydrooxazol-2-yl)pyridine gave excellent enantioselectivity in the asymmetric hydrosilylation of ketones. In addition the electronic effect of remote substituents on the catalytic activity of rhodium catalyst was studied.

ChemInform Abstract: Optically Active Bis(oxazolinyl)pyridine (Pybox) Rhodium Complexes: Asymmetric Hydrosilylation of Ketones

Optically active bis(oxazolinyl)pyridine (Pybox) was developed as a chiral nitrogen ligand for asymmetric hydrosilylation of ketones. Pybox reacted with RhCl3 in ethanol solution to produce a stable RhCl3 (Pybox) complex, which exhibited powerful catalytic activity for the hydrosilylation of ketones with diphenylsilane in the presence of silver tetrafluoroborate. Acetophenone was reduced to 1-phenylethanol in 91% yield and in 94% ee (S). Other simple ketones were reduced to optically active secondary, alcohols in ca. 90% ee on the average. Substituents on the oxazoline rings and on the pyridine ring were also examined to verify the yield and the enantioselectivity due to their steric and electronic effects.