Enantioselective Hydrosilylation Research Articles

Recent Advances in Organocatalyzed Asymmetric Hydrosilylations

Catalytic asymmetric hydrosilylation based on the use of inexpensive and handleable hydridic silanes is a very useful reaction in organic chemistry which has become the subject of special interest because of the intrinsic peculiarity of economic efficiency and environmental friendliness. Recently, a lot of structurally diverse organocatalysts have been designed and applied to this chemistry. Enantiopure organocatalysts such as formamide derivatives, picolinamide derivatives, S-chiral sulfinamide derivatives and chiral frustrated Lewis pairs (FLPs) have been proved as privileged catalysts for the enantioselective hydrosilylations. This manuscript provides an overview on the most relevant advances in this field and presents an extensive summary of the catalysis performance of the above mentioned catalysts. Keywords: Asymmetric reduction, hydrosilylations, organocatalysts, hydridic silanes, asymmetric synthesis, Lewis Bases.

Enantioselective Synthesis of Nonracemic Geminal Silylboronates by Pt-Catalyzed Hydrosilylation.

A Pt-catalyzed enantioselective hydrosilylation of alkenylboronates is described. This reaction occurs with high regio- and enantioselectivity, providing a convenient route to chiral non-racemic geminal silylboronates. These compounds are useful reagents in stereoselective synthesis.

One-pot diastereo- and enantioselective hydrosilylation–transacylation of α-acyloxy β-enamino esters

Asymmetric hydrosilylation–transacylation of α-acyloxy β-enamino esters provided various α-hydroxyl-β-acylamido esters in good yields with good diastereoselectivities and enantioselectivities.

The open d-shell enforces the active space in 3d metal catalysis: highly enantioselective chromium(ii) pincer catalysed hydrosilylation of ketones.

Bis(oxazolinyldimethylmethyl)pyrrol (PdmBox) stereodirecting ligands provided the key to the chromium(ii)-catalysed highly enantioselective hydrosilylation of ketones. A rare square planar, chiral chromium(ii) alkyl complex was found to serve as a potent precatalyst for the reduction of a broad range of aryl alkyl and dialkyl ketone derivatives. The stereoelectronic preference of the open d4 shell of chromium(ii) firmly locks the molecular catalyst in a square planar geometry giving rise to two blocked quadrants of the coordination sphere. This earth-abundant base metal catalytic platform produces the corresponding chiral alcohols in excellent isolated yields with up to 98 %ee under mild reaction conditions (-40 °C to rt) and at low catalyst loadings (as low as 0.5 mol%).

Coordination Chemistry of the PdmBOX Pincer Ligand: Reactivity at the Metal and the Ligand

The coordination chemistry of PdmBOX [2,5‐bis(2‐oxazolinyldimethylmethyl)pyrrole] pincer ligands, in which a dimethylmethylidene linker has been introduced to preclude rearrangements in the backbone, has been studied with 3d and 4d late transition metals. The in situ lithiated PdmBOX ligand was treated with MnCl2, FeCl2 and CoCl2 to give [iPr(PdmBOX)MCl] (M = Mn: 2; Fe: 3; Co: 4) which were found to adopt a distorted tetrahedral coordination geometry in the crystal. Addition of 2,6‐dimethylphenylisocyanide to 3 gave the diamagnetic tris(isocyanide) derivative [iPr(PdmBOX)Fe(CNxyl)3]Cl (5) with the chloride as the counteranion. Complexes 2–4 were found to be precatalysts for the enantioselective hydrosilylation and hydroboration of acetophenone. The synthesis of PdmBOX‐palladium(II) complexes was achieved by deprotonation of the protioligand with potassium bis(trimethylsilyl)amide (KHMDS) and subsequent reaction with [(cod)PdCl2] giving [iPr(PdmBOX)PdCl] (6) or by stirring (S)‐iPr(PdmBOX)H with palladium acetate yielding the palladium acetato complex 7. Treatment of the latter with phenylsilane at 60 °C also led to the slow formation of the metal hydride complex which was the key species in the catalytic hydrosilylation of styrene oxide, albeit with low activity. The coordination of (S)‐iPr(PdmBOX)H to rhodium(I) was accomplished by reaction with (acetylacetonato)dicarbonylrhodium(I) to give the corresponding PdmBOX‐Rh complex 8 which reacted with methyl iodide to give the octahedrally coordinated rhodium(III) complex [iPr(PdmBOX)Rh(CH3)I(CO)] 9. Reaction of 8 with one equivalent of CsBr3 led to the full conversion to the dibromorhodium(III) complex [iPr(PdmBOX)Rh(CO)Br2] 10. Upon reaction of 10 with N‐bromosuccinimide (NBS), the pyrrole backbone in 10 was brominated to give complex 11. In a similar way, a selective conversion of [iPr(PdmBOX)NiCl] to the backbone‐chlorinated [iPr(ClPdmBOX)NiCl] (12) was achieved by the treatment with N‐chlorosuccinimide.

Highly Enantioselective Hydrosilylation of Ketones Catalyzed by a Chiral Oxazaborolidinium Ion.

A highly enantioselective hydrosilylation of ketones was developed for the synthesis of a variety of chiral secondary alcohols. In the presence of a chiral oxazaborolidinium ion (COBI) catalyst, the reaction proceeded with good yields (up to 99%) with excellent enantioselectivities (up to 99% ee).

Highly Enantioselective Cobalt-Catalyzed Hydrosilylation of Alkenes.

A cobalt-catalyzed highly Markovnikov-type and enantioselective hydrosilylation of alkenes is developed for the efficient synthesis of valuable chiral dihydrosilanes. This protocol is operationally simple and atom-economy, and using relatively simple and readily available starting materials. The reaction is suitable for both aryl and aliphatic alkenes with excellent functional group tolerability. The reaction could be easily carried out in a gram-scale. The TOF and TON is up to 1800 and 860, respectively.

Hidden Enantioselective Hydrogenation of N-Silyl Enamines and Silyl Enol Ethers in Net C═N and C═O Hydrosilylations Catalyzed by Ru–S Complexes with One Monodentate Chiral Phosphine Ligand

Ruthenium thiolate complexes with one chiral monodentate phosphine ligand are applied to enantioselective hydrosilylation of enolizable imines and ketones. The structural features of the catalyst exclude the presence of more than one phosphine ligand at the ruthenium center in the enantioselectivity-determining step. The enantiomeric excesses obtained in these reduction reactions are moderate (up to 66% ee), but the stereochemical outcome enables an experimental analysis of the reaction pathways operative in this catalysis. A two-step sequence consisting of successive N–Si/O–Si dehydrogenative coupling and enamine/enol ether hydrogenation is the prevailing mechanism of action. Both steps involve cooperative bond activation at the Ru–S bond of the coordinatively unsaturated ruthenium complex: Si–H bond activation in the dehydrogenative coupling and heterolytic H–H splitting in the hydrogenation. Previously documented side reactions such as deprotonation/protonation equilibria as well as competing direct C═...

Asymmetric Copper Hydride-Catalyzed Markovnikov Hydrosilylation of Vinylarenes and Vinyl Heterocycles.

We report a highly enantioselective CuH-catalyzed Markovnikov hydrosilylation of vinylarenes and vinyl heterocycles. This method has a broad scope and enables both the synthesis of isolable silanes and the conversion of crude products to chiral alcohols. Density functional theory calculations support a mechanism proceeding by hydrocupration followed by σ-bond metathesis with a hydrosilane.

Zinc‐Catalyzed Enantioselective Hydrosilylation of Ketones and Imines under Solvent‐Free Conditions

AbstractThe zinc acetate promoted asymmetric hydrosilylation of various ketones and imines under solvent‐free conditions was examined by using an unprecedented low catalyst loading. Exposure of ketones to 0.05 mol % Zn‐based chiral diamine complex in the presence of triethoxysilane afforded enantioenriched alcohols in excellent yields (up to 98 %) and enantioselectivities (up to 97 % ee). This methodology also allowed for the chemoselective 1,2‐reduction of α,β‐unsaturated ketones and imines.

ChemInform Abstract: The Asymmetric Piers Hydrosilylation.

AbstractA new axially chiral, cyclic borane catalyst is developed for the highly enantioselective hydrosilylation of acetophenone derivatives.

ChemInform Abstract: Chiral Frustrated Lewis Pairs Catalyzed Highly Enantioselective Hydrosilylations of 1,2‐Dicarbonyl Compounds.

AbstractA variety of optically active α‐hydroxy ketones and esters is prepared utilizing the combination of tricyclohexylphosphine and chiral alkenylborane generated in situ from diyne as frustrated Lewis pair catalyst.

The Asymmetric Piers Hydrosilylation.

An axially chiral, cyclic borane decorated with just one C6F5 group at the boron atom promotes the highly enantioselective hydrosilylation of acetophenone derivatives without assistance of an additional Lewis base (up to 99% ee). The reaction is an unprecedented asymmetric variant of Piers' B(C6F5)3-catalyzed carbonyl hydrosilylation. The steric congestion imparted by the 3,3'-disubstituted binaphthyl backbone of the borane catalyst as well as the use of reactive trihydrosilanes as reducing agents are keys to success.

ChemInform Abstract: Enantioselective Hydrosilylation of Imines Catalyzed by Chiral Zinc Acetate Complexes.

AbstractZinc acetate complexes with optically pure diphenylethanediamine derived ligands are employed as enantioselective catalysts for hydrosilylation of imines.

ChemInform Abstract: Zinc Acetate‐Catalyzed Enantioselective Hydrosilylation of Ketones.

AbstractThe enantioselective hydrosilylation of a broad range of aryl ketones succeeds using a stable and inexpensive zinc salt together with easily accessible chiral diamine ligands.

Chiral Frustrated Lewis Pairs Catalyzed Highly Enantioselective Hydrosilylations of 1,2-Dicarbonyl Compounds.

A highly enantioselective hydrosilylation of 1,2-dicarbonyl compounds was successfully realized for the first time utilizing the combination of tricyclohexylphosphine and chiral alkenylborane derived in situ from diyne as a frustrated Lewis pair catalyst. A variety of optically active α-hydroxy ketones and esters were obtained in 52-98% yields with 86-99% ee's.

The design of second generation MOP-phosphonites: efficient chiral hydrosilylation of functionalised styrenes.

A series of enantiopure MOP-phosphonite ligands, with tailored steric profiles, have been synthesised and are proven to be very successful in high-yielding, regio- and enantioselective catalytic hydrosilylation reactions of substituted styrenes, affording important chiral secondary alcohols.

Enantioselective Hydrosilylation of N-Aryl Diaryl Ketimines

Lewis base catalyzed enantioselective hydrosilylation of non-ortho-substituted N-aryl diaryl ketimines was real- ized. In the presence of 20 mol% of the optimal catalyst, the reactions provided a series of (diarylmethyl)amines with high yields (up to 97%) in moderate to good enantioselectivities (up to 89% ee). The absolute configuration of one product was determined by X-ray crystallographic analysis.

Enantioselective Hydrosilylation of Imines Catalyzed by Chiral Zinc Acetate Complexes.

A series of zinc acetate complexes with optically pure diphenylethanediamine (DPEDA)-derived ligands have been employed as enantioselective catalyst for the hydrosilylation of various imines. High control of stereoselectivity (up to 97% ee) and excellent yields (up to 96%) were gained for a broad range of N-phosphinoylimines by using (R,R)-N,N'-dibenzyl-1,2-diphenylethane-1,2-diamine. This is the first successful application of an air-stable and environmentally friendly chiral Zn(OAc)2 complex instead of the previously used harmful diethylzinc in the asymmetric reduction of the C═N double bond.

Zinc Acetate‐Catalyzed Enantioselective Hydrosilylation of Ketones

AbstractZinc acetate complexes with a chiral diphenylethylenediamine (DPEDA)‐derived ligand have been proved to be efficient catalysts for the enantioselective hydrosilylation of aryl ketones. Replacing pyrophoric dialkylzinc with the readily available zinc salt simplifies the procedures and provides excellent conversions (up to >99%) and enantioselectivities (ees up to 97%).magnified image