Catalytic Enantioselective Synthesis of Alkenylhydrosilanes

Organocatalytic enantioselective construction of Si-stereocenters: recent advances and perspectives

Silicon-stereogenic chiral organosilanes have found increasing applications in synthetic chemistry, medicinal chemistry, and materials science. In this context, various asymmetric catalytic methods have been established for the diverse synthesis of silicon-stereogenic silanes. In particular, asymmetric organocatalysis is emerging as an important and complementary synthetic tool for the enantioselective construction of silicon-stereocenters, along with the rapid development of chiral-metal catalyzed protocols. Its advent provides a powerful platform to achieve functionalized silicon-stereogenic organosilanes with structural diversity, and should lead to great development in chiral organosilicon chemistry. In this Tutorial Review, we highlight these latest achievements from two aspects: desymmetrizations of prochiral tetraorganosilanes and dynamic kinetic asymmetric transformations of racemic organosilanes by employing five organocatalytic activation modes. The advantages, limitations and synthetic value of each protocol, as well as the synthetic opportunities still open for further exploration, are also discussed.

Organocatalytic asymmetric synthesis of Si-stereogenic silacycles

A strong and confined Brønsted acid catalyzed enantioselective cyclization of bis(methallyl)silanes provides enantioenriched Si-stereogenic silacycles. High enantioselectivities of up to 96.5:3.5 er were obtained for a range of bis(methallyl)silanes. NMR and ESI-MS studies reveal that the formation of a covalent adduct irreversibly inhibits turnover. Remarkably, we found that acetic acid as an additive promotes the collapse of this adduct, enabling full turnover. Experimental investigation and density functional theory (DFT) calculations were conducted to elucidate the origin of this phenomenon and the observed enantioselectivity.

Catalytic Kinetic Resolution of Monohydrosilanes via Rhodium-Catalyzed Enantioselective Intramolecular Hydrosilylation

The catalytic access of silicon-stereogenic organosilanes remains a big challenge, and largely depends on the desymmetrization of the symmetric precursors with two identical substitutes attached to silicon atom. Here we report the construction of silicon-stereogenic organosilanes via catalytic kinetic resolution of racemic monohydrosilanes with good to excellent selectivity factors. Both Si-stereogenic dihydrobenzosiloles and Si-stereogenic monohydrosilanes could be efficiently accessed in one single operation via Rh-catalyzed enantioselective intramolecular hydrosilylation, employing (R,R)-Et-DuPhos as the optimal ligand. This catalytic protocol features mild conditions, a low catalyst loading (0.1 mol % [Rh(cod)Cl]2), high stereoinduction (S factor up to 152), and excellent scalability. Moreover, further derivatizations led to the efficient synthesis of uncommon middle-size (7- and 8-membered) Si-stereogenic silacycles. Preliminary mechanistic study indicates this reaction might undergo a modified Chalk-Harrod mechanism.

Organocatalytic Asymmetric Synthesis of Si-Stereogenic Siloxanols

We report the organocatalytic synthesis of Si-stereogenic compounds via desymmetrization of a prochiral silanediol with a chiral imidazole-containing catalyst. This metal-free silylation method affords high yields with enantioselectivity up to 98:2 for various silanediol and silyl chloride substrate combinations (including secondary alkyl, vinyl, and H groups), accessing products with potential for further elaboration. NMR and X-ray studies reveal insight into the H-bonding interactions between the imidazole organocatalyst and the silanediol and the dual activating role of the Lewis basic imidazole to account for the high enantioselectivity.

Catalytic Synthesis of Silanols by Hydroxylation of Hydrosilanes: From Chemoselectivity to Enantioselectivity

As a crucial class of functional molecules in organosilicon chemistry, silanols are found valuable applications in the fields of modern science and will be a potentially powerful framework for biologically active compounds or functional materials. It has witnessed an increasing demand for non-natural organosilanols, as well as the progress in the synthesis of these structural features. From the classic preparative methods to the catalytic selective oxidation of hydrosilanes, electrochemical hydrolysis of hydrosilanes, and then the construction of the most challenging silicon-stereogenic silanols. This review summarized the progress in the catalyzed synthesis of silanols via hydroxylation of hydrosilanes in the last decade, with a particular emphasis on the latest elegant developments in the desymmetrization strategy for the enantioselective synthesis of silicon-stereogenic silanols from dihydrosilanes.

Enantioconvergent construction of stereogenic silicon via Lewis base-catalyzed dynamic kinetic silyletherification of racemic chlorosilanes

Organosilanes possessing an enantioenriched stereogenic silicon center are important in many branches of chemistry, yet they remain challenging to synthesize in a practical and scalable way. Here we report a dynamic kinetic silyletherification process of racemic chlorosilanes with (S)-lactates using 4-aminopyridine as a Lewis base catalyst. This enantioconvergent approach asymmetrically constructs the stereogenic silicon center in a different manner from traditional resolution or desymmetrization. A range of silylethers have been prepared with high diastereoselectivity on up to 10 g-scale, allowing the practical synthesis of diverse enantioenriched organosilane analogs.

Copper-Catalyzed Synthesis of S-S Bond-Containing Silanols from SCBs and Trisulfide-1,1-dioxides

In this work, an efficient method for the copper-catalyzed ring-opening hydrolysis of silacyclobutanes to silanols was developed. This strategy has the advantages of friendly reaction conditions, simple operation, and good functional group compatibility. No additional additives are required in the reaction, and the S-S bond can also be introduced into the organosilanol compounds in one step. Furthermore, the success at the gram scale demonstrates the great potential of the developed protocol for practical industrial applications.

An unusual autocatalysis with an air-stable Pd complex to promote enantioselective synthesis of Si-stereogenic enynes

Given the powerful potential of chiral-at-silicon chemistry, enantioselective synthesis of Si-stereogenic centers has attracted substantial research interest in recent years. However, the catalytic asymmetric synthesis of Si-stereogenic organosilicon compounds remains an appealing venture and is a challenging subject because of the difficulty in achieving high reactivity and stereoselectivity for "silicon-center" transformations. Herein, we disclose a highly enantioselective palladium-catalyzed hydrosilylation of 1,3-diynes with dihydrosilanes, which enables the facile preparation of Si-stereogenic enynes and an enyne-linked chiral polymer (polyenyne) in good yields and excellent ees (up to >99%) by desymmetrization. The unusual stereoselectivity in this reaction is achieved by precisely controlling the steric hindrance and electronic effect of the newly developed chiral ligands, resulting in a wide range of chiral silanes and a Si-containing polymer bearing a Si-stereogenic center which is otherwise difficult to access. The key to the high enantioselectivity relies on catalyst aggregation-induced non-covalent interaction, which exerts a remarkably positive influence on the Si-H bond activation and enhancement of enantioselectivity, in which the palladium/P-ligand complex was proved to be air-stable and moisture-insensitive in this reaction.

Chiral Silanols: Strategies and Tactics for Their Synthesis

Silanols are valuable and important compounds, which have found widespread applications in the field of materials science, synthetic chemistry, and medicinal chemistry. Although a handful of approaches have been developed for the synthesis of various silanols, access to enantioenriched silicon-stereogenic silanols remains underdeveloped. This Concept article intends to summarize and highlight recent advances in the construction of silicon-stereogenic silanols and endeavors to encourage further research in this area.

Rhodium-Catalyzed Dynamic Kinetic Asymmetric Hydrosilylation to Access Silicon-Stereogenic Center

Strategies on the construction of enantiomerically pure silicon-stereogenic silanes generally relies on desymmetrization of prochiral and symmetric substrates. However, dynamic kinetic asymmetric transformations of organosilicon compounds have remained underdeveloped and unforeseen owing to a lack of an effective method for deracemization of the static silicon stereocenters. Here we report the first Rh-catalyzed dynamic kinetic asymmetric intramolecular hydrosilylation (DyKAH) with "silicon-centered" racemic hydrosilanes that enables the facile preparation of silicon-stereogenic benzosiloles in good yields and excellent enantioselectivities. The special rhodium catalyst controlled by non-diastereopure-type mixed phosphine-phosphoramidite ligand with axial chirality and multiple stereocenters can induce enantioselectivity efficiently in this novel DyKAH reaction. Density functional theory (DFT) calculations suggest that the amide moiety in chiral ligand plays important role in facilitating the S_N 2 substitution of chloride ion to realize the chiral inversion of silicon center.

Enantioselective synthesis of acyclic monohydrosilanes by steric hindrance assisted C-H silylation

Herein, a rhodium-catalyzed desymmetrization of dihydrosilanes with heterocyclic compounds via intermolecular dehydrogenative C-H silylation is developed. The strategy tolerates a variety of thianaphthene and thiophene derivatives, giving rise to a wide range of silicon-stereogenic acyclic monohydrosilanes. Several rare skeletons featuring bis-silicon-stereogenic centers were also designed to enhance the library's diversity further. Preliminary mechanistic studies reveal that the surrounding spatial environment of the Si-center plays a crucial role in enabling intermolecular C-H silylation preferentially.

Silicon-Stereogenic Monohydrosilane: Synthesis and Applications

Optically active organosilanes have been demonstrated to be versatile chiral reagents in synthetic chemistry since the early seminal contributions by Sommer and Corriu. Among these silicon-containing chiral architectures, monohydrosilanes, which bear a Si-H bond, hold a unique position because of their facile transformations through stereospecific Si-carbon or Si-heteroatom bond-formation reactions. In addition, those compounds have also been leveraged as chiral reagents for alcohol resolution, chiral auxiliaries, mechanistic probes, as well as potential optoelectronic materials. This Minireview comprehensively summarizes the synthesis and synthetic applications of silicon-stereogenic monohydrosilanes, particularly the advances in the transition-metal-catalyzed asymmetric synthesis of this class of functional molecules.

Enantioselective Intermolecular C-H Silylation of Heteroarenes for the Synthesis of Acyclic Si-Stereogenic Silanes

Intermolecular C-H silylation for the synthesis of acyclic silanes bearing a silicon-stereogenic center in one enantiomeric form remains unknown to date. Herein, we report the first enantioselective intermolecular C-H silylation of heteroarenes for the synthesis of acyclic silicon-stereogenic heteroarylsilanes. This process undergoes a rhodium-catalyzed direct intermolecular dehydrogenative Si-H/C-H cross-coupling, giving access to a variety of acyclic heteroarylated silicon-stereogenic monohydrosilanes, including bis-Si-stereogenic silanes, in decent yields with excellent chemo-, regio-, and stereo-control, which significantly enlarge the chemical space of the optically active silicon-stereogenic monohydrosilanes.

Cobalt-Catalyzed Sequential Site- and Stereoselective Hydrosilylation of 1,3- and 1,4-Enynes

Catalytic sequential hydrosilylation of 1,3-enynes and 1,4-enynes promoted by cobalt complexes derived from bisphosphines are presented. Site- and stereoselective Si-H addition of primary silanes to 1,3-enynes followed by sequential intramolecular diastereo- and enantioselective Si-H addition afforded enantioenriched cyclic alkenylsilanes with simultaneous construction of a carbon-stereogenic center and a silicon-stereogenic center. Reactions of 1,4-enynes proceeded through sequential isomerization of the alkene moiety followed by site- and stereoselective hydrosilylation. A wide range of alkenylsilanes were afforded in high efficiency and selectivity. Functionalization of the enantioenriched silanes containing a stereogenic center at silicon delivered a variety of chiral building blocks that are otherwise difficult to access.

Regiodivergent hydrosilylation in the nickel(0)-catalyzed cyclization of 1,6-enynes

The divergent nickel(0)-catalyzed hydrosilylation/cyclization of 1,6-enynes has been developed, providing an efficient synthetic route for vinyl silanes or alkyl silanes from the same starting materials.

Cu-catalyzed efficient construction of S (Se)-containing functional organosilicon compounds

A Cu-catalyzed cascade reaction of four-membered silacyclobutanes (SCBs) and thiosulfonates to construct S (Se)-containing organosilicon compounds is described.

Enantioselective Synthesis of Silicon-Stereogenic Monohydrosilanes by Rhodium-Catalyzed Intramolecular Hydrosilylation

Enantiopure monohydrosilanes are versatile chiral reagents for alcohol resolution and mechanistic investigation. Herein, we have demonstrated the asymmetric synthesis of monohydrosilanes via an intramolecular hydrosilylation strategy. This protocol is suitable for the synthesis of five- and six-membered cyclic monohydrosilanes, including a class of chiral oxasilacycles, with excellent diastereo-, regio-, and enantioselectivities. Notably, the catalyst loading could be reduced to 0.1 mol % which makes this one of the most efficient methods to access chiral monohydrosilanes. Mechanistic studies and DFT calculations indicate this Rh-catalyzed intramolecular asymmetric hydrosilylation reaction might proceed via a Chalk-Harrod mechanism, and the enantio-determining step was predicted to be oxidative addition of Si-H bond.

Catalytic Enantioselective Dehydrogenative Si-O Coupling to Access Chiroptical Silicon-Stereogenic Siloxanes and Alkoxysilanes

A rhodium-catalyzed enantioselective construction of triorgano-substituted silicon-stereogenic siloxanes and alkoxysilanes is developed. This process undergoes a direct intermolecular dehydrogenative Si-O coupling between dihydrosilanes with silanols or alocohols, giving access to a variety of highly functionalized chiral siloxanes and alkoxysilanes in decent yields with excellent stereocontrol, that significantly expand the chemical space of the silicon-centered chiral molecules. Further utility of this process was illustrated by the construction of CPL-active (circularly polarized luminescence) silicon-stereogenic alkoxysilane small organic molecules. Optically pure bis-alkoxysilane containing two silicon-stereogenic centers and three pyrene groups displayed a remarkable g_lum value with a high fluorescence quantum efficiency (g_lum = 0.011, Φ_F = 0.55), which could have great potential application prospects in chiral organic optoelectronic materials.

Construction of Axial Chirality and Silicon-Stereogenic Center via Rh-Catalyzed Asymmetric Ring-Opening of Silafluorenes

A rhodium-catalyzed enantioselective ring-opening/acylation of silafluorenes is reported. The newly developed bulky phosphoramidite ligand, in combination with methanol as the additive, enabled the reaction to create one axial chirality and one silicon-stereogenic center in a highly selective manner by only cleavage of one Si-C bond.

Pt(ii)-Chiral diene-catalyzed enantioselective formal [4 + 2] cycloaddition initiated by C–C bond cleavage and elucidation of a Pt(ii)/(iv) cycle by DFT calculations

A Pt(ii)-chiral diene complex demonstrates a high catalytic activity in the enantioselective formal [4 + 2] cycloaddition along with C–C bond cleavage of biphenylene at room temp. DFT calculations elucidated that the present catalysis involves a rare Pt(ii)/Pt(iv) cycle.

Asymmetric total synthesis and antidepressant activity of (−)-sila-mesembranol bearing a silicon stereocenter

Asymmetric total synthesis of (−)-sila-mesembranol, the silicon analog of the natural alkaloid (−)-mesembranol has been achieved in 3.3% yield over 11 steps. The synthetic (−)-sila-mesembranol in mice exhibits better antidepressant effects than its carbon counterpart.

Enantioselective Construction of Si-Stereogenic Center via Rhodium-Catalyzed Intermolecular Hydrosilylation of Alkene

Catalytic, enantioselective synthesis of stereogenic silicon compounds remains a challenge. Herein, we report a rhodium-catalyzed regio- and enantio-selective intermolecular hydrosilylation of alkene with prochiral dihydrosilane. This new method features a simple catalytic system, mild reaction conditions and a wide functional group tolerance.

Desymmetrization of gem-diols via water-assisted organocatalytic enantio- and diastereoselective cycloetherification

The first desymmetrization of gem-diols forming chiral hemiketal carbons was accomplished via organocatalytic enantio- and diastereoselective cycloetherification, which afforded optically active tetrahydropyrans containing a chiral hemiketal carbon and tetrasubstituted stereocenters bearing synthetically versatile fluorinated groups. The desymmetrization of silanediols was also demonstrated as an asymmetric route to chiral silicon centers.

Enantioselective Silylation of Aliphatic C-H Bonds for the Synthesis of Silicon-Stereogenic Dihydrobenzosiloles

A rhodium(I)-catalyzed enantioselective silylation of aliphatic C-H bonds for the synthesis of silicon-stereogenic dihydrobenzosiloles is demonstrated. This reaction involves a highly enantioselective intramolecular C(sp³ )-H silylation of dihydrosilanes, followed by a stereospecific intermolecular alkene hydrosilylation leading to the asymmetrically tetrasubstituted silanes. A wide range of dihydrosilanes and alkenes displaying various functional groups are compatible with this process, giving access to a variety of highly functionalized silicon-stereogenic dihydrobenzosiloles in good to excellent yields and enantioselectivities.

Catalytic Asymmetric trans-Selective Hydrosilylation of Bisalkynes to Access AIE and CPL-Active Silicon-Stereogenic Benzosiloles

Chirality widely exists in a diverse array of biologically active molecules and life forms, and the catalytic constructions of chiral molecules have triggered a heightened interest in the fields of chemistry and materials and pharmaceutical sciences. However, the synthesis of silicon-stereogenic organosilicon compounds is generally recognized as a much more difficult task than that of carbon-stereogenic centers because of no abundant organosilicon-based chiral sources in nature. Herein, we reported a highly enantioselective rhodium-catalyzed trans-selective hydrosilylation of silicon-tethered bisalkynes to access chiral benzosiloles bearing a silicon-stereogenic center. This protocol featured with chiral Ar-BINMOL-Phos bearing hydrogen-bond donors as a privileged P-ligand for catalytic asymmetric hydrosilylation that is operationally simple and has 100% atom-economy with good functional group tolerability as well as high enantioselectivity (up to >99:1 er). Benefiting from the trans-selective hydrosilylation with the aid of Rh/Ar-BINMOL-Phos-based asymmetric catalysis, the Si-stereogenic benzosiloles exhibited pronounced aggregation-induced emission (AIE) and circularly polarized luminescence (CPL) activity.