Iron-Catalyzed Highly Enantioselective Hydrosilylation of Unactivated Terminal Alkenes

Iron-Catalyzed Tunable Alkene Migratory Silylation and Transposition

The example of iron-catalyzed alkenes migratory silylation and transposition has been demonstrated, affording a tunable approach to synthesize thermodynamically stable allylsilanes and internal alkenes with high efficiency and regioselectivity. These reactions showcase several advantageous features, including good functional group tolerance, excellent regioselectivity, a broad substrate scope, scalability to gram-scale synthesis, and late-stage functionalization of bio-relevant molecules. Furthermore, the relay catalytic mechanism of the migratory silylation, involving both iron-silyl and iron-hydride intermediates, provides valuable insights into iron-catalyzed coupling reactions, opening new avenues for the development of novel transformations under iron catalysis.

Iron-catalyzed sequential hydrosilylation

Highly regio-, diastereo- and enantioselective iron-catalyzed sequential hydrosilylation of o-alk-n-enyl-phenyl silanes with alkynes is reported for various 5-, 6-, and 7-membered benzosilacycles in 60-94% yields with up to 95:5 rr, 95:5 dr, and 99% ee. Chiral fully carbon-substituted silicon-stereogenic benzosilacycles could also be obtained via triple hydrosilylation reactions. The unique electronic effect of ligands is observed while adjusting the regioselectivity and enantioselectivity in hydrosilylation reactions. A possible mechanism has been proposed by variable time normalization analysis (VTNA) and H/D exchange experiment.

Copper-catalyzed intermolecular Regio- and Enantioselective Hydrosilylation of Alkenes with Prochiral Silanes

This study presents a copper-catalyzed, substrate-controlled regio- and enantioselective intermolecular hydrosilylation method capable of accommodating a broad scope of alkenes and prochiral silanes. The approach offers an efficient and versatile pathway to generate enantioenriched linear and branched alkyl-substituted Si-stereogenic silanes. Key features of this reaction include mild reaction conditions, simple catalytic systems, compatibility with diverse substrates, high yields and enantioselectivities.

Rhodium-Catalyzed Enantioselective Hydrosilylation of 1,1-Disubstituted Enamides

Catalytic hydrosilylation of 1,1-disubstituted enamides is one of the most challenging and synthetically useful processes in organosilicon chemistry and asymmetric catalysis. Herein, we report a rhodium-catalyzed enantioselective hydrosilylation of α-arylenamides with substituted hydrosilanes with the aid of chiral P-ligand, including newly developed spirophosphite ligands, giving various chiral β-silylated amides in excellent yields with good to excellent enantioselectivities (98:2 er after recrystallization). In addition, chiral β-silylated amines can be obtained by further functionalization of the hydrosilylation product.

Computational Insights into the Effect of Ligands and Transition-Metal Centers on the Mechanism and Regioselectivity of Hydrosilylation of Alkenes

Development of sustainable synthetic methods for the hydrosilylation of alkenes, catalyzed by 3d transition metals, offers a promising alternative to traditional noble metal catalysts. This study presents a computational mechanistic investigation into the hydrosilylation of alkenes, focusing on the role of ligands and metal centers in modulating the reaction's mechanism and regioselectivity. The ligand's electronic and steric properties were found to modulate the regioselectivity for cobalt catalysts, with phosphine ligand (xantphos) promoting higher linear selectivity compared to nitrogen-based ligand (mesPDI). The energy decomposition analysis reveals that the xantphos ligand favors linear products due to stronger electrostatic and orbital interactions despite increased steric repulsion. The metal center also plays a crucial role, with cobalt catalysts favoring the modified Chalk-Harrod mechanism for branched product formation in the presence of PNN ligand (iPrPCNNMe), due to lower activation barriers in alkene insertion. Beneficial electrostatic and orbital interactions predominate, rendering the alkene insertion transition state for cobalt more favorable compared to that for iron. This work provides a comprehensive understanding of how ligand and metal center effects can be harnessed to control regioselectivity in hydrosilylation reactions.

Nickel-catalyzed stereospecific reductive cross-coupling of vinyl chlorosilanes with axially chiral biaryl electrophiles

Enantioenriched organosilanes are important chiral molecules in materials science and organic synthesis. The synthesis of axially chiral organosilanes is particularly significant in terms of applications. Herein, we report a Ni-catalyzed reductive cross-electrophile coupling of vinyl chlorosilanes with sterically hindered chiral biaryl electrophiles for the synthesis of atropisomeric biaryl organosilanes. Various enantioenriched axially chiral vinylsilanes are accessible in high efficiency under mild conditions. The synthetic transformations and applications of new chiral silicon-containing alkene ligands are demonstrated.

Radical-Chain Hydrosilylation of Alkenes Enabled by Triplet Energy Transfer

Development of mild, robust and metal-free catalytic approach for the hydrosilylation of alkenes is critical to the advancement of modern organosilicon chemistry given their powerful capacity in the construction of various C-Si bonds. Herein, we wish to disclose a visible light-triggered organophotocatalytic strategy, which proceeds via a triplet energy transfer (EnT)-enabled radical chain pathway. Notably, this redox-neutral protocol is capable of accommodating a broad spectrum of electron-deficient and -rich alkenes with excellent functional group compatibility. Electron-deficient alkenes are more reactive and the reaction could be finished within a couple of minutes even in PBS solution with extremely low concentration, which suggests its click-like potential in organic synthesis. The preparative power of the transformations has been further highlighted in a number of complex settings, including the late-stage functionalization and scale-up experiments. Furthermore, although only highly reactive (TMS)3SiH is suitable hydrosilane substrate, our studies revealed the great reactivity and versatility of (TMS)3Si- group in diverse C-Si and Si-Si bond cleavage-based transformations, enabling the rapid introduction of diverse functional groups and the facile construction of valuable quaternary silicon architectures.

Nickel-catalyzed regiodivergent hydrosilylation of α-(fluoroalkyl)styrenes without defluorination

The fluoroalkyl-containing organic molecules are widely used in drug discovery and material science. Herein, we report ligand regulated nickel(0)-catalyzed regiodivergent hydrosilylation of α-(fluoroalkyl)styrenes without defluorination, providing an atom- and step-economical synthesis route of two types of fluoroalkyl substituted silanes with exclusive regioselectivity. The anti-Markovnikov addition products (β-fluoroalkyl substituted silanes) are formed with monodentate phosphine ligand. Noteworthy, the bidentate phosphine ligand promote the generation of the more challenging Markovnikov products (α-fluoroalkyl substituted silanes) with tetrasubstituted saturated carbon centers. This protocol features with easy available starting materials and commercially available nickel catalysis, a wide range of substrates and excellent regioselectivity. The structure divergent products undergo a variety of transformations. Comprehensive mechanistic studies including the inverse kinetic isotope effects demonstrate the regioselectivity controlled by ligand structure through α-CF3 nickel intermediate. DFT calculations reveal a distinctive mechanism involving an open-shell singlet state, which is crucial for generating intricate tetra-substituted Markovnikov products.

Advances in disilylation reactions to access cis/trans-1,2-disilylated and gem-disilylated alkenes

Organosilane compounds are widely used in both organic synthesis and materials science. Particularly, 1,2-disilylated and gem-disilylated alkenes, characterized by a carbon-carbon double bond and multiple silyl groups, exhibit significant potential for subsequently diverse transformations. The versatility of these compounds renders them highly promising for applications in materials, enabling them to be valuable and versatile building blocks in organic synthesis. This review provides a comprehensive summary of methods for the preparation of cis/trans-1,2-disilylated and gem-disilylated alkenes. Despite notable advancements in this field, certain limitations persist, including challenges related to regioselectivity in the incorporation and chemoselectivity in the transformation of two nearly identical silyl groups. The primary objective of this review is to outline synthetic methodologies for the generation of these alkenes through disilylation reactions, employing silicon reagents, specifically disilanes, hydrosilanes, and silylborane reagents. The review places particular emphasis on investigating the practical applications of the C-Si bond of disilylalkenes and delves into an in-depth discussion of reaction mechanisms, particularly those reactions involving the activation of Si-Si, Si-H, and Si-B bonds, as well as the C-Si bond formation.

Regulating the Flexibility to Assemble Porous Single-Atom Fe-Coordinated Metallopolymers for Efficient Heterogeneous Catalytic Oxidations

Metallopolymers as organic-inorganic hybrid materials formulated by metal embedding organic polymers show great potential for novel heterogeneous catalysis, in terms of the facile structural design and tunability. Herein, the disadvantage of nonporous stacking of one-dimensional (1D) structures has been suppressed by chain modulation of the 1D metallopolymers, allowing for the convenient construction of porous assemblies with single-atom dispersion and accessible active sites. By postmodification, the Fe/CM-1 catalyst readily synthesized by coordinating the Fe(II) to the twisted chain of 1D Schiff-base polymer possesses expedient flexibility, showing the highest porosity, remarkable heterogeneous recyclability, and thus prominent catalytic activity for the selective oxidation of benzylamine and alcohols. Moreover, control experiments supported by computational studies demonstrated that the unique pincer structure of Fe/CM-1 effectively maintains the valence state of the anchored single-atom iron, facilitating single-electron transfer and promoting efficient iron redox cycling during the catalytic process. Notably, these 1D metallopolymers have the advantage of cost-effectiveness, easy preparation in gram-scale, and utilization in continuous reaction, providing inspirations for facile synthesis of efficient heterogeneous catalysts from the well-developed 1D metallopolymers.

Insight into Radical Initiation, Solvent Effects, and Biphenyl Production in Iron-Bisphosphine Cross-Couplings

Iron-bisphosphines have attracted broad interest as highly effective and versatile catalytic systems for two- and three-component cross-coupling strategies. While recent mechanistic studies have defined the role of organoiron(II)-bisphosphine species as key intermediates for selective cross-coupled product formation in these systems, mechanistic features that are essential for catalytic performance remain undefined. Specifically, key questions include the following: what is the generality of iron(II) intermediates for radical initiation in cross-couplings? What factors control reactivity toward homocoupled biaryl side-products in these systems? Finally, what are the solvent effects in these reactions that enable high catalytic performance? Herein, we address these key questions by examining the mechanism of enantioselective coupling between α-chloro- and α-bromoalkanoates and aryl Grignard reagents catalyzed by chiral bisphosphine-iron complexes. By employing freeze-trapped ⁵⁷Fe Mössbauer and EPR studies combined with inorganic synthesis, X-ray crystallography, reactivity studies, and quantum mechanical calculations, we define the key in situ iron speciation as well as their catalytic roles. In contrast to iron-SciOPP aryl-alkyl couplings, where monophenylated species were found to be the predominant reactive intermediate or prior proposals of reduced iron species to initiate catalysis, the enantioselective system utilizes an iron(II)-(R,R)-BenzP* bisphenylated intermediate to initiate the catalytic cycle. A profound consequence of this radical initiation process is that halogen abstraction and subsequent reductive elimination result in considerable amounts of biphenyl side products, limiting the efficiency of this method. Overall, this study offers key insights into the broader role of iron(II)-bisphosphine species for radical initiation, factors contributing to biphenyl side product generation, and protocol effects (solvent, Grignard reagent addition rate) that are critical to minimizing biphenyl generation to obtain more selective cross-coupling methods.

Amide-Directed, Rhodium-Catalyzed Enantioselective Hydrosilylation of Unactivated Internal Alkenes

Despite the recent advances made in the area of asymmetric hydrosilylation, metal-catalyzed enantioselective hydrosilylation of unactivated internal alkenes remains a challenge. Here, we report a rhodium-catalyzed enantioselective hydrosilylation of unactivated internal alkenes bearing a polar group. The coordination assistance by an amide group enables the hydrosilylation to occur with high regio- and enantioselectivity.

Enantioselective Nickel-Catalyzed Hydrosilylation of 1,1-Disubstituted Allenes

Here, we report the first example of Ni-catalyzed asymmetric hydrosilylation of 1,1-disubstituted allenes with high level of regioselectivities and enantioselectivities. The key to achieve this stereoselective hydrosilylation reaction was the development of the SPSiOL-derived bisphosphite ligands (SPSiPO). This protocol features broad substrate scope, excellent functional group, and heterocycle tolerance, thus provides a versatile method for the construction of enantioenriched tertiary allylsilanes in a straightforward and atom-economic manner. DFT calculations were performed to reveal the reaction mechanism and the origins of the enantioselectivity.

Enantioselective Synthesis of Chiral Organosilicon Compounds by Organocatalytic Asymmetric Conjugate Addition of Boronic Acids to β-Silyl-α,β-Unsaturated Ketones

Herein, we report (R)-3,3'-(3,5-(CF₃)₂-C₆H₃)₂-BINOL-catalyzed enantioselective conjugate addition of organic boronic acids to β-silyl-α,β-unsaturated ketones, furnishing moderate to excellent yields of the corresponding β-silyl carbonyl compounds with stereogenic centers in excellent enantioselectivities (up to 98% ee). Moreover, the catalytic system features mild reaction conditions, high efficiency, broad substrate scope, and easy scale-up.

Cobalt-Catalyzed Regiodivergent Double Hydrosilylation of Arylacetylenes

Double hydrosilylation of alkynes represents a straightforward method to synthesize bis(silane)s, yet it is challenging if α-substituted vinylsilanes act as the intermediates. Here, a cobalt-catalyzed regiodivergent double hydrosilylation of arylacetylenes is reported for the first time involving this challenge, accessing both vicinal and geminal bis(silane)s with exclusive regioselectivity. Various novel bis(silane)s containing Si-H bonds can be easily obtained. The gram-scale reactions could be performed smoothly. Preliminarily mechanistic studies demonstrated that the reactions were initiated by cobalt-catalyzed α-hydrosilylation of alkynes, followed by cobalt-catalyzed β-hydrosilylation of the α-vinylsilanes to deliver vicinal bis(silane)s, or hydride-catalyzed α-hydrosilylation to give geminal ones. Notably, these bis(silane)s can be used for the synthesis of high-refractive-index polymers (n_d up to 1.83), demonstrating great potential utility in optical materials.

Synthetic Zwitterionic Streptococcus pneumoniae Type 1 Oligosaccharides Carrying Labile O-Acetyl Esters

We herein report the first total synthesis of the Streptococcus pneumoniae serotype 1 (Sp1) oligosaccharide, a unique zwitterionic capsular polysaccharide carrying labile O-acetyl esters. The target oligosaccharides, featuring rare α-2,4-diamino-2,4,6-trideoxy galactose (AAT) and α-galacturonic acids, were assembled up to the 9-mer level, in a highly stereoselective manner using trisaccharide building blocks. The lability of the O-acetyl esters imposed a careful deprotection scheme to prevent migration and hydrolysis. The migration was investigated in detail at various pD values using NMR spectroscopy, to show that migration and hydrolysis of the C-3-O-acetyl esters readily takes place under neutral conditions. Structural investigation showed the oligomers to adopt a right-handed helical structure with the acetyl esters exposed on the periphery of the helix in close proximity of the neighboring AAT residues, thereby imposing conformational restrictions on the AATα1-4GalA(3OAc) glycosidic linkages, supporting the helical shape of the polysaccharide, that has been proposed to be critical for its unique biological activity.

Ligand-controlled regiodivergence in cobalt-catalyzed hydrosilylation of isoprene

An atom-economical, regiodivergent hydrosilylation reaction of isoprene was developed using an Earth-abundant cobalt catalyst through variation of ligands.

Synthesis and Catalysis of Anionic Amido Iron(II) Complexes

Low-coordinate, open-shell 3d metal complexes have attracted great attention due to their critical role in several catalytic transformations but have been notoriously difficult to prepare and study due to their high lability. Here, we report the synthesis of a heteroleptic tri-coordinate amidoferrate that displays high catalytic activity in the regioselective hydrosilylation of alkenes.

Control over Selectivity in Alkene Hydrosilylation Catalyzed by Cobalt(III) Hydride Complexes

Two new bisphosphine [PCP] pincer cobalt(III) hydrides, [(L¹)Co(PMe₃)(H)(Cl)] (^L11, L¹ = 2,6-((Ph₂P)(Et)N)₂C₆H₃) and [(L²)Co(PMe₃)(H)(Cl)] (^L21, L² = 2,6-((ⁱPr₂P)(Et)N)₂C₆H₃), as well as one new bissilylene [SiCSi] pincer cobalt(III) hydride, [(L³)Co(PMe₃)(H)(Cl)] (^L31, L³ = 1,3-((PhC(^tBuN)₂Si)(Et)N)₂C₆H₃), were synthesized by reaction of the corresponding protic [PCP] or [SiCSi] pincer ligands L¹H, L²H, and L³H with CoCl(PMe₃)₃. Despite the similarities in the ligand scaffolds, the three cobalt(III) hydrides show remarkably different performance as catalysts in alkene hydrosilylation. Among the PCP pincer complexes, ^L11 has higher catalytic activity than complex ^L21, and both catalysts afford anti-Markovnikov selectivity for both aliphatic and aromatic alkenes. In contrast, the catalytic activity for alkene hydrosilylation of silylene complex ^L31 is comparable to phosphine complex ^L11, but a dependence of regioselectivity on the substrates was observed: While aliphatic alkenes are converted in an anti-Markovnikov fashion, the hydrosilylation of aromatic alkenes affords Markovnikov products. The substrate scope was explored with 28 examples. Additional experiments were conducted to elucidate these mechanisms of hydrosilylation. The synthesis of cobalt(I) complex (L¹)Co(PMe₃)₂ (^L17) and its catalytic properties for alkene hydrosilylation allowed for the proposal of the mechanistic variations that occur in dependence of reaction conditions and substrates.

Copper-Catalyzed Enantioselective Hydrosilylation of Allenes to Access Axially Chiral (Cyclohexylidene)ethyl Silanes

A novel strategy of copper-catalyzed regio- and enantioselective hydrosilylation of 4-substituted vinylidenecyclohexanes with silanes was developed. In this protocol, various allenes and silanes were used to afford the corresponding (cyclohexylidene)ethyl silanes in moderate to high yields with good enantioselectivities.

Enantioselective Cu-catalyzed double hydroboration of alkynes to access chiral gem-diborylalkanes

Chiral organoborons are of great value in asymmetric synthesis, functional materials, and medicinal chemistry. The development of chiral bis(boryl) alkanes, especially optically enriched 1,1-diboron compounds, has been greatly inhibited by the lack of direct synthetic protocols. Therefore, it is very challenging to develop a simple and effective strategy to obtain chiral 1,1-diborylalkanes. Herein, we develop an enantioselective copper-catalyzed cascade double hydroboration of terminal alkynes and highly enantioenriched gem-diborylalkanes were readily obtained. Our strategy uses simple terminal alkynes and two different boranes to construct valuable chiral gem-bis(boryl) alkanes with one catalytic and one ligand pattern, which represents the simplest and most straightforward strategy for constructing such chiral gem-diborons.

Phenanthroline-imine ligands for iron-catalyzed alkene hydrosilylation

Iron-catalyzed organic reactions have been attracting increasing research interest but still have serious limitations on activity, selectivity, functional group tolerance, and stability relative to those of precious metal catalysts. Progress in this area will require two key developments: new ligands that can impart new reactivity to iron catalysts and elucidation of the mechanisms of iron catalysis. Herein, we report the development of novel 2-imino-9-aryl-1,10-phenanthrolinyl iron complexes that catalyze both anti-Markovnikov hydrosilylation of terminal alkenes and 1,2-anti-Markovnikov hydrosilylation of various conjugated dienes. Specifically, we achieved the first examples of highly 1,2-anti-Markovnikov hydrosilylation reactions of aryl-substituted 1,3-dienes and 1,1-dialkyl 1,3-dienes with these newly developed iron catalysts. Mechanistic studies suggest that the reactions may involve an Fe(0)-Fe(ii) catalytic cycle and that the extremely crowded environment around the iron center hinders chelating coordination between the diene and the iron atom, thus driving migration of the hydride from the silane to the less-hindered, terminal end of the conjugated diene and ultimately leading to the observed 1,2-anti-Markovnikov selectivity. Our findings, which have expanded the types of iron catalysts available for hydrosilylation reactions and deepened our understanding of the mechanism of iron catalysis, may inspire the development of new iron catalysts and iron-catalyzed reactions.

Asymmetric Hydrosilylation of β-Silyl Styrenes Catalyzed by a Chiral Palladium Complex

A palladium complex coordinated with a chiral SIPHOS ligand was evaluated as an efficient catalyst for asymmetric hydrosilylation of β-silyl styrenes with trichlorosilane and 23 1,2-bis(silyl) chiral compounds were produced. Good to excellent enantioselectivities were observed with 1-aryl-2-silyl ethanols, where the trichlorosilyl groups of the hydrosilylation products were selectively converted into a hydroxyl group in the presence of pre-installed trialkylsilyl groups. Asymmetric hydrosilylation of β-silyl styrenes followed by methylation of the trichlorosilyl group gave stable 1,2-bis(silyl) chiral compounds 4 with excellent yields. DFT calculations of hydridopalladium B coordinated with a SIPHOS ligand, an intermediate of the hydrosilylation reaction, established the optical structures to be energy minima, and the structural information could well illustrate the enantioselectivity for the hydrosilylation reaction.

Metal-ligand cooperative approaches in homogeneous catalysis using transition metal complex catalysts of redox noninnocent ligands

Catalysis offers a straightforward route to prepare various value-added molecules starting from readily available raw materials. The catalytic reactions mostly involve multi-electron transformations. Hence, compared to the inexpensive and readily available 3d-metals, the 4d and 5d-transition metals get an extra advantage for performing multi-electron catalytic reactions as the heavier transition metals prefer two-electron redox events. However, for sustainable development, these expensive and scarce heavy metal-based catalysts need to be replaced by inexpensive, environmentally benign, and economically affordable 3d-metal catalysts. In this regard, a metal-ligand cooperative approach involving transition metal complexes of redox noninnocent ligands offers an attractive alternative. The synergistic participation of redox-active ligands during electron transfer events allows multi-electron transformations using 3d-metal catalysts and allows interesting chemical transformations using 4d and 5d-metals as well. Herein we summarize an up-to-date literature report on the metal-ligand cooperative approaches using transition metal complexes of redox noninnocent ligands as catalysts for a few selected types of catalytic reactions.

NiH-catalyzed C(sp3)–Si coupling of alkenes with vinyl chlorosilanes

A novel NiH-catalyzed highly selective cross-coupling of alkenes with vinyl chlorosilanes is developed. Using this practical chemistry, various benzyl organosilanes could be produced with good functional group tolerance.

Applications of iron pincer complexes in hydrosilylation reactions

Due to its abundance, low cost and low toxicity, the first-row transition metal, iron is widely preferred as a catalyst in organic synthesis. The only drawback of lower selectivity due to high reactivity and low stability of the metal centre is tuned by using pincer ligands of different types. The different iron pincer complexes thus prepared are extensively used in catalyzing different types of organic reactions with great selectivity and functional group tolerance under moderate reaction conditions. In this review, we focus on the applications of iron pincer complexes in hydrosilylation reactions, especially the hydrosilylation of carbonyl derivatives and alkene/alkynes.

Iron pincer complexes are efficient in catalyzing various organic reactions with excellent selectivity and functional group tolerance at moderate reaction conditions. This review focuses on the applications of iron pincer complexes in hydrosilylation reactions.

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.

Synthesis of aryl cobalt and iron complexes and their catalytic activity on hydrosilylation of alkenes

Four aryl Co and Fe complexes, (F4C5N)CoCl(PMe3)3 (1), (F4C5N)Fe(PMe3)4 (2), (F5C6)CoCl(PMe3)3 (3) and (F4C5)FeCl(PMe3)3 (4), were synthesized from the reactions of 3-chloro-2,4,5,6-tetrafluoro-pyridine and chloropentafluorobenzene with Co(PMe3)4 and Fe(PMe3)4, respectively.

Highly Regio- and Enantioselective Hydrosilylation of gem-Difluoroalkenes via Nickel Catalysis

The synthesis of small organic molecules with a difluoromethylated stereocenter is particularly attractive in drug discovery. Herein, we developed an efficient method for the direct generation of difluoromethylated stereocenters through Ni(0)-catalyzed regio - and enantioselective hydrosilylation of gem -difluoroalkenes. The reaction also represents the enantioselective construction of carbon(sp 3 )-silicon bonds with nickel catalysis, which provides an atom- and step-economical synthesis route of high-value optically active α-difluoromethylsilanes. This protocol features with readily available starting materials and commercial chiral catalysis, broad substrates spanning a range of functional groups with high yield (up to 99% yield) and excellent enantioselectivity (up to 96% ee). The enantioenriched products undergo a variety of stereospecific transformations. Preliminary mechanistic studies were performed.

Base Metal-terpyridine Complex Immobilized on Stationary Phase Aimed as Reusable Hydrosilylation Catalyst

The catalytic activity of a base metal-terpyridine complex immobilized on silica gel (M(tpy)X₂ @SiO₂ /H₂ O: M=Mn, Fe, Co, Ni, Cu; X=Cl, Br) for hydrosilylation was investigated. Co(tpy)Br₂ @SiO₂ /H₂ O in the presence of NaBHEt₃ exhibited the highest catalytic activity for hydrosilylation of 1-octene with diphenylsilane (Ph₂ SiH₂ ) to form the anti-Markovnikov-type hydrosilylation compound as the main product. The reusability of Co(tpy)Br₂ @SiO₂ /H₂ O activated by NaBHEt₃ was examined. It was found that the catalytic activity decreased with repeated use because of the peeling off of the Co complex anchor portion from the silica gel surface upon the attack of NaBHEt₃ . The introduction of Co(OAc)₂ instead of CoBr₂ to silica gel formed Co(tpy)(OAc)₂ - and Co(tpy)(OH)₂ -immobilized silica gel, which exhibited catalytic activity for the hydrosilylation in the absence of an activator such as NaBHEt₃ . The glassware in which Co(tpy)(OH)₂ was immobilized on the inner wall was prepared. It was found that the hydrosilylation catalytically occurred on the surface of a pretreated glassware and that the catalytic activity did not decrease even after 10 repeated uses.

Cobalt-Catalyzed Asymmetric 1,4-Hydroboration of Enones with HBpin

Herein, a series of new 8-OIQ cobalt complexes were synthesized and used for cobalt-catalyzed chemo- and enantioselective 1,4-hydroboration of enones with HBpin to access chiral β,β-disubstituted ketones with good to excellent chemo- and enantioselectivties. This protocol is operationally simple and shows a broad substrate scope.

Enantioselective Cobalt-Catalyzed Cascade Hydrosilylation and Hydroboration of Alkynes to Access Enantioenriched 1,1-Silylboryl Alkanes

Enantioenriched 1,1-silylboryl alkanes possess silyl and boryl groups that are both connected to the same stereogenic carbon center at well-defined orientations. As these chiral multifunctionalized compounds potentially offer two synthetic handles, they are highly valued building blocks in asymmetric synthesis as well as medicinal chemistry. Despite the potential usefulness, efficient synthetic approaches for their preparation are scarce. Seeking to address this deficiency, an enantioselective cobalt-catalyzed hydrosilylation/hydroboration cascade of terminal alkynes has been realized. This protocol constitutes an impressive case of chemo-, regio-, and stereoselectivity wherein the two different hydrofunctionalization events are exquisitely controlled by a single set of metal catalyst and ligand, an operation which would usually require two separate catalytic systems. Downstream transformations of enantioenriched 1,1-silyboryl alkanes led to various valuable chiral compounds. Mechanistic studies suggest that the present reaction undergoes highly regioselective and stereocontrolled sequential hydrosilylation and hydroboration processes.

Iron-Catalyzed Highly Enantioselective Hydrogenation of Alkenes

Here, we reported for the first time an iron-catalyzed highly enantioselective hydrogenation of minimally functionalized 1,1-disubstituted alkenes to access chiral alkanes with full conversion and excellent ee. A novel chiral 8-oxazoline iminoquinoline ligand and its iron complex have been designed and synthesized. This protocol is operationally simple by using 1 atm of hydrogen gas and shows good functional group tolerance. A primary mechanism has been proposed by the deuterium-labeling experiments.

Iron-Catalyzed Diborylation of Unactivated Aliphatic gem-Dihalogenoalkenes: Synthesis of 1,2-Bis(boryl)alkanes

Herein, we report the first example of iron-catalyzed defluoroborylation of unactivated gem-difluoroalkenes, gem-dichloroalkenes, and gem-dibromoalkenes, providing the 1,2-bis(boryl)alkanes in moderate to good yield. This transformation has high regioselectivity, wide substrate scope, and excellent functional group compatibility. Preliminary mechanistic studies indicate that double β-F elimination is involved in the catalytic cycle, and the 1,1-diborylated alkenes might be intermediates in this iron-catalyzed 1,2-diborylation reaction.

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Iron- and Cobalt-Catalyzed Asymmetric Hydrofunctionalization of Alkenes and Alkynes

Transition metal catalyzed asymmetric hydrofunctionalization of readily available unsaturated hydrocarbons presents one of the most straightforward and atom-economic protocols to access valuable optically active products. For decades, noble transition metal catalysts have laid the cornerstone in this field, on account of their superior reactivity and selectivity. In recent years, from an economical and sustainable standpoint, first-row, earth-abundant transition metals have received considerable attention, due to their high natural reserves, affordable costs, and low toxicity. Meanwhile, the earth-abundant metal catalyzed hydrofunctionalization reactions have also gained much interest and been investigated gradually. However, since chiral ligand libraries for earth-abundant transition-metal catalysis are limited to date, the development of highly enantioselective versions remains a significant challenge.This Account summarizes our recent efforts in developing suitable chiral ligands for iron and cobalt catalysts and their applications in the highly enantioselective hydrofunctionalization reactions (hydroboration and hydrosilylation) of alkenes and alkynes. In ligand design, we envisioned that chiral unsymmetric NNN-tridentate (UNT) ligand scaffolds could promote these enantioselective transformations with earth-abundant metals. Therefore, several types of chiral UNT ligands were designed and prepared in our laboratory, utilizing readily available natural amino acids as chiral sources. In the very beginning, chiral oxazoline iminopyridine (OIP) ligands were proposed and investigated through the rational combination of nitrogen-containing ligand scaffolds. After a systematic survey of the ligand effects, the imine moiety in the rigid OIP ligands was replaced by a conformationally more flexible amine unit, leading to the construction of reactive oxazoline aminoisopropylpyridine (OAP) ligands. Subsequently, imidazoline iminopyridine (IIP) and thiazoline iminopyridine (TIP) ligands were prepared by altering the oxygen atom of oxazoline with nitrogen and sulfur linkers, respectively. To further expand the chiral ligand library, other tridentate ligands containing a twisted pincer, anionic, and nonrigid backbone were also designed and synthesized, including iminophenyl oxazolinyl phenylamine (IPOPA) and imidazoline phenyl picolinamide (ImPPA). The efficacy of these chiral UNT ligands for asymmetric induction in iron and cobalt catalysis has been demonstrated through asymmetric hydrofunctionalization of alkenes and asymmetric sequential hydrofunctionalization of alkynes, which exhibit excellent reactivity as well as high chemo-, regio-, and stereoselectivity with broad functional group tolerance. Notably, highly regio- and enantioselective hydrofunctionalization of challenging substrates, such as 1,1-disubstituted aryl alkenes and terminal aliphatic alkenes, was also achieved. Furthermore, the development of asymmetric sequential isomerization/hydroboration of internal alkenes and sequential hydrofunctionalization of alkynes further demonstrates the synthetic power of these catalytic systems. The chiral enantioenriched products obtained by these methodologies could be potentially utilized in organic synthesis, medicinal chemistry, and materials science. We believe that our continuous efforts in this field would be beneficial to the development of asymmetric earth-abundant metal catalysis.

Asymmetric Cobalt-Catalyzed Regioselective Hydrosilylation/Cyclization of 1,6-Enynes

We report an enantioselective cobalt-catalyzed hydrosilylation/cyclization reaction of 1,6-enynes with secondary and tertiary hydrosilanes employing a catalyst generated in situ from the combination of Co(acac)₂ and (R,S_p )-Josiphos. A wide range of oxygen-, nitrogen-, and carbon-tethered 1,6-enynes reacted with Ph₂ SiH₂ , (EtO)₃ SiH, or (RO)₂ MeSiH to afford the corresponding chiral organosilane products in high yields and up to >99 % ee. This cobalt-catalyzed hydrosilylation/cyclization also occurred with prochiral secondary hydrosilane PhMeSiH₂ to yield chiral alkylsilanes containing both carbon- and silicon-stereogenic centers with excellent enantioselectivity, albeit with modest diastereoselectivity. The chiral organosilane products from this cobalt-catalyzed asymmetric hydrosilylation/cyclization could be converted to a variety of chiral five-membered heterocyclic compounds by stereospecific conversion of their C-Si and Si-H bonds without loss of enantiopurity.