Silicon-stereogenic Silanes Research Articles

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.

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 SN 2 substitution of chloride ion to realize the chiral inversion of silicon center.

Construction of Tetrasubstituted Silicon-Stereogenic Silanes via Conformational Isomerization and N-Heterocyclic Carbene-Catalyzed Desymmetrization

Disclosed here is a catalytic strategy for asymmetric access to chiral tetrasubstituted silicon-stereogenic silanes. Our reaction starts with a (covalently) symmetric silane bearing two aldehyde moieties as the substrate. Single-crystal structural analysis shows that the substrate exists as a racemate of two conformational enantiomers because of the presence of a Si/O weak interaction. Mechanistic studies assisted by DFT calculation indicate that the two conformational enantiomers can readily isomerize to each other, and one of the conformational enantiomers of the substrate is favorably activated by a N-heterocyclic carbene catalyst via an overall desymmetrization process to eventually afford optically enriched tetrasubstituted silicon-stereogenic silanes as the products. Our chiral silanes’ products can be readily transformed to a diverse set of silicon stereogenic functional molecules.

Enantioselective C–H Functionalization toward Silicon-Stereogenic Silanes

AbstractIn recent years, transition-metal-catalyzed enantioselective C–H bond functionalization has emerged as a powerful and attractive synthetic approach to access silicon-stereogenic centers, which provides impetus for the innovation of chiral organosilicon chemistry. This short review summarizes recent advances in the construction of silicon-stereogenic silanes via transition-metal-catalyzed enantioselective C–H functionalization. We endeavor to highlight the great potential of this methodology and hope that this review will shed light on new perspectives and inspire further research in this emerging area.1 Introduction2 Enantioselective C–H Functionalization Induced by Oxidative Addition­ of an Aryl-OTf Bond3 Enantioselective C–H Functionalization Induced by Oxidative Addition­ of a Silacyclobutane4 Directing-Group-Assisted Enantioselective C–H Functionalization5 Enantioselective Dehydrogenative C–H/Si–H Coupling5.1 Enantioselective C(sp2)–H Silylation5.2 Enantioselective C(sp3)–H Silylation6 Summary and Outlook

A Glimpse and Perspective of Current Organosilicon Chemistry from the View of Hydrosilylation and Synthesis of Silicon-Stereogenic Silanes

AbstractSilicon and its related organosilicon chemistry has become a mainstay in synthetic chemistry as they can participate in numerous organic transformations. Due to space limitations, this perspective is focused on a glimpse of current organosilicon chemistry from the view of catalytic hydrosilylation and synthesis of silicon-stereogenic silanes. The progress of the two topics fully illustrates that organosilicon chemistry has become a hot research field in recent years and will play a greater role in academic research and industrial applications of silicon element in the future.1 Introduction2 Hydrosilylation3 Catalytic Constructions of Silicon-Stereogenic Centers4 Conclusion and Perspective

Streamlined Construction of Silicon-Stereogenic Silanes by Tandem Enantioselective C-H Silylation/Alkene Hydrosilylation.

A rhodium-catalyzed tandem enantioselective C-H silylation/alkene hydrosilylation of dihydrosilanes, which enables the streamlined construction of a wide range of silicon-stereogenic silanes, is successfully developed. This process involves a SiH2-steered highly enantioselective C-H silylation to furnish the corresponding desymmetric monohydrosilanes, which are subsequently trapped with alkenes in a stereospecific fashion to build functionally diverse asymmetrically tetrasubstituted silanes. This general strategy combines readily available dihydrosilanes and alkenes to construct various enantioenriched silicon-stereogenic silanes, including 9-silafluorenes, Si-bridged ladder compounds, and benzosilolometallocenes, in a single step with good to excellent yields and enantioselectivities.

Enantioselective Si-H Insertion Reactions of Diarylcarbenes for the Synthesis of Silicon-Stereogenic Silanes.

We report the first example of enantioselective, intermolecular diarylcarbene insertion into Si-H bonds for the synthesis of silicon-stereogenic silanes. Dirhodium(II) carboxylates catalyze an Si-H insertion using carbenes derived from diazo compounds where selective formation of an enantioenriched silicon center is achieved using prochiral silanes. Fourteen prochiral silanes were evaluated with symmetrical and prochiral diazo reactants to produce a total of 25 novel silanes. Adding an ortho substituent on one phenyl ring of a prochiral diazo enhances enantioselectivity up to 95:5 er with yields up to 98%. Using in situ IR spectroscopy, the impact of the off-cycle azine formation is supported based on the structural dependence for relative rates of diazo decomposition. A catalytic cycle is proposed with Si-H insertion as the rate-determining step, supported by kinetic isotope experiments. Transformations of an enantioenriched silane derived from this method, including selective synthesis of a novel sila-indane, are demonstrated.

Asymmetric Synthesis of Silicon-Stereogenic Silanes by Copper-Catalyzed Desymmetrizing Protoboration of Vinylsilanes.

The catalytic asymmetric creation of silanes with silicon stereocenters is a long-sought but underdeveloped topic, and only a handful of examples have been reported. Moreover, the construction of chiral silanes containing (more than) two stereocenters is a more arduous task and remains unexploited. We herein report an unprecedented copper-catalyzed desymmetrizing protoboration of divinyl-substituted silanes with bis(pinacolato)diboron (B2 pin2 ). This method enables the facile preparation of an array of enantiomerically enriched boronate-substituted organosilanes bearing contiguous silicon and carbon stereocenters with exclusive regioselectivity and generally excellent diastereo- and enantioselectivity.

Stereocontrolled Syntheses of Functionalized cis- and trans-Siladecalins.

We report the synthesis of both diastereomers of an all-silicon analog of decalin. Carbocyclic decalin is aubiquitousbicyclic structural motif. The siladecalinsynthesis provides materials functionalized with either Si-Ph or Si-H groups, versatile entry points for further chemical diversification. The synthesis of silicon-stereogenic silanes is significantly less precedented than the synthesis of asymmetric carbon centers, and strategies for control of relative stereochemistry in oligosilanes are hardly described. This study offers insights of potential generality, such as the epimerization of the cis-isomer to the thermodynamically downhill trans-isomer via a hypothesized pentavalent intermediate. Decalin is a classic example in the conformational analysis of organic ring systems, and the carbocyclic diastereomers have highly divergent conformational profiles. Like the carbocycle, we observe different conformational properties in cis- and trans-siladecalins with consequences for NMR spectroscopy, optical properties, and vibrational spectroscopy. This study showcases the utility of targeted synthesis for preparing complex and functionalized polycyclic silanes.

Desymmetrization-Oriented Enantioselective Synthesis of Silicon-Stereogenic Silanes by Palladium-Catalyzed C-H Olefinations.

A palladium-catalyzed chelation-assisted enantioselective C-H olefination of symmetrically diaryl-substituted tetraorganosilicon derivatives was developed, enabling the generation of nitrogen-containing silicon-stereogenic tetraorganosilicon compounds with modest to good yields and good to excellent enantioselectivities (up to 95.5:4.5 e.r.). The Thorpe-Ingold effect exerted by the substituents on silicon was observed to have a profound influence on formation of olefinated products which were further converted into other relevant chiral organosilanes without the loss of enantiomeric purity, thus demonstrating the synthetic utility of the developed enantioselective olefination.

Enantioselective Construction of Silicon-Stereogenic Silanes by Scandium-Catalyzed Intermolecular Alkene Hydrosilylation.

The catalytic asymmetric construction of silicon-stereogenic silanes is of great interest and significance, but has met with only limited success to date. We herein report the enantioselective hydrosilylation of alkenes with dihydrosilanes by a chiral half-sandwich scandium catalyst, which constitutes an efficient and general route for the synthesis of a wide range of enantioenriched silicon-stereogenic silanes from easily accessible starting materials. This reaction features a broad substrate scope, high yields, and high enantioselectivity. Some of the chiral tertiary silane products were also converted into valuable derivatives, such as chiral silanol, quaternary silane, and benzosilole compounds.

Palladium-Catalyzed Desymmetrization of Silacyclobutanes with Alkynes to Silicon-Stereogenic Silanes: A Density Functional Theory Study.

The palladium-catalyzed desymmetrization of silacyclobutanes using electron-deficient alkynes proceeds with high enantioselectivity in the presence of a chiral P ligand; this provides a facile approach for the synthesis of novel silicon-stereogenic silanes. In this work, we used hybrid density functional theory (DFT) to elucidate the mechanism of the palladium-catalyzed desymmetrization of silacyclobutanes with dimethyl acetylenedicarboxylate. Full catalytic cycle including two different initiation modes that were proposed to be a possible initial step to the formation of the 1-pallada-2-silacyclopentane/alkyne intermediate-the oxidative addition of the palladium complex to the silacyclobutane Si-C bond (cycle MA) or coordination of the Pd0 complex with the alkyne C≡C bond (cycle MB)-have been studied. It was found that the ring-expansion reaction began with cycle MB is energetically more favorable. The formation of a seven-membered metallocyclic PdII intermediate was found to be the rate-determining step, whereas the enantioselectivity-determining step, oxidative addition of silacyclobutane to the three-membered metallocyclic PdII intermediate, was found to be quite sensitive to the steric repulsion between the chiral ligand and silacyclobutane.

Palladium-catalyzed Si–C bond-forming silylation of aryl iodides with hydrosilanes: an enhanced enantioselective synthesis of silicon-stereogenic silanes by desymmetrization

An enantioselective Pd-catalyzed silicon–carbon bond-forming silylation reaction of aryl iodides with hydrosilanes for the synthesis of silicon-stereogenic silanes has been developed with good enantioselectivity under mild conditions.

Insight into the mechanism of carbonyl hydrosilylation catalyzed by Brookhart's cationic iridium(III) pincer complex.

New experimental findings suggest partial revision of the currently accepted mechanism of the carbonyl hydrosilylation catalyzed by the iridium(III) pincer complex introduced by Brookhart. Employing silicon-stereogenic silanes as a stereochemical probe results in racemization rather than inversion of the configuration at the silicon atom. The degree of the racemization is, however, affected by the silane/carbonyl compound ratio, and inversion is seen with excess silane. Independently preparing the silylcarboxonium ion intermediate and testing its reactivity then helped to rationalize that effect. The stereochemical analysis together with these control experiments, rigorous multinuclear NMR analysis, and quantum-chemical calculations clearly prove that another silane molecule participates in the hydride transfer. The activating role of the silane is unexpected but, in fact, vital for the catalytic cycle to close.

Desymmetrization Catalyzed by Transition‐Metal Complexes: Enantioselective Formation of Silicon‐Stereogenic Silanes

Prochiral silanes: recent work has demonstrated the powerful potential of enantioselective desymmetrization catalyzed by transition-metal complexes in the stereoselective formation of silicon-stereogenic silanes. These methods offer new possibilities for asymmetric synthesis of any functionalized chiral silanes that possess tetraorganosilicon stereocenters.

Illuminating the Mechanism of the Borane‐Catalyzed Hydrosilylation of Imines with Both an Axially Chiral Borane and Silane

The reduction of C=O groups with silanes catalyzed by electron-deficient boranes follows a counterintuitive mechanism in which the Si-H bond is activated by the boron Lewis acid prior to nucleophilic attack of the carbonyl oxygen atom at the silicon atom. The borohydride thus formed is the actual reductant. These steps were elucidated by using a silicon-stereogenic silane, but applying the same technique to the related reduction of C=N groups was inconclusive due to racemization of the silicon atom. The present investigation now proves by the deliberate combination of our axially chiral borane catalyst and axially chiral silane reagents (in both enantiomeric forms) that the mechanisms of these hydrosilylations are essentially identical. Unmistakable stereochemical outcomes for the borane/silane pairs show that both participate in the enantioselectivity-determining hydride-transfer step. These experiments became possible after the discovery that our axially chiral C(6)F(5)-substituted borane induces appreciable levels of enantioinduction in the imine hydrosilylation.

Catalytic enantioselective dehydrogenative Si–O coupling of oxime ether-functionalized alcohols

Asymmetric silylation of alcohols is an unusual but effective approach to their kinetic resolution, and the Cu–H-catalyzed dehydrogenative Si–O coupling is particularly noteworthy as dihydrogen is formed as the sole by-product. Our laboratory had previously reported on diastereoselective (with silicon-stereogenic silanes) and enantioselective (with achiral silanes) Si–O couplings of azine donor-functionalized alcohols. The limitation, that is, the requirement of a nitrogen donor atom, prompted us to seek equally useful donor groups. Oxime ethers were identified as a suitable alternative, and we describe herein the preparation of a series of oxime ether-functionalized alcohols. To assess different substitution patterns in their kinetic resolution, these were tested in the reagent-controlled Si–O coupling using a silicon-stereogenic silane. The optimal substituents at the oxime carbon atom (dr=85:15 in the diastereoselective coupling) were then chosen for the related catalyst-controlled Si–O coupling but selectivity factors were only moderate.

ChemInform Abstract: The Recent Synthesis and Application of Silicon‐Stereogenic Silanes: A Renewed and Significant Challenge in Asymmetric Synthesis

AbstractReview: ca. 100 refs.

The recent synthesis and application of silicon-stereogenic silanes: A renewed and significant challenge in asymmetric synthesis

The synthesis of silicon-stereogenic silanes is undoubtedly one of the most intriguing and challenging aspects in organic chemistry and organosilicon chemistry and is neglected by chemists to some extent. This critical review will focus on the recent exciting advances in the synthesis of silicon-stereogenic silane and outline the application of these chiral silanes in asymmetric synthesis (89 references).

Potassium tert‐Butoxide‐Catalyzed Dehydrogenative SiO Coupling: Reactivity Pattern and Mechanism of an Underappreciated Alcohol Protection

A remarkable tert-butoxide-catalyzed coupling of alcohols and silanes is reported. Dihydrogen and not hydrochloric acid (generated in the prevalent, related coupling of alcohols and chlorosilanes) is formed as the sole by-product. A comprehensive survey of common silanes provides a reliable tool for the predictability of their reactivity under defined reaction conditions. The debated mechanism of this transformation is investigated monitoring the stereochemical course at the silicon atom by means of a silicon-stereogenic silane. On this basis, a transition state for the enantiospecific Si-O coupling step is suggested rationalizing the observed frontside attack and thus retention at the silicon atom.