Access to Chiral Silicon Centers for Application to Circularly Polarized Luminescence Materials

Hydrosilane/Organoiodine Coupling-Enabled Studies of Organosilane Physical Properties

ConspectusThis Account summarizes recent developments in metal-mediated coupling reactions between hydrosilanes and aryl iodides in the presence of Pd(P(t-Bu)3)2 and base. Alkylated organosilanes are obtained when Pt(P(t-Bu)3)2 and a base are employed in reactions between hydrosilanes and aliphatic iodides. These transformations show unusual reactivity compared to the typical behavior of hydrosilanes toward organoiodides in the presence of PdCl2 or PtCl2, and they proceed in high yields under mild conditions. In addition, the reaction demonstrates (1) high functional group tolerances, (2) stepwise introduction of substituents onto silicon atoms from secondary silanes, and (3) transformation without cleavage of weak Si-Si σ bonds in the molecules.This transformation can serve as a powerful tool for the synthesis of functional organosilicon compounds. The advantage is the flexibility in the molecular structure due to the large size compared to carbon. These compounds have relatively sparse packing in the crystalline state, unlike π-conjugated molecules, which induce dense π-π interactions. Consequently, they have significantly different physical properties in solution and in the solid state. Among them, aromatic disilanes and oligosilanes are important substance groups, because the conjugated chain within these molecules expands due to σ-π interaction between Si-Si σ bond and π orbital of aromatic ring. σ-π Conjugation is most efficient when the dihedral angle between the aromatic ring and the Si-Si bond is 90°, resulting in the overlap of σ orbital and π orbital. The conformational structure, packing, and physical properties of these compounds can change in tandem in response to external stimuli through a crystal phase transition. The interlocking changes in structure and physical properties are reversible, easily returning to their original state with different external stimulus. This account covers several important aspects, including solid-state emission with high fluorescence intensity, aggregation-induced emission (AIE) in water-THF system, mechanochromic fluorescence, organic light emitting diode (OLED), second harmonic generation (SHG) and thermosalient phenomena.This reaction can synthesize optically active tertiary and quaternary silanes by the enantioselective arylation of secondary silanes with aryl iodides using a palladium catalyst modified with a TADDOL-derived amide phosphoric acid ester as a chiral ligand. These optically active compounds can be used as useful circularly polarized luminescence (CPL) materials due to their strong luminescence intensity (Φ) and luminescence dissymmetry factor (glum) attributed to the chiral silicon atom. The efficient synthesis of sila-pharmaceuticals using this method as a key step is also described.The technique enables the design and synthesis of various silicon-containing bioactive substances and medical chemicals. Through the synthesis of organosilane compounds using this method, it is anticipated that the development of functional organic silanes will accelerate their practical applications in a wide range of fields.

Stereospecific synthesis of silicon-stereogenic optically active silylboranes and general synthesis of chiral silyl Anions

Silicon-stereogenic optically active silylboranes could potentially allow the formation of chiral silyl nucleophiles as well as the synthesis of various chiral silicon compounds. However, the synthesis of such silicon-stereogenic silylboranes has not been achieved so far. Here, we report the synthesis of silicon-stereogenic optically active silylboranes via a stereospecific Pt(PPh3)4-catalyzed Si-H borylation of chiral hydrosilanes, which are synthesized by stoichiometric and catalytic asymmetric synthesis, in high yield and very high or perfect enantiospecificity (99% es in one case, and >99% es in the others) with retention of the configuration. Furthermore, we report a practical approach to generate silicon-stereogenic silyl nucleophiles with high enantiopurity and configurational stability using MeLi activation. This protocol is suitable for the stereospecific and general synthesis of silicon-stereogenic trialkyl-, dialkylbenzyl-, dialkylaryl-, diarylalkyl-, and alkylary benzyloxy-substituted silylboranes and their corresponding silyl nucleophiles with excellent enantiospecificity (>99% es except one case of 99% es). Transition-metal-catalyzed C-Si bond-forming cross-coupling reactions and conjugate-addition reactions are also demonstrated. The mechanisms underlying the stability and reactivity of such chiral silyl anion were investigated by combining NMR spectroscopy and DFT calculations.

Highly Enantioselective Construction of Multifunctional Silicon-Stereogenic Silacycles by Asymmetric Enamine Catalysis

We report the first highly enantioselective construction of silicon-stereocenters by asymmetric enamine catalysis. An unprecedented desymmetric intramolecular aldolization of prochiral siladials was thus developed for the facile access of multifunctional silicon-stereogenic silacycles in high to excellent enantioselectivity. With an enal moiety, these adducts could be readily elaborated for the diverse synthesis of silicon-stereogenic compounds, and for late-stage modification.

Dynamically stable and amplified circularly polarized excimer emission regulated by solvation of chiral co-assembly process

Chiral supramolecular assembly has been assigned to be one of the most favorable strategies for the development of excellent circularly polarized luminescent (CPL)-active materials. Herein, we report our study of an achiral boron-containing pyrene (Py)-based chromophore (PyBO) as a circularly polarized excimer emission (CPEE) dye induced by chiral co-assemblies containing chiral binaphthyl-based enantiomers (R/S-M). Chiral co-assembly R/S-M-(PyBO)₄ fresh film spin-coated from toluene solution can exhibit orderly nanofibers and strong green CPEE (λ_em = 512 nm, g_em = ±0.45, Φ_FL = 51.2 %) resulting from an achiral PyBO excimer. In contrast, only a very weak blue CPL was observed (λ_em = 461 nm, g_em = ±  0.0125, Φ_FL = 19.0 %) after 187 h due to PyBO monomer emission as spherulite growth. Interestingly, this kind of chiral co-assembly R-M-(PyBO)₄-T film from tetrahydrofuran (THF) solution retains uniform morphology and affords the most stable and strongest CPEE performance (λ_em = 512 nm, g_em = + 0.62, Φ_FL = 53.3 %) after 10 days.

Chiral supramolecular assembly is an important strategy for the development of excellent circularly polarized luminescent (CPL)-active materials, but often they suffer from low quantum yield and luminescence dissymmetry factor. Herein, the authors we report a chiral coassembly process of achiral pyrene-based dyes leading to chiral excimers emitting with a high dissymmetry factor.

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.

Recent Progress in Transition Metal-Catalyzed Hydrosilanes-Mediated C-H Silylation

Organosilicon compounds are widely used in materials science, medicinal chemistry and synthetic chemistry. Recently, significant progress has been achieved in transition metal-catalyzed dehydrogenative C-H silylation. Particularly, recently developed monohydrosilane and dihydrosilane mediated C-H silylation have emerged as powerful tools in constructing C-Si bonds. Besides, dihydrosilane-mediated enantioselective asymmetric C-H silylation has successfully achieved the construction of central and helical silicon chirality. In addition, chiral organosilicon compounds have exhibited excellent photoelectric material properties and broad application prospects. Furthermore, organosilicon compounds could under a series of functional group transformations to enrich the diversity of silicon chemistry. This review will present a comprehensive picture of the development of transition metal-catalyzed hydrosilanes-mediated intramolecular C(sp 2 )-H and C(sp 3 )-H silylation organized by their reaction types and mechanisms. In addition, dihydrosilane-mediated enantioselective asymmetric C-H silylation to construct central and helical silicon chirality will also be highlighted in the review.

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.

Chirality in Organic and Mineral Systems: A Review of Reactivity and Alteration Processes Relevant to Prebiotic Chemistry and Life Detection Missions

Chirality is a central feature in the evolution of biological systems, but the reason for biology’s strong preference for specific chiralities of amino acids, sugars, and other molecules remains a controversial and unanswered question in origins of life research. Biological polymers tend toward homochiral systems, which favor the incorporation of a single enantiomer (molecules with a specific chiral configuration) over the other. There have been numerous investigations into the processes that preferentially enrich one enantiomer to understand the evolution of an early, racemic, prebiotic organic world. Chirality can also be a property of minerals; their interaction with chiral organics is important for assessing how post-depositional alteration processes could affect the stereochemical configuration of simple and complex organic molecules. In this paper, we review the properties of organic compounds and minerals as well as the physical, chemical, and geological processes that affect organic and mineral chirality during the preservation and detection of organic compounds. We provide perspectives and discussions on the reactions and analytical techniques that can be performed in the laboratory, and comment on the state of knowledge of flight-capable technologies in current and future planetary missions, with a focus on organics analysis and life detection.

Catalytic C(sp)–Si cross-coupling silylation of alkynyl bromides with hydrosilanes by palladium catalysis

An unprecedented and convenient Si–C(sp) bond-forming cross-coupling of alkynyl bromides with hydrosilanes has been established for the facile synthesis of alkynylsilanes in good yields and with excellent chemoselectivity.

Nickel(0)-Catalyzed Asymmetric Ring Expansion Toward Enantioenriched Silicon-Stereogenic Benzosiloles

The development of a straightforward strategy to obtain enantioenriched silicon-stereogenic benzosiloles remains a challenging yet appealing synthesis venture due to their potential future application in chiral electronic and optoelectronic devices. In this context, all of the existing methods rely on Rh-catalyzed systems and are somewhat limited in scope. Herein, we disclose the first Ni⁰ -catalyzed ring expansion process that enables the preparation of benzosiloles possessing tetraorganosilicon stereocenters in excellent yields and enantioselectivities. The presented catalysis strategy is further applied to the asymmetric synthesis of silicon-stereogenic bis-silicon-bridged π-extended systems. Preliminary studies reveal that such compounds exhibit fluorescence emission, Cotton effects and circularly polarized luminescence (CPL) activity.

Chiral exciplex dyes showing circularly polarized luminescence: extension of the excimer chirality rule

A series of axially chiral binaphthyls and quaternaphthyls possessing two kinds of aromatic fluorophores, such as pyrenyl, perylenyl, and 4-(dimethylamino)phenyl groups, arranged alternately were synthesized by a divergent method. In the excited state, the fluorophores selectively formed a unidirectionally twisted exciplex (excited heterodimer) by a cumulative steric effect and exhibited circularly polarized luminescence (CPL). They are the first examples of a monomolecular exciplex CPL dye. This versatile method for producing exciplex CPL dyes also improved fluorescence intensity, and the CPL properties were not very sensitive to the solvent or to the temperature owing to the conformationally rigid exciplex. This systematic study allowed us to confirm that the excimer chirality rule can be applied to the exciplex dyes: left- and right-handed exciplexes with a twist angle of less than 90° exhibit (-)- and (+)-CPL, respectively.

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.

Crystal structure of (R)-6-(benzo[b]thiophen-5-yl)-2-methyl-2,6-dihydrobenzo [5,6] silino[4,3,2-cd]indole, C23H17NSSi

C23H17NSSi, orthorhombic, P212121 (no. 19), a = 7.7862(5) Å, b = 9.0728(6) Å, c = 25.3797(16) Å, V = 1792.9(2) Å3, Z = 4, R gt (F) = 0.0301, wR ref (F 2) = 0.0778, T = 100.0 K.

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.

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.

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.

Enhancement of the Photofunction of Phosphorescent Pt(II) Cyclometalated Complexes Driven by Substituents: Solid-State Luminescence and Circularly Polarized Luminescence

Ligand functionalization is an attractive strategy for enhancing the performance of metal-based phosphorescent emitters. Here, we report the synthesis and characterization of cyclometalated Pt(II) complexes Pt3 and Pt4 containing organosilyl-substituted (2-(2-thienyl)pyridine) ligands and compare their properties with those of Pt1 (no substituent) and Pt2 (organocarbon substituent). The photophysical characteristics of these molecules, including their absorption and phosphorescence spectra, phosphorescence quantum yield and lifetime, were investigated. The molecular structures were revealed by X-ray diffraction analysis. Under UV light irradiation, Pt2-Pt4 emitted intense orange phosphorescence in the solid state because of the bulkiness of their side chains (up to Φ_P: 0.49). Optically pure (-)-(S)_Si-Pt4 and (+)-(R)_Si-Pt4 were prepared using the optically active ligands (+)-L4 and (-)-L4, respectively. The chiroptical properties of (+)-(R)_Si-Pt4, which has an asymmetric silicon atom, were investigated. Circular dichroism and circularly polarized luminescence measurements showed that these structural motifs are suitable for applications in chiroptical phosphorescent materials.

Competing Allosteric Mechanisms for Coordination-Directed Conformational Changes of Chiral Stacking Structures with Aromatic Rings

This work revealed that significant asymmetric nonlinear effects can be found in a coordination-directed conformational alteration through competing allosteric mechanisms. Toward this aim, we have prepared new chiral bridging ligands [( S, S)- and ( R, R)-Im₂An] containing an anthracene ring as a spacer with two ethynyl-linked chiral imidazole groups at the 9,10-positions. The ( S, S)- and ( R, R)-Im₂An ligands (L) spontaneously form the assemblies with Zn²⁺ ions (M) in solution phase, giving L₄M₂-type assemblies with a general formula [( S, S)- or ( R, R)-Im₂An]₄(Zn²⁺)₂. NMR studies revealed that the [( S, S)-Im₂An]₄(Zn²⁺)₂ assembly has an anthracene dimer structure with a parallel-displaced geometry, leading to relatively small circular dichroism (CD) signals, as expected for nonchiral objects. Conversely, subsequent addition of chiral coligands [( R)- or ( S)-Ph-box] to [( S, S)-Im₂An]₄(Zn²⁺)₂ afforded an alternative Zn²⁺ assembly with general formula [( R)- or ( S)-Ph-box]₂[( S, S)-Im₂An]₂(Zn²⁺)₂, where the chiral coligands expel two of the ( S, S)-Im₂An ligands that were singly bound to the Zn²⁺ ions in the original [( S, S)-Im₂An]₄(Zn²⁺)₂ assembly. This ligand-exchange reaction causes conformational alteration from a parallel-displaced structure to a twisted stacking between the anthracene rings inside the Zn²⁺ assembly, which results in a significant enhancement of CD signals due to excitonic interactions of the chiral anthracene dimer. Dissymmetry factor ( g_CD) for CD due to chiral stacking structures shows a significant inverse sigmoidal response to the enantiomeric excess of the chiral coligands. The observed nonlinear phenomena are results of the two conflicting mechanisms, homochiral cooperative association (homochiral self-sorting) to form CD-active assemblies [( S)- or ( R)-Ph-box]₂[( S, S)-Im₂An]₂(Zn²⁺)₂ versus heterochiral cooperative dissociation of [( S, S)-Im₂An]₄(Zn²⁺)₂ by sequestering of Zn²⁺ inside the assembly through formation of a heterochiral 2:1 Zn²⁺ complex ([( R)-Ph-box][( S)-Ph-box]Zn²⁺). The presented mechanisms provide a new strategy for generating switch-like OFF/ON states in chiral systems.

A Smart Sensing Method for Object Identification Using Circularly Polarized Luminescence from Coordination-Driven Self-Assembly

The potential use of circularly polarized luminescence for object identification in a sensor application is demonstrated. New luminescence probes using pyrene derivatives as sensor luminophores were developed. (R,R)-Im₂ Py and (S,S)-Im₂ Py contain two chiral imidazole moieties at 1,6-positions through ethynyl spacers (angle between spacers ca. 180°). The probe molecules spontaneously self-assemble into chiral stacks (P or M helicity) upon coordination to metal ions with tetrahedral coordination (Zn²⁺ ). The chiral probes display neither circular dichroism (CD) nor circularly polarized luminescence (CPL) without metal ions. However, (R,R)-Im₂ Py and (S,S)-Im₂ Py exhibit intense chiroptical activity (CD and CPL) upon self-assembly with Zn²⁺ ions. (R,R)-Im₂ Py and (S,S)-Im₂ Py with chemical stimuli-responsibility allow sensing using the CPL signal as detection output, enabling us to discriminate between a signal from the target analyte and that from non-target species.

Effects of substituents in silyl groups on the absorption, fluorescence and structural properties of 1,3,6,8-tetrasilylpyrenes

1,3,6,8-Tetrasilylpyrenes and related germyl and stannyl derivatives were synthesized, and their absorption and fluorescence spectroscopic and structural properties were elucidated. The results show that the UV-vis absorption maxima of these substances in CH2Cl2 solutions shift to longer wavelengths as the size of the alkyl groups and numbers of phenyl groups on silicon increase. Fluorescence quantum yields of tetrasilylpyrenes in cyclohexane are larger than that of pyrene, and a pentamethyldisilyl derivative has an emission efficiency of 0.79. Except in the case of the SiMe2H derivative, excimer emission was not observed in concentrated solutions of these substances. The SiMe2H and SiMe3 derivatives were shown to form CT complexes with tetracyanoethylene in CH2Cl2 solutions. The calculated energy barriers for rotation of the silyl groups about the Si-C bond increase as the steric bulk of the silyl group increases. 29Si NMR chemical shifts were found to depend on the sizes of the alkyl groups and numbers of phenyl groups. Data arising from theoretical calculations suggest that the silyl groups act as electron-donating groups, and the donating ability of the groups decreases in the order SiR3 > GeR3 > SnR3.