Conjugated Organosilicon Materials for Organic Electronics and Photonics

Mechanism and Origins of Enantioselectivity in the Nickel-catalyzed Asymmetric Synthesis of Silicon-Stereogenic Benzosiloles

As important π-skeletons, benzosiloles often possess unique electronic and optical properties and have been widely used in semiconductor materials. Therefore, great attention has been drawn to the area of developing novel synthetic methods for various benzosiloles. However, the synthesis of enantioenriched silicon-stereogenic benzosiloles is still at an early stage and remains to be explored. Herein, we performed systematic density functional theory studies on the recently reported nickel-catalyzed asymmetric synthesis of silicon-stereogenic benosiloles, which was enabled by an enantioselective desymmetrization of (2-alkenyl)aryl-substituted silacyclobutanes. Our computational study shows that the reaction mechanism involves ligand exchange, oxidative addition, alkene insertion, and hydrogen-transfer coupled reductive-demetalation steps. The proposed transmetalation and β-hydride elimination mechanism was not found, which might be due to the unfavorable ring strain of the multicyclic intermediates. The novel hydrogen-transfer coupled reductive-demetalation mechanism was shown to be reasonable for the generation of the silicon-stereogenic benzosilole. Noncovalent interactions (including C-H···π and hydrogen bonding) in the rate-determining alkene insertion transition state account for the origins of the enantioselectivity. Our computational study sheds light on the detailed reaction mechanism and also provides insights for the development of novel approaches for synthesis of high-value silicon-stereogenic compounds.

Selective Access to Silacyclopentanes and Homoallylsilanes by La-Catalyzed Hydrosilylations of 1-Aryl Methylenecyclopropanes

Methylenecyclopropanes (MCPs) have emerged as versatile building blocks in synthetic chemistry because of their unique reactivity. However, metal-catalyzed hydrosilylation of MCPs has met with very limited successes. In this paper, catalytic selective hydrosilylations of MCPs with some primary silanes using an ene-diamido lanthanum ate complex as the catalyst were described. The catalytic reactions resulted in the selective formation of silacyclopentanes and (E)-homoallylsilanes, respectively, depending on the substituents on MCPs. The formation of silacyclopentanes via a catalytic cascade inter- and intramolecular hydrosilylation mechanism is strongly supported by the control and deuteration-labeling experiments and DFT calculations. The unique reactivity and selectivity could be attributed to the large lanthanum ion and ate structure of the catalyst.

Ligand-Controlled Site- and Enantioselective Carbene Insertion into Carbon-Silicon Bonds of Benzosilacyclobutanes

We report herein a highly efficient palladium-catalyzed carbene insertion into strained Si-C bonds of benzosilacyclobutanes, which provides an efficient method to access α-chiral silanes. With a sterically hindered ligand, carbene insertion into the C(sp³ )-Si bond of benzosilacyclobutanes occurred in excellent site- and enantioselectivity, while C(sp² )-Si bond insertion occurred selectively with less sterically hindered ligands. Reaction mechanism, in particular the roles of the chiral ligands in controlling the site-selectivity of the insertion reactions, are elucidated by using hybrid density functional theory.

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.

Homogeneous catalytic C(sp3)-H functionalization of gaseous alkanes

The conversion of light alkanes into bulk chemicals is becoming an important challenge as it effectively avoids the use of prefunctionalized alkylating reagents. The implementation of such processes is, however, hampered by their gaseous nature and low solubility, as well as the low reactivity of the C-H bonds. Efforts have been made to enable both polar and radical processes to activate these inert compounds. In addition, these methodologies also benefit significantly from the development of a suitable reactor technology that intensifies gas-liquid mass transfer. In this review, we critically highlight these developments, both from a conceptual and a practical point of view. The recent expansion of these mechanistically-different methods have enabled the use of various gaseous alkanes for the development of different bond-forming reactions, including C-C, C-B, C-N, C-Si and C-S bonds.

Palladium-Catalyzed Enantioselective Carbene Insertion into Carbon-Silicon Bonds of Silacyclobutanes

We report herein a highly efficient palladium-catalyzed carbene insertion into strained Si-C bonds with excellent enantioselectivity, which provides a rapid and distinct method to access silacyclopentanes with a three- or four-substituted stereocenter asymmetrically. Mechanistic studies using hybrid density functional theory suggest a catalytic cycle involving oxidative addition, carbene migratory insertion, and reductive elimination. In addition, roles of the chiral ligands in controlling the reaction enantioselectivity are also elucidated.

Base-Mediated Borylsilylation/Silylation of Ammonium Salts with Silylborane

This work describes a base-mediated borylsilylation of benzylic ammonium salts to synthesize geminal silylboronates bearing benzylic proton under mild reaction conditions. Deaminative silylation of aryl ammonium salts was also achieved in the presence of LiO^tBu. This strategy which is featured with high efficiency, mild reaction conditions, and good functional group tolerance provides efficient routes for late-stage functionalization of amines.

Catalyst-free carbosilylation of alkenes using silyl boronates and organic fluorides via selective C-F bond activation

A regioselective carbosilylation of alkenes has emerged as a powerful strategy to access molecules with functionalized silylated alkanes, by incorporating silyl and carbon groups across an alkene double bond. However, to the best of our knowledge, organic fluorides have never been used in this protocol. Here we disclose the catalyst-free carbosilylation of alkenes using silyl boronates and organic fluorides mediated by tBuOK. The main feature of this transformation is the selective activation of the C-F bond of an organic fluoride by the silyl boronate without undergoing potential side-reactions involving C-O, C-Cl, heteroaryl-CH, and even CF3 groups. Various silylated alkanes with tertiary or quaternary carbon centers that have aromatic, hetero-aromatic, and/or aliphatic groups at the β-position are synthesized in a single step from substituted or non-substituted aryl alkenes. An intramolecular variant of this carbosilylation is also achieved via the reaction of a fluoroarene with a ω-alkenyl side chain and a silyl boronate.

Silicon and oxygen synergistic effects for the discovery of new high-performance nonfullerene acceptors

In organic electronics, an aromatic fused ring is a basic unit that provides π-electrons to construct semiconductors and governs the device performance. The main challenge in developing new π-skeletons for tuning the material properties is the limitation of the available chemical approach. Herein, we successfully synthesize two pentacyclic siloxy-bridged π-conjugated isomers to investigate the synergistic effects of Si and O atoms on the geometric and electronic influence of π-units in organic electronics. Notably, the synthesis routes for both isomers possess several advantages over the previous approaches for delivering conventional aromatic fused-rings, such as environmentally benign tin-free synthesis and few synthetic steps. To explore their potential application as photovoltaic materials, two isomeric acceptor-donor-acceptor type acceptors based on these two isomers were developed, showing a decent device efficiency of 10%, which indicates the great potential of this SiO-bridged ladder-type unit for the development of new high-performance semiconductor materials.

Crystal Structures and Phosphorescent Properties of Group 14 Dipyridinometalloles and Their Copper Complexes

We recently reported dipyridinosilole (DPyS) and dipyridinogermole (DPyG) as π-conjugated units that show low-temperature phosphorescence. A copper complex of DPG shows solid-state phosphorescence at room temperature. In this work, we prepared diphenyldipyridinostannole (DPySn1). Its optical and electrochemical properties were investigated and compared with those of DPyS1 and DPyG1 with the same phenyl substituents. DPySn1 showed the most red-shifted phosphorescence at 520 nm at 77 K in the solid-state, which is likely due to the intermolecular Sn-pyridine interaction that was confirmed by single-crystal X-ray diffraction. Copper complexes of DPyS1 and DPySn1 were also prepared and it was found that the phosphorescence of these dipyridinometallole-copper complex solids was red-shifted in the order of DPySn1-Cu (617 nm)<DPyG1-Cu (624 nm)<DPyS1-Cu (645 nm). Single-crystal X-ray diffraction studies were performed for DPyS1-Cu and DPySn1-Cu complexes. DPyS1-Cu showed a double-chained polymeric ladder-like structure with Cu₄ I₄ cluster units and DPySn1-Cu possessed a single chain polymeric structure with Cu₂ I₂ units, analogous to DPyG1-Cu.

Mechanistic Investigation of Biocatalytic Heme Carbenoid Si-H Insertions

Recent studies reported the development of biocatalytic heme carbenoid Si-H insertions for the selective formation of carbon-silicon bonds, but many mechanistic questions remain unaddressed. To this end, a DFT mechanistic investigation was performed which reveals an FeII-based concerted hydride transfer mechanism with early transition state feature. The results from these computational analyses are consistent with experimental data of radical trapping, kinetic isotope effects, and structure-reactivity data using engineered variants of hemoproteins. Detailed geometric and electronic profiles along the heme catalyzed Si-H insertion pathways were provided to help understand the origin of experimental reactivity trends. Quantitative relationships between reaction barriers and some properties such as charge transfer from substrate to heme carbene and Si-H bond length change from reactant to transition state were found. Results suggest catalyst modifications to facilitate the charge transfer from the silane substrate to the carbene, which was determined to be a major electronic driving force of this reaction, should enable the development of improved biocatalysts for Si-H carbene insertion reactions.

Synthesis of silacyclopent-2-en-4-ols via intramolecular [2 + 2] photocycloaddition of benzoyl(allyl)silanes

Organosilicon compounds are versatile units with a wide range of uses from medicinal chemistry to the field of organic electronics.

Directed evolution of cytochrome c for carbon-silicon bond formation: Bringing silicon to life

Enzymes that catalyze carbon-silicon bond formation are unknown in nature, despite the natural abundance of both elements. Such enzymes would expand the catalytic repertoire of biology, enabling living systems to access chemical space previously only open to synthetic chemistry. We have discovered that heme proteins catalyze the formation of organosilicon compounds under physiological conditions via carbene insertion into silicon-hydrogen bonds. The reaction proceeds both in vitro and in vivo, accommodating a broad range of substrates with high chemo- and enantioselectivity. Using directed evolution, we enhanced the catalytic function of cytochrome c from Rhodothermus marinus to achieve more than 15-fold higher turnover than state-of-the-art synthetic catalysts. This carbon-silicon bond-forming biocatalyst offers an environmentally friendly and highly efficient route to producing enantiopure organosilicon molecules.

Synthesis and characterization of new triphenylamino-1,8-naphthalimides for organic light-emitting diode applications

Three new triphenylamine derivatives with various numbers of N-phenyl-1,8-naphthalimide pendants were synthesized and fabricated into OLED devices.