Attachment chemistry of organic molecules on Si(111)-7 x 7

Molecular attachment chemistry is emerging as an important approach to tailor the chemical, physical, and mechanical properties of silicon surfaces, as well as to incorporate organic functions into silicon-based devices for various technological needs. The chemical bonding and reactivity of various organic molecules on Si(111)-7 x 7 were systematically studied using XPS, HREELS, TPD, UPS, STM, and DFT calculations. The spatial arrangements and unique electronic properties of reactive adatoms and rest atoms on the surface offer rich attachment chemistry for organic functionalities. Investigations demonstrated that organic molecules can be chemically bound to Si(111)-7 x 7 through several reaction pathways, including [4 + 2]- and [2 + 2]-like additions, dative bonding, and dissociative reaction. This Account reviews the recent progress and current understanding of reactivity, selectivity, and mechanisms of organic molecules on Si(111)-7 x 7.     

Interrogation of an Enzyme Library Reveals the Catalytic Plasticity of Naturally Evolved [4+2] Cyclases

Stereoselective carbon-carbon bond forming reactions are quintessential transformations in organic synthesis. One example is the Diels-Alder reaction, a [4+2] cycloaddition between a conjugated diene and a dienophile to form cyclohexenes. The development of biocatalysts for this reaction is paramount for unlocking sustainable routes to a plethora of important molecules. To obtain a comprehensive understanding of naturally evolved [4+2] cyclases, and to identify hitherto uncharacterised biocatalysts for this reaction, we constructed a library comprising forty-five enzymes with reported or predicted [4+2] cycloaddition activity. Thirty-one library members were successfully produced in recombinant form. In vitro assays employing a synthetic substrate incorporating a diene and a dienophile revealed broad-ranging cycloaddition activity amongst these polypeptides. The hypothetical protein Cyc15 was found to catalyse an intramolecular cycloaddition to generate a novel spirotetronate. The crystal structure of this enzyme, along with docking studies, establishes the basis for stereoselectivity in Cyc15, as compared to other spirotetronate cyclases.     

Organometallic-like C-H bond activation and C-S bond formation on the disulfide bridge in the RuSSRu core complexes

C-S bond formations on the disulfide bridge in the dinuclear Ru(III) complexes have been found in the reaction with unsaturated organic molecules, such as alkenes and ketones. The reactions are initiated by the organometallic-like C-H bond activation. Through the mechanistic study of these novel reactions, the specific nature of the disulfide bridging ligand has been unveiled: (i) the double bond character of the sulfur-sulfur bond, which allows the Diels-Alder-type [4 + 2] cycloaddition reaction with butadiene to form a C(4)S(2) ring, (ii) the C-H bond activation on the S-S bond forming a C-S bond. All of the C-H activation reactions and the ensuing C-S bond formation reactions suggest that the disulfide ligand can act in a manner similar to the transition metal centers of the organometallic complexes.     

[2.2]Paracyclophane as a Target of the Organic Solid State: Emergent Properties via Supramolecular Construction

In this account, we review the synthesis of [2.2]paracyclophanes in the organic solid state. Reactions in crystalline solids provide a means to obtain molecules with high degrees of stereocontrol that can also be unattainable in solution. We show that [2.2]paracyclophanes form in the solid state stereospecifically and in quantitative yield via intermolecular [2+2] double photodimerization reactions. The double cycloaddition that affords a paracyclophane in the solid state does not readily occur in solution. Small molecules in the form of hydrogen-bond-donor templates can provide access to [2.2]paracyclophanes in a solid by design. A [2.2]paracyclophane obtained using a hydrogen-bond template is shown to exhibit attractive optical properties and has been employed as a building block of a metal-organic framework (MOF).     

Synthesis of Group 9 Metal‐Olefin Complexes with Identical Ligand Frameworks and Comparison of their Catalytic Activity in [2+2+2] Cycloaddition and other Addition Reactions

The preparation and catalytic evaluation of the three group 9 (cyclopentadienyl)metal(trimethylvinylsilane) complexes of the type C₅H₅M(H₂CCHSiMe₃)₂ (M=cobalt, rhodium and iridium) are reported. The complexes were investigated in [2+2+2] cycloaddition as well as in hydrogenation, hydroformylation and hydroboration reactions. Despite the identical organic frameworks and structural parameters, the complexes display remarkable reactivity and stability differences.     

以二甲基二环戊基氢氧化铵为模板剂合成BEC分子筛

采用自主设计、合成的二甲基二环戊基氢氧化铵(DMDCpAOH)为有机模板剂,静态水热条件下,在含Ge和F的晶化体系中合成beta沸石多型体C,即BEC分子筛。考察了晶化温度、Si/Ge物质的量比、H_2O/(Si+Ge)物质的量比、HF/(Si+Ge)物质的量比对BEC分子筛晶化及形貌的影响。并采用X射线衍射(XRD)、扫描电镜(SEM)、N_2吸附-脱附和热重分析(TGA)等手段对样品进行了表征。结果表明,在投料配比为0.833SiO_2:0.167GeO_2:0.5DMDCpAOH:0.5HF:15H_2O,晶化温度为170℃的条件下晶化10d,所得样品的BEC含量达85%。BEC分子筛晶体具有尺寸为20μm左右的棒状形貌,其骨架结构经焙烧后保持稳定。     

Synthesis, vasodilating, antithrombotic and cardioprotective activity of pyridyl substituted 5-silyl(germyl)isoxazolines-2

The reactions of [2+3] cycloaddition of pyridylnitrile oxides to vinyl- and allylgermanes proceed regioselectively and afford 5-Ge-substituted isoxazolines-2. We have synthesized 9 new pyridyl substituted 5-Si(Ge)-isoxazolines-2 and investigated their biological activity. The vasodilating, anticoagulant and cardioprotective activities of 5-Si(Ge) substituted isoxazolines-2 have been studied in vitro and in vivo. Substitution of the silicon atom for the germanium one leads to the significant increase in vasodilating, antithrombotic and cardioprotective activity. The insertion of the methylene group between Ge and the isoxazoline ring reduces the vasodilating activity. The most active isoxazoline - 3-(5'-triethylgermyl-3'-isoxazolinyl)pyridine hydrochloride protects the heart from rhythm disturbances and lethality during ischaemia-reperfusion.     

Catalytic coupling of sp2- and sp-hybridized carbon-hydrogen bonds with vinylmetalloid compounds

In the Account given herein, it has been shown that silylative coupling of olefins, well-recognized as a new catalytic route for the activation of double bond C-H bond of olefins and double bond C-Si bond of vinylsilicon compounds with ethylene elimination, can be extended over both other vinylmetalloid derivatives (double bond C-E) (where E = Ge, B, and others) as well as the activation of triple bond C-H, double bond C aryl-H, and -O-H bond of alcohols and silanols. This general transformation is catalyzed by transition-metal complexes (mainly Ru and Rh) containing or initiating TM-H and/or TM-E bonds (inorganometallics). This new general catalytic route for the activation of double bond C-H and triple bond C-H as well as double bond C-E bonds called metallative coupling or trans-metalation (cross-coupling, ring-closing, and polycondensation) constitutes an efficient method (complementary to metathesis) for stereo- and regioselective synthesis of a variety of molecular and macromolecular compounds of vinyl-E (E = Si, B, and Ge) and ethynyl-E (E = Si and Ge) functionality, also potent organometallic reagents for efficient synthesis of highly pi-conjugated organic compounds. The mechanisms of the catalysis of this deethenative metalation have been supported by equimolar reactions of TM-H and/or TM-E with initial substances and reactions with deuterium-labeled reagents.     

Surface modification of amorphous carbon thin films by 1,3-dipolar cycloaddition

Amorphous carbon thin film surfaces were successfully modified by 1,3-dipolar cycloaddition of nitrones, generated by the condensation of 4-(trifluoromethyl)benzaldehyde and N-methylhydroxylamine. Amorphous carbon thin films were deposited by electron cyclotron resonance sputtering and consisted of mainly sp²-hybridized carbon. The modification of amorphous carbon thin film surfaces with organic molecules was confirmed by X-ray photoelectron spectroscopy (XPS), Raman, and atomic force microscopy (AFM). F 1s, N 1s, and C 1s electron spectra revealed the existence of organic molecules on the surface of modified amorphous carbon thin films. The surface coverage increased with reaction temperature, reactant concentration, and reaction time.     

Azo Bridges from Azine 9. [2 + 2 + 1] Cycloadditions of Parallel C=C and N=N Bonds with Dihalocarbenes

Schemes 1 and 2 are examples of type A compounds in which the groups -N=N- and -CH=CH- are held in a close parallel position. They react with dichloro- and fluorochlorocarbene to form a unique pyrazolidine moiety (3–5) in a formal [2 + 2 + 1] cycloaddition which, despite the substitution pattern, is stable to ionization and hydrolysis according to Bredt's rule. The reaction is considered to start with the addition of the carbenes to the azo group. The azomethine imine thus formed subsequently undergoes a rapid [3 + 2] dipolar cycloaddition with the bridging C=C bond. The different isomers from 3–5 are identified from their NMR spectra.     

Synthesis of Multisubstituted Silyloxy-based Donor-Acceptor Cyclobutanes by an Acid-Catalyzed [2+2] Cycloaddition

This account summarizes our recent efforts on the acid-catalyzed [2+2] cycloaddition of silyl enol ethers with α,β-unsaturated esters, giving donor-acceptor (D -A) cyclobutanes and their related reactions. The cycloaddition generally produces multisubstituted cyclobutanes in good yields with high diastereoselectivity. We found that aluminum Lewis acids and triflic imide (Tf₂NH) had good catalytic activity. The retro [2+2] cycloaddition proceeded at higher reaction temperatures, and in some cases, diastereoselectivity switching of cycloadducts was observed. A microflow protocol was established for Tf₂NH-catalyzed [2+2] cycloaddition. Although the cycloaddition usually requires cryogenic conditions in a batch reactor, the microreactor system enabled the production of D -A cyclobutanes, even at ambient temperature. [2+2] Cycloaddition of allylsilanes and alkyl enol ethers, instead of silyl enol ethers, afforded the corresponding D -A cyclobutanes.     

Organosilicium-Chemie: Vom Molekül zum Werkstoff

Organosilicon Chemistry: From Molecules to Materials The transition from basic to applied organosilicon chemistry is described for the research fields that are studied by the author's group: silaethene chemistry, organosilicon polymer chemistry with silacycles and the chemistry of higher coordinated silicon species. Early investigations started with the study of the pyrolysis of silacyclobutanes that yielded silaethenes in the gas phase. However, these investigations are tedious and are of little use for a widespread study of silaethene chemistry. Therefore the generation of silaethenes in solution (from vinylchlorosilanes and tert-butyllithium) is the preferred method. Numerous differently substituted silaethenes have been generated in situ and their reactivity, especially in cycloaddition reactions, has been studied. Silaethenes that have two chlorine substituents at the silicon atom (especially Cl₂SiCHCH₂Bu^t) show an unusual cycloaddition reactivity and yield unexpected products with dienes and trienes (e. g. from [2+2] and [6+2] reactions); some of the silenes with donorfunctional groups at the α-C-atom undergo interesting intramolecular rearrangements. The cycloadducts of Si-dichlorofunctional silenes also show rearrangements when thermolyzed (e. g. [2+2] → [4+2] adducts). The reactivity of these silaethenes and their cyclo-adducts is due to the two chlorine atoms at the silicon atom and their influence can be explained by theoretical models. Silacycles synthesized from silaethenes have been incorporated or have been transformed into silicon containing polymers. Model compounds for nucleophilic substitution reactions at silicon centers and for the hydrolysis of chlorosilanes are higher coordinated silicon species up to coordination number „seven”︁. Some structures that describe such reactions have been characterized by X-ray crystallography.     

The Thermal Rearrangement of 1,2,7-Trienes

The thermal rearrangement of 1,6,7-nonatriene and 1,2,7-octatriene at 400°C has been studied. Besides intramolecular [2 + 2] cycloaddition reactions, these hydrocarbons undergo intramolecular ene reactions, furnishing cis-1-methyl-2-(1-propynyl)cyclopentane and cis-1-ethynyl-2-methylcyclopentane, respectively.     

Synthesis and radioprotective activity of new organosilicon and germanium compounds

Silathiazolidine and metalladithioacetals (M = Si, Ge) have been prepared by the interaction of dialkyldichloro- or bis(diethylamino)dialkylsilanes and -germanes with 3-[N-(2- thioethyl)]amino-propanamide (WR-2529) and [1-thioethyl-2-(1-naphtylmethyl)]-2- imidazoline. The study of these compounds in the field of chemical radioprotection has shown a notable decrease in the toxicity and a rather large increase in the radioprotective activity of these new derivatives in comparison with the starting organic compounds.     

Substitution of Sodium and Silicon in Tricalcium Aluminate

Four types of tricalcium aluminate solid solutions with different concentrations of Na₂O and SiO₂ were prepared and examined using an electron probe microanalyzer. The atomic ratios, including those determined in a previous study, were derived from the oxide compositions and provided excellent correlations between Ca and Na + Si (i.e., Ca = 3.003 − 0.48[Na + Si]), and Al and Si (Al = 1.997 − 1.02Si). Because the replacement reactions, Ca²⁺↔ 2Na⁺ and Ca²⁺+ 2Al³⁺↔ 2Si⁴⁺, independently occur within the same crystal, these reactions have been simply combined together to generate a new formula, Na_{2 x} Ca_{3− x − y} (Al_{1− y} Si _y )₂O₆, where x is the amount of Ca substituted by Na, and y is the amount of Al substituted by Si. This formula leads to the equations Ca = 3 − 0.5[Na + Si] and Al = 2 − Si, which nicely account for the constrained chemical variation of the actual solid solutions with 0 ≤ x < 0.049 and 0 ≤ y < 0.073.     

Copper‐in‐Charcoal‐Catalyzed,Tandem One‐Pot Diazo Transfer‐Click Reactions

Copper‐in‐charcoal (Cu/C) is an effective heterogeneous catalyst for tandem diazo transfer/click reactions. In the presence of Cu/C, various azides can be generated in situ from the corresponding amines, and subsequently undergo [3+2] cycloaddition with terminal alkynes to afford triazoles in good yields. The catalyst is also easily recycled.     

Development of cascade reactions for the concise construction of diverse heterocyclic architectures

Heterocyclic structural architectures occur in many bioactive natural products and synthetic drugs, and these structural units serve as important intermediates in organic synthesis. This Account documents our recent progress in the development of cascade reactions to construct complex carbocycles and heterocycles. We describe the rational design of cascade reactions and in-depth investigations of their mechanism as well as their applications in the synthesis of drugs, natural products, and related molecular analogs. Relying on knowledge about the dipole-type reactivity of sulfur ylides, we have developed three different types of cascade reactions: a [4 + 1] annulation/rearrangement cascade, a [4 + 1]/[3 + 2] cycloaddition cascade, and a Michael addition/N-alkylation cascade. Using these processes, we can generate oxazolidinones, fused heterocycles, and pyrrolines starting with simple and readily available substances such as nitroolefins and unsaturated imines. We have also developed corresponding enantioselective reactions, which are guided by axial chirality and asymmetric H-bonding control. In addition, by relying on the reactivity characteristics of newly designed acrylate-linked nitroolefins, we have disclosed an asymmetric Michael/Michael/retro-Michael addition cascade using the combination of a protected hydroxylamine and a bifunctional organocatalyst. Using this methodology, we prepared chiral chromenes in good yields and with high enantioselectivities. Moreover, a series of double Michael addition cascade reactions with anilines, thiophenols, and benzotriazoles generated highly functionalized chromanes. Via mechanistically distinct cascade processes that start with vinyl-linked indoles, we have synthesized polycyclic indoles. Intermolecular cross-metathesis/intramolecular Friedel-Crafts alkylation cascades, promoted by either a single ruthenium alkylidene catalyst or a sequence involving Grubbs' ruthenium catalyst and MacMillan's imidazolidinone catalyst, converted ω-indolyl alkenes into tetrahydrocarbazoles, tetrahydropyranoindoles, and tetrahydrocarbolines. In addition, we constructed tetrahydrocarbazoles and tetrahydroquinones using organocatalytic Friedel-Crafts alkylation/Michael addition cascades that used 2-vinyl indoles as common starting materials. Most green plants carry out photosynthesis to produce organic substances relying on readily available and renewable solar energy. In a related manner, we have also developed two cascade reactions that merge visible light-induced aerobic oxidation with either [3 + 2] cycloaddition/oxidative aromatization or intramolecular cyclization. These processes lead to the formation of pyrrolo[2,1-a]isoquinolines and enantiopure tetrahydroimidazoles, respectively.     

A general lithography-free method of microscale/nanoscale fabrication and patterning on Si and Ge surfaces

Here, we introduce and give an overview of a general lithography-free method to fabricate silicide and germanide micro-/nanostructures on Si and Ge surfaces through metal-vapor-initiated endoepitaxial growth. Excellent controls on shape and orientation are achieved by adjusting the substrate orientation and growth parameters. Furthermore, micro-/nanoscale pits with controlled morphologies can also be successfully fabricated on Si and Ge surfaces by taking advantage of the sublimation of silicides/germanides. The aim of this brief report is to illustrate the concept of lithography-free synthesis and patterning on surfaces of elemental semiconductors, and the differences and the challenges associated with the Si and the Ge surfaces will be discussed. Our results suggest that this low-cost bottom-up approach is promising for applications in functional nanodevices.     

Cytotoxic Activity of Silyl- and Germyl-Substituted 4,4-Dioxo-3a,6a-Dihydrothieno[2,3-d]isoxazolines-2

The [2+3] dipolar cycloaddition of nitrile oxides to the double C = C bonds of thiophene-1, 1-dioxides leads to formation of the fused isoxazolines-2 (1, 2). Tumor growth inhibition of these compounds strongly depends on the nature of group IV A element increasing from slightly active tert-butyl derivatives to silicon and germanium containing analogues. The products of benzonitrile oxide cycloaddition have greater cytotoxic effect than the compounds obtained from the cycloaddition reaction of 2, 5-disubstituted thiophene-1, 1-dioxides with acetonitrile oxide. Fused silyl substituted isoxazolines-2 are stronger NO-inducers than their germyl and tert-butyl analogues.     

Clickable poly(ionic liquid)s for modification of glass and silicon surfaces

Covalent attachment of poly(ionic liquid)s (PILs) by click chemistry on glass or silicon (Si) surfaces was performed. Poly[1-(4-vinylbenzyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide] (polyVBBI⁺Tf₂N⁻), and copolymers of polyVBBI⁺Tf₂N⁻ with fluorescein O-methacrylate were synthesized by conducting an atom transfer radical polymerization (ATRP) from initiators containing azide or thioacetate groups. The azide- and thiol-terminated PILs were then successfully grafted onto alkyne and alkene modified glass/Si wafers by thermal azide–alkyne cycloaddition and photoinitiated thiol-ene click reactions, respectively. The modified surfaces were characterized by contact angle measurements and ellipsometry. The fluorescent PIL functionalized surfaces showed strong fluorescence under UV irradiation. This procedure of tethering PILs to substrates also provides an easy way to change the surface hydrophilicity by replacing the anions in the grafted PILs. The present approach could be readily applied for surface modifications with other types of PILs or their copolymers to achieve different functionalities on various surfaces.