Cleavage Of Carbon-silicon Bond Research Articles

Directed evolution of enzymatic silicon-carbon bond cleavage in siloxanes.

Volatile methylsiloxanes (VMS) are man-made, nonbiodegradable chemicals produced at a megaton-per-year scale, which leads to concern over their potential for environmental persistence, long-range transport, and bioaccumulation. We used directed evolution to engineer a variant of bacterial cytochrome P450BM3 to break silicon-carbon bonds in linear and cyclic VMS. To accomplish silicon-carbon bond cleavage, the enzyme catalyzes two tandem oxidations of a siloxane methyl group, which is followed by putative [1,2]-Brook rearrangement and hydrolysis. Discovery of this so-called siloxane oxidase opens possibilities for the eventual biodegradation of VMS.

Selective Carbanion-Pyridine Coordination of a Reactive P,N Ligand to RhI.

Ligands with reactive carbon sites in the periphery of a metal center have emerged as a powerful approach for metal–ligand bond activation. These reactive carbon sites are commonly generated by deprotonation strategies. Carbon–silicon bond cleavage is a potential alternative to access such constructs. Herein, the monodesilylation of bis‐silyl‐substituted P,N scaffold PNSi2 in the coordination sphere of [RhI(Cl)(CO)(PNSi2)] (1) with sodium azide is disclosed. This affords a unique dinucleating anionic κ2‐C,N‐κ1‐P ligand with a carbanionic methine carbon atom directly bound to rhodium as part of a four‐membered Rh‐N‐C‐C rhodacycle. This dimer undergoes meta‐pyridine C−H activation facilitated by weak bases, which leads to a desymmetrization of the system and provides a σ,π‐bridging 3‐pyridyl fragment bound to RhI. The facile Si−C cleavage strategy may pave the way to studying the reactivity and functionalization of a variety of κ2‐C,N‐coordinated pyridine scaffolds for selective transformations.

Confined Polymerization in Highly Ordered Mesoporous Organosilicas.

Hybrid mesoporous organosilica exhibiting crystal-like order in the walls provided an ideal channel reaction vessel for the confined polymerization of acrylonitrile (PAN). The resulting high-molecular-mass PAN fills the channels at high yield and forms an ordered nanostructure of polymer nanobundles enclosed into the hybrid matrix. The in situ thermal transformation of PAN into rigid polyconjugated and, eventually, into condensed polyaromatic carbon nanofibers, retains the periodic architecture. Simultaneously, the matrix evolves showing the fusion of the p-phenylene rings and the cleavage of carbonsilicon bonds: this gives rise to graphitic-carbon/silica nanocomposites containing hyper-oxydrylated silica nanophases. Interestingly, the 3D hexagonal mesostructure survives in the carbonaceous material. The exploitation of porous materials of high capacity and a hybrid nature, for polymerization in the confined state, followed by high temperature treatments, allowed us to achieve unique and precisely fabricated nanostructures, thus paving the way for the construction of fine-tuned electronic and light-harvesting materials.

Studies on Photoinduced Carbon-Silicon Bond Cleavage and Subsequent Bromination Reaction

A novel photoinduced bromination of benzylsilanes is reported. This reaction undergoes smoothly under UV irradiation at room temperature via two key steps: C—Si bond cleavage and subsequent bromination, which presents a new protocol to the synthesis of benzyl bromides. In addition, a possible mechanism of this reaction is proposed.

Mechanism of Rhodium-Catalyzed Carbon–Silicon Bond Cleavage for the Synthesis of Benzosilole Derivatives: A Computational Study

Rhodium-catalyzed carbon-silicon bond cleavage reaction is an efficient approach for the synthesis of silole derivates. The newly reported density functional theory method M11 is employed in order to elucidate how to cleave the inactive C(methyl)-Si bond. The computational results indicate that oxidative addition/reductive elimination pathway is favored over direct transmetallation in the C(methyl)-Si bond cleavage step. Alternatively, 1,4-rhodium-silicon exchange could take place before oxidative addition/reductive elimination. The rate-determining step for both pathways has been targeted on the initial transmetallation of 2-trimethylsilylphenyl boronic acid. The active catalytic species is a monomeric hydroxyrhodium complex, which could be regenerated from the hydrolysis of methylrhodium complex. In addition, theoretical calculations show that the hydrolyses of both aryl and vinyl intermediates are inhibited by intramolecular π-coordinated groups.

ChemInform Abstract: Synthesis of Six‐Membered Silacycles by Intramolecular Nucleophilic Substitution at Silicon Involving the Cleavage of Carbon—Silicon Bonds.

AbstractSix‐membered silacycles are available via the cleavage of an inert carbon‐silicon bond using organolithium reagents.

ChemInform Abstract: A New Mechanistic Path in the Reaction of Ruthenium Trichloride with Silylated Cyclopentadienes. Ionic Transition Metal Halides as Desilylating Agents.

The reaction of hydrated ruthenium trichloride with mono-, bis- and tris-(trimethylsilyl)cyclopentadiene in alcohol solvents leads to high yields of ruthenocene; these processes proceed through an unprecedented chloride ion-induced desilylation of the silylated Cyclopentadienes followed by reaction of RuIII ion with cyclopentadiene, rather than the mechanistically distinct carbon–silicon bond cleavage that has been observed in other transition metal halide systems.

The competition between Si–Si and Si–C cleavage in functionalised oligosilanes: their reactivity with elemental lithium

The reaction of aryl-substituted disilanes with elemental lithium is a common method for the preparation of lithiosilanes and the subsequent synthesis of functionalised oligosilanes, especially of enantiomerically pure compounds. A series of alkyl- and arylsubstituted di- and trisilanes has been investigated with respect to their reactivity against elemental lithium. Thereby, depending on the substituents, silicon-silicon bond cleavage of the central Si-Si unit and/or silicon-carbon bond cleavage to arenes are observed. Quantum chemical studies provide a deeper insight into the ongoing processes. The reactive centre can be estimated by both, bond elongation after electron transfer and frontier orbital interactions (pi-bonding and sigma-antibonding part). Aromatic substituents at the silicon atoms proved to be necessary for the processing of any cleavage reaction in the studied systems by stabilising the radical anion after electron transfer at the corresponding di- or trisilane. Yet, selective cleavage reactions do not depend on the number of arenes.

Gas‐phase intramolecular anion rearrangements of some trimethylsilyl‐containing systems revisited. A theoretical approach

Ab initio calculations at the CCSD(T)/6-311++G(2d,p)//B3LYP/6-311++G(d,p) level of theory have been carried out for three prototypical rearrangement processes of organosilicon anion systems. The first two are reactions of enolate ions which involve oxygen-silicon bond formation via three- and four-membered states, respectively. The overall reactions are: CH(2) = C(O(-))Si(CH(3))(3) --> (CH(3))(3)SiO(-) + CH(2)C, and (CH(3))(3)SiCH = CHO(-) --> (CH(3))(3)SiO(-) + C(2)H(2). The DeltaG (reaction) values for the two processes are +175 and +51 kJ mol(-1), with maximum barriers (to the highest transition state) of +55 and +159 kJ mol(-1), respectively. The third studied process is the following: (CH(3)O)C(=CH(2))Si(CH(3))(2)CH(2)(-) --> (CH(3))(2)(C(2)H(5))Si(-) + CH(2)CO, a process involving an S(N)i reaction between -CH(2)(-) and CH(3)O- followed by silicon-carbon bond cleavage. The reaction is favourable [DeltaG(reaction) = -39 kJ mol(-1)] with the barrier for the S(N)i process being 175 kJ mol(-1). The previous experimental and the current theoretical data are complementary and in agreement.

ChemInform Abstract: Discovery and Synthetic Applications of Novel Silicon—Carbon Bond Cleavage Reactions Based on the Coordination Number Change of Organosilicon Compounds

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Selective Si–C Bond Cleavage on a Diorganosilicon Porphyrin Complex Bearing Different Axial Ligands

Abstract Tetraphenylporphyrinato methylphenylsilicon complex (abbreviated to Si(TPP)MePh) was synthesized and structurally characterized. Although the Si–Ph bond of Si(TPP)MePh is much longer than the Si–Me bond, photolysis and chemical oxidation of Si(TPP)MePh cleaved the Si–Me bond selectively and gave new phenyl-substituted silicon porphyrins. Theoretical studies of a model compound showed that significant σ–π orbital interaction raised the HOMO and is responsible for the silicon–carbon bond cleavage. The instability of phenyl radical to methyl radical caused selective bond cleavage.

Silicon–Carbon bond cleavage reactions of Ansa tungstenocene compounds: The [Me 2 Si] bridge as a site for metallocene functionalization

[Me(2)Si(Cp(Me(2)))(2)]W(H)Cl is obtained via reaction of WCl(6) with a mixture of [Me(2)Si(Cp(Me(2)))(2)]Li(2) and NaBH(4), from which the dichloride [Me(2)Si(Cp(Me(2)))(2)]WCl(2) is obtained via treatment with CHCl(3). [Me(2)Si(Cp(Me(2)))(2)]WCl(2) provides a means to access other ansa tungstenocene compounds, such as [Me(2)Si(Cp(Me(2)))(2)]WH(2), [Me(2)Si(Cp(Me(2)))(2)]WMe(2), and [Me(2)Si(Cp(Me(2)))(2)]WCO. Of most interest, the reactions of [Me(2)Si(Cp(Me(2)))(2)]W(H)Cl with organolithium reagents do not yield simple ansa tungstenocene derivatives. Specifically, the reactions of [Me(2)Si(Cp(Me(2)))(2)]W(H)Cl with MeLi, Bu(n)Li, or PhLi result in the formation of mixed-ring tungstenocene compounds resulting from C-Si cleavage and functionalization of the ansa bridge, namely (Cp(Me(2)))(eta(5),kappa(1)-C(5)H(2)Me(2)SiMe(2)CH(2))WH, (Cp(Me(2)))[eta(5),kappa(1)-C(5)H(2)Me(2)Si(Me)(Bu(n))CH(2)]WH, and (Cp(Me(2)))[eta(5),kappa(1)-C(5)H(2)Me(2)SiMe(2)(C(6)H(4))]WH, respectively. In contrast to the C-Si cleavage achieved by MeLi, Bu(n)Li, and PhLi, the ansa bridge of [Me(2)Si(Cp(Me(2)))(2)]W(H)Cl is inert to Bu(t)Li and the product obtained is the fulvene ("tuck-in") complex [Me(2)Si(Cp(Me(2)))(eta(6)-C(5)MeH(2)CH(2))]WH derived from dehydrohalogenation.

シリレンの環化付加反応の有機合成化学的利用

In this short review, a new methodology of preparation of the silacyclic compounds using silylene and their application for organic synthesis are described. Silylene transfer from silacyclopropane to alkene, alkyne, imine, and α, β-unsaturated carbonyl compounds was catalyzed by silver salt to give the corresponding [1+2] or [1+4] cycloadducts in good yields. Combination of the cycloaddition, Ireland-Claisen rearrangement, and oxidative cleavage of silicon-carbon bond provided the regio-and stereoselective synthesis of polyol and α-hydroxyacid in a mild condition.

Discovery and synthetic applications of novel silicon-carbon bond cleavage reactions based on the coordination number change of organosilicon compounds

Some synthetically useful transformations of organosilicon compounds have been developed since the mid 1970s, based on the new concept that the silicon-carbon bonds are activated toward electrophilic cleavage via the formation of penta- and hexa-coordinate species. This review mainly consists of the following aspects: (1) a general concept for the activation of the silicon-carbon bond via penta- and hexa-coordinate species, (2) synthetic application of hexa-coordinate organopentafluorosilicates, and (3) development of the H2O2 oxidation of the silicon-carbon bond and its synthetic applications via the intramolecular hydrosilylation, silicon-tethered intramolecular radical cyclization and Diels-Alder reaction, and some silicon-containing organometallic reagents for nucleophilic hydroxymethylation and hydroxyallylation synthons.

Synthesis of ansa‐[n]Silacyclopentadienyl–Cycloheptatrienyl–Chromium Complexes (n = 1, 2): Novel Precursors for Polymers Bearing Chromium in the Backbone

Reaction of [(eta5-C5H4Li)(eta7-C7H6Li)Cr]tmeda with a variety of dialkyl(dichloro)silanes in aliphatic solvents afforded the corresponding [1]silatrochrocenophanes. Structural characterization by X-ray diffraction analysis of the [1]silatrochrocenophanes bearing Me2Si, (iPr)2Si, and silacyclobutane bridges revealed tilt angles alpha of 15.56(12) degrees , 15.8(1) degrees , and 16.33(17) degrees , respectively. Analogously, a [2]silatrochrocenophane (6) was prepared in excellent yield by reaction of [(eta5-C5H4Li)(eta7-C7H6Li)Cr]tmeda with 1,2-dichloro-1,1,2,2-tetramethyldisilane. This complex also was characterized structurally and exhibited a tilt angle alpha of 2.60(15) degrees. The [1]silatrochrocenophane bearing the Me2Si bridge underwent facile and regioselective carbon-silicon bond cleavage with [Pt(PEt3)4] to give a very high yield of an oxidative addition product. The ring-opening polymerization of these novel [1]silatrochrocenophanes afforded ring-opened chromium-based polymers.

Structural Changes in RSiO3/2-TiO2 Hybrid Films with UV Irradiation and Their Photocatalytic Micropatterning

UV light irradiation induced structural changes of the cleavage of silicon-carbon bonds and elimination of organic groups in organosilsesquioxane-titania (RSiO3/2-TiO2, R=methyl, ethyl, phenyl and benzyl) hybrid films due to the photocatalytic effect of TiO2 component. Phenyl and benzyl groups tended to remain in the films after the cleavage of Si-C bonds presumably due to their higher stability of aromatic rings and lager steric effect. The refractive index and dynamic hardness of all the hybrid films increased, and the thickness and contact angle for water decreased by the structural changes induced by the UV light irradiation. On the basis of these changes in film properties, micropatterning was successfully performed on the RSiO3/2-TiO2 hybrid films by UV light irradiation through a photomask. The micropatterns thus obtained should be applicable to a printing plate using the surface energy differences as well as a micro-optical component using the surface profiles and refractive index changes.

The Carbon–Silicon Bond Cleavage of Organosilicon Compounds in Supercritical Water

Abstract Arylsilanes, alkenylsilanes, allylic silanes, and alkylsilanes were found to undergo extremely facile and rapid C–Si bond cleavage in supercritical water. Rapid C–Si bond cleavage occurred even with robust unactivated tetraalkylsilanes. The control experiments revealed the dramatic difference between the supercritical and subcritical conditions, and that between supercritical water and supercritical methanol, attesting to a unique reactivity of supercritical water in C–Si bond cleavage. It was also found that acids, such as HCl, HBr, and H2SO4, promote C–Si bond cleavage in supercritical water.

The CeCl3×nH2O/NaI System in Organic Synthesis: An Efficient Water Tolerant Lewis Acid Promoter

AbstractFor Abstract see ChemInform Abstract in Full Text.

Photoprotodesilylation of 9-trimethylsilylanthracene in alcohols.

It was found that the anomalous fluorescence quenching in methanol (as compared to other solvents) of 9-trimethylsilylanthracene (2) is accompanied by a chemical reaction leading to anthracene. This ipso substitution is due to a hydrogen bonding formation in the excited singlet state preceding the carbon-silicon bond cleavage. The mechanism was studied using stationary and dynamic fluorescence, laser flash photolysis and reactivity kinetics as a function of temperature. The kinetic parameters were determined for the formation of an intermediate "X" (A(X) approximately 2.5 x 10(11) s(-1) Ea(X) approximately 19.5 kJ mol(-1)) and for the global reaction (A(R) approximately 4 x 10(10) s(-1) and Ea(R) approximately 19.2 kJ mol(-1)), indicating that, after the formation of X, the reaction does not involve any step with an activation energy larger than 19 kJ mol(-1). The intermediate X undergoes partitioning between the product (a approximately 0.18) and the starting material. The isotopic effect for the quenching by methanol was found to be kX(H)/kX(D) approximately 1.9, in keeping with the proposed protonation in the excited singlet state (S1). The reactivity ratio between the S1 state and the ground state, kR(S1)/ kR(S0) approximately 10(12) is in line with those observed by other authors. This unusual photoreaction can proceed for 2 with other alcohols. ipsoProtodesilylation of aromatic silanes are known to occur in the ground state but in acidic media.

The CeCl3·nH2O/NaI System in Organic Synthesis: An Efficient Water Tolerant Lewis Acid Promoter

The ability of CeCl 3 .nH 2 O/NaI system to work as an useful water tolerant promoter and/or reagent in many organic transformations is reviewed. They include Michael additions of 1,3-dicarbonyl compounds, amines and indoles to α,β-unsaturated ketones, and related systems, the cleavage of carbon-oxygen and carbon-silicon bonds, the conversion of alcohols into the corresponding iodides, as well as the stereospecific dehydration of β-hydroxy carbonyl compounds and the hydroxacyclization of unsaturated 3-hydroxy esters. Generally the reactions are carried out at reflux in wet CH 3 CN, however, in some case, it is possible to work under solvent free conditions and/or in the presence of the catalyst supported on SiO 2 . In the last case the simple experimental procedures allow the complete recovery of the catalyst and its recycling without loss of activity. Many of these protocols found wide application in organic synthesis and are now methods of common choice, since they are competitive and in some cases superior to the pre-existing procedures. The reasons for this success can be ascribed to the fact that this reagent allows the reaction to be carried out in close to neutral conditions and thus allows the survival of a large variety of functionalities sensitive to acidic hydrolysis. In addition, CeCl 3 .7H 2 O and NaI are cheap, non toxic, and stable compounds, and therefore the protocols based on their use represent an environmentally benign alternative to current chemical processes using water intolerant Lewis acids.