Silicon Substituents Research Articles

Synthesis of cyclic organosilicon compounds by palladium-catalyzed reactions of 2-silylaryl triflates

Abstract 2-Silylaryl triflates are widely employed as effective aryne precursors in organic synthesis, but their use as substrates for the synthesis of organosilicon compounds by retaining their silicon substituents is another attractive usage of these reagents. In particular, cyclic compounds with a silicon atom in the ring are promising candidates for various biologically active substances and optoelectronic functional materials. In this context, new synthetic methods for silicon-containing cyclic compounds have been actively investigated through the development of palladium-catalyzed reactions of 2-silylaryl triflates without generating aryne intermediates. As a result, selective synthesis of various silacyclic compounds has been achieved via C–H and/or C–Si bond activations as well as intramolecular exchange between these bonds and C–Pd bonds that are formed as reaction intermediates. An overview of this topic is described, including the mechanistic insights.

Molecular Engineering of Stabilized Silicon-Rosindolizine Shortwave Infrared Fluorophores.

Fluorescence-based biological imaging in the shortwave infrared (SWIR, 1000-1700 nm) is an attractive replacement for modern in vivo imaging techniques currently employed in both medical and research settings. Xanthene-based fluorophores containing heterocycle donors have recently emerged as a way to access deep SWIR emitting fluorophores. A concern for xanthene-based SWIR fluorophores though is chemical stability toward ambient nucleophiles due to the high electrophilicity of the cationic fluorophore core. Herein, a series of SWIR emitting silicon-rosindolizine (SiRos) fluorophores with emission maxima >1300 nm (up to 1550 nm) are synthesized. The SiRos fluorophore photophysical properties and chemical stability toward nucleophiles are examined through systematic derivatization of the silicon-core alkyl groups, indolizine donor substitution, and the use of o-tolyl or o-xylyl groups appended to the fluorophore core. The dyes are studied via absorption spectroscopy, steady-state emission spectroscopy, solution-based cyclic voltammetry, time-dependent density functional theory (TD-DFT) computational analysis, X-ray diffraction crystallography, and relative chemical stability over time. Optimal chemical stability is observed via the incorporation of the 2-ethylhexyl silicon substituent and the o-xylyl group to protect the core of the fluorophore.

β‐Silylacrylate as a Multipurpose Reagent in Organic Synthesis

AbstractAcrylates are well known electrophilic alkenes having multitude of applications in organic synthesis. They are very good acceptors in Michael addition reactions and are good enophile/dienophile/dipolarophile partners in cycloaddition reactions. Replacing the β‐alkyl/aryl groups in acrylates by a silicon group would be interesting. In addition to the conventional reactions displayed by acrylates, β‐silylacrylates (β‐SAs) can show reactivity specifically related to the silicon group. Many conventional organic reactions such as hydrodimerization, organocatalytic asymmetric Michael additions, inter‐ and intra‐molecular Diels–Alder reactions, and asymmetric 1,3‐dipolar cycloadditions have been used to generate the complex chemical entities from β‐SAs. Some of the reaction outcomes were vastly influenced by the silicon substituent. This review describes the practical synthesis β‐SAs and their use as starting point in complex molecule generation including total synthesis of some natural products/bioactive molecules.

Photocatalytic Synthesis of Diarylmethyl Silanes via 1,6-Conjugate Addition of Silyl Radicals to p-Quinone Methides.

A novel photocatalytic method for the preparation of diarylmethyl silanes was reported through silyl radicals addition strategy to p-QMs (p-quinone methides). This protocol could tolerate a variety of functional groups affording the corresponding silylation products with moderate to excellent yields. The resulting silylation products could be easily converted into a series of bioactive GPR40 agonists and useful p-QMs precursors for the synthesis of compounds possessing both quaternary carbon centers and silicon substituents through simple operation. A plausible mechanism of silyl radicals to p-QMs was proposed on the basis of experimental results and previous literature.

O,O-Silyl Group Migrations in Quinic Acid Derivatives: An Opportunity for Divergent Synthesis.

The O,O-silyl group migrations on a quinic acid-derived cyclitol have been studied, and the ease of migration was observed to be dependent on the silicon substituents and reaction conditions. Conditions were found to improve the formation of a main isomer during the O,O-silyl group migrations that could be integrated into the formal synthesis of vitamin D receptor modulator VS-105 and in the first total synthesis of a metabolite from the African ant Crematogaster nigriceps.

Reversible C(sp3)-Si Oxidative Addition of Unsupported Organosilanes: Effects of Silicon Substituents on Kinetics and Thermodynamics.

The intermolecular oxidative addition of unactivated C(sp3)-Si bonds is reported for a family of organosilanes at a cationic pincer-supported iridium complex. To our knowledge, no examples of oxidative addition to give analogous unsupported (alkyl)metal silyl complexes have been previously reported. The generality of this transformation is excellent, with successful examples demonstrated for tetraorganosilanes, mono- and poly alkoxysilanes, and two siloxysilanes. Oxidative addition is found to be completely reversible, with the product of reductive elimination being subject to trapping by triethylsilane. The successful isolation of these metal silyl complexes has allowed for an in-depth kinetic analysis of C(sp3)-Si reductive elimination, a process with strong implications in both catalytic C-H silylation and olefin hydrosilylation. The apparent order of reactivity is SiMe3 > SiMe2(CF3) > SiMe2OSiMe3 > SiMe2OSiMe2OSiMe3 > SiMe2(OMe) > SiMe2(OEt) > SiMe(OMe)2. A DFT analysis of the oxidative addition products shows that the thermodynamic stability of the (alkyl)metal silyl complexes span a range of ca. 10 kcal·mol-1, which relate closely with the experimentally determined rates of C(sp3)-Si reductive elimination and trapping, though a clear kinetic distinction exists between methoxy- and siloxysilyl complexes.

New Adhesive Materials Based on Silicon-Substituted Polynorbornenes

Two metathesis homopolymers with tri(n-propoxy)silyl and tri(n-butoxy)silyl side substituents are synthesized, and their adhesive properties are investigated. High-molecular-weight polymers are prepared in the presence of the first-generation Grubbs catalyst in a good yield. Adhesive properties are studied by the probe tack method at different temperatures. Using the example of the studied polymers, for the first time, it is shown that pressure-sensitive adhesives can be obtained on the basis of polynorbornenes. The effect of the length of the side silicon substituent on the work of adhesion is evaluated.

Intercepting the Disilene-Silylsilylene Equilibrium.

The equilibrium between disilenes (R2 Si=SiR2 ) and their silylsilylene (R3 Si-SiR) isomers has previously been inferred but not directly observed, except in the case of the parent system H2 Si=SiH2 . Here, we report a new method to prepare base-coordinated disilenes with hydride substituents. By varying the bulk of the coordinating base and other silicon substituents, we have been able to control the rearrangement of disilene adducts to their silylsilylene tautomers. Remarkably, 1,2 migration of a trimethylsilyl group is preferred over hydrogen migration. A DFT study of the reaction mechanism provides a rationale for the observed reactivity and detailed information on the bonding situation in base-stabilized disilenes.

Intercepting the Disilene‐Silylsilylene Equilibrium

AbstractThe equilibrium between disilenes (R2Si=SiR2) and their silylsilylene (R3Si−SiR) isomers has previously been inferred but not directly observed, except in the case of the parent system H2Si=SiH2. Here, we report a new method to prepare base‐coordinated disilenes with hydride substituents. By varying the bulk of the coordinating base and other silicon substituents, we have been able to control the rearrangement of disilene adducts to their silylsilylene tautomers. Remarkably, 1,2 migration of a trimethylsilyl group is preferred over hydrogen migration. A DFT study of the reaction mechanism provides a rationale for the observed reactivity and detailed information on the bonding situation in base‐stabilized disilenes.

Arylation of hydrocarbons enabled by organosilicon reagents and weakly coordinating anions.

Over the past 80 years, phenyl cation intermediates have been implicated in a variety of C-H arylation reactions. Although these examples have inspired several theoretical and mechanistic studies, aryl cation equivalents have received limited attention in organic methodology. Their high-energy, promiscuous reactivity profiles have hampered applications in selective intermolecular processes. We report a reaction design that overcomes these challenges. Specifically, we found that β-silicon-stabilized aryl cation equivalents, generated via silylium-mediated fluoride activation, undergo insertion into sp3 and sp2 C-H bonds. This reaction manifold provides a framework for the catalytic arylation of hydrocarbons, including simple alkanes such as methane. This process uses low loadings of Earth-abundant initiators (1 to 5 mole percent) and occurs under mild conditions (30° to 100°C).

An unprecedented 1,4-diphospha-2,3-disila butadiene (-P[double bond, length as m-dash]Si-Si[double bond, length as m-dash]P-) derivative and a 1,3-diphospha-2-silaallyl anion, each stabilized by the amidinate ligand.

The first acyclic 4π-electron -P[double bond, length as m-dash]Si-Si[double bond, length as m-dash]P- motif with two four coordinate silicon substituents supported by the amidinate ligand and two coordinate phosphorus has been synthesized from the reaction of heteroleptic chlorosilylene LSiCl (1), TripPCl2 (Trip = 2,4,6-iPr3C6H2) and KC8 in a 1 : 1 : 3 ratio. The same reaction in a 1 : 2 : 6 ratio in the presence of one equivalent of 18-crown-6 ether affords the 1,3-diphospha-2-silaallyl anion.

Cyclopentadienyl–Silsesquioxane Titanium Catalysts: Factors Affecting Their Formation and Activity in Olefin Epoxidation with Aqueous Hydrogen Peroxide

The reaction of titanium chlorosilyl‐substituted cyclopentadienyl (Cp) complexes, Ti(η5‐C5H3R′SiMe2Cl)Cl3 (R′ = H, 1b; SiMe3, 1c), with 1 equiv. of various silsesquioxane trisilanols, R7Si7O9(OH)3 (R = iBu, 2a; Ph, 2b), affords either corner‐capped Cp derivatives, Ti(η5‐C5H3R′SiMe2Cl)(R7Si7O12‐κ3O3) (R′ = H, R = iBu, 5a, Ph, 5b; R′ = SiMe3, R = iBu, 7a, Ph, 7b), or cyclopentadienyl–silsesquioxane complexes, Ti(η5‐C5H4SiMe2OR7Si7O11‐κ2O2)Cl (R′ = H, R = iBu, 6a, Ph, 6b; R′ = SiMe3, R = iBu, 8a, Ph, 8b), depending on the reaction conditions. In any case, upon heating, kinetic products 5 and 7 are transformed into the corresponding thermodynamic products 6 and 8, respectively. The electron‐donating ability of the Cp ring is a relevant controlling parameter: a strong π‐donor character facilitates the isomerization process. In addition, the nature of the silicon substituents in the silsesquioxane framework, the type of solvent, and the reaction temperature are also factors that significantly affect this process. Cyclopentadienyl–silsesquioxane complexes 6b and 8b are efficient and selective catalysts for epoxidation with aqueous hydrogen peroxide under mild conditions. Such a catalytic efficiency is attributed to the hydrophobic environment generated about the titanium atom by the Cp ring incorporated into the cyclopentadienyl–silsesquioxane ligand.

ChemInform Abstract: Indium‐Catalyzed [2 + 2] Cycloaddition of Allylsilanes to Internal Alkynones.

AbstractTransformation of the cyclobutenone products via thermal ring opening and oxidative cleavage of the silicon substituent is exemplified.

Stereoselective construction of 1,3-disilylcyclopentane derivatives by scandium-catalyzed [3+2] cycloaddition of allylsilanes to β-silylenones.

The Sc(OTf)3-catalyzed [3+2] cycloaddition of allylsilanes to β-silyl-α,β-unsaturated ketones (β-silylenones) has been developed to form five-membered syn-1,3-disilylketones diastereoselectively through the rearrangement of the silicon substituents on the allylsilane. Stabilization of the carbocation intermediates by a double silicon effect plays a key role in directing the course of the reaction to favor the [3+2] cycloaddition pathway over simple allylation.

Stereoselective Construction of 1,3‐Disilylcyclopentane Derivatives by Scandium‐Catalyzed [3+2] Cycloaddition of Allylsilanes to β‐Silylenones

AbstractThe Sc(OTf)3‐catalyzed [3+2] cycloaddition of allylsilanes to β‐silyl‐α,β‐unsaturated ketones (β‐silylenones) has been developed to form five‐membered syn‐1,3‐disilylketones diastereoselectively through the rearrangement of the silicon substituents on the allylsilane. Stabilization of the carbocation intermediates by a double silicon effect plays a key role in directing the course of the reaction to favor the [3+2] cycloaddition pathway over simple allylation.

ChemInform Abstract: Acid‐Catalyzed Direct Conjugate Alkenylation of α,β‐Unsaturated Ketones.

AbstractTreatment of the product (IIIa) with HBF4 gives the compound (IV) via 1,4‐phenyl shift.

Remote Control by Steric Effects

A rhodium-catalyzed reaction places a silicon substituent on the site farthest away from the largest group present on an aromatic ring. [Also see Research Article by Cheng and Hartwig ]

Rhodium-Catalyzed Intermolecular C–H Silylation of Arenes with High Steric Regiocontrol

Regioselective C-H functionalization of arenes has widespread applications in synthetic chemistry. The regioselectivity of these reactions is often controlled by directing groups or steric hindrance ortho to a potential reaction site. Here, we report a catalytic intermolecular C-H silylation of unactivated arenes that manifests very high regioselectivity through steric effects of substituents meta to a potential site of reactivity. The silyl moiety can be further functionalized under mild conditions but is also inert toward many common organic transformations, rendering the silylarene products useful building blocks. The remote steric effect that we observe results from the steric properties of both the rhodium catalyst and the silane.

Acid‐Catalyzed Direct Conjugate Alkenylation of α,β‐Unsaturated Ketones

New tricks, old reaction: The title reaction proceeds under mild and transition-metal-free conditions (see scheme; Tf=trifluoromethanesulfonyl). Various combinations of substrates are applicable to the synthesis of γ,δ-unsaturated ketones by employing silicon substituents at the β-position of the enones. The silicon substituents play a key role in stabilization of the cationic intermediate by hyperconjugation.

ChemInform Abstract: SmI2‐Mediated Radical Cyclizations Directed by a C—Si Bond.

AbstractThe silicon substituent, connected to carbon, plays a crucial role as controlling element to achieve the diastereoselective cyclization with formation of 2 or 3 stereocenters.