Silyl Ethers Research Articles

Crosslinking of polyethylene with polysilsesquioxane via carbamate linkages: Synthesis, shape recovery, and reprocessing properties

AbstractIn this contribution, we reported the crosslinking of polyethylene (PE) with dynamic silyl ether bonds. First, polycyclooctene copolymers bearing hydroxyl groups were synthesized via the ring‐opening metathesis copolymerization of cyclooctene with 5‐norbornene‐2‐methanol. The hydroxy‐functionalized polycyclooctene copolymers were further hydrogenated into the corresponding hydroxy‐functionalized PE copolymers. The PE copolymers were allowed to react with 3‐isocyanatopropyltriethoxysilane to obtain the PE copolymers bearing triethoxysilane groups. By taking advantage of hydrolysis and condensation of triethoxysilane groups grafted onto PE chains, the PE copolymers were well crosslinked; polysilsesquioxane (PSSQ) segments behaved as the crosslinking linkages in the networks. Notably, the PE networks were reprocessable (or recyclable) while zinc trifluoromethanesulfonate [Zn(OTf)2] was incorporated. The reprocessing properties of the PE networks were quite dependent on the crosslinking densities. The reprocessing behavior of the networks is attributable to the introduction of two dynamic chemistries: (i) the metathesis of silyl ether bonds which was catalyzed with Zn(OTf)2 and (ii) transcarbamoylation of carbamates which was catalyzed with dibutyltin dilaurate. Owing to the crosslinking, the PE networks displayed excellent shape memory properties. For the shape memory networks, notably, the original shapes can be reprogrammed with the aid of the exchange of the dynamic covalent bonds introduced into the systems.

Environmentally Friendly High‐Performance Polydimethylsiloxane Composites Reinforced with Renewable Cellulose Nanocrystals Based on Dynamic Silyl Ether Linkages

AbstractEnvironmentally friendly high‐performance composites made from either reprocessable polymer matrix or renewable fillers have become increasingly significant in order to reach a sustainable future. Herein, polydimethylsiloxane/modified cellulose nanocrystals (PDMS/mCNC) composites showing outstanding mechanical and reprocessable properties via silyl ether linkages are prepared. With only 1 wt% mCNC, the PDMS/mCNC composite exhibits the tensile strength of 2.96 MPa, which is twice that of the pure PDMS elastomer, while the toughness can achieve 7.32 MJ m−3. Remarkably, the reprocessing efficiency can be as high as 100% even for triple reprocessing process. Additionally, the favorable effect of mCNC on constructing hybrid filler network within vitrimers is explored for the first time. Selection of boron nitride nanosheet (BNNS) as the functional filler, 40 wt% BNNS is uniformly dispersed by only a small amount of mCNC and a homogeneous thermally conductive network is built, with the thermal conductivity of the PDMS/mCNC@BNNS composite increasing remarkably to 2.901 W m−1 K−1. In a word, this work paves the way to develop sustainable, mechanically strong, and reprocessable composites with mCNC as a renewable bio‐filler.

Pot-economical total synthesis of prostaglandins via organocatalyst-mediated asymmetric reactions

Abstract Prostaglandins are a family of molecules that possess important biological properties, and some of the analogues are used as medicines. Many synthetic methods have been developed for the efficient synthesis of prostaglandins. In this review, we will describe recent advances in the synthesis of prostaglandins and our endeavors in the effective synthesis of prostaglandins. Our group has developed three [3 + 2] cycloaddition reactions catalyzed by diphenylprolinol silyl ether, affording chiral substituted cyclopentane frameworks with excellent diastereo- and enantioselectivities. By using these cyclopentanes as key intermediates and applying the pot reactions, several prostaglandins were synthesized efficiently in a small number of pots.

Self-Amplified HF Release and Polymer Deconstruction Cascades Triggered by Mechanical Force.

Hydrogen fluoride (HF) is a versatile reagent for material transformation, with applications in self-immolative polymers, remodeled siloxanes, and degradable polymers. The responsive in situ generation of HF in materials therefore holds promise for new classes of adaptive material systems. Here, we report the mechanochemically coupled generation of HF from alkoxy-gem-difluorocyclopropane (gDFC) mechanophores derived from the addition of difluorocarbene to enol ethers. Production of HF involves an initial mechanochemically assisted rearrangement of gDFC mechanophore to α-fluoro allyl ether whose regiochemistry involves preferential migration of fluoride to the alkoxy-substituted carbon, and ab initio steered molecular dynamics simulations reproduce the observed selectivity and offer insights into the mechanism. When the alkoxy gDFC mechanophore is derived from poly(dihydrofuran), the α-fluoro allyl ether undergoes subsequent hydrolysis to generate 1 equiv of HF and cleave the polymer chain. The hydrolysis is accelerated via acid catalysis, leading to self-amplifying HF generation and concomitant polymer degradation. The mechanically generated HF can be used in combination with fluoride indicators to generate an optical response and to degrade polybutadiene with embedded HF-cleavable silyl ethers (11 mol %). The alkoxy-gDFC mechanophore thus provides a mechanically coupled mechanism of releasing HF for polymer remodeling pathways that complements previous thermally driven mechanisms.

Poly(silyl ether)s as Degradable and Sustainable Materials: Synthesis and Applications.

Polymer research is currently focused on sustainable and degradable polymers which are cheap, easy to synthesize, and environmentally friendly. Silicon-based polymers are thermally stable and can be utilized in various applications, such as columns and coatings. Poly(silyl ether)s (PSEs) are an interesting class of silicon-based polymers that are easily hydrolyzed in either acidic or basic conditions due to the presence of the silyl ether Si-O-C bond. Synthetically, these polymers can be formed in several different ways, and the most effective and environmentally friendly synthesis is dehydrogenative cross coupling, where the byproduct is H2 gas. These polymers have a lot of promise in the polymeric materials field due to their sustainability, thermal stability, hydrolytic degradability, and ease of synthesis, with nontoxic byproducts. In this review, we will summarize the synthetic approaches for the PSEs in the recent literature, followed by the properties and applications of these materials. A conclusion and perspective will be provided at the end.

Benzylic C(sp3)−H Alkoxylation through Visible‐Light‐Driven Oxidative Radical‐Polar Crossover

AbstractWe report a metal‐free method for benzylic C(sp3)−H alkoxylation with a silyl ether as an alkoxylating agent. The reaction is considered to proceed through a sequence consisting of hydrogen atom transfer (HAT) and oxidative radical‐polar crossover, wherein Selectfluor serves as a one‐electron oxidant, a HAT agent, and a fluoride ion source. The fluoride ion formed in situ activates the silyl ether to give the alkoxide nucleophile, which reacts with a benzyl cation intermediate to provide the benzyl ether product. The present protocol has a broad substrate scope, enabling C−H alkoxylation of primary, secondary, and tertiary benzylic C(sp3)−H bonds. Furthermore, it is applicable for late‐stage C−H alkoxylation of bioactive molecules.

Selective O-Acylation of Enol Silyl Ethers with Acyl Fluorides Catalyzed by Fluoride Ions Derived from Potassium Fluoride and 18-Crown-6.

The fluoride ion-catalyzed selective O-acylation of enol silyl ethers with acyl fluorides using KF and 18-Crown-6 is described herein. This catalytic system facilitated the practical and facile reaction of a variety of enol silyl ethers derived from aromatic/aliphatic ketones and aldehydes with acyl fluorides to afford useful and valuable enol esters.

Recyclable epoxy resins with different silyl ether structures: structure–property relationships and applications in diverse functional composites

In this work, three liquid silyl ether-containing cycloaliphatic epoxides (CESI1, CESI2 and CESI3) with different numbers of pendent phenyl groups were synthesized and cured by methylhexahydrophthalic anhydride. The high glass transition temperatures of the cured epoxy resins range from 150 to 184 °C, which increase with the increasing number of pendent phenyl groups. However, the tensile strengths of the cured epoxy resins decrease as the number of pendent phenyl groups increase. Incorporating dynamic silyl ether bonds enables the cured epoxy resins to be reprocessable and degradable. It is demonstrated that the pendent phenyl groups have a significant impact on the dynamic behaviors and degradation rates of cured epoxy resins. The reprocessing efficiencies and degradation rates of the three epoxy resins monotonously decrease with increasing the number of pendent phenyl groups. Finally, the CESI1 resin with the lowest viscosity was used as the polymer matrix for carbon fiber reinforced composites, electromagnetic wave absorbing coatings and silver-filled conductive adhesives. Notably, non-destructive recovery of carbon fibers, ferromagnetic particles and silver flakes can be achieved through the degradation of the epoxy matrix.

NFSI mediated C3-ether oxidation of glycals for the synthesis of hex-3-enuloses

Hex-3-enuloses constitute a vital carbohydrate synthetic intermediate that provide access to wide range of chiral molecules through diverse derivatizations. Herein we report synthesis of these fascinating scaffolds by oxidation of C3-ether protections on glycals in presence of N-fluorobenzenesulfonimide (NFSI) under Cu(I) catalysed conditions. Benzyl, methyl and silyl ethers have been efficiently oxidized to the carbonyl group. The oxidation has been found to be highly regioselective where an array of protecting groups were tolerant to the reaction conditions. Pyranosyl glycals from various commercially available sugars have been studied in this work to evaluate the broad substrate scope.

Synthesis of the C1–C13 Segment of Poecillastrin C

AbstractA stereoselective synthesis of the C1–C13 segment of poecillastrin C has been achieved. The C1–C4 moiety was derived from diallyl l-tartrate, and the amide group at the C3 position was constructed by means of a traceless Staudinger reaction. The C1–C13 segment was submitted to model studies, including esterification with a bulky alcohol at the C1 position and Stille coupling with vinyl iodide at the C13 position. The reactivity of the C1 position was affected by the neighboring C2-protective group. When the C2 hydroxy group was protected as a TBS ether, the C1 carboxylic acid did not undergo esterification with a bulky secondary alcohol, whereas the p-methoxybenzylidene N,O-acetal afforded a 2,4-dimethyl-3-pentyl ester. Stille coupling of the C1–C13 segment with 1-iodohept-1-ene gave the southern part of the poecillastrin C macrolactam attached to simplified eastern and western parts.

Silyloxymethylation of Aryl Halides via Metallaphotoredox Catalyzed Desulfinative Cross‐Coupling with Silyl Rongalite

AbstractA visible‐light‐triggered silyloxymethylation of aryl halides has been developed by the merger of photoredox and transition metal catalysis. Under the Ru/Ni dual catalytic conditions, sodium tert‐butyl‐dimethylsilyloxymethanesulfinate (SOMS) undergoes facile extrusion of sulfur dioxide to engender a silyloxymethyl radical that engages in the cross‐coupling with a range of aryl halides to produce benzyl silyl ether derivatives. The protocol is also effective with alkenyl halides and triflates, thus offering an efficient synthetic approach to the silyl‐protected allylic alcohols.

Visible-light-promoted desulfonylative radical difluoroalkylation between difluoroenol silyl ethers and difluoroalkyl sulfones to construct functionalized aryltetrafluoroethane derivatives

A visible-light-promoted desulfonylative radical difluoroalkylation between difluoroenol silyl ethers and difluoroalkyl sulfones has been developed.

Nickel-Catalyzed C(sp3)-Sb Coupling of Chlorostibines with Unactivated Alkyl Chlorides and In Vitro Anticancer Activity of Products.

In this study, we present a nickel-catalyzed reductive C(sp3)-Sb coupling of unactivated alkyl chlorides with chlorostibines. This approach is highly versatile, tolerating various functional groups such as acetal, alkene, nitrile, amine, ester, silyl ether, thioether, and various heterocyclic compounds. Notably, the late-stage modification of bioactive molecules and the satisfactory anticancer activity against cancerous MDA-MB-231 also demonstrate the potential application.

Polyhydroxyurethane and Poly(ethylene oxide) Multiblock Copolymer Networks: Crosslinking with Polysilsesquioxane, Reprocessing and Solid Polyelectrolyte Properties.

This contribution reports the synthesis of polyhydroxyurethane (PHU)-poly(ethylene oxide) (PEO) multiblock copolymer networks crosslinked with polysilsesquioxane (PSSQ). First, the linear PHU-PEO multiblock copolymers were synthesized via the step-growth polymerization of bis(6-membered cyclic carbonate) (B6CC) with α,ω-diamino-terminated PEOs with variable molecular weights. Thereafter, the PHU-PEO copolymers were allowed to react with 3-isocyanatopropyltriethoxysilane (IPTS) to afford the derivatives bearing triethoxysilane moieties, the hydrolysis and condensation of which afforded the PHU-PEO networks crosslinked with PSSQ. It was found that the PHU-PEO networks displayed excellent reprocessing properties in the presence of trifluoromethanesulfonate [Zn(OTf)2]. Compared to the PHU networks crosslinked via the reaction of difunctional cyclic carbonate with multifunctional amines, the organic-inorganic PHU networks displayed the decreased reprocessing temperature. The metathesis of silyl ether bonds is responsible for the improved reprocessing behavior. By adding lithium trifluoromethanesulfonate (LiOTf), the PHU-PEO networks were further transformed into the solid polymer electrolytes. It was found that the crystallization of PEO chains in the crosslinked networks was significantly suppressed. The solid polymer electrolytes had the ionic conductivity as high as 7.64 × 10-5 S × cm-1 at 300 K. More importantly, the solid polymer electrolytes were recyclable; the reprocessing did not affect the ionic conductivity.

Rhodium-Catalyzed Enantioselective Formal [4+1] Cyclization of Benzyl Alcohols and Benzaldimines: Facile Access to Silicon-Stereogenic Heterocycles.

The carbon-to-silicon switch in formation of bioactive sila-heterocycles with a silicon-stereogenic center has garnered significant interest in drug discovery. However, metal-catalyzed synthesis of such scaffolds is still in its infancy. Herein, a rhodium-catalyzed enantioselective formal [4+1] cyclization of benzyl alcohols and benzaldimines has been realized by enantioselective difunctionalization of a secondary silane reagent, affording chiral-at-silicon cyclic silyl ethers and sila-isoindolines, respectively. Mechanistic studies reveal a dual role of the rhodium-hydride catalyst. The coupling system proceeds via rhodium-catalyzed enantio-determining dehydrogenative OH silylation of the benzyl alcohol or hydrosilylation of the imine to give an enantioenriched silyl ether or silazane intermediate, respectively. The same rhodium catalyst also enables subsequent intramolecular cyclative C-H silylation directed by the pendent Si-H group. Experimental and DFT studies have been conducted to explore the mechanism of the OH bond silylation of benzyl alcohol, where the Si-O reductive elimination from a Rh(III) hydride intermediate has been established as the enantiodetermining step.

Rhodium‐Catalyzed Enantioselective Formal [4+1] Cyclization of Benzyl Alcohols and Benzaldimines: Facile Access to Silicon‐Stereogenic Heterocycles

AbstractThe carbon‐to‐silicon switch in formation of bioactive sila‐heterocycles with a silicon‐stereogenic center has garnered significant interest in drug discovery. However, metal‐catalyzed synthesis of such scaffolds is still in its infancy. Herein, a rhodium‐catalyzed enantioselective formal [4+1] cyclization of benzyl alcohols and benzaldimines has been realized by enantioselective difunctionalization of a secondary silane reagent, affording chiral‐at‐silicon cyclic silyl ethers and sila‐isoindolines, respectively. Mechanistic studies reveal a dual role of the rhodium‐hydride catalyst. The coupling system proceeds via rhodium‐catalyzed enantio‐determining dehydrogenative OH silylation of the benzyl alcohol or hydrosilylation of the imine to give an enantioenriched silyl ether or silazane intermediate, respectively. The same rhodium catalyst also enables subsequent intramolecular cyclative C−H silylation directed by the pendent Si‐H group. Experimental and DFT studies have been conducted to explore the mechanism of the OH bond silylation of benzyl alcohol, where the Si‐O reductive elimination from a Rh(III) hydride intermediate has been established as the enantiodetermining step.

1,6‐Addition Reactions of Difluoroenol Silyl Ethers with Benzoquinones and Quinonediimines To Access Difluorooxymethyl and Difluoroaminomethyl Compounds

AbstractThe etherification and amination of difluoroenol silyl ethers with benzoquinones and quinonediimines were achieved by using hexafluoroisopropanol (HFIP) as the solvent. This 1,6‐addition reaction could construct difluorooxymethyl and difluoroaminomethyl compounds under metal‐free conditions. It is preliminarily speculated that the reaction took place through a single electron transfer (SET) process.

Organocatalytic Asymmetric Vinylogous Michael Addition of Electron-Deficient Aryl Alkane Nucleophiles to Enals.

We report herein a protocol for an organocatalyzed asymmetric vinylogous Michael addition of aryl alkane nucleophiles with enals under base- and additive-free conditions. A series of allylic building blocks were obtained in 60%-93% yield and 88-99% ee with 20 mol % diphenylprolinol silyl ether as catalyst. This protocol has advantages such as excellent chemoselectivity and regioselectivity, good tolerance of functionalities, and simple reaction conditions.

Deconstructable and Chemically Recyclable High-Density Polyethylene (HDPE)-Like Materials Based on Si-O Bonds.

Polyethylene (PE) is the most widely produced synthetic polymer. By installing chemically cleavable bonds into the backbone of PE, it is possible to produce chemically deconstructable PE derivatives; to date, however, such designs have primarily relied on carbonyl- and olefin-related functional groups. Bifunctional silyl ethers (BSEs; SiR2(OR'2)) could expand the functional scope of PE mimics as they possess strong Si-O bonds, excellent oxidative stabilities, and facile chemical tunability. Here, we report BSE-containing high-density polyethylene (HDPE)-like materials synthesized through a one-pot catalytic ring-opening metathesis polymerization (ROMP) and hydrogenation sequence. The crystallinity of these materials can be adjusted by varying the BSE concentration or the steric bulk of the Si-substituents, providing handles to control thermomechanical properties while maintaining the high thermal stability of HDPE. Two methods for chemical recycling of HDPE mimics are introduced, including a circular approach that leverages acid-catalyzed Si-O bond exchange with 1-propanol. Additionally, despite the fact that the starting HDPE mimics were synthesized by chain-growth polymerization (ROMP), we show that it is possible to recover the molar mass and dispersity of recycled HDPE products using step-growth Si-O bond formation or exchange, generating high molecular weight recycled HDPE products with mechanical properties similar to commercial HDPE.

Photoredox Synthesis of Silyl Ethers via Radical Coupling of Ketones with Hydrosilanes and [1, 2]‐Brook Rearrangement

AbstractAn expedient synthesis of silyl ethers is reported via photocatalytic free radical coupling of ketones with hydrosilanes and subsequent [1, 2]‐Brook rearrangement. This strategy features the advantages of 100 % atom economy, environmental friendliness, easily available starting materials as well as late‐stage functionalization of complex molecules.