By-Product-Free Siloxane-Bond Formation and Programmed One-Pot Oligosiloxane Synthesis

A Clean One-Pot Synthesis of Structurally Ordered Linear, Monocyclic, and Bicyclic Oligosiloxanes

A highly selective cross-coupling reaction between Si-OAc (AcO=acetoxy) and Si-OH compounds that generates unsymmetrical and symmetrical oligosiloxanes concurrent with the release of acetic acid has been developed. The high selectivity arises from the reactivity difference that depends on the varying number of acetoxy groups present, thus facilitating a clean one-pot synthesis of oligosiloxanes. For instance, the reactions of di-, tri-, or tetraacetoxysilanes with silanols furnish acetoxy-containing di- and trisiloxanes in high yield. Two equivalents of tetraacetoxysilane can react with various silanediols to form 1,1,1,3,3,3-hexaacetoxytrisiloxanes, which subsequently react with a second molecule of a silanediol to selectively afford 1,1,3,3-tetraacetoxycyclotetrasiloxanes. The cyclotetrasiloxanes further react with a third molecule of silanediol to provide unprecedented bicyclic pentasiloxanes with acetoxy groups at the bridgehead silicon atoms. Applications of the acetoxy-containing products as efficient surface-treatment agents and new building blocks for highly heat-resistant materials are demonstrated.

Application of Silicone in Ophthalmology: A Review

This paper reviews the latest trends and applications of silicone in ophthalmology, especially related to intraocular lenses (IOLs). Silicone, or siloxane elastomer, as a synthetic polymer, has excellent biocompatibility, high chemical inertness, and hydrophobicity, enabling wide biomedical applications. The physicochemical properties of silicone are reviewed. A review of methods for mechanical and in vivo characterization of IOLs is presented as a prospective research area, since there are only a few available technologies, even though these properties are vital to ensure medical safety and suitability for clinical use, especially if long-term function is considered. IOLs represent permanent implants to replace the natural lens or for correcting vision, with the first commercial foldable lens made of silicone. Biological aspects of posterior capsular opacification have been reviewed, including the effects of the implanted silicone IOL. However, certain issues with silicone IOLs are still challenging and some conditions can prevent its application in all patients. The latest trends in nanotechnology solutions have been reviewed. Surface modifications of silicone IOLs are an efficient approach to further improve biocompatibility or to enable drug-eluting function. Different surface modifications, including coatings, can provide long-term treatments for various medical conditions or medical diagnoses through the incorporation of sensory functions. It is essential that IOL optical characteristics remain unchanged in case of drug incorporation and the application of nanoparticles can enable it. However, clinical trials related to these advanced technologies are still missing, thus preventing their clinical applications at this moment.

Hidden silylium-type reactivity of a siloxane-based phosphonium-hydroborate ion pair

A new class of siloxane-based cations with hidden silylium-type reactivity is provided, which, in combination with an arylborate counteranion, initiates a highly selective para-C(sp²)-F defunctionalization of a perfluorinated aryl group. The hydrodefluorinated aryl borane is obtained as a crystalline solid via continuous sublimation during the reaction. The heterocyclic six-membered cation could be obtained single-crystalline after dehydrogenative anion exchange. DFT calculations give insight into the bonding within the siloxane-based cation and the mechanism of the ion pair reaction.

Molecular Scissors for Tailor-Made Modification of Siloxane Scaffolds

The controlled design of functional oligosiloxanes is an important topic in current research. A consecutive Si-O-Si bond cleavage/formation using siloxanes that are substituted with 1,2-diaminobenzene derivatives acting as molecular scissors is presented. The method allows to cut at certain positions of a siloxane scaffold forming a cyclic diaminosilane or -siloxane intermediate and then to introduce new functional siloxy units. The procedure could be extended to a direct one-step cleavage of chlorooligosiloxanes. Both siloxane formation and cleavage proceed with good to excellent yields, high regioselectivity, and great variability of the siloxy units. Control of the selectivity is achieved by the choice of the amino substituent. Insight into the mechanism was provided by low temperature NMR studies and the isolation of a lithiated intermediate.

Siloxane-containing derivatives of benzoic acid: chemical transformation of the carboxyl group

This research presents a scalable method for chemical transformation of Si-containing derivatives of benzoic acid to a wide range of corresponding esters, thioesters, amides, etc. Some of them form HOF-like structures in the crystalline state.

Well-defined hydrogen and organofunctional polysiloxanes with spiro-fused siloxane backbones

Structurally well-defined macrocyclic polysiloxanes with unique spirosiloxane units and regularly arranged Si–H groups were synthesized by B(C₆F₅)₃-catalyzed dehydrocarbonative cross-couplings.

Facile Synthesis of Sequence-Defined Oligo(Dimethylsiloxane-co-Diphenylsiloxane)s

1,1,3,3,5,5,7,7-Octamethyltetrasiloxane (^H MD₂ M^H ), which is reported to release Me₂ SiH₂ via a B(C₆ F₅ )₃ -catalyzed redistribution, acts as a good Me₂ SiH₂ precursor in the B(C₆ F₅ )₃ -catalyzed dehydrocarbonative condensation of alkoxysilanes. A series of oligo(dimethylsiloxane-co-diphenylsiloxane)s that are uniformly sized and sequence-defined at the atomic level are synthesized by a one-pot controlled iteration of a B(C₆ F₅ )₃ -catalyzed dehydrocarbonative condensation of alkoxysilanes with ^H MD₂ M^H or Ph₂ SiH₂ and a B(C₆ F₅ )₃ -catalyzed hydrosilylation of carbonyl compounds, followed by the subsequent B(C₆ F₅ )₃ -catalyzed dehydrogenative condensation of silanols.

A catalyst- and additive-free synthesis of alkoxyhydrosiloxanes from silanols and alkoxyhydrosilanes

A convenient method for the selective synthesis of alkoxyhydrosiloxanes that bear SiH and SiOR² groups on the same silicon atom, R¹₃Si-O-SiR³_2-n(OR²)_nH (n = 0, 1, or 2), via a simple catalyst- and additive-free dealcoholization reaction between silanols and alkoxyhydrosilanes has been developed. These alkoxyhydrosiloxanes can be easily converted into Si(OR²)₃-containing siloxanes by zinc catalyzed alkoxylation and alkoxy-containing silphenylene polymers by platinum catalyzed hydrosilylation.

Synthesis of structured polysiloxazanes via a Piers-Rubinsztajn reaction

A series of siloxazanes were successfully prepared by a Piers-Rubinsztajn reaction between methoxydisilazanes and the corresponding hydrosilanes. Polysiloxazanes with narrow dispersion were also synthesized from methoxydisilazanes and Si-H terminated oligosiloxanes. The possible interaction mechanism between tris(pentafluorophenyl)borane and the methoxydisilazane was investigated.

Chemistry of a 1,5-Oligosilanylene Dianion Containing a Disiloxane Unit

Synthesis of a number of disiloxane containing cyclo- and bicyclooligosilanes is described starting from the dipotassium 1,5-oligosiloxanylene diide derived from 1,3-bis[tris(trimethylsilyl)silyl]tetramethyldisiloxane. In addition, the use of this particular fragment as ligand for zinc and group 4 metallocene complexes was studied. Both types of compounds exhibit marked structural differences compared to related compounds containing Si-Si-Si units instead of the Si-O-Si fragment.

A Highly Effective Route to Si-O-Si Moieties through O-Silylation of Silanols and Polyhedral Oligomeric Silsesquioxane Silanols with Disilazanes

A simple and highly practical catalyst-free O-silylation of silanols with commercially available disilazanes has been developed under mild conditions. In the case of polyhedral oligomeric silsesquioxane (POSS) silanols and some other silanols, it was necessary to use catalytic amounts of inexpensive Bi(OTf)₃ as additional catalyst. This efficient chlorine-free protocol involves the synthesis of a wide range of important organosilicon derivatives such as unsymmetrical disiloxanes and functionalized silsesquioxanes.

Selective hydrosiloxane synthesis via dehydrogenative coupling of silanols with hydrosilanes catalysed by Fe complexes bearing a tetradentate PNNP ligand

A well-defined iron complex system was established using PNNP-R (R = Ph and Cy) as a strong σ-donating ligand with a rigid meridional tetradentate structure. Reactive Fe(0) complexes [{Fe(PNNP-R)}₂(μ-N₂)] were synthesized by a reaction of the corresponding iron dihalide with NaBEt₃H and structurally characterized. The reaction proceeded via the iron dihydride intermediate [Fe(H)₂(PNNP-R)], which underwent H₂ reductive elimination, supporting the hemilabile behavior of PNNP-R. [{Fe(PNNP-R)}₂(μ-N₂)] catalyzed the dehydrogenative coupling of silanols with silanes to selectively form various hydrosiloxanes, which are important building blocks for the synthesis of a range of siloxane compounds. This system exhibited higher catalytic efficiency than the previously reported precious-metal-catalyzed systems.

Sequence-Controlled Catalytic One-Pot Synthesis of Siloxane Oligomers

Silicones are highly valuable poly- and oligomeric materials with a broad range of applications due to their outstanding physicochemical properties. The core framework of silicone materials consists of siloxane (Si-O-Si) bonds, and thus, the development of efficient siloxane-bond-forming reactions has attracted much attention. However, these reactions, especially "catalytic" siloxane-bond-forming reactions that enable the selective formation of unsymmetrical siloxane bonds, remain relatively underdeveloped. On the other hand, controlled iteration has become a powerful tool for the sequence-controlled synthesis of poly- and oligomeric compounds. Recently, control over the siloxane sequence has been achieved by the one-pot iteration of a B(C₆ F₅ )₃ -catalyzed dehydrocarbonative cross-coupling of alkoxysilanes with hydrosilanes and a B(C₆ F₅ )₃ -catalyzed hydrosilylation of carbonyl compounds. Thus, it is now possible to generate linear, branched, and cyclic sequence-specific oligosiloxanes in a highly selective manner under chloride-free conditions.

Carbonyl and ester C–O bond hydrosilylation using κ4-diimine nickel catalysts

(^Ph2PPrDI)Ni chemoselectively catalyzes α-allyl ester C–O bond hydrosilylation to prepare silyl esters with turnover frequencies of up to 990 h⁻¹.

Facile synthesis and bridgehead-functionalization of bicyclo[3.3.3]pentasiloxanes

Various bicyclo[3.3.3]pentasiloxanes (BPSO) were successfully synthesized via regioselective functionalization at the bridgehead positions.