POSS supported C2-symmetric bisprolinamide as a recyclable chiral catalyst for asymmetric Aldol reaction

Synthesis of Octachloro- and Octaazido-Functionalized T8-Cages and Application to Recyclable Palladium Catalyst

Unprecedented T₈-cages bearing eight chloromethyldimethylsilylethyl substituents were obtained in excellent yield from the readily and commercially available octavinylsilsesquioxane. The chloro groups can be quantitatively substituted by azido ones to yield the corresponding octaazido T₈ without rearrangement of the cage. The syntheses of both functionalizable POSSs are scalable (gram-scale). The azido-functionalized T₈ compound constitutes a versatile building block able to undergo copper-catalyzed azide-alkyne [3 + 2] cycloaddition. As a proof of concept, it was allowed to react with 2-ethynylpyridine to give rise to a multidentate ligand bearing eight 2-pyridyl-triazole moieties (N,N-pincers). The coordination of the eight N,N-bidentate ligands to palladium(II) led to the corresponding octa-palladium complex shown to successfully promote the coupling reaction between anisole and phenylboronic acid. The low solubility of this catalytic complex in the reaction medium enabled (or facilitated or made possible) its straightforward recovery and recycling with four cycles with no loss of activity.

Large Polyhedral Oligomeric Silsesquioxane Cages: The Isolation of Functionalized POSS with an Unprecedented Si18 O27 Core

The synthesis of organo-functionalized polyhedral oligomeric silsesquioxanes (POSS, (R-SiO_1.5 )_n , T_n ) is an area of significant activity. To date, T₁₄ is the largest such cage synthesized and isolated as a single isomer. Herein, we report an unprecedented, single-isomer styryl-functionalized T₁₈ POSS. Unambiguously identified among nine possible isomers by multinuclear solution NMR (¹ H, ¹³ C, and ²⁹ Si), MALDI-MS, FTIR, and computational studies, this is the largest single-isomer functionalized T_n compound isolated to date. A ring-strain model was developed to correlate the ²⁹ Si resonances with the number of 6-, 5-, and/or 4-Si-atom rings that each non-equivalent Si atom is part of. The model successfully predicts the speciation of non-equivalent Si atoms in other families of T_n compounds, demonstrating its general applicability for assigning ²⁹ Si resonances to Si atoms in cage silsesquioxanes and providing a useful tool for predicting Si-atom environments.

Synthesis of Silsesquioxanes with Substituted Triazole Ring Functionalities and Their Coordination Ability

A synthesis of a series of mono-T8 and difunctionalized double-decker silsesquioxanes bearing substituted triazole ring(s) has been reported within this work. The catalytic protocol for their formation is based on the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) process. Diverse alkynes were in the scope of our interest-i.e., aryl, hetaryl, alkyl, silyl, or germyl-and the latter was shown to be the first example of terminal germane alkyne which is reactive in the applied process' conditions. From the pallet of 15 compounds, three of them with pyridine-triazole and thiophenyl-triazole moiety attached to T8 or DDSQ core were verified in terms of their coordinating properties towards selected transition metals, i.e., Pd(II), Pt(II), and Rh(I). The studies resulted in the formation of four SQs based coordination compounds that were obtained in high yields up to 93% and their thorough spectroscopic characterization is presented. To our knowledge, this is the first example of the DDSQ-based molecular complex possessing bidentate pyridine-triazole ligand binding two Pd(II) ions.

Functionalised polyhedral oligomeric silsesquioxane with encapsulated fluoride - first observation of fluxional SiF interactions in POSS

The synthesis of a styryl functionalised POSS incorporating an encapsulated fluoride ion within a (SiO_1.5)₈ cage (T₈-F) is reported. It was characterised by single crystal XRD, MALDI-MS, FTIR, solution (²⁹Si, ¹⁹F, ¹³C, ¹H) and solid state (²⁹Si, ¹⁹F) NMR. In the absence of ¹H decoupling, the ²⁹Si solution NMR spectrum exhibited a triplet of doublets. In contrast, ¹H, ¹⁹F and ¹H/¹⁹F double-decoupling resulted in two, three and one signal, respectively, being consistent with a single Si site whose ²⁹Si NMR signal is modulated by both the proximal aromatic-ring protons and fluoride. The associated SiF coupling constant (2.5 Hz) is substantially lower than expected for a covalent Si-F bond and arises from a fluxional SiF covalent effect in which the F^- interacts equivalently with all eight Si atoms. Additional variable temperature NMR studies demonstrated a threshold at -5 °C below which no SiF interactions are observed, and above which an increasing SiF covalent character occurs.

Post-synthesis modification of functionalised polyhedral oligomeric silsesquioxanes with encapsulated fluoride – enhancing reactivity of T8-F POSS for materials synthesis

The first successful approach for modifying poorly reactive POSS containing F⁻ (T₈-F) and incorporating intact T₈-F within a nanohybrid material is described.

Synthesis of Tetrachloro, Tetraiodo, and Tetraazido Double-Decker Siloxanes

A convenient and scalable (gram-scale) route to unprecedented T₈D₂-double-decker siloxanes (DDSQs) bearing four chloro (3b) or four azido (5b) groups is reported. Both compounds were characterized and proved to undergo successful nucleophilic substitution for 3b (with iodide or azide) and copper-catalyzed azide-alkyne [3 + 2] cycloaddition for 5b. All of these transformations occurred under mild conditions, and the corresponding DDSQs were prepared in very high yields. Beyond the enhanced multivalency as compared to the previously described disubstituted D₂T₈ structures, the reported tetrafunctional DDSQs are formed as a single isomer and readily isolated in very high yields. Moreover, the tetra-azido DDSQ 5b constitutes a multipurpose nanobuilding block for the further preparation of new inorganic-organic hybrid materials where the covalent incorporation of a DDSQ moiety brings valuable properties.

POSS nanostructures in catalysis

In this review we highlight the use of appealing POSS-based nanostructures for both homogeneous and heterogeneous catalytic applications.

Self-Assembly and Applications of Amphiphilic Hybrid POSS Copolymers

Understanding the mechanism of molecular self-assembly to form well-organized nanostructures is essential in the field of supramolecular chemistry. Particularly, amphiphilic copolymers incorporated with polyhedral oligomeric silsesquioxanes (POSSs) have been one of the most promising materials in material science, engineering, and biomedical fields. In this review, new ideas and research works which have been carried out over the last several years in this relatively new area with a main focus on their mechanism in self-assembly and applications are discussed. In addition, insights into the unique role of POSSs in synthesis, microphase separation, and confined size were encompassed. Finally, perspectives and challenges related to the further advancement of POSS-based amphiphilics are discussed, followed by the proposed design considerations to address the challenges that we may face in the future.

Silsesquioxane cages as fluoride sensors

Pyrene functionalized silsesquioxane cages (PySQ) not only provide significant fluorescence from pyrene-pyrene excimers with a very large Stokes shift (Δλ = 143 nm, 69 930 cm^-1) in DMSO but also exhibit fluoride capture results coincidentally with a π-π* fluorescence enhancement. On the other hand, PySQ-F^- in THF significantly exhibits π-π* fluorescence quenching and a color change can be observed with the naked eye from light yellow to deep orange by forming a charge-transfer (CT) complex among the pyrenyl rings. Moreover, PySQ selectively captures F^- with a response time of <2 min and with a very low detection limit (1.61 ppb), while ¹⁹F NMR is used to confirm encapsulation of F^- with Δδ = 19 ppm.

Chiral catalysts immobilized on achiral polymers: effect of the polymer support on the performance of the catalyst

Achiral polymeric supports can have important positive effects on the activity, stability and selectivity of supported chiral catalysts.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Synthesis and isolation of non-chromophore cage-rearranged silsesquioxanes from base-catalyzed reactions

Carbonate (CO₃²⁻) anion as a stronger base but poorer in nucleophilicity gives rise exclusively to a cage expansion of unsubstituted products.