Tris(pentafluorophenyl)borane-catalyzed Hydride Transfer Reactions in Polysiloxane Chemistry-Piers-Rubinsztajn Reaction and Related Processes

Tris(pentafluorophenyl)borane (TPFPB) is a unique Lewis acid that catalyzes the condensation between hydrosilanes (Si-H) and alkoxysilanes (Si-OR), leading to the formation of siloxane bonds (Si-OSi) with the release of hydrocarbon (R-H) as a byproduct-the so-called Piers-Rubinsztajn reaction. The analogous reactions of hydrosilanes with silanols (Si-OH), alcohols (R-OH), ethers (R-OR') or water in the presence of TPFPB leads to the formation of a siloxane bond, alkoxysilane (Si-OR or Si-OR') or silanol (Si-OH), respectively. The above processes, often referred to as Piers-Rubinsztajn reactions, provide new synthetic tools for the controlled synthesis of siloxane materials under mild conditions with high yields. The common feature of these reactions is the TPFPB-mediated hydride transfer from silicon to carbon or hydrogen. This review presents a summary of 20 years of research efforts related to this field, with a focus on new synthetic methodologies leading to numerous previously difficult to synthesize well-defined siloxane oligomers, polymers and copolymers of a complex structure and potential applications of these new materials. In addition, the mechanistic aspects of the recently discovered reactions involving hydride transfer from silicon to silicon are discussed in more detail.

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.

Effects of ethanol content on the properties of silicone rubber foam

In this study, silicone rubber foam (SF) was prepared through cross-linking and foaming. The effects of ethanol content on the SF were investigated in terms of the physical properties, static cushioning properties, dynamic thermomechanical properties, and dynamic fatigue properties. The cell structure was characterized using scanning electron microscopy and its relationship with the SF properties was analyzed. With increasing ethanol content, the cell diameter increases gradually and its uniformity deteriorates. Moreover, the density, tensile strength, and elongation at breaking of the SF samples gradually decrease. In addition, with the increase of strain and stress, the cushioning coefficient of SF decreases initially and then increases, and the fatigue times worsens with increasing ethanol content. However, fatigue process has little effect on the cushioning performance of SF, which means the SF can be used as reusable packaging materials and thereby reduce environmental pollution.

Formation and application of electrochemically active cross-linked triarylamine–siloxane films using the Piers–Rubinsztajn reaction

Cross-linked triarylamine–siloxane hybrid thin film have been formed using Piers–Rubinsztajn chemistry. Key to this approach was the use of a ring-opening reaction to prevent the evolution of volatile small molecules. A representative cyclic ether containing biphenyl triarylamine compound was synthesized and on ring-opening was shown to form a smooth, glassy, and electroactive films by cross-linking with tetrakis(dimethylsiloxy)silane (QM*4). It was found that the films were electrochemically active with low glass transition temperatures. Cross-linked films were incorporated into organic light emitting diodes (OLEDs) under various conditions and functionality within OLEDs was confirmed. Finally, the resistance of the system to dissolution (orthogonality) was considered by casting F8T2, a p-type emitting polymer, from solution on top of the cross-linked film, which formed a working OLED.

Transition metal-free hydrodefluorination of acid fluorides and organofluorines by Ph3GeH promoted by catalytic [Ph3C][B(C6F5)4]

The germylium cation [Ph₃Ge]⁺converts aryl and aliphatic acid fluorides directly to their corresponding aldehydes. Hydrodefluorination of organofluorine compounds by [Ph₃Ge]⁺was also observed.

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

New Control Over Silicone Synthesis using SiH Chemistry: The Piers-Rubinsztajn Reaction

There is a strong imperative to synthesize polymers with highly controlled structures and narrow property ranges. Silicone polymers do not lend themselves to this paradigm because acids or bases lead to siloxane equilibration and loss of structure. By contrast, elegant levels of control are possible when using the Piers-Rubinsztajn reaction and analogues, in which the hydrophobic, strong Lewis acid B(C₆ F₅ )₃ activates SiH groups, permitting the synthesis of precise siloxanes under mild conditions in high yield; siloxane decomposition processes are slow under these conditions. A broad range of oxygen nucleophiles including alkoxysilanes, silanols, phenols, and aryl alkyl ethers participate in the reaction to create elastomers, foams and green composites, for example, derived from lignin. In addition, the process permits the synthesis of monofunctional dendrons that can be assembled into larger entities including highly branched silicones and dendrimers either using the Piers-Rubinsztajn process alone, or in combination with hydrosilylation or other orthogonal reactions.

A novel synthetic approach to poly(hydrosiloxane)s via hydrolytic oxidation of primary organosilanes with a AuNPs-stabilized Pickering interfacial catalyst

A simple and versatile approach based on AuNPs-stabilized Pickering catalyst in water–chloroform biphasic medium has been developed for the synthesis of poly(alkyl/arylhydrosiloxane)s.

A unified survey of Si-H and H-H bond activation catalysed by electron-deficient boranes

The bond activation chemistry of B(C6F5)3 and related electron-deficient boranes is currently experiencing a renaissance due to the fascinating development of frustrated Lewis pairs (FLPs). B(C6F5)3's ability to catalytically activate Si-H bonds through η(1) coordination opened the door to several unique reduction processes. The ground-breaking finding that the same family of fully or partially fluorinated boron Lewis acids allows for the related H-H bond activation, either alone or as a component of an FLP, brought considerable momentum into the area of transition-metal-free hydrogenation and, likewise, hydrosilylation. This review comprehensively summarises synthetic methods involving borane-catalysed Si-H and H-H bond activation. Systems corresponding to an FLP-type situation are not covered. Aside from the broad manifold of C=X bond reductions and C=X/C-X defunctionalisations, dehydrogenative (oxidative) Si-H couplings are also included.

Photo Lewis acid generators: photorelease of B(C6F5)3 and applications to catalysis

A series of molecules capable of releasing of the strong organometallic Lewis acid B(C₆F₅)₃ upon exposure to 254 nm light have been developed.

Cyclotetrasiloxane frameworks for the chemoenzymatic synthesis of oligoesters

The lipase-mediated synthesis of branched and polycyclic polyester systems based on a cyclotetrasiloxane core.

Printing silicone-based hydrophobic barriers on paper for microfluidic assays using low-cost ink jet printers

Inkjet printed silicone resins provide hydrophobic barriers for paper-based microfluidic assays.

Silicone dendrons and dendrimers from orthogonal SiH coupling reactions

Iterative assembly of highly branched silicone dendrons and dendrimers, alternatively using Piers–Rubinsztajn and Pt-catalyzed hydrosilylation reactions, was achieved in high yield to give materials of up to 13 770 molecular weight by combining divergent and convergent approaches.

Siloxane-Bond Formation Promoted by Lewis Acids: A Nonhydrolytic Sol-Gel Process and the Piers-Rubinsztajn Reaction

Siloxane formation reactions of both the nonhydrolytic sol-gel process and Piers-Rubinsztajn reaction can be integrated as Lewis acid promoted siloxane syntheses without involving silanol groups. The former was developed in the field of inorganic materials chemistry and the latter was initiated in polymer chemistry. We have realized both reactions are quite similar, in terms of 1) the nonhydrolytic reaction, 2) the use of alkoxysilanes, 3) the group-exchange reactions competing with the siloxane formation, and 4) the proposed reaction mechanisms. This Minireview focuses on the above two reactions. The evolution of both reactions should realize a more sophisticated molecular design of siloxane compounds, which surely contributes to the development of advanced functional materials.

Rhodium carbene complexes as versatile catalyst precursors for Si-H bond activation

Rhodium(III) complexes comprising monoanionic C,C,C-tridentate dicarbene ligands activate Si-H bonds and catalyse the hydrolysis of hydrosilanes to form silanols and siloxanes with concomitant release of H(2). In dry MeNO(2), selective formation of siloxanes takes place, while changing conditions to wet THF produces silanols exclusively. Silyl ethers are formed when ROH is used as substrate, thus providing a mild route towards the protection of alcohols with H(2) as the only by-product. With alkynes, comparably fast hydrosilylation takes place, while carbonyl groups are unaffected. Further expansion of the Si-H bond activation to dihydrosilanes afforded silicones and polysilyl ethers. Mechanistic investigations using deuterated silane revealed deuterium incorporation into the abnormal carbene ligand and thus suggests a ligand-assisted mechanism involving heterolytic Si-H bond cleavage.

Mechanistic aspects of bond activation with perfluoroarylboranes

In the mid-1990s, it was discovered that tris(pentafluorophenyl)borane, B(C(6)F(5))(3), was an effective catalyst for hydrosilylation of a variety of carbonyl and imine functions. Mechanistic studies revealed a counterintuitive path in which the function of the borane was to activate the silane rather than the organic substrate. This was the first example of what has come to be known as "frustrated Lewis pair" chemistry utilizing this remarkable class of electrophilic boranes. Subsequent discoveries by the groups of Stephan and Erker showed that this could be extended to the activation of dihydrogen, initiating an intense period of activity in this area in the past 5 years. This article describes the early hydrosilylation chemistry and its subsequent applications to a variety of transformations of importance to organic and inorganic chemists, drawing parallels with the more recent hydrogen activation chemistry. Here, we emphasize the current understanding of the mechanism of this process rather than focusing on the many and emerging applications of hydrogen activation by fluoroarylborane-based frustrated Lewis pair systems.