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.

Chemo- and Diastereoselective Hydrosilylation of Amorphadiene toward the Synthesis of Artemisinin

A formal synthesis of artemisinin starting from amorphadiene is described. This new route relies on the development of a catalytic chemo- and diastereoselective hydrosilylation. The practicability of this method is demonstrated by converting amorphadiene to dihydroartemisinic aldehyde using a one-pot hydrosilylation/oxidation sequence, minimizing the number of purifications and maximizing the productivity through a practical one-pot procedure. In addition, this approach can be coupled with a crystallization-induced diastereoselective transformation (CIDT) to enhance the optical purity of the key target intermediate, dihydroartemisinic aldehyde.

Catalytic Synthesis of Oligosiloxanes Mediated by an Air Stable Catalyst, (C6F5)3B(OH2)

The utility of (C₆F₅)₃B(OH₂) as catalyst for the simple and environmentally benign synthesis of oligosiloxanes directly from hydrosilanes, is reported. This protocol offers several advantages compared to other methods of synthesizing siloxanes, such as mild reaction conditions, low catalyst loading, and a short reaction time with high yields and purity. The considerable H₂O-tolerance of (C₆F₅)₃B(OH₂) promoted a catalytic route to disiloxanes which showed >99% conversion of three tertiary silanes, Et₃SiH, PhMe₂SiH, and Ph₃SiH. Preliminary data on the synthesis of unsymmetrical disiloxanes (Si-O-Si') suggests that by modifying the reaction conditions and/or using a 1:1 combination of silane to silanol the cross-product can be favored. Intramolecular reactions of disilyl compounds with catalytic (C₆F₅)₃B(OH₂) led to the formation of novel bridged siloxanes, containing a Si-O-Si linkage within a cyclic structure, as the major product. Moreover, the reaction conditions enabled recovery and recycling of the catalyst. The catalyst was re-used 5 times and demonstrated excellent conversion for each substrate at 1.0 mol% catalyst loading. This seemingly simple reaction has a rather complicated mechanism. With the hydrosilane (R₃SiH) as the sole starting material, the fate of the reaction largely depends on the creation of silanol (R₃SiOH) from R₃SiH as these two undergo dehydrocoupling to yield a disiloxane product. Generation of the silanol is based on a modified Piers-Rubinsztajn reaction. Once the silanol has been produced, the mechanism involves a series of competitive reactions with multiple catalytically relevant species involving water, silane, and silanol interacting with the Lewis acid and the favored reaction cycle depends on the concentration of various species in solution.

On Silylated Oxonium and Sulfonium Ions and Their Interaction with Weakly Coordinating Borate Anions

Attempts have been made to prepare salts with the labile tris(trimethylsilyl)chalconium ions, [(Me3 Si)3 E]+ (E=O, S), by reacting [Me3 Si-H-SiMe3 ][B(C6 F5 )4 ] and Me3 Si[CB] (CB- =carborate=[CHB11 H5 Cl6 ]- , [CHB11 Cl11 ]- ) with Me3 Si-E-SiMe3 . In the reaction of Me3 Si-O-SiMe3 with [Me3 Si-H-SiMe3 ][B(C6 F5 )4 ], a ligand exchange was observed in the [Me3 Si-H-SiMe3 ]+ cation leading to the surprising formation of the persilylated [(Me3 Si)2 (Me2 (H)Si)O]+ oxonium ion in a formal [Me2 (H)Si]+ instead of the desired [Me3 Si]+ transfer reaction. In contrast, the expected homoleptic persilylated [(Me3 Si)3 S]+ ion was formed and isolated as [B(C6 F5 )4 ]- and [CB]- salt, when Me3 Si-S-SiMe3 was treated with either [Me3 Si-H-SiMe3 ][B(C6 F5 )4 ] or Me3 Si[CB]. However, the addition of Me3 Si[CB] to Me3 Si-O-SiMe3 unexpectedly led to the release of Me4 Si with simultaneous formation of a cyclic dioxonium dication of the type [Me3 Si-μO-SiMe2 ]2 [CB]2 in an anion-mediated reaction. DFT studies on structure, bonding and thermodynamics of the [(Me3 Si)3 E]+ and [(Me3 Si)2 (Me2 (H)Si)E]+ ion formation are presented as well as mechanistic investigations on the template-driven transformation of the [(Me3 Si)3 E]+ ion into a cyclic dichalconium dication [Me3 Si-μE-SiMe2 ]2 2+ .

DFT study of trialkylborohydride-catalysed hydrosilylation of alkenes – the mechanism and its implications

A detailed DFT study reveals the mechanism of trialkylborohydride-catalysed Markovnikov hydrosilylation of aromatic alkenes.

Unsupported metal silyl ether coordination

Silyl ethers surrender to magnesium: the very weak Lewis base hexamethyldisiloxane is forced to ligate a “naked” cationic Mg species. Agostic interactions and van der Waals attraction contribute to stability.