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

Abstract

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.

Keywords: Piers–Rubinsztajn reaction; dehydrocarbonative condensation; dehydrogenative condensation; hydride transfer polymerization; mechanism; tris(pentafluorophenyl)borane.

Figure 1

General structure of silicones and the most important properties of these materials.

Scheme 1

Industrially practiced processes to manufacture siloxane polymers.

Figure 2

Polarization of C-H and Si-H bonds.

Scheme 2

Interaction of the Si-H reagent with TPFBP.

Figure 3

Isolated adduct of 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene with Et₃SiH.

Scheme 3

The Piers mechanism of carbonyl compound reduction (hydrosilylation) catalyzed by TPFPB.

Scheme 4

Mechanism of the Piers–Rubinsztajn reaction.

Figure 4

Conversion–time plot for the reaction of Ph₂MeSiH with Ph₂MeSin-Oct in the presence of TPFPB. (A)—UV spectra of the reaction mixture before the reaction started; (B)—UV spectra of the reaction mixture after the reaction started.

Scheme 5

Decomposition under UV irradiation of carbamate borate triphenylsulfonium salt with the release of TPFPB.

Scheme 6

Example of a synthetic route to a dendritic polymer by combining the P-R reaction with Pt(0) hydrosilylation.

Scheme 7

Synthesis of functional siloxane copolymers with spatially regularly spaced functional groups.

Scheme 8

Synthesis of branched siloxane oligomers with aminoalkyl functional groups for modification of sugar lactones.

Scheme 9

Synthesis of siloxane copolymers with regularly spaced phenylmethyl and diphenyl siloxane units.

Scheme 10

Synthesis of well-defined siloxane oligomers with a sequence-specific structure using a one-pot process involving two reactions catalyzed by TPFPB.

Scheme 11

Synthesis of the polysiloxanes with pendant benzocyclobutene groups using the combination of the P-R reaction and Heck reaction.

Scheme 12

Three examples of the synthesis of multi-arm siloxanes with different thermolabile groups and their conversion to highly cross-linked carbo-siloxane materials. (A)—Benzocyclobutene group; (B)—Trifluorovinyloxyphenyl group; (C)—p-Vinylbenzene group.

Scheme 13

Synthesis of siloxane macromers with trifluoromethylphenyl group and two styryl functions; their conversion to highly cross-linked material with excellent thermal stability and good mechanical and dielectric properties.

Scheme 14

Synthesis of the regular branched polysiloxanes of a controlled structure with the two-step P-R process.

Scheme 15

Example of the synthesis of dendrimers by combining the P-R reaction with Pt(0)-catalyzed hydrosilylation.

Scheme 16

Example of the synthesis of the dendritic polysiloxane with chelating multidentate ligands.

Scheme 17

Symbols used to describe the functionalities of the structural units in siloxane materials.

Scheme 18

Synthesis of highly branched alkoxy-functional DQ and T^PhQ resins.

Scheme 19

Example of the preparation of MTD resin with the combination of the P-R reaction and hydrosilylation process.

Scheme 20

Example of the preparation of the MDTQ resins with the two-step P-R reaction.

Scheme 21

Example of the one-pot synthesis of the hyperbranched polysiloxanes from dimethoxy metylsilane.

Scheme 22

Example of the synthesis of cyclic polycyclotetrasiloxane polymers with the P-R process.

Scheme 23

Example of the synthesis of spirocyclosiloxanes in cyclohexane in the presence of TPFPB.

Scheme 24

Synthesis of cyclotetrasiloxanes with two side SiH groups and their conversion to a linear polymer via Pt-catalyzed hydrosilylation process with preserved cyclotetrasiloxane structure.

Scheme 25

Synthesis of the poly(siloxane/double-decker silsesquioxane) copolymer.

Scheme 26

Synthesis of liquid polysiloxane with preserved cyclotetrasiloxane rings and reactive Si-H groups.

Scheme 27

Synthesis of the super large hyperbranched siloxane structures from Q₈M^H₈ silicate.

Scheme 28

Synthesis of the fluoro-containing polysiloxane thermoset resin from vanillin.

Scheme 29

Synthesis of the siloxane copolymer with 1,4-bis(dimethylsilyl)benzene units.

Scheme 30

Synthesis of the silphenylenesiloxane copolymers with pendant vinyl and benzocyclobutene groups.

Scheme 31

Two examples of the synthesis of transparent silphenylene elastomers developed by Brook and co-workers.

Scheme 32

Conversion of 1,3-dimethoxytetraphenyldisilazane to polysiloxazanes with the P-R process.

Scheme 33

Synthesis of polyaryloxysiloxanes with the P-R condensation of hydroquinone dimethyl ether and diphenylsilane.

Scheme 34

Synthesis of the tri-block polysiloxane with hydroquinone units (C_x(AB)_yC_x using the two-step P-R process.

Scheme 35

Reactivity of the eugenol molecule in the hydrosilylation process and P-R reaction.

Scheme 36

Synthesis of the siloxane copolymers with pendant catechol groups.

Scheme 37

Opening of the epoxide ring using siloxanes with Si-H functional groups in the presence of TPFPB.

Scheme 38

Synthesis of Janus-type silsesquioxanes with P-R reactions.

Scheme 39

New process of the cross-linking of PDMHMS developed by the Hawker group.

Scheme 40

Synthesis of triarylamines with oligosiloxy functional groups from their methoxy derivatives using the P-R reaction.

Scheme 41

Functionalization of the porphyrine–Zn complex with oligosiloxane side groups.

Scheme 42

Modification of the carbon nanotubes with Si-H-terminated oligosiloxanes in P-R conditions.

Scheme 43

Synthesis of epoxy-functional poly(silphenylenesilyloxanes) with dehydrogenative condensation catalyzed by TPFPB.

Scheme 44

Functionalization of phthalocyanines with SiH-functional silanes using dehydrogenative condensation catalyzed by TPFPB.

Scheme 45

Synthesis of well-defined polysiloxanes using the dehydrogenative condensation of siloxanol-functional dendrons with SiH-functional polysiloxanes.

Scheme 46

An example of the dehydrogenative coupling of POSS silanols with Si-H-functional compounds in the presence of TPFPB.

Scheme 47

Synthesis of the polycarbosiloxane copolymers with dehydrogenative condensation.

Scheme 48

Rapid modification of the surface of amorphous silica particles with functional organosilanes.

Scheme 49

Synthesis of high-molecular-weight linear polysiloxane from Si-H-terminated siloxane oligomers in the presence of a controlled amount of water using the partial hydrolysis of Si-H groups and dehydrogenative condensation.

Scheme 50

Examples of polyaryloxysiloxanes prepared with the dehydrogenative condensation: (A)—TMDS with TMBP; (B)—TMDS with BPF; (C)—BDMSB with TMBP; (D)—TMDS with DMBPF.

Scheme 51

Dehydrogenative coupling of binol with SiH-terminated oligosiloxanes.

Scheme 52

Synthesis of novel siloxanes functionalized with oligoethylene glycol side groups.

Scheme 53

Modification of the graphene oxide (GO) surface with siloxane side groups.

Scheme 54

Mechanism of the P-R reaction showing three alternative pathways of this process. (1)—P-R reaction; (2)—back to substrates; (3)—metathesis.

Scheme 55

Reactions of model triorganoalkoxysilane with triorganosilane in the presence of TPFPB.

Scheme 56

Inter- and intra-molecular metathesis of SiH-terminated oligosiloxanes in the presence of TPFPB.

Scheme 57

Hydride-transfer ring-opening polymerization of D₃. A—chain propagation; B—termination via metathetic chain coupling.

Scheme 58

Initiation step of the hydride-transfer ring-opening polymerization of D^H₄.

Scheme 59

Termination step of the hydride-transfer ring-opening polymerization of D^H₄.

Scheme 60

The general equation of the self-restructuration of PHMS in the presence of TPFPB.