abstractHydrosilylationreactions, also known as the catalytic hydrosilylation, the reaction usuallyproduces anti-Markovnikov additions alkane1 (i.e., silicon on the terminal carbon) by the addition ofSi-H bonds to unsaturated bonds.
It also present in compounds in the form ofalkenes and alkynes, offering numerous unique and advantageous properties forthe preparation of polymeric materials, such as high yields andstereoselectivity. Hydrosilylationreactions requires to be catalyzed, and for which the most used catalyst areplatinum compounds. Popular industrial catalysts are “Speier’scatalyst,” H2PtCl6, and Karstedt’s catalyst, analkene-stabilized platinum(0) catalyst2. Hydrosilylation has been called the”most important application of platinum in homogeneous catalysis.
“3This review addresses and discusses selected current trends of the scientificresearch in the area, namely non-thermal stimulated hydrosilylation reactions(highlighting microwave-initiated, electrochemically-initiated), and(potential) industrial applications (highlighting the catalysts used andproducts manufactured). This review focuses on the hydrosilylation reactions involvingalkene reactants.Keywords: hydrosilylation;catalysis; anti-Markovnikov addition; stereoselectivity IntroductionHydrosilylationreactions are widely implemented for the production of functional silanes andsiloxanes. First report of hydrosilylation in history was from 1947 (70 yearsago) 4 and the first report of Speier’s platinum catalyst was in 1957 (60years ago) 5, the time of first application of a hydrosilylation reaction inpolymer science can be dated back to 19676. The current trend along with theacademia and industry are seeking for both price-efficient and durablecatalysts that meets the increasing demand of silicon-based polymers.
Theproperties are targeted at catalysts selectivity, activity (TOF) and stability(TON). The majority of industrial hydrosilylation reactions are using Speier’scatalyst (H2PtCl6) and Karstedt’s catalysts (fig.1) dueto their high performance in activity and selectivity. In the section ofhydrosilylation catalysis, including triggered catalysis, such asmicrowave-initiated hydrosilylation, application potential to hydrosilylatedproducts for industrial applications. Non-ThermalStimulated Hydrosilylation ReactionMaking ahydrosilylation reaction switchable between external triggering andself-triggering is a huge challenge as it usually react immediately at a highspeed in the presence of most catalysts. For the purpose of some industrialapplications, it is required to have a curable formulation composed of catalystand crosslinking components that are stable for storage for months. Suchspecialized and growing demands were the main driving force for the developmentof hydrosilylation systems that starts upon a certain external trigger.
Heattriggering has been the most commonly used trigger in industrial applicationsso far. In this section, non-thermally triggered hydrosilylation reactions willbe described. Microwave-Initiated HydrosilylationReactionsMicrowave irradiationtransfers heat to any material with the characteristic of separated or partialseparated charges such as metals, ions and dipoles, it is hard to differentiatebetween visible thermal initiation and the non-thermal microwave effects. WeiSun and coworkers has been researching and proven that microwave-assistedheating had no visible or distinct acceleration effect on the reaction rate ofhydrosilylation compared relatively to conventional thermal heating7.
Rabah Boukherroub and coworkers reported that the functionalization ofhydrogen-terminated porous silicon surfaces with functional alk-1-enes undermicrowave irradiation8. Organic monolayers covalently attached to surface bySi–C bonds were made assumption that originate from microwaves as a source ofenergy and it accelerated the hydrosilylation reaction and yielded a highersurface coverage compared to conventional heating. The reaction was performedat 180?C in an oil bath and ithardly yielded any chemical grafting on the particles’ surfaces. Undermicrowave irradiation for 30 min at 170?C, the efficiency wasreported to be 38%, but was not further improved upon increasing thetemperature. Electrochemically-InitiatedHydrosilylation ReactionsEdward G. Robins andcoworkers demonstrated that terminal alkynes can be electrochemically graftedto porous silicon surfaces with either positive or negative bias9. Throughthe process of cathodic electrografting(CEG), alkynes were directly attached tothe surface, but with anodic electrografting(AEG), an alkyl surface wasgenerated. The authors proposed that CEG proceeded via a silyl anionintermediate formed by the reduction of surficial Si–H bonds.
The subsequentin-situ generation of a carbanion from deprotonation of weak acidic alkyne leadsto a nucleophilic Si–Si bond attack. It is likely that surface-initiatedcationic hydrosilylation mechanism is responsible for Si–C bond formation inAEG reactions (fig.2).Industrial ApplicationsFor the implementationof hydrosilylation into industrial processes, catalysts have to show highselectivity, efficiency and stability. However, with the increasing demand andalso the existence of high efficient/low-cost catalysts, industrial applicationsof hydrosilylation reactions of functional products will be described.
CoatingsHu Liu and coworkersprepared an anti-graffiti film by incorporating aPMHS(poly-methyl-hydro-siloxane) polymer grafted with hexa-fluorobutyl acrylateinto polyurethane10. The free surface energy was reduced from 30.7(polyurethane) to 21.4 mN·m?1(anti-graffiti-polyurethane).
Due to the lower free surface energies, thewetting capabilities were correspondingly degraded. Cleaning tests shown thatanti-graffiti-coated areas could be completely cleaned using water andisopropanol. Ding Wang and coworkersdeveloped a method for synthesizing a highly transparent, durable,superhydrophobic and nanoporous coating.
They used polysiloxane containing Si–Hand vinyl–Si groups as precursor and methyl-terminated PDMS as reagents11.Due to the Si–CHn groups abundantly present in the polymer, furtherfluorination to enhance the hydrophobicity is not required. Due to its physicaland mechanical properties, the siloxane-coating may possibly be applied inwindshields, safety goggles, etc. Printings and InksMicro-contact printingis a method to transfer a master mold pattern onto a substrate. The master moldis often created by 2.5-dimensional photolithography. The transfer mold is madeby applying a layer of PDMS followed by curing it through hydrosilylationreactions at a rising temperature to yield an elastomeric PDMS mold(fig.3)12.
One disadvantage of PDMS is that it swells in common organic solvents,rendering the necessity of thermal curing. In addition, due to the hydrophobicsurface of PDMS, the use of water-based inks containing compounds such asbiomolecules or inorganic complexes is challenging13.ConclusionsBy choosing carefullythe catalyst, pure products can be synthesized in high yields. Commonly, thesecatalysts are platinum-based; Notably, external stimulation other than heat canbe utilized to start the hydrosilylation in a formulated reaction mixture.Despite the progress of the research in recent years, challenges exist to beovercomed for industrial applications. In my opinion, these approaches havelarge potential to advance the applicability of the hydrosilylation:? Triggered hydrosilylation: Stimuli otherthan heat give further flexibility to the processing schedule and eventuallyenhance the storage stability.
? Solvent-free processes: Thesehydrosilylation reactions represent a big step towards green chemistry.? Selectivity: Chemoselective hydrosilylationcan pave the way to novel materials from a broad range of different substrates. References1 “Hydrosilylation AComprehensive Review on Recent Advances” B. Marciniec (ed.
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