Hydrosilylation Research Articles

The Role of the Fluoro–Hydrido–Phosphorane Intermediate in Catalytic Hydrosilylation of Acetophenone: Computational Study

The fluorophosphonium cation [(C6F5)3PF]+ (1) is used as a catalyst in many chemical transformations including the hydrosilylation of ketones. In these reactions the fluoro–hydrido–phosphorane (C6F5)3PFH, which is formed upon a hydride abstraction by 1 from a hydrosilane, was proposed as the key intermediate. One of the most important features attributed to (C6F5)3PFH in the catalytic hydrosilylation of ketones is its ability to transfer the hydride to an electrophile. However, (C6F5)3FPH was not observed experimentally and its properties and in particular its hydride‐donating ability was not studied in detail. In this work the mode of formation of (C6F5)3FPH, its properties and its interactions with substrates in the catalytic hydrosilylation of acetophenone were investigated by density functional theory (DFT), and possible routes of this catalytic reaction were studied. Calculations show that (C6F5)3PFH can act both as a fluoride and hydride donor, and that these processes can exist in thermodynamic equilibrium. However, only the donation of the hydride results in intermediates that allow the catalytic cycle to proceed.

Enantioselective catalytic hydrosilylation of propiophenone with a simple combination of a cationic iridium complex and a chiral azolium salt

This study aims to propose a simple procedure for the development of enantioselective hydrosilylation of a ketone using catalytic amounts of [Ir(cod)2]BF4 and chiral azolium salt. Previously, catalytic asymmetric hydrosilylation reactions have used well-defined metal-N-heterocyclic carbene (NHC) complexes. The proposed method offers an important advantage of avoiding preparation of NHC-metal species. Several reaction parameters including the amount of reductant, solvent, catalyst loading and ligand structure were evaluated. In addition, the investigation of the reaction progress as a function of time revealed that an iridium species, which was generated after 5 h of reaction time, catalyzed the stereoselective reduction with almost perfect facial selection of the ketone. An attempt to obtain a catalytic active species from the reaction of [Ir(cod)2]BF4 and chiral azolium salt has been made. The newly obtained iridium species promoted the hydrosilylation of a ketone with high yield and enantioselectivity.

Cobalt(0) and Iron(0) Isocyanides as Catalysts for Alkene Hydrosilylation with Hydrosiloxanes

Iron and cobalt isocyanides, Fe(CNR)5 (1) and Co2(CNR)8 (2), where R = t-butyl (tBu), adamantyl (Ad), and mesityl (Mes), were prepared by reduction of FeBr2 or CoI2 in the presence of CNR by C8K or silica-Na. These complexes were subjected to catalytic hydrosilylation of alkenes with hydrosiloxanes, and the results are compared with those obtained by previously reported Fe(OPiv)2/CNAd or Co(OPiv)2/CNAd catalyst systems. Hydrosilylation of allylic ethers with 1,1,1,3,3-pentamethyldisiloxane (PMDS) catalyzed by 1 and the reaction of several alkenes with PMDS or 1,1,1,3,5,5,5-heptamethyltrisiloxane (MD′M) catalyzed by 2 exhibited greater catalytic activity than that observed for the Fe(OPiv)2 or Co(OPiv)2/CNR catalyst system. Complexes 1 and 2 were effective for catalytic chemical modification of silicone fluids containing Si–H groups and for two-component silicone curing. In all cases, selectivity of the reaction in terms of formation of the desired product by hydrosilylation and of byproducts due to dehydr...

Hypervalent Hydrosilicates Connected to Light Alkali Metal Amides: Synthesis, Structure, and Hydrosilylation Catalysis.

When light alkali metal amides [M(Me3 TACD)]n (M=Li, Na, K; (Me3 TACD)H=1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane) were treated with H2 SiPh2 in THF, [M{(H2 SiPh2 )Me3 TACD}(thf)x ] containing a pendant hypervalent dihydridosilicate were formed and characterized by elemental analysis, NMR/IR spectroscopy, and single-crystal X-ray crystallography. The lithium complex catalyzed the hydrosilylation of styrene derivatives under mild conditions with anti-Markovnikov regioselectivity.

Fe and Co Complexes of Rigidly Planar Phosphino-Quinoline-Pyridine Ligands for Catalytic Hydrosilylation and Dehydrogenative Silylation

Co and Fe dihalide complexes of a new rigidly planar PNN ligand platform are prepared and examined as precatalysts for hydrosilylation of alkenes. Lithiation of Thummel’s 8-bromo-2-(pyrid-2′-yl)quinoline followed by treatment with (i-Pr)2PCl and (C6F5)2PCl afforded the phosphine-quinoline-pyridine ligands, abbreviated RPQpy for R = i-Pr and C6F5, respectively. These ligands form 1:1 adducts with the dichlorides and dibromides of iron and cobalt. Crystallographic characterization of FeBr2(iPrPQpy), FeBr2(ArFPQpy), CoCl2(iPrPQpy), CoBr2(iPrPQpy), and CoCl2(ArFPQpy) confirmed that the M–P–C–C–N–C–C–N portion of these complexes is planar within 0.078 A unlike previous generations of PNN complexes where deviations from planarity were ∼0.35 A. Bond distances as well as magnetism indicate that the Fe complexes are high spin and the cobalt complexes are high spin or participate in spin equilibria. Also investigated were the NNN analogues of the RPQpy ligands, wherein the phosphine group was replaced by the mesity...

Recent Advances in Organocatalyzed Asymmetric Hydrosilylations

Catalytic asymmetric hydrosilylation based on the use of inexpensive and handleable hydridic silanes is a very useful reaction in organic chemistry which has become the subject of special interest because of the intrinsic peculiarity of economic efficiency and environmental friendliness. Recently, a lot of structurally diverse organocatalysts have been designed and applied to this chemistry. Enantiopure organocatalysts such as formamide derivatives, picolinamide derivatives, S-chiral sulfinamide derivatives and chiral frustrated Lewis pairs (FLPs) have been proved as privileged catalysts for the enantioselective hydrosilylations. This manuscript provides an overview on the most relevant advances in this field and presents an extensive summary of the catalysis performance of the above mentioned catalysts. Keywords: Asymmetric reduction, hydrosilylations, organocatalysts, hydridic silanes, asymmetric synthesis, Lewis Bases.

Earth-Abundant Transition Metal Catalysts for Alkene Hydrosilylation and Hydroboration: Opportunities and Assessments.

The addition of a silicon-hydrogen or a boron-hydrogen bond across a carbon-carbon multiple bonds is a well-established method for the introduction of versatile silane and borane functional groups to base hydrocarbon feedstocks. Transition metal catalysis, historically with precious second- and third- row transition metals, has been used to broaden the scope of the hydrofunctionalization reaction, improve reaction rate and enhance selectivity. The anti-Markovnikov selectivity of platinum-catalyzed hydrosilylation of alkenes, for example, is an enabling synthetic technology in the multibillion-dollar silicones industry. Increased emphasis on sustainable catalytic methods and more economic processes has shifted focus to catalysis with more earth-abundant transition metals such as iron, cobalt and nickel. This review describes contemporary approaches and offers a contextual analysis of catalytic alkene hydrosilylation and hydroboration reactions using first-row transition metals. Emphasis is placed on defining advances in the field, what constitutes catalyst cost, safety, and important design features to enable precious metal-like reactivity, as well as new chemistry that is unique to first-row transition metals.

Specific properties of in situ ruthenium‐catalyzed polyamide 12/polydimethylsiloxane compatibilized blend

ABSTRACTThe main objective of this work focused on the chemical modification of polyamide 12 (PA12) properties through the reaction with a hydride‐terminated polydimethylsiloxane (PDMS‐SiH). The investigated PA12/PDMS‐SiH blend was compatibilized by ruthenium derivative catalyzed hydrosilylation reaction in molten state. This original route enhanced interfacial adhesion and avoid PDMS‐SiH leaching phenomenon between the two immiscible phases. More specifically, the size of PDMS‐SiH domains in the blend decreased from around 4 μm to 800 nm and from 30 to 1 μm after compatibilization with 10 and 20 wt % PDMS‐SiH, respectively. For the best compatibilized PA12/PDMS‐SiH blend, the introduction of PDMS lowered the surface free energy and the PA12‐based blend turned from hydrophilic to hydrophobic behavior, as evidenced by the water contact angle measurements. Gas permeability and CO2/H2 and CO2/He gas selectivity were also improved with the increase in PDMS content. Besides, the mechanical properties were enhanced with 13% increase in Young's modulus after in situ compatibilization with 15 wt % PDMS‐SiH. Thermal stability was also improved after compatibilization as the initial degradation temperature of reactive blends obviously increased compared with nonreactive ones. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 978–988

Asymmetric Synthesis of Silanediol Inhibitors for the Serine Protease Coagulation Cascade Enzyme FXIa.

Silanediol peptidomimetics have been prepared, designed to inhibit the serine protease enzyme Factor XIa (FXIa) for treatment of thrombosis without complete interruption of normal hemostasis. These Arg-[Si]-Ala analogues of the FXIa substrate (FIX) are the first silanediol dipeptide analogues to carry a basic guanidine group. Control of stereochemistry was accomplished using catalytic asymmetric hydrosilylation and addition of a silyllithium intermediate to the Davis-Ellman sulfinimine.

Scandium Terminal Imido Chemistry.

Research into transition metal complexes bearing multiply bonded main-group ligands has developed into a thriving and fruitful field over the past half century. These complexes, featuring terminal M═E/M≡E (M = transition metal; E = main-group element) multiple bonds, exhibit unique structural properties as well as rich reactivity, which render them attractive targets for inorganic/organometallic chemists as well as indispensable tools for organic/catalytic chemists. This fact has been highlighted by their widespread applications in organic synthesis, for example, as olefin metathesis catalysts. In the ongoing renaissance of transition metal-ligand multiple-bonding chemistry, there have been reports of M═E/M≡E interactions for the majority of the metallic elements of the periodic table, even some actinide metals. In stark contrast, the largest subgroup of the periodic table, rare-earth metals (Ln = Sc, Y, and lanthanides), have been excluded from this upsurge. Indeed, the synthesis of terminal Ln═E/Ln≡E multiple-bonding species lagged behind that of the transition metal and actinide congeners for decades. Although these species had been pursued since the discovery of a rare-earth metal bridging imide in 1991, such a terminal (nonpincer/bridging hapticities) Ln═E/Ln≡E bond species was not obtained until 2010. The scarcity is mainly attributed to the energy mismatch between the frontier orbitals of the metal and the ligand atoms. This renders the putative terminal Ln═E/Ln≡E bonds extremely reactive, thus resulting in the formation of aggregates and/or reaction with the ligand/environment, quenching the multiple-bond character. In 2010, the stalemate was broken by the isolation and structural characterization of the first rare-earth metal terminal imide-a scandium terminal imide-by our group. The double-bond character of the Sc═N bond was unequivocally confirmed by single-crystal X-ray diffraction. Theoretical investigations revealed the presence of two p-d π bonds between the scandium ion and the nitrogen atom of the imido ligand and showed that the dianionic [NR]2- imido ligand acts as a 2σ,4π electron donor. Subsequent studies of the scandium terminal imides revealed highly versatile and intriguing reactivity of the Sc═N bond. This included cycloaddition toward various unsaturated bonds, C-H/Si-H/B-H bond activations and catalytic hydrosilylation, dehydrofluorination of fluoro-substituted benzenes/alkanes, CO2 and H2 activations, activation of elemental selenium, coordination with other transition metal halides, etc. Since our initial success in 2010, and with contributions from us and across the community, this young, vibrant research field has rapidly flourished into one of the most active frontiers of rare-earth metal chemistry. The prospect of extending Ln═N chemistry to other rare-earth metals and/or different metal oxidation states, as well as exploiting their stoichiometric and catalytic reactivities, continues to attract research effort. Herein we present an account of our investigations into scandium terminal imido chemistry as a timely summary, in the hope that our studies will be of interest to this readership.

Recent Advances in Amide Reductions

This short review highlights recent advances in amide reductions. The last two years have witnessed important developments with milestones reached in catalytic hydrogenations and hydrosilylations. While metal-catalyzed processes still focus tremendous efforts from the scientific community, methodologies based on the metal-free hydrosilylation of amides have definitively joined the competition.1 Introduction2 Recent Use of Stoichiometric Reducing Reagents3 Hydrogenations4 Hydrosilylations5 Conclusion and Outlook

Hydride Source in Ethers Hydrosilylation Reaction Catalyzed by Brookhart's Ir(Ⅲ) Pincer Complex

采用DFT方法对Ir（Ⅲ）螯合物催化乙醚硅氢化生成乙烷和乙基硅醚的反应展开理论研究.反应中[H-Ir-H]，[H-Ir-Si]，[Ir（HSiEt 3 ）]和[Et 3 Si-Ir-（H） 3 ]化合物均为可能的负氢来源.理论研究表明[H-Ir-H]化合物是最优势的负氢来源.通过扭曲-相互作用能分析，发现其他三种可能的负氢来源不优势的原因在于HSiEt 3 或SiEt 3 基团对铱中心的络合.更为重要的，我们发现[H-Ir-H]化合物中适中的Ir-H键解离能，小位阻以及SiEt 3 对醚的络合而产生的促进作用共同使得[H-Ir-H]化合物上的负氢转移相对优势.

Mechanistic insights into the catalytic carbonyl hydrosilylation by cationic [CpM(CO)2(IMes)]+ (M = Mo, W) complexes: the intermediacy of η1-H(Si) metal complexes

The ionic SN2-type mechanistic pathway initiated by silane end-on coordination on the metal centers, forming η1-H(Si) Mo/W complexes, is the preferred reaction pathway for the two cationic cyclopentadienyl molybdenum/tungsten complexes, [CpM(CO)2(IMes)]+ (M = Mo, W) in catalyzing carbonyl hydrosilylation.

Tuning the redox non-innocence of a phenalenyl ligand toward efficient nickel-assisted catalytic hydrosilylation.

In this report, a ligand-redox assisted catalytic hydrosilylation has been investigated. A phenalenyl ligand coordinated nickel complex has been utilized as an electron reservoir to develop a base metal-assisted catalyst, which very efficiently hydrosilylates a wide variety of olefin substrates under ambient conditions. A mechanistic investigation revealed that a two-electron reduced phenalenyl based biradical nickel complex plays the key role in such catalysis. The electronic structure of the catalytically active biradical species has been interrogated using EPR spectroscopy, magnetic susceptibility measurements, and electronic structure calculations using a DFT method. Inhibition of the reaction by a radical quencher, as well as the mass spectrometric detection of two intermediates along the catalytic loop, suggest that a single electron transfer from the ligand backbone initiates the catalysis. The strategy of utilising the redox reservoir property of the ligand ensures that the nickel is not promoted to an unfavorable oxidation state, and the fine tuning between the ligand and metal redox orbitals elicits smooth catalysis.

Synthesis and characterization of an iron complex bearing a hemilabile NNN-pincer for catalytic hydrosilylation of organic carbonyl compounds.

A low-coordinate iron(ii) complex (CztBu(PztBu)2)Fe[N(SiMe3)2], 1 bearing an NNN-pincer ligand was prepared and fully characterized. Intramolecular C-H activation on the 5-position of a pyrazole at elevated temperatures was observed. Complex 1 was found to be an efficient and chemoselective pre-catalyst for the hydrosilylation of organo carbonyl substrates.

A β-diketiminate manganese catalyst for alkene hydrosilylation: substrate scope, silicone preparation, and mechanistic insight.

The dimeric β-diketiminate manganese hydride compound, [(2,6-iPr2PhBDI)Mn(μ-H)]2, was prepared by treating [(2,6-iPr2PhBDI)Mn(μ-Cl)]2 with NaEt3BH. This compound was characterized by single crystal X-ray diffraction and found to feature high-spin Mn centres that exhibit strong magnetic coupling by EPR spectroscopy. Once characterized, [(2,6-iPr2PhBDI)Mn(μ-H)]2 was found to mediate the hydrosilylation of a broad scope of alkenes at elevated temperature. Aliphatic alkenes were found to undergo anti-Markovnikov hydrosilylation, while the hydrosilylation of styrenes using [(2,6-iPr2PhBDI)Mn(μ-H)]2 afforded Markovnikov's product. Importantly, this catalyst has also been employed for the cross-linking of industrially-relevant silicones derived from vinyl-terminated poly(dimethylsiloxane) and 1,2,4-trivinylcyclohexane with catalyst loadings as low as 0.05 mol%. To gain a mechanistic understanding of [(2,6-iPr2PhBDI)Mn(μ-H)]2-catalyzed olefin hydrosilylation, 4-tert-butylstyrene was added to [(2,6-iPr2PhBDI)Mn(μ-H)]2 and conversion to the monomeric Mn alkyl complex, (2,6-iPr2PhBDI)Mn(CH(CH3)(4- t BuPh)), was observed. Isolation of this secondary alkyl intermediate confirms that olefin insertion into the Mn-H bond dictates the observed regioselectivities. The importance of our mechanistic findings as they relate to recent advances in Mn hydrosilylation catalysis is described herein.

Zinc and Magnesium Catalysts for the Hydrosilylation of Carbon Dioxide.

The terminal zinc and magnesium hydride compounds, [κ3-TismPriBenz]ZnH and [TismPriBenz]MgH, which feature the tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, react with B(C6F5)3 to afford the ion pairs, {[TismPriBenz]M}[HB(C6F5)3] (M = Zn, Mg), which are rare examples of these metals in trigonal monopyramidal coordination environments. Significantly, in combination with B(C6F5)3, {[TismPriBenz]M}-[HB(C6F5)3] generates catalytic systems for the hydrosilylation of CO2 by R3SiH to afford sequentially the bis(silyl)acetal, H2C(OSiR3)2, and CH4 (R3SiH = PhSiH3, Et3SiH, and Ph3SiH). In contrast to many other catalysts for these transformations, both the zinc and magnesium catalytic systems are active at room temperature, and the latter provides the first example of catalytic hydrosilylation of CO2 involving a magnesium compound. Also of note, the selectivity of the catalytic systems may be controlled by the nature of the silane, with PhSiH3 favoring CH4, and Ph3SiH favoring the bis(silyl)acetal, H2C(OSiPh3)2.

Coordination Chemistry of the PdmBOX Pincer Ligand: Reactivity at the Metal and the Ligand

The coordination chemistry of PdmBOX [2,5‐bis(2‐oxazolinyldimethylmethyl)pyrrole] pincer ligands, in which a dimethylmethylidene linker has been introduced to preclude rearrangements in the backbone, has been studied with 3d and 4d late transition metals. The in situ lithiated PdmBOX ligand was treated with MnCl2, FeCl2 and CoCl2 to give [iPr(PdmBOX)MCl] (M = Mn: 2; Fe: 3; Co: 4) which were found to adopt a distorted tetrahedral coordination geometry in the crystal. Addition of 2,6‐dimethylphenylisocyanide to 3 gave the diamagnetic tris(isocyanide) derivative [iPr(PdmBOX)Fe(CNxyl)3]Cl (5) with the chloride as the counteranion. Complexes 2–4 were found to be precatalysts for the enantioselective hydrosilylation and hydroboration of acetophenone. The synthesis of PdmBOX‐palladium(II) complexes was achieved by deprotonation of the protioligand with potassium bis(trimethylsilyl)amide (KHMDS) and subsequent reaction with [(cod)PdCl2] giving [iPr(PdmBOX)PdCl] (6) or by stirring (S)‐iPr(PdmBOX)H with palladium acetate yielding the palladium acetato complex 7. Treatment of the latter with phenylsilane at 60 °C also led to the slow formation of the metal hydride complex which was the key species in the catalytic hydrosilylation of styrene oxide, albeit with low activity. The coordination of (S)‐iPr(PdmBOX)H to rhodium(I) was accomplished by reaction with (acetylacetonato)dicarbonylrhodium(I) to give the corresponding PdmBOX‐Rh complex 8 which reacted with methyl iodide to give the octahedrally coordinated rhodium(III) complex [iPr(PdmBOX)Rh(CH3)I(CO)] 9. Reaction of 8 with one equivalent of CsBr3 led to the full conversion to the dibromorhodium(III) complex [iPr(PdmBOX)Rh(CO)Br2] 10. Upon reaction of 10 with N‐bromosuccinimide (NBS), the pyrrole backbone in 10 was brominated to give complex 11. In a similar way, a selective conversion of [iPr(PdmBOX)NiCl] to the backbone‐chlorinated [iPr(ClPdmBOX)NiCl] (12) was achieved by the treatment with N‐chlorosuccinimide.

Synthesis and characterization of new (dimethylsilyl)phenoxy and (dimethyl(vinyl)silyl)phenoxy substituted cyclotriphosphazenes

The routes of synthesis, as well as, spectroscopic and X-ray diffraction characterization of a new group of bis- or hexakis-4-(dimethyl(vinyl)silyl)phenoxy and 4-(dimethylsilyl)phenoxy-substituted cyclotriphosphazenes as a versatile starting substrates for the synthesis of a variety of organic cyclophosphazene derivatives, via catalytic hydrosilylation or silylative coupling processes, is presented.

N-heterocyclic carbene platinum complexes functionalized with a polyether chain and silyl group: Synthesis and application as a catalyst for hydrosilylation

GRAPHICAL ABSTRACT