Hydrosilylation Products Research Articles

Platinum(II) Phenylpyridyl Schiff Base Complexes as Latent, Photoactivated, Alkene Hydrosilylation Catalysts.

Photoactivated catalysts for the hydrosilylation of alkenes with silanes offer temporal control in manufacturing processes that require silicone curing. We report the development of a range of air-stable Pt(II) (salicylaldimine)(phenylpyridyl), [Pt(sal)(ppy)], complexes as photoinitiated hydrosilylation catalysts. Some of these catalysts show appreciable latency in thermal catalysis and can also be rapidly (10 s) activated by a LED UV-light source (365 nm), to give systems that selectively couple trimethylvinylsilane and hexamethylsiloxymethylsilane to give the linear hydrosilylation product. Although an undetectable (by NMR spectroscopy) amount of precatalyst is converted to the active form under UV-irradiation in the timescale required to initiate hydrosilylation, clean and reliable kinetics can be measured for these systems that allow for a detailed mechanism to be developed for Pt(sal)(ppy)-based photoactivated hydrosilylation. The suggested mechanism is shown to have close parallels with, but also subtle differences from, those previously proposed for thermally-activated Karstedt-type Pt(0) systems.

Hydroboration and hydrosilylation of alkenes catalyzed by an unsymmetrical magnesium methyl complex.

The hydroboration and hydrosilylation of alkenes catalyzed by the unsymmetrical β-diketiminate magnesium methyl complex [(DippXylNacnac)MgMe (THF)] (1) have been reported. When complex 1 was employed as a highly efficient catalyst in the hydroboration of various alkenes with HBpin, only the anti-Markovnikov hydroboration products were obtained in high yields and with high regioselectivities under mild reaction conditions (60 °C). To our surprise, it showed different regioselectivities in the hydrosilylation of a range of alkenes with PhSiH3. Aromatic alkene substrates afforded the corresponding branched Markovnikov hydrosilylation products in high yields and with high regioselectivities; conversely, aliphatic alkenes produced the linear anti-Markovnikov products in moderate yields. This is completely consistent with the corresponding density functional theory (DFT) calculations. In addition, the practical utility was demonstrated via scale-up reactions of boronate esters and a preliminary plausible mechanism of hydroboration and hydrosilylation have been investigated as well.

Photo-Triggered, Copper(II) Chloride-Catalyzed Radical Hydroalkylation and Hydrosilylation of Vinylboronic Esters To Access Alkylboronic Esters.

Alkyl boronic acids and their derivatives constitute vital building blocks in organic synthesis and are important motifs identified in medicinal chemistry. Herein, we present a phototriggered, CuCl2-catalyzed radical hydroalkylation and hydrosilylation of vinylboronic esters to alkylboronic esters. This approach exhibits mild reaction conditions, utilization of easily accessible reagents, and scalability up to a gram scale. Further synthetic transformations of the hydrosilylation products and mechanistic studies are also demonstrated.

The Chemistry of [1,1′‐bis(o‐Carboranyl)]Borane η2‐σ‐Silane Adduct

AbstractWe report here a new route for the preparation of [1,1′‐bis(o‐carboranyl)]borane, a 3D analogue of 9‐borafluorene, η2‐σ‐silane adduct (1‐H‐SiEt3) in high yields (>80 %). The nature of B−H−Si bonding was studied by density functional theory (DFT) computations. Preliminary reactivity study of 1‐H‐SiEt3 with small molecules was carried out. Interestingly, 1‐H‐SiEt3 reacts with benzophenone in hexane producing the deoxygenation product Ph2CH2. While in benzene, 1‐H‐SiEt3 reacts with benzophenone leading to mixture of Ph2CH2 and Ph3CH (3 : 1). Ph3CH is formed via addition of benzene to the carbonyl followed by deoxygenation. In contrast, the reaction of benzophenone with Et3SiH in presence of a catalytic amount of 1‐H‐SiEt3 leads to hydrosilylation product Ph2(H)C−OSiEt3.

Mechanism of Silylation of Vinyl Arenes by Hydrodisiloxanes Driven by Stoichiometric Amounts of Sodium Triethylborohydride-A Combined DFT and Experimental Study.

The reactions of vinyl arenes with hydrodisiloxanes in the presence of sodium triethylborohydride were studied using experimental and computational methods. The expected hydrosilylation products were not detected because triethylborohydrides did not exhibit the catalytic activity observed in previous studies; instead, the product of formal silylation with dimethylsilane was identified, and triethylborohydride was consumed in stoichiometric amounts. In this article, the mechanism of the reaction is described in detail, with due consideration given to the conformational freedom of important intermediates and the two-dimensional curvature of the potential energy hypersurface cross sections. A simple way to reestablish the catalytic character of the transformation was identified and explained with reference to its mechanism. The reaction presented here is an example of the application of a simple transition-metal-free catalyst in the synthesis of silylation products, with flammable gaseous reagents replaced by a more convenient silane surrogate.

Cobalt-Catalyzed Regio- and Stereoselective Hydrosilylation of Alk-2-ynes with Tertiary Silanes

Selectivity control in the addition reactions of internal alkynes that bear two similar substituents presents a big challenge in modern organic synthesis. Herein we report that β-(E)-selective hydrosilylation of alk-2-ynes with tertiary silanes can be achieved by using Co2(CO)8 as the catalyst. Under the catalytic reaction conditions, a wide range of alk-2-ynes can be converted into vinylsilanes with the β-(E) isomers as the major or sole hydrosilylation products. Mechanistic studies suggest that alkyne-bridged dicobalt species are the likely intermediates. Steric repulsion between substituents in alkenyl-bridged dicobalt silyl intermediates is proposed as the key factor inducing the observed regioselectivity.

Effect of liquid confinement on regioselectivity in the hydrosilylation of alkynes with cationic Rh(I) N-heterocyclic carbene catalysts.

Polymeric mesoporous monoliths were prepared via ring-opening metathesis polymerization (ROMP) from norbornene (NBE), 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalene (DMN-H6), tris(norborn-2-enylmethylenoxy)methylsilane and the 1st-generation Grubbs catalyst [RuCl2(PCy3)2(CHC6H5)] in the presence of 2-propanol and toluene and surface grafted with 1-(2-((norborn-5-ene-2-carbonyl)oxy)ethyl)-3-ethyl-1H-imidazol-3-ium tetrafluoroborate. Subsequently, a supported ionic-liquid-phase (SILP) system was created by immobilizing the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] with the cationic catalyst [Rh((1-pyrid-1-yl)-3-mesitylimidazol-2-ylidene)(COD)+BF4-] (Rh-1; COD = 1,4-cyclooctadiene) dissolved therein. The regio- and stereoselectivity of Rh-1 dissolved in the IL and supported on the mesoporous monolith, referred to as Rh@SILPROMP, in the hydrosilylation of 1-alkynes with HSiMe2Ph was studied and compared to that of the homogeneous catalyst Rh-1 under biphasic conditions using methyl tert-butyl ether (MTBE) as a second organic phase. Different amounts of IL were used, which allowed for the creation of SILPs with different layer thicknesses. Rh@SILPROMP provided by far better β-(Z) selectivity for both aromatic and aliphatic 1-alkynes in comparison to Rh-1 used under biphasic conditions. The highest β-(Z) selectivity was obtained with the thinnest IL layer. No leaching of the IL or rhodium from the SILP system into the organic phase was observed, resulting in virtually metal-free hydrosilylation products. The data obtained with Rh@SILPROMP were also compared with those from previous studies with Rh-1 in the same IL supported on polyurethane-derived mesoporous monolithic supports (Rh@SILPPUR) and on mesoporous SBA-15 (Rh@SILPSBA-15). For the first time, the use of a liquid confinement created by both a SILP and the support itself to tune the transition state of an organometallic catalyst by non-covalent interactions and thus stereo- and regioselectivity is outlined.

Nickel‐Catalyzed Transfer Hydrogenation of Unactivated Aryl Alkenes with Hydrosilane as Hydrogen Source

AbstractA nickel‐catalyzed transfer hydrogenation of aryl alkenes has been developed using hydrosilane as the hydrogen source. Sterically hindered disubstituted or trisubstituted alkenes were employed to test the efficiency of this transformation. Mechanism studies showed that hydrogenation took place smoothly with more sterically‐hinderedtrans‐alkenes. While uncongestedcis‐alkenes could hardly convert to the alkanes but hydrosilylation products were generated instead. Moreover, certain amount of water was found to be a promotion for the protonation process and could partly serve as the hydrogen source. Not only for simple stilbenes, this easy handle protocol also showed tolerance for substrates involving other unsaturated groups as well as heterocycles.

The catalytic activity difference of bis(phosphinite) pincer ligated Pt(II) thiolate and hydride complexes against hydrosilylation of aldimines

• POCOP pincer Pt thiolate and hydride complexes are good catalysts for hydrosilylation of aldimines • The thiolate complexes are more active than the hydride complexes • The reactions feature with good functional group compatibility and wide substrate scope • The expected amine products can be isolated in good to excellent yields In order to get information about the catalytic activities of bis(phosphinite) (POCOP) pincer ligated group 10 metal thiolate and hydride complexes against the hydrosilylation of unsaturated carbon-heteroatom bonds, platinum(II) complexes [2,6-( t Bu 2 PO) 2 C 6 H 3 ]PtX (X = SH, 1a; SPh, 1b; H, 1c) and [2,6-( i Pr 2 PO) 2 C 6 H 3 ]PtH (2) were applied to the catalytic hydrosilylation of aldimines. It was found that both auxiliary ligand and donor substituent play very important roles in affecting the catalytic activity of the platinum center. The hydride complex 1c is almost inactive in catalyzing the reactions; however, the hydride complex 2, which processes less bulky donor substituents ( i Pr vs t Bu), is an active catalyst. Meanwhile, the thiolate complexes 1a and 1b, which have the same donor substituents as 1c, are active catalysts and 1a is among most effective catalysts for this type of reaction. The catalytic reactions are featured with a wide substrate scope with relatively high turnover frequencies. Both aromatic and aliphatic aldimines were successfully hydrosilylated and the expected amines were isolated in good to excellent yields after basic hydrolysis of the hydrosilylation products. Functional groups such as OMe, halogens and internal C=C double bond were unaffected during the reactions. Although the hydride complex 2 is an active catalyst, it is less effective than the thiolate complex 1a despite the fact that the metal center in 2 is less sterically hindered by the donor substituents. As a result, the thiolate complexes are more active than the hydride complexes in catalyzing the reactions. The possible reasons for the different catalytic activities of these complexes are discussed.

Asymmetric Hydrosilylation of β-Silyl Styrenes Catalyzed by a Chiral Palladium Complex.

A palladium complex coordinated with a chiral SIPHOS ligand was evaluated as an efficient catalyst for asymmetric hydrosilylation of β-silyl styrenes with trichlorosilane and 23 1,2-bis(silyl) chiral compounds were produced. Good to excellent enantioselectivities were observed with 1-aryl-2-silyl ethanols, where the trichlorosilyl groups of the hydrosilylation products were selectively converted into a hydroxyl group in the presence of pre-installed trialkylsilyl groups. Asymmetric hydrosilylation of β-silyl styrenes followed by methylation of the trichlorosilyl group gave stable 1,2-bis(silyl) chiral compounds 4 with excellent yields. DFT calculations of hydridopalladium B coordinated with a SIPHOS ligand, an intermediate of the hydrosilylation reaction, established the optical structures to be energy minima, and the structural information could well illustrate the enantioselectivity for the hydrosilylation reaction.

Mn2(CO)10 and UV light: a promising combination for regioselective alkene hydrosilylation at low temperature.

The low temperature regioselective hydrosilylation of various alkenes with (1,1,1,3,5,5,5-heptamethyltrisiloxane) MDHM is described using Mn2(CO)10 under UV irradiation with Mn loadings as low as 1 mol%, in the absence of additives and with excellent selectivity and yields. The generation of a manganese radical allowed the anti-Markovnikov hydrosilylation products to be selectively obtained in yields up to 99%.

Markovnikov Hydrosilylation of Alkynes with Tertiary Silanes Catalyzed by Dinuclear Cobalt Carbonyl Complexes with NHC Ligation.

Metal-catalyzed hydrosilylation of alkynes is an ideal atom-economic method to prepare vinylsilanes that are useful reagents in the organic synthesis and silicone industry. Although great success has been made in the preparation of β-vinylsilanes by metal-catalyzed hydrosilylation reactions of alkynes, reported metal-catalyzed reactions for the synthesis of α-vinylsilanes suffer from narrow substrate scope and/or poor selectivity. Herein, we present selective Markovnikov hydrosilylation reactions of terminal alkynes with tertiary silanes using a dicobalt carbonyl N-heterocyclic carbene (NHC) complex [(IPr)2Co2(CO)6] (IPr = 1,3-di(2,6-diisopropylphenyl)imidazol-2-ylidene) as catalyst. This cobalt catalyst effects the hydrosilylation of both alkyl- and aryl-substituted terminal alkynes with a variety of tertiary silanes with good functional group compatibility, furnishing α-vinylsilanes with high yields and high α/β selectivity. Mechanistic study revealed that the stoichiometric reactions of [(IPr)2Co2(CO)6] with PhC≡CH and HSiEt3 can furnish the dinuclear cobalt alkyne and mononuclear cobalt silyl complexes [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)3], [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)2(IPr)], and [(IPr)Co(CO)3(SiEt3)], respectively. Both dicobalt bridging alkyne complexes can react with HSiEt3 to yield α-triethylsilyl styrene and effect the catalytic Markovnikov hydrosilylation reaction. However, the mono(NHC) dicobalt complex [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)3] exhibits higher catalytic activity over the di(NHC)-dicobalt complexes. The cobalt silyl complex [(IPr)Co(CO)3(SiEt3)] is ineffective in catalyzing the hydrosilylation reaction. Deuterium labeling experiments with PhC≡CD and DSiEt3 indicates the syn-addition nature of the hydrosilylation reaction. The absence of deuterium scrambling in the hydrosilylation products formed from the catalytic reaction of PhC≡CH with a mixture of DSiEt3 and HSi(OEt)3 hints that mononuclear cobalt species are less likely the in-cycle species. These observations, in addition to the evident of nonsymmetric Co2C2-butterfly core in the structure of [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)3], point out that mono(IPr)-dicobalt species are the genuine catalysts for the cobalt-catalyzed hydrosilylation reaction and that the high α selectivity of the catalytic system originates from the joint play of the dicobalt carbonyl species to coordinate alkynes in the Co(μ-η2:η2-HCCR')Co mode and the steric demanding nature of IPr ligand.

B(C6F5)3-Catalyzed Diastereoselective Formal (4 + 1)-Cycloaddition of Vinylcyclopropanes and Et2SiH2.

A formal (4 + 1)-cycloaddition of vinylcyclopropanes and Et2SiH2 to afford 3,4-disubstituted silolanes is reported. The reaction sequence commences with the known B(C6F5)3-catalyzed alkene hydrosilylation with dihydrosilanes. Cleavage of the remaining Si-H bond in the hydrosilylation product assisted by B(C6F5)3 leads to formation of a cyclopropane-stabilized silylium ion. The activated cyclopropane ring is then opened by the in situ-generated borohydride accompanied by ring closure to the silolane. The diastereoselectivity is rationalized by a mechanistic model.

Catalytic hydrosilylation of olefins and ketones by base metal complexes bearing a 2,2′:6′,2″-terpyridine ancillary ligand

The activities of [M(tpy)Br2] (M = Mn, Co, Ni, or Cu) for the hydrosilylation of olefins and ketones were investigated in the presence of NaBHEt3 as an activator. [Co(tpy)Br2] and [Ni(tpy)Br2] showed catalytic activities for the hydrosilylation of 1-octene with diphenylsilane. In particular, [Co(tpy)Br2] showed good catalytic activity. The linear anti-Markovnikov hydrosilylation product was predominant; however, a small quantity of the branched Markovnikov product was also produced. Generation of the branched product was suppressed by the addition of THF and pyridine as solvents. The reactivities of other olefins and silanes were also examined. In the hydrosilylation of ketones, unlike the hydrosilylation of 1-octene, all four complexes used in this study exhibited catalytic activity. Interestingly, in the reaction between diphenylsilane and acetophenone, the dominant product was the 1:2 double hydrosilylation product, not the 1:1 single hydrosilylation product, even in a reaction mixture of equal parts diphenylsilane and acetophenone.

Functionalization of polyhydromethylsiloxane with nitrogen-containing organic compounds

Linear polyhydromethylsiloxane (PHMS) was functionalized with nitrogen-containing organic compounds: N-allylaniline (Naa), N-allylcyclohexylamine (Nach), N-allylpiperidine (Nap) and 4-vinylpyridine (4VP) via hydrosilylation reaction in the presence of Pt0 complex (Karstedt’s catalyst) under mild conditions. Reaction course was followed by FTIR spectroscopy and final hydrosilylation products were characterized by FTIR, 1H, 29Si NMR and 29Si MAS-NMR spectroscopies as well as by elemental analysis. Results showed that functionalization of PHMS with N-allyl amines took place but in none of the systems it was complete. Hydrosilylation of Naa, Nach and Nap with PHMS led both, to the β and α addition products. 29Si NMR spectroscopy showed unequivocally that the reaction of PHMS with 4VP did not occur and the only reactions in the systems were hydrolysis of Si–H groups of PHMS followed by condensation of the silanol groups resulting in cross-linking of the polymer. All the functionalized polymers studied in the work contained reactive amine moieties prone to further modifications, therefore exhibit a great potential for various applications.

14-Electron Rh and Ir silylphosphine complexes and their catalytic activity in alkene functionalization with hydrosilanes.

Herein we report an experimental and computational study of a family of four coordinated 14-electron complexes of Rh(iii) devoid of agostic interactions. The complexes [X-Rh(κ3(P,Si,Si)PhP(o-C6H4CH2SiiPr2)2], where X = Cl (Rh-1), Br (Rh-2), I (Rh-3), OTf (Rh-4), Cl·GaCl3 (Rh-5); derive from a bis(silyl)-o-tolylphosphine with isopropyl substituents on the Si atoms. All five complexes display a sawhorse geometry around Rh and exhibit similar spectroscopic and structural properties. The catalytic activity of these complexes and [Cl-Ir(κ3(P,Si,Si)PhP(o-C6H4CH2SiiPr2)2], Ir-1, in styrene and aliphatic alkene functionalizations with hydrosilanes is disclosed. We show that Rh-1 catalyzes effectively the dehydrogenative silylation of styrene with Et3SiH in toluene while it leads to hydrosilylation products in acetonitrile. Rh-1 is an excellent catalyst in the sequential isomerization/hydrosilylation of terminal and remote aliphatic alkenes with Et3SiH including hexene isomers, leading efficiently and selectively to the terminal anti-Markonikov hydrosilylation product in all cases. With aliphatic alkenes, no hydrogenation products are observed. Conversely, catalysis of the same hexene isomers by Ir-1 renders allyl silanes, the tandem isomerization/dehydrogenative silylation products. A mechanistic proposal is made to explain the catalysis with these M(iii) complexes.

Cobalt- and iron-catalyzed regiodivergent alkene hydrosilylations

Regiodivergent olefin hydrosilylation products are accessed through a simple change in the metal center. Linear products (anti-Markovnikov) are obtained using cobalt-based catalysts whereas branched (Markovnikov) products result from an iron-based analogue.

Application of bis(phosphinite) pincer nickel complexes to the catalytic hydrosilylation of aldehydes

A series of bis(phosphinite) (POCOP) pincer ligated nickel complexes, [2,6-(tBu2PO)2C6H3]NiX (X = SH, 1; SCH2Ph, 2; SPh, 3; NCS, 4; N3, 5), were used to catalyse the hydrosilylation of aldehydes. It was found that both complexes 1 and 2 are active in catalysing the hydrosilylation of aldehydes with phenylsilane and complex 1 is comparatively more active. The expected alcohols were isolated in good to excellent yields after basic hydrolysis of the resultant hydrosilylation products. However, no reaction was observed when complex 3 or 4 or 5 was used as the catalyst. The results are consistent with complexes 1 and 2 serving as catalyst precursors, which generate the corresponding nickel hydride complex [2,6-(tBu2PO)2C6H3]NiH in situ, and the nickel hydride complex is the active species that catalyses this hydrosilylation process. The in situ generation of the nickel hydride species was supported by both experimental results and DFT calculation.

Silsesquioxanes-Based Nanolubricant Additives with High Thermal Stability, Superhydrophobicity, and Self-cleaning Properties

Nanoadditives are promising materials for long-envisioned next-generation lubricants to achieve excellent tribological performance and thermal stability. Here, an instigative and novel approach has been scrutinized to facilely prepare the nanolubricant additive. For this purpose, three synthetic strategies were designed for i) preparation of uniform-sized poly(methyl silsesquioxane) (PMSQ) nanoparticles, ii) hydrosilylation of the long carbon chain of ethyl 10-undecenoate and iii) modification of PMSQ nanoparticles with hydrosilylation product through a condensation reaction, in order to obtain long-carbon-chain grafted nanohybrids. The morphology, composition, and properties of these nanohybrids were confirmed by 1H-NMR, FTIR, SEM, EDS, and TGA. The effects of different concentrations of unmodified and modified PMSQ nanoparticles on the tribological properties of silicone oil were discussed. In the comparison of unmodified PMSQ nanoparticles, the modified one performs very well to reduce the coefficient of friction and wear scar diameter at low concentration. The TGA results revealed the extraordinary thermal stability of these particles, as their weight loss was only 19% at 800 °C which is remarkably higher than other solid lubricant additives. In this research, we tried to fill the deficiency of thermally stable material in the field of heavy machinery and industry. In addition, the environment-friendly (fluorine-free), superhydrophobic and self-cleaning surface effect of modified PMSQ nanoparticles was also observed. These silsesquioxane-based nanohybrids having synergistic effects, advantageous scientific values, and promising application prospects are expected to be more useful with other longer carbon chains.

Regioselective Hydrosilylation of Olefins Catalyzed by Co-Iminobipyridine Complexes: The Role of Cyclohexyl Substituent on the Imino Nitrogen

Abstract Regioselective hydrosilylation of aliphatic olefins catalyzed by Co-iminobipyridine complexes, Co〈R〉, were investigated, where R indicates a substituent on the imino nitrogen in an iminobipyridine ligand (iminobypyridine = [2,2′-bpy]-6-C(Me)=N-R). Syntheses of two complexes, Co〈Mes〉 (Mes = 2,4,6-trimethylphenyl) and Co〈Cy〉 (Cy = cyclohexyl), and comparison of their catalytic activity for hydrosilylation of 1-octene with diphenylsilane revealed that the reaction system using Co〈Mes〉 produced a mixture of anti-Markovnikov and Markovnikov hydrosilylation products, whereas that using Co〈Cy〉 produced the anti-Markovnikov product selectively. Crystal structures of these complexes implied that a catalytically active species derived from Co〈Cy〉 has narrower active space for hydrosilylation than that from Co〈Mes〉. According to the Chalk-Harrod mechanism, there are two types of hydrosilylated products (anti-Markovnikov and Markovnikov products), which stem from the way of terminal olefin insertion into an M-H bond (that is 1,2- or 2,1-insertion). One of the intermediates derived from Co〈Cy〉 in the Chalk-Harrod mechanism has hydride and silyl ligands. In the step of olefin insertion into the Co-H bond, 1,2-insertion is more likely to occur from the steric point of view, leading to selective formation of the anti-Markovnikov product.