Synthesis and characterization of a free phenylene bis(N-heterocyclic carbene) and its di-Rh complex: Catalytic activity of the di-Rh and CCC–NHC Rh pincer complexes in intermolecular hydrosilylation of alkynes

1,3-Bis(3-butylimidazolium-1-yl)benzene diiodide (1) was reacted with Li(2,2,6,6-tetramethylpiperidine) yielding the free bis-carbene, 1,3-bis(3-butylimidazol-2-ylidene-1-yl)benzene (3), which has been spectroscopically characterized. Combining the free bis-carbene with [Rh(COD)Cl]₂ yielded the corresponding di-Rh bis(N-heterocyclic carbene) complex (4) that was structurally characterized. The di-Rh bis-carbene complex was found to exhibit complex solution ¹³C and ¹H NMR spectra that have been assigned as a mixture of diastereomers. The crystal structure of the di-Rh bis-carbene compound 4 was composed of a pair of enantiomeric atropisomers. The diastereomeric atropisomers were assigned as the source of the spectral complexities. The di-Rh di-carbene complex 4 and the CCC–NHC Rh pincer complex 2 were applied as catalysts in hydrosilylation reactions of terminal and internal alkynes. Both catalysts are highly active, regioselective, stereoselective, and chemoselective: terminal alkynes give predominantly the β-(Z) isomer and internal alkynes afford the β-(E) isomer in chloroform or benzene. One of the strongest attributes of the catalyst systems is that the results were achieved without exclusion of air and without purification of commercially available reagents.     

UV‐activated hydrosilylation: a facile approach for synthesis of hyperbranched polycarbosilanes

A facile approach for synthesis of hyperbranched polycarbosilane from AB₂ monomer via UV‐activated hydrosilylation is presented in this communication. The polymerization process was monitored using real‐time FTIR spectroscopy and the resulting hyperbranched polycarbosilanes were characterized using ¹H‐NMR, ¹³C‐NMR, ²⁹Si‐NMR and SEC/MALLS. It is found that hyperbranched polycarbosilane can be synthesized from methyldiallylsilane via UV‐activated hydrosilylation with bis(acetylacetonato)platinum(II) as catalyst. The polymerization activated by UV irradiation was much faster than that under thermal conditions. The similar degree of branching, average number of branch units and the exponent of the Mark–Houwink equation demonstrate that the hyperbranched polycarbosilane synthesized via UV‐activated polyhydrosilylation possesses almost the same branching structure as that synthesized via thermal‐activated polyhydrosilylation. Copyright © 2009 John Wiley & Sons, Ltd.     

Cobalt-Catalyzed Regiodivergent Double Hydrosilylation of Arylacetylenes

Double hydrosilylation of alkynes represents a straightforward method to synthesize bis(silane)s, yet it is challenging if α-substituted vinylsilanes act as the intermediates. Here, a cobalt-catalyzed regiodivergent double hydrosilylation of arylacetylenes is reported for the first time involving this challenge, accessing both vicinal and geminal bis(silane)s with exclusive regioselectivity. Various novel bis(silane)s containing Si−H bonds can be easily obtained. The gram-scale reactions could be performed smoothly. Preliminarily mechanistic studies demonstrated that the reactions were initiated by cobalt-catalyzed α-hydrosilylation of alkynes, followed by cobalt-catalyzed β-hydrosilylation of the α-vinylsilanes to deliver vicinal bis(silane)s, or hydride-catalyzed α-hydrosilylation to give geminal ones. Notably, these bis(silane)s can be used for the synthesis of high-refractive-index polymers (n_d up to 1.83), demonstrating great potential utility in optical materials.     

Synthesis and application of a new selenoplatinum catalyst

The reaction of 4-methyl-5-ethoxycarbonyl-1,2,3-selenadiazole with (PPh₃)₄Pt leads to the formation of a new platinum-containing heterocyclic system. It was found that the selenoplatinum complex is a selective catalyst for the hydrosilylation of terminal alkynes to yield β-(Z)- and β-(E)-silylethylenes.     

Reaction of hydrosilanes with alkynes catalyzed by gold nanoparticles supported on TiO2

Gold nanoparticles supported on TiO₂ (0.8–1.4 mol %) catalyze the β-(E) regioselective hydrosilylation of a variety of functionalized terminal alkynes with alkylhydrosilanes in 1,2-dichloroethane (70 °C). The product yields are excellent, and the reaction times relatively short, while almost equimolar amounts of alkynes and hydrosilanes can be used. Minor side-products in up to 35% relative yield of cis-oxidative (dehydrogenative) disilylation, an unprecedented reaction pathway, are formed in the cases of the less hindered hydrosilanes and alkynes. Triethoxysilane reacts faster and affords apart from β-(E) addition products, minor α-hydrosilylation regio-isomers in upto 15% relative yield. Internal alkynes are generally less reactive or even unreactive. It is proposed that cationic Au(I) species stabilized by the support are the reactive catalytic sites, forming in the presence of hydrosilanes either silyl–Au(III)–H (hydrosilylation pathway) or Au(III)–disilyl species (dehydrogenative disilylation pathway). Regarding the mechanism of hydrosilylation, kinetic experiments are in agreement with silyl carbometallation of the triple bond in the rate determining step of the reaction.     

Platinum-catalyzed double silylations of alkynes with bis(dichlorosilyl)methanes

Bis(dichlorosilyl)methanes 1 undergo the two kind reactions of a double hydrosilylation and a dehydrogenative double silylation with alkynes 2 such as acetylene and activated phenyl-substituted acetylenes in the presence of Speier’s catalyst to give 1,1,3,3-tetrachloro-1,3-disilacyclopentanes 3 and 1,1,3,3-tetrachloro-1,3-disilacyclopent-4-enes 4 as cyclic products, respectively, depending upon the molecular structures of both bis(dichlorosilyl)methanes (1) and alkynes (2). Simple bis(dichlorosilyl)methane (1a) reacted with alkynes [R¹-CC-R²: R¹ = H, R² = H (2a), Ph (2b); R¹ = R² = Ph (2c)] at 80 °C to afford 1,1,3,3-tetrachloro-1,3-disilacyclopentanes 3 as the double hydrosilylation products in fair to good yields (33-84%). Among these reactions, the reaction with 2c gave a trans-4,5-diphenyl-1,1,3,3-tetrachloro-1,3-disilacyclopentane 3ac in the highest yield (84%). When a variety of bis(dichlorosilyl)(silyl)methanes [(Me_nCl_3 − nSi)CH(SiHCl₂)₂: n = 0 (1b), 1 (1c), 2 (1d), 3 (1e)] were applied in the reaction with alkyne (2c) under the same reaction conditions. The double hydrosilylation products, 2-silyl-1,1,3,3-tetrachloro-1,3-disilacyclopentanes (3), were obtained in fair to excellent yields (38-98%). The yields of compound 3 deceased as follows: n = 1 > 2 > 3 > 0. The reaction of alkynes (2a-c) with 1c under the same conditions gave one of two type products of 1,1,3,3-tetrachloro-1,3-disilacyclopentanes 3 and 1,1,3,3-tetrachloro-1,3-disilacyclopent-4-enes (4): simple alkyne 2a and terminal 2b gave the latter products 4ca and 4cb in 91% and 57% yields, respectively, while internal alkyne 2c afforded the former cyclic products 3cc with trans form between two phenyl groups at the 3- and 4-carbon atoms in 98% yield, respectively. Among platinum compounds such as Speier’s catalyst, PtCl₂(PEt₃)₂, Pt(PPh₃)₂(C₂H₄), Pt(PPh₃)₄, Pt[ViMeSiO]₄, and Pt/C, Speier’s catalyst was the best catalyst for such silylation reactions.     

Recent advances in the synthetic and mechanistic aspects of the ruthenium-catalyzed carbon-heteroatom bond forming reactions of alkenes and alkynes

The group’s recent advances in catalytic carbon-to-heteroatom bond forming reactions of alkenes and alkynes are described. For the C-O bond formation reaction, a well-defined bifunctional ruthenium-amido catalyst has been successfully employed for the conjugate addition of alcohols to acrylic compounds. The ruthenium-hydride complex (PCy₃)₂(CO)RuHCl was found to be a highly effective catalyst for the regioselective alkyne-to-carboxylic acid coupling reaction in yielding synthetically useful enol ester products. Cationic ruthenium-hydride catalyst generated in-situ from (PCy₃)₂(CO)RuHCl/HBF₄·OEt₂ was successfully utilized for both the hydroamination and related C-N bond forming reactions of alkenes. For the C-Si bond formation reaction, regio- and stereoselective dehydrosilylation of alkenes and hydrosilylation of alkynes have been developed by using a well-defined ruthenium-hydride catalyst. Scope and mechanistic aspects of these carbon-to-heteroatom bond forming reactions are discussed.     

Regio- and stereoselective hydrosilylation of immobilized terminal alkynes

Regio- and stereoselective hydrosilylation of terminal alkynes on solid support using diisopropyl hydrosilanes yielding β-(E)-vinyl silanes with excellent selectivity is reported. The hydrosilylation is catalyzed by Pt(DVDS)/P(ⁱBuNCH₂CH₂)₃N (DVDS = 1,3-divinyl-1,1,3,3-tetramethyl-disiloxane), in which the bulky proazaphosphatrane ligand plays a key role for the selectivity. The immobilized products are characterized with gel phase¹³C NMR and ¹H high resolution magic angle spinning NMR.     

Highly Chemo‐, Regio‐, and Stereoselective Cobalt‐Catalyzed Markovnikov Hydrosilylation of Alkynes

A highly chemo‐, regio‐ and stereoselective cobalt‐catalyzed Markovnikov hydrosilylation of alkynes was developed. Various functionalized groups, such as halides, free alcohols, free aniline, ketones, esters, amides, and nitriles are tolerated, which may lead to further applications and late‐stage derivatizations. To date, this is the most efficient cobalt catalytic system (TOF=65 520 h^?1; TOF=turnover frequency) for hydrosilylation of alkynes. The Hiyama–Denmark cross‐coupling reactions of vinylsilanes with aryl iodides underwent smoothly to afford 1,1‐diarylethenes. A unique regioselectivity‐controllable hydrosilylation/hydroboration reaction of alkynes was also described.     

Copper (II)-catalyzed regio- and stereoselective addition of H/P(O)R2 to alkynes

Cu(acac)₂ is the new universal catalyst for β-E regio- and stereoselective syn-addition of the H–P(O)-bond of diphenylphosphine oxide, H-phosphinates, dialkylphosphites to various alkynes in the synthesis of P(O)-containing alkenes. Without additives and ligands Cu(II)-compounds showed better results than CuI or Ni(acac)₂. The catalytic system developed is tolerant to typical organic functional groups present in the alkynes and to the nature of different substituents in the H–P(O)-compounds.     

Polymerizations of chloral,styrene oxide,and methyl methacrylate initiated by the binary system of bis(acetylacetonato)metal(II) and chloral

The binary system of bis(acetylacetonato)metal(II) [M(acac)₂] and chloral induced the polymerization of chloral [M = Mn(II), Co(II), Mg(II), and Cu(II)], the ring-opening polymerization of styrene oxide [M = Co(II) and Mg(II)], and the radical polymerization of methyl methacrylate [M = Mn(II) and Co(II)]. The similar order of activity of M(acac)₂ as the catalyst for the polymerization of chloral and for the aldol reaction of chloral with acetylacetone, the deactivation of the catalyst by the introduction of a substituent at the 3-position of M(acac)₂, the presence of saturated β-diketone at the end of the polymer of chloral and that of styrene oxide, and the visible light spectral data supporting the formation of the β-ketoalcoholate intermediate in the binary system of Co(acac)₂ and chloral are all experimental findings which suggest that M(acac)₂ is subject to the aldol addition by chloral at the 3-position of the chelated acetylacetone and that the resultant β-ketoalcoholate is a common active species for these polymerizations.     

Synthesis of New Methylsiloxane Oligomers with Pendant Trialkoxysilylethyl Groups for Preparation of Silicon hard Coatings

For the purpose to prepare precursor materials for the silicon hard coatings, the hydrosilylation reactions of α, ω–bis(trimethylsiloxy)methylhydridesiloxane to trialkoxyvinylsilanes in the presence of platinum hydrochloric acid (0.1 M solution in THF), Karstedt's catalyst (Pt₂[(VinSiMe₂)₂O]₃) and platinum on the carbon (5%) were investigated. Hydrosilylation reactions at different ratios of initial compounds and at various temperatures (40–60 °C) were investigated and methylsiloxane oligomers with pendant trialkoxy fragments have been obtained. It was shown that completely hydrosilylation of all active Si H groups do not take place. The hydrosilylation reaction order, activation energy and rate constants were determined. The synthesized oligomers were characterized by ¹H, ¹³C NMR and FTIR spectra data. Gel-permeation chromatography, differential scanning calorimetric, thermogravimetric and wide-angle X-ray investigations of synthesized oligomers were carried out. Synthesized oligomers together with tetraethoxysilane were used for preparation of silicon hard coatings via sol-gel processes.     

Exploring Bis(cyclometalated) Ruthenium(II) Complexes as Active Catalyst Precursors: Room‐Temperature Alkene–Alkyne Coupling for 1,3‐Diene Synthesis

Described is the development of a new class of bis(cyclometalated) ruthenium(II) catalyst precursors for C? C coupling reactions between alkene and alkyne substrates. The complex [(cod)Ru(3‐methallyl)₂] reacts with benzophenone imine or benzophenone in a 1:2 ratio to form bis(cyclometalated) ruthenium(II) complexes ( 1 ). The imine‐ligated complex 1 a promoted room‐temperature coupling between acrylic esters and amides with internal alkynes to form 1,3‐diene products. A proposed catalytic cycle involves C? C bond formation by oxidative cyclization, β‐hydride elimination, and C? H bond reductive elimination. This Ru^II/Ru^IV pathway is consistent with the observed catalytic reactivity of 1 a for mild tail‐to‐tail methyl acrylate dimerization and for cyclobutene formation by [2+2] norbornene/alkyne cycloaddition.     

Cobalt‐Catalyzed Alkyne Hydrosilylation and Sequential Vinylsilane Hydroboration with Markovnikov Selectivity

A pyridinebis(oxazoline) cobalt complex is a very efficient precatalyst for the hydrosilylation of terminal alkynes with Ph₂SiH₂, providing α‐vinylsilanes with high (Markovnikov) regioselectivity and broad functional‐group tolerance. The vinylsilane products can be further converted into geminal borosilanes through Markovnikov hydroboration with pinacolborane and a bis(imino)pyridine cobalt catalyst.     

Design and Synthesis of Isocyanide Ligands for Catalysis: Application to Rh‐Catalyzed Hydrosilylation of Ketones

New isocyanide ligands with meta‐terphenyl backbones were synthesized. 2,6‐Bis[3,5‐bis(trimethylsilyl)phenyl]‐4‐methylphenyl isocyanide exhibited the highest rate acceleration in rhodium‐catalyzed hydrosilylation among other isocyanide and phosphine ligands tested in this study. ¹H NMR spectroscopic studies on the coordination behavior of the new ligands to [Rh(cod)₂]BF₄ indicated that 2,6‐bis[3,5‐bis(trimethylsilyl)phenyl]‐4‐methylphenyl isocyanide exclusively forms the biscoordinated rhodium–isocyanide complex, whereas less sterically demanding isocyanide ligands predominantly form tetracoordinated rhodium–isocyanide complexes. FTIR and ¹³C NMR spectroscopic studies on the hydrosilylation reaction mixture with the rhodium–isocyanide catalyst showed that the major catalytic species responsible for the hydrosilylation activity is the Rh complex coordinated with the isocyanide ligand. DFT calculations of model compounds revealed the higher affinity of isocyanides for rhodium relative to phosphines. The combined effect of high ligand affinity for the rhodium atom and the bulkiness of the ligand, which facilitates the formation of a catalytically active, monoisocyanide–rhodium species, is proposed to account for the catalytic efficiency of the rhodium–bulky isocyanide system in hydrosilylation.     

Synthesis,cytotoxicity and antiviral activity studies of the conjugates of cobalt bis(1,2-dicarbollide)(-I) with 5-ethynyl-2′-deoxyuridine and its cyclic derivatives

A series of novel conjugates of cobalt bis(1,2-dicarbollide)(-I) with 5-ethynyl-2′-deoxyuridine and its cyclic derivatives were synthesized. Conjugates with 5-ethynyl-2-deoxyuridine were prepared by the direct Sonogashira coupling of a series of cobalt bis(1,2-dicarbollide)(-I) terminal alkynes and 5-iodo-2′-deoxyuridine. Their furo[2,3-d]pyrimidin-2(3H)-one isomers were obtained either by intermolecular cyclization of the above conjugates or by Sonogashira coupling using Pd/C as a catalyst. Action of ammonia on these furo[2,3-d]pyrimidin-2(3H)-one conjugates resulted in pyrrolo[2,3-d]pyrimidin-2(3H)-one conjugates. Most of the designed compounds have shown low cytotoxicity in several cell lines. Some 5-ethynyl-2-deoxyuridine and furo[2,3-d]pyrimidin-2(3H)-one conjugates have also presented antiviral activity.     

Z-Selective hydroamidation of terminal alkynes with secondary amides and imides catalyzed by a Ru/Yb-system

A catalyst system formed in situ from bis(2-methallyl)cycloocta-1,5-diene-ruthenium(II) [(cod)Ru(met)₂], 1,4-bis(dicyclohexylphosphino)butane (dcypb) and ytterbium(III) triflate hydrate (Yb(OTf)₃) was found to catalyze the addition of nitrogen nucleophiles to terminal alkynes under mild conditions to stereoselectively form the Z-enamide or Z-enimide products. Various secondary amides and imides could be added across the triple bond of a range of aliphatic and aromatic alkynes. The new bimetallic catalyst system sets new standards with regard to scope and selectivity for the synthesis of Z-configured anti-Markovnikov enamides.     

Silicon hydrides and nickel complexes : II. Mechanism of the hydrosilylation catalyzed by nickel-phosphine complexes

Two ethylene-nickel(0) complexes, viz., [1,2-bis(diphenylphosphino)ethane]-(ethylene)nickel(0) and bis(triphenylphosphine)(ethylene)nickel(0) have been used in a comparison of their catalytic activities in hydrosilylation reactions with those of the corresponding nickel(II) complexes, viz., dichloro [1,2-bis(diphenylphosphino)-ethane]nickel(II) and dichlorobis(triphenylphosphine)nickel(II). The reaction profiles are similar, apart from a significant difference in the induction period; the nickel(II) catalysts requiring a substantially longer time. A mechanism involving a nickel(0) species is proposed for the hydrosilylation.The interchange of hydrogen and chlorine on silicon accompanying the hydrosilylation is related to a high electron density at the nickel atom bearing the phosphine, olefin, and silicon hydride ligands.     

Catalytic asymmetric hydrosilylation of ketones by rhodium(I) complexes containing the enantiomers of (R1, R1)-(±)-1,2-phenylenebis(methylphenylarsine) and their phosphorus isosteres

Soluble (bicyclo[2.2.1]hepta-2,5-diene)rhodium(I) complexes containing the enantiomers of (R¹,R¹-(±)-1,2-phenylenebis(methylphenyl-arsine) or its phosphorus isosteres have been found to be highly efficient catalysts for the asymmetric hydrosilylation of prochiral ketones. The rates of reaction for both the bis(tertiary arsine)- and the bis(tertiary phosphine)-containing catalysts appear to be amongst the fastest of their type hitherto reported, with both catalysts giving comparable optical yields of asymmetric silyl ethers. This is the first successful use of a tertiary arsine-containing complex for catalytic asymmetric hydrosilylation. The optical yields varied between 18 – 41% for C₆H₅COCH₃ and CH₃COC(CH₃)₃, depending upon conditions and catalyst, but with o-MeOC₆H₄COCH₃ as substrate, the optical yield dropped to 1 – 2% for both catalysts.     

Übergangsmetall—carbin-komplexe : II. trans-halogeno-diäthylaminocarbin-tetracarbonyl-komplexe des wolframs aus äthoxydiathylaminocarbenpentacarbonyl-wolfram(0)

A crystal structure analysis shows that hexafluoro-2-butyne reacts with bis(acetylacetonato)palladium(II) to give Pd[OC(Me)CH(COMe)C(CF₃)C(CF₃₎]₂ in which the hexafluoro-2-butyne links the γ-CH of the β-diketonato ligands to the palladium. Other palladium(II)β-diketonato systems behave similarly.