Hydrosilylation Of Terminal Alkynes Research Articles

Stepwise Construction of Multimetallic Complexes Supported by Heteroditopic NHC/MIC Ligands

AbstractWe report the stepwise synthesis of a novel series of heterometallic gold(I)/palladium(II) complexes supported by heteroditopic imidazol‐2‐ylidene/triazol‐5‐ylidene ligands. All new compounds were characterized by means of 1H and 13C NMR spectroscopy, melting points, and elemental analyses. The versatility of the ligand platforms, additionally allows for the preparation of cationic chelate biscarbenic palladium(II) complexes through full deprotonation of the mixed azolium precursors. Characterization studies were completed by the determination of the molecular structures of some synthetic intermediates. The new biscarbenic palladium(II) complexes were tested as catalysts in the cross‐coupling of boronic acid to a variety of chloroarenes and the hydrosilylation of terminal alkynes. Both catalytic processes provide good to excellent yields under low catalyst loadings.

Ruthenium, rhodium and iridium complexes of a silyl-substituted N-heterocyclic carbene ligand and their catalytic performance in hydrosilylation of terminal alkynes

Herein, [Ru(NHC)(p-cymene)Cl2] (3) and [MCl(NHC)(cod)] (M=Rh (4) or Ir (5)) (cod=1,5‒cyclooctadiene) complexes of a silyl‒substituted N-heterocyclic carbene ligand (NHC) have been prepared by the transmetalation reaction between Ag‒NHC (2) and corresponding starting metal compound. The characterization of the complexes has been completed by NMR spectroscopy and elemental analyses. Solid state structures of Ag- (2) and Rh-NHC (4) have also been determined by single crystal X-ray analysis. [RhCl(NHC)(CO)2] type bis-carbonyl complex (6) has also been prepared and CO stretching frequencies pointed the strong σ‒donor ability of the silyl-substituted NHC. The catalytic activity of all complexes have been examined in hydrosilylation of terminal alkynes. Among the complexes, Ir-NHC has been found out as active catalyst with around 90% conversion and β‒(Z) selectivity at room temperature. Further studies revealed that this complex is able to maintain its stability in catalytic reaction conditions, chemoselective to alkynes in the presence of alkenes, and operable in scale-up reactions. In contrast, bis-carbonyl Rh-NHC performed 82% β‒(Z) selectivity with full conversion of alkyne in first 4 h of reaction, but when reaction time was extended to 24 h, almost all β‒(Z) vinylsilane isomerised to thermodynamically more stable β‒(E).

Rhodium(I) Complexes Bearing Hydroxyl‐Functionalized 1,2,3‐Triazolylidenes and Their Catalytic Application

AbstractWe report the straightforward synthesis and full characterization of a novel series rhodium(I) complexes (3 a–c) supported by hydroxyl‐functionalized 1,2,3‐triazolylidenes. All new compounds were fully characterized by means of 1H and 13C NMR spectroscopy, melting points, and elemental analyses. The new triazolylidene rhodium complexes 3 a–c were tested as catalysts in the hydrosilylation of terminal alkynes and the 1,4‐addition of boronic acid to 2‐cyclohexenone. Both catalytic processes provide good to excellent yields under low catalyst loadings and in the case of the hydrosilylation, a good selectivity toward the E‐isomer is observed.

Rh(I)/(III)-N-Heterocyclic Carbene Complexes: Effect of Steric Confinement Upon Immobilization on Regio- and Stereoselectivity in the Hydrosilylation of Alkynes.

Rh(I) NHC and Rh(III) Cp* NHC complexes (Cp*=pentamethylcyclopentadienyl, NHC=N‐heterocyclic carbene=pyrid‐2‐ylimidazol‐2‐ylidene (Py−Im), thiophen‐2‐ylimidazol‐2‐ylidene) are presented. Selected catalysts were selectively immobilized inside the mesopores of SBA‐15 with average pore diameters of 5.0 and 6.2 nm. Together with their homogenous progenitors, the immobilized catalysts were used in the hydrosilylation of terminal alkynes. For aromatic alkynes, both the neutral and cationic Rh(I) complexes showed excellent reactivity with exclusive formation of the β(E)‐isomer. For aliphatic alkynes, however, selectivity of the Rh(I) complexes was low. By contrast, the neutral and cationic Rh(III) Cp* NHC complexes proved to be highly regio‐ and stereoselective catalysts, allowing for the formation of the thermodynamically less stable β‐(Z)‐vinylsilane isomers at room temperature. Notably, the SBA‐15 immobilized Rh(I) catalysts, in which the pore walls provide an additional confinement, showed excellent β‐(Z)‐selectivity in the hydrosilylation of aliphatic alkynes, too. Also, in the case of 4‐aminophenylacetylene, selective formation of the β(Z)‐isomer was observed with a neutral SBA‐15 supported Rh(III) Cp* NHC complex but not with its homogenous counterpart. These are the first examples of high β(Z)‐selectivity in the hydrosilylation of alkynes by confinement generated upon immobilization inside mesoporous silica.

Immobilization of Pyrene‐Adorned N‐Heterocyclic Carbene Complexes of Rhodium(I) on Reduced Graphene Oxide and Study of their Catalytic Activity

AbstractTwo pyrene‐tagged N‐heterocyclic carbene (NHC) complexes of rhodium(I) were obtained and characterized. The two complexes were supported onto reduced graphene oxide (rGO), generating two new materials in which the molecular complexes are immobilized by π–π stacking interactions onto the surface of the solid. The catalytic activity of both complexes and solid hybrid materials were studied in the 1,4‐addition of phenylboronic acid to cyclohex‐2‐one, and in the hydrosilylation of terminal alkynes. The studies showed that for both reactions, the dimetallic complex displayed better catalytic performances than the monometallic one. This accounted for both the reactions performed in homogeneous conditions and for the reactions performed with the solid. In the case of the addition of phenylboronic acid to cyclohexanone, the solid containing the dimetallic catalyst could be effectively recycled up to five times, with negligible loss of activity, whereas the monometallic catalyst rapidly became inactive. In the hydrosilylation of terminal alkynes, the selectivity towards the β‐(Z)vinylsilane was improved if the immobilized dimetallic catalyst was used, although the catalyst started to lose activity after the second run.

A pronounced ligand effect on platinum‐catalyzed Hydrosilylation of terminal alkynes

The use of ligands to tune the reactivity and regioselectivity of transition metal‐based catalyst for silicon‐mediated organic synthesis (SiMOS) is an important topic and challenge in organosilicon chemistry and synthetic organic chemitstry. In this manuscript, the dual ligand platinum catalyst system was developed for the hydrosilylation of alkynes. It was found that regioselectivity enhancement was determined with combinational use of phosphine ligand (6a) and multifunctional Py‐BINMOL ligand (4a). The corresponding vinylsilanes were obtained in high yields and high regioselectivities.

Metal Complexes of a Redox‐Active [1]Phosphaferrocenophane: Structures, Electrochemistry and Redox‐Induced Catalysis

The synthesis and characterisation of several metal complexes of a redox-active, mesityl(Mes)-substituted [1]phosphaferrocenophane, FcPMes (1), are reported. Cyclic voltammetry studies on the bimetallic complexes [M(κ1 P-1)(cod)Cl] (M=Rh: 2; M=Ir: 4), [Rh(κ1 P-1)2 (CO)Cl] (3) and [AuCl(κ1 P-1)] (5), in conjunction with DFT calculations, provided indications for a good electronic communication between the metal atoms. To confirm that the ferrocenophane unit might be able to electrochemically influence the reactivity of the coordinated transition metal, the rhodium complex 2 was employed as stimuli-responsive catalyst in the hydrosilylation of terminal alkynes. All reactions were greatly accelerated with in situ generated 2+ as a catalyst as compared to 2. Even more importantly, a markedly different selectivity was observed. Both factors were attributed to different mechanisms operating for 2 and 2+ (alternative Chalk-Harrod and Chalk-Harrod mechanism, respectively). DFT calculations revealed relatively large differences for the activation barriers for 2 and 2+ in the reductive elimination step of the classical Chalk-Harrod mechanism. Thus, the key to the understanding is a cooperative "oxidatively induced reductive elimination" step, which facilitates both a higher activity and a markedly different selectivity.

Hydrosilylation of Terminal Alkynes

Hydrosilylation of Terminal Alkynes

Hydrosilylation of Terminal Alkynes with a Copper/Carbene-Functionalized Polymer

Key words copper catalysis - hydrosilylation - alkynes - boryldisiloxane - vinyldisiloxanes - carbenes

ChemInform Abstract: Preferential α‐Hydrosilylation of Terminal Alkynes by Bis‐N‐heterocyclic Carbene Rhodium(III) Catalysts.

AbstractRhodium complexes featuring one or two trifluoroacetato ligands catalyze the unusual formation of a variety of α‐vinylsilanes in good selectivities by non‐directed hydrosilylation of terminal alkynes.

Preferential α‐Hydrosilylation of Terminal Alkynes by Bis‐N‐Heterocyclic Carbene Rhodium(III) Catalysts

AbstractWe describe a bis‐N‐heterocyclic carbene rhodium(III) complex, featuring two trifluoroacetato ligands, that affords a variety of α‐vinylsilanes in good yields by hydrosilylation of terminal alkynes. Selectivities around 7:1 α/β‐(E) were reached, while the β‐(Z) product was only marginally obtained. This example sharply contrasts with the β‐(Z)‐selectivity observed for its parent diiodido complexmagnified image

Solvent‐Free Hydrosilylation of Terminal Alkynes by Reaction with a Nonclassical Ruthenium Hydride Pincer Complex

AbstractUpon the simple addition of substrates, the ruthenium pincer complex [Ru(tBuPNP)(H2)(H)2] [1; tBuPNP = 2,6‐bis(di‐tert‐butylphosphinomethyl)pyridine] is an active and selective catalyst system for the hydrosilylation of terminal alkyl alkynes under mild, solvent‐free conditions. The reactivity of this system for other functionalized terminal alkynes was also investigated, and we observed competing catalytic cycles that produce both alkyne dimers and dehydrogenative silylation products. Kinetic measurements for the hydrosilylation of 1‐octyne show that the catalyst has an initial turnover frequency of 121 h–1 at room temperature. The stoichiometric reaction between 1 and H2SiPh2 yields [Ru(tBuPNP)(H)2(H2SiPh2)], which undergoes Si–H bond activation to yield the catalytically active species [Ru(tBuPNP)(HSiPh2)(H)]. The reaction of 1 with phenylacetylene yielded [Ru(tBuPNP)(H)2(HC≡CPh)] and [Ru(tBuPNP)(H)(C≡CPh)(HC≡CPh)], and we propose that the latter is the active species in the dimerization reaction.

N-Heterocyclic carbene rhodium(i) complexes containing an axis of chirality: dynamics and catalysis

Novel rhodium(i) complexes [RhCl(NBD)(NHC)] [NHC = 1-benzyl-3-R-imidazolin-2-ylidene; R = Me, Bz, Tr, tBu]: determination of the rotation barriers about the Rh-carbene and catalytic activity in the hydrosilylation of terminal alkynes.

An Alternative Mechanistic Paradigm for the β‐Z Hydrosilylation of Terminal Alkynes: The Role of Acetone as a Silane Shuttle

The β-Z selectivity in the hydrosilylation of terminal alkynes has been hitherto explained by introduction of isomerisation steps in classical mechanisms. DFT calculations and experimental observations on the system [M(I)2{κ-C,C,O,O-(bis-NHC)}]BF4 (M=Ir (3a), Rh (3b); bis-NHC=methylenebis(N-2-methoxyethyl)imidazole-2-ylidene) support a new mechanism, alternative to classical postulations, based on an outer-sphere model. Heterolytic splitting of the silane molecule by the metal centre and acetone (solvent) affords a metal hydride and the oxocarbenium ion [R3Si-O(CH3)2](+), which reacts with the corresponding alkyne in solution to give the silylation product [R3Si-CH=C-R](+). Thus, acetone acts as a silane shuttle by transferring the silyl moiety from the silane to the alkyne. Finally, nucleophilic attack of the hydrido ligand over [R3Si-CH=C-R](+) affords selectively the β-(Z)-vinylsilane. The β-Z selectivity is explained on the grounds of the steric interaction between the silyl moiety and the ligand system resulting from the geometry of the approach that leads to β-(E)-vinylsilanes.

ChemInform Abstract: Reaction of Hydrosilanes with Alkynes Catalyzed by Gold Nanoparticles Supported on TiO2.

AbstractThe hydrosilylation of terminal alkynes under conditions A) affords predominantly β‐(E)‐isomers.

ChemInform Abstract: A Synthon for a 14‐Electron Ir(III) Species: Catalyst for Highly Selective β‐(Z) Hydrosilylation of Terminal Alkynes.

AbstractThe reaction also works with diphenylmethylsilane and dimethylphenylsilane, respectively.

Catalytic applications of recyclable silica immobilized NHC–ruthenium complexes

A monosilylated Hoveyda–Grubbs ruthenium alkylidene has been prepared and grafted through the NHC ligand to a mesostructured silica, in refluxing toluene or at room temperature, giving two new organic–inorganic hybrid silica materials M2 and M3, respectively. While M3 exhibited good performances in several metathesis reactions, M2 showed good selectivity in the hydrosilylation of terminal alkynes, where the β-(Z)-vinylsilane was obtained as major product. Recycling of the supported catalysts without significant decrease in activity and selectivity was proven for at least three cycles in both transformations.

A Simple Route to Chelating, Structurally Different Triazole‐Based Bis(N‐heterocyclic carbene) Ligands and Their Coordination to PtII

AbstractTwo new bis(triazolium) salts have been prepared and used as N‐heterocyclic carbene (NHC) precursors. The structure of the salts is highly dependent on the synthetic route, which implies the sequential alkylation of two of the three N atoms at the triazole rings. This method allows the easy preparation of bistriazolium salts with the bridge at the 1,1′ or 4,4′ positions. The two bis(triazolium) salts have been used for the preparation of three different bis(NHC)PtII complexes that have been fully characterized and the molecular structures of all three new complexes have been determined. The new Pt compounds have been tested in the hydrosilylation of terminal alkynes.

Synthesis, Structure, and Catalytic Activity of Bimetallic PtII–IrIII Complexes Bridged by Cyclooctane‐1,2‐dithiolato Ligands

AbstractCationic PtII–IrIII heterobimetallic complexes bridged by cyclooctane‐1,2‐dithiolato ligands [(PPh3)2Pt(μ2‐SRS)IrCl(η5‐Cp*)][SbF6] [R = cis‐C8H14 (5), trans‐C8H14 (6), Cp* = C5Me5] were synthesized by reaction of (cyclooctane‐1,2‐dithiolato)PtII complexes [Pt(SRS)(PPh3)2] [R = cis‐C8H14 (3), trans‐C8H14 (4)] with [IrCl(μ‐Cl)(η5‐Cp*)]2 in thf in the presence of AgSbF6. The structures of complexes 5 and 6 were fully characterized by their NMR spectroscopic data. Moreover, X‐ray crystallographic analysis of 5 revealed that the PtS2Ir core exhibits a hinged arrangement, in which the cis‐cyclooctane‐1,2‐dithiolate acts as a bridging ligand between the platinum and iridium metals. Complexes 5 and 6 served as a catalyst in the hydrosilylation of terminal alkynes R′CCH (R′ = Ph, Bu, CO2Me) with tertiary hydrosilanes such as Et3SiH and Ph3SiH to afford selectively β‐(Z)‐vinylsilanes 8 in high yields.

Novel Pt(II) Mono- and Biscarbene Complexes: Synthesis, Structural Characterization and Application in Hydrosilylation Catalysis

Reactions of imidazolium and benzimidazolium halides with PtBr2 and PtI2 in DMSO afford cis-Pt(II) heterocyclic carbene DMSO mixed ligand complexes 1−3 and biscarbene complexes 4−6. The reaction between 1,3-dibenzylbenzimidazolium bromide and PtBr2 in CH3CN gives no carbene complex but a benzimidazolium hexabromoplatinate(IV) salt 7. All the products have been fully characterized by NMR and ESI-MS spectroscopy as well as X-ray single-crystal diffraction study. Both intramolecular γ-hydride interactions with metal or its coordinated bromide and intermolecular H-bonding between heterocyclic proton with bromide are evident in some of these structures. These complexes are active in the hydrosilylation of terminal alkynes (phenylacetylene and trimethylsilylacetylene) with triethylsilane and bis(trimethylsiloxy)methylsilane. When phenylacetylene is used, the monocarbene complexes (1−3) show higher catalytic activity than chelating biscarbene analogues (5 and 6). Non-chelating biscarbene complex 4 gives a different selectivity pattern favoring the β(Z) and dehydrogenative silylation products when compared to 1−3, 5 and 6, and 7. In contrast, 4 and 5 are more active than 1 and 2 toward trimethylsilylacetylene. In general, monocarbene complexes give higher conversions over a short duration in the hydrosilylation of phenylacetylene with triethylsilane and bis(trimethylsiloxy)methylsilane, whereas in the hydrosilylation of trimethylsilylacetylene with triethylsilane, the chelating and biscarbene tend to be more active.