Primary Silanes Research Articles

Enantioselective Copper-Catalyzed Sequential Hydrosilylation of Arylmethylenecyclopropanes.

Despite impressive advances in the construction of enantioenriched silacarbocycles featuring silicon-stereogenic centers via a selection of well-defined sila-synthons, the development of a more convenient and economic method with readily available starting materials is significantly less explored and remains a considerable challenge. Herein, we report the first example of copper-catalyzed sequential hydrosilylation of readily accessible methylenecyclopropanes (MCPs) and primary silanes, affording an efficient and convenient route to a wide range of chiral silacyclopentanes bearing consecutive silicon- and carbon-stereogenic centers with excellent enantio- and diastereoselectivities (generally ≥98 % ee, >25 : 1 dr). Mechanistic studies reveal that these reactions combine copper-catalyzed intermolecular ring-opening hydrosilylation of aryl MCPs and intramolecular asymmetric hydrosilylation of the resultant Z/E mixture of homoallylic silanes.

Selective Access to Silacyclopentanes and Homoallylsilanes by La-Catalyzed Hydrosilylations of 1-Aryl Methylenecyclopropanes.

Methylenecyclopropanes (MCPs) have emerged as versatile building blocks in synthetic chemistry because of their unique reactivity. However, metal-catalyzed hydrosilylation of MCPs has met with very limited successes. In this paper, catalytic selective hydrosilylations of MCPs with some primary silanes using an ene-diamido lanthanum ate complex as the catalyst were described. The catalytic reactions resulted in the selective formation of silacyclopentanes and (E)-homoallylsilanes, respectively, depending on the substituents on MCPs. The formation of silacyclopentanes via a catalytic cascade inter- and intramolecular hydrosilylation mechanism is strongly supported by the control and deuteration-labeling experiments and DFT calculations. The unique reactivity and selectivity could be attributed to the large lanthanum ion and ate structure of the catalyst.

Rhodium-Mediated Dehydrogenation of Hydroboranes and Group 14 Compounds: Base-Stabilized Silylene and Germylene Complexes vs. Transmetalation.

Monocarbonyl rhodium complex LRh(CO), 1, which is stabilized by a pyrrole-based bis(phosphinimine) pincer ligand (L=κ3 -NNN'=2,5-[i Pr2 P=N(4-i PrC6 H4 )]2 -N'(C4 H2 )- ), serves as a versatile platform for the dehydrogenation of group 14 substrates. Reaction with primary and secondary silanes and germanes (MesSiH3 , Et2 SiH2 , Ph2 GeH2 , t BuGeH3 ; Mes=mesityl) liberates H2 and yields base-stabilized tetrylene compounds of the form κ2 -L(CO)Rh(ER2 ) (E=Si: R=Mes, H, 2; R=Et, 5; E=Ge: R=Ph, 6; R=t Bu, H, 8). The ":ER2 " fragment in these species bridges between the rhodium center and a phosphinimine donor. Preliminary reactions between pinacol (Pin) and κ2 -L(CO)Rh(ER2 ), E=Si, Ge, indicate that such complexes can serve as silylene and germylene synthons, releasing :ER2 and catalytically generating PinER2 . In contrast, combination of complex 1 and MesGeH3 does not yield the anticipated dehydrogenation product, but rather, transmetalation similar to that observed upon reaction between 1 and 3,5-dimethylphenylborane prevails.

Cover Picture

In nature, differences in temperature create completely distinguishable landscapes in winter and spring. Similarly, by controlling the reaction temperature, efficient regioselective control of nickel‐catalyzed hydrosilylation of conjugated dienes with primary silanes can be achieved, facilitating the divergent synthesis of various homoallylic silanes and allylic silanes under mild conditions. More details are discussed in the article by Zhang et al. on page 13—19.image

Nickel‐Catalyzed Regioselective Hydrosilylation of Conjugated Dienes†

Comprehensive SummaryWith the increasing demand for homoallylic silanes and allylic silanes, the highly efficient and regioselective hydrosilylations of conjugated dienes are urgently needed. Herein, we developed a Ni‐catalyzed regiodivergent hydrosilylation of aromatic conjugated dienes by adjusting the temperature and ligands. Under low temperature (–30 °C), an eternal‐ligand‐free system (Ni/t‐BuOK) can efficiently facilitate the 3,4‐anti‐Markovnikov hydrosilylation to provide homoallylic silanes via electrophilic activation process; under room temperature (25 °C), a ligand‐controlled system (Ni/t‐BuOK/PPh3) can eventuate the 3,4‐Markovnikov hydrosilylation to produce allylic silanes via Chalk‐Harrod process. Both systems are compatible with various conjugated dienes and primary silanes in excellent yields and regioselectivities.

Group I Alkoxides and Amylates as Highly Efficient Silicon-Nitrogen Heterodehydrocoupling Precatalysts for the Synthesis of Aminosilanes.

Group I alkoxides are highly active precatalysts in the heterodehydrocoupling of silanes and amines to afford aminosilane products. The broadly soluble and commercially available KOt Amyl was utilized as the benchmark precatalyst for this transformation. Challenging substrates such as anilines were found to readily couple primary, secondary, and tertiary silanes in high conversions (>90 %) after only 2 h at 40 °C. Traditionally challenging silanes such as Ph3 SiH were also easily coupled to simple primary and secondary amines under mild conditions, with reactivity that rivals many rare earth and transition-metal catalysts for this transformation. Preliminary evidence suggests the formation of hypercoordinated intermediates, but radicals were detected under catalytic conditions, indicating a mechanism that is rare for Si-N bond formation.

Lithium Triethylborohydride (LiHBEt3)-Promoted Hydrosilylation of Allenes to Prepare (E)-Allylsilanes.

A transition-metal-free hydrosilylation of allenes is reported herein by using commercially available lithium triethylborohydride (LiHBEt3) as the catalyst. Both mono- and disubstituted allenes could be hydrosilylated with primary or secondary silanes effectively. This reaction represents an environmental and economic method to prepare (E)-allylsilanes in good yields along with decent selectivities.

Photocatalyzed regioselective hydrosilylation for the divergent synthesis of geminal and vicinal borosilanes

Geminal and vicinal borosilanes are useful building blocks in synthetic chemistry and material science. Hydrosilylation/hydroborylation of unsaturated systems offer expedient access to these motifs. In contrast to the well-established transition-metal-catalyzed methods, radical approaches are rarely explored. Herein we report the synthesis of geminal borosilanes from α-selective hydrosilylation of alkenyl boronates via photoinduced hydrogen atom transfer (HAT) catalysis. Mechanistic studies implicate that the α-selectivity originates from a kinetically favored radical addition and an energetically favored HAT process. We further demonstrate selective synthesis of vicinal borosilanes through hydrosilylation of allyl boronates via 1,2-boron radical migration. These strategies exhibit broad scopes across primary, secondary, and tertiary silanes and various boron compounds. The synthetic utility is evidenced by access to multi-borosilanes in a diverse fashion and scaling up by continuous-flow synthesis.

Si-H Bond Activation and Dehydrogenative Coupling of Silanes across the Iron-Amide Bond of a Bis(amido)bis(phosphine) Iron(II) Complex.

Despite the utility of Si-Si bonds, there are relatively few examples of Si-Si bond formation by base metals. In this work, a four-coordinate iron complex, (PNNP)FeII, is shown to strongly activate the Si-H bonds in primary silanes across the Fe-amide bonds in a metal-ligand cooperative fashion. Upon treatment with excess silane, Si-Si dehydrogenative homocoupling is shown to occur across the Fe-Namide bond without concomitant oxidation and spin state changes at the Fe center.

Lanthanide-catalyzed regio- and enantioselective hydrosilylation of aryl-substituted terminal and internal alkenes

Highly regio- and enantioselective hydrosilylation of aryl-substituted terminal/internal alkenes with primary silanes was achieved using a chiral La(iii)-bisoxazoline complex.

Selective oxidation of silanes into silanols with water using [MnBr(CO)5] as a precatalyst.

The development of earth-abundant catalysts for the selective conversion of silanes to silanols with water as an oxidant generating valuable hydrogen as the only by-product continues to be a challenge. Here, we demonstrate that [MnBr(CO)5] is a highly active precatalyst for this reaction, operating under neutral conditions and avoiding the undesired formation of siloxanes. As a result, a broad substrate scope, including primary and secondary silanes, could be converted to the desired products. The turnover performances of the catalyst were also examined, yielding a maximum TOF of 4088 h-1. New light was shed on the debated mechanism of the interaction between [MnBr(CO)5] and Si-H bonds based on the reaction kinetics (including KIEs of PhMe2SiD and D2O) and spectroscopic techniques (FT-IR, GC-TCD, 1H-, 29Si-, and 13C-NMR). The initial activation of [MnBr(CO)5] was found to result from the formation of a manganese(i) hydride species and R3SiBr, and the experimental data are most consistent with a catalytic cycle comprising a cationic tricarbonyl Mn(i) unit as the active framework.

σ-GeH and Germyl Cationic Pt(II) Complexes.

The low electron count Pt(II) complexes [Pt(NHC')(NHC)][BArF] (where NHC is a N-heterocyclic carbene ligand and NHC' its metalated form) react with tertiary hydrogermanes HGeR3 at room temperature to generate the 14-electron platinum(II) germyl derivatives [Pt(GeR3)(NHC)2][BArF]. Low-temperature NMR studies allowed us to detect and characterize spectroscopically some of the σ-GeH intermediates [Pt(η2-HGeR3)(NHC')(NHC)][BArF] that evolve into the platinum-germyl species. One of these compounds has been characterized by X-ray diffraction studies, and the interaction of the H-Ge bond with the platinum center has been analyzed in detail by computational methods, which suggest that the main contribution is the donation of the H-Ge to a σ*(Pt-C) orbital, but backdonation from the platinum to the σ*(Ge-H) orbital is significant. Primary and secondary hydrogermanes also produce the corresponding platinum-germyl complexes, a result that contrasts with the reactivity observed with primary silanes, in which carbon-silicon bond-forming reactions have been reported. According to density functional theory calculations, the formation of Pt-Ge/C-H bonds is both kinetically and thermodynamically preferred over the competitive reaction pathway leading to Pt-H/C-Ge bonds.

Heterodehydrocoupling of Silanes and Amines Catalyzed by a Simple Lanthanum-Based Complex

We report an attractive and efficient synthetic approach to form Si–N bonds via the heterodehydrocoupling of silanes and amines catalyzed by a lanthanum-based N,N-dimethylbenzylamine complex [La(DMBA)3]. A wide variety of primary and secondary amines were well-suited coupling partners with primary, secondary, and tertiary silanes for this catalytic transformation, resulting in the formation of numerous silylamines in high efficiency and moderate-to-excellent yields under mild conditions.

Metal‐Free Catalyst, Fast Curing, Stretchable, Self‐Healing Siloxane Modified Polyurea Elastomer with Tunable Properties Based on Hydrogen Bonds

AbstractIn the past decade, self‐healing polymer materials, being researched and utilized in many fields, have attracted more and more attention. In this work, self‐healing siloxane elastomer (SPUR) with flexible cross‐linked networks were designed through the reaction of amino‐functionalized polyether, isocyanate and α‐siloxane, which endowed elastomer with intermolecular hydrogen bond. Herein, these hydrogen bonds can break and reorganize via a simple thermal treatment to gain self‐healing and post‐processing actions. The self‐healing performance of the elastomer is confirmed through tensile test and SEM surface analysis. Consistent with our experiments, the researches show that siloxane modified polyurea elastomers (SPUR) congaing higher content of amino‐functionalized polyether with 2000 molecular weights (PEA20) show much better flexibility and healing efficiency. Also, the polyurea can be quickly cured without any metal catalyst due to the presence of α‐siloxane, which is a highlight of this article, and it is better than the related literature reported so far. Through tensile test and healing performance test, SPUR1 possessing the both remarkable strength and elasticity was obtained (elongation at break 530 %, tensile stress 5.26 MPa). In addition, it has relatively satisfied healing efficiency 30 % and excellent thermal stability. In view of such intermolecular hydrogen bond, we have developed a special self‐healing polymer material with tunable performance by varying the monomer ratio between amino‐functionalized polyether PEA9 (the molecular weight Mn=900 Da) and PEA20 (Mn=2000 Da). A variety of self‐healing polymer materials can be lightly synthesized on account of the wide extent of commercial activities of raw materials such as primary amine, silane and isocyanate. Therefore, self‐healable polymer elastomers are exceptionally popular in a rank of fields, such as coatings, construction, aerospace, flexible electronics, automobile, medical device and lithium‐ion batteries.

Cobalt-Catalyzed Sequential Site- and Stereoselective Hydrosilylation of 1,3- and 1,4-Enynes.

Catalytic sequential hydrosilylation of 1,3-enynes and 1,4-enynes promoted by cobalt complexes derived from bisphosphines are presented. Site- and stereoselective Si-H addition of primary silanes to 1,3-enynes followed by sequential intramolecular diastereo- and enantioselective Si-H addition afforded enantioenriched cyclic alkenylsilanes with simultaneous construction of a carbon-stereogenic center and a silicon-stereogenic center. Reactions of 1,4-enynes proceeded through sequential isomerization of the alkene moiety followed by site- and stereoselective hydrosilylation. A wide range of alkenylsilanes were afforded in high efficiency and selectivity. Functionalization of the enantioenriched silanes containing a stereogenic center at silicon delivered a variety of chiral building blocks that are otherwise difficult to access.

Iron-Catalyzed H/D Exchange of Primary Silanes, Secondary Silanes, and Tertiary Siloxanes.

A synthetic study into the catalytic hydrogen/deuterium (H/D) exchange of 1° silanes, 2° silanes, and 3° siloxanes is presented, facilitated by iron-β-diketiminato complexes (1a and 1b). Near-complete H/D exchange is observed for a variety of aryl- and alkyl-containing hydrosilanes and hydrosiloxanes. The reaction tolerates alternative hydride source pinacolborane (HBpin), with quantitative H/D exchange. A synthetic and density functional theory (DFT) investigation suggests that a monomeric iron-deuteride is responsible for the H/D exchange.

Cobalt-Catalyzed Dehydrogenative C-H Silylation of Alkynylsilanes.

Herein, we report that a cobalt catalyst permits the general synthesis of substituted alkynylsilanes through dehydrogenative coupling of alkynylsilanes and hydrosilanes. Several silylated alkynes, including di‐ and trisubstituted ones, were prepared in a one‐step procedure. Thirty‐seven compounds were synthesized for the first time by applying our catalyst system. The alkynylsilanes bearing hydrosilyl moieties provide an opportunity for further functionalization (e. g., hydrosilylation). The use of primary silanes as substrates and precatalyst activators permits the use of inexpensive and easily accessible 3d metal precatalysts, and avoids the presence of additional activators.

Cyclic (Alkyl)(amino)carbene Lanthanide Amides: Synthesis, Structure, and Catalytic Selective Hydrosilylation of Alkenes.

The first examples of cyclic (alkyl)(amino)carbene (CAAC) lanthanide (Ln) complexes were synthesized from the reaction of CAAC with Yb[N(SiMe3)2]2 and Eu[N(SiMe3)2]2(THF)2 (THF = tetrahydrofuran). The structures of (CAAC)Yb[N(SiMe3)2]2 (2) and (CAAC)Eu[N(SiMe3)2]2(THF) (3) were determined by X-ray diffraction analysis. Density functional theory calculations of 2 revealed the predominantly ionic bond between the Ln ion and CAAC. Complex 3 enabled catalytic hydrosilylation of aryl- and silylalkenes with primary and secondary silanes in high yields and Markovnikov selectivity.

Iron-Catalyzed Regio- and Stereoselective Hydrosilylation of 1,3-Enynes To Access 1,3-Dienylsilanes.

A regio- and stereoselective hydrosilylation of 1,3-enynes with primary and secondary silanes to access 1,3-dienylsilanes is accomplished by employing an iron precatalyst bearing iminopyridine-oxazoline (IPO) ligand. The hydrosilylation proceeds via syn-addition of a Si-H bond to the alkyne group of 1,3-enynes, incorporating the silyl group at the site proximal to the alkene. The reaction features mild conditions, broad substrate scope, and good functional group tolerance. The synthetic utility was demonstrated by gram-scale reactions and further transformations.

Nickel-Catalyzed Hydrosilylation of Terminal Alkenes with Primary Silanes via Electrophilic Silicon-Hydrogen Bond Activation.

We report a simple and effective nickel-based catalytic system, NiCl2·6H2O/tBuOK, for the electrophilically activated hydrosilylation of terminal alkenes with primary silanes. This protocol provides excellent performance under mild reaction conditions: exclusive anti-Markovnikov selectivity, broad functional group tolerance (36 examples), and good scalability (TON = 5500). However, the secondary and tertiary silanes are not suitable. Mechanistic studies revealed that this homogeneous catalytic hydrosilylation includes an electrophilically activated Si-H bond process without the generation of nickel hydrides.