Recent advances in catalytic hydrosilylation

Notable Catalytic Activity of Transition Metal Thiolate Complexes against Hydrosilylation and Hydroboration of Carbon-Heteroatom Bonds

Chemists tend to use transition metal hydride complexes rather than thiolate complexes to catalyse chemical transformations because the hydride complexes possess diverse catalytic reactivity, although most of them are air/moisture-sensitive and difficult to prepare. By comparing the catalytic performances of pincer ligated group 10 metal thiolate and hydride complexes in catalysing the hydroboration and hydrosilylation of C=O and C=N bonds, we demonstrate in this review that transition metal thiolate complexes are much better catalysts than the corresponding hydride complexes in catalysing this type of reactions. Many hydroboration and hydrosilylation reactions catalysed by pincer ligated group 10 metal hydride complexes can also be catalysed by the corresponding thiolate complexes and the thiolate systems are far more active. Therefore, the applications of transition metal thiolate complexes in the catalytic hydroboration and hydrosilylation of unsaturated carbon-heteroatom bonds deserve special attention in future work.

Direct synthesis and applications of solid silylzinc reagents

The increased synthetic utility of organosilanes has motivated researchers to develop milder and more practical synthetic methods. Silylzinc reagents, which are typically the most functional group tolerant, are notoriously difficult to synthesize because they are obtained by a pyrophoric reaction of silyllithium, particularly Me3SiLi which is itself prepared by the reaction of MeLi and disilane. Furthermore, the dissolved LiCl in silylzinc may have a detrimental effect. A synthetic method that can avoid silyllithium and involves a direct synthesis of silylzinc reagents from silyl halides is arguably the simplest and most economical strategy. We describe, for the first time, the direct synthesis of PhMe2SiZnI and Me3SiZnI reagents by employing a coordinating TMEDA ligand, as well as single crystal XRD structures. Importantly, they can be obtained as solids and stored for longer periods at 4 °C. We also demonstrate their significance in cross-coupling of various free alkyl/aryl/alkenyl carboxylic acids with broader functional group tolerance and API derivatives. The general applicability and efficiency of solid Me3SiZnI are shown in a wide variety of reactions including alkylation, arylation, allylation, 1,4-addition, acylation and more.

Pt-Ligand single-atom catalysts: tuning activity by oxide support defect density

Metal–ligand coordination stabilizes single atom Pt on pristine and defective TiO₂ supports to impact local coordination and catalytic hydrosilylation activity.

Magnesium hydride alkene insertion and catalytic hydrosilylation

The β-diketiminato magnesium hydride, [(BDI)MgH]]₂, reacts with alkenes and catalyses their hydrosilylation with PhSiH₃.

The dimeric β-diketiminato magnesium hydride, [(BDI)MgH]₂, reacts at 80 °C with the terminal alkenes, 1-hexene, 1-octene, 3-phenyl-1-propene and 3,3-dimethyl-butene to provide the respective n-hexyl, n-octyl, 3-phenylpropyl and 3,3-dimethyl-butyl magnesium organometallics. The facility for and the regiodiscrimination of these reactions are profoundly affected by the steric demands of the alkene reagent. Reactions with the phenyl-substituted alkenes, styrene and 1,1-diphenylethene, require a more elevated temperature of 100 °C with styrene providing a mixture of the 2-phenylethyl and 1-phenylethyl products over 7 days. Although the reaction with 1,1-diphenylethene yields the magnesium 1,1-diphenylethyl derivative as the sole reaction product, only 64% conversion was achieved over a 21 day timeframe. Reactions with the α,ω-dienes, 1,5-hexadiene and 1,7-octadiene, provided divergent results. The initial 5-alkenyl magnesium reaction product of the shorter chain diene undergoes 5-exo-trig cyclisation via intramolecular carbomagnesiation to provide a cyclopentylmethyl derivative, which was shown by X-ray diffraction analysis to exist as a three-coordinate monomer. In contrast, 1,7-octadiene provided a mixture of two compounds, a magnesium oct-7-en-1-yl derivative and a dimagnesium-octane-1,4-diide, as a result of single or two-fold activation of the terminal C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C double bonds. The magnesium hydride was unreactive towards internal alkenes apart from the strained bicycle, norbornene, allowing the characterisation of the resultant three-coordinate magnesium norbornyl derivative by X-ray diffraction analysis. Computational analysis of the reaction between [(BDI)MgH]₂ and 1-hexene using density functional theory (DFT) indicated that the initial Mg–H/C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C insertion process is rate determining and takes place at the intact magnesium hydride dimer. This exothermic reaction (ΔH = –14.1 kcal mol^–1) traverses a barrier of 18.9 kcal mol^–1 and results in the rupture of the dinuclear structure into magnesium alkyl and hydride species. Although the latter three-coordinate hydride derivative may be prone to redimerisation, it can also provide a further pathway to magnesium alkyl species through its direct reaction with a further equivalent of 1-hexene, which occurs via a lower barrier of 15.1 kcal mol^–1. This Mg–H/C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C insertion reactivity provides the basis for the catalytic hydrosilylation of terminal alkenes with PhSiH₃, which proceeds with a preference for the formation of the anti-Markovnikov organosilane product. Further DFT calculations reveal that the catalytic reaction is predicated on a sequence of Mg–H/C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C insertion and classical Si–H/Mg–C σ-bond metathesis reactions, the latter of which, with a barrier height of 24.9 kcal mol^–1, is found to be rate determining.

Chemoselective nitro reduction and hydroamination using a single iron catalyst

An amine-bis(phenolate) iron(iii) complex catalyses both the chemoselective reduction of nitroarenes and the formal hydroamination of highly substituted olefins.

The reduction and reductive addition (formal hydroamination) of functionalised nitroarenes is reported using a simple and bench-stable iron(iii) catalyst and silane. The reduction is chemoselective for nitro groups over an array of reactive functionalities (ketone, ester, amide, nitrile, sulfonyl and aryl halide). The high activity of this earth-abundant metal catalyst also facilitates a follow-on reaction in the reductive addition of nitroarenes to alkenes, giving efficient formal hydroamination of olefins under mild conditions. Both reactions offer significant improvements in catalytic activity and chemoselectivity and the utility of these catalysts in facilitating two challenging reactions supports an important mechanistic overlap.

Hydrosilylation reaction of olefins: recent advances and perspectives

This review focuses on the recent development of efficient, selective, and cheaper hydrosilylation catalyst systems appearing in the last decade.

Tandem deuteration/hydrosilylation reactions catalyzed by a rhodium carbene complex under solvent-free conditions

The complex [Rh(I(t)Bu)(2)HCl] has been shown to be an active catalyst in the hydrosilylation of carbonyl and imine complexes. This reactivity, combined with the previously reported H/D exchange catalyzed by these complexes allows for a one pot, two step reaction using a single catalyst for both H/D exchange and hydrosilylation. Using triethylsilane, [Rh(I(t)Bu)(2)Cl] catalyst, and D(2) gas, deuterated silyl-ethers can be synthesized in an atom-economical, solvent-free reaction.

Probing the catalytic potential of chloro nitrosyl rhenium(I) complexes

The reduction of the mononitrosyl Re(II) salt [NMe(4)](2)[ReCl(5)(NO)] (1) with zinc in acetonitrile afforded the Re(i) dichloride complex [ReCl(2)(NO)(CH(3)CN)(3)] (2). Subsequent ligand substitution reactions with PCy(3), PiPr(3) and P(p-tolyl)(3) afforded the bisphosphine Re(i) complexes [ReCl(2)(NO)(PR(3))(2)(CH(3)CN)] (3, R = Cy a, iPr b, p-tolyl c) in good yields. The acetonitrile ligand in 3 is labile, permitting its replacement with H(2) (1 bar) to afford the dihydrogen Re(I) complexes [ReCl(2)(NO)(PR(3))(2)(η(2)-H(2))] (4, R = Cy a, iPr b). The catalytic activity of 2, 3 and 4 in hydrogen-related catalyses including dehydrocoupling of Me(2)NH·BH(3), dehydrogenative silylation of styrenes, and hydrosilylation of ketones and aryl aldehydes were investigated, with the main focus on phosphine and halide effects. In the dehydrocoupling of Me(2)NH·BH(3), the phosphine-free complex 2 exhibits the same activity as the bisphosphine-substituted systems. In the dehydrogenative silylation of styrenes, 3a and 4a bearing PCy(3) ligands exhibit high catalytic activities. Monochloro Re(I) hydrides [Re(Cl)(H)(NO)(PR(3))(2)(CH(3)CN)] (5, R = Cy a, iPr b) were proven to be formed in the initiation pathway. The phosphine-free complex 2 showed in dehydrogenative silylations even higher activity than the bisphosphine derivatives, which further emphasizes the importance of a facile phosphine dissociation in the catalytic process. In the hydrosilylation of ketones and aryl aldehydes, at least one rhenium-bound phosphine is required to ensure high catalytic activity.

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 (PCy(3))(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 (PCy(3))(2)(CO)RuHCl/HBF(4)·OEt(2) 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.

Hydrosilation of Carbonyl-Containing Substrates Catalyzed by an Electrophilic η-Silane Iridium(III) Complex

Hydrosilation of a variety of ketones and aldehydes using the cationic iridium catalyst, (POCOP)Ir(H)(acetone)(+), 1, (POCOP = 2,6-bis(di-tert-butyl phosphinito)phenyl) is reported. With triethyl silane, all but exceptionally bulky ketones undergo quantitative reactions employing 0.5 mol% catalyst in 20-30 min at 25 °C. Hydrosilation of esters and amides results in over-reduction and cleavage of C-O and C-N bonds, respectively. The diastereoselectivity of hydrosilation of 4-tert-butyl cyclohexanone has been examined using numerous silanes and is highly temperature dependent. Using EtMe(2)SiH, analysis of the ratio of cis:trans hydrosilation products as a function of temperature yields values for ΔΔH(‡) (ΔH(‡) (trans) - ΔH(‡) (cis)) and ΔΔS(‡) (ΔS(‡) (trans) - ΔS(‡)(cis)) of -2.5 kcal/mol and -6.9 e.u., respectively. Mechanistic studies show that the ketone complex, (POCOP)Ir(H)(ketone)(+), is the catalyst resting state and is in equilibrium with low concentration of the silane complex, (POCOP)Ir(H)(HSiR(3))(+). The silane complex transfers R(3)Si(+) to ketone forming the oxocarbenium ion, R(3)SiOCR'(2) (+), which is reduced by the resulting neutral dihydride 3, (POCOP)Ir(H)(2), to yield product R(3)SiOCHR'(2) and (POCOP)IrH(+) which closes the catalytic cycle.

Reactivity of Rhodium-Triflate Complexes with Diphenylsilane: Evidence for Silylene Intermediacy in Stoichiometric and Catalytic Reactions

Addition of Ph2SiH2 to [Rh(iPr3P)2(OTf)] (1) yielded the thermally unstable RhIII adduct [Rh(iPr3P)2(OTf)(H)(SiPh2H)] (2), which decomposed to [Rh(iPr3P)2(H)2(OTf)] (3), liberating (unobserved) silylene. The silylene was trapped by 1, resulting in the RhI-silyl complex [Rh(iPr3P)2(SiPh2OTf)]. Complex 3 was converted to 2 by addition of diphenylsilane, providing a basis for a possible catalytic cycle. The last reaction did not involve a RhI intermediate, as shown by a labeling study. Complex 1 catalyzed the dehydrogenative coupling of Ph2SiH2 to Ph2HSi--SiHPh2. A mechanism involving a silylene intermediate in this catalytic cycle is proposed. The mechanism is supported by complete lack of catalysis in the case of the tertiary silanes Ph2MeSiH and PhMe2SiH, and by a study of individual steps of the catalytic cycle. The outcome of the reaction of Ph2SiH2 with styrene in the presence of 1 depends on the complex/substrate ratio; under stoichiometric conditions olefin hydrogenation prevailed over hydrosilylation, whereas with excess of substrates hydrosilylation prevailed. Catalytic hydrosilylation resulted in double addition giving Ph2Si(CH2CH2Ph)2. Mechanistic aspects of the reported processes are discussed, and a new hydrosilylation mechanism based on silylene intermediacy is proposed.