Rhodium-Catalyzed Hydrosilylation of Ketones: Catalyst Development and Mechanistic Insights

Unveiling the Reactivity of Part Per Million Levels of Cobalt-Salen Complexes in Hydrosilylation of Ketones

A series of air-stable cobalt(III)salen complexes Co-1 to Co-4 have been synthesized and employed in the hydrosilylation of ketones. Notably, the most intricately tailored Co-3 pre-catalyst exhibited exceptional catalytic activity under mild reaction conditions. The developed catalytic hydrosilylation protocol proceeded with an unusual ppm level (5 ppm) catalyst loading of Co-3 and achieved a maximum turnover number (TON) of 200,000. A wide variety of aromatic, aliphatic, and heterocyclic ketones encompassing both electron-donating and electron-withdrawing substituents were successfully transformed into the desired silyl ethers or secondary alcohols in moderate to excellent yields.

Hydrosilylation of Terminal Alkynes Catalyzed by an Air-Stable Manganese-NHC Complex

In recent years, catalysis with base metal manganese has received a significant amount of interest. Catalysis with manganese complexes having N-heterocyclic carbenes (NHCs) is relatively underdeveloped in comparison to the extensively investigated manganese catalysts possessing pincer ligands (particularly phosphine-based ligands). Herein, we describe the synthesis of two imidazolium salts decorated with picolyl arms (L₁ and L₂) as NHC precursors. Facile coordination of L₁ and L₂ with MnBr(CO)₅ in the presence of a base resulted in the formation manganese(I)-NHC complexes (1 and 2) as an air-stable solid in good isolated yield. Single-crystal X-ray analysis revealed the structure of the cationic complexes [Mn(CO)₃(NHC)][PF₆] with tridentate N,C,N binding of the NHC ligand in a facile fashion. Along with a few known manganese(I) complexes, these Mn(I)-NHC complexes 1 and 2 were tested for the hydrosilylation of terminal alkynes. Complex 1 was proved to be an effective catalyst for the hydrosilylation of terminal alkynes with good selectivity toward the less thermodynamically stable β-(Z)-vinylsilanes. This method provided good regioselectivity (anti-Markovnikov addition) and stereoselectivity (β-(Z)-product). Experimental evidence suggested that the present hydrosilylation pathway involved an organometallic mechanism with manganese(I)-silyl species as a possible reactive intermediate.

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.

Aluminium-Catalyzed Selective Hydroboration of Esters and Epoxides to Alcohols: C-O Bond Activation

In this work, the molecular aluminium dihydride complex bearing an N, N'-chelated conjugated bis-guanidinate (CBG) ligand is used as a catalyst for reducing a wide range of aryl and alkyl esters with good tolerance of alkene (C=C), alkyne (C≡C), halides (Cl, Br, I and F), nitrile (C≡N), and nitro (NO₂ ) functionalities. Further, we investigated the catalytic application of aluminium dihydride in the C-O bond cleavage of alkyl and aryl epoxides into corresponding branched Markovnikov ring-opening products. In addition, the chemoselective intermolecular reduction of esters over other reducible functional groups, such as amides and alkenes, has been established. Intermediates are isolated and characterized by NMR and HRMS studies, which confirm the probable catalytic cycles for the hydroboration of esters and epoxides.

Unified Total Synthesis of the Limonoid Alkaloids: Strategies for the De Novo Synthesis of Highly Substituted Pyridine Scaffolds

Highly substituted pyridine scaffolds are found in many biologically active natural products and therapeutics. Accordingly, numerous complementary de novo approaches to obtain differentially substituted pyridines have been disclosed. This article delineates the evolution of the synthetic strategies designed to assemble the demanding tetrasubstituted pyridine core present in the limonoid alkaloids isolated from Xylocarpus granatum, including xylogranatopyridine B, granatumine A and related congeners. In addition, NMR calculations suggested structural misassignment of several limonoid alkaloids, and predicted their C3-epimers as the correct structures, which was further validated unequivocally through chemical synthesis. The materials produced in this study were evaluated for cytotoxicity, anti-oxidant effects, anti-inflammatory action, PTP1B and Nlrp3 inflammasome inhibition, which led to compelling anti-inflammatory activity and anti-oxidant effects being discovered.

Applications of catalysis in hydroboration of imines, nitriles, and carbodiimides

The catalytic hydroboration of imines, nitriles, and carbodiimides is a powerful method of preparing amines which are key synthetic intermediates in the synthesis of many value-added products. Imine hydroboration has perennially featured in notable reports while nitrile and carbodiimide hydroboration have gained attention recently. Initial developments in catalytic hydroboration of imines and nitriles employed precious metals and typically required harsh reaction conditions. More recent advances have shifted toward the use of base metal and main group element catalysis and milder reaction conditions. In this survey, we review metal and nonmetal catalyzed hydroboration of these unsaturated organic molecules and group them into three distinct categories: precious metals, base metals, and main group catalysts. The TON and TOF of imine hydroboration catalysts are reported and summarized with a brief overview of recent advances in the field. Mechanistic and kinetic studies of some of these protocols are also presented.

Hydroelementation of diynes

This review highlights the hydroelementation reactions of conjugated and separated diynes, which depending on the process conditions, catalytic system, as well as the type of reagents, leads to the formation of various products: enynes, dienes, allenes, polymers, or cyclic compounds. The presence of two triple bonds in the diyne structure makes these compounds important reagents but selective product formation is often difficult owing to problems associated with maintaining appropriate reaction regio- and stereoselectivity. Herein we review this topic to gain knowledge on the reactivity of diynes and to systematise the range of information relating to their use in hydroelementation reactions. The review is divided according to the addition of the E-H (E = Mg, B, Al, Si, Ge, Sn, N, P, O, S, Se, Te) bond to the triple bond(s) in the diyne, as well as to the type of the reagent used, and the product formed. Not only are the hydroelementation reactions comprehensively discussed, but the synthetic potential of the obtained products is also presented. The majority of published research is included within this review, illustrating the potential as well as limitations of these processes, with the intent to showcase the power of these transformations and the obtained products in synthesis and materials chemistry.

Hydrosilylation of carbonyls over electron-enriched Ni sites of intermetallic compound Ni3Ga heterogeneous catalyst

Nanoparticulate intermetallic compound Ni3Ga supported on SiO2 has emerged as a highly efficient catalyst for the hydrosilylation of carbonyls, such as aldehydes and ketones, at room temperature. Formation of electron-enriched Ni via alloying with Ga is the key to the catalytic performance.

2-Pyridone-stabilized iridium silylene/silyl complexes: structure and QTAIM analysis

Iridium(iii) complexes of the general formula [Ir(X)(κ2-NSiiPr2)2] (NSiiPr2 = (4-methyl-pyridine-2-yloxy)diisopropylsilyl; X = Cl, 3; CF3SO3, 5; CF3CO2, 6) have been prepared and fully characterized, including X-ray diffraction studies and theoretical calculations. The presence of isopropyl substituents at the silicon atom favours the monomeric structure found in complexes 3 and 5. The short Ir-Si bond distances (2.25-2.28 Å) indicate some degree of base-stabilized silylene character of the Ir-Si bond in 3, 5 and 6 assisted by the 2-pyridone moiety. However, the shortening of these Ir-Si bonds might be a consequence of the constrained 2-pyridone geometry, and consequently the silyl character of these bonds can not be excluded. A DFT theoretical study on the nature of the Ir-Si bonds has been performed for complex 3 as well as for four other iridium complexes finding representative examples of different bonding situations between Ir and Si atoms: silylene, base-assisted silylene (both with an anionic base and with a neutral base), and silyl bonds, using the topological properties of the electron charge density. The results of these studies show that the Ir-Si bonds in Ir-NSiiPr2 complexes can be considered as an intermediate between the base-stabilized silylene and silyl cases, and therefore they have been proposed as 2-pyridone-stabilized iridium silylene/silyl bonds.

A Redox-Active Heterobimetallic N-Heterocyclic Carbene Based on a Bis(imino)pyrazine Ligand Scaffold

A new redox-active N-heterocyclic carbene (NHC) architecture is obtained using N-methylated pyrazinediimine iron complexes as precursors. The new species exhibit strong π-accepting/σ-donating properties and are able to ligate two metal centres simultaneously. The redox activity was demonstrated by the reversible chemical oxidation of a heterobimetallic Fe0 /RhI example, which affords an isolable ligand-based radical cation. The reversible redox process was then applied in the catalytic hydrosilylation of 4,4'-difluorobenzophenone, where the reaction rate could be reversibly controlled as a function of the catalyst oxidation state. The new NHC exhibits high electrophilicity and nucleophilicity, which was demonstrated in the reversible activation of alcohols and amines. The electronic structure of the resulting complexes was investigated through various spectroscopic and computational methods.

Zinc Hydride-Catalyzed Hydrofuntionalization of Ketones

Three new dimeric bis-guanidinate zinc(II) alkyl, halide, and hydride complexes [LZnEt]₂ (1), [LZnI]₂ (2) and [LZnH]₂ (3) were prepared. Compound 3 was successfully employed for the hydrosilylation and hydroboration of a vast number of ketones. The catalytic performance of 3 in the hydroboration of acetophenone exhibits a turnover frequency, reaching up to 5800 h^-1, outperforming that of reported zinc hydride catalysts. Notably, both intra- and intermolecular chemoselective hydrosilylation and hydroboration reactions have been investigated.

Pseudo-enantiomeric carbohydrate-based N-heterocyclic carbenes as promising chiral ligands for enantiotopic discrimination

The practical synthesis of carbohydrate-based NHC-Rh complexes bearing C1 or C3 sterically differentiated positions, accessed by glycosylation or SNAr strategies, is reported. These catalysts exhibit pseudo-enantiomeric behaviour in the hydrosilylation of acetophenone. We show that steric bulk at C1 gives preference for (S)-phenyl-1-ethanol, while bulk at C3 leads to the (R)-enantiomer. These results represent the first example of pseudo-enantiomeric carbohydrate-based NHC ligands leading to enantiotopic discrimination.

Hydrosilylation of Esters Catalyzed by Bisphosphine Manganese(I) Complex: Selective Transformation of Esters to Alcohols

Selective and efficient hydrosilylations of esters to alcohols by a well-defined manganese(I) complex with a commercially available bisphosphine ligand are described. These reactions are easy alternatives for stoichiometric hydride reduction or hydrogenation, and employing cheap, abundant, and nonprecious metal is attractive. The hydrosilylations were performed at 100 °C under solvent-free conditions with low catalyst loading. A large variety of aromatic, aliphatic, and cyclic esters bearing different functional groups were selectively converted into the corresponding alcohols in good yields.

Catalytic Reductive Aldol and Mannich Reactions of Enone, Acrylate, and Vinyl Heteroaromatic Pronucleophiles

Catalytic reductive coupling of enone, acrylate, or vinyl heteroaromatic pronucleophiles with carbonyl or imine partners offers an alternative to base-mediated enolization in aldol- and Mannich-type reactions. In this review, direct catalytic reductive aldol and Mannich reactions are exhaustively catalogued on the basis of metal or organocatalyst. Stepwise processes involving enone conjugate reduction to form discrete enol or (metallo)enolate derivatives followed by introduction of carbonyl or imine electrophiles and aldol reactions initiated via enone conjugate addition are not covered.

Enantioselective hydrosilylation of unsaturated carbon-heteroatom bonds (C[double bond, length as m-dash]N, C[double bond, length as m-dash]O) catalyzed by [Ru-S] complexes: a theoretical study

A detailed theoretical study on the mechanism of enanthioselective hydrosilylation of imines and ketones catalyzed by the ruthenium(ii) thiolate catalyst [Ru–S] ([L*-Ru(SDmp)]⁺[BAr₄^F]⁻) with a chiral monodentate phosphine ligand is carried out in this work. We elucidate all the pathways leading to the main products or by products mediated by the [Ru–S] complex in order to have deep understanding of the chemoselectivity and enantioselectivity. The DFT (Density Functional Theory) calculations show that the reaction mechanism including: (1) Si–H bond cleavage by the dual activity of Ru–S bond; (2) the generation of a sulfur-stabilized silane cation; (3) the electrophilic attack of silane cation to NC/OC; (4) hydrogen transfer from Ru to carbon cation. The hydrosilylation products are found to be the final products rather than the dehydrogenative ones, which is consistent with the experimental results. The dehydrogenative silylation reaction pathways which give N- or O-silylated enamine/enol ether are reversible according to our calculations. The computational results also show that the electrophilic attack of silicon to NC/OC is the rate-determining step and the ee value can be improved significantly with more bulky model phosphine ligand based on the same calculation methods.

A detailed theoretical study on the mechanism of enanthioselective hydrosilylation of imines and ketones catalyzed by the ruthenium(ii) thiolate catalyst with a chiral monodentate phosphine ligand is carried out in this work.

Supersilyl as an effective monodentate ligand to stabilize four-coordinate manganese(ii) complexes

Supersilyl, –Si(SiMe₃)₃, serves as an effective ligand to afford a series of four-coordinate manganese(ii) complexes.

Emergence and Applications of Base Metals (Fe, Co, and Ni) in Hydroboration and Hydrosilylation

Base metal catalysis offers an alternative to reactions, which were once dominated by precious metals in hydrofunctionalization reactions. This review article details the development of some base metals (Fe, Co, and Ni) in the hydroboration and hydrosilylation reactions concomitant with a brief overview of recent advances in the field. Applications of both commercially available metal salts and well-defined metal complexes in catalysis and opportunities to further advance the field is discussed as well.

Theoretical Study on the Rhodium-Catalyzed Hydrosilylation of C═C and C═O Double Bonds with Tertiary Silane

Reaction mechanisms of hydrosilylation of ketone and alkene with tertiary silane using the Wilkinson-type catalyst were theoretically investigated on the basis of density functional calculations using ωB97XD functional. Previously proposed three mechanisms, the Chalk-Harrod (CH) mechanism, the modified Chalk-Harrod (mCH) mechanism, and the outer-sphere mechanism were examined. Besides, we also found two mechanisms, the alternative CH (aCH) mechanism and the double hydride (DH) mechanism. In the aCH mechanism, a four-coordinate rhodium hydride complex formed through the elimination of R₃Si-Cl is a catalytically active species. In the DH mechanism, the active species is a six-coordinate complex with two Rh-H bonds. For the C═O double bond hydrosilylation, the rate-determining steps of the aCH and DH mechanisms are both acetone insertion into the Rh-H bond, and the order of the activation barrier is DH < aCH ≈ CH < mCH. For the C═C double bond hydrosilylation, except for the mCH pathway whose rate-determining step is the hydrosilane addition reaction, the rate-determining steps of the CH, aCH, and DH pathways are Si-C reductive elimination reactions. The order of the energy barrier is DH ≈ mCH < aCH ≈ CH. In the outer-sphere mechanism, no stable intermediate or transition state was found. Consequently, we concluded that the DH mechanism is adopted as the mechanism for the Rh-catalyzed hydrosilylation of the carbonyl group while the mCH or DH mechanism is adopted as the mechanism for alkenes under conditions where their active intermediates are formed. The present result revises a hypothesis that the hydrosilylation of the carbonyl group is in general accomplished by the mCH mechanism. The active species in the DH mechanism has one more extra Rh-H bond compared to that of the other pathways, and its interaction with a silyl group, trans-influence, and small steric effect are the origin of the highly efficient catalytic activity, which was not reported before.

New anionic rhodium complexes as catalysts for the reduction of acetophenone and its derivatives

New anionic rhodium(iii) complexes, obtained by a simple reaction of RhCl3 with organic chlorides (derivatives of imidazole and pyridine), have been employed as catalysts for hydrosilylation (reduction) of acetophenone derivatives. The reactions, in which 1,1,1,3,5,5,5-heptamethyltrisiloxane was a reducing agent, proceeded in a biphasic system because the above complexes are insoluble in the reaction medium. Thereby easy isolation of the complexes from post-reaction mixtures was possible after reaction completion. This is the first example of the application of rhodium complexes of this type as catalysts for ketone reduction. The complexes have shown high activity and enabled obtaining the hydrosilylation product in a very short time and in the range of low concentrations (0.1 mol%). By using FT-IR in situ analysis that enables measuring product concentrations in real time, a comparison has been made of the catalytic activity for hydrosilylation of acetophenone and methoxyacetophenone isomers shown by four rhodium complexes ([C+][RhCl4 -]) differing in cations and the most effective catalyst for this process has been distinguished.

A phosphine-stabilized silylene rhodium complex

The first example of a phosphine stabilized silylene rhodium complex.

[Rh(L-alaninate)(1,5-Cyclooctadiene)] Catalyzed Helix-Sense-Selective Polymerizations of Achiral Phenylacetylenes

The [Rh(L-alaninate)(cod)] (cod = 1,5-Cyclooctadiene) complex was synthesized and characterized. Asymmetric polymerizations of achiral phenylacetylene with two hydroxyl groups and a dodecyl group (DoDHPA) were performed by using the rhodium complex as the catalyst to provide polymers with a higher molecular weight (>10⁵) than the polymers obtained using the [Rh(cod)Cl]₂ initiator systems. The resulting polymers showed circular dichroism (CD) signals at approximately 310 and 470 nm, indicating that they have a preferential one-handed helical structure. The helix sense in the polymer main chain was controlled by the sign of the catalyst chirality. These findings suggest that the rhodium complex with a chiral amine is the true active species for the helix-sense-selective polymerization of DoDHPA. The [Rh(L-alaninate)(cod)] complex also exhibits high catalytic activity in the polymerization of phenylacetylene (PA) to give a high yield and molecular weight. All these results demonstrate that this Rh complex is an excellent catalyst for the polymerization of phenylacetylene monomers.

Manganese-Catalyzed Hydrosilylation Reactions

Over the past few decades, manganese-catalyzed hydrosilylation of C=O or C=C/C≡C unsaturated bonds have undergone enormous developments. In this focus review, the hydrosilylation reactions of alkenes, alkynes, and carbonyl-containing substrates catalyzed by manganese complexes are summarized. Moreover, the mechanisms of the manganese-catalyzed hydrosilylation are briefly discussed.

Cobalt–nickel alloy catalysts for hydrosilylation of ketones synthesized by utilizing metal–organic framework as template

The facile synthesis of CoNi@NC materials from a MOF precursor is reported along with the catalytic properties in ketone hydrosilylation.

Unveiling the role of ancillary ligands in acceptorless benzyl alcohol dehydrogenation and etherification mediated by mesoionic carbene iridium complexes

We synthesized a set of triazolylidene iridium(iii) complexes [IrCp*(C^N)L]ⁿ⁺ (Cp* = pentamethylcyclopentadienyl, C^N = C,N-bidentate coordinating pyridyl-triazolylidene) containing different neutral or anionic ancillary ligands L and evaluated their impact on the catalytic activity in alcohol conversion. We demonstrate that these ancillary ligands have a strong influence on the catalytic selectivity and direct whether the iridium center preferentially catalyzes either the dehydrogenation or the dehydration of benzyl alcohol. Ligand exchange experiments provide a direct correlation of ligand lability with catalytic activity and selectivity. These results underline the relevance of ancillary ligands and provide a rational approach to tailor the catalytic activity of the iridium center towards aldehyde formation (loss of H₂) or etherification (elimination of H₂O).

Rh-catalyzed regiodivergent hydrosilylation of acyl aminocyclopropanes controlled by monophosphine ligands

A Rh-catalyzed regiodivergent hydrosilylation of acyl aminocyclopropanes has been developed. Acyl aminocyclopropanes were reacted with hydrosilanes in the presence of Rh catalysts to afford ring-opened hydrosilylated adducts through carbon-carbon (C-C) bond cleavage of the cyclopropane ring. The regioselectivity of the addition of silanes (linear or branched) can be switched by changing the monophosphine ligand. This C-C bond cleavage/hydrosilylation methodology is applicable to the synthesis of silanediol precursors.

Ether formation through reductive coupling of ketones or aldehydes catalyzed by a mesoionic carbene iridium complex

A promiscuous triazolylidene iridium complex efficiently catalyses the etherification of ketones and aldehydes and also promotes deoxygenation and hydrosilylation.