Highly Efficient Solvent-Free Cyanosilylation of Aldehydes Catalyzed by Bilayered Two-Dimensional Metal-Organic Frameworks

While metal-organic frameworks (MOFs) are promising for heterogeneous catalysis and mechanistic investigations due to their well-defined active sites, high porosity, large surface area, and structural tunability, two-dimensional (2D) MOFs exhibit superior catalytic efficiency over three-dimensional (3D) MOFs by offering higher specific surface areas and reduced diffusion limitations, thus enhancing active site exposure and substrate accessibility. In this work, two novel bilayered 2D MOFs, Mg3(TPHB)2(H2O)6·(DMF)12 (Mg-TPHB-MOF) and Mn3(TPHB)2(H2O)6·(DMF)16 (Mn-TPHB-MOF), were synthesized, and their efficacy as Lewis acid catalysts in the solvent-free cyanosilylation of aldehydes was investigated. Both MOFs exhibited remarkable catalytic activity, with Mg-TPHB-MOF achieving complete conversion of benzaldehyde within 10 min at room temperature. This catalytic performance surpasses that of many previously reported MOF catalysts for the same reaction, highlighting the potential of 2D MOFs in catalysts. To gain deeper insights into the catalytic mechanism and the superior performance of Mg-TPHB-MOF, computational studies were conducted to determine the free energy profiles of the reaction pathways with and without the catalyst. The results of these calculations corroborated the experimental findings, indicating a lower energy barrier for the Mg-TPHB-MOF-catalyzed reaction. Furthermore, both MOFs demonstrated good reusability over five catalytic cycles, maintaining high conversion rates and structural integrity.

Computational study on catalyst-free BCl3-promoted chloroboration of carbonyl compounds

DFT calculations were performed to investigate the possible reaction mechanisms underlying catalyst-free chloroboration reactions of carbonyl compounds with BCl3. The interaction between BCl3 and the C[double bond, length as m-dash]O moiety of carbonyl compounds is a two-step reaction. In the first step, B of BCl3 forms a bond with the O of the C[double bond, length as m-dash]O moiety, followed by the 1,3-Cl migration process from BCl3 to the C of the carbonyl group. To indicate the versatility of our synthetic methodology, a catalyst-free chloroboration of a variety of aldehydes and ketones with a broad range of electron-donating and electron-withdrawing groups with BCl3 was checked. According to DFT results, BCl3-induced chloroboration of aldehydes and ketones progressed under a kinetically favorable condition with <20 kcal mol-1 of activation free energy.

Low-valent germanium and tin hydrides as catalysts for hydroboration, hydrodeoxygenation (HDO), and hydrodesulfurization (HDS) of heterocumulenes

The low-valent germanium and tin hydrides, [LMH; L = {(ArHN)(ArN)-CN-C(NAr)(NHAr); Ar = 2,6-Et2-C6H3}; M = Ge; (Ge-1), Sn (Sn-2)] bearing bis-guanidinato anions are employed as catalysts for chemoselective reduction of heterocumulenes via hydroboration reactions. This protocol demonstrates that a wide range of carbodiimides (CDI), isocyanates, isothiocyanates, and isoselenocyanates undergo partial reduction, yielding the corresponding N-boryl formamidine, N-boryl formamide, N-boryl thioformamide, and N-boryl selenoformamide products, respectively. Isocyanates and isothiocyanates are further converted into N-boryl methyl amines through hydrodeoxygenation (HDO) and hydrodesulfurization (HDS) reactions in the presence of catalyst Ge-1. Additionally, catalyst Sn-2 exhibits excellent inter and intra-molecular chemoselectivity over other functional groups. Based on stoichiometric experiments, a plausible catalytic cycle for chemoselective hydroboration of heterocumulenes is proposed.

Alkylaluminum Complexes Featuring Bridged Bis-Formylfluorenimide Ligands for Hydroboration of Aldehyde, Ketone, and Imines

Three bis-formylfluorenimide ligands with different bridging groups were designed and synthesized, leading to the successful preparation of six novel alkylaluminum complexes through their reaction with alkylaluminum reagents (AlMe3 or AlEt3). Complexes 1 and 2 were obtained by the reaction of 1,2-propylene-bridged diamine (L1) with AlMe3 or AlEt3. By reacting 1,2-cyclohexylene-bridged diamine (L2) with AlMe3 or AlEt3 to obtain complexes 3 and 4. The above ligands formed a bidentate four-coordinate structure with alkylaluminum, which involved the elimination of one alkyl group as the ligand reacted with alkylaluminum. The complexes 5 and 6 were synthesized through the reaction of 1,2-phenylene-bridged diamine (L3) with AlEt3 in toluene or tetrahydrofuran. Notably, L3 exhibited unique reactivity compared with the other ligands, which formed a tridentate four-coordinated structure when reacting with alkylaluminum. The formation of the tridentate complex resulted from the introduction of a benzimidazole derivative or tetrahydrofuran (THF) molecule along with the elimination of two alkyl groups during its coordination with alkylaluminum. All complexes were characterized via 1H NMR, 13C NMR, and elemental analysis, with structural determination confirmed through X-ray. Furthermore, the catalytic activity in the hydroboration reaction of aldehyde, ketone, and imines was investigated with these complexes as catalysts. Among them, complex 1 demonstrated excellent catalytic performance (up to 99% yield) and broad substrate compatibility (more than 30 substrates) at low catalyst loading (1 mol %) under mild reaction conditions.

Hydroboration and hydrosilylation of alkenes catalyzed by an unsymmetrical magnesium methyl complex

The hydroboration and hydrosilylation of alkenes catalyzed by the unsymmetrical β-diketiminate magnesium methyl complex [(DippXylNacnac)MgMe (THF)] (1) have been reported. When complex 1 was employed as a highly efficient catalyst in the hydroboration of various alkenes with HBpin, only the anti-Markovnikov hydroboration products were obtained in high yields and with high regioselectivities under mild reaction conditions (60 °C). To our surprise, it showed different regioselectivities in the hydrosilylation of a range of alkenes with PhSiH3. Aromatic alkene substrates afforded the corresponding branched Markovnikov hydrosilylation products in high yields and with high regioselectivities; conversely, aliphatic alkenes produced the linear anti-Markovnikov products in moderate yields. This is completely consistent with the corresponding density functional theory (DFT) calculations. In addition, the practical utility was demonstrated via scale-up reactions of boronate esters and a preliminary plausible mechanism of hydroboration and hydrosilylation have been investigated as well.

Applications of low-valent compounds with heavy group-14 elements

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

Controlled reduction of isocyanates to formamides using monomeric magnesium

This work describes a transition metal-free methodology involving an efficient and controlled reduction of isocyanates to only formamide derivatives using pinacolborane (HBpin) as the hydrogenating agent and a bis(phosphino)carbazole ligand stabilized magnesium methyl complex (1) as the catalyst. A large number of substrates undergo selective hydroboration and give exclusively N-boryl formamides.

Multifaceted behavior of a doubly reduced arylborane in B-H-bond activation and hydroboration catalysis

Alkali-metal salts of 9,10-dimethyl-9,10-dihydro-9,10-diboraanthrancene (M₂[DBA-Me₂]; M⁺ = Li⁺, Na⁺, K⁺) activate the H-B bond of pinacolborane (HBpin) in THF already at room temperature. For M⁺ = Na⁺, K⁺, the addition products M₂[4] are formed, which contain one new H-B and one new B-Bpin bond; for M⁺ = Li⁺, the H^- ion is instantaneously transferred from the DBA-Me₂ unit to another equivalent of HBpin to afford Li[5]. Although Li[5] might commonly be considered a [Bpin]^- adduct of neutral DBA-Me₂, it donates a [Bpin]⁺ cation to Li[SiPh₃], generating the silyl borane Ph₃Si-Bpin; Li₂[DBA-Me₂] with an aromatic central B₂C₄ ring acts as the leaving group. Furthermore, Li₂[DBA-Me₂] catalyzes the hydroboration of various unsaturated substrates with HBpin in THF. Quantum-chemical calculations complemented by in situ NMR spectroscopy revealed two different mechanistic scenarios that are governed by the steric demand of the substrate used: in the case of the bulky Ph(H)C[double bond, length as m-dash]NtBu, the reaction requires elevated temperatures of 100 °C, starts with H-Bpin activation which subsequently generates Li[BH₄], so that the mechanism eventually turns into "hidden borohydride catalysis". Ph(H)C[double bond, length as m-dash]NPh, Ph₂C[double bond, length as m-dash]O, Ph₂C[double bond, length as m-dash]CH₂, and iPrN[double bond, length as m-dash]C[double bond, length as m-dash]NiPr undergo hydroboration already at room temperature. Here, the active hydroboration catalyst is the [4 + 2] cycloadduct between the respective substrate and Li₂[DBA-Me₂]: in the key step, attack of HBpin on the bridging unit opens the bicyclo[2.2.2]octadiene scaffold and gives the activated HBpin adduct of the Lewis-basic moiety that was previously coordinated to the DBA-B atom.

Donor free stibenium cation as an efficient cyanosilylation catalyst

The synthesis of novel stibenium cations and their catalytic application in cyanosilylation of carbonyl compounds have been described. Treatment of chlorostibine L₁SbCl [L₁ = 1,2-C₆H₄{N(CH₂^tBu)}₂] (2) with 1 equiv. of AgOTf and AgSbF₆ resulted in the formation of donor free L₁SbOTf (3) and [L₁Sb]⁺[SbF₆]^- (4), respectively. Among these three compounds, 4 exhibits excellent catalytic activity towards the cyanosilylation of aldehydes and ketones.

Electrochemical hydroboration of carbonyl compounds

A green and sustainable electrochemical hydroboration of carbonyl compounds with HBpin has been reported for the first time. Under catalyst-free and additive-free mild reaction conditions the corresponding boronic esters were obtained in excellent yields via the simple electrochemical hydroboration of various aldehydes and ketones with HBpin at room temperature. The scale-up reaction demonstrated potential practical applications. A plausible reaction mechanism was proposed based on the corresponding deuterium-labelling, radical inhibition and cyclic voltammetry experiments.

s-Block Metal Catalysts for the Hydroboration of Unsaturated Bonds

The addition of a B-H bond to an unsaturated bond (polarized or unpolarized) is a powerful and atom-economic tool for the synthesis of organoboranes. In recent years, s-block organometallics have appeared as alternative catalysts to transition-metal complexes, which traditionally catalyze the hydroboration of unsaturated bonds. Because of the recent and rapid development in the field of hydroboration of unsaturated bonds catalyzed by alkali (Li, Na, K) and alkaline earth (Mg, Ca, Sr, Ba) metals, we provide a detailed and updated comprehensive review that covers the synthesis, reactivity, and application of s-block metal catalysts in the hydroboration of polarized as well as unsaturated carbon-carbon bonds. Moreover, we describe the main reaction mechanisms, providing valuable insight into the reactivity of the s-block metal catalysts. Finally, we compare these s-block metal complexes with other redox-neutral catalytic systems based on p-block metals including aluminum complexes and f-block metal complexes of lanthanides and early actinides. In this review, we aim to provide a comprehensive, authoritative, and critical assessment of the state of the art within this highly interesting research area.

Lithium and magnesium complexes from the employment of pyridyl-pendanted unsymmetrical β-diketiminates: syntheses and utilization as catalysts for the hydroboration of carbonyl compounds

The push for environmentally benign and sustainable chemical processes has reinforced the demand to displace transition metals with cheap, nontoxic and naturally abundant metals. To fulfil this requirement, we endeavored...

Pyridylpyrrolido ligand in Ge(II) and Sn(II) chemistry: synthesis, reactivity and catalytic application

In our previous communication, we have reported the synthesis of a new chlorogermylene (B) featuring a pyridylpyrrolido ligand. This study details the preparation of a series of new germylenes and stannylenes starting from B. A transmetallation reaction between B and SnCl₂ led to the analogous chlorostannylene (1) with the simultaneous elimination of GeCl₂. This is a very unusual example of transmetallation between two elements of the same group. The preparation of 1via lithiation led to the formation of 2 as a side product, where the ortho C-H bond of the pyridine ring was activated and functionalized with a ⁿBu moiety. Subsequently, B and 1 were used as precursors to generate germylene (4) and stannylene (5) featuring tris(trimethylsilyl)silyl (hypersilyl) moieties. We also prepared tetrafluoropyridyl germylene (6) by reacting 4 with C₅F₅N with the simultaneous elimination of (Me₃Si)₃SiF by utilizing the fluoride affinity of the silicon atom. As there is scarcity of Sn(II) compounds as single-site catalysts, we investigated 5 as a catalyst towards the hydroboration of aldehydes, ketones, alkenes and alkynes. All the compounds have been characterized by single-crystal X-ray diffraction and by state of the art spectroscopic studies.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Catalytic hydroboration of carbonyl derivatives by using phosphinimino amide ligated magnesium complexes

Reduction of carbonyl derivatives by using Earth-abundant, cheap, and environmentally benign metal-based catalysts through an atom-efficient method is a challenging task. Herein, we report the synthesis and characterization of dinuclear magnesium complexes 1-3 chelated by a phosphinimino amide skeleton. In combination with pinacolborane (HBpin) as a reducing agent, complex 1 bearing an ortho-methyl substituent on the phenyl ring of the ligand showed excellent reduction capability for a broad range of carbonyl derivatives under mild reaction conditions. Aldehydes, ketones, and acrolein substrates were efficiently reduced to the corresponding alkoxy-borane products with a record high TOF. Besides, acrolein derivatives were exclusively reduced to 1,2-regioselective products. Using two equiv. of HBpin, ester substrates were reduced to two kinds of alkoxy-borane products. Carbonate reduction accomplished by using complex 1 and three equiv. of HBpin afforded diols and a methanol precursor, respectively. When chiral substrates such as (S)-1,2-propanediol carbonate and L-lactide or polymeric P(L-LA) were employed, the chirality was almost retained in their reductive products.

n-Butyllithium as a highly efficient precatalyst for cyanosilylation of aldehydes and ketones

A highly efficient cyanosilylation protocol mediated by the easily available n-BuLi with a wide range of aldehydes and ketones was developed. This protocol features excellent yields with very low n-BuLi loadings (0.01-0.05 mol%) at room temperature, solvent-free process, good chemo-/regio-selectivity and functional group tolerance and scalability. A possible reaction pathway based upon stoichiometric reactivity was put forward.

Germyliumylidene: A Versatile Low Valent Group 14 Catalyst

Bis‐NHC stabilized germyliumylidenes [RGe(NHC)₂]⁺ are typically Lewis basic (LB) in nature, owing to their lone pair and coordination of two NHCs to the vacant p‐orbitals of the germanium center. However, they can also show Lewis acidity (LA) via Ge−C^NHC σ* orbital. Utilizing this unique electronic feature, we report the first example of bis‐NHC‐stabilized germyliumylidene [^MesTerGe(NHC)₂]Cl (1), (^MesTer=2,6‐(2,4,6‐Me₃C₆H₂)₂C₆H₃; NHC= IMe₄=1,3,4,5‐tetramethylimidazol‐2‐ylidene) catalyzed reduction of CO₂ with amines and arylsilane, which proceeds via its Lewis basic nature. In contrast, the Lewis acid nature of 1 is utilized in the catalyzed hydroboration and cyanosilylation of carbonyls, thus highlighting the versatile ambiphilic nature of bis‐NHC stabilized germyliumylidenes.

Manganese-Catalyzed Hydroborations with Broad Scope

Reductive transformations of easily available oxidized matter are at the heart of synthetic manipulation and chemical valorization. The applications of catalytic hydrofunctionalization benefit from the use of liquid reducing agents and operationally facile setups. Metal‐catalyzed hydroborations provide a highly prolific platform for reductive valorizations of stable C=X electrophiles. Here, we report an especially facile, broad‐scope reduction of various functions including carbonyls, carboxylates, pyridines, carbodiimides, and carbonates under very mild conditions with the inexpensive pre‐catalyst Mn(hmds)₂. The reaction could be successfully applied to depolymerizations.

Expected and Unexpected Reactivities of Homoleptic LiNacNac and Heteroleptic NacNacMg(TMP) β-Diketiminates toward Various Small Unsaturated Organic Molecules

Homoleptic LiNacNac forms simple donor-acceptor complexes with N,N'-dicyclohexylcarbodiimide (CyN═C═NCy), triphenylphosphine oxide (Ph₃P═O), and benzophenone (Ph₂CO). These crystallographically characterized compounds could be regarded as model intermediates en route to reducing the N═C, P═O, and C═O bonds of unsaturated substrates. Heteroleptic NacNacMg(TMP) intriguingly functions as a TMP nucleophile both with t-BuNCO and t-BuNCS, producing a urea or thiourea derivative respectively attached to Mg, though the NacNac ligand in the former reaction also engages noninnocently with a second t-BuNCO molecule via insertion at the reactive NacNac backbone γ-carbon site.

Lithium compound catalyzed deoxygenative hydroboration of primary, secondary and tertiary amides

A selective and efficient route for the deoxygenative reduction of primary to tertiary amides to corresponding amines has been achieved with pinacolborane (HBpin) using simple and readily accessible 2,6-di-tert-butyl phenolate lithium·THF (1a) as a catalyst. Both experimental and DFT studies provide mechanistic insight.

CeO2–nanocubes as efficient and selective catalysts for the hydroboration of carbonyl groups

An efficient and reusable CeO2 nanocatalyst has been developed for the selective hydroboration of carbonyl compounds, including aromatic, heteroaromatic, aliphatic, and (hetero)aliphatic aldehydes and ketones.

Cyanosilylation by Compounds with Main-Group Elements: An Odyssey

The past few decades have seen remarkable headways in the structural and reaction chemistry of compounds with heavier main-group elements. In recent years, there is an ongoing effort to derive catalytic chemistry involving main-group compounds, driven by their lower costs and higher terrestrial abundances. Here, a survey on the state-of-the-art in the development of cyanosilylation methodology by compounds with heavier main-group elements has been given. Once dominated by transition metals, the field has matured to embrace the majority of the main-group elements including aluminum, silicon, and calcium. Of particular focus will be how the mechanism of cyanosilylation involving compounds with main-group elements differs from those of transition metals.

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.

Lithium amide catalyzed hydroboration of nitriles

An operationally facile hydroboration of nitriles is reported that utilizes the stable and inexpensive catalyst LiN(SiMe₃)₂. The reaction displayed good tolerance of functional groups and also converted carbonyl derivatives.

Asymmetric ketone hydroboration catalyzed by alkali metal complexes derived from BINOL ligands

The ability of alkali metal complexes featuring functionalized BINOL-derived ligands to catalyze ketone hydroboration reactions was explored. The reduced products were formed in excellent yields and with variable enantioselectivities dependent upon the nature of the ligand and the alkali metal cation.

Syntheses of asymmetrical magnesium(i) complexes and their catalytic application in epoxide hydroboration

A series of novel structural magnesium(i) complexes stabilized by cyclopentyl and cyclohexyl substituted β-diketiminate ligands have been synthesized and used as highly active and regioselective pre-catalysts for various epoxides hydroboration.

Green hydroboration of carboxylic acids and mechanism investigation

A catalyst-free and solvent-free method for the hydroboration of a variety of carboxylic acids with pinacolborane was developed. The hydroboration of various aromatic and aliphatic carboxylic acids as well as dicarboxylic acids with HBpin could be completed within 6 h at room temperature or within 1 h at 60 °C to give the products in quantitative yields under neat conditions without the need for any solvent or metal catalyst. The possible reaction mechanism was investigated in detail based on the corresponding DFT calculations and the stoichiometric reaction of acetic acid with different equivalents of HBpin (at room temperature and 0 °C) and it revealed the stepwise nature of the protocol.

Catalyst-Free Approach for Hydroboration of Carboxylic Acids under Mild Conditions

Herein, we present a facile method for deoxygenative hydroboration of a broad range of carboxylic acids under very mild conditions. The most striking feature of this attractive hydroboration is that this elusive and challenging transformation was realized without catalyst and solvent. The investigation of solvent effect showed that tetrahydrofuran was also suitable for this kind of reaction. Moreover, a successful gram-scale trial may provide a very promising toolkit for carboxylic acid reduction at a large scale.