Redox-Noninnocent Ligand-Supported Vanadium Catalysts for the Chemoselective Reduction of C═X (X = O, N) Functionalities

One-Pot, Tandem Reductive Amination/Alkylation-Cycloamidation for Lactam Synthesis from Keto or Amino Acids

Monotrifluoroacetoxyborane-amines, prepared by treating borane-amines with trifluoroacetic acid, have been shown to be efficient reagents for a one-pot, tandem reductive amination/alkylation-cycloamidation of keto or amino acids to achieve the synthesis of 5-aryl or 5-methyl pyrrolidin-2-ones and 6-aryl or 6-methyl piperidin-2-ones.

Homoleptic octahedral CoII complexes as precatalysts for regioselective hydroboration of alkenes with high turnover frequencies

Homoleptic complexes adopting octahedral coordination modes are usually less active in catalysis due to the saturated coordination around metal centers that prevents substrate activation in a catalytic event. In this work, we demonstrated that a homoleptic octahedral cobalt complex (1) of 4'-pyridyl-2,2';6',2''-terpyridine that experienced monoprotonation at the non-coordinating pyridyl moiety upon crystallization could serve as a highly efficient precatalyst for the hydroboration of styrene derivatives with Markovnikov selectivity. The solid-state structure of this precatalyst along with relevant homoleptic CoII and FeII complexes has been characterized by X-ray crystallography. In the solid state, 1 features one-dimensional hydrogen-bonded chains that are further stacked by interchain π⋯π interactions. The newly synthesized complexes (1-3) along with several known analogues (4-6) were examined as precatalysts for the hydroboration of alkenes. The best-performing system, 1/KOtBu was found to promote Markovnikov hydroboration of substituted styrenes with high turnover frequencies (TOFs) up to ∼47 000 h-1, comparable to the most efficient polymeric catalyst [Co(pytpy)Cl2]n reported to date. Although some limitations in substrate scope as well as functional group tolerance exist, the catalyst shows good promise for several relevant hydrofunctionaliation reactions.

Vanadium-catalysed regioselective hydroboration of epoxides for synthesis of secondary alcohols

Regioselective epoxide ring-opening through hydroboration catalysed by a vanadium(III) dialkyl complex supported by a redox-active terpyridine ligand is reported. Secondary alcohols were obtained in high yields via effective Markovnikov hydroboration of terminal epoxides, showcasing a new catalytic application of an earth-abundant vanadium(III) complex.

Metallated dihydropyridinates: prospects in hydride transfer and (electro)catalysis

Hydride transfer (HT) is a fundamental step in a wide range of reaction pathways, including those mediated by dihydropyridinates (DHP-s). Coordination of ions directly to the pyridine ring or functional groups stemming therefrom, provides a powerful approach for influencing the electronic structure and in turn HT chemistry. Much of the work in this area is inspired by the chemistry of bioinorganic systems including NADH. Coordination of metal ions to pyridines lowers the electron density in the pyridine ring and lowers the reduction potential: lower-energy reactions and enhanced selectivity are two outcomes from these modifications. Herein, we discuss approaches for the preparation of DHP-metal complexes and selected examples of their reactivity. We suggest further areas in which these metallated DHP-s could be developed and applied in synthesis and catalysis.

Charge and Solvent Effects on the Redox Behavior of Vanadyl Salen-Crown Complexes

The incorporation of charged groups proximal to a redox active transition metal center can impact the local electric field, altering redox behavior and enhancing catalysis. Vanadyl salen (salen = N,N'-ethylenebis(salicylideneaminato)) complexes functionalized with a crown ether containing a nonredox active metal cation (V-Na, V-K, V-Ba, V-La, V-Ce, and V-Nd) were synthesized. The electrochemical behavior of this series of complexes was investigated by cyclic voltammetry in solvents with varying polarity and dielectric constant (ε) (acetonitrile, ε = 37.5; N,N-dimethylformamide, ε = 36.7; and dichloromethane, ε = 8.93). The vanadium(V/IV) reduction potential shifted anodically with increasing cation charge compared to a complex lacking a proximal cation (ΔE_1/2 > 900 mV in acetonitrile and >700 mV in dichloromethane). In contrast, the reduction potential for all vanadyl salen-crown complexes measured in N,N-dimethylformamide was insensitive to the magnitude of the cationic charge, regardless of the electrolyte or counteranion used. Titration studies of N,N-dimethylformamide into acetonitrile resulted in cathodic shifting of the vanadium(V/IV) reduction potential with increasing concentration of N,N-dimethylformamide. Binding constants of N,N-dimethylformamide (log(K_DMF)) for the series of crown complexes show increased binding affinity in the order of V-La > V-Ba > V-K > (salen)V(O), indicating an enhancement of Lewis acid/base interaction with increasing cationic charge. The redox behavior of (salen)V(O) and (salen-OMe)V(O) (salen-OMe = N,N'-ethylenebis(3-methoxysalicylideneamine) was also investigated and compared to the crown-containing complexes. For (salen-OMe)V(O), a weak association of triflate salt at the vanadium(IV) oxidation state was observed through cyclic voltammetry titration experiments, and cation dissociation upon oxidation to vanadium(V) was identified. These studies demonstrate the noninnocent role of solvent coordination and cation/anion effects on redox behavior and, by extension, the local electric field.

Combined effects of the lewis acidity and electric field of proximal redox innocent metal ions on the redox potential of vanadyl Schiff base complexes: an experimental and theoretical study

The reactivity of biological or synthetic metalloenzymes is modulated in the presence of redox innocent Lewis acidic metal ions as they change the redox potential of the redox active metal ions present in the active site of metalloenzymes. To study this effect, we synthesised a mono-nuclear V(IV) complex (VOL, 1) with an N₂O₄ donor bicompartmental ligand, characterized it by single-crystal X-ray crystallography and recorded its cyclic voltammogram in acetonitrile. The CV revealed a reversible redox process for the V(IV)/V(V) couple. The potential of the V(IV)/V(V) couple shifted to a more positive value when equivalent amounts of Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺ and Ba²⁺ ions were added separately to its acetonitrile solution, but the extent of shift for Li⁺ and Mg²⁺ was much less than that of the other metal ions. The guest metal ions except Li⁺ and Mg²⁺ were accommodated in the outer compartment of VOL as confirmed by IR and UV-Vis spectral analysis. Single-crystal structural analysis of [(VOL)KPF₆]₂, (1·K) and [(VOL)Ba(ClO₄)₂(H₂O)]_n, (1·Ba) also confirmed the hetero-metallic adduct formation. The correlation of the shift of the V(IV/V) redox potential with the Lewis acidity of respective metal ions deviated appreciably from linearity. DFT calculations suggest that the shift in potential is probably controlled by local electric fields induced by those ions, as indicated by 2D vector electric field maps.

Synthesis and catalytic performance of nickel phosphinite pincer complexes in deoxygenative hydroboration of amides

A series of imino-POCN^R, amino-POCN^R2, and bis(phosphinite) POCOP pincer complexes of Ni(II) were prepared and tested in catalytic deoxygenative hydroboration of amides with HBPin to the corresponding amines. In contrast to the deoxygenative hydrosilylation approach, primarily developed for tertiary amides, superior reactivity in Ni-catalyzed deoxygenative hydroboration was demonstrated for secondary carboxamides. The bis(phosphinite) hydride complex (POCOP)NiH proved the most active in these reactions, tolerating potentially reducible functionalities such as internal alkenes, esters, nitriles, heteroaromatic compounds, and tertiary amides. Preferable hydroboration of secondary amides was also demonstrated in the presence of primary amide functionalities. The reactions were conducted at 60-80 °C, representing a rare example of a base-metal catalytic system for selective deoxygenation of secondary amides to the corresponding amines under mild conditions. In contrast to secondary amides, deoxygenative hydroboration of primary amides was demonstrated using an iminophosphinite pre-catalyst (POCN^Dmp)Ni(CH₂TMS) (Dmp = 2,6-Me₂C₆H₃). Deoxygenation reactions were suggested to proceed via a direct C-O bond cleavage mechanism, which is triggered by dehydrogenative N-borylation to access more electrophilic N-borylamides amenable to the addition of HBPin to the carbonyl group.

Exploration of Ligand-Centered Hydride Transfer in La/Y-Catalyzed Deoxygenative Reduction of Tertiary Amides with Pinacolborane

A number of rare-earth metals and actinides have proven to be active in a wide variety of atom-efficient transformations. As compared to the related organometallic catalysts, the detailed mechanisms for the rare-earth metal-catalyzed reactions remain largely unexplored. Herein, the detailed catalyst activation process and reaction mechanisms of deoxygenative reduction of amides with pinacolborane (HBpin) catalyzed by Y[N(TMS)₂]₃ and La[N(TMS)₂]₃ complexes as well as a La₄(O)acac₁₀ cluster are investigated by density functional theory calculations. The M(III)-hemiaminal complex is disclosed to be the active catalyst for both the complexes and the cluster. During catalyst activation for both the Y and La complexes, the H-B bond polarity results in the formation of a transient M(III)-hydride intermediate, which is converted into an on-cycle M(III)-hemiaminal complex via facile migratory insertion. However, this kind of La(III)-hydride species cannot be formed for the La cluster. Starting from the M(III)-hemiaminal complex, the reaction proceeds via the ligand-centered hydride transfer mechanism that involves B-O bond formation, hydride transfer to B, C-O cleavage within the hemiaminal borane, hydride transfer to C, and σ-bond metathesis. The additional HBpin molecule is vital for the first hydride transfer that leads to the formation of [H₂Bpin]^- species. Our calculations reveal several important cooperative effects of the HBpin component during the hydride transfer processes. The improved mechanistic insights will be helpful for further development of selective C═O reduction.

An ionic Fe-based metal-organic-framework with 4'-pyridyl-2,2':6',2''-terpyridine for catalytic hydroboration of alkynes

An ionic metal-organic-framework (MOF) containing nanoscale channels was readily assembled from ditopic 4'-pyridyl-2,2':6',2''-terpyridine (pytpy) and a simple iron(ii) salt. X-ray structural analysis revealed a two-dimensional grid-like framework assembled by classic octahedral (pytpy)2FeII cations as linkers (with pytpy as a new ditopic pyridyl ligand) and octa-coordinate FeCl2 centers as nodes. The layer-by-layer assembly of the 2-D framework resulted in the formation of 3-D porous materials consisting of nano-scale channels. The charges of the cationic framework were balanced with anionic Cl3FeOFeCl3 in its void channels. The new Fe-based MOF material was employed as a precatalyst for syn-selective hydroboration of alkynes under mild, solvent-free conditions in the presence of an activator, leading to the synthesis of a range of trans-alkenylboronates in good yields. The larger scale applicability and recyclability of the new MOF catalyst was further explored. This represents a rare example of an ionic MOF material that can be utilized in hydroboration catalysis.

Mild and Chemoselective Carboxylic Acid Reduction Promoted by Borane Catalysis

Although considerable advances have been made in developing chemoselective transformations of ubiquitous carboxylic acid groups, many challenges still exist. For instance, their selective reduction is problematic if both more nucleophilic and more electrophilic groups are present in the starting material. Here, we address this problem with a simple and mild protocol using bench-stable reagents at ambient temperatures. This platform is able to tolerate a diverse range of functionality, leaving ketones, esters, nitro-groups, olefins, nitriles and amides untouched. A combination of experimental and computational mechanistic experiments demonstrate that this reaction proceeds via hidden borane catalysis with small quantities of in situ generated BH3 playing a key role in the exquisite selectivity that is observed.

Diplumbane-catalysed solvent- and additive-free hydroboration of ketones and aldehydes

A new diplumbane, namely [Pb(CH2SiMe3)3]2, was synthesized and structurally characterized. This group 14 element compound was found to catalyse the hydroboration of ketones and aldehydes under mild conditions without the use of additives and solvents, leading to the synthesis of a range of alcohols in high yields after hydrolysis.

Azo-oximate metal-carbonyl to metallocarboxylic acid via the intermediate Ir(III) radical congener: quest for co-ligand driven stability of open- and closed-shell complexes

The redox non-innocent behavior of the diaryl-azo-oxime ligand L^NOH1 has been accentuated via the synthesis of metastable anion radical complexes of type trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 (CO is trans to azo group of the ligand) by the oxidative coordination reaction of 1 with Vaska's complex. The stereochemical role of co-ligands vis-à-vis the interplay of π-bonding has been found to be decisive in controlling the aptitude of the coordinated redox non-innocent ligand to accept or reject an electron. This has been clarified via the isolation of quite a few complexes as well as the failure to synthesize some others. The oxidized analogues of type trans-[Ir(L^NO-)Cl(CO)(PPh₃)₂]⁺2+ (CO and azo group of the ligand are trans) as well as its cis isomer cis-[Ir(L^NO-)Cl(CO)(PPh₃)₂]⁺3+ (CO and azo group of the ligand are cis) have been structurally characterized but the radical anion congener of the latter could not be synthesized. Furthermore, the closed shell complexes [Ir(L^NO-)Cl₂(PPh₃)₂] 4 and [Ir(L^NO-)₂Cl(PPh₃)] 5 have been well characterized by diffraction as well as spectral techniques but their corresponding azo anion radical complexes could not be isolated and this is attributed to the trans influence of ancillary ligands. The anion radical complexes trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 may be rapidly transformed to the metallocarboxylic acids trans-[Ir(L^NO-)Cl(CO₂H)(PPh₃)₂] 6via a proton-coupled electron transfer (PCET) process, thereby demonstrating the role of odd electron over the coordinated ligand framework to trigger metal-mediated carbonyl to carboxylic acid functionalization. Complexes 6 are further stabilized via intramolecular -CO₂H⋯ON- (carboxylic acid⋯oximato) H-bonding. The optoelectronic properties as well as the origin of transitions in the complexes were analyzed by TD-DFT and theoretical analysis, which further disclose that the odd electron in trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 is primarily azo-oxime centric with very low contribution from the iridium center.

Assembly of a 3D Cobalt(II) Supramolecular Framework and Its Applications in Hydrofunctionalization of Ketones and Aldehydes

A ditopic nitrogen ligand (E)-N′-(pyridin-4-ylmethylene)isonicotinohydrazide (L) containing both divergent pyridyl coordination sites and a hydrogen-bonding hydrazide–hydrazone moiety was synthesized. The Co(NCS)2-mediated self-assembly of L has resulted in the synthesis of a novel 3-dimensional (3D) supramolecular framework (1) that features both coordination and hydrogen bonding interactions. X-ray structural analysis reveals the formation and coordination mode of 1 in the solid state. The rational utilization of coordination bonds and hydrogen bonding interactions is confirmed and responsible for constructing the 3D materials. Catalytic studies using 1 in the presence of an activator are performed for the hydroboration and hydrosilylation reactions of ketones and aldehydes, and the results are compared with previously reported cobalt-based polymeric catalysts.

N-Heterocyclic Carbene-Phosphinidenide Complexes as Hydroboration Catalysts

The reactions of the N-heterocyclic carbene-phosphinidene adducts (NHC)PSiMe₃ and (NHC)PH with the dinuclear ruthenium and osmium complexes [(η⁶-p-cymene)MCl₂]₂ (M = Ru, Os) afforded the half-sandwich complexes [(η⁶-p-cymene){(NHC)P}MCl] and [(η⁶-p-cymene){(NHC)PH}MCl₂] with two- and three-legged piano-stool geometries, respectively (NHC = IDipp, IMes; IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). The complexes were initially tested as precatalysts for the hydroboration of benzonitrile, and the most active species, the ruthenium complex [(η⁶-p-cymene){(IMes)P}RuCl], was further used for the efficient hydroboration of a wide range (ca. 50 substrates) of nitriles, carboxylic esters, and carboxamides in neat pinacolborane (HBpin) under comparatively mild reaction conditions (60-80 °C, 3-5 mol % catalyst loading). Preliminary mechanistic and kinetic studies are reported, and stoichiometric reactions with HBpin indicate the initial formation of the monohydride complex [(η⁶-p-cymene){(IMes)P}RuH] as the putative catalytically active species.

An efficient catalytic method for the borohydride reaction of esters using diethylzinc as precatalyst

A cheap and easily available ZnEt2 is an effective precatalyst, which can be used for the hydroboration reaction of various organic carbonates and esters with HBpin.

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.

Highly efficient Ti-catalyst for deoxygenative reduction of esters at ambient conditions: experimental and mechanistic insights from DFT Study

In this paper, we report the synthesis of dianionic amidophosphineborane–supported titanium chloride [{Ph2P(BH3)N}2C6H4}TiCl2] (1) and TiIV alkyl complex [{Ph2P(BH3)N}2C6H4}Ti(CH2SiMe3)2] (2) using salt metathesis reaction. The TiIV complex 1 was obtained...

Hydroborative reduction of amides to amines mediated by La(CH2C6H4NMe2-o)3

La(CH2C6H4NMe2-o)3/HBpin is an efficient catalytic system for the deoxygenative reduction of primary, secondary and tertiary amides to amines.

Rh-Catalyzed Coupling of Aldehydes with Allylboronates Enables Facile Access to Ketones

We present herein a novel strategy for the preparation of ketones from aldehydes and allylic boronicesters. This reaction involves the allylation of aldehydes with allylic boronicesters and the Rh-catalyzed chain-walking of homoallylic alcohols. The key to this successful development is the protodeboronation of alkenyl borylether intermediate via a tetravalent borate anion species in the presence of KHF 2 and MeOH. This approach features mild reaction conditions, broad substrate scope, and excellent functional group tolerance. Mechanistic studies also supported that the tandem allylation and chain-walking process was involved.

Hydrosilylation of Aldehydes and Ketones Catalyzed by a 2-Iminopyrrolyl Alkyl-Manganese(II) Complex

A well-defined and very active single-component manganese(II) catalyst system for the hydrosilylation of aldehydes and ketones is presented. First, the reaction of 5-(2,4,6-iPr₃C₆H₂)-2-[N-(2,6-iPr₂C₆H₃)formimino]pyrrolyl potassium (KL) and [MnCl₂(Py)₂] afforded the binuclear 2-iminopyrrolyl manganese(II) pyridine chloride complex [Mn₂{κ²N,N'-5-(2,4,6-iPr₃C₆H₂)-NC₄H₂-2-C(H)═N(2,6-iPr₂C₆H₃)}₂(Py)₂(μ-Cl)₂] 1. Subsequently, the alkylation reaction of complex 1 with LiCH₂SiMe₃ afforded the respective (trimethylsilyl)methyl-Mn(II) complex [Mn{κ²N,N'-5-(2,4,6-iPr₃C₆H₂)-NC₄H₂-2-C(H)═N(2,6-iPr₂C₆H₃)}(Py)CH₂SiMe₃] 2 in a good yield. Complexes 1 and 2 were characterized by elemental analysis, ¹H NMR spectroscopy, Evans' method, FTIR spectroscopy, and single-crystal X-ray diffraction. While the crystal structure of complex 1 has been identified as a binuclear entity, in which the Mn(II) centers present pentacoordinate coordination spheres, that of complex 2 corresponds to a monomer with a distorted tetrahedral coordination geometry. Complex 2 proved to be a very active precatalyst for the atom-economic hydrosilylation of several aldehydes and ketones under very mild conditions, with a maximum turnover frequency of 95 min^-1, via a silyl-Mn(II) mechanistic route, as asserted by a combination of experimental and theoretical efforts, the respective silanes were cleanly converted to the respective alcoholic products in high yields.

Deoxygenative hydroboration of carboxamides: a versatile and selective synthetic approach to amines

Deoxygenative reduction of amides is considered as an attractive method for preparation of synthetically valuable amines. However, the low electrophilicity of the amide carbonyl group, high thermodynamic stability and kinetic inertness of the amides make their reduction a challenging task. Until recently, most efforts for catalytic deoxygenation of amides to amines were concentrated on hydrogenation and hydrosilylation approaches, which mainly employed precious metal catalysts and often required harsh reaction conditions and showed insufficient selectivities. Moreover, these reactions are mostly limited to secondary and tertiary amides, whereas direct reduction of primary amides to primary amines remained arduous. In contrast, deoxygenative hydroboration of amides, although it appeared less then a decade ago, has already proved advantageous in terms of the amide scope, reaction conditions and selectivity of transformations. This article provides an overview of the developments in hydroboration of amides, focusing on mechanistic aspects of these transformations and advantages of hydroboration compared to hydrogenation and hydrosilylation approaches.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Reduction of Amides to Amines with Pinacolborane Catalyzed by Heterogeneous Lanthanum Catalyst La(CH2C6H4NMe2-o)3@SBA-15

Hydroboration of amides is a useful synthetic strategy to access the corresponding amines. In this contribution, it was found that the supported lanthanum benzyl material La(CH₂C₆H₄NMe₂-o)₃@SBA-15 was highly active for the hydroboration of primary, secondary, and tertiary amides to amines with pinacolborane. These reactions selectively produced target amines and showed good tolerance for functional groups such as -NO₂, -halogen, and -CN, as well as heteroatoms such as S and O. This reduction procedure exhibited the recyclable and reusable property of heterogeneous catalysts and was applicable to gram-scale synthesis. The reaction mechanisms were proposed based on some control experiments and the previous literature. This is the first example of hydroborative reduction of amides to amines mediated by heterogeneous catalysts.

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.

Recent developments in highly basic N-heterocyclic iminato ligands in actinide chemistry

In the last decade, major conceptual advances in the chemistry of actinide molecules and materials have been made to demonstrate their distinct reactivity profiles as compared to lanthanide and transition metal compounds, but some difficult questions remain concerning the intriguing stability of low-valent actinide complexes, and the importance of the 5f-orbitals in reactivity and bonding. The imidazolin-2-iminato moiety has been extensively used in ligands for the advancement of actinide chemistry owing to its unique capability of stabilizing the reactive and highly electrophilic metal ions by virtue of its strong electron donation and steric tunability. The current review article describes recent developments in the chemistry of light actinide metal ions (thorium and uranium) bearing these N-heterocyclic iminato moieties as supporting ligands. In addition, the effect of ring expansion of the N-heterocycle on the catalytic aptitude of the organoactinides is also described herein. The synthesis and reactivity of actinide complexes bearing N-heterocyclic iminato ligands are presented, and promising apposite applications are also presented. The current review focuses on addressing the catalytic behavior of actinide complexes with oxygen-containing substrates such as in the Tishchenko reaction, hydroelementation processes, and polymerization reactions. Actinide complexes have also found new catalytic applications, as demonstrated by the potent chemoselective carbonyl hydroboration and tandem proton-transfer esterification (TPTE) reaction, featuring coupling between an aldehyde and alcohol.

Hydroboration of Nitriles, Esters, and Carbonates Catalyzed by Simple Earth-Abundant Metal Triflate Salts

During the past decade earth-abundant metals have become increasingly important in homogeneous catalysis. One of the reactions in which earth-abundant metals have found important applications is the hydroboration of unsaturated C-C and C-X bonds (X=O or N). Within these set of transformations, the hydroboration of challenging substrates such as nitriles, carbonates and esters still remain difficult and often relies on elaborate ligand designs and highly reactive catalysts (e. g., metal alkyls/hydrides). Here we report an effective methodology for the hydroboration of challenging C≡N and C=O bonds that is simple and applicable to a wide set of substrates. The methodology is based on using a manganese(II) triflate salt that, in combination with commercially available potassium tert-butoxide and pinacolborane, catalyzes the hydroboration of nitriles, carbonates, and esters at room temperature and with near quantitative yields in less than three hours. Additional studies demonstrated that other earth-abundant metal triflate salts can facilitate this reaction as well, which is further discussed in this report.

A benzene-bridged divanadium complex-early transition metal catalyst for alkene alkylarylation with PhI(O2CR)2via decarboxylation

The synthesis, structure and catalytic activity of a benzene-bridged divanadium complex were comprehensively studied. The reduction of (Nacnac)VCl2 (1) (Nacnac = (2,6-iPr2C6H3NCMe)2HC) supported by β-diketiminate with potassium graphite (KC8) by employing benzene as the solvent allows access to the benzene-bridged inverted-sandwich divanadium complex (μ-η6:η6-C6H6)[V(Nacnac)]2 (2a), which can catalyze alkene alkylarylation with hypervalent iodine(iii) reagents (HIRs) via decarboxylation to generate regioselectively diverse indolinones. Furthermore, the mild nature of this reaction was amenable to a wide range of functionalities on alkenes and HIRs. Mechanistic studies revealed a relay sequence of decarboxylative radical alkylation/radical arylation/oxidative re-aromatization.

Visible Light Induced Reduction and Pinacol Coupling of Aldehydes and Ketones Catalyzed by Core/Shell Quantum Dots

We present an efficient and versatile visible light-driven methodology to transform aryl aldehydes and ketones chemoselectively either to alcohols or to pinacol products with CdSe/CdS core/shell quantum dots as photocatalysts. Thiophenols were used as proton and hydrogen atom donors and as hole traps for the excited quantum dots (QDs) in these reactions. The two products can be switched from one to the other simply by changing the amount of thiophenol in the reaction system. The core/shell QD catalysts are highly efficient with a turn over number (TON) larger than 4 × 10⁴ and 4 × 10⁵ for the reduction to alcohol and pinacol formation, respectively, and are very stable so that they can be recycled for at least 10 times in the reactions without significant loss of catalytic activity. The additional advantages of this method include good functional group tolerance, mild reaction conditions, the allowance of selectively reducing aldehydes in the presence of ketones, and easiness for large scale reactions. Reaction mechanisms were studied by quenching experiments and a radical capture experiment, and the reasons for the switchover of the reaction pathways upon the change of reaction conditions are provided.

Chemoselective hydroborative reduction of nitro motifs using a transition-metal-free catalyst

The first transition-metal-free catalysts for deoxygenative reduction of nitroarenes, nitro heteroarenes and even notoriously challenging nitroalkanes using pinacolborane have been reported.

La(CH2C6H4NMe2-o)3-catalyzed reduction of esters to alcohols with pinacolborane

La(CH2C6H4NMe2-o)3/HBpin is an efficient catalytic system for the hydroborative reduction of esters to alcohols under mild conditions.

Molybdenum(VI) bis-imido Complexes of Dipyrromethene Ligands

We report the synthesis of high-valent molybdenum(VI) bis-imido complexes 1-4 with dipyrromethene (DPM) supporting ligands of the general formula (DPM^R)Mo(NR')₂Cl (R, R' = mesityl (Mes) or tert-butyl (^tBu)). The electrochemical and chemical properties of 1-4 reveal unexpected ligand noninnocence and reactivity. ¹⁵N NMR spectroscopy is used to assess the electronic properties of the imido ligands in the tert-butyl complexes 1 and 3. Complex 1 is inert toward ligand (halide) exchange with bulky phenolates such as KOMes or amides (e.g., KN(SiMe₃)₂), whereas the use of the lithium alkyl LiCH₂SiMe₃ results in a rare nucleophilic β-alkylation of the DPM ligand. While the reductions of the complexes occur at molybdenum, the oxidation is centered at the DPM ligand. Quantum-chemical calculations (complete active space self-consistent field, density functional theory) suggest facile (near-infrared) interligand charge transfer to the imido ligand, which might preclude the isolation of the oxidized complex [1]⁺ in the experiment.

Homoleptic Bis(trimethylsilyl)amides of Yttrium Complexes Catalyzed Hydroboration Reduction of Amides to Amines

Homoleptic lanthanide complex Y[N(TMS)₂]₃ is an efficient homogeneous catalyst for the hydroboration reduction of secondary amides and tertiary amides to corresponding amines. A series of amides containing different functional groups such as cyano, nitro, and vinyl groups were found to be well-tolerated. This transformation has also been nicely applied to the synthesis of indoles and piribedil. Detailed isotopic labeling experiments, control experiments, and kinetic studies provided cumulative evidence to elucidate the reaction mechanism.

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.