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

Base- and Additive-Free Carbon Dioxide Hydroboration to Methoxyboranes Catalyzed by Non-Pincer-Type Mn(I) Complexes

Well-defined, bench stable Mn(I) non-pincer-type complexes were tested as earth-abundant transition metal catalysts for the selective reduction of CO₂ to boryl-protected MeOH in the presence of pinacolborane (HBpin). Essentially, quantitative yields were obtained under mild reaction conditions (1 bar CO₂, 60 °C), without the need of any base or additives, in the presence of the alkylcarbonyl Mn(I) bis(phosphine) complexes fac-[Mn(CH₂CH₂CH₃)(dippe)(CO)₃] [Mn1, dippe = 1,2-bis(diisopropylphosphino)ethane] and [Mn(dippe)(CO)₂{(μ-H)₂(Bpin)}] (Mn4), that is obtained by reaction of the bench-stable precatalyst Mn1 with HBpin via elimination of butanal. Preliminary mechanistic details were obtained by a combination of NMR experiments and monitoring of the catalytic reactions.

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.

Zn-Catalyzed Regioselective and Chemoselective Reduction of Aldehydes, Ketones and Imines

An operationally convenient Zn-catalyzed synthesis of alcohols by the reduction of aldehydes, ketones, and α,β-unsaturated aldehydes/ketones is reported. It is a rare example of using mild and sustainable HBpin as a reductant for catalytic reduction of carbonyl compounds in the absence of acid or base as hydrolysis reagent. The reaction is upscalable and proceeds in high selectivity without the formation of boronate ester by-products, and tolerates sensitive functionalities, such as iodo, bromo, chloro, fluoro, nitro, trifluoromethyl, aminomethyl, alkynyl, and amide. The Zn(OAc)₂/HBpin combination has been also proved to be chemoselective for the C=N reduction of imine analogs.

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.

Organoaluminum hydrides catalyzed hydroboration of carbonates, esters, carboxylic acids, and carbon dioxide

The reductive functionalization of the CO unit of carbonates, carboxylic acids, esters, and CO₂, respectively has received great attention since its introduction. This method is often used industrially for the synthesis of high value-added energy products in chemistry. This opens up a new way forward to reduce greenhouse gases and the consumption of traditional energy sources. Herein, we report an earth-abundant, cheap, and readily available aluminum dihydride, which can catalyze the reduction of a range of carbonates, esters, carboxylic acids, and CO₂, respectively in the presence of pinacolborane as a reducing agent. Moreover, we demonstrate that the reaction can proceed to obtain good yield products under mild conditions, with low catalyst loading and solvent-free reactions. The mechanism of the catalytic reduction of carbonates has been investigated.

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.

Controlling Chemoselectivity of Catalytic Hydroboration with Light

The ability to selectively react one functional group in the presence of another underpins efficient reaction sequences. Despite many designer catalytic systems being reported for hydroboration reactions, which allow introduction of a functional handle for cross‐coupling or act as mild method for reducing polar functionality, these platforms rarely deal with more complex systems where multiple potentially reactive sites exist. Here we demonstrate, for the first time, the ability to use light to distinguish between ketones and carboxylic acids in more complex molecules. By taking advantage of different activation modes, a single catalytic system can be used for hydroboration, with the chemoselectivity dictated only by the presence or absence of visible light.

Reported Catalytic Hydrofunctionalizations that Proceed in the Absence of Catalysts: The Importance of Control Experiments

The enlarged landscape of catalysis lies in the heart of chemistry. As the journey has set a milestone in organic synthesis, its darker side has not entered into the limelight. Studies disclose that the reported reactions by using catalysts were also attainable in the absence of catalysts in many cases. This article presents a literature collection that includes the significance of control experiments in hydrofunctionalization reactions. Systematic analysis reveals that the catalysts are ambiguous and might be unessential in chemical reactions enlisted here.

Boric acid as a precatalyst for BH3-catalyzed hydroboration

We report that boric acid, BO₃H₃, is a good precatalyst for the BH₃-catalyzed hydroboration of esters using pinacolborane as a borylation agent. Using microwave irradiation as an energy source, we demonstrated that a dozen esters were converted into the corresponding boronate ethers in good yields. It was also possible to use boric acid as a precatalyst to reduce carbonates and alkynes. Considering the hazardous and pyrophoric nature of BH₃ solutions, boric acid proves to be a safe and green precatalyst for the metal-free reduction of unsaturated species.

Cheap and air-stable boric acid is shown to be a good precatalyst for BH₃ hydroboration of esters and carbonates.

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.

Catalytic and non-catalytic hydroboration of carbonyls: quantum-chemical studies

The addition of hydroboranes across several unsaturated moieties is a universal synthetic tool for the reduction or functionalization of unsaturated moieties. Given the sustainable nature of this process, the development of more environmentally-benign approaches (main-group catalysis or uncatalysed approaches) for hydroboration has gained considerable recent momentum. The present paper examines both catalyst-free and KF-mediated hydroboration of carbonyl compounds with the use of quantum-chemical methods. The results of computations for several potential reaction pathways are juxtaposed with experiment-based calculations, which leads to stepwise mechanisms and energy profiles for the reactions of pinacolborane with benzaldehyde and acetophenone (in the presence of KF). For each step of these reactions, we provide an accurate description of the geometric and electronic structures of corresponding stationary points. Five different levels of theory are employed to select the most applicable theoretical approach and develop a computational protocol for further research. Upon selection of the best-performing methods, larger molecular systems are studied to explore possible more complex pathways at the M06-2X/6-311++G(2d,p) and ωB97XD/6-311++G(2d,p) levels of theory, which brings up multi-pathway, overlapping catalytic cycles. The mechanism of solvent-free, catalyst-free hydroboration of aldehydes is also revisited through the prism of the elaborated methodology, which leads to a whole new perspective on the pathways of this and similar reactions, with a multimolecular cascade of hydride transfers being more energetically favoured than a four-membered transition state.

Hydroboration Reaction and Mechanism of Carboxylic Acids using NaNH2(BH3)2, a Hydroboration Reagent with Reducing Capability between NaBH4 and LiAlH4

Hydroboration reactions of carboxylic acids using sodium aminodiboranate (NaNH₂[BH₃]₂, NaADBH) to form primary alcohols were systematically investigated, and the reduction mechanism was elucidated experimentally and computationally. The transfer of hydride ions from B atoms to C atoms, the key step in the mechanism, was theoretically illustrated and supported by experimental results. The intermediates of NH₂B₂H₅, PhCH═CHCOOBH₂NH₂BH₃^-, PhCH═CHCH₂OBO, and the byproducts of BH₄^-, NH₂BH₂, and NH₂BH₃^- were identified and characterized by ¹¹B and ¹H NMR. The reducing capacity of NaADBH was found between that of NaBH₄ and LiAlH₄. We have thus found that NaADBH is a promising reducing agent for hydroboration because of its stability and easy handling. These reactions exhibit excellent yields and good selectivity, therefore providing alternative synthetic approaches for the conversion of carboxylic acids to primary alcohols with a wide range of functional group tolerance.

Benzoic Acid-Promoted C2-H Borylation of Indoles with Pinacolborane

A benzoic acid-promoted C2-H borylation of indoles with pinacolborane to afford C2-borylated indoles is developed. Preliminary mechanistic studies indicate BH₃-related borane species formed via the decomposition of pinacolborane to be the probable catalyst. This transformation provides a prompt route toward the synthesis of diverse C2-functionalized indoles.

Magnesium-catalyzed hydroboration of organic carbonates, carbon dioxide and esters

A magnesium(i) complex was employed as a highly efficient precatalyst for the hydroboration of carbonates and esters under mild conditions.

Catalyst-free and solvent-free hydroboration of alkynes

The hydroboration of alkynes with pinacolborane (HBpin) under catalyst- and solvent-free conditions was demonstrated.

Strategies and mechanisms of metal–ligand cooperativity in first-row transition metal complex catalysts

The use of metal–ligand cooperation (MLC) by transition metal bifunctional catalysts has emerged at the forefront of homogeneous catalysis science.

Catalytic Hydroboration of Aldehydes, Ketones, and Alkenes Using Potassium Carbonate: A Small Key to Big Transformation

An efficient transition-metal-free protocol for the hydroboration of aldehydes and ketones (reduction) was developed. The hydroboration of a wide range of aldehydes and ketones with pinacolborane (HBpin) under the K₂CO₃ catalyst has been studied. The reaction system is practical and reliable and proceeds under extremely mild and operationally simple conditions. No prior preparation of the complex metal catalyst was required, and hydroboration occurred stoichiometrically. Further, the chemoselective reduction of aldehydes over ketones was carried out. Moreover, we demonstrated the use of K₂CO₃ as an efficient catalyst for the hydroboration of alkenes. The operational simplicity, inexpensive and transition-metal-free catalyst, and the applicability to gram-scale synthesis strengthen its potential applications for hydroboration (reduction) at an industrial scale.

Catalyst-Free and Solvent-Free Facile Hydroboration of Imines

A facile process for the catalyst-free and solvent-free hydroboration of aromatic as well as heteroaromatic imines is reported. This atom-economic methodology is scalable, compatible with sterically and electronically diverse imines, displaying excellent tolerance towards various functional groups, and works efficiently at ambient temperature in most of the cases, affording secondary amines in good to excellent yield after hydrolysis.

Catalyst-free and solvent-free hydroboration of ketones

The hydroboration of a wide range of ketones with HBpin under catalyst-free and solvent-free conditions was reported in high yields.