La[N(SiMe 3 ) 2 ] 3 ‐Catalyzed Hydroboration of Esters and Other Challenging Unsaturated Groups

Zinc hydride catalyzed hydroboration of esters

The conjugated bis-guanidinate (CBG)-supported zinc hydride {LZnH}2; L = {(ArHN)(ArN)-CN-C(NAr)(NHAr); Ar = 2,6-Et2-C6H3} (I) is utilized as a catalyst for the hydroboration of esters with pinacolborane (HBpin) under mild reaction conditions. Various aryl and alkyl substrates containing electron-donating, withdrawing, and cyclic groups of esters are effectively converted into alkoxy boronate esters as products upon hydroboration. Furthermore, stoichiometric experiments have been performed to understand the plausible reaction mechanism for the hydroboration of esters. Additionally, complex (I) was used for the hydroboration of carbonate, carboxylic acid, and anhydride substrates to showcase the broad substrate scope.

Grignard Reagent-Catalyzed Hydroboration of Esters, Nitriles, and Imines

The reduction in esters, nitriles, and imines requires harsh conditions (highly reactive reagents, high temperatures, and pressures) or complex metal-ligand catalytic systems. Catalysts comprising earth-abundant and less toxic elements are desirable from the perspective of green chemistry. In this study, we developed a green hydroboration protocol for the reduction in esters, nitriles, and imines at room temperature (25 °C) using pinacolborane as the reducing agent and a commercially available Grignard reagent as the catalyst. Screening of various alkyl magnesium halides revealed MeMgCl as the optimal catalyst for the reduction. The hydroboration and subsequent hydrolysis of various esters yielded corresponding alcohols over a short reaction time (~0.5 h). The hydroboration of nitriles and imines produced various primary and secondary amines in excellent yields. Chemoselective reduction and density functional theory calculations are also performed. The proposed green hydroboration protocol eliminates the requirements for complex ligand systems and elevated temperatures, providing an effective method for the reduction in esters, nitriles, and imines at room temperature.

Palladium-Catalyzed Multicomponent Assembly of (Z)-Alkenylborons via Carbopalladation/Boronation/Retro-Diels-Alder Cascade Reaction

A palladium-catalyzed multicomponent cascade reaction of aryl iodides, oxanorbornadiene, and diborns to access (Z)-alkenylborons is reported. This transformation proceeds through the sequential carbopalladation/boronation/retro-Diels-Alder domino reaction. The oxanorbornadiene used in this reaction serves as an acetylene surrogate, which is generated via a retro-Diels-Alder reaction. Such a stereoselective and scalable approach has a wide range of functional group tolerance and good substrate universality.

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.

t BuOLi-Promoted Hydroboration of Esters and Epoxides

Commercially available and inexpensive lithium tert-butoxide ( ^t BuOLi) acts as a good precatalyst for the hydroboration of esters, lactones, and epoxides using pinacolborane as a borylation agent. Functional groups such as cyano-, nitro-, amino-, vinyl, and alkynyl are unaffected under the presented hydroboration process, representing high chemoselectivity. This transformation has also been effectively applied to the synthesis of key intermediates of Erlotinib and Cinacalcet. Preliminary investigations of the mechanism show that the hydroboration proceeds through the in situ formed BH₃ species.

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.

Reductive depolymerization of polyesters and polycarbonates with hydroboranes by using a lanthanum(III) tris(amide) catalyst

The homogeneous reductive depolymerization of polyesters and polycarbonates with hydroboranes is achieved with the use of an f-metal complex catalyst. These polymeric materials are transformed into their value-added alcohol equivalents. Catalysis proceeds readily, under mild conditions, with La[N(SiMe₃)₂]₃ (1 mol%) and pinacolborane (HBpin) and shows high selectivity towards alcohols and diols, after hydrolysis.

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.

Synthesis and characterization of rare-earth metallate amido complexes bearing 2-amidate-functionalized indolyl ligand and their application in the hydroboration of esters with pinacolborane

The reactions of 2-amidate-functionalized indolyl proligand 2-(2,6-iPr2C6H3NHC=O)C8H5NH (H2L) with [(Me3Si)2N]3RE(μ-Cl)Li(THF)3 were studied leading to the synthesis and characterization of a series of novel discrete trinuclear rare-earth metal metallate amido complexes...

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.

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.

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.

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.

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.

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.