Large-Scale Carbonyl Reductions in the Pharmaceutical Industry

Efficient hydroboration of carbonyls by an iron(ii) amide catalyst

An easily prepared iron(ii) amide precatalyst enables the selective hydroboration of carbonyls with HBpin (pinacolborane) in the absence of any additive. The reactions proceed with low catalytic loading (1-3 mol%) under mild reaction conditions and display wide functional group compatibility. Aldehydes are selectively hydroborated in the presence of other reducible functional groups, such as ketones, alkenes, nitriles, esters, amides, acids and halides.

Selective Hydrogenation of Cyclic Imides to Diols and Amines and Its Application in the Development of a Liquid Organic Hydrogen Carrier

Direct hydrogenation of a broad variety of cyclic imides to diols and amines using a ruthenium catalyst is reported here. We have applied this strategy toward the development of a new liquid organic hydrogen carrier system based on the hydrogenation of bis-cyclic imide that is formed by the dehydrogenative coupling of 1,4-butanediol and ethylenediamine using a new ruthenium catalyst. The rechargeable system has a maximum gravimetric hydrogen storage capacity of 6.66 wt%.

Room temperature catalytic carbon-hydrogen bond alumination of unactivated arenes: mechanism and selectivity

We report the first catalytic methods for the transformation of C-H bonds of unactivated arenes into C-Al bonds. The catalytic reactions occur at 25 °C (benzene, toluene and xylenes) with palladium loadings as low as 0.1 mol%. Remarkably, the C-H activation of toluene and xylenes proceeds with ortho- and meta-selectivity. This selectivity is highly unusual and complementary to both Friedel-Crafts and the majority of C-H borylation methods. Through a detailed mechanistic analysis (Eyring analysis, KIE, DFT, QTAIM) we show that unusual Pd-Al intermetallic complexes are on the catalytic cycle and that the selectivity is determined by weak attractive dispersion forces in the transition state for C-H bond breaking.

LiAlH4 : From Stoichiometric Reduction to Imine Hydrogenation Catalysis

Imine-to-amine conversion with catalytic instead of stoichiometric quantities of LiAlH₄ is demonstrated (85 °C, catalyst loading≥2.5 mol %, pressure≥1 bar). The effects of temperature, pressure, solvent, and catalyst modifications, as well as the substrate scope are discussed. Experimental investigations and preliminary DFT calculations suggest that the catalytically active species is generated in situ: LiAlH₄ +Ph(H)C=NtBu→LiAlH₂ [N(tBu)CH₂ Ph]₂ . A cooperative mechanism in which Li and Al both play a prominent role is proposed.

V, Mukku N, Chanda K, Maiti B. Rapid Construction of an Imidazo[4,5-b]pyridine Skeleton from 2-Chloro-3-nitropyridine via Tandem Reaction in H2O-IPA Medium

A highly efficient, clean, and simple procedure for the synthesis of a privilege imidazo[4,5-b]pyridine scaffold from 2-chloro-3-nitropyridine in combination with environmentally benign H₂O-IPA as a green solvent is presented. The scope of the novel method has been demonstrated through the tandem sequence of S_NAr reaction with substituted primary amines followed by the in situ nitro group reduction and subsequent heteroannulation with substituted aromatic aldehydes to obtain functionalized imidazo[4,5-b]pyridines with only one chromatographic purification step. The synthesis pathway appears to be green, simple, and superior compared with other already reported procedures, with the high abundance of reagents and great ability in expanding the structural diversity.

Lewis-base-catalysed selective reductions of ynones with a mild hydride donor

Nucleophilic phosphines catalyze efficient 1,2-reductions of ynones employing pinacolborane as a mild hydride donor in the presence of alcohol additives.

Reusable Fe2O3-nanoparticle catalysed efficient and selective hydroboration of carbonyl compounds

We present readily accessible Fe₂O₃ nanoparticles as an efficient catalyst for the selective hydroboration of carbonyl compounds, which represents the first example of the use of nanoparticles as a catalyst for this process.

Methanol as hydrogen source: transfer hydrogenation of aromatic aldehydes with a rhodacycle

A rhodacycle catalyses efficient hydrogenation of aldehydes, deriving the hydrogen from methanol.

Small bite-angle 2-phosphinophosphinine ligands enable rhodium-catalysed hydroboration of carbonyls

2-Phosphinophosphinine ligands generate Rh catalysts for the hydroboration of ketones and imines in contrast to standard phosphine ligands.

Iron Catalyzed Hydroboration of Aldehydes and Ketones

We report an operationally convenient room temperature hydroboration of aldehydes and ketones employing Fe(acac)₃ as precatalyst. The hydroboration of aldehydes and ketones proceeded efficiently at room temperature to yield, after work up, 1° and 2° alcohols; chemoselective hydroboration of aldehydes over ketones is attained under these conditions. We propose a σ-bond metathesis mechanism in which an Fe-H intermediate is postulated to be a key reactive species.

Tris(cyclopentadienyl)lanthanide Complexes as Catalysts for Hydroboration Reaction toward Aldehydes and Ketones

It was found that homoleptic cyclopentadienyl lanthanide complexes Cp₃Ln (Ln = Y (1), Yb (2), Sm (3), Nd (4), La (5), Cp = cyclopentadienyl) can be employed as excellent catalysts for the hydroboration of various aldehydes and ketones toward pinacolborane. These robust lanthanide catalysts exhibited high reactivity with low catalyst loadings (0.01-1 mol %) under mild conditions and good functional group tolerability. These complexes also demonstrated uniquely carbonyl-selective hydroboration in the presence of alkenes and alkynes.

Why Does Alkylation of the N-H Functionality within M/NH Bifunctional Noyori-Type Catalysts Lead to Turnover?

Molecular metal/NH bifunctional Noyori-type catalysts are remarkable in that they are among the most efficient artificial catalysts developed to date for the hydrogenation of carbonyl functionalities (loadings up to ∼10^-5 mol %). In addition, these catalysts typically exhibit high C═O/C═C chemo- and enantioselectivities. This unique set of properties is traditionally associated with the operation of an unconventional mechanism for homogeneous catalysts in which the chelating ligand plays a key role in facilitating the catalytic reaction and enabling the aforementioned selectivities by delivering/accepting a proton (H⁺) via its N-H bond cleavage/formation. A recently revised mechanism of the Noyori hydrogenation reaction (Dub, P. A. et al. J. Am. Chem. Soc. 2014, 136, 3505) suggests that the N-H bond is not cleaved but serves to stabilize the turnover-determining transition states (TDTSs) via strong N-H···O hydrogen-bonding interactions (HBIs). The present paper shows that this is consistent with the largely ignored experimental fact that alkylation of the N-H functionality within M/NH bifunctional Noyori-type catalysts leads to detrimental catalytic activity. The purpose of this work is to demonstrate that decreasing the strength of this HBI, ultimately to the limit of its complete absence, are conditions under which the same alkylation may lead to beneficial catalytic activity.

Chemoenzymatic one-pot reaction of noncompatible catalysts: combining enzymatic ester hydrolysis with Cu(i)/bipyridine catalyzed oxidation in aqueous medium

Combination of a lipase (CALB) with a Cu/bipyridine catalyst for environmentally benign synthesis of aldehydes from their corresponding esters.

Catalytic carbonyl hydrosilylations via a titanocene borohydride–PMHS reagent system

Catalytic amounts of titanocene(iii) borohydride, generated under mild conditions from commercially available titanocene dichloride, in concert with a stoichiometric hydride source is shown to effectively reduce aldehydes and ketones to their respective alcohols in aprotic media.

Organocatalytic hydroborylation promoted by N-heterocyclic olefins

N-Heterocyclic olefins (NHO) effectively catalyze the hydroborylation of selected ketones and aldehydes under mild conditions.