Rhenium complex-catalyzed coupling reaction of enol acetates with alcohols

The reaction of enol acetates with alcohols in the presence of a catalytic amount of a rhenium complex, such as ReBr(CO)₅, produced the corresponding ketones and aldehydes in moderate to good yields. It was suggested that the preparation of an ether, an intermolecular dehydrated product, was the first step of the reaction.     

Rhenium-catalyzed synthesis of naphthalene derivatives via insertion of aldehydes into a C-H bond

A rhenium complex, [ReBr(CO)₃(THF)]₂, catalyzed reactions of aromatic ketimines and aldehydes with dienophiles, followed by dehydration, to give naphthalene derivatives in good to excellent yields. This reaction proceeds via C-H bond activation, insertion of an aldehyde, intramolecular nucleophilic cyclization, reductive elimination, elimination of aniline and Diels-Alder reaction. After dehydration, naphthalene derivatives were formed.     

Rhenium complex-catalyzed allylation of acetals with allyltrimethylsilane

It was confirmed that the treatment of acetals with allyltrimethylsilane in the presence of a catalytic amount of the rhenium complex, ReBr(CO)₅, gave the corresponding homoallylic ethers in excellent to good yields.     

A Reusable Unsupported Rhenium Nanocrystalline Catalyst for Acceptorless Dehydrogenation of Alcohols through γ‐C–H Activation

Rhenium nanocrystalline particles (Re NPs), of 2 nm size, were prepared from NH₄ReO₄ under mild conditions in neat alcohol. The unsupported Re NPs convert secondary and benzylic alcohols to ketones and aldehydes, respectively, through catalytic acceptorless dehydrogenation (AD). The oxidant‐ and acceptor‐free neat dehydrogenation of alcohols to obtain dihydrogen gas is a green and atom‐economical process for making carbonyl compounds. Secondary aliphatic alcohols give quantitative conversion and yield. Transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS), Re K‐edge X‐ray absorption near‐edge structure (XANES), and X‐ray absorption fine structure (EXAFS) data confirmed the characterization of the Re NPs as metallic rhenium with surface oxidation to rhenium(IV) oxide (ReO₂). Isotope labeling experiments revealed a novel γ‐CH activation mechanism for AD of alcohols.     

Simple System for Cleavage of Alkene C–C Bond Using Aqueous Sodium Paraperiodate

A simple and mild method for direct conversion of alkenes to corresponding aldehydes has been developed using sodium paraperiodate (Na₃H₂IO₆) in water. The reagent was found to give excellent yields of corresponding aldehydes.     

Rhenium complex-catalyzed acylative cleavage of ethers with acyl chlorides

It was found that rhenium complex was an efficient catalyst for the acylative cleavage of C-O bond of ethers with acyl chlorides. When acyclic ethers were allowed to react with acyl chlorides in the presence of a catalytic amount of ReBr(CO)₅, acylative cleavage of C-O bond of acyclic ethers smoothly proceeded to give the corresponding esters in moderate to good yields. Similarly, cyclic ethers were acylative cleaved by acyl chlorides to give the corresponding chloro substituted esters in good yields by the use of Re₂O₇ catalyst.     

Dimerization of aldehydes to carboxylic esters catalyzed by K2[Fe(CO)4]–crown ether system

K₂[Fe(CO)₄] (1) with a crown ether was found to be an efficient catalyst for the dimerization of aldehydes to carboxylic esters. Several aromatic aldehydes including furfural gave the corresponding esters in good yields. This reaction also proceeded intramolecularly to give phthalide from phthalaldehyde. However, aliphatic aldehydes gave aldol-condensation products instead of the corresponding esters. In the reactions of p-substituted benzaldehydes with 1, the reactivity decreased with increase of the electron-releasing ability of the substituents. On the basis of these results, the reaction mechanism including the nucleophilic attack of tetracarbonylferrate dianion to the carbonyl carbon is discussed.     

Kinetics of catalytic Meerwein-Ponndorf-Verley reduction of aldehydes and ketones using boron triethoxide

Catalytic Meerwein-Ponndorf-Verley (MPV) reduction of various aliphatic, aromatic, and unsaturated aldehydes and ketones to corresponding alcohols (analyzed by GC-MS) in the presence of boron triethoxide (B(OEt)₃) were studied. Kinetics of this reduction reaction was also studied and the respective rate constants were determined. It was found that B(OEt)₃ catalyzes the reduction of aliphatic aldehydes and ketones to alcohols at room temperature while aromatic aldehydes and ketones were not reduced under the same conditions. In addition, MPV reduction using B(OEt)₃ was found to be chemoselective as unsaturated aldehydes and ketones afforded the corresponding alcohols without affecting unsaturated groups. The mechanism proposed involves a six-membered transition state in which both the alcohol and the carbonyl are coordinated to the same boron centre of a boron alkoxide catalyst.     

Oxidative Cleavage of Epoxides Using Aqueous Sodium Paraperiodate

A simple and mild method in water for direct conversion of epoxides to corresponding aldehydes has been developed using sodium paraperiodate (Na₃H₂IO₆). High yields of corresponding aldehydes were obtained.     

Chloronitrosyl complexes of rhenium

Preparation and properties of several highly stable nitroso complexes of rhenium viz. ReCl₃(NO), Re(OH)₃(NO)₃ M₂ReCl₅(NO) where M is Cs, K and NH₄ are described. These are analogous to the corresponding ruthenium complexes. The oxidation number of the ‘ReNO’ unit as a whole, is +3 but that of the rhenium atom cannot be determined. The nitric oxide stretching band lies within the range of 1710–1920 cm^?1. The cesium and potassium salts are weakly paramagnetic at room temperature.     

Synthesis of carboxylic esters from aldehydes using metal carbonyl anions,part 2. Dimerization of aldehydes to carboxylic esters catalyzed by disodium pentacarbonylchromate,na2[cr(co)5]

Na₂[Cr(CO)₅] (1) was found to be an efficient catalyst for the dimerization of aldehydes to carboxylic esters. Several aromatic aldehydes including furfural gave the corresponding esters in good yields. This reaction also proceeded intramolecularly to give phthalide from phthalaldehyde. Compared with M₂[Fe(CO)₄] (M = Na, K), 1 was found to be a more efficient catalyst for this reaction. However, aliphatic aldehydes gave aldolcondensation products instead of the corresponding esters. In the reactions of p-substituted benzaldehydes with 1, the reactivity decreased with the increase of the electron-releasing ability of the substituents. However, even p-anisaldehyde, which hardly reacted with M₂[Fe(CO)₄], reacted with 1 to give the ester in moderate yield. The reaction mechanism, including the nucleophilic attack of the pentacarbonylchromate dianion on the carbonyl carbon, is discussed.     

Extraction of rhenium(VII) with aliphatic alcohols from acid solutions

Extraction of rhenium(VII) with C₇–C₁₀ aliphatic alcohols from HCl and H₂SO₄ solutions was examined. The rhenium(VII) distribution coefficients were examined in relation to the acidity and temperature. The composition of the extracted complexes and the thermodynamic parameters of extraction were determined. The extraction method of recovery and preconcentration of rhenium(VII) from H₂SO₄ solutions with secondary octyl alcohol was tested in the counterflow mode.     

Transition metal-free synthesis of primary amides from aldehydes and hydroxylamine hydrochloride

Primary aromatic amides can be synthesized from aldehydes and hydroxylamine hydrochloride in the presence of Cs₂CO₃. Various aromatic aldehydes (include some heteroaromatic aldehydes) are able to generate the corresponding aromatic amides in moderate to excellent yields.     

Photoelectron spectroscopy (XPS) and thermogravimetry (TG) of pure and supported rhenium oxides 1. Pure rhenium compounds

To investigate the interaction of rhenium with several supports, a preliminary study on pure rhenium compounds has been carried out in order to achieve a better understanding of their surface chemical properties. To this end X-ray photoelectron spectroscopy (XPS) and thermogravimetry (TG) methods have been applied. The results show that metallic rhenium, ReO₂ and ReO₃ are covered with Re(VII). By heating in H₂, NH₄ReO₄ and rhenium oxides are reduced to metallic rhenium in different temperature ranges. In the case of Re₂O₇ a spillover effect has been found when platinum is present. For ReO₃ and ReO₂ a morphological model has been described on the basis of combined XPS, TG and surface area measurements. The binding energy values of the different rhenium oxidation states have been assessed and their variation with the oxidation number briefly discussed.     

Rhenium-catalyzed insertion of aldehyde into a C-H bond: synthesis of isobenzofuran derivatives

A rhenium complex, [ReBr(CO)3(thf)]2, catalyzed reactions of aromatic ketimines with aldehydes to give isobenzofuran derivatives in good to excellent yields. In contrast to ruthenium and rhodium catalysts, aldehydes, which are polar unsaturated molecules, inserted into the C-H bond after activation by the rhenium complex.     

Asymmetric synthesis of 2,6-substituted dihydropyrone catalyzed by 3-monosubstituted and 3,3′-bisubstituted BINOL titanium complexes

Asymmetric hetero-Diels-Alder (HDA) reactions of aromatic aldehydes with Danishefsky’s diene derivative were carried out smoothly in the presence of the Ti(IV)-(R)-BINOL (1:1.2) complex to give the corresponding chiral 2,6-disubstituted dihydropyrones under mild conditions. The readily accessible I-Ti(OCH(CH₃)₂)₄ (1.2:1) complex was found to be an effective catalyst for this reaction. Aromatic aldehydes afforded the corresponding products in moderate yields (up to 72 %) with good enantioselectivities (up to 80 % ee). Aromatic aldehydes in presence of the TiCl₄-NaOCH₃-II (1:4.2:1.2) complex gave the products in higher yields (up to 73 %) with better enantioseletivities (up to 84 % ee).     

Probing Surface Sites of TiO2: Reactions with [HRe(CO)5] and [CH3Re(CO)5]

Two carbonyl complexes of rhenium, [HRe(CO)₅] and [CH₃Re(CO)₅], were used to probe surface sites of TiO₂ (anatase). These complexes were adsorbed from the gas phase onto anatase powder that had been treated in flowing O₂ or under vacuum to vary the density of surface OH sites. Infrared (IR) spectra demonstrate the variation in the number of sites, including Ti⁺³? OH and Ti⁺⁴? OH. IR and extended X‐ray absorption fine structure (EXAFS) spectra show that chemisorption of the rhenium complexes led to their decarbonylation, with formation of surface‐bound rhenium tricarbonyls, when [HRe(CO)₅] was adsorbed, or rhenium tetracarbonyls, when [CH₃Re(CO)₅] was adsorbed. These reactions were accompanied by the formation of water and surface carbonates and removal of terminal hydroxyl groups associated with Ti⁺³ and Ti⁺⁴ ions on the anatase. Data characterizing the samples after adsorption of [HRe(CO)₅] or [CH₃Re(CO)₅] determined a ranking of the reactivity of the surface OH sites, with the Ti⁺³? OH groups being the more reactive towards the rhenium complexes but the less likely to be dehydroxylated. The two rhenium pentacarbonyl probes provided complementary information, suggesting that the carbonate species originate from carbonyl ligands initially bonded to the rhenium and from hydroxyl groups of the titania surface, with the reaction leading to the formation of water and bridging hydroxyl groups on the titania. The results illustrate the value of using a family of organometallic complexes as probes of oxide surface sites.     

Optimizing the preparation of an alumina-supported rhenium catalyst for olefin metathesis

The processes involved in the formation of the alumina-supported rhenium catalyst for olefin metathesis, from the impregnation of the support (thermally activated alumina) with ammonium perrhenate to thermal activation, are studied. The monolayer coverage of the Al₂O₃ surface is observed at a rhenium content of 10 wt % (on Re₂O₇ basis), and the surplus rhenium is sublimed as heptoxide from the support upon thermal activation. In the metathesis of both linear α-olefins and methylenecyclobutanes, the optimum supported rhenium content of the catalyst is 10 wt % on Re₂O₇ basis.     

Rhodium‐catalyzed ortho‐C‐H olefination of aromatic aldehydes employing transient directing strategy

A Rhodium(III)‐catalyzed ortho‐C‐H olefination of aromatic aldehydes in the presence of catalytic amount of TsNH₂ has been developed. The in situ generated imine intermediate from aldehyde and TsNH₂ worked as a transient directing group. Both electron‐rich and electron‐deficient aromatic aldehydes were tolerated, affording the corresponding products in moderate to good yields. Importantly, the present protocol provides a straightforward access to olefinated aromatic aldehydes with aldehydes as the simple starting materials.     

Selective H2 and CO production with rhenium(I) biscarbonyl complexes as photocatalyst

Rhenium(I) biscarbonyl complexes with two phosphine ligands photocatalyzed not only CO₂ reduction under CO₂ atmosphere but also H₂ evolution under Ar. The reductant 1-benzyl-1,4-dihydronicotinamide (BNAH) worked only as a one-electron donor, and it was quantitatively converted to its corresponding oxidized dimer (BNA₂). The photocatalytic reactions required addition of a base such as triethanolamine, because deprotonation from the oxidized BNAH (BNAH^?+) is essential for the suppression of the back electron transfer from the reduced rhenium(I) complex to BNAH^?+. ¹H, ¹³C, and ³¹P NMR studies under vacuum or ¹³CO₂ atmosphere indicated that the rhenium(I) complex is relatively stable under the CO₂ reduction conditions, but it is converted to some other complexes under the H₂ evolution conditions.