The “Borrowing Hydrogen Strategy” by Supported Ruthenium Hydroxide Catalysts: Synthetic Scope of Symmetrically and Unsymmetrically Substituted Amines

The N‐alkylation of ammonia (or its surrogates, such as urea, NH₄HCO₃, and (NH₄)₂CO₃) and amines with alcohols, including primary and secondary alcohols, was efficiently promoted under anaerobic conditions by the easily prepared and inexpensive supported ruthenium hydroxide catalyst Ru(OH)_x/TiO₂. Various types of symmetrically and unsymmetrically substituted “tertiary” amines could be synthesized by the N‐alkylation of ammonia (or its surrogates) and amines with “primary” alcohols. On the other hand, the N‐alkylation of ammonia surrogates (i.e., urea and NH₄HCO₃) with “secondary” alcohols selectively produced the corresponding symmetrically substituted “secondary” amines, even in the presence of excess amounts of alcohols, which is likely due to the steric hindrance of the secondary alcohols and/or secondary amines produced. Under aerobic conditions, nitriles could be synthesized directly from alcohols and ammonia surrogates. The observed catalysis for the present N‐alkylation reactions was intrinsically heterogeneous, and the retrieved catalyst could be reused without any significant loss of catalytic performance. The present catalytic transformation would proceed through consecutive N‐alkylation reactions, in which alcohols act as alkylating reagents. On the basis of deuterium‐labeling experiments, the formation of the ruthenium dihydride species is suggested during the N‐alkylation reactions.     

An Environmentally Benign Catalytic Method for Efficient and Selective Nucleophilic Ring Opening of Oxiranes by Zirconium Tetrakis(dodecyl Sulfate)

An operationally simple and environmentally benign protocol for a highly regio‐ and chemoselective preparation of β‐substituted alcohols by means of ring‐opening reactions of oxiranes with various aliphatic alcohols, H₂O, NaN₃, and NaCN as nucleophiles in the presence of catalytic amounts of zirconium tetrakis(dodecyl sulfate) as Lewis acid/surfactant‐combined catalysts (LASCs) was developed. The high efficiency of the catalyst was confirmed by the high product yields obtained within desired times and, in particularly by the reusability of the Zr^IV complex.     

Selective tetrahydropyranylation of alcohols and phenols using titanium(IV) salophen trifluoromethanesulfonate as an efficient catalyst

Titanium(IV) salophen trifluoromethanesulfonate, [Ti^IV(salophen)(OSO₂CF₃)₂], as a catalyst enables selective tetrahydropyranylation of alcohols and phenols with 3,4‐dihydro‐2H‐pyran. Using this catalytic system, primary, secondary and tertiary alcohols, as well as phenols, were converted to their corresponding tetrahydropyranyl ethers in high yields and short reaction times at room temperature. Investigation of the chemoselectivity of this method showed discrimination between the activity of primary alcohols in the presence of secondary and tertiary alcohols and phenols. This heterogenized catalyst could be reused several times without loss of its catalytic activity. Copyright © 2016 John Wiley & Sons, Ltd.     

Rapid Synthesis of 2‐Alkanol‐substituted Pyrrolo[2,3‐b]quinoxalines from Propargylic Alcohols via Copper‐free Sonogashira Coupling Reaction at Room Temperature

A practical and efficient procedure is established for the synthesis of 2‐alkanol‐substituted pyrrolo[2,3‐b]quinoxalines by the reaction of N‐alkyl‐3‐chloroquinoxaline‐2‐amines with propargylic alcohols. The reaction is carried out in the absence of any copper salt but in the presence of a catalytic amount of Pd(PPh₃)₂Cl₂ at room temperature. The Sonogashira coupling reaction step in this procedure is fast, producing clean products with high yields without contamination by unwanted homocoupling Glaser reaction products. The synthesized pyrroloquinoxaline derivatives are also screened against the three bacterial strains Micrococcus luteus, pseudomonas aeruginosa, and Bacillus subtilis.     

PMA‐SiO2: A Heterogenous Catalyst for O‐, S‐, and N‐Nucleophilic Substitution Reactions of Aryl Propargyl Alcohols

Aryl‐propargylic alcohols undergo O‐, S‐, and N‐nucleophilic substitution reactions in the presence of a catalytic amount of PMA‐SiO₂.     

The lewis acidic ruthenium-complex-catalyzed addition of beta-diketones to alcohols and styrenes is in fact Brønsted acid catalyzed

The perchlorate salt of the dicationic bipy–ruthenium complex cis‐[Ru(6,6′‐Cl₂bipy)₂(H₂O)₂]²⁺ effectively catalyzes addition of β‐diketones to secondary alcohols and styrenes to yield the α‐alkylated β‐diketones. In a catalytic addition reaction of acetylacetone to 1‐phenylethanol, the κ²‐acetylacetonate complex [Ru(6,6′‐Cl₂bipy)₂(κ²‐acac)]ClO₄ was isolated after the catalysis; this complex is readily synthesized by reacting cis‐[Ru(6,6′‐Cl₂bipy)₂(H₂O)₂](ClO₄)₂ with acetylacetone. [Ru(6,6′‐Cl₂bipy)₂(κ²‐acac)]ClO₄ is unreactive toward 1‐phenylethanol in the presence of HClO₄; it also fails to catalyze the addition of acetylacetone to 1‐phenylethanol. On the basis of these observations, it is proposed and confirmed by independent experiments that the catalytic addition of β‐diketones to the secondary alcohols is in fact catalyzed by the Brønsted acid HClO₄, which is generated by the reaction of cis‐[Ru(6,6′‐Cl₂bipy)₂(H₂O)₂](ClO₄)₂ with the β‐diketone.     

Domino Rhodium/Palladium‐Catalyzed Dehydrogenation Reactions of Alcohols to Acids by Hydrogen Transfer to Inactivated Alkenes

The combination of the d⁸ Rh^I diolefin amide [Rh(trop₂N)(PPh₃)] (trop₂N=bis(5‐H‐dibenzo[a,d]cyclohepten‐5‐yl)amide) and a palladium heterogeneous catalyst results in the formation of a superior catalyst system for the dehydrogenative coupling of alcohols. The overall process represents a mild and direct method for the synthesis of aromatic and heteroaromatic carboxylic acids for which inactivated olefins can be used as hydrogen acceptors. Allyl alcohols are also applicable to this coupling reaction and provide the corresponding saturated aliphatic carboxylic acids. This transformation has been found to be very efficient in the presence of silica‐supported palladium nanoparticles. The dehydrogenation of benzyl alcohol by the rhodium amide, [Rh]N, follows the well established mechanism of metal–ligand bifunctional catalysis. The resulting amino hydride complex, [RhH]NH, transfers a H₂ molecule to the Pd nanoparticles, which, in turn, deliver hydrogen to the inactivated alkene. Thus a domino catalytic reaction is developed which promotes the reaction R‐CH₂‐OH+NaOH+2 alkene→R‐COONa+2 alkane.     

Graphene oxide‐bound electron‐deficient tin(IV) porphyrin: a highly efficient and selective catalyst for trimethylsilylation of alcohols and phenols with hexamethyldisilazane

The catalytic activity of graphene oxide‐bound tetrakis(p ‐aminophenyl)porphyrinatotin(IV) trifluoromethanesulfonate, [Sn^IV(TNH₂PP)(OTf)₂], in the trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS) is reported. The prepared catalyst was characterized using inductively coupled plasma analysis, scanning electron microscopy, transmission electron microscopy, and Fourier transform infrared and diffuse reflectance UV–visible spectroscopies. This heterogeneous catalyst was used for selective trimethylsilylation of various alcohols and phenols with HMDS in short reaction times and high yields. Also, the catalyst is of high reusability and stability, in that it was recovered several times without loss of its initial activity. The chemoselectivity of this catalytic system in the silylation of primary alcohols in the presence of secondary and tertiary alcohols and also phenols was investigated.     

Chloride‐Bridged Dinuclear Rhodium(III) Complexes Bearing Chiral Diphosphine Ligands: Catalyst Precursors for Asymmetric Hydrogenation of Simple Olefins

Efficient rhodium(III) catalysts were developed for asymmetric hydrogenation of simple olefins. A new series of chloride‐bridged dinuclear rhodium(III) complexes 1 were synthesized from the rhodium(I) precursor [RhCl(cod)]₂, chiral diphosphine ligands, and hydrochloric acid. Complexes from the series acted as efficient catalysts for asymmetric hydrogenation of (E)‐prop‐1‐ene‐1,2‐diyldibenzene and its derivatives without any directing groups, in sharp contrast to widely used rhodium(I) catalytic systems that require a directing group for high enantioselectivity. The catalytic system was applied to asymmetric hydrogenation of allylic alcohols, alkenylboranes, and unsaturated cyclic sulfones. Control experiments support the superiority of dinuclear rhodium(III) complexes 1 over typical rhodium(I) catalytic systems.     

Commercial Zinc Oxide (Zn2+) as an Efficient and Environmentally Benign Catalyst for Homogeneous Benzoylation of Hydroxyl Functional Groups

The solvent‐free O‐acylation of some alcohols with benzoyl chloride was carried out to the corresponding benzoylated products in good to excellent yields by the mediation of a catalytic amount (5 mol%) of the commercially available and inexpensive zinc oxide in short reaction times. This methodology represents an eco‐friendly and simple catalytic alternative for benzoylation of primary, secondary, tertiary, and benzylic alcohols with ZnO. This catalytic system was homogeneous because of high solubility of zinc oxide in the reaction medium. Findings showed that ZnO was dissolved in hydrochloric acid, created in situ, after a few minutes. Although, others argued on the catalytic role of solid phase zinc oxide under a heterogeneous condition, it is not surprising to emphasize on the catalytic function of Zn²⁺ in the benzoylation of alcohols under homogeneous reaction conditions. Zinc oxide served as pre‐catalyst to form Zn²⁺, as the catalytically active species.     

Platinum‐Catalyzed Multi‐Step Reaction of Propargyl Alcohols with N‐Heteroaromatics

N‐Methyl indole reacts with but‐2‐yn‐1‐ol in the presence of PtCl₂ in MeOH giving indole derivatives having a substituted 3‐oxobutyl group at the 3‐position in good yield. Under the reaction conditions, various substituted indoles and substituted propargyl alcohols are successfully involved in the reaction giving the corresponding addition products in good to moderate yields. The catalytic reaction can be further extended to N‐phenyl pyrrole. In the present multi‐step reaction, PtCl₂ likely plays dual roles: as the catalyst for the rearrangement of propargyl alcohols to the corresponding alkenyl ketones and as the catalyst for the addition of indoles to the alkenyl ketones. Experimental evidence is provided to support the proposed mechanism.     

Catalytic activity of nickel(II), copper(II) and oxovanadium(II)‐dihydroindolone complexes towards homogeneous oxidation reactions

Three novel paramagnetic metal complexes (MH₂ID) of Ni²⁺, Cu²⁺ and VO²⁺ ions with 3‐hydroxy‐3,3’‐biindoline‐2,2’‐dione (dihydroindolone, H₄ID) were synthesized and characterized by different spectroscopic methods. The ligand (H₄ID) was synthesized via homocoupling reaction of isatin in presence of phenylalanine in methanol. Complexation of low valent Ni²⁺, Cu²⁺ ions and high valent VO²⁺ ions with H₄ID carried out in 1: 2 molar ratios. A comparison in the catalytic potential of paramagnetic complexes of low and high valent metal ion was explored in the oxidation processes of cis‐cyclooctene, benzyl alcohol and thiophene by an aqueous H₂O₂, as a green terminal oxidant, in the presence and absence of acetonitrile, as an organic solvent, at 85 °C. NiH₂ID, CuH₂ID and VOH₂ID show good catalytic activity, i.e. good chemo‐ and regioselectivity. VOH₂ID has the highest catalytic potential compared to both Ni²⁺‐ and Cu²⁺‐species in the same homogenous aerobic atmosphere. Catalytic oxidation of other alkenes and alcohols was also studied using NiH₂ID, CuH₂ID or VOH₂ID as a pre‐catalyst by an aqueous H₂O₂. A mechanistic pathway for those oxidation processes was proposed.     

Practical Enantioselective Arylation and Heteroarylation of Aldehydes with In Situ Prepared Organotitanium Reagents Catalyzed by 3‐Aryl‐H8‐BINOL‐Derived Titanium Complexes

A highly efficient and practical method for the catalytic enantioselective arylation and heteroarylation of aldehydes with organotitanium reagents, prepared in situ by the reaction of aryl‐ and heteroaryllithium reagents with ClTi(OiPr)₃, is described. Titanium complexes derived from DPP‐H₈‐BINOL ( 3 d ; DPP=3,5‐diphenylphenyl) and DTBP‐H₈‐BINOL ( 3 e ; DTBP=3,5‐di‐tert‐butylphenyl) exhibit excellent catalytic activity in terms of enantioselectivity and turnover efficiency for the transformation, providing diaryl‐, aryl heteroaryl‐, and diheteroarylmethanol derivatives in high enantioselectivity at low catalyst loading (0.2–2 mol %). The reaction begins with a variety of aryl and heteroaryl bromides through their conversion into organolithium intermediates by Br/Li exchange with nBuLi, thus providing straightforward access to a range of enantioenriched alcohols from commercially available starting materials. Various 2‐thienylmethanols can be synthesized enantioselectively by using commercially available 2‐thienyllithium in THF. The reaction can be carried out on a 10 mmol scale at 0.5 mol % catalyst loading, demonstrating its preparative utility.     

Chiral Borate Esters in Asymmetric Synthesis. Part 2

Asymmetric catalytic activity of the chiral spiroborate esters 1 – 9 with a O₃BN framework (see Fig. 1) toward borane reduction of prochiral ketones was examined. In the presence of 0.1 equiv. of a chiral spiroborate ester, prochiral ketones were reduced by 0.6 equiv. of borane in THF to give (R)‐secondary alcohols in up to 92% ee and 98% isolated yields (Scheme 1). The stereoselectivity of the reductions depends on the constituents of the chiral spiroborate ester (Table 2) and the structure of the prochiral ketones (Table 1). The configuration of the products is independent of the chirality of the diol‐derived parts of the catalysts. A mechanism for the catalytic behavior of the chiral spiroborate esters (R,S)‐ 2 and (S,S)‐ 2 during the reduction is also suggested.     

Natural hydroxyapatite‐supported MnO2: a green heterogeneous catalyst for selective aerobic oxidation of alkylarenes and alcohols

Natural hydroxyapatite‐supported MnO₂ (MnO₂@NHAp) was easily prepared in situ from reduction of potassium permanganate with natural hydroxyapatite derived from cow bones in water at room temperature, and its structure was characterized using flame atomic absorption spectroscopy, X‐ray diffraction, thermogravimetric analysis, scanning electron microscopy and energy dispersive spectroscopy. The catalytic activity of the synthesized catalyst was investigated for the aerobic oxidation of alkylarenes and alcohols. MnO₂@NHAp shows excellent catalytic performance for the oxidation of alkylarenes and alcohols to their corresponding carbonyl compounds without using any other oxidizing agent. This catalyst can be readily recycled and reused for several runs without any significant loss of efficiency. Copyright © 2016 John Wiley & Sons, Ltd.     

Enantioselective addition of diethylzinc to aldehydes catalyzed by β‐amino alcohols derived from (1R,2S)‐norephedrine

β‐Amino alcohols derived from (1R,2S)‐norephedrine were synthesized and used as ligands in the catalytic enantioselective diethylzinc addition to benzaldehydes. N‐alkylated (1R,2S)‐norephedrine‐based derivative 3a gave the highest enantioselectivity. The effects of different parameters on the enantioselectivity of the product were investigated. Copyright © 2009 John Wiley & Sons, Ltd.     

Efficient asymmetric addition of diethylzinc to aldehydes using C2‐novel chiral pyridine β‐amino alcohols as chiral ligands

A series of novel C₂‐symmetric chiral pyridine β‐amino alcohol ligands have been synthesized from 2,6‐pyridine dicarboxaldehyde, m‐phthalaldehyde and chiral β‐amino alcohols through a two‐step reaction. All their structures were characterized by ¹H NMR, ¹³C NMR and IR. Their enantioselective induction behaviors were examined under different conditions such as the structure of the ligands, reaction temperature, solvent, reaction time and catalytic amount. The results show that the corresponding chiral secondary alcohols can be obtained with high yields and moderate to good enantiomeric excess. The best result, up to 89% ee, was obtained when the ligand 3c (2S,2′R)‐2,2′‐((pyridine‐2,6‐diylbis(methylene))bisazanediyl))bis(4‐methyl‐1,1‐diphenylpentan‐1‐ol) was used in toluene at room temperature. The ligand 3g (2S,2′R)‐2,2′‐((1,3‐phenylenebis(methylene))bis(azanediyl))bis(4‐methyl‐1,1‐diphenylpentan‐1‐ol) was prepared in which the pyridine ring was replaced by the benzene ring compared to 3c in order to illustrate the unique role of the N atom in the pyridine ring in the inductive reaction. The results indicate that the coordination of the N atom of the pyridine ring is essential in the asymmetric induction reaction. Copyright © 2014 John Wiley & Sons, Ltd.     

Palladium‐catalyzed carbonylative cyclization of 3‐bromoallyl alcohols leading to furan‐2(5H)‐ones

3‐Bromoallyl alcohols are carbonylatively cyclized under carbon monoxide pressure in toluene in the presence of a catalytic amount of Pd(OAc)₂ and PPh₃ along with Na₂CO₃ to give furan‐2(5H)‐ones in good yields. Copyright © 2012 John Wiley & Sons, Ltd.     

Ruthenium(II) Complexes Bearing Schiff Base Ligands for Efficient Acceptorless Dehydrogenation of Secondary Alcohols†

Four ruthenium(II) complexes 1—4 [RN=CH‐(2,4‐(^tBu)₂C₆H₂O)]RuH(PPh₃)₂(CO) (R = C₆H₅, 1; R = 4‐MeC₆H₄, 2; R = 4‐ClC₆H₄, 3; R = 4‐BrC₆H₄, 4) bearing Schiff base ligands were prepared by treating RuHClCO(PPh₃)₃ with RN=CH‐(2,4‐(^tBu)₂C₆H₂OH (L1—L4) in the presence of triethylamine. Their structures were fully characterized by elemental analysis, IR, NMR spectroscopy and X‐ray crystallography. These Ru(II) complexes exhibit high catalytic performance and good functional‐group compatibility in the acceptorless dehydrogenation of secondary alcohols, affording the corresponding ketones in 82%—94% yields.     

Highly Efficient Redox Isomerisation of Allylic Alcohols Catalysed by Pyrazole‐Based Ruthenium(IV) Complexes in Water: Mechanisms of Bifunctional Catalysis in Water

The catalytic activity of ruthenium(IV) ([Ru(η³:η³‐C₁₀H₁₆)Cl₂L]; C₁₀H₁₆=2,7‐dimethylocta‐2,6‐diene‐1,8‐diyl, L=pyrazole, 3‐methylpyrazole, 3,5‐dimethylpyrazole, 3‐methyl‐5‐phenylpyrazole, 2‐(1H‐pyrazol‐3‐yl)phenol or indazole) and ruthenium(II) complexes ([Ru(η⁶‐arene)Cl₂(3,5‐dimethylpyrazole)]; arene=C₆H₆, p‐cymene or C₆Me₆) in the redox isomerisation of allylic alcohols into carbonyl compounds in water is reported. The former show much higher catalytic activity than ruthenium(II) complexes. In particular, a variety of allylic alcohols have been quantitatively isomerised by using [Ru(η³:η³‐C₁₀H₁₆)Cl₂(pyrazole)] as a catalyst; the reactions proceeded faster in water than in THF, and in the absence of base. The isomerisations of monosubstituted alcohols take place rapidly (10–60 min, turn‐over frequency=750–3000 h^?1) and, in some cases, at 35 °C in 60 min. The nature of the aqueous species formed in water by this complex has been analysed by ESI‐MS. To analyse how an aqueous medium can influence the mechanism of the bifunctional catalytic process, DFT calculations (B3LYP) including one or two explicit water molecules and using the polarisable continuum model have been carried out and provide a valuable insight into the role of water on the activity of the bifunctional catalyst. Several mechanisms have been considered and imply the formation of aqua complexes and their deprotonated species generated from [Ru(η³:η³‐C₁₀H₁₆)Cl₂(pyrazole)]. Different competitive pathways based on outer‐sphere mechanisms, which imply hydrogen‐transfer processes, have been analysed. The overall isomerisation implies two hydrogen‐transfer steps from the substrate to the catalyst and subsequent transfer back to the substrate. In addition to the conventional Noyori outer‐sphere mechanism, which involves the pyrazolide ligand, a new mechanism with a hydroxopyrazole complex as the active species can be at work in water. The possibility of formation of an enol, which isomerises easily to the keto form in water, also contributes to the efficiency in water.