Complex formation between catalysts,alcohols, and isocyanates in the preparation of urethanes

The mechanism of the catalyzed reaction between alcohols and isocyanates was investigated by means of NMR, infrared, and ultraviolet spectroscopy. The shift of the ? OH proton resonance in the NMR spectra indicated the existence of a 1 : 1 complex in the system dibutyltin dilaurate (DBTDL)–1-methoxy-2-propanol. Complex formation was also observed when lead naphthenate or triethylamine (TEA) were substituted for the DBTDL. Mixtures of the DBTDL–TEA catalysts caused a shift of the ? OH proton resonance greater than that observed for either catalyst alone. This correlates with the synergistic effect noted when preparing urethanes with a mixture of these catalysts. No direct evidence of alcohol–catalyst complex formation could be obtained by infrared spectroscopy. Efforts were also made to detect complex formation in mixtures of phenyl isocyanate and catalysts. These complexes could not be detected by NMR, infrared, or ultraviolet spectroscopy.     

Kinetics and mechanism of urethane reactions: Phenyl isocyanate–alcohol systems

Urethane reactions of phenyl isocyanate alcohol systems with toluene as solvent and various aprotic polar solvents (including tertiary amines) as additives were carried out at constant temperature of 10–40°C. Analysis of the variation of the second order rate constants of these systems and those available in the literature indicates that formation of the hydrogen bonding complexes (alcohol with phenyl isocyanate and with aprotic solvent) and electron donor number (DN) of the aprotic solvent are the two factors allowing satisfactory explanation of the catalysis and inhibition effects of the wide range of aprotic solvents (including amines, amides, etc.). Based on these considerations, an ion-pair mechanism and the resulting kinetic equation for the urethane reaction are proposed. Verification on the kinetic equation with experimental results for the systems of phenyl isocyanate with alcohol in toluene (for the self catalysis of the alcohol), with dimethyl formamide and dimethyl sulfoxide in toluene (for the catalysis of the aprotic solvents), and with triethylamine in toluene (for the catalysis of the tertiary amines) shows satisfactory. In the mechanism, the aprotic solvent is considered to solvate the complex of phenyl isocyanate/alcohol at the active hydrogen to form an ion-pair which can undergo the urethane reaction more easily.     

Kinetics of urethane formation from isophorone diisocyanate: The catalyst and solvent effects

The dependence of the kinetic parameters of urethane formation in the reaction between isophorone diisocyanate and alcohols of different structure (n-propanol, isopropanol, propargyl alcohol, 1,3-diazidopropan-2-ol, and phenol) in diluted solutions on the natures of solvent (toluene, carbon tetrachloride) and catalyst (dibutyltin dilaurate, diazobicyclooctane) was found using an original IR spectroscopic procedure. The ratio of the apparent rate constants for the reactions involving the aliphatic and cycloaliphatic NCO groups of isophorone diisocyanate was determined, and the efficiency of catalysis in these reactions was estimated. The reaction conditions under which the difference between the reactivities of isocyanate groups can reach 40 were determined.     

Highly Efficient,Catalytic Meerwein–Ponndorf–Verley Reduction with a Novel Bidentate Aluminum Catalyst

Double electrophilic activation of carbonyl groups allows a modern variant of the Meerwein–Ponndorf–Verley reduction to be carried out under mild conditions with bidentate catalyst 1 (see reaction). Various carbonyl substrates can be reduced efficiently at room temperature in CH₂Cl₂ with 2-propanol or sec-phenethyl alcohol in the presence of a catalytic amount of 1 . This system is also applicable to the Oppenauer oxidation of secondary alcohols to the corresponding ketones.     

A silica gel-supported ruthenium complex of 1,4,7-trimethyl-1,4,7-triazacyclononane as recyclable catalyst for chemoselective oxidation of alcohols and alkenes by tert-butyl hydroperoxide

A silica gel-immobilized [(Me(3)tacn)Ru(III)(CF(3)COO)(2)(H(2)O)]CF(3)CO(2) complex (1-SiO(2), Me(3)tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) was prepared by simple impregnation, and the catalyst was characterized by powdered X-ray diffraction, nitrogen adsorption/desorption, Raman, and diffuse reflectance UV-vis spectroscopies. The supported Ru catalyst can effect facile oxidation of alcohols by tert-butyl hydroperoxide (TBHP). Primary and secondary benzyl, allylic, and propargylic alcohols were transformed to their corresponding aldehydes and ketones in excellent yields; no oxidation of the C=C and Ctbd1;C bonds was observed for the allylic and propargylic alcohol oxidations. Likewise alkene epoxidation by TBHP can be achieved by 1-SiO(2); cycloalkenes such as norbornene and cyclooctene were oxidized to their exo-epoxides exclusively in excellent yields (>95%). The 1-SiO(2) catalyst can be recycled and reused for consecutive alcohol and alkene oxidations without significant loss of catalytic activity and selectivity; over 9000 turnovers have been attained for the oxidation of 1-phenyl-1-propanol to 1-phenyl-1-propanone. 4-Substituted phenols were oxidized by the "1 + TBHP" protocol to give exclusively ruthenium-catecholate complexes, which were characterized by UV-vis and ESI-MS spectroscopies. No (tert-butyldioxy)cyclohexadienone and other radical coupling/overoxidation products were produced using the "1 + TBHP" protocol. The formation of ruthenium-catecholate is proposed to proceed via ortho-hydroxylation of phenol.     

Nitrogen–Carbon Bond Formation by Reactions of a Titanium–Potassium Dinitrogen Complex with Carbon Dioxide,tert‐Butyl Isocyanate,and Phenylallene

Nitrogen–carbon bond‐forming reactions at coordinated dinitrogen in a bifunctional titanium–potassium system are reported. A titanium atrane complex with a tris(aryloxide)methyl ligand ( 1 ) was treated with two equivalents of potassium naphthalenide under N₂ atmosphere to generate a bifunctional complex ( 2 ) in which N₂ binds end‐on to two titanium centers and side‐on to three potassium cations. Dinitrogen complex 2 reacted with carbon dioxide, tert ‐butyl isocyanate, and phenylallene, forming nitrogen–carbon bonds and affording diverse N‐functionalized products. The reaction of 2 with CO₂ followed by addition of Me₃SiCl resulted in the formation of the starting complex 1 with concomitant release of silylated carboxyl hydrazines while the reaction with two equivalents of tert ‐butyl isocyanate proceeded by insertion into the Ti−N bonds. Treatment of 2 with phenylallene afforded vinyl‐substituted hydrazido complexes.     

Isothermal vapour–liquid equilibria in the binary and ternary systems composed of 2-propanol, diisopropyl ether and 1-methoxy-2-propanol

Vapour pressures for 1-methoxy-2-propanol are reported as well as the vapour–liquid equilibrium data in the two binary 2-propanol + 1-methoxy-2-propanol, and diisopropyl ether + 1-methoxy-2-propanol systems, and in the ternary 2-propanol + diisopropyl ether + 1-methoxy-2-propanol system. The data were measured isothermally at 330.00 and 340.00 K covering the pressure range 5–98 kPa. The binary vapour–liquid equilibrium data were correlated using the Wilson, NRTL, and Redlich–Kister equations; resulting parameters were then used for calculation of phase behaviour in the ternary system and for subsequent comparison with experimental data.     

Nanopalladium on polyethylenimine–grafted starch: An efficient and ecofriendly heterogeneous catalyst for Suzuki–Miyaura coupling and transfer hydrogenation reactions

Functionalized natural polysaccharides are attractive supports for colloidal metal nanocatalysts due to their abundance, cheapness, biocompatibility and biodegradability. In this study, isocyanate–functionalized starch was prepared by treating with diisocyanate. Polyethylenimine grafted onto starch via the formation of urea linker. The palladium nanoparticles deposited starch PEIS@Pd(0) was obtained through a chelating–in situ reduction procedure. Characterization of these materials was done using Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X–ray diffraction, and inductive coupled plasma atomic emission spectrometry. The catalytic activity of PEIS@Pd(0) was then tested in two series of model reactions: Suzuki–Miyaura coupling and transfer hydrogenation. The catalyst could be recovered by simple filtration and was reused for five times without significant loss of catalytic activity, which confirmed the good stability of the catalyst.     

Effect of temperature conditions on the mechanism of formation,molecular structure,and properties of polyurethanes

The formalized kinetic approach to the mechanism of urethane formation in the system of prepolymer with end NCO groups–1,4-butanediol together with the dispersion analysis of kinetic curves have helped to establish that, at moderate temperatures, the mechanism may be represented by a scheme comprising a noncatalytic stage of urethane formation and a catalytic one including the formation of a double complex of a catalyst with an isocyanate or a hydroxyl group. As the temperature rises the proportion of the noncatalytic stage decreases significantly, while a mechanism, including the formation of the triple complex isocyanate group–catalyst–hydroxyl, becomes the most probable. It has been shown that the formation of a triple complex at elevated temperatures is thermodynamically more advantageous than the formation of double complexes; hence these changes of mechanism with temperature. It has been found that the temperature conditions of polyurethane production greatly affect the degree of its branching. Two crosslinked polyurethanes were obtained, one under the temperature conditions providing for the minimum degree of branching, the other under isothermal conditions. It has been discovered that the degree of branching of a prepolymer may greatly affect the density of chemical and physical bonds and a range of physical and chemical properties of the polyurethane.     

Spectroscopic study of complex formation by methanol and phenyl isocyanate with bis(acetylacetonato) copper

It has been shown that the reaction of phenyl isocyanate with methanol in dioxane in the presence of bis(acetylacetonato) copper involves the formation of intermediate complexes. The isocyanate is coordinated to the copper ion of the catalyst, and the alcohol is joined to the oxygen of the chelate rings of bis(acetylacetonato) copper together and hydrogen bond. This brings the reacting molecules closer together and orients them relative to one another, thus facilitating their interaction. The phenylmethylurethan formed is also coordinated to the catalyst, and when it appears in the system, equilibrium is established between the complexes formed by the catalyst with the solvent, the isocyanate, and the urethan.     

Palladium–cadmium sulfide nanopowder at oil–water interface as an effective catalyst for Suzuki–Miyaura reactions

A simple and effective strategy is described for the synthesis of Pd–CdS nanopowder by the reduction of an organopalladium(II) complex, [PdCl₂(cod)] (cod = cis ,cis ‐1,5‐cyclooctadiene), in the presence of CdS quantum dots (QDs) at a toluene–water interface. We investigated the impact of addition of CdS QDs on catalytic activity of Pd nanoparticles (NPs). The Pd–CdS nanopowder functions as an efficient catalyst for Suzuki–Miyaura reactions for the formation of carbon–carbon bonds. There is a high electron density on Pd NPs and due to their high electron affinity they behave as an electron scavenger from CdS increasing the rate of oxidative addition, which is the rate‐determining step of the catalytic cycle, and, just as we expect, the C─C coupling reaction with the Pd–CdS nanopowder is faster and occurs in less time than that with Pd nanocatalysts. Compared to classical reactions, this method consistently has the advantages of short reaction times, high yields in a green solvent, reusability of the catalyst without considerable loss of catalytic activity and low cost, and is a facile method for the preparation of the catalyst.     

Mechanistic characterization of aerobic alcohol oxidation catalyzed by Pd(OAc)(2)/pyridine including identification of the catalyst resting state and the origin of nonlinear [catalyst] dependence

The Pd(OAc)(2)/pyridine catalyst system is one of the most convenient and versatile catalyst systems for selective aerobic oxidation of organic substrates. This report describes the catalytic mechanism of Pd(OAc)(2)/pyridine-mediated oxidation of benzyl alcohol, which has been studied by gas-uptake kinetic methods and (1)H NMR spectroscopy. The data reveal that turnover-limiting substrate oxidation by palladium(II) proceeds by a four-step pathway involving (1) formation of an adduct between the alcohol substrate and the square-planar palladium(II) complex, (2) proton-coupled ligand substitution to generate a palladium-alkoxide species, (3) reversible dissociation of pyridine from palladium(II) to create a three-coordinate intermediate, and (4) irreversible beta-hydride elimination to produce benzaldehyde. The catalyst resting state, characterized by (1)H NMR spectroscopy, consists of an equilibrium mixture of (py)(2)Pd(OAc)(2), 1, and the alcohol adduct of this complex, 1xRCH(2)OH. These in situ spectroscopic data provide direct support for the mechanism proposed from kinetic studies. The catalyst displays higher turnover frequency at lower catalyst loading, as revealed by a nonlinear dependence of the rate on [catalyst]. This phenomenon arises from a competition between forward and reverse reaction steps that exhibit unimolecular and bimolecular dependences on [catalyst]. Finally, overoxidation of benzyl alcohol to benzoic acid, even at low levels, contributes to catalyst deactivation by formation of a less active palladium benzoate complex.     

The role of tin(II) octoate – azobisisobutyronitrile complex in the formation rate of the respective networks in simultaneous interpenetrating polymer networks

Simultaneous interpenetrating polymer networks (IPNs) based on polyether polyurethane (PUR) and poly(methyl methacrylate‐co‐trimethylol‐propane trimethacrylate) (PMMA) were prepared in bulk at 60°C, using tin(II) octoate and azobisisobutyronitrile (AIBN) as pur polymerization catalyst and free‐radical initiator, respectively. The kinetics of the PUR network formation, PMMA network formation as well as PUR/PMMA IPN formation were studied independently by Fourier transform infra‐red spectroscopy. The simultaneous formation of the two networks interfered with each other, although they follow different polymerization mechanisms. Mainly two effects concerning the free‐radical polymerization have been seen: a decrease of the initiation period and an earlier appearance of the Trommsdorff effect when increasing the concentration of the catalyst. On the other hand, the presence of AIBN in the reaction medium drastically reduced the catalytic efficiency of the organotin compound. An explanation of these results for this particular activating system could be the formation of a cyclic equimolar complex by coordination of the nitrile groups of AIBN with the Sn(II) atom. Complexation both reduces the effective catalyst concentration and induces steric constraints in the azo bond of AIBN, rendering this linkage weaker and more easily cleavable and allowing an early decomposition into radicals of the complexed AIBN. The maximum rate corresponds to a 1:1 complex. Further, decomposition into radicals leads to tin oxidation and formation of a new tetravalent organotin compound, the catalytic activity of which is lower than that of pure tin(II) octoate for the isocyanate‐alcohol reaction.     

Polymerization of propylene with TiCl3–AlEt2Cl–polymeric electron donor catalyst

Polymeric donors having ether or carbonyl groups were added to the TiCI₃–AlEt₂CI catalyst system as the third component, and the effects on the polymerization of propylene were investigated in comparison with the effect of the electron donors with low molecular weight. The polymeric donors were effective in making the catalyst more active, but the donors of low molecular weight depressed the catalyst activity. In the case of poly(propylene glycol dimethyl ether) (PPG-DME), PPG–DME with a number of propylene oxide units (n) of more than 6.7 was effective in enhancing the catalyst activity. These effects were considered to be due to the different reactivities between TiCI₃ and AlEt₂CI-polymeric donor complexes having various chain lengths.     

Cyclotrimerization of butyl and cyclohexyl isocyanate—catalysis and kinetics

The cyclotrimerization of model aliphatic and cycloaliphatic isocyanates (butyl and cyclohexyl isocyanate) was carried out using an ammonium carboxylate and a salicylaldehyde-potassium complex as catalysts. The kinetics of the cyclotrimerization of butyl isocyanate in both 2-ethoxyethyl acetate and dimethylformamide (DMF) using the 2-ethylhexanoate salt of trimethylaminopropanol-2 was found to be of first order with respect to the isocyanate and also of first order with respect to the catalyst. The reaction rate in DMF was considerably greater than in 2-ethoxyethyl acetate, as could be expected. Employing the salicylaldehyde-potassium catalyst, the cyclotrimerization of butyl isocyanate followed second-order kinetics with respect to the isocyanate and first order with regard to the catalyst. Due to the fact that the cyclotrimerization of cyclohexyl isocyanate was found to be slower than that of butyl isocyanate, the cyclotrimerization of this isocyanate was carried out only in DMF using the 2-ethylhexanoate salt of trimethylaminopropanol-2 as the catalyst. The kinetics of this reaction was found to follow second order with respect to the isocyanate and first order with regard to the catalyst. The products of the reactions were identified by IR, ¹H-NMR, and mass spectrometry.     

Asymmetric hydrogenation of salicyl alcohol catalyzed by methylsulfo–sodium carboxymethylcellulose–Pt complex

A new chiral polymer–metal complex, methylsulfo–sodium carboxymethyl–cellulose–Pt complex (MS‐NaCMC‐Pt), has been prepared by the reaction of sodium carboxymethylcellulose with methylsulfonyl chloride and H₂PtCl₆·6H₂, which was found to be able to catalyze the asymmetric hydrogenation of salicyl alcohol to give (1S,2S)‐2‐(hydroxymethyl)‐cyclohexanol at 28 °C and under 1 atm H₂, in > 90% product and optical yields, respectively. The catalyst could be reused many times without any remarkable changes in optical catalytic activity. Copyright © 2000 John Wiley & Sons, Ltd.     

Fluoropolyethers end‐capped by polar functional groups. III. Kinetics of the reactions of hydroxy‐terminated fluoropolyethers and model fluorinated alcohols with cyclohexyl isocyanate catalyzed by organotin compounds

The kinetics of the dibutyltin dilaurate (DBTDL)‐catalyzed urethane formation reactions of cyclohexyl isocyanate (CHI) with model monofunctional fluorinated alcohols and fluoropolyether diol Z‐DOL H‐1000 of various molecular weights (100–1084 g mol^?1) in different solvents were studied. IR spectroscopy and chemical titration methods were used for measuring the rate of the total NCO disappearance at 30–60 °C. The effects of the reagents and DBTDL catalyst concentrations, the solvent and hydroxyl‐containing compound nature, and the temperature on the reaction rate and mechanism were investigated. Depending on the initial reagent concentration and solvent, the reactions could be well described by zero‐order, first‐order, second‐order, or more complex equations. The reaction mechanism, including the formation of intermediate ternary or binary complexes of reagents with the tin catalyst, could vary with the concentration and solvent and even during the reaction. The results were treated with a rate expression analogous to those used for enzymatic reactions. Under the explored conditions, the rate of the uncatalyzed reaction of fluorinated alcohols with CHI was negligible. Moreover, there was no allophanate formation, nor were there other side reactions, catalysis by urethane in the absence of DBTDL, or a synergetic effect in the presence of the tin catalyst. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3771–3795, 2002     

Half‐sandwich ruthenium‐based versatile catalyst for both alcohol oxidation and catalytic hydrogenation of carbonyl compounds in aqueous media

A series of half‐sandwich ruthenium‐based catalysts for both alcohol oxidation and carbonyl compounds hydrogenation have been synthesized through metal‐induced C–H bond activation based on benzothiazole ligands. The neutral ruthenium complexes 1 – 4 were fully characterized by UV–vis, NMR, IR, and elemental analysis. Molecular structures of complexes 1 and 3 were further confirmed by X‐ray diffraction analysis. All complexes exhibited high activity for the catalytic oxidation of a variety of alcohols with ^tBuOOH as oxidants to give carbonyl compounds with high yields in water. Moreover, these half‐sandwich complexes also showed high efficiency for the catalytic hydrogenation of carbonyl compounds in a methanol–water mixture. The catalyst could be reused for at least five cycles without any loss of activity. The catalytic system also worked well for various kinds of substrates with either electron‐donating or electron‐withdrawing groups.     

手性双胺双膦-铱体系催化芳香酮的高对映选择性氢转移氢化

在异丙醇溶液中,从[Ir(COD)Cl]2和C2-对称的手性双胺双膦配体原位制备了手性能-Ir(Ⅰ)配合物,并直接用于催化几种芳香酮的不对称氢转移氢化。结果表明,该配合物是异丙基苯基酮不对称转移氢化的优秀催化剂,当底物酮与催化剂的摩尔比(S/C)为1200:1时,在室温下反应4h后,得到相应的手性芳香醇的转化率和对映选择性分别高达98%和98%ee.     

CuO-NiO/SiO2催化氧化1-甲氧基-2-丙醇合成甲氧基丙酮

 采用浸渍法制备了CuO－NiO／SiO2负载型催化剂．以空气为氧源，对CuO－NiO／SiO2催化体系催化氧化1－甲氧基－2－丙醇合成甲氧基丙酮反应的催化活性进行了考察．实验结果表明，NiO组分的负载量对催化活性影响较大；NiO和CuO两者之间有很强的协同催化效应．TPR和XRD结果表明，添加镍组分可促进铜在载体表面上分散，使氧化物还原温度降低，提高催化活性．在优化条件下，1－甲氧基－2－丙醇的转化率可达74．3％，甲氧基丙酮的收率可达63％．