Monomer Catalyst Research Articles

A broadly applicable and practical oligomeric (salen)Co catalyst for enantioselective epoxide ring-opening reactions

The (salen)Co catalyst (4a) can be prepared as a mixture of cyclic oligomers in a short, chromatography-free synthesis from inexpensive, commercially available precursors. This catalyst displays remarkable enhancements in reactivity and enantioselectivity relative to monomeric and other multimeric (salen)Co catalysts in a wide variety of enantioselective epoxide ring-opening reactions. The application of catalyst 4a is illustrated in the kinetic resolution of terminal epoxides by nucleophilic ring-opening with water, phenols, and primary alcohols; the desymmetrization of meso epoxides by addition of water and carbamates; and the desymmetrization of oxetanes by intramolecular ring opening with alcohols and phenols. The favorable solubility properties of complex 4a under the catalytic conditions facilitated mechanistic studies, allowing elucidation of the basis for the beneficial effect of oligomerization. Finally, a catalyst selection guide is provided to delineate the specific advantages of oligomeric catalyst 4a relative to (salen)Co monomer 1 for each reaction class.

Synthesis of Cinchona Alkaloid-Derived Chiral Polymers by Mizoroki–Heck Polymerization and Their Application to Asymmetric Catalysis

To facilitate the asymmetric catalysis process, we designed novel polymeric chiral catalysts. Since quaternary ammonium salts of cinchona alkaloid derivatives show efficient catalytic activity in various asymmetric transformations, we have synthesized novel chiral polymer catalysts containing cinchonidinium moieties in the main chain of the polymer. Repetitive Mizoroki–Heck coupling reactions between the cinchona alkaloid-derived dimer and diiodide afforded the chiral polymer catalysts, which were subsequently used as catalysts in asymmetric benzylation reactions to yield the corresponding phenylalanine derivatives in higher yields and levels of enantioselectivity than can be obtained with a monomeric catalyst. Because of the insolubility of the polymeric catalysts, they were easily recovered from the reaction mixture and reused several times.

ChemInform Abstract: Application of Pulse Radiolysis to Mechanistic Investigations of Water Oxidation Catalysis

AbstractReview: <50 refs.

Mechanistic analysis of water oxidation catalyzed by mononuclear copper in aqueous bicarbonate solutions

We characterize a mechanism for a monomeric copper catalyst reported to oxidize water in bicarbonate solution when subject to sufficiently high external potentials at near neutral pH values.

Rationally Designing a Polymerization Catalyst for Electron-Deficient Monomers

Rationally Designing a Polymerization Catalyst for Electron-Deficient Monomers

Platinum Phosphinito Catalysts for Nitrile Hydration

The reaction of PEt2OH with K2PtCl4 yields [PtCl{(PEt2O)2H}]2 (2). The X-ray crystal structure of 2 shows that the two chloride ligands bridge the two Pt atoms. The structure is unusual in that the PtCl2Pt unit is bent, which is the first example of a platinum phosphinito chloride-bridged dimer that is bent. Dimer 2 is a poor catalyst for the hydration of acetonitrile (and presumably other nitriles). Likewise, the monomeric catalyst that results from the reaction of 2 with sodium hydroxide is not a good hydration catalyst. In an attempt to form a more reactive monomeric catalyst, PtCl(PMe2OH){(PMe2O)2H} (1) was reacted with sodium hydroxide. This reaction resulted in the formation of two species, tentatively characterized as Na[PtCl(PMe2O){(PMe2O)2H}] (5) and Na[PtCl(OH){(PMe2O)2H}] (6). Complexes 5 and 6 could not be separated. However, the mixture of the two complexes quickly hydrated acetonitrile. The catalyst mixture of 5 and 6 was efficiently poisoned by cyanide, which prevented its use as a catalyst for the hydration of cyanohydrins.

Monomeric Ferrocene Bis-Imidazoline Bis-Palladacycles: Variation of Pd–Pd Distances by an Interplay of Metallophilic, Dispersive, and Coulombic Interactions

Monomeric ferrocene bis-imidazoline bis-palladacycles (FBIP) have recently been reported to be efficient bimetallic catalysts in different sorts of asymmetric reactions by the cooperation of two Pd(II) centers. A crucial parameter regarding the efficiency of reactions catalyzed in a bimetallic mode is—in general—the intermetallic distance of both catalytically relevant metal centers. In this article we describe the structural elucidation of the monomeric FBIP catalyst type (usually generated in situ from a catalytically inactive dimeric chloride bridged precatalyst) by X-ray crystal structure analysis. Two dicationic monomeric complexes are compared to a neutral complex. The solid-state structures reveal varying Pd–Pd distances ranging from 3.15 to 5.27 A for the doubly charged complexes, whereas for the neutral complex a distance of 3.28 A has been found. This variability is supposed to be one of the key advantages of a ferrocene backbone in a bimetallic catalyst system, since the Fe–Cp bonds allow the b...

ChemInform Abstract: Catalysis on Gold‐Nanoparticle‐Passivating Monolayers

AbstractReview: 48 refs.

Radical polymerization of methyl methacrylate (MMA) initiated by KHSO5 – BTBAC system - a kinetic study

Kinetics of free radical polymerization of methyl methacrylate using potassium peroxomonosulfate as initiator in the presence of benzyltributylammonium chloride (BTBAC) as phase transfer catalyst was studied. The polymerization reactions were carried out under nitrogen atmosphere and unstirred conditions at a constant temperature of 60°C in ethyl acetate/water bi-phase system. The role of concentrations of monomer, initiator, catalyst, temperature, acid and ionic strength on the rate of polymerization (Rp) was ascertained. The orders with respect to monomer, initiator and phase transfer catalyst were found to be 1.5, 0.5 and 0.5 respectively. The rate of polymerization (Rp) is independent of ionic strength and pH. Based on the kinetic results, a suitable mechanism is proposed.

The OO Bonding in Water Oxidation: the Electronic Structure Portrayal of a Concerted Oxygen Atom–Proton Transfer Pathway

The two-phased reaction: Calculations on the monomeric ruthenium catalyst with 1,10-phenanthroline-2,9-dicarboxylic acid reveals an interaction of the RuV=O complex with water that proceeds through ...

ChemInform Abstract: Chiral Linear Polymers Bonded Alternatively with Salen and 1,4‐Dialkoxybenzene: Synthesis and Application for Ti‐Catalyzed Asymmetric TMSCN Addition to Aldehydes.

AbstractA polymeric titanium/salen catalyst (ca. 15 repeating units) promotes the asymmetric addition of TmsCN to aldehydes with enantioselectivities comparable to those of the corresponding monomeric catalyst.

Co(III)-Porphyrin-Mediated Highly Regioselective Ring-Opening of Terminal Epoxides with Alcohols and Phenols

Co-T(p-OMe)PP-SbF6 (1-SbF6, T(p-OMe)PP = 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrinato) is demonstrated to be a highly active and regioselective catalyst in the ring-opening of various epoxides with alcohols and phenols. 1-SbF6 is demonstrated to be twice as active as the established Co-salen-SbF6 (2-SbF6, salen = trans-N,N-bis(3,5-ditertbutylsalicylidene)-1,2-cyclohexanediamine) catalyst, exhibiting >99% regioselectivity in many cases. The catalyst is suggested to promote the ring-opening via a cooperative bimolecular mechanism because the Co-porphyrin dimer, 1d-OTs, is significantly more active compared with the monomeric catalyst, 1-OTs.

ChemInform Abstract: Heterogenization of Homogeneous Catalysts on Dendrimers

AbstractReview: 60 refs.

ChemInform Abstract: Supported Monomeric Vanadium Catalyst for Dehydration of Amides to Form Nitriles.

AbstractA monomeric vanadium oxide species is created on the surface of hydrotalcite.

Self‐Assembly Approach toward Chiral Bimetallic Catalysts: Bis‐Urea‐Functionalized (Salen)Cobalt Complexes for the Hydrolytic Kinetic Resolution of Epoxides

A series of novel bis-urea-functionalized (salen)Co complexes has been developed. The complexes were designed to form self-assembled structures in solution through intermolecular urea-urea hydrogen-bonding interactions. These bis-urea (salen)Co catalysts resulted in rate acceleration (up to 13 times) in the hydrolytic kinetic resolution (HKR) of rac-epichlorohydrin in THF by facilitating cooperative activation, compared to the monomeric catalyst. In addition, one of the bis-urea (salen)Co(III) catalyst efficiently resolves various terminal epoxides even under solvent-free conditions by requiring much shorter reaction time at low catalyst loading (0.03-0.05 mol %). A series of kinetic/mechanistic studies demonstrated that the self-association of two (salen)Co units through urea-urea hydrogen bonds was responsible for the observed rate acceleration. The self-assembly study with the bis-urea (salen)Co by FTIR spectroscopy and with the corresponding (salen)Ni complex by (1)H NMR spectroscopy showed that intermolecular hydrogen-bonding interactions exist between the bis-urea scaffolds in THF. This result demonstrates that self-assembly approach by using non-covalent interactions can be an alternative and useful strategy toward the efficient HKR catalysis.

ChemInform Abstract: Mn(III) Salen Complexes‐Catalyzed Enantioselective Addition of Trimethyl Silylcyanide to N‐Benzylimines in the Presence of 4‐Phenyl Pyridine‐N‐oxide as an Additive.

Abstract Chiral monomeric and dimeric Mn(III) salen complexes viz., [(S,S)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminato manganese(III) chloride and 5,5-methylene di-[(S,S)-{N-(3-tert-butyl salicylidine)-N′-(3′,5′-di-tert-butyl salicylidene)}-1,2-cyclohexanediaminato manganese(III) chloride were used as catalysts for the highly enantioselective Strecker reaction of various imines (derived by the condensation of aldehydes and amine) with TMSCN as a source of cyanide in toluene. Excellent yield (95%) of α-amino nitrile with ee >99% was achieved when N-(2-methoxybenzylidine)-1-phenylmethanamine was used as a substrate and 4-phenyl pyridine-N-oxide (4-PPNO) as an additive at −55 °C in 28 h. The dimeric catalyst was found to be more reactive and enantioselective than the monomeric catalyst. The chiral dimeric catalyst used in the present study was recoverable and recyclable several times with retention of its performance.

Enantioselective desymmetrization of meso-epoxides with anilines catalyzed by polymeric and monomeric Ti(IV) salen complexes

The active catalysts for the enantioselective ring opening (ARO) of meso-stilbene oxide, cis-butene oxide, cyclohexene oxide, cyclopentene oxide, and cyclooctene oxide with various substituted anilines were generated in situ by the reaction of Ti(O(i)Pr)(4) with poly-[(R,R)-N,N'-bis-{3-(1,1-dimethylethyl)-5-methylene salicylidene} cyclohexane-1,2-diamine]-1 and (1R,2R)-N,N'-bis[3,5-di(tert-butyl)salicylidene] cyclohexane-1,2-diamine-2. These catalysts in the presence of nonracemic imine as an additive provided β-amino alcohol in excellent yield (99%) and chiral purity (enantiomeric excess (ee) up to 99%) for the ARO of meso-stilbene oxide with aniline. The same protocol was less effective for the ARO of cyclic epoxides; however, when triphenylphosphine was used as an additive, there was a significant improvement in catalyst performance for the ARO of cyclohexene oxide (yield, 85-90%; ee, 63-67%). Both in situ generated polymeric and monomeric catalysts performed in a similar manner except that the polymeric catalyst Ti(IV)-1 was more active and recycled several times with retention of enantioselectivity when compared with the monomeric catalyst Ti(IV)-2, which was nonrecyclable.

Epoxy Resins Cross-Linked with Rosin Adduct Derivatives. Cross-Linking and Thermal Behaviors

Rosin adduct derivatives were obtained by chemical reaction of the Diels−Alder adduct of rosin acid with maleic anhydride (RAMA) with aspartic acid (ASP) and p-aminobenzoic acid (p-ABA), at 1/1 molar ratio. The structure of the obtained monomers was established by means of elemental analysis, FT-IR spectroscopy, and 1H NMR. These acids were used as cross-linking agents, in the presence of triethylbenzylammonium chloride as catalyst (TEBAC) for diglycidyl monomers (diglycidyl ether of bisphenol A, DGEBA, and diglycidyl ether of hydroquinone, DGEHQ). Using differential scanning calorimetry (DSC) at different heating rates and the literature methods, the kinetic parameters of cross-linking reactions were obtained. The thermal stability of the cross-linked polymers investigated by using thermogravimetric analysis (TGA) showed they are reasonably thermostable.

Injectable, dual cross-linkable polyphosphazene blend hydrogels

A new class of injectable, self cross-linkable, and thermosensitive polyphosphazene-based blending systems of functional thiolated and acrylated polymers was designed and synthesized to develop an ideal injectable carrier, and to overcome many barriers associated with developing the injectable carriers, such as the uses of monomeric crosslinkers, catalysts, oxidants, pH adjustments, initiators, UV light, heat production and organic solvent. The aqueous solutions of the polymer blends were exhibited a solution state at low temperature, and transformed into a hydrogel state with desired mechanical strength at body temperature via thermosensitive hydrophobic interactions. The mechanical strength was further improved by the cross-linking of thiol groups with acrylate groups in the polymer network under physiological conditions. The thermoresponsive hydrophobic interactions in the polymer network accelerated the chemical cross-linking to improve the mechanical property. The mechanical strength, inner three-dimensional network, and degradation rate can be tuned through the degree of cross-linking between the thermosensitive and functional blended polymers. The results suggest that the self cross-linkable thermosensitive polyphosphazene blend systems have great potentials to play a crucial role as an injectable carrier because of their improved suitable mechanical properties for application potentials, in addition to their inherent advantages such as injectable, biodegradable and thermosensitive properties.

Mn(III) salen complexes-catalyzed enantioselective addition of trimethyl silylcyanide to N-benzylimines in the presence of 4-phenyl pyridine-N-oxide as an additive

Chiral monomeric and dimeric Mn(III) salen complexes viz., [( S,S)- N,N′-bis(3,5-di- tert-butylsalicylidene)-1,2-cyclohexanediaminato manganese(III) chloride and 5,5-methylene di-[( S,S)-{ N-(3- tert-butyl salicylidine)- N′-(3′,5′-di- tert-butyl salicylidene)}-1,2-cyclohexanediaminato manganese(III) chloride were used as catalysts for the highly enantioselective Strecker reaction of various imines (derived by the condensation of aldehydes and amine) with TMSCN as a source of cyanide in toluene. Excellent yield (95%) of α-amino nitrile with ee >99% was achieved when N-(2-methoxybenzylidine)-1-phenylmethanamine was used as a substrate and 4-phenyl pyridine-N-oxide (4-PPNO) as an additive at −55 °C in 28 h. The dimeric catalyst was found to be more reactive and enantioselective than the monomeric catalyst. The chiral dimeric catalyst used in the present study was recoverable and recyclable several times with retention of its performance.