Ethylene polymerization reactions with Ziegler–Natta catalysts. I. Ethylene polymerization kinetics and kinetic mechanism

Kinetics of ethylene homopolymerization reactions and ethylene/1-hexene copolymerization reactions using a supported Ziegler–Natta catalyst was carried out over a broad range of reaction conditions. The kinetic data were analyzed using a concept of multicenter catalysis with different centers that respond differently to changes in reaction parameters. The catalyst contains five types of active centers that differ in the molecular weights of material they produce and in their copolymerization ability. In ethylene homopolymerization reactions, each active center has a high reaction order with respect to ethylene concentration, close to the second order. In ethylene/α-olefin copolymerization reactions, the centers that have poor copolymerization ability retain this high reaction order, whereas the centers that have good copolymerization ability change the reaction order to the first order. Hydrogen depresses activity of each type of center in the homopolymerization reactions in a reversible manner; however, the centers that copolymerize ethylene and α-olefins well are not depressed if an α-olefin is present in the reaction medium. Introduction of an α-olefin significantly increases activity of those centers, which are effective in copolymerizing it with ethylene but does not affect the centers that copolymerize ethylene and α-olefins poorly. To explain these kinetic features, a new reaction scheme is proposed. It is based on a hypothesis that the Ti—C₂H₅ bond in active centers has low reactivity due to the equilibrium formation of a Ti—C₂H₅ species with the H atom in the methyl group β-agostically coordinated to the Ti atom in an active center. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4255–4272, 1999     

Hydroxynitrile lyase-catalyzed addition of HCN to 2- and 3-substituted cyclohexanones

The addition of HCN to monosubstituted cyclohexanones yielding cyanohydrins is strongly catalyzed by hydroxynitrile lyases (HNLs). With PaHNL from bitter almonds, the addition to 2-alkyl cyclohexanones 1b-g is highly (R)-selective, whereas the methyl compound 1a reacts (S)-selectively. With MeHNL from cassava, all 2-alkyl derivatives 1 react (S)-selectively. The catalytic activity of both PaHNL and MeHNL decreases with increasing size of the substituent in substrates 1. The diastereoselectivity of HCN additions to 2-alkoxy cyclohexanones 4 and 3-substituted cyclohexanones 6, however, is only moderate. The absolute configuration of the synthesized cyanohydrins was determined by X-ray crystallography of O-p-bromobenzoyl derivatives.     

不对称双酸催化3,4-二氢-2H-吡喃的反电子Hetero-Diels-Alder反应

研究了双酸催化剂不对称催化烯醚和β,γ-不饱和α-酮酸酯的反电子Hetero-Diels-Alder (HDA)反应, 为手性合成3,4-二氢-2H-吡喃类化合物提供了一种新的催化合成方法. InBr₃与手性磷酸钙盐Ca(1c)₂组合的手性双路易斯酸催化体系能够有效催化3,4-二氢-2H-吡喃和β,γ-不饱和α-酮酸酯的反电子HDA反应, 反应给出优秀的产率(最高达98%), 中等到良好的非对映选择性(最高达89:11)和良好到优秀的对映选择性(最高可达94%). 并且该双酸催化体系也能成功实现其它烯醚(如: 2,3-二氢-2H-呋喃, 乙烯基乙醚)的HDA反应, 获得优秀的非对映选择性(>94:6)和良好的对映选择性.     

O-glycoligases, a new category of glycoside bond-forming mutant glycosidases, catalyse facile syntheses of isoprimeverosides

A mutant α-xylosidase that lacks its catalytic acid/base residue efficiently catalyses the synthesis of O-linked isoprimeverosides (Xyl-α-1,6GlcOR) in near quantitative yields.     

Direct Catalytic Asymmetric Mannich‐type Reaction of α,β‐Unsaturated γ‐Butyrolactam with Ketimines

We report a direct catalytic asymmetric Mannich‐type addition of α,β‐unsaturated γ‐butyrolactam to α‐ethoxycarbonyl ketimines promoted by a soft Lewis acid/Brønsted base cooperative catalyst. A thiophosphinoyl group on the nitrogen of ketimines was crucial for both electrophilic activation and α‐addition of γ‐butyrolactams. The obtained aza‐Morita–Baylis–Hillman‐type products bear an α‐amino acid architecture with a tetra‐substituted stereogenic center.     

Computational Investigation on the Ethylene-induced Esterase from Citrus Sinensis

In order to understand the interaction between ethylene-induced esterase(EIE, EC 3.1.1.) from Citrus sinensis and α-naphthyl acetate, a 3D model of EIE was generated based on the crystal structure of the tobacco salicylic acid binding protein 2(SABP2). With the aid of the molecular mechanics and molecular dynamics methods, the final refined model was obtained and its reliability was further assessed. In this study, the docking results show that the main-chain amide of residue His85 and residue Val18 can form hydrogen bonds to the carbonyl oxygen group of α-naphthyl acetate. MM-PBSA method was applied to calculating the binding free energy between EIE mutants and α-naphthyl acetate. Our calculated binding free energy of each of the two mutant complexes is increased compared with that of the one of the wild type, which is unfavorable to the reaction. It is well consistent with the experimental data. The above results clearly indicate that His85 and Val18 in EIE function as the oxyanion role and take part in the catalytic reaction. The new structural insights obtained from this computational study are expected to stimulate further biochemical studies on the structures and mechanisms of EIE and other members of the plant α/β hydrolases.     

Anti-Markovnikov hydration of α-olefins and the other pathways to n-alcohols

Three possible mechanisms of the direct hydration of α-olefins under conditions of catalysis with metal complexes in solution are considered, and information that confirms the possible realization of the reaction steps of various types is presented. Factors determining the regioselectivity of reaction steps are considered in the framework of various mechanisms, and the available published data about catalytic systems capable of ensuring the direct hydration of α-olefins are analyzed. New approaches to organization of the synthesis of n-alcohols from olefins and dienes are discussed, which are based on the systems of consecutive reactions, conjugated, and combined (associated) processes.     

Evidence that protons can be the active catalysts in Lewis acid mediated hetero-Michael addition reactions

The mechanism of Lewis acid catalysed hetero‐Michael addition reactions of weakly basic nucleophiles to α,β‐unsaturated ketones was investigated. Protons, rather than metal ions, were identified as the active catalysts. Other mechanisms have been ruled out by analyses of side products and of stoichiometric enone–catalyst mixtures and by the use of radical inhibitors. No evidence for the involvement of π‐olefin–metal complexes or for carbonyl–metal‐ion interactions was obtained. The reactions did not proceed in the presence of the non‐coordinating base 2,6‐di‐tert‐butylpyridine. An excellent correlation of catalytic activities with cation hydrolysis constants was obtained. Different reactivities of mono‐ and dicarbonyl substrates have been rationalised. A ¹H NMR probe for the assessment of proton generation was established and Lewis acids have been classified according to their propensity to hydrolyse in organic solvents. Brønsted acid‐catalysed conjugate addition reactions of nitrogen, oxygen, sulfur and carbon nucleophiles are developed and implications for asymmetric Lewis acid catalysis are discussed.     

Click chemistry in materials synthesis. II. Acid‐swellable crosslinked polymers made by copper‐catalyzed azide–alkyne cycloaddition

A highly crosslinked hyperbranched polymer that rapidly swells and shrinks in a halogenated solvent in response to the addition of an acid or base has been prepared by Cu(I) catalysis of the reaction between a diazide and an amine‐containing trialkyne. The triazole linkages in the polymer are highly stable and may also play a role in the swelling behavior. The swelling–deswelling process is reversible. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5513–5518, 2006     

Enantioselective De Novo Synthesis of α,α-Diaryl Ketones from Alkynes

Chiral α,α-diaryl ketones are structural motifs commonly present in bioactive molecules, and they are also valuable building blocks in synthetic organic chemistry. However, catalytic asymmetric synthesis of α,α-diaryl ketones bearing a tertiary stereogenic center remains largely unsolved. Herein, we report a catalytic de novo enantioselective synthesis of α,α-diaryl ketones from simple alkynes via chiral phosphoric acid (CPA) catalysis. A broad range of enolizable α,α-diaryl ketones are prepared in good yields and with excellent enantioselectivities. The described protocol also serves as an efficient deuteration method for the preparation of enantiomerically enriched deuterated α,α-diaryl ketones. Using the methodology reported, bioactive molecules, including one of the best-selling anti-breast cancer drugs, tamoxifen, are readily synthesized.     

Helical peptide models for protein glycation: proximity effects in catalysis of the Amadori rearrangement.

INTRODUCTION: Non-enzymatic glycation of proteins has been implicated in various diabetic complications and age-related disorders. Proteins undergo glycation at the N-terminus or at the epsilon-amino group of lysine residues. The observation that only a fraction of all lysine residues undergo glycation indicates the role of the immediate chemical environment in the glycation reaction. Here we have constructed helical peptide models, which juxtapose lysine with potentially catalytic residues in order to probe their roles in the individual steps of the glycation reaction. RESULTS: The peptides investigated in this study are constrained to adopt helical conformations allowing residues in the i and i+4 positions to come into spatial proximity, while residues i and i+2 are far apart. The placing of aspartic acid and histidine residues at interacting positions with lysine modulates the steps involved in early peptide glycation (reversible Schiff base formation and its subsequent irreversible conversion to a ketoamine product, the Amadori rearrangement). Proximal positioning of aspartic acid or histidine with respect to the reactive lysine residue retards initial Schiff base formation. On the contrary, aspartic acid promotes catalysis of the Amadori rearrangement. Presence of the strongly basic residue arginine proximate to lysine favorably affects the pK(a) of both the lysine epsilon-amino group and the singly glycated lysine, aiding in the formation of doubly glycated species. The Amadori product also formed carboxymethyl lysine, an advanced glycation endproduct (AGE), in a time-dependent manner. CONCLUSIONS: Stereochemically defined peptide scaffolds are convenient tools for studying near neighbor effects on the reactivity of functional amino acid sidechains. The present study utilizes stereochemically defined peptide helices to effectively demonstrate that aspartic acid is an efficient catalytic residue in the Amadori arrangement. The results emphasize the structural determinants of Schiff base and Amadori product formation in the final accumulation of glycated peptides.     

Complex new materials from simple chemistry: Combining an amino‐substituted polysiloxane and carboxylic acids

Changes in rheological, adhesive, and swelling properties of quaternary salts, made by adding one of eight mono‐ or six α,ω‐alkanedioic acids (the latter with two to six or nine carbon atoms) to 6‐7PSil (a polysiloxane with 6%–7% of the monomer units contain a 3‐aminopropyl group) have been correlated with the salt structures. The simple acid‐base chemistry initiates drastic changes in the bulk properties of the materials that depend on the amount and type of the added acid. Thus, the quaternized forms of the 6‐7PSil have significantly enhanced viscoelastic and adhesive properties compared to those of the initial polysiloxane, and they can swell selectively liquids based on their polarity. Also, interpenetrating networks have been made in situ by photopolymerization of salts with vinylic carboxylic acids. DFT calculations on model systems consisting of dimethylammonium α,ω‐alkanedioate salts with two to six carbon atoms provide insights into the interactions responsible for the unexpected dependence of the properties of the 6‐7PSil salts on the chain lengths of the diacids. The potential for applying the methodologies described here to systems with other amino‐substituted polymers and other acid types is noted. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3851–3861     

The kinetics,catalysis and inhibition of the Baeyer—Villiger reaction in the processes of liquid-phase oxidation of organic compounds

Literary data on kinetics, catalysis and inhibition of the oxidation reaction of carbonyl compounds with peroxy acids according to the Baeyer—Villiger reaction under aerobic liquidphase oxidation conditions have been considered and discussed. The main reaction channel involves a reversible formation of α-hydroxyperoxy ester and its rearrangement to an ester or a lactone. In the case of homolytic decomposition of α-hydroxyperoxy ester no esters are formed. At all steps the formation and transformation of α-hydroxyperoxy ester are catalyzed by carboxylic acids. The possibility of formation of the second intermediate, presumably dioxirane, is shown. Catalysts of the oxidation processes such as variable-valency metal salts influence the kinetics at all steps in the Baeyer—Villiger reaction. Inhibition of ester formation in the presence of cobalt and manganese salts is associated with catalysis of homolytic decomposition of peroxy acid and α-hydroxyperoxy ester.     

Histidine in enzyme active centers.

The presence of histidine in the active center of an enzyme can be demonstrated by kinetic measurements, chemical modification, NMR spectroscopy or X-ray structure analysis. Histidine is the only naturally occurring amino acid to contain an imidazole residue as a side chain. The role of histidine in enzyme catalysis depends, inter alia, upon the special features of the imidazole residue: it thus tends to form hydrogen bonds, combines donor and acceptor properties and can take part in either nucleophilic or base catalysis. In some of these enzymes the position of each atom is known; however, the theories as to how the catalysis proceeds at a molecular level are controversial.     

Controlled racemization and asymmetric transformation of α-substituted carboxylic acids in the melt

The racemization and asymmetric transformation of a series of α-substituted carboxylic acids, viz. mandelic acid, hydratropic acid, ibuprofen and naproxen, were studied. Several racemization methods for mandelic acid were studied and it was found that base-catalyzed racemization in aprotic polar solvents was the most efficient method. Moreover, a fast and mild base-catalyzed racemization reaction in the melt was developed. DBN turned out to be a very efficient racemizing base for the substrates studied. Combination of the base-catalyzed racemization in the melt with known resolution processes resulted in crystallization-induced asymmetric transformations. Treating racemic ibuprofen or hydratropic acid with 1.5–2.5 equivalents of enantiopure α-methylbenzylamine and a catalytic amount of DBN resulted in the isolation of enantiomerically enriched ibuprofen or hydratropic acid with ees of up to 75% and almost quantitative yields.     

Asymmetric biocatalytic hydrocyanation of pyrrole carboxaldehydes

The asymmetric hydrocyanation of pyrrole-2- and -3-carboxaldehydes substituted with either methyl, benzyl or phenyl in the 1-position catalyzed by the hydroxynitrile lyases from Hevea brasiliensis (HbHNL) and Prunus amygdalus (PaHNL) is reported. The products could be isolated—after O-silylation—with moderate to good enantiomeric purity although the carbonyl activity of the substrates was found to be very low, which is supported by quantum-chemical calculations. Structural effects concerning substrate size and regiochemistry are discussed considering docking calculations based on the X-ray crystal structures of the two enzymes. From these calculations one particular amino acid residue (Trp-128) in the active site of HbHNL could be identified, which plays a major role for the appropriate binding of structurally demanding carbonyl compounds.     

Micellar kinetics in aqueous acetonitrile. Part A. Sodium-hydrogen ion-exchange constant for 1-dodecanesulfonate surfactant. Part B. Substrate structural effects for micellar catalysis by perfluorooctanoic acid as reactive counterion surfactant

The sodium-hydrogen ion exchange constant for the system sodium 1-dodecanesulfonate-hydrochloric acid in aqueous acetonitrile has been determined from the pseudo-phase ion exchange model for surfactant catalytic effects. The results indicate that the micellar system behaves similarly for the aqueous and the aqueous acetonitrile (2.106 M) solvent systems. The influence of substrate molecular structure on micellar catalysis by perfluorooctanoic acid of the hydrolysis of hydroxamic acids (R—CO—NHOH) in aqueous acetonitrile has been explored. Data for substrate structures of fifteen compounds with R=alkyl, aralkyl, alicyclylalkyl, phenylalkyl, alkyl-substituted phenylalkyl, and with chain branching at the α, β, and γ positions are compared. Relative binding constant values indicate that substrates with aromatic groups are less well solubilized in the perfluoro micellar environment than are substrates with saturated groups. There is now evidence for specific micellar effects on the reaction rate as well as general micellar catalysis. © 1997 John Wiley & Sons, Inc.     

Atom‐Specific Mutagenesis Reveals Structural and Catalytic Roles for an Active‐Site Adenosine and Hydrated Mg2+ in Pistol Ribozymes

The pistol RNA motif represents a new class of self‐cleaving ribozymes of yet unknown biological function. Our recent crystal structure of a pre‐catalytic state of this RNA shows guanosine G40 and adenosine A32 close to the G53–U54 cleavage site. While the N1 of G40 is within 3.4 Å of the modeled G53 2′‐OH group that attacks the scissile phosphate, thus suggesting a direct role in general acid–base catalysis, the function of A32 is less clear. We present evidence from atom‐specific mutagenesis that neither the N1 nor N3 base positions of A32 are involved in catalysis. By contrast, the ribose 2′‐OH of A32 seems crucial for the proper positioning of G40 through a H‐bond network that involves G42 as a bridging unit between A32 and G40. We also found that disruption of the inner‐sphere coordination of the active‐site Mg²⁺ cation to N7 of G33 makes the ribozyme drastically slower. A mechanistic proposal is suggested, with A32 playing a structural role and hydrated Mg²⁺ playing a catalytic role in cleavage.     

Unifying the Current Data on the Mechanism of Cleavage–Transesterification of RNA

The various mechanisms by which RNA is hydrolyzed are currently under intense investigation. The first step in the hydrolysis pathway is a cleavage–transesterification in which a 2′-OH group attacks a 3′,5′-phosphodiester linkage with departure of the 5′ group. The second step involves the opening of a 2′,3′-cyclic phosphodiester. Complications in these steps arise from multiple possible pathways involving specific acid and base as well as general acid and base catalysis. In addition, controversy exists concerning the protonation states of the phosphodiesters and any intermediate phosphoranes under various experimental conditions. A summary of mechanistic studies involving general and specific acid/base catalysis of RNA hydrolysis and the hydrolysis of RNA analogs is presented herein, along with the interpretations given by the original authors. Included are theoretical calculations, kinetic studies, pK_a determinations, isotope effects, Hammett and Brønsted correlations, and model studies. Recent analyses of the mechanism of RNase A are also briefly reviewed. Two limiting mechanisms for the cleavage–transesterification step that unify the data in the literature and differ only in the role of the phosphorane and its protonation state are given at the outset. An analysis of the literature studies supporting these mechanisms is then provided.