Copolyethers with controlled structure: Mechanism of formation and microstructure

Cationic copolymerization of 3-membered cyclic ethers (oxiranes) with 5-membered cyclic ether (tetrahydrofuran) leads to linear copolyethers containing significant amount of cyclic fraction. When the copolymerization is conducted in the presence of diols, telechelic copolymers are formed by the process in which oxirane is incorporated into copolymer irreversibly by Activated Monomer mechanism, while tetrahydrofuran is incorporated reversibly by Active Chain End mechanism. Thus both kinetic and thermodynamic factors can be used to control the rate of the incorporation of comonomers. Studies of the kinetics of the competing reactions leading to formation of copolymer, studies of copolymer microstructure by ¹³C-NMR and analysis of the composition of the cyclic fraction, led to the correlation between the copolymer microstructure and the amount of cyclic fraction formed. The dependence of the cyclic fraction content on the reaction conditions was therefore explained.     

Polymerization chemistry of the family of cyclic imino ethers

Characteristics of the polymerization mechanisms of the family of cyclic imino ethers are described. The variety of the mechanism of propagation has been systematized on the basis of the nature of propagating species, i.e., cationic or electrophilic covalent (dipole) species. In the polymerizations of 2-alkyl cyclic imino ethers (5- and 6-membered), propagation mechanism via one of these two different species has been established, which is dependent upon the relative nucleophilic reactivities of the monomer and the counter anion derived from initiator. The polymerization of cyclic pseudoureas having a cyclic amine substituent at 2-position proceeds in two different ways. Ionic propagation leads to the single isomerization/ring-opening polymerization involving only the cyclic imino ether ring. On the other hand, covalent propagation gives rise to the double isomerization ring-opening polymerization involving the two rings of cyclic imino ether and cyclic amine. Polymerization of 5-membered cyclic iminocarbonate with a sulfonate initiator proceeds through the isomerization/ring-opening of 2-oxazoline ring. The same monomer was isomerized to the corresponding cyclic urethane when it was treated with benzyl bromide.     

Thermal decomposition of cyclo-linear methylsiloxane polymers

The pyrolysis products from two cyclo-linear methylsiloxane polymers have been analysed, and formation of the important products interpreted in terms of a simple decomposition mechanism. This mechanism is very similar to that of the polydimethylsiloxane degradation; thus cyclic oligomers are formed from the linear segments, and polycyclic compounds from the cyclic segments of the cyclolinear methylsiloxane polymers.     

A novel pathway in the photooxygenation of cyclic allenes

[equation--see text] The photooxidation of cyclic allenes gives rise to cyclic 1,2, 3-trione hydrates. The formation of these compounds points to a novel photooxidation mechanism involving both singlet and triplet oxygen. Upon placement of a methyl group on the allene, the mechanism shifts to predominantly an "ene" reaction. The corresponding cycloadditions with 4-methyl-1,3,4-triazoline-3, 5-dione (MTAD) with cyclic allenes involve 2 equiv of MTAD. The dipolar intermediates are trapped with H(2)O to give alpha-urazole-substituted 2-cycloalkenones.     

Kinetics and mechanism of epoxy oligomers curing

A review of new results concerning kinetics, thermodynamics and mechanism of epoxy oligomers curing with various nucleophilic agents (amines, phenols, carboxylic acids) has been made. The concept of the mechanism of such reactions is formulated on the basis of this study. Kinetic data obtained are interpreted within the framework of cyclic transition state. The addition of nucleophilic agent to epoxy ring is rather complicated. It results from preliminary formation of homo- and heteroassociates of reaction system components which finally form reactive cyclic heteroassociates. Reaction products are formed as a result of monomolecular rearrangement with synchronous transfer of electrons in the reactive cyclic heteroassociates.     

Cyclic amine initiation of polypivalolactone

Certain monocyclic and polycyclic tertiary amines initiate polymerization of, and copolymerize with, pivalolactone. These comprise three-, four-, and five-membered monocyclic amines and 1-azapolycyclic amines. The polymerization proceeds in three steps: (a) initiation to form a cyclic amine/pivalate betaine, (b) propagation to yield a polylactone zwitterion by an anionic mechanism, and (c) interlinking by a carboxylate end attacking a cyclic amine end to incorporate amine as a comonomer in the chain. When polymerization is carried out at low temperatures, the more stable cyclic amine ring systems yield isolatable step b polymers. These polymeric zwitterions undergo step c by heating, even in the solid state. The mechanism suggests that, for the more labile cyclic amines, a lactone could be the initiator for polyamine formation by a cationic mechanism. Interlinking would lead to incorporation of lactone initiator. In intermediate cases, block copolymers would result. With a proper balance of reactivities, 1:1 alternating copolymers would be possible.     

Enantioselective synthesis of β-substituted cyclic ketones via cobalt-catalyzed asymmetric reductive coupling of alkynes with alkenes

A CoI(2)/(R)-BINAP, Zn, ZnI(2), H(2)O system efficiently catalyzes the intermolecular asymmetric reductive coupling of alkynes with cyclic enones to afford highly regio- and enantioselective β-alkenyl cyclic ketones. A possible mechanism that involves the formation of a cobaltacyclopentene intermediate from the alkyne and cyclic enone is proposed.     

肉桂腈电化学还原反应机理

采用循环伏安法研究了肉桂腈在乙腈溶液中的电还原行为,其在-1.46和-2.0V处各存在一个还原峰.第一个峰处恒电位电解得到了线性、环化氢化二聚产物以及苯基戊二腈;第二个峰处电解得到了饱和二氢还原产物苯丙腈.结合循环伏安模拟判定了整个电还原的具体反应机理是肉桂腈通过电化学-电化学-化学-化学(EECC)反应机理生成苯丙腈,同时经历自由基-自由基(RR)过程得到线性和环化二聚产物,肉桂腈还可以与乙腈的共轭碱反应得到苯基戊二腈.最终通过循环伏安模拟求得相应反应的动力学常数,自由基-自由基耦合反应速率常数为104L·mol-.1s-1,第二个电子转移反应速率常数为0.3cm.s-1,其后质子化反应的速率常数为105s-1.     

A critical analysis of the cyclic and open alternatives of the transmetalation step in the stille cross-coupling reaction

The transmetalation step of the Stille cross-coupling reaction catalyzed by PdL(2) (L = PH(3), AsH(3)) has been analyzed by means of DFT methods for PhBr as the electrophile and CH(2)=CHSnMe(3) as the nucleophile. Both experimentally proposed mechanisms (cyclic and open) were theoretically studied. For the case of the cyclic mechanism, the associative and dissociative ligand substitution alternatives were both analyzed. For the case of the open mechanism, the cis and the trans pathways were evaluated. All the reaction pathways were also studied taking into account the solvent effects by means of continuum models, for THF and PhCl as solvents. In selected cases, explicit solvent molecules were introduced to account for their potential role as ligands. Theoretical analysis indicates that the open reaction mechanism is preferred for organotriflate systems, whereas the cyclic mechanism is favored for the reaction with organohalide systems.     

Hyperbranched polyethers by ring‐opening polymerization: Contribution of activated monomer mechanism

Propagation in the cationic ring‐opening polymerization of cyclic ethers involves nucleophilic attack of oxygen atoms from the monomer molecules on the cationic growing species (oxonium ions). Such a mechanism is known as the active chain‐end mechanism. If hydroxyl groups containing compounds are present in the system, oxygen atoms of HO? groups may compete with cyclic ether oxygen atoms of monomer molecules in reaction with oxonium ions. At the proper conditions, this reaction may dominate, and propagation may proceed by the activated monomer mechanism, that is, by subsequent addition of protonated monomer molecules to HO? terminated macromolecules. Both mechanisms may contribute to the propagation in the cationic polymerization of monomers containing both functions (i.e., cyclic ether group and hydroxyl groups) within the same molecule. In this article, the mechanism of polymerization of three‐ and four‐membered cyclic ethers containing hydroxymethyl substituents is discussed in terms of competition between two possible mechanisms of propagation that governs the structure of the products—branched polyethers containing multiple terminal hydroxymethyl groups. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 457–468, 2003     

Isolation and identification of the linear and cyclic oligomers of poly(ethylene terephthalate) and the mechanism of cyclic oligomer formation

Chromatographic techniques are described which can be used to isolate and identify the linear and the cyclic oligomers of poly(ethylene terephthalate). Extraction of the oligomers from high molecular weight polymer produces at least eight different cyclic species, some of which are isolated and identified. The cyclic dimer, the cyclic trimer, and the cyclic tetramer of poly(ethylene terephthalate) have also been prepared by acid chloride esterification and transesterification. Similar materials can be isolated from the ethylene glycol distillate obtained from the polymer melt. The mechanism of cyclic oligomer formation has been studied by determining the rate of formation of the cyclic oligomers during polymerization and during melt extrusion of polyesters which did not initially contain cyclic oligomers. The rate of formation depends upon the concentration of hydroxyl groups; hence, the cyclic oligomers are formed by transesterification from the chain ends or cyclodepolymerization. Therefore oligomers are inevitably produced during polymerization.     

Pummerer反应在有机合成中的新进展

综述了Pummerer反应的机理、类型及其在有机合成和不对称合成中的新进展。     

Direct conversion of β-hydroxyketones to cyclic disiloxanes

β-Hydroxyketones can be directly converted to cyclic disiloxanes using diphenylchlorosilane in the presence of imidazole and an amine base. The reaction is proposed to proceed via a nucleophilic activation mechanism through a cyclic chairlike transition state affording hydrosilylated products with high diastereoselectivity.     

Investigation of Reaction Mechanism of Amino Acids and Phosphorus Trichloride by 31P NMR and ESI‐MS/MS

The reaction of amino acids and phosphorus trichloride in THF was studied by ³¹P NMR tracing and ESI‐MS/MS. A series of hydridophoranes and cyclic dipeptides were obtained. The reaction presented interesting diversity and the reaction mechanism was proposed. The mechanism suggests that phosphorus plays an important role in the synthesis of amino acid hydridophorane and cyclic dipeptides. The results also show that ³¹P NMR and ESI‐MS/MS are useful tools for the investigation of reaction mechanism.     

Inhibitory effect of extracts of muscles of mackerel (Scomber japonicus Houttuyn) on hepatic glycogenolysis in rats

The effects of extracts of muscles of mackerel (Scomber japonicus; M-ext) on hepatic glycogenolysis were investigated by a rat liver perfusion method. M-ext inhibited glucagon- and cyclic adenosine monophosphate (AMP)-induced glycogenolysis but was ineffective on phenylephrine-induced glycogenolysis. The contents of hepatic glycogen and cyclic AMP, and phosphorylase and glycogen synthase activities in liver were measured after perfusion with glucagon. M-ext inhibited the increase of cyclic AMP and activation of phosphorylase. It is considered that M-ext inhibits hepatic glycogenolysis caused by glucagon through a cyclic AMP-dependent mechanism.     

A facile and one-pot electrochemical method for the synthesis of a new anthraquinonethioether

The reaction of electrochemically generated anthradiquinone as a Michael acceptor with 2-mercaptobenzothiazole and 2- mercaptobenzoxazole,as nucleophiles in ethanol/water mixtures has been studied by means of cyclic voltammetry.Our voltammetric data indicate that produced anthradiquinone participates in Michael addition reaction with nucleophiles and via an ECEC mechanism converts to the new anthraquinonethioether derivatives.Based on an EC mechanism,the observed homogeneous rate constant of the Michael reaction of anthradiquinone with nucleophiles were estimated by comparing the experimental cyclic voltammograms with the digital simulated results.     

碱性溶液中分子氧在多晶银上的多步还原

 Oxygen reduction on a polycrystalline silver electrode was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The reaction occurred by a two-electron pathway. The steps in the mechanism were observed in the cyclic voltammograms recorded with different scan rates. The intermediates formed in the steps were detected by electrochemical impedance spectroscopy.     

Construction of Chiral Cyclic Compounds Enabled by Enantioselective Photocatalysis

Chiral cyclic molecules are some of the most important compounds in nature, and are widely used in the fields of drugs, materials, synthesis, etc. Enantioselective photocatalysis has become a powerful tool for organic synthesis of chiral cyclic molecules. Herein, this review summarized the research progress in the synthesis of chiral cyclic compounds by photocatalytic cycloaddition reaction in the past 5 years, and expounded the reaction conditions, characters, and corresponding proposed mechanism, hoping to guide and promote the development of this field.     

NiCl2-catalyzed radical cross decarboxylative coupling between arylpropiolic acids and cyclic ethers

A direct alkenylation of cyclic ethers via radical cross decarboxylative coupling process catalyzed by NiCl₂ and using DTBP as radical initiator and oxidant was developed. A variety of arylpropiolic acids and cyclic ethers were transformed into the corresponding 2-arylvinyl cyclic ethers in moderate to excellent yields. Mechanistic experiments were conducted to determine the nature of the reaction intermediates, and a plausible reaction mechanism involving NiCl₂-promoted radical process was proposed.     

Infrared spectroscopy studies of the cyclic anhydride as the intermediate for the ester crosslinking of cotton cellulose by polycarboxylic acids. I. Identification of the cyclic anhydride intermediate

The mechanism of esterification of cotton cellulose by a polycarboxylic acid was investigated using Fourier transform infrared spectroscopy (FT-IR). The infrared spectroscopic data indicate that a polycarboxylic acid esterifies with cotton cellulose through the formation of an acid anhydride intermediate. A five-member cyclic anhydride intermediate was identified in the cotton fabric treated with poly(maleic acid). The five-member cyclic anhydride is a reactive intermediate and readily esterifies when reaction sites are available. We also found that those polycarboxylic acids, which form five-member cyclic anhydride intermediates, crosslink cotton cellulose more effectively than those polycarboxylic acids which form six-member cyclic anhydride intermediates. © 1993 John Wiley & Sons, Inc.