A HIGHLY ACTIVE NEODYMIUM CHLORIDE ISOPROPANOL COMPLEX/MODIFIED METHYLALUMINOXANE CATALYST FOR PREPARING POLYISOPRENE WITH HIGH CIS-1,4 STEREOSPECIFICITY AND NARROW MOLECULAR WEIGHT DISTRIBUTION

Neodymium chloride isopropanol complex （NdCl3.3＇prOH） activated by modified methylaluminoxane （MMAO） was examined in isoprene polymerization in hexane, with regards to Nd compounds, aluminum （Al） compounds, [Al]/[Nd] ratio, polymerization temperature and time. NdCl3＇3iprOH exhibited high activity producing polymers feasting high cis-l,4 stereospecificity （〉 96%）, very high molecular weight （Mn 〉 1.0 × 10^6） and fairly narrow molecular weight distribution （MWD, Mw/Mn 〈 2.0） simultaneously. In comparison, neodymium isopropoxide also showed high activity providing polymers with narrow MWD （Mw/Mn = 2.07）, but somewhat low cis-1,4 content （ca. 92%）, while neodymium chloride had no activity under present polymerization conditions. The Al compounds affected the polymer yield in the order of Al（i-Bu）3 〉 MMAO 〉 Al（i-Bu）2H. MMAO as cocatalyst afforded polyisoprene with high Mn over 1.0 × 10^6, whereas as stronger chain transfer agent than MMAO, AI（i-Bu）3 and AI（i-Bu）EH yielded polymers with low Mn （1.0 × 10^5-8.0 × 10^5）. NdCl3·3＇PrOH/MMAO catalyst showed a fairly good catalytic activity even at relatively low [Al]/[Nd] ratio of 30, and the produced polymer remained high cis-1,4 content of 95.8% along with high Mn over 1.0× 10^6 even at elevated temperatures up to 70℃. The polymerization rate is of the first order with respect to the concentration of isoprene. The mechanism of active species formation was discussed preliminarily.

Molecular Basis for Stereospecific Hydrolysis of Ethyl Mandelate by Thermophilic Esterase

The stereospecific hydrolysis of mandelate can be effectively catalyzed by hyperthermophilic acylpeptide esterase APE 1547（Aeropyrum pernix esterase 1547）. APE 1547 used in this reaction showed a remarkable stereodi-scrimination in favour of R-mandelic acid（99% e.e.） with an enantiomeric ratio E〉200. The results of computer simulation are consistent with the experimental results. It can be inferred that the R-substrate adopted a binding mode productive of the reaction due to the formation of the hydrogen bond at the active site of APE 1547.

Higher Order Cuprate Species( Ⅱ )——Stereospecific Carbocupration of Terminal Alkynes and Its Synthetic Utility for ( ± )-Ipsenol and (E)-β-Farnesene

The stereospecific carbocupration of terminal alkynes via higher order cuprates to give 1,1'-disubstituted olefins with 99% configuration purity is described. Its synthetic utility as a general method for the preparation of substituted olefins is further illustrated by the direct synthesis of (±)-ipsenol and (E)-β-farnesene with highly geometric purity of the carbon-carbon double bond.

NATURAL PRODUCTS SYNTHESIS BY RETRO-DIELS-ALDER REACTION Ⅷ~1 A CONCISE STEREOSPECIFIC FORMAL SYNTHESIS OF TETRAHYDROCHLOROVULONE

A potent antineoplastic tetrahydrochlorovulone 3 was synthesized with retro-Diels Alder reaction as a key step.

Stereospecific Synthesis of Drospirenone

A procedure for the stereospecific synthesis of drospirenone has been developed. The key steps included the stereospecific reduction of the C7-tertiary alcohol with ZnI2/Et3SiH, a novel mild and stereospecific tertiary alcohol reduction system, and the tandem oxidation/cyclopropanation reactions.

A new strategy for the stereospecific syntheses of C-furanosides

This reaction provided an efficient method to synthesize C-furanoside derivatives in high yields and stereospecificity under mild conditions. The cyclization mechanism regarding benzyl group as a leaving group was discussed and their stereochemistry was deduced by H-1-H-1 NOESY data.

A HIGHER ACTIVE LANTHANIDE CHLORIDE CATALYST FOR STEREOSPECIFIC POLYMERIZATION OF CONJUGATED DIENE

It is known that the first lanthanide （Ln） catalyst used for stereospecific polymeriza-tion of conjugated diene is composed of LnCl3 and R3Al.It initiates stereospecific polymeri-zation of butadiene with a very high cis-1, 4 content（98%）, yet owns poor activity. This paper presents the new results obtained in our laboratory. By adding alcohol tothis binary system （LnCl3-R3Al）, the catalytic activity is greatly enhanced without any losson its high stereospecificity. It is shown that the catalytic activity is dependent on the ratioof its components and their order of addition. Furthermore, we have ascertained that itis the alcohol complex of LnCl3 with a definite composition LnCl3.R3OH, that turns out tobe an active catalyst when treated with R3Al.

Stereospecific synthesis of(E)-furfurylalkenes from alkenyldisiamylboranes and furfuryl chloride

The Pd(PPh_3)_4-catalyzed coupling reaction of alkenyl-disiamylboranes with furfuryl chloride in the presence of t-BuOK afforded(E)-furfurylalkenes stereospecifically in good yields.

Stereospecific hydrohalogenation reactions of electron-deficient alkynes

Stereospecific hydrohalogenation reactions of electron-deficient alkynes, i.e., 2-alkynoic acids, 2-alkynoates, 2-propynamides, 2-propynenitrile, 1-alkynyl phosphonates/phosphine oxides, 1-alkynyl sulfoxides/triflones, and 1-alkynyl iodonium salts with lithium or sodium halides to afford Z-alkenyl derivatives and their applications in synthesis of important intermediates as well as natural products were reviewed.

Stereocontrolled Synthesis of Optically Active Aggregation Pheromone of Lesser Grain Borer

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Stereoselective Syntheses of Some β-D-1-Thioglycopyranosides and Bis(β-D-glycopyranosyl) Sulfides by Phase Transition Catalysis

The present paper covers the stereoselective syntheses of four 1 thiogalactopyranosides 1—4 and three thio disaccharides 5—7 through the PTC methods and their structures confirmed by means of elementary analysis, IR and NMR spectral data.

Synthesis and Steric Structure of 1, 5-Benzothiazepine-Phenyl-β-Lactams

1, 5-Benzothiazepines 1 react with phenylacetyl chloride to give the title compounds.The structures of these new compounds were confirmed by elemental analysis, ~1H NMR, ^(13)C NMRand MS spectroscopy, and their configuration (the mutual positions of the substituents relative to theβ-lactam ring) and conformation of the compounds were determined by X-ray crystal analysis.

Synthesis and Stereostructure of New β-Lactam Derivatives of1,5-Benzothiazepines

Reaction of 1,5-benzothiazepine with N-protected glycine gives new a-amino-β-lactamderivatives of 1.5-benzothiazepine. The configuration and conformation of the products wereconfirmed by x-ray diffraction. The result further reveals that the reaction of 1.5-benzothiazepineswith derivatives of carboxylic acid stereospecific.

Synthesis of 1,2:4,5-di-O-Isopropylidene-3-C-Cyano-β-D-ribo-hex-2-ulo Pyranose by Phase Transition Catalysis

Introduction The cyano branched carbohydrates are useful, versatil intermediaries for the synthesis of branched sugars having biological activity in nature. Owing to the good yield, mild reaction conditions and high stereospecificity, the phase transition catalysis(PTC)method has been widely used to the syntheses of C-

Impact of Leucine 278 Residue on Fatty Acid Length Specificity of <i>Candida antarctica</i>Lipase B

Structural analysis of Candida antarctica lipase B (CALB) indicates that side chain of leucine at 278 site lies above the entrance of the catalytic pocket, which prognosticates its potential role on substrate specificity of the enzyme. To verify this presumption, shortened side chain of glycine or proline was rational designed and mutants were constructed by site-directed mutagenesis method. The colorimetric assay using p-nitrophenyl esters of fatty acids with various chain-lengths was used to study the substrate preference of lipases. Results indicated that L278G or L278P mutations both induced the drift of substrate specificity of CALB from p-nitrophenyl caprylate (pNP-C8) to longer carbon chain length of p-nitrophenyl caprate (pNP-C10). Meanwhile, Vmax value of two mutants to pNP-C10 was both higher than that of wild-type. Docking results also indicated that shortened side chain of glycine or proline residues substitution at this site could get rid of the space block present above the catalytic pocket, and made longer chain substrate (pNP-C10) enter into the catalytic pocket easier. The modulation of specificity observed allowed for building substrate binding model and opened new possibilities for designing ligand specific lipases.

Polymerization of 1,3-Butadiene with Catalysts Based on Cobalt Dichloride Complexes with Aminophosphines: Switching the Regioselectivity by Varying the MAO/Co Molar Ratio

Several novel cobalt dichloride complexes with amino-phosphine bidentate ligands were synthesized and characterized.For some of them single crystals were obtained and their molecular structure was determined by X-ray diffraction method.All the complexes were then used in combination with MAO for the polymerization of 1,3-butadiene,obtaining polymers with different structures(i.e.,predominantly 1,2 or cis-1,4)mainly depending on the type of ligand and on the MAO/Co molar ratio.The behavior of these novel catalysts was compared with that exhibited,in the polymerization of the same monomer,by the systems CoCl_(2)(PR3)2-MAO and CoCl_(2)(PRPh2)2-MAO(R=alkyl or cycloalkyl group),and by the systems based on cobalt dichloride complexes with various bi-and tridentate ligands(e.g.,diphosphines,bis-imines,pyridyl-imines,bis-iminopyridines).The comparison between the different systems allowed us to make some clarity about the actual and effective role played by the various types of ligands in the polymerization of conjugated dienes with catalytic systems CoCl_(2)(L)-MAO,in which L=mono-,bi-,or tri-dentate ligand.

Synthetic rubbers prepared by lanthanide coordination catalysts

China is rich in rare earth resources. Rare earth elements, also named lanthanides, are number 58 to number 81 elements in the elemental periodic table. They have unique electronic structures and may form various coordination compounds. In the early 1960s, researchers at the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CIAC) found the catalytic activities of lanthanide compounds in stereospecific polymerization of conjugated dienes, and published the first paper on this topic in 1964. On the basis of this finding, CIAC launched extensive research activities on lanthanide compounds as diene polymerization catalysts, from a series of fundamental research to the efforts of industrializing the rare earth catalyzed cis-1,4-polybutatine rubber and cis-1,4-polyisoprene rubber. This review aims to summarize the progress in this field in the past half century.

PREPARATION OF POLYPROPYLENE/CLAY NANOCOMPOSITES BY in situ POLYMERIZATION WITH TiCl_4/MgCl_2/CLAY COMPOUND CATALYST

TiCl4/MgCl2/clay compound catalyst was prepared by chemical reaction. Exfoliated polypropylene （PP）/clay nanocomposites were synthesized by in situ polymerization with this compound catalyst. Effects of polymerization temperature, polymerization time, propylene pressure, solvent consumption and pre-treat time of catalyst on catalyst activity and catalytic stereospecificity were studied. Under optimal conditions, activity of the nano-compound catalyst is about 88.3 kg/（mol Ti-h）. Isotacticity of PP obtained in the nanocomposites is in the range of 89%-99%, and its melting temperature is about 159℃. The weight-average molecular weight of PP can reach 6.7 × 10^5 - 7.8 × 10^5, and the molecular weight distribution is between 7.7 and 7.9.

EHMO QUANTITATIVE VERIFICATION OF QUANTUM THEORY MECHANISM FOR ELECTROPHILIC ADDITIONS

Most of the electrophilic additions on carbon-carbon multiply bond can be summarized as: where Y are certain atoms on the fight of the periodic table; Z may be null, atom or group. Regardless of the rate law of Ad_E2 Ad_E3 or the more complex kinetics, the micromechanisms of transition state of electrophilic addition can be divided into two sorts, i.e. open ion and bridged ion mechanisms. The former one allows free rotation of