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991.
Reactions of the oxorhenium(V) complexes [ReOX3(PPh3)2] (X = Cl, Br) with the N‐heterocyclic carbene (NHC) 1,3,4‐triphenyl‐1,2,4‐triazol‐5‐ylidene (LPh) under mild conditions and in the presence of MeOH or water give [ReOX2(Y)(PPh3)(LPh)] complexes (X = Cl, Br; Y = OMe, OH). Attempted reactions of the carbene precursor 5‐methoxy‐1,3,4‐triphenyl‐4,5‐dihydro‐1H‐1,2,4‐triazole ( 1 ) with [ReOCl3(PPh3)2] or [NBu4][ReOCl4] in boiling xylene resulted in protonation of the intermediately formed carbene and decomposition products such as [HLPh][ReOCl4(OPPh3)], [HLPh][ReOCl4(OH2)] or [HLPh][ReO4] were isolated. The neutral [ReOX2(Y)(PPh3)(HLPh)] complexes are purple, airstable solids. The bulky NHC ligands coordinate monodentate and in cis‐position to PPh3. The relatively long Re–C bond lengths of approximate 2.1Å indicate metal‐carbon single bonds. 相似文献
992.
A series of novel hexacoordinate silicon(IV) complexes with an SiO2N4 skeleton (compounds (OC-6-12)-3, (OC-6-12)-4, (OC-6-12)-5, (OC-6-12)-6, and (OC-6-2'2)-7) were synthesized, starting from Si(NCO)4 or Si(NCS)4. These compounds contain (i) two bidentate O,N-chelate ligands (or one tetradentate O,N,N,O-chelate ligand) derived from 4-aminopent-3-en-2-ones of the formula type Me-C(NRH)=CH-C(O)-Me (R = organyl) and (ii) two monodentate cyanato-N or thiocyanato-N ligands. Formally, the bidentate singly negatively charged O,N-chelate ligands (tetradentate 2-fold negatively charged O,N,N,O-chelate ligand) behave as ligands of the imino-enolato type. In addition, the adduct trans-8 was synthesized by reaction of Si(NCS)4 with 2 molar equiv of Me-C(Ni-PrH)=CH-C(O)-Me. This hexacoordinate silicon(IV) complex contains (i) four monodentate thiocyanato-N ligands and (ii) two neutral monodentate ligands of the iminio-enolato type. All compounds synthesized were structurally characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. To get more information about the stereochemistry of these compounds, the experimental investigations were complemented by computational studies. 相似文献
993.
Oliver Jungmann Markus Beier Anatol Luther HoanK. Huynh Marc‐Olivier Ebert Bernhard Jaun Ramanarayanan Krishnamurthy Albert Eschenmoser 《Helvetica chimica acta》2003,86(5):1259-1308
Among the members of a family of diastereoisomeric pentopyranosyl‐(4′→2′)‐oligonucleotide systems derived from D ‐ribose, D ‐xylose, L ‐lyxose, and L ‐arabinose, the α‐arabinopyranosyl system shows by far the strongest Watson? Crick base pairing. The system is, in fact, one of the strongest oligonucleotide‐type base‐pairing systems known. It undergoes efficient cross‐pairing with all the other members of the pentopyranosyl family, but not with RNA and DNA. The paper describes the synthesis and pairing of the properties of α‐L ‐arabinopyranosyl‐(4′→2′)‐oligonucleotides. 相似文献
994.
Various reactions of 2-oxo(or thioxo)-1,2,3,4-tetrahydropyrimidine-5-carboxylic acid derivatives (Biginelli-compounds) were investigated. The site of methylation and acylation on 6-methyl-4-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylic acid ethyl ester 1a and its 2-oxo derivative 9a was studied. The synthesis of pyrimido[2,3-b]thiazines and thiazolo[3,2-a]pyrimidines was accomplished by condensation of 1a with 1,3-and 1,2-dielectrophiles. A Dimroth-like rearrangement yielding 6H-1,3-thiazines can be observed when 1a was treated with dimethylformamide and phosphorus oxychloride. The formation of indeno[1,2-d]pyrimidines can be achieved by intramolecular Friedl-Crafts acylation of 9a and 13 , respectively. Finally a route for the preparation of 4,6-disubstituted-pyrimidine-5-carbonitriles is presented, starting with Biginelli-compound 25 . 相似文献
995.
The complex trans-[Rh(CO)(NH3)(PiPr3)2]PF6 (2) was prepared from [(η3-C3H5)Rh(PiPr3)2] (1), NH4PF6 and CO or from 1 and NH4PF6 in presence of an excess of methanol. With an excess of CO, the dicarbonyl and tricarbonyl compounds trans-[Rh(CO)2(PiPr3)2]PF6 (3) and [Rh(CO)3(PiPr3)2]PF6 (4) were obtained. Displacement of one CO ligand in 3 by pyridine and acetone led to the formation of trans-[Rh(CO)(py)PiPr3)2]PF6 (5a) and trans-[Rh(CO) (O=CMe2(PiPr3)2]PF6 (6), respectively. Treatment of 1 with [pyH]BF4 and pyridine gave trans-[Rh(py)2(PiPr3)2]BF4 (7); in presence of H2 the dihydrido complex [RhH2(py)2(PiPr3)2]BF4 (8) was formed. The reaction of 1 with NH4PF6 and ethylene produced trans [Rh(C2H4(NH3(PiPr3)2]PF6(9) whereas with methylvinylketone and acetophenone the octahedral hydridorhodium(III) complexes [RhH(η2-CH=CHC(=O)CH3 (NH3(PiPr3)2]PF6(11) and [RhH(η2-C6H4C(=O)CH3(NH3(Pipr3)2]PF6 (13) were obtained. The synthesis of the cationic vinylidenerhodium(I) compounds trans-[Rh(=C=CHR)(py)(PiPr3)2]BF4 (14–16) and trans-[Rh(=C=CHR)(NH3)(PiPr3) 2]PF6 (17–19) was achieved either on treatment of 1 with [pyH]BF4 or NH4PF6 in presence of 1-alkynes or by ethylene displacement from 9 by HCCR. With tert-butylacetylene as substrate, the alkinyl(hydrido)rhodium(III) complex [RhH(CCtBu)(NH3)(O=CMe2)(PiPr3) 2]PF6 (20) was isolated which in CH2Cl2 solution smoothly reacted to give 19 (R =tBu). The cationic but-2-yne compound trans-[Rh(MeCCMe)(NH3)(Pi Pr3)2]PF6 (21) was prepared from 1, NH4PF6 and C2Me2. The molecular structures of 3 and 14 were determined by X-ray crystallography; in both cases the square-planar coordination around the metal and the trans disposition of the phosphine ligands was confirmed.
Abstract
Der Komplex trans-[Rh(CO)(NH3)(PiPr3)2]PF6 (2) wurde aus [(η3-C3H5)Rh(PiPr3)2] (1), NH4PF6 und CO oder aus 1, NH4PF6 und Methanol hergestellt. In Gegenwart von überschüssigem CO wurden die Dicarbonyl- und Tricarbonyl-Verbindungen trans-[Rh(CO)2(PiPr3)2]PF6 (3) und [Rh(CO)3(PiPr3)2]PF6 (4) erhalten. Die Verdrängung eines CO-Liganden in 3 durch Pyridin oder Aceton führte zur Bildung von trans-[Rh(CO)(py)(PiPr3)2]PF6 (5a) bzw. trans-[Rh(CO)(O=CMe2)(PiPr3)2]PF6 (6). Bei Einwirkung von [pyH]BF4 und Pyridin auf 1 entstand trans-[Rh(py)2(PiPr3)2]BF4 (7); in Gegenwart von H2 bildete sich der Dihydrido-Komplex [RhH2(py)2(PiPr3) 2]BF4 (8). Die Reaktion von 1 mit NH4PF6 und Ethen lieferte trans-[Rh(C2H4)(NH3)(PiPr3)2] PF6 (9) während mit Methylvinylketon und Acetophenon die oktaedrischen Hydridorhodium(III)-Komplexe [RhH(η2-CH=CHC(=O)CH3 (NH3)-(PiPr3)2]PF6 (11) und [RhH(η-2-C6H4C(=O)CH3(NH3)(PiPr3)2)2]PF6 (13) erhalten wurden. Die Synthese der kationischen Vinyli-denrhodium(I)-Verbindungen trans-[Rh(=C=CHR(py)(PiPr3)2]BF4 (14–16) und trans-[Rh(=C=CHR)(NH3)(PiPr3)2]PF6 (17–19) gelang durch Einwirkung von [pyH]BF4 bzw. NH4PF6 auf 1 in Gegenwart von 1-Alkinen oder durch Ethen-Verdrängung aus 9 mit HCCR. Mit tert-Butylacetylen als Reaktionspartner wurde der Alkinyl(hydrido)rhodium(III)-Komplex [RhH(CCtBu)(NH3(O=CMe2)(PiPr3)2]PF6 (20) isoliert, der in CH2Cl2-Lösung sofort zu 19 (R =tBu) reagiert. Die kationische 2-Butin-Verbindung trans -[Rh(MeCCMe)(NH3)PiPr3)2]PF6 (21) wurde aus 1, NH4PF6 und C2Me2 hergestellt. Die Strukturen von 3 und 14 wurden kristallographisch bestimmt; in beiden Fa len ließ sich die quadratisch-planare Koordination des Metalls und die trans-Anordnung der Phosphanliganden bestätigen. 相似文献996.
The synthesis, characterization, and X-ray crystal structures of [Re(diimine)(CO)(3)(dpe)](PF(6)) (dpe = 1,2-di(4-pyridyl)ethylene) compounds are reported. The cis-dpe complexes exhibit yellow luminescence after UV excitation, whereas the trans-dpe counterparts are nonluminescent. The luminescence quantum yields of the cis-dpe complexes are strongly dependent on the identity of the diimine ligand. Irradiation (350 nm) of the trans-dpe complexes induces trans --> cis dpe-ligand isomerization with quantum yields on the order of 0.2, and this process leads to an on-switching of yellow luminescence. After long 350-nm irradiation times, a steady state composed of roughly 70% cis- and 30% trans-dpe complexes is reached. The reverse cis --> trans photoisomerization reaction is induced by irradiating the cis-dpe complexes at 250 nm, switching off the yellow luminescence. For 250-nm excitation, photodecomposition of the [Re(diimine)(CO)(3)(dpe)](+) complexes competes efficiently with photoisomerization. 相似文献
997.
The concentration and glycosylation of alpha(1)-acid glycoprotein (AGP) alter significantly during inflammation. A definitive physiological role for AGP remains elusive and is the subject of extensive investigation. This study investigated the influence of AGP on the activity of collagenase-3, an important mediator of cartilage destruction in rheumatoid arthritis. AGP was isolated from normal and rheumatoid plasma. Fucosylation was determined by high pH anion-exchange chromatography; sialylation was assessed following enzymatic digest. Rheumatoid AGP displayed elevated fucosylation and sialylation compared with normal. The influence of each sample on collagenase-3 activity was measured fluorometrically. AGP influenced collagenase-3 catalysis and collagen binding, with catalytic activity correlating with fucosylation. Rheumatoid AGP exhibited less efficient inhibition than normal plasma AGP. It is hypothesized that AGP within rheumatoid synovial fluid may be inadequate to prevent excessive cartilage destruction and hence may exacerbate the disease process. 相似文献
998.
Stephen E. Rudolph Raymond C. Glowaky 《Journal of polymer science. Part A, Polymer chemistry》1978,16(9):2129-2140
Mixed esters of hydrolyzed starch represent a new class of chemically modified natural polymers demonstrating a broad range of properties. Members of this class of polymers have both neutral aliphatic ester side chains and carboxyl-functional half-ester side chains. Use of hydrolyzed starch as the backbone polymer results in products that are considerably lower in molecular weight than whole starch derivatives, but which are still polymeric in character. Synthesis proceeds smoothly in pyridine using anhydrides as acylating agents and the pure solid mixed ester products are isolated by precipitation in water. Measurement of degree of substitution (D.S.) by NMR or hydrolysis characterizes the chemical composition of the polymers. The actual D.S. achieved in the synthesis depends upon the competition between starch and residual water for anhydrides, which can be quantitatively evaluated by monitoring the acid content of the reaction mixture. The Tg and Ts of starch mixed esters vary with both D. S. and length of the aliphatic ester side chain. As the composition changes from acetate—phthalate to butyrate—phthalate a Tg range of greater than 100°C is observed. Hydrolyzed starch mixed esters are hydrophobic and organic-soluble, but may be readily solubilized in aqueous base through the half-ester groups. Solutions show surface activity which varies according to the type and extent of substitution. 相似文献
999.
Allen JW Barker PD Daltrop O Stevens JM Tomlinson EJ Sinha N Sambongi Y Ferguson SJ 《Dalton transactions (Cambridge, England : 2003)》2005,(21):3410-3418
This perspective seeks to discuss why biology often modifies the fundamental iron-protoporphyrin IX moiety that is the very versatile cofactor of many heme proteins. A very common modification is the attachment of this cofactor via covalent bonds to two (or rarely one) sulfur atoms of cysteine residue side chains. This modification results in c-type cytochromes, which have diverse structures and functions. The covalent bonds are made in different ways depending on the cell type. There is little understanding of the reasons for this complexity in assembly routes but proposals for the rationale behind the covalent modification are presented. In contrast to the widespread c-type cytochromes, the d1 heme is restricted to a single enzyme, the cytochrome cd1 nitrite reductase that catalyses the one-electron reduction of nitrite to nitric oxide. This is an extensively derivatised heme; a comparison is drawn with another type of respiratory nitrite reductase in which the active site is a c-type heme, but the product ammonia. 相似文献
1000.