Showing papers in "ChemInform in 1982"

Polyhedral oligosilsesquioxanes and their homo derivatives

Abstract: A critical analysis of the present state of synthetic methods and mechanisms of the formation of polyhedral oligosilsesquioxanes and their homo derivatives is given. The data available on the structure, physico-chemical properties, reactivity, and application of the above compounds are summarized and systematized.

Flavones and Flavonols

Abstract: Most reports on the identification of known flavonoids in new plant sources as well as those on novel flavonoids naturally deal with the two most abundant classes, the flavones and flavonols. For this chapter we have listed all references to the occurrence of flavone and flavonol aglycones which have appeared from 1975 to 1980, and also include such earlier references as were overlooked in the relevant chapters of The Flavonoids’ (1975). Emphasis is given to the natural distribution of these compounds. The data are collected in Tables 4.1 and 4.2 which cite substances, plant sources and references. Since 1974, some 50 flavones and 60 flavonols with common substitution patterns have been described as novel compounds. Many are simply new methyl ethers of known compounds. The structures are arranged throughout the Tables by number and arrangement of substituents in ring A, followed by ring B. Wherever possible, the plant organs from which the compounds have been isolated are cited. In cases where authors have extracted the ‘whole plant’ or ‘aerial parts’, it does not necessarily follow that the compound is present in all parts, since these substances are often specifically located in certain organs only. In all cases where free aglycones are encountered, especially highly methylated flavonoids, it is likely that they occur externally, or are associated with secretory structures and are correlated with the production of other lipophilic secondary compounds, mainly of terpenoid origin.

Total synthesis of (-)-domoic acid. A revision of the original structure

Abstract: Die ursprunglich angenommene Struktur (I) von Domosaurewird, ausgehend von (III), durch eine Totalsynthese revidiert.

Transition Metal Formyl Complexes

Abstract: Publisher Summary This chapter discusses the synthesis of transition-metal formyl complexes, their physical properties, their reactions, and topics relating to the stability and decomposition of transition-metal formyl complexes. The synthesis, chemical characterization, and physical characterization of transition-metal complexes of new carbon-containing ligands constitute important ongoing objectives of organometallic chemists. In the presence of metallic heterogeneous or homogeneous catalysts, synthesis gas can be converted to a variety of organic molecules (methane, methanol, higher alkanes and alcohols, and glycols). In advance of any experiment, formyl complexes appear deceptively easy to synthesize. Comparisons to methods for transition metal acyl synthesis are instructive. Unfortunately, formic acetic anhydride is not a general reagent for formyl complex synthesis. Formyl complexes are characterized by distinct spectroscopic features. Transition-metal formyl complexes are capable of donating hydride to several classes of substrates, of which ketones and aldehydes are prototypical. A unique reaction of formyl complexes is “formyl transfer,” in which the formyl ligand undergoes apparent migration from one metal to another. Reactions of neutral formyl complexes with alkylating agents can follow different courses. Formyl complexes show varying behavior when treated with protonating agents. Anionic formyl complexes can be reduced when treated with the more potent trialkylborohydride nucleophiles. Neutral formyl complexes that contain ligating CO often decompose by decarbonylation; however, several exceptions exist. It is evident that transition-metal formyl complexes possess a unique and rich chemistry.

Reaction of tungsten(VI) alkylidyne complexes with acetylenes to give tungstenacyclobutadiene and cyclopentadienyl complexes

Abstract: Das Tetrachloroalkylidinwolframat (I) reagiert mit Alkinen zu Produkten, fur die aufgrund einer Rontgenstrukturanalyse des Methyl-Derivats (RG P21/c, Z=4) die Cyclopentadienylalkin-Struktur (III) bestatigt wird.

Structure and reactivity of monomeric, molecular Tin(II) compounds

Abstract: 3 General Structural Aspects of Tin(II) Chemistry . . . . . . . . . . . . . 7 3.1 Compar ison between Structures of Sn(II) and Sn(IV) Compounds . . . 7 3.2 The Stereochemical Effect o f the Lone Electron Pair at Tin(II) . . . . 8 3.2.1 Coordinat ion Polyhedra . . . . . . . . . . . . . . . . . . . 9 3.2.2 Correlat ion between Tin-Ligand Distances and Coordinat ion Numbers . . . . . . . . . . . . . . . . . . . 13

NMR spectroscopic studies on chalcogen compounds. 4. Carbon-13 isotope effect on selenium-77 and tellurium-125 nuclear shielding and its correlation with carbon-selenium bond distances. Tellurium-123 isotope effect on tellurium-125 nuclear shielding

Abstract: A one-bond isotope effect of /sup 13/C on the nuclear shielding of /sup 77/Se and /sup 125/Te was observed for the first time. Thirty values for Se(II) and Se(IV) compounds containing sp/sup 3/-, sp/sup 2/-, and sp-hybridized carbon were measured, and a wide range of low frequency shifts was found between -0.012 and -1.099 ppM. A correlation was established between the isotope shift and the C-Se bond distance for CF/sub 3/- and CH/sub 3/-substituted selenium compounds. The isotope shift of the linear molecules SeCO, SeCS, and SeCSe is inversely dependent on the force constant of the C=Se bond. This is attributed to different vibrational amplitudes. The CSe bond order plays an important role for the isotope shift and appreciable substituent effects were observed. Five Te(II) and Te(IV) compounds were studied. The isotope shifts are larger than for the corresponding Se compounds, and the results can be qualitatively interpreted in analogy to those found for the selenium compounds. The isotope shift can be temperature dependent; this was shown, for example, on liquid samples of CF/sub 3/SeCN and SeCO. This behavior is explained by a substantial contribution of vibrationally excited states. Furthermore, an isotope effect of /sup 123/Te on the screening ofmore » /sup 125/Te (and vice versa) in CF/sub 3/TeTeCF/sub 3/ was detected, and the coupling constant /sup 1/J(/sup 125/Te-/sup 123/Te) was determined.« less

The minor flavonoids

Abstract: It is the purpose of this chapter to discuss the five remaining flavonoid structural types, here collectively known as the minor flavonoids, chalcones, aurones, dihydrochalcones, flavanones and dihydroflavonols. In order to orientate the reader it should be mentioned that chalcones, aurones and dihydrochalcones were discussed in Chapter 9 of The Flavonoids (1975) while flavanones and dihydroflavonols were treated in Chapter 11. In The Flavonoids-Advances in Research (1982) the five structural types were brought together in Chapter 6 under the present name. As far as is practicable I have presented the material here in the same order and format as used in the earlier two volumes. This makes it possible for the reader to compare current material easily and directly with material in the corresponding sections in the other two books. For the most part each structural type is presented in terms of increasing structural complexity: the primary classification is based upon B-ring oxygenation with the presentation of compounds in order of degree of A-ring oxygenation, O-alkylation and C-alkylation.

Multiply Bonded Germanium Species

Abstract: Publisher Summary The fact that intermediates with silicon doubly bonded to carbon, oxygen, sulfur, nitrogen, or silicon have been described and fully characterized for several years, and no such germanium intermediate was described until 1973. Intermediates with germanium doubly bonded to carbon, oxygen, sulfur, nitrogen, phosphorus, and germanium are, like their silaanalogs, very unstable. The germanones thus generated were identified by various trapping reactions with methoxygermanes, oxagermacyclopentane, and ethylene oxide. The reactivity of azide increases with the electrophilic character of germylenes. This fact is consistent with a nucleophilic attack of azide on germylene leading to bipolar intermediate, which decomposes generating nitrogen and forming the germaimine. Germaphosphimine and silaphosphimine species with germanium or silicon doubly bonded to dicoordinated phosphorus can be obtained from 2-germa or 2- silaphosphetanes by a thermal β-decomposition reaction. In the study of GeH 2 , Kraus claimed it likely that dihydride is a dimer with doubly bonded germanium atoms and that upon the addition of sodium one bond is broken to form the salt NaH 2 GeGeH 2 Na. The first theoretical studies on germanium doubly bonded intermediates were carried out by Gowenlock and Hunter. The instability of germanium doubly bonded intermediates makes the spectroscopic observation and measurement very difficult. Doubly bonded germanium species, as well as germylenes, are of great fundamental and practical interest. These intermediates are powerful synthetic reagents in organometallic chemistry.

The Biological Methylation of Metals and Metalloids

Abstract: Publisher Summary This chapter discusses the biological methylation of metals and metalloids, their chemical rate studies, other aqueous transmethylations, biological transmethylation, mechanisms of transmethylation, and other aspects of biological transmethylation. The reaction between ethylcobalamin and Hg(OAc) 2 is also reported. Despite the importance of S-adenosylmethionine in biological methylation, very little research has been done in the laboratory on its reaction with arsenic or other metalloids, nor has there been much effort to explore possible methylsulfur model compounds. For various reasons, and, especially the Minamata disaster, most kinetic studies on in vivo transmethylation have concentrated on mercury. An enzyme system isolated from a strain of Pseudomonas catalyzes reductive cleavage of the mercury–carbon linkage in methylmercuric compounds. Human volunteers who drank wine containing inorganic arsenic excreted methylarsonic and cacodylic acids in their urine. At some point during any transmethylation, biotic or abiotic, the linkage between the methyl group and the atom holding it must break. Stereochemical retention of configuration around the saturated α-carbon can only result from the closed transition state, whereas the open transition state can lead to inversion. Water-soluble methylmetals frequently show marked lability and undergo facile transformations.