四正丁基过硫酸铵氧化裂解2，4-二硝基苯腙衍生物的反应研究

四正丁基过硫酸铵在1,2二氯乙烷溶剂中可高产率地使醛和酮的2,4-二硝基苯腙衍生物氧化裂解成相应的羰基化合物,其分子中碳碳双键,氰基,酯基和缩醛基等基团均不受影响。同时探讨了可能的反应机理和反应溶剂对氧化裂解反应的影响。     

Reductive cleavage of dichalcogenide bonds

The kinetics of reductive cleavage of the dichalcogenide bond in dimeric Schiff's bases ArEEAr (E=S, Te) induced by indirect electron transfer byin situ generated bridged bispyridinium radical cations and biradicals was studied by cyclic voltammetry. It was shown that the dependence between the apparent rate constants of intermolecular electron transfer and the electron-withdrawing properties of diaryldichalcogenides is violated. The mechanisms of homogeneous (chemical) and heterogeneous (electrochemical) reductive cleavage of diaryldichalcogenides were discussed. For Part 2, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1522–1526, August, 1999.     

Electroreductive cleavage of carbon-sulphur bonds in dithioacetals

The carbon-sulphur bond in aliphatic diphenyldithio-acetals, α-carbonyldiphenyldithioacetals and α-carbonylketene dimethyldithioacetals can be cleaved cathodlcally on mercury (Hg) or glassy carbon (GC) electrodes.In the presence of tetra-butylammoniumhydrogensulphate (TBAHSO₄) as a proton donor one thio-phenyl- respectively thiomethyl group is substituted by hydrogen to provide the corresponding phenylthioethers (phenylthiomethyl) ketones or methylthiovinylketones in very good yields. At a Hg cathode the electroreduction of α-carbonyldiphenyldi-thioacetals is self-catalyzed.     

A practical oxidative method for the cleavage of hydrazide N[bond]N bonds

The selective N-oxidation of the most nucleophilic amino nitrogen atom in hydrazides is central to the development of an unprecedented methodology for the cleavage of their N[bond]N bonds under oxidative conditions. Treatment of a series of hydrazides 1-9 with peracids such as magnesium monoperoxyphtalate hexahydrate (MMPP.6 H(2)O) or meta-chloroperbenzoic acid (m-CPBA) afforded the corresponding amides 10-16 in good-to-excellent yields (80-92 %). The extension of the methodology to carbamate-like substrates such as 17 and 18 was also investigated, but in this case the process is synthetically useless in view of the low yields observed of carbamates 19 and 20 (approximately 15 %). Experiments carried out with equivalent amounts of oxidant produced nitrones, such as 26, proceeding from the dialkylamino moiety, and (1)H NMR experiments indicated that this product is formed by fast conversion of the parent hydrazide, without detection of the expected hydrazide N-oxides. In addition, the over oxidation of 26 into nitronate 25 proceeds through an unknown intermediate. This oxidative N[bond]N bond cleavage by peracids is an alternative method for the deamination of hydrazides, and constitutes the only solution compatible with substrates carrying functionalities sensitive to reducing conditions.     

Total Synthesis of the 7,10‐Epimer of the Proposed Structure of Amphidinolide N,Part I: Synthesis of the C1–C13 Subunit

Amphidinolide N, the structure of which has been recently revised, is a 26‐membered macrolide featuring allyl epoxide and tetrahydropyran moieties with 13 chiral centers. Due to its challenging structure and extraordinary potent cytotoxicity, amphidinolide N is a highly attractive target of total synthesis. During our total synthesis studies of the 7,10‐epimer of the proposed structure of amphidinolide N, we have synthesized the C1–C13 subunit enantio‐ and diastereoselectively. Key reactions include an l ‐proline catalyzed enantioselective intramolecular aldol reaction, Evans aldol reaction, Sharpless asymmetric epoxidation and Tamao–Fleming oxidation. To aid late‐stage manipulations, we also developed the 4‐(N‐benzyloxycarbonyl‐N‐methylamino)butyryl group as a novel ester protective group for the C9 alcohol.     

A Short Sequence for the Iterative Synthesis of Fused Polyethers

A simple and efficient four‐step sequence for the synthesis of fused polyether arrays has been developed. Cyclic ethers are installed by sequential alkynyl ether formation, carbocupration, ring‐closing metathesis and hydroboration with acidic workup. Crucially, the alkene required for the subsequent ring formation by ring‐closing metathesis is present in the substrate but is masked in the form of a vinylic silane, which prevents competitive metathesis of the side chain. Generation of the reactive alkene from the unreactive vinylic silane is accomplished by hydroboration and subsequent acid‐mediated Peterson elimination of the intermediate hydroxysilane.     

Alkaline cleavage of silicon-hydrogen bonds in pentamethyl- and tetramethyldisiloxane

The rates of cleavage of the Si–H bond in penta-or tetramethyldisiloxane, catalyzed by potassium hydroxide have been studied in moist ethanol at 25°C. The reaction is first order in the catalyst and in the silane. The apparent kinetic constant varies with the inverse of the square root of the initial silane concentration. The mechanism involves either a fast equilibrium between hydroxide ion and the silane, followed by the approach ofa hydroxylated molecule in the rate-determining step, or a concerted attack on the silicon atom.

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Si–H - , , 25 °C. , . . , .

     

Role of metal-oxo complexes in the cleavage of C-H bonds

The functionalization of C-H bonds has yet to achieve widespread use in synthetic chemistry in part because of the lack of synthetic reagents that function in the presence of other functional groups. These problems have been overcome in enzymes, which have metal-oxo active sites that efficiently and selectively cleave C-H bonds. How high-energy metal-oxo transient species can perform such difficult transformations with high fidelity is discussed in this tutorial review. Highlighted are the relationships between redox potentials and metal-oxo basicity on C-H bond activation, as seen in a series of bioinspired manganese-oxo complexes.     

Selective cleavage of the carbon-sulphur and carbon-oxygen bonds in methoxythioanisoles

Selective cleavage of thioether of ether functions in methoxythioanisoles in hexamethylphosphoramide (HMPA) with sodium gives methoxythiophenols by cleavage of the carbon-sulphur bond. Reactions with sodium isopropanethiolate give instead the thiomethoxyphenols by dealkylation of the methoxy function. When the methoxythioanisoles were treated first with sodium isopropanethiolate and then with sodium complete dealkylation was achieved with formation of mercaptophenols. The present methods have considerable advantages over existing procedures for the synthesis of methoxythiophenols, thiomethoxyphenols and mercaptophenols. The mechanistic implications of the reactions investigated are also discussed.     

Metal-mediated formation of carbon-halogen bonds

Organic halides represent basic starting materials for numerous metal-catalyzed organic transformations. Generally, the carbon-halogen is broken in the first step, that is, an oxidative addition reaction, of the catalytic cycle. On the other hand, very little is known about the reverse reaction, carbon-halogen reductive elimination from a transition-metal center. In this Concept article, we describe the examples of C(sp(3))-halide and C(sp(2))-halide reductive-elimination reactions which demonstrate that this type of reactivity can be quite common in organometallic chemistry. Although the thermodynamic driving force for the formation of carbon-halogen bonds is relatively small, the kinetic barrier for these reactions can also be low. Thus, C-halide reductive elimination can compete favorably with the more established organic transformations, such as C-C reductive elimination.     

Bond-coupled electron transfer processes: cleavage of Si-Si bonds in disilanes

Photooxidation of 1,1,2,2-tetra-tert-butyl-1,2-diphenyldisilane, 1, by triplet sensitizers in CHCl3/CCl4 solutions yields chlorosilane, 2, in high chemical yield. The quantum yield for formation of 2 depends on the energy of the ion radical pair formed following initial electron transfer. Dissociative return electron transfer (DRET) is proposed as the mechanism for the highly efficient Si-Si bond cleavage in 1. DRET may be a useful strategy for the fragmentation of other such bonds in di-, oligo-, and polysilanes as well as other group 4A compounds using a variety of sensitizers with different spectral properties.     

Oxidative cleavage of ribofuranose 5-(α-hydroxyphosphonates): a route to erythrofuranose-based nucleoside phosphonic acids

We report here an oxidative cleavage of (5R)- and (5S)-ribofuranosyl-5-C-phosphonate derivatives with periodate anion under both strong acidic and neutral conditions. In both cases, only (5R)-configured compound underwent the expected oxidation reaction and afforded the desired (4R)-erythrofuranosylphosphonate, whereas the second epimer, (5S)-ribofuranosyl-5-C-phosphonate did not provide the corresponding (4S)-erythrofuranosylphosphonate derivative. This different behavior of epimers toward oxidative cleavage is an important phenomenon. The obtained (4R)-erythrofuranosylphosphonate was used for the preparation of phosphonate mimic of adenosine 5′-phosphate via classical nucleosidation reaction. Condensation of the protected shortened AMP analogue with adenosine derivatives, however, provided only the 2′,5′-linked ApA analogue. Study on hybridization of the modified 2′-5′ ApA with polyU revealed its ability to form stable triplex-like complex, similar to natural 2′-5′ r(ApA) and 3′-5′ r(ApA). NMR spectroscopy study showed that the erythrofuranose part of the phosphonate nucleotide unit of modified 2′-5′ ApA was predominantly in the C2′-endo conformation, which is characteristic for B-DNA.     

Aerobic photo-oxidative cleavage of the C-C double bonds of styrenes

Carbon tetrabromide enables us to carry out oxidative cleavage of the C-C double bonds of styrenes under aerobic photo-irradiation conditions. Oxidative cleavage of the various β-substituted styrenes produced benzoic acid in good yields. Since this reaction is found to be applicable to the α- or β-substituted styrenes, which showed very low reactivity under our previous cleavage reaction condition with FSM-16 and I₂, this reaction can be used complementarily.     

Oxidative fluorination of P-H bonds in the coordination sphere of tungsten

The reaction of triphenylcarbenium tetrafluoroborate with primary or secondary phosphine pentacarbonyltungsten complexes gives the corresponding mono or diflurorophosphine complexes. The mechanism probably involves transient phosphenium complexes. The reaction is compatible with P-O but not with P-N bonds.     

Oxidative dehydrogenation of propane

At temperatures near 650°C and residence times ofca. 3 s, the homogeneous oxidative dehydrogenation (OXD) of propane to propylene and ethylene approached oxygen limiting conditions, even when the reactor was filled with quartz chips. The addition of catalysts that are known to be effective in the OXD of ethane slightly increased the reaction rate, but the selectivities at a given conversion level were the same as those that were achieved in the homogeneous reaction.