Mechanistic studies on the water-oxidizing reaction of homogeneous manganese-based catalysts: isolation and characterization of a suggested catalytic intermediate

The synthesis, isolation, and characterization of two high-valent manganese dimers with isomeric ligands are reported. The complexes are synthesized and crystallized from solutions of low-valent precursors exposed to tert-butyl hydroperoxide. The crystal structures display centrosymmetric complexes consisting of Mn(2)(IV,IV)(μ-O)(2) cores, with one ligand coordinating to each manganese. The ligands coordinate with the diaminoethane backbone, the carboxylate, and one of the two pyridines, while the second pyridine is noncoordinating. The activity of these complexes, under water oxidation conditions, is discussed in light of a proposed mechanism for water oxidation, in which this type of complexes have been suggested as a key intermediate.     

Mechanistic implications of an asymmetric intermediate in catalytic C-C coupling by a dinuclear nickel complex

An asymmetric nickel-nickel bonded intermediate was isolated in the reaction of biphenylene with bis(1,5-cyclooctadiene)nickel and i-Pr(3)P, where three of the four carbons are σ-bonded to one nickel. Mechanistic investigations support reactivity as a formal Ni(III)-Ni(I) complex; reductive elimination of cis-disposed Ni-C bonds from a single nickel centre directly provides a dinuclear Ni(I)-Ni(I) complex, a reaction relevant to dinuclear catalysis.     

Organocatalytic asymmetric allylic alkylation of sulfonylimidates with Morita-Baylis-Hillman carbonates

The asymmetric allylic alkylation reaction of sulfonylimidates with various Morita-Baylis-Hillman (MBH) carbonates was accomplished by the catalysis of commercially available cinchona alkaloids catalyst (DHQD)2AQN.The corresponding allylic alkylation products were obtained in good yields with high stereoselectivities (up to 99% ee,89:11 dr).     

Gas-phase conversion of tetrazoles to oxadiazoles: isolation and characterization of the N-acylated intermediate

The 5-substituted tetrazole ring was reacted in the gas phase with an acyl ion generated as the secondary reactive chemical ionization plasma in the mass spectrometer. The product ions obtained, among others, were proposed as the rearranged 2,5-disubstituted-1,3,4-oxadiazole nucleus. Its structure was demonstrated by comparison of the product ion spectrum of the 2,5-disubstituted-1,3,4-oxadiazole derivative obtained by condensed-phase reaction and the product ion spectrum of the oxadiazole derivative generated in situ by reaction of the 5-substituted tetrazole derivative with the acyl plasma. It was proposed that the mechanism of this transformation involved the presence of an acylated tetrazole intermediary, which could not be isolated in the condensed phase, followed by the rearrangement, with nitrogen loss, to afford the oxadiazole derivative. Under our conditions we were able to isolate the intermediate ion in the first field free region and demonstrate its structure by collision induced dissociation-tandem mass spectrometry.     

First isolation and characterization of an anti-apicophilic spirophosphorane bearing an oxaphosphetane ring: a model for the possible reactive intermediate in the Wittig reaction

An anti-apicophilic phosphorane bearing an oxaphosphetane ring, in which the ring carbon is apical and the ring oxygen is equatorial (C-apical), has been prepared as a thermally less stable stereoisomer of a phosphorane with an ordinary equatorial carbon-apical oxygen array in the oxaphosphetane ring (O-apical). This novel C-apical phosphorane, which could be considered to be a model compound of the reactive intermediate in the Wittig reaction, was fully characterized by NMR and X-ray structural analysis. The compound was found to easily isomerize to its more stable O-apical isomer, especially in the presence of proton sources, and the latter O-apical compound was found to furnish olefin at elevated temperatures.     

Mechanistic insights into the palladium-induced domino reaction leading to ketones from benzyl beta-ketoesters: first characterization of the enol as an intermediate

The monitoring by UV spectroscopy of the Pd-catalyzed hydrogenolysis in acetonitrile of 2-methyl-2-benzyloxycarbonyl-1-indanone and 2-methyl-2-benzyloxycarbonyl-1-tetralone showed the successive formation of corresponding beta-ketoacids and enols to deliver finally the ketones. Some factors which influence the stability of the intermediates are determined. In contrast to the above benzyl beta-ketoesters, the enol was not detected from benzyl (2-methylinden-3-yl) carbonate.     

Palladium-catalyzed regio- and enantioselective allylic alkylation of bis allylic carbonates derived from Morita-Baylis-Hillman adducts

Morita-Baylis-Hillman diene adducts are used as substrates in the palladium-catalyzed asymmetric allylic alkylation reaction with oxygen and carbon nucleophiles in good regio- and enantioselectivity.     

Transnitrosation of nitrosothiols: characterization of an elusive intermediate

We report an investigation of the reaction between (S)-nitroso-l-cysteine ethyl ester and l-cysteine ethyl ester as a model of physiologically relevant transnitrosation processes. Our theoretical and experimental evidence clearly supports the existence of a nitroxyl disulfide intermediate in solution.     

Mechanistic investigation of the iridium-catalysed alkylation of amines with alcohols

The [Cp*IrCl(2)](2)-catalysed alkylation of amines with alcohols was investigated using a combination of experimental and theoretical methods. A Hammett study involving a series of para-substituted benzyl alcohols resulted in a line with a negative slope. This clearly documents that a positive charge is built up in the transition state, which in combination with the measurement of a significant kinetic isotope effect determines hydride abstraction as being the selectivity-determining step under these conditions. A complementary Hammett study using para-substituted anilines was also carried out. Again, a line with a negative slope was obtained suggesting that nucleophilic attack on the aldehyde is selectivity-determining. A computational investigation of the entire catalytic cycle with full-sized ligands and substrates was performed using density functional theory. The results suggest a catalytic cycle where the intermediate aldehyde stays coordinated to the iridium catalyst and reacts with the amine to give a hemiaminal which is also bound to the catalyst. Dehydration to the imine and reduction to the product amine also takes place without breaking the coordination to the catalyst. The fact that the entire catalytic cycle takes place with all the intermediates bound to the catalyst is important for the further development of this synthetic transformation.     

Highly enantioselective allylic alkylation of 5H-oxazol-4-ones with Morita-Baylis-Hillman carbonates

An organocatalytic enantioselective allylic alkylation of 5H-oxazol-4-ones with Morita-Baylis-Hillman carbonates has been developed. With 10?mol% of commercially available cinchonidine, a wide range of substituted 5H-oxazol-4-one derivatives were constructed in good-to-excellent yields with high diastereo- and enantioselectivities. The allylic alkylation adducts obtained are valuable precursors for the synthesis of chiral α-alkyl α-hydroxycarboxylic acid derivatives, which represent a series of versatile building blocks in many biologically active compounds.     

Mechanistic implications of plastic degradation

Plastics have become an indispensable ingredient of human life. Their enormous use is a matter of great environmental and economic concern, which has motivated the researchers and the technologists to induce different degrees of degradations in the plastic. These degradations can be induced in a better way if their mechanistic implications are properly understood. A better understanding of the mechanism for these degradations is also advocated in order to facilitate the proper use of the alternative waste disposal strategies. In view of the facts concerning the plastic degradation, in this review article, we have discussed various types of polymeric degradations along with their mechanisms, which include photo-oxidative degradation, thermal degradation, ozone-induced degradation, mechanochemical degradation, catalytic degradation and biodegradation. This article also discusses the different methods used to study these degradations and the factors that affect these degradations.     

Coke precursor as an intermediate during the alkylation of isobutene/butene over a solid superacid

~~Coke precursor as an intermediate during the alkylation of isobutene/butene over a solid superacid~~     

Coke precursor as an intermediate during the alkylation of isobutane/butene over a solid superacid

Alkylation of isobutane/butene was conducted on a Brønsted-Lewis conjugated solid superacid. It is found that some hydrocarbons accumulated on the catalyst surface. These hydrocarbons, as called coke precursor, played an intermediate role at the initial stage of the alkylation before they lead the catalyst to lose its activity. The presence of the intermediate is beneficial to the alkylation between isobutane and butene, while increase the TMP content and TMP/DMH ratio in the products     

Mechanistic studies of the biosynthesis of 2-thiosugar: evidence for the formation of an enzyme-bound 2-ketohexose intermediate in BexX-catalyzed reaction

The first mechanistic insight into 2-thiosugar production in an angucycline-type antibiotic, BE-7585A, is reported. d-Glucose 6-phosphate was identified as the substrate for the putative thiosugar biosynthetic protein, BexX, by trapping the covalently bonded enzyme-substrate intermediate. The site-specific modification at K110 residue was determined by mutagenesis studies and LC-MS/MS analysis. A key intermediate carrying a keto functionality was confirmed to exist in the enzyme-substrate complex. These results suggest that the sulfur insertion mechanism in 2-thiosugar biosynthesis shares similarities with that for thiamin biosynthesis.     

Mechanistic studies of the radical S-adenosyl-L-methionine enzyme DesII: EPR characterization of a radical intermediate generated during its catalyzed dehydrogenation of TDP-D-quinovose

DesII, a radical S-adenosyl-l-methionine (SAM) enzyme from Streptomyces venezuelae, catalyzes the deamination of TDP-4-amino-4,6-dideoxy-D-glucose to TDP-3-keto-4,6-dideoxy-D-glucose in the desosamine biosynthetic pathway. DesII can also catalyze the dehydrogenation of TDP-D-quinovose to the corresponding 3-keto sugar. Similar to other radical SAM enzymes, DesII catalysis has been proposed to proceed via a radical mechanism. This hypothesis is now confirmed by EPR spectroscopy with the detection of a TDP-D-quinovose radical intermediate having a g-value of 2.0025 with hyperfine coupling to two spin 1/2 nuclei, each with a splitting constant of 33.6 G. A significant decrease in the EPR line width is observed when the radical is generated in reactions conducted in D(2)O versus H(2)O. These results are consistent with a C3 α-hydroxyalkyl radical in which the p-orbital harboring the unpaired electron spin at C3 is periplanar with the C-H bonds at both C2 and C4.     

Mechanistic insight into alkylation of the ethyl acetoacetate anion with different ethyl halides

The alkylation reactions of the ambident ethyl acetoacetate anion with C₂H₅X (X = F, Cl, Br, and I) in the O2, C3, and O4 positions of the anion were investigated at the B3LYP/6-311+G(d,p) level of theory. It was found that the ethylation reaction does not occur in the position O4, as well as with ethyl fluoride in any position of the anion, due to very high activation energies and thermodynamic instability of the hypothetic products. The activation energies for the reactions in the position O2 are lower in comparison to the position C3, but the products of the reactions in the C3 position are more stable than those in the position O4, implying that the C/O products ratio is controlled by both thermodynamic and kinetic factors, leading to the O2-product with the chloride, and C3-product with the iodide as leaving group.