Rapid annulation of tropolone units via a pyrylium ylide 1,3-dipolar cycloaddition reaction

The tropolone subunit of the naturally occurring alkaloid rubrolone aglycon is synthesized via a short reaction sequence starting with a 1,3-dipolar cycloaddition of a pyrylium ylide and indenone, followed by enone oxidation, oxygen bridge elimination and finally hydroxy group oxidation.

Enzyme-catalysed synthesis and reactions of benzene oxide/oxepine derivatives of methyl benzoates

A series of twelve benzoate esters was metabolised, by species of the Phellinus genus of wood-rotting fungi, to yield the corresponding benzyl alcohol derivatives and eight salicylates. The isolation of a stable oxepine metabolite, from methyl benzoate, allied to evidence of the migration and retention of a carbomethoxy group ( the NIH Shift), during enzyme-catalysed ortho-hydroxylation of alkyl benzoates to form salicylates, is consistent with a mechanism involving an initial arene epoxidation step. This mechanism was confirmed by the isolation of a remarkably stable, optically active, substituted benzene oxide metabolite of methyl 2-( trifluoromethyl) benzoate, which slowly converted into the racemic form. The arene oxide was found to undergo a cycloaddition reaction with 4-phenyl-1,2,4-triazoline-3,5-dione to yield a crystalline cycloadduct whose structure and racemic nature was established by X-ray crystallography. The metabolite was also found to undergo some novel benzene oxide reactions, including epoxidation to give an anti-diepoxide, base-catalysed hydrolysis to form a trans-dihydrodiol and acid-catalysed aromatisation to yield a salicylate derivative via the NIH Shift of a carbomethoxy group.

Nucleoside Azide-Alkyne Cycloaddition Reactions Under Solvothermal Conditions or Using Copper Vials in a Ball Mill

Novel nucleoside analogues containing photoswitchable moieties were prepared using 'click' cycloaddition reactions between 5 '-azido-5 '-deoxythymidine and mono- or bis-N-propargylamide-substituted azobenzenes. In solution, high to quantitative yields were achieved using 5mol% Cu(I) in the presence of a stabilizing ligand. 'Click' reactions using the monopropargylamides were also effected in the absence of added cuprous salts by the application of liquid assisted grinding (LAG) in metallic copper reaction vials. Specifically, high speed vibration ball milling (HSVBM) using a 3/32('') (2.38mm) diameter copper ball (62mg) at 60Hz overnight in the presence of ethyl acetate lead to complete consumption of the 5 '-azido nucleoside with clean conversion to the corresponding 1,3-triazole.

General rules for predicting where a catalytic reaction should occur on metal surfaces: A density functional theory study of C-H and C-O bond breaking/making on flat, stepped, and kinked metal surfaces

To predict where a catalytic reaction should occur is a fundamental issue scientifically. Technologically, it is also important because it can facilitate the catalyst's design. However, to date, the understanding of this issue is rather limited. In this work, two types of reactions, CH4 CH3 + H and CO C + 0 on two transition metal surfaces, were chosen as model systems aiming to address in general where a catalytic reaction should occur. The dissociations of CH4 - CH3 + H and CO --> C + O and their reverse reactions on flat, stepped, and kinked Rh and Pd surfaces were studied in detail. We find the following: First, for the CH4 Ch(3) + H reaction, the dissociation barrier is reduced by similar to0.3 eV on steps and kinks as compared to that on flat surfaces. On the other hand, there is essentially no difference in barrier for the association reaction of CH3 + H on the flat surfaces and the defects. Second, for the CO C + 0 reaction, the dissociation barrier decreases dramatically (more than 0.8 eV on Rh and Pd) on steps and kinks as compared to that on flat surfaces. In contrast to the CH3 + H reaction, the C + 0 association reaction also preferentially occurs on steps and kinks. We also present a detailed analysis of the reaction barriers in which each barrier is decomposed quantitatively into a local electronic effect and a geometrical effect. Our DFT calculations show that surface defects such as steps and kinks can largely facilitate bond breaking, while whether the surface defects could promote bond formation depends on the individual reaction as well as the particular metal. The physical origin of these trends is identified and discussed. On the basis of our results, we arrive at some simple rules with respect to where a reaction should occur: (i) defects such as steps are always favored for dissociation reactions as compared to flat surfaces; and (ii) the reaction site of the association reactions is largely related to the magnitude of the bonding competition effect, which is determined by the reactant and metal valency. Reactions with high valency reactants are more likely to occur on defects (more structure-sensitive), as compared to reactions with low valency reactants. Moreover, the reactions on late transition metals are more likely to proceed on defects than those on the early transition metals.

On the importance of steady-state isotopic techniques for the investigation of the mechanism of the reverse water-gas-shift reaction

The formation and reactivity of surface intermediates in the reverse water-gas-shift reaction on a Pt/CeO2 catalyst are critically dependent on the reaction conditions so that conclusionsregarding the reaction mechanism cannot be inferred using ex operando conditions.