Novel pathways for oxygen insertion into unactivated C-H bonds by dioxiranes. Transition structures for stepwise routes via radical pairs and comparison with the concerted pathway

The oxygen insertion into C-H bonds (of methane, isobutane, and acetone) by dioxiranes (parent dioxirane and dimethyldioxirane) to give alcohols was studied with the DFT theory, using both restricted and unrestricted B3LYP methods, and 6-31G(d) and 6-311+G(d,p) basis sets to evaluate the feasibility of stepwise mechanisms and their competition with the concerted counterpart. Confirming previous results by other authors, we have located, with the RB3LYP method, concerted TSs in which the oxygen bound to be inserted interacts very strongly with the hydrogen atom and very weakly with the carbon atom of the C-H bond. These TSs nicely explain all the experimental observations (e.g., configuration retention at the chiral centers), but all of them exhibit an RHF --> UHF wave function instability that preclude considering them as genuine transition structures. We also were able to characterize, with UB3LYP methods, two alternative two-step processes that can lead to final products (alcohol + carbonyl compound) via singlet radical pair intermediates. For the first step of both processes we located genuine diradicaloid TSs, namely, TSs rad,coll and TSs rad,perp, that have stable wave functions. In TSs rad,coll the alkane C-H bond tends to be collinear with the breaking O(1)- - -O(2) bond while in TSs rad,perp the alkane C-H bond is almost perpendicular to the O(1)- - -O(2) bond. The first step, of both processes, can represent an example of a "molecule induced homolysis" reaction: collision between alkane and dioxirane brings about the homolytic cleavage of the dioxirane O-O bond and the hydrogen abstraction follows afterward to produce the diradicaloid TS that then falls down to a singlet radical pair. This hypothesis was fully confirmed by IRC analysis in the case of TSs rad,coll. The possible pathways that lead from the intermediate radical pair to final products are discussed as well as the hypothesis that the radical collinear TSs may collapse directly to products in a "one-step nonconcerted" process. However, diradical mechanisms cannot explain the experimental data as satisfactorily as the concerted pathway does. As for computational predictions about competition of diradical vs concerted mechanisms, they strongly depend (i) on the alkane C-H type, (ii) on whether gas phase or solution is considered, and (iii) on the basis set used for calculations. In short, the concerted TS benefits, with respect to the corresponding diradicaloid TSs, of alkyl substitution at the C-H center, solvation effects, and basis set extension. Actually, in the case of DMD reactions with methane and acetone, the diradicaloid TSs are always (both in gas phase and in solution and with both the basis sets used) strongly favored over their concerted counterpart. In the case of DMD reaction with isobutane tertiary C-H bond the large favor for the diradicaloid TSs over the concerted TS, predicted in gas phase by the B3LYP/6-31G(d) method, progressively decreases as a result of basis set extension and introduction of solvent effects: the higher theory level [B3LYP/6-311+G(d,p)] suggests that in acetone solution TS conc has almost the same energy as TS rad,perp while TS rad,coll resides only 2 kcal/mol higher.     

Quantitative determination of fluorine in toothpastes

The quantitative CE analysis of fluorine and monofluorophosphate in toothpastes is described. Separation is performed using a Waters CIA-Pak chemistry kit with Waters Electro Osmotic Flow Modifiers on a CE instrument from Spectra Physics (Spectra-phoresis 500) at elevated temperatures up to 40 °C and at a low temperature of 10 °C. The quantitative results and the simplicity are compared for the CE method, the existing standard GC procedure described in the Official Journal of the European Communities, and the procedure employing a fluoride-ion-specific electrode.     

A fluorous chiral dirhodium(II) complex as a recyclable asymmetric catalyst

[reaction: see text] The chiral fluorous complex tetrakis-dirhodium(II)-(S)-N-(n-perfluorooctylsulfonyl)prolinate has been prepared and used as a catalyst in homogeneous or fluorous biphasic fashion. The catalyst displays good chemo- and enantioselectivity in intermolecular cyclopropanation and C-H bond activation reactions. The catalyst can be simply and thoroughly separated from the reaction mixture and is recyclable.     

Stereomutations of two-bladed propeller derivatives: ortho-substituted diaryl ethylene and diaryl ketone

Stereochemical analysis, supported by ab initio computations, predicts the existence of three possible stable helical conformers for o,o'-diisopropyl-1,1'-diphenylethylene (1) and o,o'-diisopropylbenzophenone (2). At low temperature the NMR spectra of 1 showed distinct sets of signals for these conformers, thus allowing the measurement of the three barriers involved in the related stereomutation processes to be obtained (DeltaG=6.45, 4.65, and < or =4.0 kcal mol(-1)). The NMR spectra also indicate that the asymmetric conformer (C1 point group) is the most stable one in solution, as anticipated by calculations. X-ray diffraction confirmed that this structure is that adopted in the crystalline state. On the other hand, o,o'-diisopropylbenzophenone (2) is predicted by calculations to exist essentially as a C2-type conformer, a result that was confirmed by the low-temperature NMR spectra. The interconversion barrier for the enantiomeric forms of this conformer was also measured (DeltaG=6.3(5) kcal mol(-1)).     

α-Amido sulfones from natural α-amino acids and their reaction with carbon nucleophiles

Amides obtained from N-carbamoyl α-amino acids react with aldehydes in the presence of benzenesulfinic acid to give α-amido sulfones in good yield. These derivatives act as equivalents of N-acylimines in the reaction with nucleophiles leading to the corresponding addition products. The utilization of the lithium enolate of alkyl acetates as a nucleophile allows the preparation of α,β-dipeptides, while a two-step procedure involving nitromethylation and Nef conversion leads to the synthesis of α,α-dipeptides.     

Combinatorial bases of modules for affine Lie algebra B 2 (1)

We construct bases of standard (i.e. integrable highest weight) modules L(Λ) for affine Lie algebra of type B ₂ ⁽¹⁾ consisting of semi-infinite monomials. The main technical ingredient is a construction of monomial bases for Feigin-Stoyanovsky type subspaces W(Λ) of L(Λ) by using simple currents and intertwining operators in vertex operator algebra theory. By coincidence W(kΛ₀) for B ₂ ⁽¹⁾ and the integrable highest weight module L(kΛ₀) for A ₁ ⁽¹⁾ have the same parametrization of combinatorial bases and the same presentation P/I.     

Connecting and Integrating Theoretical Frames: The TELMA Contribution

This paper presents the methodology developed within TELMA for connecting and integrating the theoretical frames used by the different teams for studying the design and use of interactive learning environments in mathematics education. Two case studies are then analysed and compared in order to illustrate the methodology and the results it can lead to. The papers ends by a more general discussion about the outcomes of the experimental work developed within TELMA and the perspectives it offers for approaching theoretical fragmentation.