Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ‐donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus‐based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon‐based ligands (which are not NHCs), and which feature even stronger σ‐donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.     

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.     

Functional [6]pericyclynes: synthesis through [14+4] and [8+10] cyclization strategies

Critical analysis of possible strategies for the synthesis of novel carbo-benzene derivatives suggests several [(18-n)+n] routes for the preparation of hexaoxy[6]pericyclyne precursors. Beyond the previously attempted [9+9] symmetrical scheme (n=9), the a priori most selective strategies are those for which n=1, 4, 7, 10, 13, and 16. They involve a cyclizing double-propargylation of a C(18-n) omega-bis-terminal-skipped oligoyne containing (19-n)/3 triple bonds with a C(n) omega-dicarbonyl-skipped oligoyne containing (n-1)/3 triple bonds. To complement the previously used [11+7] strategy, the [14+4] and [8+10] strategies were thus explored. They proved to be efficient, affording seven novel hexaoxy[6]pericyclynes corresponding to six different substitution patterns. These compounds were obtained in 7-15 steps as mixtures of stereoisomers. Thus, by using dibenzoylacetylene as the C(4) electrophile, a [14+4] strategy allowed the synthesis of two hexaphenyl representatives with two or four free carbinol vertices. This approach also afforded tetraphenyl representatives in which the two remaining carbinoxy vertices were substituted with either two alkynyl or one 4-anisyl and one 4-pyridyl groups. By using the hexacarbonyldicobalt complex of butynedial as the C(4) electrophile, a [14+4] strategy also allowed the isolation of a tetraphenylhexaoxy[6]pericyclyne with two adjacent unsubstituted carbinol vertices. A regioisomer with two opposite unsubstituted carbinol vertices was obtained through an [8+10] strategy and its oxidation afforded the corresponding pericyclynedione. Several attempts at synthesizing diphenylhexaoxy[6]pericyclynes with four unsubstituted carbinoxy vertices are described. Only an [8+10] strategy allowed the generation of a fragile diphenylhexaoxy[6]pericyclyne with four adjacent unsubstituted carbinoxy vertices, which could be partly characterized. These results show that the synthesis of the nonsubstituted hexahydroxy[6]pericyclyne, the ring carbo-mer of [6]cyclitol, is a difficult challenge.     

Model-based measurement uncertainty evaluation,with applications in testing

Basic concepts and implementations of the model-based approach to uncertainty evaluation are discussed. One implementation is that of the law of propagation of uncertainty with the use of the central limit theorem as recommended in the Guide to the expression of uncertainty in measurement (GUM). Another is the propagation of distributions, the subject of a supplemental guide to the GUM, which is at an advanced stage of development. It falls in the category of other analytical and numerical methods indicated in the GUM. Two testing applications are used to illustrate the principles: tennis-ball rebound and tensile strength.

Calcul stochastique non adapté pour des processus à deux paramètres: formules de changement de variables de type Stratonovitch et de type Skorohod

Résumé Nous établissons une formule de changement de variables de type Stratonovitch pour des processus non adaptés indéxés par [0, 1]² qui est analogue à celle existant en calcul déterministe. Pour cela nous définissons une intégrale de Stratonovitch simple (resp. double): Nous en déduisons une formule de changement de variables de type Skorohod ne contenant pas d'intégrale curviligne. Nous utilisons des régularisations du processus de Wiener obtenues par convolution avec une approximation de l'unité.

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Summary We prove a Stratonovitch-type change of variable formula for anticipative processes on [0, 1]². The formula is the same as the existing one from deterministic calculus. In order to do so we define simple and double Stratonovitch integrals. We deduce a Skorohod-type change of variable formula which does not contain any line integral. Our method consists in using regularization of the Wiener process obtained by convolution.

     

Collapse temperature of bacterial suspensions: the effect of cell type and concentration

The characterisation of the physical state of frozen and freeze dried biological products delivers powerful information for freeze-drying process optimisation. The influence of lactic acid bacterial cell size, shape and concentration on collapse temperature of concentrated bacterial suspensions was investigated. Lactobacillus bulgaricus (long rods), and Streptococcus thermophilus (small spherical cells) were used as cellular models for this study. Whatever the strain, when lactic acid bacterial cells were added to protective solutions, the collapse temperature increased, thus allowing the use of higher sublimation temperatures during primary drying than expected from the protective medium alone. Moreover, the higher the cell concentration, the greater the effect, linear relationships existing between the collapse temperatures and the total dried matter. Cells of both strains gave a kind of robustness to the freeze-dried product, but the increase observed in collapse temperature was considerably higher (3 - 5 degree C) for L. bulgaricus compared to S. thermophilus. This result was ascribed to the different size and shape of the strains.     

Crystallographic/experimental electron density characterizations and reactions with nucleophiles of beta-enaminonitriles possessing a pyrrolobenzazepine core

In connection with a total synthesis of cephalotaxine (1a), we have examined the addition of various nucleophilic reagents to [ABC] subunits 2 and 7 possessing a pyrrolobenzazepine core. In fact, this reaction implicates invariably the carbonyl group of 2. Regarding the reaction of 7 with nucleophiles, the most striking aspect is the complete lack of reactivity of the enaminonitrile moiety. For instance, the condensation of 7 with methylmagnesium bromide involves exclusively the cleavage of the dioxole ring, yielding regioisomers 9 and 10. With the aim of understanding the unexpected reactivity of 2 and 7 toward nucleophiles, crystallographic studies of 2 and 7 and an experimental electron density determination of 7 were carried out. The marked reactivity of the carbonyl group of 2 was interpreted by invoking the weakness of the amide resonance, due to a pronounced delocalization of the N(9) lone pair over the enaminonitrile moiety. The electron density study of 7 reveals this electron delocalization along the enaminonitrile fragment, highlighted and quantified through the bond geometries, topological indicators, and atomic charges, a phenomenon that is responsible for the failure of the addition of nucleophilic species.