Homogeneous and heterogeneous catalysis: bridging the gap through surface organometallic chemistry

Surface organometallic chemistry is an area of heterogeneous catalysis which has recently emerged as a result of a comparative analysis of homogeneous and heterogeneous catalysis. The chemical industry has often favored heterogeneous catalysis, but the development of better catalysts has been hindered by the presence of numerous kinds of active sites and also by the low concentration of active sites. These factors have precluded a rational improvement of these systems, hence the empirical nature of heterogeneous catalysis. Catalysis is primarily a molecular phenomenon, and it must involve well-defined surface organometallic intermediates and/or transition states. Thus, one must be able to construct a well-defined active site, test its catalytic performance, and assess a structure-activity relationship, which will be used, in turn-as in homogeneous catalysis-to design better catalysts.By the transfer of the concepts and tools of molecular organometallic chemistry to surfaces, surface organometallic chemistry can generate well-defined surface species by understanding the reaction of organometallic complexes with the support, which can be considered as a rigid ligand. This new approach to heterogeneous catalysis can bring molecular insight to the design of new catalysts and even allow the discovery of new reactions (Ziegler-Natta depolymerization and alkane metathesis). After more than a century of existence, heterogeneous catalysis can still be improved and will play a crucial role in solving current problems. It offers an answer to economical and environmental problems faced by industry in the production of molecules (agrochemicals, petrochemicals, pharmaceuticals, polymers, basic chemicals).     

Synthesis of (−)-xialenon A by enantioselective α-deprotonation-rearrangement of a meso-epoxide

The first total synthesis of (−)-xialenon A was achieved via enantioselective transannular desymmetrisation of a substituted cycloctene oxide using an organolithium/(−)-α-isosparteine combination. Oxidation of the resulting bicyclo[3.3.0]octanol gave an enone which underwent stereoselective conjugate allylation; subsequent α′-hydroxylation on the more hindered face of a derived enone using hypervalent iodine chemistry led to the natural product.     

Determination of soluble lignin and proteins in the presence of each other

Several methods for the differential determination of lignin and of proteins solubilized together in the same solution are compared. It is shown that lignin is best determined by spectrophotometry at 280 nm, the absorptivity of the polymer being an order of magnitude greater than those of proteins. In contrast, a turbidimetric method applied to acid-precipitated lignin is strongly influenced by both the nature and the amounts of the proteins present in the mixture. Ninhydrin reagents permits the determination of proteins without interference from soluble lignin. Phenol and Coomassie Blue reagents react strongly with lignin, thus masking their interaction with proteins. The soluble lignin preparations used were solubilized from stake lignin in buffers adjusted to different pH values. They were analysed by exclusion chromatography and the results suggest that the lignin fragments obtained at higher pH are larger than those solubilized at lower pH. Turbidimetry showed that these larger soluble fragments form larger aggregates after acid precipitation.     

The Acetolysis of exo- and endo-5,6-Dimethylidene-2-Norbornyl p-Bromobenzenesulfonates and of their Optically Active and Deuterium-Labelled Derivatives

The buffered (AcOK) acetolyses of exo, (11) and endo-5, 6-dimethylidene-2-norbornyl brosylate (12) yielded exo5, 6-dimethylidene-2-norbornyl (16) and (3-methylidene-2-nortricyclyl)methyl acetates (18) . Endo-5, 6-dimethylidene-2-norbornyl (17) and 2-methylidene-3-tricyclo [3.2.1.0^3,6]octyl acetates (20) could not be detected. The titrimetric rate constants of the acetolysis of 11 (k_t(exo)=4.49 ± 0.02) · 10^?5 s^?1 at 25°, ΔH^≠=23.6 ±0.7 kcal mol^?1, ΔS^≠=0.7 ±2 calmol^?1 K^?1 and 12 (k_t(endo)=1.9 ±0.08) · 10^?9 s^?1 at 25°, ΔH^≠=27 ±1 kcal mol^?1, ΔS^≠=-8 ±2.5 calmol^?1 K^?1) were measured and compared with the polarimetric rate constants (k_α/k_(exo)=6.8 at 25°,(k_α/k_(exo)=1.0 at 121°) of the buffered acetolyses of the optically active brosylates (+)- 11 and (+)- 12 . Neither a common-ion (KOBs) nor a special ion effect (LiClO₄) on k_t(endo) could be detected, although external return might well intervene as some exo-5,6-dimethylidene-2-norbornyl tosylate (21) was formed upon solvolysis in the presence of KOTs. Acetolysis of (+)- 11 yielded completely racemized products, whereas (+)- 12 led to incomplete racemization. The buffered acetolysis of exo-(3exo-D)-5,6-dimethylidene-2-norbornyl brosylate (24) furnished (3exo-D)-( 26 :37.5%), exo-(7syn-D)-5,6-dimethylidene-2-norbornyl brosylate (27 : 37.5%) and [(5anti-D)-3-methylidene-2-nortricyclyl]methyl acetates (28 : 25 %). The acetolysis of endo-(2exo-D)-5,6-dimethylidene-2-norbornyl brosylate (25) yielded (2endo-D)-( 29 : 54%), exo-(1-D)-5,6-dimethylidene-2-norbornyl ( 30 : 36%) and [(6-D)-3-methylidene-2-nortricyclyl]methyl acetates ( 31 : 10%). Product analysis and deuterium label distribution was established by a combination of GC., ¹H-NMR., ²H-{¹H}-NMR. and MS. techniques. The results are rationalized by invoking anchimerically assisted ionization of the exo-brosylate 11 to symmetrical ion-pairs (cyclopropylcarbinyl cation intermediates) which undergo internal (and probably also external) return. Acetolysis of the endo-brosylate 12 is not anchimerically assisted and leads initially to non-symmetrical ion pairs. These evolve to symmetrical ion pair intermediates or, to a minor extent, are intercepted by solvent.     

Biodegradable materials by reactive extrusion: from catalyzed polymerization to functionalization and blend compatibilization

This contribution aims at reviewing how reactive extrusion (REx) technique can participate in the design and development of biodegradable polymeric materials more particularly based on aliphatic polyesters issued from both renewable (agrochemical) and fossil (petrochemical) resources. On one side, REx will be approached for producing in a continuous way biodegradable aliphatic polyesters by ring-opening polymerization of lactones. Tin and aluminum-based catalysts will be considered for quantitatively converting the cyclic monomer in high molecular weight polyester chains within residence time of a few minutes only. On the other hand, such polyesters will undergo various chemical modifications again performed by REx. Self-branching reactions will be investigated in order to compensate the characteristic molecular weight reduction of the polyester chains as triggered by hydrolysis and/or transesterification side-reactions. Controlled functionalization reactions by maleic anhydride treatment will be studied as well. Finally biodegradable composite materials will be produced by REx and will rely upon the interface compatibilization between the polyester matrix and natural fillers such as starch granules or talc microparticles. To cite this article: J.-M. Raquez, et al., C. R. Chimie 9 (2006).     

Theoretical study of the second-order nonlinear optical properties of [N]helicenes and [N]phenylenes

The second-order nonlinear optical properties of helicenes and phenylenes have been theoretically investigated at the time-dependent Hartree-Fock level using the Austin model 1 semiempirical Hamiltonian. Both the antisymmetric isotropic component of the first hyperpolarizability (beta) and its projection on the dipole moment (beta(parallel)) have been determined for increasingly large helical systems as well as for their analogs substituted by donor/acceptor pairs. It is found that (i) in nonsubstituted helicenes and phenylenes, beta increases monotonically with the size of the system and slightly depends on the nature of the helix; (ii) the corresponding beta(parallel) is mostly determined by the radial component of the first hyperpolarizability vector; (iii) in helicenes, beta(parallel) is positive and presents quasiperiodic oscillations with the helix; (iv) in phenylenes, beta(parallel) depends upon the size of the helix and it can be either positive or negative as a result of the differences in evolution with N of the radial components of the dipole moment and first hyperpolarizability. Substituting the helicenes and phenylenes by the prototypical NH2/NO2 donor/acceptor pair provides a diversity of effects on beta and beta(parallel) that encompasses decrease, increase, and change in sign.     

Evidence for the Generation of Singlet Oxygen (<Superscript>1</Superscript>O<Subscript>2</Subscript>, <Superscript>1</Superscript>Δ<Subscript>g</Subscript>) from Ozone Promoted by Inorganic Salts

Summary.  Evidence for novel chemical sources of singlet oxygen based on the conversion of ozone by tin(II) in acetic acid or nitrite ions in water was found by screening a selection of inorganic salts by means of a sensitive detector for exothermic processes (SEDEX) and a N₂-cooled Ge-diode for singlet oxygen luminescence measurements. Corresponding author. E-mail: heinz.falk@jku.at Received July 18, 2002; accepted August 23, 2002     

Molecular Recognition of NADP(H) and ATP by Macrocyclic Polyamines Bearing Acridine Groups

The macrocyclic polyamine-based receptor BA bearing two acridine units makes use of combined electrostatic and stacking interactions for the binding of nucleotide polyphosphates and for the recognition of ATP and of NADPH (k_a > 3 · 10⁸ M), with a high selectivity for NADPH vs. NADP (ca. 10³) and NAD(H) (> 10⁶). The binding properties of this receptor towards a variety of substrates led to its in vitro application as a fluorescent probe for ATP. BA also interacts strongly with nucleic acids as shown by spectrophotometric, spectrofluorimetic, and electrophoretic mobility methods.     

Lifetime of fluorescent pyrene butyric acid probe in single living cells for measurement of oxygen fluctuation

We study the fluorescence lifetime of the well-known 1-pyrene butyric acid (PBA) to assess oxygen concentrations in living cells. The behavior of the probe is first studied in water, ethanol, protein solution and liposome suspension. The Stern-Volmer plot of these solutions is linear, and the bimolecular reaction rate constant agrees with previous observations. In single living cells, the PBA lifetime decreases with oxygen concentration (185 to 55 ns). The probe lifetime differences between living cells and liposome suspension, especially under nitrogen atmosphere, suggest a supplemental pathway for the deactivation of the probe. We simplify further the complex living cells system by stopping the cell functions and studying freshly fixed cells. In this case, we obtained an increase of PBA lifetime under nitrogen atmosphere (215 ns).