Unprecedented polymerization of epsilon-Caprolactone initiated by a single-site lanthanide borohydride complex, [Sm(eta-C5Me5)2(BH4)(thf)]: mechanistic insights

The monoborohydride lanthanide complex [Sm(Cp*)2(BH4)(thf)] (1a) (Cp* = eta-C5Me5), has been successfully used for the controlled ring-opening polymerization of epsilon-caprolactone (epsilon-CL). The organometallic samarium(III) initiator 1 a produces, in quantitative yields, alpha,omega-dihydroxytelechelic poly(epsilon-caprolactone) displaying relatively narrow polydispersity indices (<1.3) within a short period of time (30 min). The polymers have been characterized by 1H and 13C NMR, SEC, and MALDI-TOF MS analyses. Use of the single-site initiator 1 a allows a better understanding of the polymerization mechanism, in particular with the identification of the intermediate compound [Sm(Cp*)2(BH4)(epsilon-CL)] (1b). Indeed, one molecule of epsilon-CL initially displaces the coordinated THF in 1 a to give 1 b. Then, epsilon-CL opening (through cleavage of the cyclic ester oxygen-acyl bond) and insertion into the Sm--HBH3 bond followed by reduction of the carbonyl function by the BH3 end-group ligand, leads to the samarium alkoxyborane derivative [Sm(Cp*)2[O(CH2)6O(BH2)]] (2). This compound subsequently initiates the polymerization of epsilon-CL through a coordination-insertion mechanism. Finally, upon hydrolysis, alpha,omega-dihydroxypoly(epsilon-caprolactone), HO(CH2)5C(O)[O(CH2)5C(O)]nO(CH2)6OH (4) is recovered. The stereoelectronic contribution of the two Cp* ligands appears to slow down the polymerization and to limit transesterification reactions.     

Enhanced second-harmonic generation by metal surfaces with nanoscale roughness: nanoscale dephasing, depolarization, and correlations

On the basis of spectral-expansion Green's function theory, we theoretically describe the topography, polarization, and spatial-coherence properties of the second-harmonic (SH) local fields at rough metal surfaces. The spatial distributions of the fundamental frequency and SH local fields are very different, with highly enhanced hot spots of the SH. The spatial correlation functions of the amplitude, phase, and direction of the SH polarization all show spatial decay on the nanoscale in the wide range of the metal fill factors. This implies that SH radiation collected from even nanometer-scale areas is strongly depolarized and dephased, i.e., has the nature of hyper-Rayleigh scattering, in agreement with recent experiments. The present theory is applicable to nanometer-scale nonlinear-optical illumination, probing, and modification.     

Einige Auswerteverfahren bei spannungsoptischen Plattenuntersuchungen

Zusammenfassung Es sollte gezeigt werden, da\ sich bei der spannungsoptischen Untersuchung elastischer Platten, die innerhalb der Kirchhoffschen Bedingungen auf Biegung beansprucht werden, eine ganze Reihe von Auswertemöglichkeiten ableiten lassen, die es gestatten, die Biegungsmomente einzeln zu bestimmen. Um spannungsoptische Bilder zu bekommen, ist es erforderlich, die Modelplatte aus zwei Schichten verschiedenen Materials zusammenzukleben (Zweischichtenverfahren) oder in der PlattenmittelflÄche eine reflektierende Schicht anzubringen (Reflexionsverfahren). Die Me\grö\en sind die Differenz der Hauptbiegungsmomente und deren Richtungen. So wie man bei Scheibenauswertungen je nach dem vorliegenden Versuchsobjekt das eine oder das andere Auswerteverfahren bevorzugen wird, so hat man auch bei Platten die gleiche Auswahl an Auswerteverfahren. Einige davon wurden hier ausführlich wiedergegeben. Ausgehend von der Integration lÄngs geradliniger Wege wurde die Bestimmung der Momentensumme durch Integration lÄngs beliebiger Wege beschrieben, woraus sich wiederum eine Reihe von speziellen Methoden herleiten lie\. Dazu gehört zunÄchst die Integration lÄngs einer Hauptbiegungsmomentenlinie, die sich im Falle rotationssymmetrisch belasteter Kreisplatten als besonders zweckmÄ\ig erweist, und die Integration lÄngs einer Haupttorsionsmomentenlinie. Weiterhin wurde eine Möglichkeit zur Bestimmung der Momentensumme beschrieben, die sich an ein von Föppl für Scheiben angegebenes Verfahren anlehnt, sowie ein von Neuber für Scheiben entwickeltes graphisches Verfahren zur Konstruktion der Linien konstanter Momentensumme bei Platten erweitert. Zum Schlu\ wurden noch weitere Beziehungen angegeben, mit deren Hilfe sich die Richtungen der Linien konstanter Momentensumme errechnen lassen.Der Verfasser dankt dem Leiter des Spannungsoptischen Laboratoriums, Herrn Dr. H. Schwieger, für fruchtbare Diskussionen.     

Primary products and mechanistic considerations in alkane metathesis

Alkane metathesis, a reaction catalyzed by the silica-supported tantalum hydride [(SiO)2Ta-H], 1, which transforms acyclic alkanes into their higher and lower homologues, was reported in 1997. New studies conducted in a continuous flow reactor in the case of propane indicate that, by varying the contact time, hydrogen and olefins are primary products. This crucial observation, as well as the known properties of tantalum alkyls to perform alpha-H or beta-H eliminations, supports the proposition of a new mechanism involving metallacyclobutane intermediates just like in olefin metathesis. The observed selectivities for linear and branched Cn+1 and Cn+2 products as well as the linear/branched ratio can be well-explained on the basis of the minimization of steric interactions between 1,2- or 1,3-substituents in the various tantallacyclobutane intermediates or during their formation. Hydrogen plays a specific role in the cleavage of metal alkyls to complete the catalytic cycle.     

Sterically biased 3,3-sigmatropic rearrangement of azides: efficient preparation of nonracemic alpha-amino acids and heterocycles

[reaction: see text] Homochiral alpha-amino acids, heterocycles, and carbocycles are efficiently constructed via a short sequence of reactions starting from the chiral auxiliary p-menthane-3-carboxaldehyde. The key feature of the sequence is a highly selective tandem Mitsunobu/3,3-sigmatropic rearrangement of hydrazoic acid that procures enantiomerically enriched allylic azides. The sequence is either terminated by oxidative cleavage to provide amino acids or by ring-closing metathesis to provide heterocycles or carbocycles bearing nitrogen.     

Study of the Inclusion Complexes of β-Cyclodextrin with the Sodium Salt of Trisulfonated Triphenylphosphine

The formation of inclusion complexesbetween the sodium salt of trisulfonatedtriphenylphosphine and -cyclodextrin has beeninvestigated at two temperatures by high field nuclearmagnetic resonance, electrospray mass and UV-visspectroscopies. At 268 K, titration experiments andJob's method suggest that the major species insolution is a 1 : 1 inclusion complex. The moleculargeometry of this inclusion complex was studied usingthe ROESY NMR technique complemented by molecularmodelling. All these methods converged towards thestructure attained by inserting one aromatic ring intothe hydrophobic cavity of the host from the side ofthe secondary hydroxyls. At 298 K, a higher proportionof 2 : 1 and 3 : 1 complexes induces strong alterations ofthe NMR signals, preventing an easy and reliabledetermination of association constants. Nevertheless,an apparent association constant can be determinedfrom UV-vis data by assuming a 1 : 1 equilibrium. Thegeometry of the 2 : 1 and 3 : 1 complexes is also brieflydiscussed from ROESY NMR experiments.     

Estimation de la densité dans un espace de dimension infinie : Application aux diffusionsDensity estimation in an infinite dimensional space: Application to diffusion processes

This Note presents a nonparametric density function estimator in an infinite dimensional space. We consider two estimators of the density. Asymptotic results are stated. Finally we give a rate of convergence in the case of a diffusion process's density relative to a Wiener's measure. To cite this article: S. Dabo-Niang, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 213–216.