Regioselective allylic alkylation and etherification catalyzed by in situ generated N-heterocyclic carbene ruthenium complexes

Benzimidazolium halides are used for the first time as ligand precursors in ruthenium-catalyzed substitution of allylic carbonates and chlorides by carbon nucleophiles and phenols, respectively. After generation of diaminocarbene species upon deprotonation by tBuOK, their association with [Cp*Ru(MeCN)₃]PF₆ induces a very high regioselectivity in favor of the branched isomers when cinnamyl derivatives are used as starting substrates. They also provide good regioselectivities for the allylation of phenols by unsymmetrical aliphatic allylic substrates such as 3-chloro-4-phenylbut-1-ene, and thus provide a straightforward access to new allylic phenyl ethers.     

One-pot rhodium(i)-catalyzed hydroboration of alkenes: radical conjugate addition

B-Alkylcatecholboranes, prepared by rhodium(I)-catalyzed hydroboration of alkenes, are suitable radical precursors for conjugate addition to activated olefins. This procedure proved to be particularly useful for the control of the regio- and chemoselectivity of such tandem processes. Enantioselective hydroboration has also been successfully coupled with radical chain reaction in a one-pot process.     

Use of bis(trifluoromethyl)peroxy dicarbonate as initiator in the radical homopolymerisation of vinylidene fluoride (VDF) and copolymerisation of VDF with hexafluoropropylene

The radical homopolymerisation in acetonitrile of vinylidene fluoride (or 1,1-difluoroethylene, VDF) and the copolymerisation of VDF with hexafluoropropylene (HFP) initiated by bis(trifluoromethyl)peroxy dicarbonate are presented. Different reactions and different reactants were chosen to monitor the polymerisation in terms of initiating radicals generated from this initiator. Homopolymers and copolymers thus obtained were characterised by and NMR spectroscopy. From the assignments of the characteristic signals, an overall reaction mechanism is proposed that explains each step of the polymerisation. Particularly, an interpretation of the polymer microstructures and the presence of end-groups arising from the radical initiator as well as from eventual transfers is suggested. Among some of the microstructures, the trifluoromethoxy end-group was noted to be present in both PVDF and poly(VDF-co-HFP) (co)polymers, as generated from the decomposition of the initiator. This trifluoromethoxy end-group enabled the assessment of the molecular weights of PVDF and poly(VDF-co-HFP) (co)polymers. Thermal properties of the copolymers were also determined, showing that original fluoroelastomers possessing CF₃ end-groups are obtained endowed with low T_g and good thermal stability.     

Zeolitic properties of mixed hexacyanoferrates/II/: Adsorption isotherms and differential heats of adsorption of water vapour

The series of mixed hexacyanoferrates M₂Zn₃[Fe/CN/₆]₂.xH₂O (M=K⁺, Na⁺, Cs⁺) present zeolitic properties. The adsorption water vapour isotherms were determined by TG at 303 K. The differential heats of adsorption of water vapour were measured by microcalorimetry; the molar integral entropy of the sorbed phase was calculated. The data obtained show that (i) the H₂O-sorbent interactions are very weak at the beginning of cavity filling, whereas the sorbed molecules exhibit the same mobility as in the vapour phase; (ii) during the filling, the H₂O-H₂O interactions increase up to a maximum; (iii) close to total filling, the adsorption phenomenon is comparable to a vapour-liquid transition and the sorbed phase has the same molar entropy as that of the liquid phase.     

Probing polymers with single fluorescent molecules

The use of single molecules to study local, nanoscale polymer dynamics is presented. Fluorescence lifetime fluctuations were used to extract the number of polymer segments (N_s) taking part in the rearranging volume around the probe molecule below the glass transition temperature. N_s was dependent on the temperature and it decreased with increasing temperature. Above the glass transition, rotational motion of single molecules was followed in time and typical time-scales of the rotational diffusion were extracted. These two approaches allowed us to obtain non-averaged information about the heterogeneous dynamics present in polymer systems, on the nanoscale, above and below glass transition temperatures.     

Thiophenol-mediated hydrogen atom abstraction: an efficient tin-free procedure for the preparation of cyclopentane derivatives

[reaction: see text] An efficient procedure for running a cascade reaction involving 1,5-abstraction of a hydrogen atom followed by a radical cyclization is reported. Alkenyl radicals are generated from easily available terminal alkynes and thiophenol. This procedure eliminates the need of using the toxic tributyltin hydride and gives a greater amount of radical translocation products.     

Enantiomerenreine Pyrrolidin-Derivate aus trans-4-Hydroxy-L-prolin durch elektrochemische oxidative Decarboxylierung und Titantetrachlorid-vermittelte Umsetzung mit Nukleophilen

Enantiomerically Pure Pyrrolidine Derivatives from trans-4-Hydroxy-L -proline by Electrochemical Oxidative Decarboxylation and Titanium-Tetrachloride-Mediated Reaction with Nucleophiles Preparative electrolysis of N-methoxycarbonyl-O-[(t-butyl)dimethylsilyl]hydroxyproline 4 in MeOH leads to substitution of the COOH by a MeO group (oxidative decarboxylation). The mixture 5 of the two diastereoisomers (ca 1:1) thus obtained was reacted in CH₂Cl₂ with nucleophilic silylated compounds (such as allylsilane, silyl cyanide and 1-phenyl-1-silyloxyethane) or with trimethyl phosphite in the presence of TiCl₄ to give 2-allyl-, 2-cyano-, 2-(2-oxo-2-phenylethyl)- and 2-phosphono-substituted hydroxypyrrolidines, respectively, with high diastereoselectivities (≥ 90%, products 6-12 ). The configuration of two of the products ( 6/7 and 8/9 ) was shown to be cis.     

Elektrochemische Oxidation von (S)-Äpfelsäure-Derivaten: ein Weg zu enantiomerenreinen alkylierten Malonaldehydsäure-estern

Electrochemical Oxidation of (S)-Malic-Acid Derivatives: a Route to Enantiomerically Pure Alkylmalonaldehydic Esters The 3,3-dialkymalic-acid diesters, prepared by the previously described diastereoselective alkylations through dilithium alkoxide enolates, are saponified to the monoesters containing a free α-hydroxycarboxylic-acid moiety(Scheme 3). The monoesters are subjected to electrochemical oxidative decarboxylation in MeOH . If the intermediate monoacids are purified, the malonaldehydic esters (2-formy1-2-alkylcaroxylates) Obtained by this procedure are enantiomerically pure; they have the same structural features, i.e. two enantiotopic functionalized branches on the (persubstituted) stereogenic center, as the well known 3-hydroxy-2-methylpropanoic acid (‘Roche acid’) which was employed frequently as a starting material for the preparation of either enantiomer of various target molecules.     

The impact of pyrrolidine hydroxylation on the conformation of proline-containing peptides

[structure: see text] A series of eight dipeptides of the general formula Ac-Phe-Pro-NHMe was synthesized and the thermodynamics of the cis --> trans isomerization about the central amide bond were studied by NMR. Pro* represents the following prolines: l-proline (Pro), l-trans-4-hydroxyproline (Hyp), l-cis-4-hydroxyproline (hyp), l-cis-4-methoxyproline (hyp[OMe]), l-trans-3-hydroxyproline (3-Hyp), l-cis-3-hydroxyproline (3-hyp), l-2,3-trans-3,4-cis-3,4-dihydroxyproline (DHP), and l-2,3-cis-3,4-trans-3,4-dihydroxyproline (dhp). The conformation of the pyrrolidine ring in each case is discussed in light of previous structural studies, analysis of potential stereoelectronic effects, and NMR data. Hydroxy substituents at C-4 have a greater impact on cis --> trans isomerization than analogous substituents at C-3 as a result of the intervening bond distances and bridging groups. The position of the equilibrium and its dependence on temperature are a reflection of both enthalpic and entropic factors, the latter being complicated in this study by an Ar-Pro interaction in the cis conformation. The substituents on the pyrrolidine ring determine the conformation of the five-membered ring, which in turn influences the strength of the Ar-Pro interaction, backbone dihedral angles, and the relative energy of the cis and trans species. The ultimate position of the equilibrium depends on a complex blend of steric, electronic, and conformational factors.     

Studies on quinones. Part 37. Synthesis and biological activity of o-aminoester functionalised benzo- and naphtho[2,3-b]thiophenequinones

The synthesis of 3-amino-2-methoxycarbonyl-4,7-dimethoxybenzo[b]thiophene (5) and benzothieno[3,2-d][1,3]oxazin 15 from 3,6-dimethoxy-2-nitrobenzaldehyde (1) is reported. Benzo[b]thiophene-4,7-quinones 9 and 10 were prepared in good yields by oxidative deprotection of the corresponding dimethoxybenzothiophenes 8 and 7. Cycloaddition reaction of quinone 8 with 1-(E)-trimethylsilyloxy-1,3-butadiene followed by acid-induced aromatization provides access to naphtho[2,3-b]thiophene-4,9-quinone 13 and 14. The in vitro activity of the new quinones against Leishmania amazonensis and human-T-cell leukemia virus type 1 (HTLV-1) is reported.