bluuebunny/crossref_metadata_2025_split huggingface dataset:
Molecular‐mechanics and ‐dynamics simulations of trivalent europium complexes of calix[4]arene derivatives and a bislariat diazacrown ether
Authors: van Veggel Frank C. J. M. , Reinhoudt David N. 
Published: September 2010

AbstractMolecular‐mechanics (MM) and molecular‐dynamics (MD) simulations (in MeOH) were performed on Eu3+ complexes of two derivatized calix[4]arenes (2b and 2d) and a bislariat diazacrown ether (3). The Quanta/CHARMm Lennard‐Jones parameters of Ca2+ proved suitable as a model for the Lennard‐Jones parameters of Eu3+. The radial distribution function (RDF) of Eu3+ … MeOH (O) agrees well with experimental data. The MD of 2b · Eu3+ showed that the phenolic oxygens cannot compete with the solvent, although they are preorganized. Three molecules of MeOH are within the first coordination shell. The MD of 2d · Eu3+ showed two transitions. After the first the Eu3+ was coordinated by one molecule of MeOH and after the second by three molecules of MeOH. The pyridine ring was not involved in coordination to Eu3+, which explained the measured lifetimes and derived coordination shells in methanol. The calculations on 3 · Eu3+ revealed that the conformation with the pendant arms in an anti configuration is unlikely to exist in solution, whereas the syn form calculated by MM rearranged to a structure with two molecules of MeOH in the first coordination sphere. This is consistent with experimental data, which show that the two dominant species in methanol solution have two molecules of MeOH coordinated to the Eu3+. All calculations were performed with the Quanta/CHARMm package.
Synthesis of nitroxide containing polyenes: Two chemically modified retinals and their interaction with bacterioopsin
Authors: Groesbeek M. , Lugtenburg J. 
Published: September 2010

AbstractThe synthesis and spectroscopic characterization of two retinal analogues is described, the paramagnetic 3‐pyrrolin‐1‐yloxy analogue 1 and its diamagnetic equivalent, the 3‐pyrroline analogue 2. Various aspects of the synthesis of the aminoxy group containing polyenes are discussed. Upon interaction with bacterioopsin, both 1 and 2 are incorporated in the protein and form a system with λmax 459 nm. Neither of the two bacteriorhodopsin analogues is photoactive. ESR spectroscopy data of the system containing 1 show that the ring part of the chromophore in the protein is rigidly fixed in orientation.
Reactivity and regioselectivity in the reaction of allylsilanes and some analogous all‐carbon alkenes with sulfur trioxide
Authors: Cerfontain Hans , Kramer Johannes B. , Schonk Ruud M. , Bakker Bert H. 
Published: September 2010

AbstractSulfonations of the 1‐(trimethylsilyl)alk‐2‐enes 1a‐12a, the 1‐(trimethylsilyl)‐ω‐phenyl‐alk‐2‐enes 13a–18a, the triisopropylsilylalk‐2‐enes 19a–21a, 1‐(trimethylsilyl)penta‐2,4‐diene (22a), and the allylsilane 23a with sulfur trioxide were studied in the temperature range ‐60 to 25°C using dichloromethane‐d2 as solvent and 1.5 mol‐equiv. of dioxane‐d8 relative to the amount of SO3 as reactivity moderator. For comparison, the sulfonation of the all carbon analogues 24a–27a were also studied. Reaction of the 1‐(trialkylsilyl)alk‐2‐enes 1a–11a and 13a–17a with SO3 at −60°C all afford the corresponding trialkylsilyl‐alk‐1‐ene‐3‐sulfonate esters c within 20 min in yields of > 95%. 1‐(Trimethylsilyl)‐2‐cycloheptylideneethane (12a) at −60°C gives a mixture of the trimethylsilyl sulfonate ester 12c and the isomeric 1‐(trimethylsilyl)‐2‐(1‐cycloheptenyl)ethane‐2‐sulfonic acid (12f) in a molar ratio of 4:6. With 1‐(trimethylsilyl)‐2‐(1,2,3,4‐tetrahydro‐1‐naphthylidene)‐ ethane (18a) the exclusive product is 1‐(trimethylsilyl)‐2‐(3,4‐dihydro‐1‐naphthyl)ethane‐2‐sulfonic acid (18f). Direct NMR evidence for the formation of β‐sultones as initial intermediates was observed in the sulfonation of the 1‐(trimethylsilyl)‐2‐cycloalkylideneethanes 11a and 12a. Upon sulfonation of the 1‐(triisopropylsilyl)‐alk‐2‐enes 19a–21a at — 60°C the initial products are the β‐sultone 20b‐II and the carbyl sulfates 19h‐II and 21h–II, of which the relative yields depend on the amount of SO3 employed. The SO3 sulfonation of 1‐(trimethylsilyl)penta‐2,4‐diene (22a) gives trimethylsilyl penta‐1,3‐diene‐5‐sulfonate (22i). The three homologous non‐silicon containing 1‐ cycloalkylidenehexanes 24a–26a at — 60°C afford the respective 1‐cycloalkylhexane‐1,1′‐sultones 24b‐III–26b‐III which, at higher temperatures, isomerize to give the 1‐(cycloalk‐1′‐enyl)hexane‐1‐ sulfonic acids 24m–26m. 1‐Methylene‐1,2,3,4‐tetrahydronaphthalene (27a) reacts with SO3 at −60°C to give (3,4‐dihydro‐1‐naphthyl)methanesulfonic acid (27o). Treatment of the various products containing a trialkylsilyl sulfonate ester substituent with aqueous KOH gives quantitatively the corresponding potassium sulfonate salts.Mechanisms for the formation of the various products are suggested and the observed selectivities are discussed.
The lowest triplet state of tetradecahexaenal
Authors: Kok Peter , Groenen Edgar J. J. 
Published: September 2010

AbstractThe lowest triplet state, T0, of all‐trans‐tetradecahexaenal has been investigated by means of electron‐spin‐echo spectroscopy in a magnetic field and in zero field. The directions of the principal axes and the eigenvalues of the fine‐structure tensor have been determined. The triplet state corresponds to a π π* excitation. In zero field, T0 is populated predominantly through the upper spin sublevel which is found to decay fastest. Comparison of the relative energies and the decay rates of the sublevels of T0 with those obtained previously for shorter polyenals reveals that both the zero‐field splitting and the logarithm of the decay rates are inversely proportional to the total number of double bonds.
Excited‐state intramolecular charge transfer in donor/acceptor‐substituted aromatic hydrocarbons and in biaryls. The significance of the redox potentials of the D/A subsystems
Authors: von Der Haar Thomas , Hebecker Axel , Il'Ichev Yurii , Jiang Yun‐Bao , Kühnle Wolfgang , Zachariasse Klaas A. 
Published: September 2010

AbstractThe energy of the charge transfer (CT) emission maximum of a series of dual fluorescent 4‐aminobenzonitriles in diethyl ether and acetonitrile does not show a correlation with the redox potentials of the amino (D) and benzonitrile (A) subgroups. It is therefore concluded that these electron donor and acceptor subgroups cannot be treated in the same way as the A and D molecules in exciplexes1(A−D+). This means that in the intramolecular charge transfer (ICT) state of the aminobenzonitriles, a considerable electronic coupling between the A−and D+parts exists, in contradiction with the twisted intramolecular charge transfer (TICT) hypothesis. For 9,9′‐bianthryl, 10‐cyano‐9,9′‐bianthryl and 10,10′‐dicyano‐9,9′‐bianthryl, photostationary and time‐resolved measurements are presented, which show that the influence of the cyano substituents on the ICT reaction in the excited state can be understood by considering the redox potentials of the anthryl groups. The introduction of an additional cyano group in an aminobenzonitrile, however, leads to the disappearance of ICT and dual fluorescence, indicating that the photophysics of these molecules cannot be understood on the basis of the redox potentials of the D and A constituents.
Selectivity in the direction of photoisomerization reactions in liquid‐crystalline guest‐host systems
Authors: Meier Herbert , Lifka Thorsten , Müller Klaus 
Published: September 2010

AbstractE ⇄ Z photoisomerization reactions of stilbenes la‐c were studied in liquid‐crystalline (LC) phases and in the corresponding isotropic melts. Whereas viscosity has a moderate influence on the photostationary states, a strong effect caused by the regular incorporation of the stilbene molecules in the LC phases was observed. The direction of the isomerization can be reversed, up to the case of a one‐way isomerization Z → E for the tailored guest‐host system 1b/BS. Such an LC‐matrix effect is of typical highly ordered (smectic) phases with free volumina which are strictly limited by hydrophobic interactions; it could not be detected in a much more mobile nematic phase. Textures based on the birefringence of doped LC phases, differential‐scanning‐calorimetry (DSC) measurements and phase‐dependent absorption spectra were used for the interpretation.
Stereochemical features of meta photocycloadducts derived from 3‐alkyl‐4‐phenoxybut‐1‐enes
Authors: van der Eycken Erik , de Keukeleire Denis , de Bruyn André , van der Eycken Johan , Gilbert Andrew 
Published: September 2010

Abstract3‐Alkyl‐4‐phenoxybut‐1‐enes (alkyl: methyl, isopropyl, tert‐butyl) on excitation at 254 nm in cyclohexane give rise to 4‐alkyl‐2‐oxatetracyclo[5.4.0.01,8.05,11]undec‐ 9‐enes via intramolecular meta photocycloaddition. Prevailing stereochemical features are the configuration of the alkyl group at C‐4 and the conformation of the tetrahydropyran ring contained in the tetracyclic system. These are revealed by detailed 1H‐NMR analysis and interpretation of the coupling constants. Whereas the alkyl group at C‐4 invariably occurs in the exo configuration, the conformation of the tetrahydropyran ring depends on the bulkiness of the alkyl substituent. Thus, the C‐4 methyl group occupies an equatorial position in a chair conformation, while a flattened chair conformation accomodates appropriately the C‐4 isopropyl group in the pertaining structure. The required equatorial position of the tert‐butyl group at C‐4, on the other hand, forces the tetrahydropyran ring into a boat conformation, thereby alleviating steric constraints.
Photoaddition reactions of 1,3‐dioxin‐4‐ones
Authors: Graalfs Heiner , Mattay Jochen 
Published: September 2010

AbstractIrradiation of 1,3‐dioxin‐4‐ones having l‐menthone as a chiral auxiliary in the 2‐position in 1,3‐dioxolane and oxolane, respectively, affords adducts with good facial selectivities. The benzophenone‐initiated photoaddition of isopropanol is also investigated.
Photochemical rearrangement of 2‐(N‐allyl‐N‐alkylamino)cyclohex‐2‐enones
Authors: Meyer Catherine , Pete Jean‐Pierre , Piva Olivier 
Published: September 2010

AbstractIrradiation at 366 nm of 2‐(N‐allyl‐N‐alkylamino)cyclohex‐2‐enones led, surprisingly, to very unstable 1‐azaspiro[2.5]octan‐4‐ones rather than to the expected [2 + 2] intramolecular photo‐cycloaddition. The corresponding alcohols could be isolated in good yield by reduction of the crude reaction mixture. When the reaction was carried out in a mixture of acetone/D2O (9/1), incorporation of one deuterium atom was observed. The proposed mechanism involves an intramolecular γ‐H abstraction in the primary step, followed by the molecular rearrangement of a 7‐azabicyclo[4.2.0]octan‐1‐ol intermediate.
Kinetic salt effects in the quenching of the excited state of the tris(2,2′‐bipyridine)ruthenium(II) complex by 2‐methylanthraquinone
Authors: Frank Rudolf , Rau Hermann 
Published: September 2010

AbstractIn the title system, quenching rate constants kq were determined as a function of c(Et4NClO4), c(NaClO4), and c(LiClO4) in acetonitrile solution. A general and a specific salt effect on kq can be distinguished. The 2‐methylanthraquinone radical anion produced by the photoinduced electron transfer forms strong associates with Na+ and even stronger ones with Li+, the formation constants of these complexes being determined from electrochemical measurements. Transient absorption spectra of the methylanthraquinone radical anion and of its associates with Na+ and Li+ are presented. The rate constant of the electron transfer step could be determined in selected systems and effΔG° values of the electron transfer step were developed. Apparent activation parameters ΔH≠, ΔS≠, and ΔG≠ were determined from the kq data. The negative values of ΔH≠ can be rationalized by the mechanism of the photoinduced electron transfer reaction. The back electron transfer in the successor complex to the ground state molecules shows Marcus‐inverted behaviour.
Role of an electron‐transfer chain reaction in the unusual photochemical formation of five‐coordinated anions [Mn(CO)3(α‐diimine)]− from fac‐[Mn(X)(CO)3(α‐diimine)] (X = halide) at low temperatures
Authors: Hartl František , Rossenaar Brenda D. , Stor Gerard J. , Stufkens Derk J. 
Published: September 2010

AbstractThe 16e− radicals [Mn(CO)3(α‐diimine)] are the key transients in the complex mechanism of formation of five‐coordinated anions [Mn(CO)3(α‐diimine)]− in 2‐MeTHF at 135K by visible excitation of the complexes fac‐[Mn(X)(CO)3(α‐diimine)] [X = halide; α‐diimine = 2,2′‐ bipyridine (bpy), pyridine‐2‐carbaldehyde N‐isopropylimine (iPr‐PyCa), and pyridine‐2‐carbaldehyde N‐p‐tolylimine (pTol‐PyCa)] into their low‐energy MLCT/XLCT transitions. This article describes the as yet unresolved electron‐transfer step in the mechanism which converts the radicals [Mn(CO)3(α‐diimine)] into the corresponding anions. Cyclic voltammetry and IR spectroelectro‐chemistry in optically transparent thin‐layer electrochemical cells were employed at variable temperatures in order to study the temperature‐dependent stability of the radicals [Mn(CO)3(α‐diimine)], their redox properties and their interaction with a coordinating solvent (Sv) to give the 18e− adducts fac‐[Mn(Sv)(CO)3(α‐diimine)]. The latter radicals are strong reductors capable of electron transfer to their five‐coordinated precursors which are quite stable at temperatures below 220K. This behaviour was demonstrated in the complex fac‐[Mn(Cl)(CO)3(bpy)] and the related complexes fac‐[Mn(Br)(CO)3(iPr‐DAB)] and fac‐[Mn(PrCN)(CO)3(iPr‐DAB)]+(PrCN = n‐ butyronitrile; iPr‐DAB = N, N′‐diisopropyl‐1,4‐diaza‐1,3‐butadieneChem. Abstracts name: N, N′‐diisopropyl‐ethane‐1,2 diimine (iPr‐ED)
which were used as reference. The (spectro)electrochemical results have revealed that the formation of [Mn(CO)3(α‐diimine)]− in the course of the above photoreaction is most probably accompanied by parallel generation of the cations fac‐[Mn(Sv)(CO)3(α‐diimine)]+. The cations in turn react back with the free halide to give the starting complex fac‐[Mn(X)(CO)3(α‐diimine)], thereby closing the catalytic cycle. The overall mechanism of the formation of [Mn(CO)3(α‐diimine)]− from the neutral Mn(halide) complexes can therefore be viewed as a photo‐assisted/ETC reaction. No anions [Mn(CO)3(α‐diimine)]− are formed in the presence of excess PR3 which is reasonably ascribed to a complete conversion of the 16e− transients [Mn(CO)3(α‐diimine)] into the stable 18e− adducts fac‐[Mn(PR3)(CO)3(α‐diimine)], the final photoproducts observed in this case.
Photolytic behaviour of tricarbonyl[(1,2,5,6‐η)‐cycloocta‐1,5‐diene]iron in solution and low‐temperature matrices
Authors: Grevels Friedrich‐Wilhelm , Klotzbücher Werner E. , Russell Graham , Schaffner Kurt 
Published: September 2010

AbstractThe photo‐induced isomerization of Fe(CO)3[(1,2,5,6‐η)‐1,5‐COD] (1) yields Fe(CO)3[(1,2,3,4‐η)‐1,3‐COD] (2) with a quantum yield of Φ 0.04 ± 0.005 (λexc 405 nm). Photolysis of 1 in low‐temperature matrices generates the fragment Fe(CO)2[(1,2,5,6‐η)‐1,5‐COD] (3) as the only primary photoproduct observed by conventional IR spectroscopy. The species 3 is also produced upon laser‐flash photolysis (λexc 308 nm) of 1 in solution at ambient temperature, as detected by time‐resolved IR spectroscopy. Accordingly, 12CO/ 13CO exchange occurs upon photolysis of 1 in the presence of 13CO. However, the isomerized complex 2 is initially formed without 13CO incorporation, thus discounting the possible involvement of the dicarbonyl fragment 3 in this process. It is concluded that photolytic generation of the de‐chelated species Fe(CO)3[(1,2‐η)‐1,5‐COD)] (4) is the initial step in the photo‐induced isomerization of 1 to 2.
The dipolar 1,3 cycloaddition of C,N‐dialkylnitrone to olefins with hydroxyl group and the derivatives
Authors: Naito Takeaki , Shirakawa Mitsuko , Ikai Miho , Ninomiya Ichiya , Kiguchi Toshiko 
Published: September 2010

AbstractThe reaction rate and stereoselectivity of intermolecular dipolar 1,3 cycloaddition of the C,N‐dialkylnitrone to olefins with hydroxyl groups and their derivatives were found to depend upon the type of hydroxyl group. The rate is accelerated in the order: magnesium alkoxide (OMgBr); free hydroxyl group (OH); ether (OR). Selectivities are improved moderately when magnesium alkoxide is used.
Synthesis and Diels‐Alder reaction of 5β‐methyl‐β‐dihydrothebaine and its 6‐demethoxy analogue (chemistry of opium alkaloids, Part XLII)
Authors: de Grauw M. , Muller E. F. , Woudenberg R. H. , Maat L. 
Published: September 2010

AbstractThe scission of the 4,5α‐epoxy bridge of 5β‐methylthebaine (2) and 5β‐methyl‐6‐demethoxythebaine (5), yielded 5β‐methyl‐β‐dihydrothebaine (4) and 5β‐methyl‐6‐demethoxy‐β‐dihydrothebaine (6), respectively. The scission was carried out by zinc reduction in a potassium hydroxide solution. It took place with retention of the 6,8‐diene system and the configuration at the C‐5 atom. To perform the Diels‐Alder reaction on 4, the hydroxyl substituent at C‐4 had to be protected with a phenyl group, yielding 5β‐methyl‐4‐O‐phenyl‐β‐dihydrothebaine (7). The Diels‐Alder reaction of 7 was carried out in boiling ethyl propenoate for three weeks. Two 6β,14β‐ethenomorphinans (8 and 9) were obtained. For the Diels‐Alder reaction of 6 no protection of the hydroxyl substituent at C‐4 was necessary. Cycloaddition gave after one day two 6β,14β‐ethenomorphinans (10 and 11).
Synthesis and characterization of a novel macrocyclic ligand containing nine donor atoms
Authors: Giovenzana G. B. , Jommi G. , Pagliarin R. , Sisti M. , Aime S. , Botta M. , Crich S. Geninatti 
Published: September 2010

AbstractThe synthesis of a novel macrocyclic ligand containing nine potential coordination sites (six nitrogens and three oxygens) formed by a triazacyclododecane ring trisubstituted with CH2CH2N(CH2Ph)CH2COOH groups is reported. The protonation scheme of this ligand has been elucidated by measuring the proton NMR shifts of the various methylenic resonances upon changing the pH of the solution. This ligand affords stable metal complexes with Cd2+ ion, whose 1H‐ and 13C‐NMR spectra suggest a remarkable kinetic inertia and stereochemical rigidity.
Reaction of D‐fructose, D‐glucose and inulin with alcohols in the presence of iodine; a novel glycosidation procedure
Authors: Verhart Cor G. J. , Fransen Carel T. M. , Zwanenburg Binne , Chittenden Gordon J. F. 
Published: September 2010

AbstractAn efficient procedure for the glycosidation of D‐fructose and D‐glucose catalyzed by iodine is described. The reaction yields mainly alkyl glycofuranosides. Treatment of inulin under similar conditions leads to inter‐glycosidic bond cleavage and to formation of alkyl D‐fructofuranosides. The reaction conditions are particularly mild and relatively selective.
Asymmetric induction in kinetically controlled zirconium‐catalysed Meerwein‐Ponndorf‐Verley reductions
Authors: Krohn Karsten , Knauer Birgit 
Published: September 2010

AbstractTwelve chiral ligands 718 were tested in the new kinetically controlled variation of zirconium alkoxide catalysed Meerwein‐Ponndorf‐Verley reduction of 2′‐chloroacetophenone (2) using rac‐4 as the reducing alcohol (Scheme 3). The tetraaryl‐1,3‐dioxolanes 11 (ca. 30% enantiomeric excess, ee) and the triphenylethanediol 14 (62% ee, 99% yield) gave the best results. A relatively low dependence on the amount of the catalyst (aluminum alkoxide gave no asymmetric induction) and the ligand excess was observed compared with a very high dependence on incubation time of Zr(OtBu)4 with the chiral ligand 14 and also on the reaction temperature.
Synthesis and properties of a series of strained [2.2]paracyclophane‐4,15‐disulfonimides
Authors: van Lindert Helen C. A. , van Doorn Johannes A. , Bakker Bert H. , Cerfontain Hans 
Published: September 2010

AbstractThe series of N‐alkyl‐, N‐cycloalkyl‐, N‐allyl‐, N‐benzyl‐, and N‐phenyl‐[2.2]paracyclophane‐4,15‐disulfonimides 4b − 4m was synthesized, starting from [2.2]paracyclophane ([2.2]PCP) (1). The initial step is sulfonation of 1 with chlorosulfuric acid to give [2.2]PCP‐4,15‐disulfonic anhydride (2) in 97% yield. Subsequent reaction of 2 with the appropiate primary amine quantitatively affords the corresponding alkylammonium 4‐(alkylsulfamoyl)[2.2]PCP‐15‐sulfonate salt (3) which, upon treatment with phosphoric anhydride in dichloromethane, gives the corresponding N‐alkyl[2.2]PCP‐4,15‐disulfonimide 4 in yields up to 56%. The parent [2.2]PCP‐4,15‐disulfonimide (4a) was obtained by pyrolysis of N‐isopropyl‐ (4d), N‐cyclopentyl‐ (4h), and N‐cyclohexyl[2.2]PCP‐4,15‐disulfonimide (4i) at 200°C for 20 min in almost quantitative yields with simultaneous expulsion of the corresponding alkene.As determined by X‐ray analysis, the distances between the two p‐phenylene rings are the same for the 4,15‐disulfonimide 4a and the corresponding 4,15‐disulfonic anhydride 2, but they are both significantly different from those of the N‐isopropyl derivative 4d. The sum of the three bond angles around N is ≈︁ 5° smaller for 4d than 4a as result of non‐bonded interaction between the methyls of the isopropyl and the neighbouring sulfonyl oxygens. Evidence for non‐bonded interaction was also obtained from dynamic 1H‐NMR studies on the N‐cyclohexyl and N‐phenyl derivatives 4i and 4l, the barrier of rotation around the C‐N bond of 4l at 353K, i.e. the temperature of coalescence, being 71 kJ.mol−1.Pyrolysis of crystals of N‐isopropyl‐, N‐cyclopentyl‐ and N‐cyclohexyl‐[2.2]PCP‐4,15‐disulfonimide (4d, 4h and 4i) in an argon or nitrogen atmosphere at 200‐210°C for 20 min leads to the formation of [2.2]PCP‐4,15‐disulfonimide (4a) in yields of > 95% under the simultaneous expulsion of the corresponding alkene. Under the same conditions the corresponding N‐propyl, N‐cyclopropyl and N‐allyl derivatives 4c, 4e and 4f were found to be stable.The homogeneous thermolysis of the N‐alkyl[2.2]PCP‐4,15‐disulfonimides 4c − 4i in DMSO as solvent, leading to the formation of [2.2]PCP‐4,15‐disulfonimide (4a), was studied quantitatively and appeared to exhibit first order kinetics. At 105‐150°C, the first order rate coefficients decrease in the order N‐cyclopentyl (4h) > N‐cyclohexyl (4i) > N‐isopropyl (4d) >> N‐allyl (4f) >> N‐propyl (4c); the N‐cyclopropyl derivative 4e does not yield any disulfonimide 4a. The mechanism of the thermolysis of the N‐alkyl[2.2]PCP‐4,15‐disulfonimides is discussed, and the differences in rate are explained in terms of (i) differences in the C(α)‐H bond strengths of the N‐alkyl moiety and (ii) relief of steric strain for the intramolecularly overcrowded disulfonimides 4d, 4h and 4i.