Die Addition von Nucleophilen an Alkinderivate mit Push-pull-Gruppen

The preparative aspects of the addition of nucleophiles to alkyne-derivatives having both push- and pull-groups ( 1 ) are discussed: The addition of primary and secondary amines as well of aliphatic alcohols to the alkyne-derivatives 1 in most cases predominantly yields the Michael-adducts 2 – 10 . The importance of a side-reaction consisting in an addition of amines to C(2) of the alkyne-derivatives 1 increases in the series 1c ? 1b < 1a and is controlled by stereoelectronic effects. The spectroscopic properties of the compounds 2 – 10 are given.     

Der Mechanismus der Umlagerung substituierter Aminoacrylderivate

Thioacetic acid and dithioacetic acid react with alkynederivatives of the type (CH₃)₂N? C?C? CO? R ( 1 ) in the same way as other carboxylic acids: The addition to dimethylaminopropinal ( 1a ) at low temperatures yields, after rearrangement of the very instable primary adducts, Z-3-acetoxy-N,N-dimethyl-thioacrylamide ( Z-16 ) and Z-3-thioacetoxy-N,N-dimethylthioacrylamide ( Z-17 ) respectively. The structure of the two compounds can be proved by spectroscopic evidence of 16 and 18 , the latter being formed by elimination of thioketene from 17 . According to the distribution of S-atoms in 16 and 17 , two reaction pathways including 4-membered rings can be ruled out. Thus the rearrangement of 3-acyloxy-N,N-dimethyl-acrylamides most probably proceeds by a mechanism including a dipolar six-membered intermediate. This mechanism cannot be valid for the rearrangement of the adducts 2 of hydrohalogen acids, alcohols and amines to the alkyne-derivatives 1 . The acid-catalysed reaction of 3-chloro-3-dimethylamino-propenal ( 2 , X?Cl), labelled at position 1 with ¹³C, yields 3-chloro-N,N-dimethyl-acrylamide ( 3 , X?Cl), containing the label exclusively at position 3 . This result supports a mechanism including an immonium-oxetene 21 (X?Cl) as intermediate. - The experiments are in accord with kinetic investigations.     

Die Addition von Säuren an Alkinderivate mit Push-pull-Gruppen,

The addition of proton acids as HF, HCl, HBr, HOAc and phenol to alkyne-derivatives of the type (CH₃)₂N? C?C? CO? R( 1 ) yielding the adducts 2 to 6 is investigated. The stereochemical course of the reaction is mainly influenced by the structure of the alkyne 1 . Kinetic investigations show that the rate of the third-order-reaction increases from 1 a (R?H) to 1 b (R ? CH₃) and 1 c (R ? OCH₃) and decreases drastically in polar solvents. According to these results a reaction mechanism is outlined and discussed.     

Influence of structure-property relationship on the optical,thermal and mechanical properties of castor oil based transparent polyurethane for catheter applications

Castor oil based transparent polyurethane elastomers were synthesized, which can be used as an advanced catheter material. The effect of NCO: OH ratio on the structural, optical, thermal and physicomechanical properties of polyurethanes (PU) has been studied. The optical properties of the PU was analysed by studying its percentage transmittance and haze. The results showed a high transparency of 90.7% for the PU with a NCO: OH ratio of 0.9:1. Differential scanning calorimetry (DSC) analysis revealed an increase in the glass transition temperature (T_g) of PU with increasing hard segment content whereas thermogravimetric analysis (TGA) shows an increase in the initial decomposition temperature of PU from 262 to 268°C upon increasing the NCO: OH ratio from 0.9 to 1.5. A similar trend of increment in the tensile properties of PU has been observed as a consequence of increasing the molar ratio of NCO: OH. In vitro cytotoxicity analysis of PU was studied using human embryonic kidney (HEK293) cell line that revealed the nontoxic character of PU.     

S-shaped para-Quinodimethane-Embedded Double [6]Helicene and Its Charged Species Showing Open-Shell Diradical Character

Helicenes and extended helical π-conjugated compounds have been widely studied, but most of the systems contain only aromatic benzene or heterocyclic rings, showing local aromatic character. Herein, new S-shaped double [6]helicene 1 , which has two embedded para-quinodimethane (p-QDM) units, is reported. Due to the existence of a proaromatic quinoidal substructure, it has open-shell diradical character. Its model compound, C-shaped single [6]helicene 2 containing one p-QDM unit, was also synthesized and compared. Their ground-state structures and electronic properties were systematically studied by a combination of various experimental methods assisted by theoretical calculations. Compound 1 has a double-helical structure in the crystal, with the two terminal [6]helicene units bent in opposite directions (i.e., anti form). However, an anti/syn isomerization process with a moderate interconversion energy barrier was observed on the NMR timescale. Compound 1 shows amphoteric redox behavior. It also exhibits open-shell diradical character (y₀=12.1 %) and a small singlet–triplet gap. On the other hand, compound 2 has a typical closed-shell nature. The dication and dianion of 1 also show open-shell diradical character. The dianion of 2 and the tetraanion of 1 exhibit similar electronic structures to their respective isoelectronic structures, that is, [6]helicene and a double [6]helicene. This work provides some insights into the design and synthesis of stable helical π systems with open-shell diradical character and magnetic activity.     

[La(CCl3COO)3·dipy·H2O]2配合物的电子结构和化学键

作者曾系统研究[Ln(CCl₃COO)₃·dipy·H₂O]₂配合物的合成和性质,并测定了[La(CCl₃COO)₃·dipy·H₂O]₂的晶体结构(待发表)。     

Thermosensitive and redox-active polymers: Preparation and properties of poly(N-ethylacrylamide-co-vinylferrocene) and poly(N,N-diethylacrylamide-co-vinylferrocene)

Thermosensitive and redox-active polymers were prepared by copolymerization of N-ethyl- or N,N-diethylacrylamides with vinylferrocene (VFc). LCST (lower critical solution temperature) of the aqueous copolymer solution was decreased by increasing the ferrocene content in the copolymer. The oxidation of ferrocene led to a significant increase in LCST due to the transition from hydrophobic to hydrophilic character of the ferrocene moiety in the copolymer. The ferrocene content in the copolymer increases with increasing differences between the LCST's of the oxidation and reduction states. The transition could be made reversible by redox reaction using L -ascorbic acid as an oxidant and cerium sulfate as a reductant. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1967–1972, 1997     

Photochemically Mediated Ring Expansion of Indoles and Pyrroles with Chlorodiazirines: Synthetic Methodology and Thermal Hazard Assessment

We demonstrate that arylchlorodiazirines serve as photo-activated halocarbene precursors for the selective one-carbon ring expansion of N-substituted pyrroles and indoles to the corresponding pyridinium and quinolinium salts. Preliminary investigations indicate that the same strategy also enables the conversion of N-substituted pyrazoles to pyrimidinium salts. The N-substituent of the substrate plays an essential role in: (1) increasing substrate scope by preventing product degradation, (2) enhancing yields by suppressing co-product inhibition, and (3) activating the azinium products towards subsequent synthetic manipulations. This latter point is illustrated by subjecting the quinolinium salts to four complementary partial reductions, which provide concise access to ring-expanded products with different degrees of increased C(sp³) character. Thermal analysis of the diazirines by differential scanning calorimetry (DSC) provides detailed insight into their energetic properties, and highlights the safety benefits of photolyzing—rather than thermolyzing—these reagents.     

Polydispersions radiation envelope: Influence of particle size and assumed distribution function

Summary The influence of polydispersity, particle size and of assumed distribution on angular dependence of theMie intensity functions in systems with relative refractive indexm = 1.10 was studied theoretically. Two distribution functions were used; Stevenson-Heller-Wallach (SHWD) and logarithmic distribution of negative order (NOLD). It was found, that for highly polydisperse systems the functional dependences obtain also undulative character. By means of NOLD the angular shift of extremes on the curves studied with increasing polydispersity was not observed. The application of this distribution in comparison with SHWD shows more structured angular scattering patterns with increasing particle size and this fact can also be employed in modelling and in distribution analysis of dispersion with higher degree of polydispersity.With 4 figures     

Tetraepoxy[32]annulene(4.4.4.4) und `Tetraoxa[30]porphyrin(4.4.4.4)'‐Dikationen

Tetraepoxy[32]annulenes(4.4.4.4) and `Tetraoxa[30]porphyrin(4.4.4.4)' Dications Of the tetraepoxy[32]annulenes as well as the `tetraoxa[30]porphyrin' dications, hithertoo only the (8.0.8.0) and the (6.2.6.2) systems are known to exist in several geometric isomers and to possess antiaromatic and aromatic character, respectively. Here we describe the still missing symmetric member of the [32]annulenes, the tetraepoxy[32]annulene(4.4.4.4) 1 and the corresponding `tetraoxa[30]porphyrin(4.4.4.4)' dication 2 . The cyclizing Wittig reaction of the dialdehyde 3 with the bis‐phosphonium salt 7 at 70° yields the configurational isomers 1a (ZE,EE,EZ,EE), 1b (ZE,EE,EE,EE), and 1c (EZ,EE,EZ,EE). All isomers are antiaromatic; in 1a and 1c , the two (E,E)‐buta‐1,3‐diene‐1,4‐diyl bridges rotate around the adjacent σ‐bonds; the rigidity of 1b with 3 (E,E) bridges prevents any dynamic character. The Wittig reaction of 3 with 7 at 20° only yields the kinetically controlled annulene 1c , and at 120°, an excess of the thermodynamically most stable isomer 1a is formed. The structure of 1 is elucidated mainly by COSY and NOESY experiments, and the dynamic character of 1a and 1c is established by temperature‐dependent <sup>1</sup>H‐NMR spectroscopy. The oxidation of the isomer mixture 1a – c with 4,5‐dichloro‐3,6‐dioxocyclohexa‐1,4‐diene‐1,2‐dicarbonitrile (DDQ) gives two isomeric `tetraoxa[30]porphyrin(4.4.4.4)' dications 2′ and 2″ , which are frozen conformers with the same (EZ,EE,EZ,EE)‐configuration and geometrically related to 1c . Semiempirical calculations of 1 and 2 are in full agreement with the experimental results.     

Synthesis and Qualitative Olfactory Evaluation of Benzodioxepine Analogues

Marine fragrances, particularly Calone 1951^® (=7‐methyl‐2H‐1,5‐benzodioxepin‐3(4H)‐one; 1 ) has carved a minor but distinct niche in the broad field of fragrance chemistry. By focusing on the polar structure fragment of the benzodioxepinone parent compound, we set out to determine the molecular influence on the dominant marine note attributed to the Calone 1951^® structure. A selection of one‐step modifications of the ketone 1 resulted in a range of odor‐active conformers with diverse olfactory attributes. The synthesis of a range of benzodioxepine analogues, i.e., of 3 – 11 , is presented alongside olfactory evaluation (Tables 2 and 3). Removal of the carbonyl group of 1 and increasing the size of the aliphatic ring portion (see 6 and 7 ) introduced sweetness and a predominant loss of the marine character.     

Furan derivatives. Part 9. Synthesis and properties of cyclohepta[cd]benzofurans

Cyclohepta[cd]benzofurans 1a-c were synthesized by heating (5-oxo-5H-benzocyclohepten-4-yloxy)acetic acids 7a-c with sodium acetate in acetic anhydride or by irradiation of 7a-c in acetonitrile. Several reactions such as protonation, catalytic hydrogenation, Diels-Alder reaction, acylation, and photoreaction were examined for 1a-b . The results show that cyclohepta[cd]benzofurans have both properties of heptafulvene and benzofuran. The carbon-carbon double bond in the furan ring of 1a has aromatic character, however, the carbon-carbon double bonds in the seven-membered ring have olefinic character.     

The diradical character of polyacenequinododimethides

Abstract  

An electronic structure study of singlet and triplet states of two series of polyacenequinododimethides was performed using the B3LYP method. It was found that the ground state of all examined polyacenequinododimethides is a singlet with significant diradical character. The diradical character of the compounds under investigation was estimated using the unrestricted symmetry-broken and complete active space methods. It was shown that polyacene-2,3-quinododimethides have more pronounced diradical character than polyacene-2,x-quinododimethides. The diradical character of polyacene-2,x-quinododimethides monotonically increases with their increasing molecular size. Within the series of polyacene-2,3-quinododimethides the diradical character is not a monotonic function of the number of hexagons. It was found that pentacene-2,3-quinododimethide has the most pronounced diradical character in this series. It can be predicted on the basis of the singlet–triplet gap values that even higher polyacenequinododimethides will be singlet, but not triplet molecules.     

Transition from Charge‐Transfer to Largely Locally Excited Exciplexes,from Structureless to Vibrationally Structured Emissions

Exciplexes of 9,10‐dicyanoanthracene (DCA) with alkylbenzene donors in cyclohexane show structureless emission spectra, typical of exciplexes with predominantly charge‐transfer (CT) character, when the donor has a relatively low oxidation potential (E_ox), e.g. hexamethylbenzene (HMB). With increasing E_ox and stronger mixing with a locally excited (LE) state, vibrational structure begins to appear with 1,2,3,5‐tetramethylbenzene and becomes prominent with p‐xylene (p‐Xy). A simple theoretical model reproduces the spectra and the radiative rate constants, and it reveals several surprises: Even in this nonpolar solvent, the fractional CT character of a highly mixed exciplex varies widely in response to fluctuations in the microscopic environment. Environments that favor the LE (or CT) state contribute more to the blue (or red) side of the overall spectrum. It is known that sparsely substituted benzene radical cations, e.g., p‐Xy^?+, are stabilized more in acetonitrile than the heavily substituted HMB^?+. Remarkably, ion pairing with DCA^?– in cyclohexane leads to even larger differences in the stabilization of these radical cations. The spectra of the low‐E_ox donors are almost identical except for displacements that approximately equal the differences in E_ox, even though the exciplexes have varying degrees of CT character. These similarities result from compensation among several nonobvious, but quantified factors.     

Can One Assess the π Character of a C–C Bond with the Help of the NMR Spin–Spin Coupling Constants?

Measured one‐bond spin–spin coupling constants (SSCC) ¹J(CC) can be used to describe the nature of the C–C bond, provided one is able to separate the various coupling mechanisms leading to ¹J(CC). The Fermi‐contact (FC) term probes the first‐order density at the positions of the coupling nuclei, whereas the noncontact terms (the paramagnetic spin orbit (PSO) and the spin–dipole (SD) terms) probe the π character of the C–C bond (the diamagnetic spin orbit (DSO) term can mostly be neglected). A model is tested, in which the value of the FC(CC) term is estimated with the help of measured SSCCs ¹J(CH). The difference between the measured J(CC) and the estimated FC(CC) values, Δ(CC)=PSO(CC)+SD(CC)+DSO(CC), provides a semiquantitative measure of the π character of a C–C multiple bond. The applicability and limitations of this approach are discussed by partitioning the four Ramsey terms of the SSCC ¹J(CC) into one‐ and two‐orbital contributions. The FC, PSO, and SD terms of ¹J(CC) are explained and analyzed with regard to their relationship to other C–C bond properties. It is shown that empirical relationships between measured SSCCs and the s character of a bond need reconsideration.     

Investigation on Electronic Property of Passive Film on Nickel in Bicarbonate/Carbonate Buffer Solution

The electronic properties of passive film formed on nickel in bicarbonate/carbonate buffer solution were studied by electrochemical impedance spectra (EIS) and Mott‐Schottky plot. The film composition was analyzed by X‐ray photoelectron spectroscopy (XPS). The results showed that passive film exhibited p‐type semi‐conductive character, and the acceptor density (N_A) decreased with increasing potential, prolonging time, decreasing temperature, increasing pH value and decreasing chloride/sulfur ions concentration. The transfer resistance and film resistance increased with the above factors changing. XPS results showed that passive film was composed of NiO and a little amount of Ni₂O₃.     

A Theoretical Study of NBO,NICS, and 14N NQR Parameters of Adenine Tautomers in the Gas Phase via DFT

The natural bond orbital (NBO) analysis, nucleus independent chemical shift (NICS), and ¹⁴N NQR parameters of the most stable tautomers of adenine in the gas phase were predicted using density functional theory method. The NBO analysis revealed that the resonance interaction between lone pair of the nitrogen atom and empty non‐Lewis NBO increases with increasing the p character of the nitrogen lone pair. The present investigation indicated the π clouds in both the considered heterocyclic rings containing six electrons, and these tautomers has the aromatic character. The NICS study utilizing the gauge‐invariant atomic orbital method showed that there are diatropic currents in the heterocyclic rings of the tautomers, so we determined the order of overall aromaticity of these tautomers. The results of NQR parameter calculations showed three parameters are effective on nuclear quadrupole coupling constant; the p character value of lone pair electrons of nitrogens, and the related occupancies and whenever, the lone pair electrons of nitrogens participate in the formation of chemical bond and/or π system of the ring, the q_zz and consequently its χ decreases.     

Stable Expanded Porphycene‐Based Diradicaloid and Tetraradicaloid

The synthesis of a bithiophene‐bridged 34π conjugated aromatic expanded porphycene 1 and a cyclopentabithiophene bridged 32π conjugated anti‐aromatic expanded porphycene 2 by a McMurry coupling strategy is presented. Magnetic measurements and theoretical calculations reveal that both 1 and 2 exhibit an open‐shell singlet ground state with significant radical character (y₀=0.63 for 1 ; y₀=0.68, y₁=0.18 for 2 ; y₀: diradical character, y₁: tetraradical character) and a small singlet–triplet energy gap (ΔE_S‐T=?3.25 kcal mol^?1 for 1 and ΔE_S‐T=?0.92 kcal mol^?1 for 2 ). Despite the open‐shell radical character, both compounds display exceptional stability under ambient air and light conditions owing to effective delocalization of unpaired electrons in the extended cyclic π‐conjugation pathway.     

Triplet excited states of nitronaphthalenes. IV. 1,2- and 1,8-dinitronaphthalenes

Nanosecond flash photolysis of 1,2- and 1,8-dinitronaphthalenes (1,2-DNO₂N; 1,8-DNO₂N) in nonpolar and polar solvents shows transient species with absorption maxima and lifetimes dependent on solvent polarity. In deaerated n-hexane the absorption maxima and lifetimes (1/K) are 490 nm and 1.0 μsec for 1,2-DNO₂N and 550 nm and 2.5 μsec for 1,8-DNO₂N. In deaerated ethanol the corresponding values are 550 nm and 4.3 μsec for 1,2-DNO₂N and 590 nm and 5.3 μsec for 1,8-DNO₂N. The transient absorptions are attributed to the lowest triplet excited states T₁ of the 1,2- and 1,8-DNO₂N. The observed red shifts in the absorption maxima of the T₁ states are indicative of the extent to which electronic charge is transferred intramolecularly during the T₁ → T_n transitions. Furthermore, the increased lifetime of the T₁ states with increasing solvent polarity indicates the intramolecular charge transfer character of the T₁ states. Changes of dipole moments accompanying the T₁ → T_n transitions as well as rate constants for electron or proton transfer and hydrogen abstraction reactions involving the T₁ states of 1,2- and 1,8-DNO₂N and tributyl tin hydride (Bu₃SnH) as the hydrogen donor were determined together with the activation energy of the hydrogen abstraction reaction for the case of 1,2-DNO₂N. The spectroscopic and kinetic data obtained in this work demonstrate that the triplet states of 1,2- and 1,8-DNO₂N behave like n → π* states in nonpolar media while in polar solvents the n → π* character of these states is reduced with a simultaneous increase in their intramolecular charge transfer character.     

Kinetics and mechanism of propene hydroformylation catalyzed by rhodium complexes with a diphosphite ligand

The kinetics of propene hydroformylation in the presence of the catalytic system Rh(acac)(CO)₂/nL (L = 2,2′-bis[(1,1′-diphenyl-2,2′-diyl)phosphito]-3,3′,5,5′-tetra-tert-butyl-1,1′-diphenyl, 0.5 < n < 20) in para-xylene at 90°C is reported. At n ≥ 2, the rate and regioselectivity of the process are independent of the L concentration. The reaction is of positive fractional order with respect to propene and hydrogen and of negative order with respect to CO. The molar ratio between the linear product and the branched product decreases with an increasing CO pressure and increases with an increasing H₂ pressure. The kinetic data are consistent with a process mechanism involving irreversible propene addition to the unsaturated hydride complex HRh(CO)L with the formation of the π-complex HRh(CO)L(C₃H₆). The insertion of coordinated propene into the H-Rh bond of this complex is reversible in the linear aldehyde formation route and is quasi-equilibrium in the branched isomer formation route. The conclusions as to the character of these reaction steps are corroborated by the compositions of the but-1-ene and but-2-ene hydro-formylation products.