Photolyse von 2-oxo-[2-13c]-1-diazocyclohexan. Ein beitrag zum oxiren-problem

The synthesis of 2 - oxo - [2 - ¹³C] -1 - diazocyclohexan (10) starting with K¹³CN is described. Photolysis of 10 in dioxan-water yields cyclopentancarboxylic acid containing all the label in the carboxy group, which has been proved by C-13 NMR spectroscopy and electron impact induced fragmentation. The absence of isotope scrambling in the photolytic ring contraction excludes an oxiren participation.     

Absence of triazirine intermediates in the photolytic formation of azides from mesoionic 3-substituted 1,2,3,4-oxatriazolylio-5-oxides : Isotope labelling experiments

Irradiation of 3-phenyl-1,2,3,4-oxatriazolylio-5-oxide (1) leads to formation of CO₂, N₂O, phenyl azide and phenyl isocyanate. The two latter compounds are observed only in low yields because of secondary photolytic reactions. Photolysis in CCl₄ or Cl₂CCCl₂ of 2-¹⁵N labelled (1) leads almost exclusively to the formation of 3-¹⁵H labelled phenyl azide identified by IR spectroscopy on comparison with authentic 1-¹⁵N, 2-¹⁵N and 3-¹⁵N labelled phenyl azides, respectively. These results show that phenyl azide is formed photolytically from (1) via phenyl migration and not via “antiaromatic”, phenyl triazirine (2).     

Photochemical Reactions of Alkyl Phenylglyoxylates(1)

Three new photoproducts, ethyl O-benzoyl mandelate (5a), ethyl O-acetylmandelate (6a), and biphenyl triketone (7a) are isolated and identified in the reactions of ethyl phenylglyoxylate (1a) in benzene. Quantum yields and initial rate constants of product formation are shown to be concentration dependent. For the formation of carbonyl product 3 at lower starting material concentrations (<0.01 M), quantum yields greater than 1 are observed. Variations in the quantum yields as a function of reaction time are due to the accumulation of alpha-hydroxyphenyl ketene (D). The relative reactivities of triplet excited states of phenylglyoxylates 1 and phenyl ketones are compared. A mechanism involving both intramolecular gamma-H abstraction and intermolecular H abstraction, which leads to radical chain reaction, is proposed. Rate constants for intramolecular gamma-H abstraction (k(N)) and intermolecular H abstraction (k(I)) of methyl phenylglyoxylate (1d) are measured.     

C-nitroso compounds—XXIV: Photolysis of some geminal nitro-nitrosoalkanes (“pseudonitroles”)

Photolysis of the “pseudonitrole” 1-nitro-1-nitrosocyclohexane with red light in the absence of oxygen occurs by homolytic C-NO fission, and generates a radical pair: NO and the α-nitro-cyclohexyl radical. If one radical of this pair adds to the NO-group of unchanged starting material, there results a paramagnetic nitroxide, that can combine with the other radical of the pair to a diamagnetic intermediate, forming two isomeric intermediate adducts to the NO double bond. The ratio in which these isomers are formed depends on solvent. In methanol, solvolysis leads to equimolar amounts of cyclohexanone and cyclohexanone-oxime, together with methyl nitrite. In benzene, the major product is cyclohexanone. Small amounts are formed of 1,1-dinitrocyclohexane, 1-nitrocyclohexene and nitrocyclohexane; arising directly from the α-nitrocyclohexyl radical. The nitroxide derived from this radical and the starting material can be detected by ESR.The photolytic behaviour of 1-nitro-1-nitroso-1-cyclopropylethane illustrates the strong interaction between a nitrogroup and an adjacent C-radical centre: only ring-closed products are formed, showing considerable spin delocalisation away from the cyclopropyl ring.     

The effect of flexibility on the photodegradation of aromatic diisocyanate-based polyurethanes

The photolytic degradation of 2,4-TDI/aliphatic diol polyurethanes is directly dependent on the flexibility of the polymer backbone. The extent of photodegradation is accelerated above the glass transition temperature, indicating the role of chain flexibility and/or oxygen diffusion in the decomposition process. Photolysis of the model compound ethyl N-phenylcarbamate (EPC) in neutral host polymer matrices indicates that the para photo-Fries to ortho photo-Fries product ratio experiences an accelerated increase with temperature above the glass transition of the polymer matrix.     

Asymmetric Induction in [3 + 2] Dipolar Cycloaddition Reactions of Nitrile Oxides with Chiral (α-Oxyallyl)silanes

The dipolar cycloaddition reactions of(α-oxyallyl)silanes 12a-g with 2,2-dimethylpropanenitrile oxide and benzonitrile oxide have been studied. Mixtures of anti (14a-g and 16a-g) and syn (15a-g and 17a-g) Δ²-isoxazolines are formed. The direction and magnitude of asymmetric induction depends on the allylie oxygen substituent: a free hydroxy provides a modest excess of the syn diastereomer, a silyl ether shows good selectivity for the anti diastereomer, and various acyl derivatives show low diastereoselectivity. The significance of these results is discussed in terms of two current models for asymmetric induction.     

Microwave-induced generation and reactions of nitrile sulfides: an improved method for the synthesis of isothiazoles and 1,2,4-thiadiazoles

The 1,3-dipolar cycloaddition reactions of nitrile sulfides, generated by microwave-assisted decarboxylation of 1,3,4-oxathiazol-2-ones, have been investigated. By this approach ethyl 1,2,4-thiadiazole-5-carboxylates 3 were prepared in good yield by cycloaddition of the nitrile sulfides to ethyl cyanoformate. Similarly, reaction of benzonitrile sulfide with dimethyl acetylenedicarboxylate (DMAD) afforded dimethyl 3-phenylisothiazole-4,5-dicarboxylate (5). In contrast, o-hydroxybenzonitrile sulfide, generated from the corresponding oxathiazolone 2d, reacted with DMAD to give methyl 4-oxo-4H-[1]benzopyrano[4,3-c]isothiazole-3-carboxylate (8) in high yield. A ca. 1:1 mixture of ethyl 3-phenylisothiazole-4- and 5-carboxylates (6,7) was formed from benzonitrile sulfide and ethyl propiolate. The corresponding reaction with diethyl fumarate gave diethyl trans-4,5-dihydro-3-phenylisothiazole-4,5-dicarboylate (10). 3-Arylisothiazoles, unsubstituted at both the 4- and 5-positions, were prepared from the reaction of 5-aryl-1,3,4-oxathiazolones with norbornadiene by a pathway involving cycloaddition of the nitrile sulfide to the norbornadiene, followed by retro-Diels-Alder extrusion of cyclopentadiene from the resulting isothiazoline cycloadduct 12. In summary, the use of microwave irradiation, rather than conventional heating methods, allows nitrile sulfide generation and reactions to be carried out in shorter times, with easier work-up and, in some cases, in higher yields.     

Photolysis of 10,10-difluorophenanthren-9(10H)-one. Evidence for solvent-assisted α-cleavage

The photolysis of 10,10-difluorophenanthren-9(10H)-one 1 in different solvents shows that the major competing reaction of the diradical formed by α-cleavage: recombination and hydrogen atom abstraction depends on the hydrogen atom donating ability of the solvent. Photolysis of 1 in cyclohexane in the presence of air or oxygen leads mainly to α-cleavage while photoreduction with the formation of 10-fluoro-9-phenanthrol occurs when the solution is deaerated prior to irradiation. In acetonitrile, a poor hydrogen donor, recombination of the diradical back to starting compound 1 is the sole process.     

Through bond mechanism versus exciplex formation in the photochemistry of fullerene / ferrocene donor-bridge-acceptor dyads

A systematic fluorescence and flash photolytic investigation of a series of covalently linked fullerene / ferrocene based donor-bridge-acceptor dyads is reported as a function of the nature of the bridge between the donor site and acceptor site. The fluorescence of the investigated dyads 2 (Φ_rel = 0.17 × 10^?4, 3 (Φ_rel = 0.78 × 10^?4), 4 (Φ_rel = 1.5 × 10^?4), 5 (Φ_rel = 0.7 × 10^?4), and 6 (Φ_rel = 2.9 × 10^?4) were substantially quenched, relative to N-methyl fulleropyrrolidine (1) (Φ_rel = 6.0 × 10^?4). Photolysis of N-methyl fulleropyrrolidine (1) in toluene revealed formation of the excited singlet state which was followed by a rapid intersystem crossing to the excited triplet state. On the other hand, the fate of the excited singlet state of 2, 3, 4, 5, and 6 was found to be governed by rapid intramolecular quenching, with rate constants of 28×10⁹ s^?1, 6.9×10⁹ s^?1, and 3.4×10⁹ s^?1, 14×10⁹ s^?1, 2.3×10⁹ s^?1 respectively. The electron transfer process and the charge separation were confirmed by monitoring the characteristic π-radical anion bands at λ_max = 400 and 1055 nm in degassed benzonitrile with τ_1/2 = 1.8 μs (3) and 2.5 μs (4).     

Gating the mechanistic pathway to the elusive 4-membered ring azeteoporphyrin

Photolysis of metalated (Cu and Ni) and free base 2-diazo-3-oxochlorins within a frozen matrix (λ = 457.9 nm, toluene, 80 K) generates a single photointermediate with a hypsochromically shifted electronic absorption spectrum relative to the starting diazochlorins. The appearance of ketene (~2131 cm(-1)) and azete (~1670 cm(-1)) vibrations in infrared absorption and Raman spectra, respectively, identifies this intermediate as resulting from the Wolff rearrangement of the diazochlorins upon N(2) loss. Computational modeling of the vibrational spectra and TDDFT simulation of the electronic transitions of potential photointermediates corroborate this assignment. Isolation and analysis of photoproducts of these diazochlorins formed within n-butanol-doped frozen toluene matrices indicate near exclusive formation of azeteoporphyrins. In sharp contrast, room temperature laser photolysis of these materials yields a mixture of photoproducts deriving from the presence of both carbene and ketene intermediates. Computational modeling of the intramolecular reactivity of the proposed sp(2) carbene intermediate shows exclusive bond insertion to the adjacent phenyl group, and no evidence of Wolff rearrangement. Computational reaction profile analyses reveal that the barrierless Wolff rearrangement proceeds via an out-of-plane carbene electronic configuration that is generated directly during the loss of N(2). The formation of out-of-plane carbene, resulting in the exclusive formation of the observed ketene photointermediate at low temperatures, is consistent with orbital symmetry considerations and by the geometric constraints imposed by the frozen matrix. Combined, this leads to a model showing that azeteoporphyrin formation via the Wolff rearrangement is dependent upon the structural disposition of the adjacent framework, and the specific reaction intermediate formed is very sensitive to this feature.     

Deracemization of compounds possessing a sec-alcohol or -amino group through a cyclic oxidation–reduction sequence: a kinetic treatment

Deracemization of compounds bearing a chiral sec-hydroxy or -amino group can be achieved in a one-pot reaction via a novel process consisting of a cyclic oxidation–reduction sequence. Thus, in the first step, one enantiomer from the racemic starting material (R+S) is enantioselectively oxidized forming an achiral intermediate product (P, a ketone or imine, respectively). In the second step, the latter is non-selectively reduced to give again R+S in racemic form. Cyclic repetition of this oxidation–reduction sequence leads to an overall chiral inversion of the faster reacting enantiomer from the racemic starting material to yield the slower reacting enantiomer as the final product in a theoretical 100% chemical and enantiomeric yield. Mathematical treatment of the kinetics of this `cyclo-process' allowed the estimation of the practical feasibility at hand of two crucial parameters: (i) the maximal obtainable enantiomeric excess and (ii) the number of cycles required (expressed as theoretical turnover) to reach equilibrium. A computer program for analysis and optimization of such processes was developed, which is available via the Internet.     

Spectroscopic and Theoretical Insights on Non-covalent Binding of PyC60 with a Designed Diporphyrin in Solution

The present paper reports the spectroscopic and theoretical investigations on non-covalent interaction of a functionalized fullerene, namely, C₆₀ pyrrolidine tris-acid ethyl ester (PyC₆₀) with a designed diporphyrin (1) in solvents having varying polarity: toluene and benzonitrile. UV–Vis studies reveal that PyC₆₀ undergoes an appreciable amount of ground state electronic interaction with 1 as the intensity of the Soret absorption band of 1 suffers a considerable decrease in the presence of PyC₆₀ in both solvents. Steady state fluorescence studies elicit efficient quenching of the fluorescence of 1 in the presence of PyC₆₀. The binding constant (K) values of the PyC₆₀/1 complex follow the trend: PyC₆₀/1 (in toluene, K = 2,825 dm³·mol^?1) < PyC₆₀/1 (in benzonitrile, K = 3,540 dm³·mol^?1). Time resolved emission studies establish a relatively long-lived charge separated state for the PyC₆₀/1 complex in benzonitrile. The magnitude of the quantum yield of the charge separated state for the PyC₆₀/1 complex indicates that, while energy transfer is feasible in toluene, there is strong propensity of electron transfer in benzonitrile.     

Photochemistry of 3-aryl-2-isoxazoline1

3-Phenyl-2-isoxazoline (1) was irradiated to give 4-phenyl-2-oxazoline (3), β-aminoaldehyde (14) and benzonitrile from its π-π^* singlet excited state. Several related derivatives afforded similar photoproducts on irradiation. The quantum yields of the photoreactions were dependent on the magnitudes of the singlet energies of the 2-isoxazolines. p-Cyanophenyl-2-isoxazoline (1c) formed a one-to-one photoadduct (22) with benzene.     

2-acyl-4-alk(en)yltetronic acid,a new oxidation product of hop α-acids

Singlet oxygen transforms(?)(R)-humulone 2b and (?)(R)-tetrahydrohumulone 2a into ketene radical intermediates via a radical reaction, which are intramolecularly trapped to give the 2-acyl-4-alkyl- or akenyl-tetronic acids. These are isolated as a 50:50 mixture of the two enol forms 5 and 6.     

Synthesis and photoreaction of Schiff bases derived from p-nitro cinnamaldehyde and diamines in Langmuir and Langmuir–Blodgett films

Monolayers of Schiff bases derived from ethylene diamine and o-phenylene diamine with p-nitro cinnamaldehyde, (compounds 1 and 2) at air/water interface have been studied. Photolysis of 1 in chloroform solution undergoes cis–trans isomerization on irradiation of white light while compound 2 does not undergo isomerization under photolytic conditions. The photolysis of 1 and 2 in Langmuir–Blodgett films (LB films) transferred to quartz plates form dimers. The change in product distribution is attributed to the influence of bridging group of the cinnamaldehyde moieties, molecular configuration and mobility of the compounds in solution, solid state and the aggregation of molecules in monolayer assemblies.     

Zu den Umlagerungsreaktionen des Acetyl-benzoylcarbens

Labelled acetyl-benzoylcarbene (13) formed by photolytic nitrogen elimination from [2-¹³C]1-phenyl-2-diazo-1,3-butadione (8) rearranges in the presence of ethanol to ethyl 2-phenylacetoacetate (26%) and ethyl 2-benzoylpropionate (74%). The¹³C-labelling in ethyl 2-phenylacetoacetate is found to the extend of ca.58% in the ester carbonyl function (20) and to ca. 42% in the adjacent tertiary carbon atom (21). This proves the isomerisation of carbene13 to carbene11 via the intermediate acetyl-phenyloxirene (12). The [1-¹³C]-labelled ester20 is formed by acetyl migration in11 to the ketene16 and subsequent addition of ethanol. In contrast, ethyl 2-benzoylpropionate is exclusively generated by migration of the methyl group in carbene13 since it contains the complete¹³C-labelling at the tertiary carbon atom (22).

     

Direct electrochemical reduction of a bromo-propargyloxy ester at vitreous carbon cathodes in dimethylformamide

Cyclic voltammograms for the reduction of ethyl 2-bromo-3-(3^′,4^′-dimethoxyphenyl)-3-(propargyloxy)propanoate (1) at glassy carbon electrodes in dimethylformamide containing tetraalkylammonium salts exhibit three prominent waves corresponding to cleavage of the carbon–bromine bond and to subsequent reduction of ethyl trans-3-(3^′,4^′-dimethoxyphenyl)-prop-2-enoate (4). Controlled-potential electrolyses of 1 at potentials corresponding to reduction of the carbon–bromine bond afford 4 as the major product with an average yield of 56%. In the presence of a proton donor (1,1,1,3,3,3-hexafluoro-2-propanol), the quantity of 4 decreases slightly, and 2-(3^′,4^′-dimethoxyphenyl)-3-(ethoxycarbonyl)-4-methyl-2,5-dihydrofuran (3) is obtained in moderate amount (26%). We propose a mechanistic scheme whereby the major products are formed via a combination of one- and two-electron processes.     

Identification of esters by collisional charge inversion of their enolate ions

The collisional charge inversion and neutralization-reionization (^?NR) mass spectra of the enolate ions of m/z 115 derived from the four butyl acetates, the two propyl propionates, ethyl butyrate, ethyl isobutyrate, methyl valerate, methyl 2-methylbutyrate and methyl 3-methylbutyrate were recorded. The major primary fragmentation reactions of the unstable carbenium ion formed by charge inversion involve elimination of an alkoxy radical to form a ketene or alkylketene molecular ion and formation of an alkyl ion consisting of the R¹ group of RCOOR¹. A minor fragmentation reaction involves elimination of an alkyl radical by cleavage of a C? C bond α to the ether oxygen. The alkylketene ions fragment by β-cleavage eliminating an alkyl radical to form an olefinic acylium ion. In most cases the charge inversion mass spectra of the enolate ions allow identification of the ester.     

Synthesis,structural characterization and reactivity of copper(I)-nitrile complexes: Crystal and molecular structure of [Cu(BzCN)4][BF4]

Complexes of general formula [CuL₄][BF₄] (L = benzonitrile – PhCN 2 or phenylacetonitrile – BzCN 3) have been prepared and structurally characterized by NMR spectroscopy and X-ray crystallography. Their structure and reactivity have been compared to the well known [Cu(MeCN)₄][BF₄] (1). The ⁶³Cu line width and the ⁶³Cu chemical shift have been evaluated by varying the temperature and the concentration of the complex 2 in benzonitrile solutions. The phenylacetonitrile solutions of the complex 3 give extremely broad signals which are beyond detection. Accordingly, compound 3 has been studied by ⁶³Cu MAS NMR spectroscopy. The solution NMR data are consistent to the prevalence of dynamic equilibrium between tetra- and low-coordinated species in both complexes. The X-ray structure of 3 revealed that the copper(I) atom sits in a slightly distorted tetrahedral geometry, surrounded by four BzCN ligands.     

Etude de la reaction chlorocarbene-acetals de cetenes : II. Stereoselectivite de la reaction

We have studied the stereoselectivity of the addition reaction of chloro and chloromethyl carbenoids with ketene alkylsilyl acetals. The best stereoselectivity was generally observed with the dimethyl tertiobutylsilyloxy group. With the chlorocarbenoid, using an E ketene acetal we obtained in majority (8?0%) an E α,β-ethylenic ester and using a Z ketene acetal we obtained in majority (7?0%) a Z α,β-ethylenic ester. In the case of the chloromethyl carbenoid the two ketene acetal isomers led to the same E α-substituted α,β-ethylenic ester (8?8% of selectivity). With the chlorophenylcarbenoid, formation of 9?0% of E α phenyl α,β-ethylenic ester is observed.