SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS—VI. IDENTIFICATION OF THE FREE RADICALS GENERATED DURING THE PHOTOLYSIS OF MUSK AMBRETTE, MUSK XYLENE AND MUSK KETONE

Musk ambrette (4-tert-butyl-3-methoxy-2,5-dinitrotoluene), a common component of perfumes, soaps, and some food flavorings, can cause cutaneous photosensitization reactions including photoallergy. These may be mediated through free radicals formed during photolysis. When musk ambrette was photolyzed under nitrogen in basic methanol, two distinct nitro anion radicals were identified by electron spin resonance. One radical was centered on a nitro group in the plane of the aromatic ring, while the other was centered on a nitro group twisted out of the plane of the ring due to steric hindrance by bulky substituents on either side of the group: the two radicals appeared to interconvert and maintain an equilibrium concentration ratio. Two closely related compounds which are also used in perfumes, but have not been reported to cause photosensitizing reactions, also produced free radicals during photolysis. Musk xylene (2,4,6-trinitro-1,3-dimethyl-5-tert-butylbenzene) generated two nitro anion radicals, both of which were centered on twisted nitro groups, while musk ketone (3,5-dinitro-2,6-dimethyl-4-tert-butylacetophenone) produced only one nitro anion radical, which is also twisted. Athough these nitro anion radicals are probably the first step in the photolysis of these nitroarornatic molecules, it seems likely that in vivo they will undergo further reduction to produce more reactive species including the corresponding nitroso and hydroxylamine derivatives. In addition, autoxidation of the nitro anion radical intermediate forms superoxide.     

Studies of the electron-promoted cope cyclization of 2,5-phenyl-substituted 1,5-hexadiene radical anions

This work describes studies of the electron-promoted Cope cyclization of 2,5-phenyl-1,5-hexadiene radical anions in a flowing afterglow triple quadrupole mass spectrometer. The electronic properties of the hexadienes have been systematically modified by using aromatic substituents at the 2- and 5-positions of the hexedienes, including those with nitro, trifluoromethyl, fluoro, chloro, and acetyl groups. Ions were formed by the thermal attachment of electrons in the gas phase. Structures of the molecular radical anions were probed to determine whether they undergo cyclization to a cyclohexane-1,4-diyl anion structure by examining chemical reactivity with neutral reagents including carbon dioxide, carbon disulfide, and nitric oxide. First-order rate constants for the reactions of ions were measured, and the reaction efficiencies were determined. Based on the reactivity results, a thermochemical model has been developed, which predicts the reaction thermochemistry by using thermochemical properties of model systems. The observed reactivity from ion-molecule reactions and the study of reaction rates show that the ion of 2,5-dicyanohexadiene and 2,5-di(4,4'-trifluoromethyl phenyl)-1,5-hexadiene undergo Cope cyclization, whereas the radical anions having substituents such as the fluoro, nitro, chloro, and acetyl groups do not.     

醌类化合物与抗坏血酸反应生成醌自由基负离子的ESR研究

醌类化合物广泛存在于动植物体内,许多抗癌药物含有醌核。醌类可经单电子还原生成相应的的醌自由基负离子,后者可转移单电子给分子氧,产生超氧负离子基,并进一步产生对生物细胞有损伤作用的羟基自由基,这被认为是醌类化合物生物毒性的主要来源之一。抗坏血酸作为重要的生物抗氧化剂,在生物保护过程中具有十分重要的意义,本文首     

Alkylation of nitroaromatics with trialkyborane

When p-dinitrobenzene is reacted with Et(3)B in t-BuOH or THF in the presence of t-BuOK, it yields p-nitroethylbenzene. In this report we examine the scope of this transformation by monitoring the effect of various parameters on the reaction. It has been found that the reaction is extremely sensitive to temperature and rather insensitive to the base-solvent combination used. It is also insensitive to the steric hindrance of the base: good yields were obtained using sodium 2,6-diisopropylphenoxide or when using NaH. Alkylation was obtained with a large variety of alkylboranes ranging from linear to polycyclic. Yields drop significantly if one of the nitro groups is replaced by another electron-withdrawing group. In all cases studied (CHO, PHCO, SO(2)Ph, and CN), it is the latter group which was preferentially displaced by the alkyl group. According to the suggested mechanism, the radical anion of the substrate combines with the alkyl radical released from the boranyl radical to form a Meisenheimer complex. The reaction takes place at the ring carbon bearing the highest spin density in accordance with ab initio calculations at the B3LYP/6-31+G level.     

Effect of conformational changes on a one-electron reduction process: evidence of a one-electron P-P bond formation in a bis(phosphinine)

EPR spectra show that one-electron reduction of bis(3-phenyl-6,6-(trimethylsilyl)phosphinine-2-yl)dimethylsilane (1) on an alkali mirror leads to a radical anion that is localized on a single phosphinine ring, whereas the radical anion formed from the same reaction in the presence of cryptand or from an electron transfer with sodium naphthalenide is delocalized on the two phosphinine rings. Density functional theory (DFT) calculations show that in the last species the unpaired electron is mainly confined in a loose P-P bond (3.479 A), which results from the overlap of two phosphorus p orbitals. In contrast, as attested by X-ray spectroscopy, the P-P distance in neutral 1 is large (5.8 A). As shown by crystal structure analysis, addition of a second electron leads to the formation of a classical P-P single bond (P-P 2.389 A). Spectral modifications induced by the presence of cryptand or by a change in the reaction temperature are consistent with the formation of a tight ion pair that stabilizes the radical structure localized on a single phosphinine ring. It is suggested that the structure of this pair hinders internal rotation around the C-Si bonds and prevents 1 from adopting a conformation that shortens the intramolecular P-P distance. The ability of the phosphinine radical anion to reversibly form weak P-P bonds with neutral phosphinines in the absence of steric hindrance is confirmed by EPR spectra obtained for 2,6-bis(trimethylsilyl)-3-phenylphosphinine (2). Moreover, as shown by NMR spectroscopy, in this system, which contains only one phosphinine ring, further reduction leads to an intermolecular reaction with the formation of a classical P-P bond.     

Activated Sterically Strained C=N Bond in N-Substituted p-Quinonemono- and -diimines: XI. Redox Potentials of 2,6- and 3,5-Dimethyl-N-phenylsulfonyl-1,4-benzoquinonimines

Due to steric effect of the methyl groups the lone electron pair on the nitrogen atom in 3,5-dimethyl-N-phenylsulfonyl-1,4-benzoquinonimine is forced out of the quinoid ring plane, so that the nitrogen atom adopts a near-sp hybridization. As a result, its electrophilicity and hence redox potential increase, leading to activation of the CÍN bond to addition of nucleophiles, as compared to the corresponding 2,6-dimethyl isomer.     

Substitution Reactions Which Proceed via Radical Anion Intermediates

A new type of substition process at a saturated carbon atom is described. These reactions, which proceed via a chain sequence in which radical anions and free radicals are intermediates, are noteworthy for providing novel and powerful means of synthesis: they occur readily under mild conditions, they give excellent yields of pure products, and, in contrast to S_N2 displacements, they are rather insensitive to steric hindrance. As a consequence, radical anion processes are especially valuable for the preparation of highly branched structures. Many inorganic and organic anions readily enter into these displacements and, indeed, amines are also effective. Systems which undergo substitutions via this electron transfer mechanism include benzylic, cumylic, strictly aliphatic, and heterocyclic molecules. It is of interest that a number of groups which do not behave as leaving groups in S_N2 displacements are readily displaced at room temperature from a satureted carbon atom via the radical anion-free radical pathway, e.g., nitro, azide, sulfone, and ether groups.     

Direct determination of rate constants for coupling between aromatic radical anions and alkyl and benzyl radicals by laser-flash photolysis

Coupling rates between the radicals methyl, n-, sec-, tert-butyl and benzyl (R.) and the aromatic radical anions of 1,4-dicyanonaphthalene, 9,10-dicyanoanthracene and fluorenone (A-.) have been obtained using a new laser-flash photolysis method. The radicals R. and the radical anions A-. were generated by a photoinduced electron transfer reaction between the aromatic compound A and the alkyl or benzyl triphenylborate anion RB(Ph)3-. For the first time the rate constants of the coupling reaction between methyl and benzyl radicals with aromatic radical anions have been obtained. For all the measured coupling rate constants an average value of k1 = 1.9 x 10(9) M-1 s-1 was found with a relatively small variation in the coupling rates (0.8-2.9 x 10(9) M-1 s-1). The results demonstrate that the coupling rate k1 is insensitive to changes in the steric and electronic properties of the radicals and the structure and standard potentials of the aromatic radical anions.     

Polarography of some 2,1,3-benzothiadiazoles,benzofurazans, 2,1,3-benzoselenadiazoles,and 3,4-disubstituted and fused 1,2,5-thiadiazoles

The redox behavior has been determined in acetonitrile solutions at a mercury and platinum electrode for 2,1,3-benzo(group VI)diazoles, and 3,4-disubstituted and fused 1,2,5-thiadiazoles. The derivatives studied contained alkyl, phenyl, bromo, chloro, cyano, nitro, methylsulfonyl, and trifluoromethylsulfonyl groups. All ring systems and their derivatives are reversibly reduced initially in a one-electron step, to their respective radical anion, but the nitro and bromo derivatives are reduced preferentially at the substituent group. The potential at which the production of the radical anion occurred became more anodic as the electron withdrawing ability of the substituent and the number of substituents increased.     

A theoretical study of the structure and vibrations of 2,4,6-trinitrotolune

Theoretical calculations of the structure, internal rotations and vibrations of 2,4,6-trinitrotolune, TNT, in the gas phase were performed at the B3LYP/6-31G* and B3LYP/6-311+G** levels of theory. Two genuine energy minimum structures were found. In both structures the 4-nitro group is planar to the phenyl ring, while the 2,6-nitro groups are slightly out of plane with the phenyl ring due to steric interaction with the methyl group. The two structures are related by internal rotations of the methyl and 2, or 6-nitro group. The lowest energy route for interconversion between them is a concerted motion of the methyl group and 2 or 6 nitro group in a ‘cog wheel’ type of mechanism. The geometry of the low energy structure A is closest to that observed in the crystal structures of TNT, where all three nitro groups are out of plane with the phenyl ring. FTIR and Raman spectra of solid TNT and ¹³C, ¹⁵N enriched TNT are presented and assigned with the help of the B3LYP/6-311+G** calculations on A. The lower level B3LYP/6-31G* calculation fails to predict the correct vibrational coupling between the nitro and phenyl groups. The B3LYP/6-311+G** calculation gives a good prediction of the nitro vibrations and the isotopic shifts observed for TNT isotopomers.     

1‐Chloro‐3,6‐di­methoxy‐2,5‐di­methyl­benzene and 1‐chloro‐3,6‐di­methoxy‐2,4‐di­methyl­benzene

The title compounds, 1‐chloro‐3,6‐di­methoxy‐2,5‐di­methyl­benzene, (IIIa), and 1‐­chloro‐3,6‐di­methoxy‐2,4‐di­methyl­benzene, (IIIb), both C₁₀H₁₃ClO₂, were obtained from 2,5‐ and 2,6‐di­methyl‐1,4‐benzo­quinone, respectively, and are intermediates in the synthesis of ammonium quinone derivatives. The isomers have different substituents around the methoxy groups and crystallize in different space groups. In both mol­ecules, the methoxy groups each have different orientations with respect to the benzene ring. In both cases, one methoxy group lies in the plane of the ring and can participate in conjugation with the aromatic system, while the second is almost perpendicular to the plane of the aromatic ring. The C—O—C bond angles around these substituents are also different: 117.5 (4) and 118.2 (3)° in (IIIa) and (IIIb), respectively, when the methoxy groups lie in the plane of the ring, and 114.7 (3) and 113.6 (3)° in (IIIa) and (IIIb), respectively, when they are out of the plane of the ring.     

13C FT-NMR spectra of alkyl substituted furans. A study of the influence of steric interactions

The ¹³C FT-NMR spectra of thirteen furans, monodi- or trisubstituted with methyl and/or t-butyl groups, were studied in detail. Substituent effects of methyl and t-butyl groups on the chemical shifts of ring carbon atoms are additive in nonsterically hindered furans. Steric shifts for the ring carbon atoms are found in furans with bulky neighbouring substituents, but the hybridisation of the carbon atoms in these hindered furans is not changed. The chemical shifts of the substituents are calculated according to the Grant-Cheney formula. No simple relationship between steric shift and steric hindrance can be ascertained.     

Photoinduced Color Change and Photomechanical Effect of Naphthalene Diimides Bearing Alkylamine Moieties in the Solid State

Photoinduced color change of naphthalene diimides (NDIs) bearing alkylamine moieties has been observed in the solid state. The color change is attributed to the generation of a NDI radical‐anion species, which may be formed through a photoinduced electron‐transfer process from the alkylamine moiety to the NDI. The photosensitivity of NDIs is highly dependent on the structures of the alkylamine moieties. Crystallographic analysis, kinetic analysis, UV/Vis/NIR spectroscopic measurements, and analysis of the photoproduct suggested that a radical anion was formed through an irreversible process initiated by proton abstraction between an amine radical cation and the neutral amine moiety. The radical anions formed stacks including mixed‐valence stacks and radical‐anion stacks, as shown by the broad absorption bands in near‐IR spectra. These photosensitive NDIs also showed crystal bending upon photoirradiation, which may be associated with a change in the intermolecular distance of the NDI stacks by the formation of monomeric radical anions, mixed‐valence stacks, and radical‐anion stacks.     

The decomposition of substituted benzoyl peroxides in the presence of dimethoxybenzenes

Benzoyl peroxides, particularly those containing electron withdrawing substituents, undergo rapid decomposition in the presence of m-dimethoxybenzene, p-dimethoxybenzene, 2,5-di-t-butyl-1,4-dimethoxybenzene, and 2,5-dimethyl-1,4-dimethoxybenzene. Reactions are first order in peroxide and dimethoxy benzene, increasing in the order given. Identified products are the acids corresponding to the peroxide and esters involving ring substitution, ring-substitution with elimination of a t-Bu group, and benzylic substitution. It is proposed that reaction involves a rate-determining charge transfer transition state leading to radical ion pairs which collapse to products. No free radicals have been detected.     

系列水溶性磺酸卟啉的制备、表征及催化性能

改进磺化路线制备了5种取代基及取代位置不同的水溶性磺酸卟啉,利用氢核磁共振波谱(1H NMR)、傅里叶变换红外光谱(FTIR)、紫外-可见吸收光谱(UV-Vis)及质谱(MS)等手段对产物进行表征及性质研究.将该系列卟啉作为光催化剂,用于催化氧化1,5-萘二酚的反应,主催化产物为5-羟基-1,4-萘二醌,转化率为78%~95%.实验发现,磺酸基团在苯环上的位置以及取代基的数目、电子效应及立体位阻效应均会对催化结果产生影响,其中磺酸根的位置至关重要.动力学研究结果表明该催化氧化过程为一级反应.探讨了该反应的反应机理.     

Voltammetric Study of Nitro Radical Anion Generated from Some Nitrofuran Compounds of Pharmacological Significance

The electrochemical behavior of 2‐(5‐amino‐ 1,3,4‐oxadiazolyl)‐5‐nitrofuran (NF359) and its comparison with well‐known drugs such as nifurtimox (NFX) and nitrofurazone (NFZ) in protic, mixed and aprotic media by cyclic voltammetry, tast and differential pulse polarography was studied. All the compounds were electrochemically reducible in all media being the reduction of the nitrofuran group the main voltammetric signal. The one‐electron reduction couple due to the nitro radical anion formation was visualized in mixed (for NF359 and NFZ) and aprotic media (for all compounds). By applying a cyclic voltammetric methodology we have calculated the decay constants (k₂) of the corresponding nitro radical anions in mixed and aprotic media. In mixed medium data fit well with a disproportionation reaction of the nitro radical anion but in aprotic medium fit better with a dimerization reaction. Also, considering cyclic voltammetric measurements in aprotic media we have estimated the reduction potential of the RNO₂/RNO₂^.? couple in aqueous medium, pH 7 (E¹₇ values) finding very good correlation with E¹₇ values obtained by pulse radiolysis. Furthermore we have calculated the equilibrium constants from the electron transfer from nitro radical anion to oxygen (k_O2) finding that nitro radical anion from NF359 is thermodynamically favored to react with oxygen in respect to both NFZ and NFX.     

Solvation Dependences of Isotropic Hyperfine Interaction Constants and out-of-Plane Distortions of ortho-Substituted Nitrobenzene Radical Anions

For a number of ortho-substituted nitrobenzene radical anions (RAs) generated in DMF and its binary mixtures with water, it is shown that for radical anions with a substituent of minor effective volume in one ortho position to the nitro group, the dependences of the isotropic hyperfine interaction (ihfi) constants on the mole fraction of water are S-like and dictated by the medium composition and the concerted out-of-plane rotational and pyramidal distortions of the nitro group of the radical anion. The S-like shape of the solvation dependences of the ihfi constants is dictated by the dominant rotational distortions of the nitro group. For most radical anions with two ortho substituents or with one ortho substituent with a large effective volume, the S-like dependences are not observed, and the values of the nitrogen ihfi constants depend on the dominant pyramidal distortion of the nitro group. For the 2-tert-butylnitrobenzene radical anion in water, the nitrogen ihfi constant is a_N=25.62 G, which is typical of nitroaliphatic radical anions. This effect is explained based on the pyramidal structure of the nitro group in the case of its large rotation angles.     

Interfacial adsorption state of protonated lipophilic porphyrins in a liquid-liquid system by using reflection spectrometry.

Analysis by reflection spectrometry was performed to clarify the interfacial adsorption of protonated lipophilic tetraphenylporphyrin derivatives in a dodecane-aqueous sulfuric acid system, and to confirm the utility of partial reflection spectroscopy. Interfacial adsorption was not observed for porphyrins substituted at the 2,6 positions of meso-phenyl groups, suggesting that the substituents prevent porphyrins from forming aggregates by steric hindrance. Polymorphous J-aggregates of acid dications were produced by tetra-p-tolylporphyrin with a saturated interfacial molecular density of 1.0 x 10(-10) mol cm(-2), which could yield 48 degrees as a mean tilting angle of the pyrrole ring plane from the interface normal. Partial-reflection spectrometry can provide sensitive detection and molecular orientation analysis of interfacial adsorbates.     

Electron acceptors of the fluorene series. 10.(1) novel acceptors containing butylsulfanyl, butylsulfinyl, and butylsulfonyl substituents: synthesis, cyclic voltammetry, charge-transfer complexation with anthracene in solution, and X-ray crystal structures of two tetrathiafulvalene complexes

2,4,5,7-Tetranitro-9-fluorenone (1b) reacts readily with n-butanethiol in dipolar aprotic solvents with selective substitution of nitro groups by butylsulfanyl groups in positions 2 and 7 (2, 3); the 2,5-isomer 4 was formed only as a minor product (<1%). Condensation of fluorenones 2-4 with malononitrile yielded 9-dicyanomethylene derivatives 5-7, which showed strong intramolecular charge transfer (lambda approximately 510-560 nm) and were found to sensitize the photoconductivity of carbazole-containing polymer films. Oxidation of sulfides 2-4 gave sulfoxide 8 or sulfones 9-11, which then were converted into their corresponding dicyanomethylene derivatives 12-15. All these novel acceptors showed three reversible single-electron reduction waves (cyclic voltammetry) yielding radical anion, dianion, and radical trianion; moreover, acceptors 13-15 showed also a fourth reduction wave, representing reversible tetraanion formation. Substitution of the oxygen of the carbonyl group in the fluorenones by a dicyanomethylene group increased the thermodynamic stability (K(SEM) growth) of the radical anion; K(SEM) ranged from 3 x 10(5) to 3 x 10(9) M(-1). CV measurements characterize compounds 3, 4 (EA = 1. 86-1.89 eV) as poor acceptors, 2, 6-11 (EA = 2.13-2.31 eV) as moderate acceptors, and 5, 12-15 (EA = 2.53-2.66 eV) as strong electron acceptors. Charge-transfer complex (CTC) formation between acceptors 9, 10, 13, 14, and anthracene as a donor was monitored by the appearance of additional low-energy bands in the visible region (CTC bands) of their electron absorption spectra. Increasing the EA of the acceptors from 9-fluorenones to the corresponding 9-dicyanomethylenefluorenes increases the complexation constants K(CTC) by 2.5-3 times, while sulfonyl substituents present substantial steric hindrance for complexation (as compared to the nitro group), decreasing K(CTC) values. Two CTCs for acceptors 14 and 17 with tetrathiafulvalene (TTF) were obtained, and their structures were solved by single-crystal X-ray diffractometry, giving the stoichometries 14:TTF, 2:3, and 17:TTF:PhCl, 1:1:0.5. In the former complex the packing motif is a mixed.DDAD'A. stack; in the latter complex the D and A moieties form unusually close CT pairs, which pack in a herringbone motif.     

Density functional calculations on dissociation reactions of radical anions of 5-fluorouracil derivatives

Fragmentation reactions upon electron attachment to 5-fluorouracil with CH2R substituents at N1 have been evaluated by means of density functional calculations. The present results show that electron attachment to R = F, HC=O or CN derivatives follows a stepwise pathway with radical anions as intermediates. For these compounds, the most stable species formed is the pi radical anion which bears an unpaired spin density at the C6=C5-C4=O pi-conjugated system of the uracil ring. Cleavage of the N1-CH2R or N1CH2-R bond of these intermediates proceeds through the mixing of the pi and sigma states by means of proper geometrical fluctuations along the reaction coordinate. No sigma radical anion could be characterised on any of these sigma basal potential surfaces. A noticeable decrease in the activation energy for the N1-CH2R bond dissociation was observed for R = H-C=O or CN. Therefore, such derivatives with unsaturated groups positioned vicinal to the N1-C1' bond are identified as targets for the development of novel radiation-activated antitumour drugs. On the other hand, the electron transfer to the compounds with R = Cl, Br is dissociative, i.e. it occurs without the mediation of radical anions. For compounds with R = halides or R = NO2, the fragmentation of the N1CH2-R bond is the preferred dissociation pathway.