Control of NO Concentration in Solutions of Nitrosothiol Compounds by Light

We studied the thermal and photolytic decomposition of two S -nitrosothiols, S -nitrosoglutathione (GSNO) and S -nitroso- N -acetylpenicillamine (SNAP), in water or propanol solutions. A "concentration clamp" (relatively constant concentration of NO as a function of time) could be implemented in a closed volume by varying the pH, concentration of nitrovasodilator and intensity of the light source. Depending on the conditions, the light either stimulated NO release or sharply decreased NO concentration in the test solutions. Changes in the absorption spectra of GSNO solutions were monitored as a function of light exposure. Generation of superoxide as a product of a photolytic decomposition reaction of S -nitrosothiols and further oxidation of NO is the most likely mechanism for light suppression of NO concentration.     

TRAPPING OF NITRIC OXIDE FORMED DURING PHOTOLYSIS OF SODIUM NITROPRUSSIDE IN AQUEOUS AND LIPID PHASES: AN ELECTRON SPIN RESONANCE STUDY

Abstract— Photolytic decomposition of sodium nitroprusside (SNP), a widely used nitrovasodilator, produced nitric oxide (NO), which was continuously monitored by electron spin resonance (ESR) spectroscopy. The NO present in the aqueous or the lipid phase was trapped by either a hydrophilic or a hydrophobic nitronyl nitroxide, respectively, to form the corresponding imino nitroxide. The conversion of nitronyl nitroxide to imino nitroxide was monitored by ESR spectrometry. The quantum yield for the generation of NO from SNP, measured from the rate of decay of nitronyl nitroxide, was 0.201 ± 0.007 and 0.324 ± 0.01 (¯± SD, n = 3) at 420 nm and 320 nm, respectively. The action spectrum for NO generation was found to overlap the optical absorption spectrum of SNP closely. A mechanism for the reaction between SNP and nitronyl nitroxide in the presence of light is proposed and computer-aided simulation of this mechanism using published rate constants agreed well with experimental data. The methodology described here may be used to assay NO production continuously during photoactivation of NO donors in aqueous and lipid environments. Biological implications of this methodology are discussed.     

THE PHOTOLYSIS OF CYSTINE IN ACIDIC AQUEOUS SOLUTION

Abstract— Quantum yields of cysteine, ammonia, 1-amino,1'-oxo,2,2'-dithiodipropionicacid (AODT–DPA), alanine, alanine 3-sulphinic acid, cysteic acid, and serine have been determined in aqueous oxygenated and deaerated cystine solutions irradiated with 254 nm radiation. From the effect of methanol, ethanol and propanol-2 on the quantum yields of cysteine, ammonia, AODT-DPA and alanine, it is concluded that (a) the S–S bond is broken with high quantum efficiency, (b) C–S and C–N bonds do not undergo primary photolytic fission, and (c) all the AODT–DPA, but only about 12 per cent of the ammonia, is free-radical in origin. The production of pyruvic acid at the expense of AODT–DPA in irradiated cystine solutions containing alanine provides further evidence that AODT–DPA has free-radical precursors. Reaction schemes are proposed for the radical-induced production of keto acid and ammonia in oxygenated and deaerated solutions.     

X‐ray Radiation‐Controlled NO‐Release for On‐Demand Depth‐Independent Hypoxic Radiosensitization

Multifunctional stimuli‐responsive nanotheranostic systems are highly desirable for realizing simultaneous biomedical imaging and on‐demand therapy with minimized adverse effects. Herein, we present the construction of an intelligent X‐ray‐controlled NO‐releasing upconversion nanotheranostic system (termed as PEG‐USMSs‐SNO) by engineering UCNPs with S‐nitrosothiol (R‐SNO)‐grafted mesoporous silica. The PEG‐USMSs‐SNO is designed to respond sensitively to X‐ray radiation for breaking down the S N bond of SNO to release NO, which leads to X‐ray dose‐controlled NO release for on‐demand hypoxic radiosensitization besides upconversion luminescent imaging through UCNPs in vitro and in vivo. Thanks to the high live‐body permeability of X‐ray, our developed PEG‐USMSs‐SNO may provide a new technique for achieving depth‐independent controlled NO release and positioned radiotherapy enhancement against deep‐seated solid tumors.     

VISIBLE LIGHT PHOTOCHEMICAL RELEASE OF NITRIC OXIDE FROM S-NITROSOGLUTATHIONE: POTENTIAL PHOTOCHEMOTHERAPEUTIC APPLICATIONS

Abstract Some aspects of the physiological role of NO may be mediated by stable NO-carriers such as S -nitrosoglutathione and related S -nitrosothiols. In this report we show that irradiation of S-nitrosoglutathione at either absorption band (λ_max= 340 nm or 545 nm) results in the release of nitric oxide. Photolysis of S -nitrosoglutathione at 545 nm exhibited a quantum yield of 0.056 ± 0.002 and was best approximated by a first-order process with k _obs= 4.9 × 10⁻⁷± 0.3 × 10⁻⁷ s⁻¹. The photolytic release of NO from S -nitrosoglutathione resulted in an enhanced cytotoxic effect of S -nitrosoglutathione on HL-60 leukemia cells. That the cytotoxic effect of S -nitrosoglutathione was diminished by the addition of oxyhemoglobin strongly suggests that NO is the cytotoxic species. The finding that NO can be readily liberated from S -nitrosoglutathione by visible radiation indicates that the photochemical properties of this compound in the visible spectrum must be considered in order to obtain meaningful data as to its physiological role and the S -nitrosoglutathione and related compounds may find use as photochemotherapeutic agents.     

Nitroso group transfer in s-nitrosocysteine: evidence of a new decomposition pathway for nitrosothiols

The rate of S-nitrosocysteine decomposition in a pH range between 0.7 < pH < 13 exhibits first- and second-order dependence on total cysteine concentration. The second-order term is only observed for pH values between 6.9 < pH < 12. Both first- and second-order terms show a complex dependence on the acidity of the medium. They increase with increasing pH, reaching a maximum value around pH = 8 and then decrease with further increase in pH. An analysis of the reaction products reveals the absence of nitrite ion and ammonia. No evidence of catalysis by copper ions is observed. These results suggest the existence of a new decomposition pathway for S-nitrosocysteine, which proceeds via an intramolecular nitroso group transfer producing a primary N-nitrosamine that decomposes rapidly to give the corresponding diazonium salt. The nitroso group transfer reaction occurs intermolecularly for the decomposition pathway exhibiting a quadratic dependence on cysteine concentration. Both nitroso group transfer pathways are subject to acid catalysis by cysteine. Kinetic results indicate that the extent of S...NO bond cleavage in the transition state is ahead of protonation of the AH...S sulfur atom. The results obtained show the existence of a new decomposition pathway for the S-nitrosocysteine where NO is not released, and hence, it has a significant biological impact due to the potential use of nitrosothiols as NO donors.     

Insights into mechanistic photodissociation of acetyl chloride by ab initio calculations and molecular dynamics simulations

The potential energy surfaces of dissociation and elimination reactions for CH(3)COCl in the ground (S0) and first excited singlet (S1) states have been mapped with the different ab inito calculations. Mechanistic photodissociation of CH(3)COCl has been characterized through the intrinsic reaction coordinate and ab initio molecular dynamics calculations. The alpha-C-C bond cleavage along the S1 pathway leads to the fragments of COCl((2)A' ') and CH(3) ((2)A') in an excited electronic state and a high barrier exists on the pathway. This channel is inaccessible in energy upon photoexcitation of the CH(3)COCl molecules at 236 nm. The S1 alpha-C-Cl bond cleavage yields the Cl((2)P) and CH(3)CO(X(2)A') fragments in the ground state and there is very small or no barrier on the pathway. The S1 alpha-C-Cl bond cleavage proceeds in a time scale of picosecond in the gas phase, followed by CH(3)CO decomposition to CH(3) and CO. The barrier to the C-Cl bond cleavage on the S1 surface is significantly increased by effects of the argon matrix. The S1 alpha-C-Cl bond cleavage in the argon matrix becomes inaccessible in energy upon photoexcitation of CH(3)COCl at 266 nm. In this case, the excited CH(3)COCl(S1) molecules cannot undergo the C-Cl bond cleavage in a short period. The internal conversion from S1 to S0 becomes the dominant process for the CH(3)COCl(S1) molecules in the condensed phase. As a result, the direct HCl elimination in the ground state becomes the exclusive channel upon 266 nm photodissociation of CH(3)COCl in the argon matrix at 11 K.     

The hydrolysis of 4-acyloxy-4-substituted-2,5-cyclohexadienones: limitations of aryloxenium ion chemistry

The title compounds serve as potential precursors to aryloxenium ions, often proposed, but primarily uncharacterized intermediates in phenol oxidations. The uncatalyzed and acid-catalyzed decomposition of 4-acetoxy-4-phenyl-2,5-cyclohexadienone, 2a, generates the quinol, 3a. (18)O-Labeling studies performed in (18)O-H(2)O, and monitored by LC/MS and (13)C NMR spectroscopy that can detect (18)O-induced chemical shifts on (13)C resonances, show that 3a was generated in both the uncatalyzed and acid-catalyzed reactions by C(alkyl)-O bond cleavage consistent with formation of an aryloxenium ion. Trapping with N(3)(-) and Br(-) confirms that both uncatalyzed and acid-catalyzed decompositions occur by rate-limiting ionization to form the 4-biphenylyloxenium ion, 1a. This ion has a shorter lifetime in H(2)O than the corresponding nitrenium ion, 7a (12 ns for 1a, 300 ns for 7a at 30 degrees C). Similar analyses of the product, 3b, of acid- and base-catalyzed decomposition of 4-acetoxy-4-methyl-2,5-cyclohexadienone, 2b, in (18)O-H(2)O show that these reactions are ester hydrolyses that proceed by C(acyl)-O bond cleavage processes not involving the p-tolyloxenium ion, 1b. Uncatalyzed decomposition of the more reactive 4-dichloroacetoxy-4-methyl-2,5-cyclohexadienone, 2b', is also an ester hydrolysis, but 2b' undergoes a kinetically second-order reaction with N(3)(-) that generates an oxenium ion-like substitution product by an apparent S(N)2'mechanism. Estimates based on the lifetimes of 1a, 7a, and the p-tolylnitrenium ion, 7b, and the calculated relative stabilities of these ions toward hydration indicate that the aqueous solution lifetime of 1b is ca. 3-5 ps. Simple 4-alkyl substituted aryloxenium ions are apparently not stable enough in aqueous solution to be competitively trapped by nonsolvent nucleophiles.     

Autowaves as a basis for spatial and temporal regulation of the biological action of nitric oxide in living systems (a hypothesis)

It was hypothesized that autowaves can appear in the NO + Fe²⁺ + thiol system in aqueous solutions characterized by oscillatory changes in the concentrations of paramagnetic dinitrosyl iron complexes with thiolcontaining ligands and S-nitrosothiols formed in the system. The autowave mode of functioning of the system allows spatial and temporal regulation of the biological activity of nitric oxide, dinitrosyl iron complexes, and S-nitrosothiols in cells and tissues.     

FTIR investigation of the intermediates formed in the reaction of nitroprusside and thiolates

We have studied the outer-sphere reduction of [Fe(CN)5(NO)]2- by several reagents including dithionite and have for the first time measured the IR spectra of [Fe(CN)5(NO)]3- and [Fe(CN)4(NO)]2- in aqueous media. The spectra of [Fe(CN)5(NO)]3- and [Fe(CN)4(NO)]2- are consistent with bent six-coordinate {MNO}7 and linear five-coordinate {MNO}7 species, respectively. We have measured the UV-visible and IR spectra that evolve after [Fe(CN)5(NO)]2- is reacted with thiolate. These spectra permit us to assign the molecular structure of the so-called "red product" as [Fe(CN)5(eta1-N-RSNO)]3-. We have followed the decomposition of the [Fe(CN)5(eta1-N-RSNO)]3- by IR. Importantly, there is a 1:1 correspondence between the disappearance of the [Fe(CN)5(eta1-N-RSNO)]3- and the formation of [Fe(CN)5(NO)]3-. Thus, we conclude under the conditions of this study, reduction of [Fe(CN)5(NO)]2- by thiolate takes place via a (dark) inner-sphere mechanism that yields [Fe(CN)5(NO)]3- via homolytic N-S bond cleavage.     

A comparison of the decomposition of electronically excited nitro-containing molecules with energetic moieties C-NO2, N-NO2, and O-NO2

Decomposition of electronically excited nitro-containing molecules with different X-NO(2) (X = C, N, O) moieties has been intensively investigated over the past decades; however, their decomposition behavior has not previously been compared and contrasted. Comparison of their unimolecular decomposition behavior is important for the understanding of the reactivity differences among electronically excited nitro-containing molecules with different X-NO(2) (X = C, N, O) bond connections. Nitromethane (NM), dimethylnitramine (DMNA), and isopropylnitrate (IPN) are used as model molecules for C-NO(2), N-NO(2), and O-NO(2) active moieties, respectively. Ultraviolet lasers at different wavelengths, such as 226, 236, and 193 nm, have been employed to prepare the excited states of these molecules. The decomposition products are then detected by resonance enhanced two photon ionization (R2PI), laser induced fluorescence (LIF) techniques, or single photon ionization at 10.5 eV. NO molecules are observed to be the major decomposition product from electronically excited NM, DMNA, IPN using R2PI techniques. The NO products from decomposition of electronically excited (226 and 236 nm) NM and IPN display similar rotational (600 K) and vibrational distributions [both (0-0) and (0-1) bands of the NO molecule are observed]. The NO product from DMNA shows rotational (120 K) and vibrational distributions (only (0-0) transition is observed) colder than those of NM and IPN. At the 193 nm excitation, electronically excited NO(2) products are observed from NM and IPN via fluorescence detection, while no electronically excited NO(2) products are observed from DMNA. Additionally, the OH radical is observed as a minor dissociation product from all three compounds. The major decomposition pathway of electronically excited NM and IPN involves fission of the X-NO(2) bond to form electronically excited NO(2) product, which further dissociates to generate NO. The production of NO molecules from electronically excited DMNA is proposed to go through a nitro-nitrite isomerization pathway. Theoretical calculations show that a nitro-nitrite isomerization for DMNA occurs on the S(1) surface following a (S(2)/S(1))(CI) conical intersection (CI), whereas NO(2) elimination occurs on the S(1) surface following the (S(2)/S(1))(CI) conical intersection for NM and IPN. The present work provides insights for the understanding of the initiation of the decomposition of electronically excited X-NO(2) energetic systems. The presence of conical intersections along the reaction coordinate plays an important role in the detailed mechanism for the decomposition of these energetic systems.     

Mechanisms of acid decomposition of dithiocarbamates. 3. Aryldithiocarbamates and the torsional effect

The acid decomposition of some p-substituted aryldithiocarbamates (arylDTCs) was observed in 20% aqueous ethanol at 25 degrees C, mu = 1.0 (KCl, for pH > 0). The pH-rate profiles showed a dumbell shape with a plateau where the observed first-order rate constant k(obs) was equal to k(o), the rate constant of the decomposition of the dithiocarbamic acid species. The acid dissociation constants of the dithiocarbamic acids (pK(a)) and their conjugate acids (pK(+)) were calculated from the pH-rate profiles. Comparatively, k(o) was more than 10(4)-fold faster than alkyldithiocarbamates (alkDTCs) with similar pK(N) (the acid dissociation constant of the parent amine). It was observed that the values of pK(a) and pK(+)were 5 and 8 units of pK, respectively, higher than the expected values from the pK(N) of alkylDTCs. The higher values were attributed to the inhibition of the delocalization of the nitrogen electron pair into the benzene ring because of the strong electron withdrawal effect of the thiocarbonyl group. Comparison of the activation parameters showed that the rate acceleration was due to a decrease in the enthalpy of activation. Proton inventory indicated the existence of a multiproton transition state, and it was consistent with an S to N proton transfer through a water molecule. There are two hydrogens contributing to a secondary SIE, and there are also two protons that are being transferred at the transition state to form a zwitterion followed by fast C-N bond cleavage. The mechanism could also be a concerted asynchronic process where the N-protonation is more advanced than the C-N bond breakdown. The kinetic barrier is similar to the torsional barrier of thioamides, suggesting that the driving force to reach the transition state is the needed torsion of the C-N bond that inhibits the resonance with the thiocarbonyl group and the aromatic moiety, increasing the basicity of the nitrogen and making the proton transfer thermodynamically favorable.     

S-亚硝基-N-乙酰基-D,L-青霉胺二肽分子中S—NO键断裂能的测定

利用滴定量热技术并结合适当的热力学循环测定了乙腈溶液中7个S-亚硝基-N-乙酰基-D,L-青霉胺二肽化合物中S—NO键的异裂能和均裂能, 其能量范围分别为234.5—246.2 kJ/mol和101.6—122.1 kJ/mol. 结果表明, 所研究的亚硝基硫醇化合物更容易通过S—NO键的均裂释放NO自由基(NO·). 通过热力学循环对7个亚硝基硫醇化合物自由基负离子中S—NO键的异裂能和均裂能进行估算, 能量范围分别为19.2—35.5 kJ/mol和-4.2—22.6 kJ/mol, 表明这些自由基负离子在室温下不稳定, 容易通过S—NO键的异裂释放出NO-.     

PROPERTIES OF SILK FIBROIN/POLY(ETHYLENE GLYCOL)400 BLEND FILMS

The blend film of silk fibroin (SF) and poly(ethylene glycol)400 (PEG400) with a blend ratio of 2/1 (wt/wt) wasprepared simply by dropping a little PEG400 into the SF solution and then casting the mixed aqueous solution at 50℃. Theresulting film exhibited much better mechanical properties in the dry and wet state than SF itself, owing to theconformational change of SF in the blends from the random coil to the β-sheet structure and intermolecular hydrogen bondformation between SF and PEG400. Thermogravimetric analysis showed that the initial thermal decomposition temperatureof the blend film was 170℃, which was 80℃ lower than that of SF (250℃) and 20℃ higher than that of PEG400 (150℃),and indicated a Strong interaction between two components of the blend. No crystalline peaks were observed in the X-raydiffraction curve of the blend film. Cell culture test showed that SF/PEG400 was a suitable substrate for the growth of humanumbilical vein endothelial cells (HUVEC).     

Endgroup-assisted siloxane bond cleavage in the gas phase

Unimolecular dissociation of H(2)N(CH(2))(3)SiOSi(CH(2))(3)NH(3)(+) generates SiC(5)H(16)NO(+) and SiC(5)H(14)N(+). The formation of SiC(5)H(16)NO(+) involves dissociation of a Si[bond]O bond and formation of an O[bond]H bond through rearrangement. The fragmentation mechanism was investigated utilizing ab initio calculations and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry in combination with hydrogen/deuterium (H/D) exchange reactions. Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) studies of the fully deuterated ion D(2)N(CH(2))(3)SiOSi(CH(2))(3)ND(3)(+) provided convincing evidence for a backbiting mechanism which involves hydrogen transfer from the terminal amine group to the oxygen to form a silanol-containing species. Theoretical calculations indicated decomposition of H(2)N(CH(2))(3)SiOSi(CH(2))(3)NH(3)(+) through a backbiting mechanism is the lowest energy decomposition channel, compared with other alternative routes. Two mechanisms were proposed for the fragmentation process which leads to the siloxane bond cleavage and the SORI-CID results of partially deuterated precursor ions suggest both mechanisms should be operative. Rearrangement to yield a silanol-containing product ion requires end groups possessing a labile hydrogen atom. Decomposition of disiloxane ions with end groups lacking labile hydrogen atoms yielded product ions from direct bond cleavages.     

Reconstructing Hydrogen Bond Network Enables High Voltage Aqueous Zinc-Ion Supercapacitors

High-voltage aqueous rechargeable energy storage devices with safety and high specific energy are hopeful candidates for the future energy storage system. However, the electrochemical stability window of aqueous electrolytes is a great challenge. Herein, inspired by density functional theory (DFT), polyethylene glycol (PEG) can interact strongly with water molecules, effectively reconstructing the hydrogen bond network. In addition, N, N-dimethylformamide (DMF) can coordinate with Zn²⁺, assisting in the rapid desolvation of Zn²⁺ and stable plating/stripping process. Remarkably, by introducing PEG400 and DMF as co-solvents into the electrolyte, a wide electrochemical window of 4.27 V can be achieved. The shift in spectra indicate the transformation in the number and strength of hydrogen bonds, verifying the reconstruction of hydrogen bond network, which can largely inhibit the activity of water molecule, according well with the molecular dynamics simulations (MD) and online electrochemical mass spectroscopy (OEMS). Based on this electrolyte, symmetric Zn cells survived up to 5000 h at 1 mA cm⁻², and high voltage aqueous zinc ion supercapacitors assembled with Zn anode and activated carbon cathode achieved 800 cycles at 0.1 A g⁻¹. This work provides a feasible approach for constructing high-voltage alkali metal ion supercapacitors through reconstruction strategy of hydrogen bond network.     

Thermal analysis of monothiocarbonohydrazones

Thermal analysis of monothiocarbonohydrazones carried out using DTA and TG techniques show that they all decompose exothermally soon after melting. The exothermal decomposition is attributed to the presence of a hydrazino (NN) bond. The decomposition process has been studied by examining the products of decomposition of benzaldehydethio-carbonohydrazone. Nitrogen, ammonia, hydrogen sulphide, benzonitrile, thiobenzaldehyde, 2,4,6-triphenyl-s-triazine and complex condensation products containing CN linkages, have been found to be the main products of decomposition. A probable mechanism of decomposition is proposed based on the formation of these products, assuming the homolytic cleavage of the NN bond as a primary step.     

2,4-二硝基咪唑铅配合物Pb(DNI)2(H2O)4的热分解

应用TG, TG-DSC-FTIR-MS联用技术和热裂解原位RSFT-IR技术研究了2,4-二硝基咪唑铅(PDNI)的热分解机理. 结果表明, PDNI在102 ℃附近脱除分子内配位水, 生成Pb(DNI)2; 在284 ℃附近C—NO2断裂, 生成NO2, 咪唑环开环, 伴随发生强烈的氧化放热反应, 生成CO2, N2O和铅盐与咪唑残余基团形成的复杂混合物或多聚烃类化合物; 在300～400 ℃范围内, PDNI继续缓慢分解, 生成CO2, N2O和Pb[NCO]2; 升温至410 ℃以上, PDNI分解生成CO和Pb[CN]2.     

Influence of thiolate ligands on reductive N-O bond activation. Probing the O2(-) binding site of a biomimetic superoxide reductase analogue and examining the proton-dependent reduction of nitrite

Nitric oxide (NO) is frequently used to probe the substrate-binding site of "spectroscopically silent" non-heme Fe(2+) sites of metalloenzymes, such as superoxide reductase (SOR). Herein we use NO to probe the superoxide binding site of our thiolate-ligated biomimetic SOR model [Fe(II)(S(Me(2))N(4)(tren))](+) (1). Like NO-bound trans-cysteinate-ligated SOR (SOR-NO), the rhombic S = 3/2 EPR signal of NO-bound cis-thiolate-ligated [Fe(S(Me(2))N(4)(tren)(NO)](+) (2; g = 4.44, 3.54, 1.97), the isotopically sensitive ν(NO)(ν((15)NO)) stretching frequency (1685(1640) cm(-1)), and the 0.05 ? decrease in Fe-S bond length are shown to be consistent with the oxidative addition of NO to Fe(II) to afford an Fe(III)-NO(-) {FeNO}(7) species containing high-spin (S = 5/2) Fe(III) antiferromagnetically coupled to NO(-) (S = 1). The cis versus trans positioning of the thiolate does not appear to influence these properties. Although it has yet to be crystallographically characterized, SOR-NO is presumed to possess a bent Fe-NO similar to that of 2 (Fe-N-O = 151.7(4)°). The N-O bond is shown to be more activated in 2 relative to N- and O-ligated {FeNO}(7) complexes, and this is attributed to the electron-donating properties of the thiolate ligand. Hydrogen-bonding to the cysteinate sulfur attenuates N-O bond activation in SOR, as shown by its higher ν(NO) frequency (1721 cm(-1)). In contrast, the ν(O-O) frequency of the SOR peroxo intermediate and its analogues is not affected by H-bonds to the cysteinate sulfur or other factors influencing the Fe-SR bond strength; these only influence the ν(Fe-O) frequency. Reactions between 1 and NO(2)(-) are shown to result in the proton-dependent heterolytic cleavage of an N-O bond. The mechanism of this reaction is proposed to involve both Fe(II)-NO(2)(-) and {FeNO}(6) intermediates similar to those implicated in the mechanism of NiR-promoted NO(2)(-) reduction.     

Synthesis of a stereochemically defined 1,2-diazetine N,N'-dioxide and a study of its thermal decomposition

Diazetine dioxide 1a has been synthesized in a single step via oxidation of meso-2,3-diphenyl-1,2-ethanediamine with dimethyldioxirane, albeit in low yield (7%). Thermal decomposition of 1a afforded predominantly either trans-stilbene or diphenyl glyoxime depending on solvent, temperature, and the presence of an amine catalyst. Reaction in chloroform at 69 degrees C favored elimination of NO and formation of trans-stilbene. The stereospecific formation of trans-stilbene suggests a mechanism of decomposition in which C-N bond cleavage leads to a diradical intermediate stabilized by the phenyl group. Bond rotation followed by cleavage of the second C-N bond accounts for the trans-stilbene. At 25 degrees C in chloroform, while trans-stilbene was still the major product, some diphenyl glyoxime was also observed (4% yield). However, 1a as a solution in chloroform in the presence of Et3N, or 1a as a solution in DMSO-d6, afforded predominantly diphenyl glyoxime. These results are interpreted in terms of two closely competing reactions subject to the effects of entropic contributions.