The effect of solvent on the reactions of allyltin (IV) compounds with singlet oxygen, 4-phenyl-1,2,4-triazoline-3,5-dione and diethyl azodicarboxylate

The effect of the influence on the increase of polarity of the solvent on the selectivity and rate of metalloene reactions of different allyltin compounds with singlet oxygen, 4-phenyl-1,2,4-triazoline-3,5-dione (TD) and diethyl azodicarboxylate has been studied.     

The fluoride ion effect in the reactions of singlet oxygen with enols

The fluoride ion effect in the reactions of enolic systems with singlet oxygen has been investigated. β-Dicarbonyl compounds yielded 1,2,3-tricarbonyl derivatives, some of which underwent further hydration, whereas α-diketones suffered oxidative decarboxylation to give open-chain aldehydo-acids or keto-acids.     

Temperature and solvent control of the stereoselectivity in the reactions of singlet oxygen with oxazolidinone-substituted enecarbamates

Oxazolidinone-functionalized enecarbamates react stereoselectively with singlet oxygen to give methyldesoxybenzoin (MDB) in moderate to high enantiomeric excess. The stereochemical outcome depends on the E/Z substrate geometry, temperature, and solvent variables. The analysis of the differential activation parameters suggests a large contribution from the entropy term in determining the enantioselectivity. We demonstrate the utility of the temperature and solvent variables in determining the degree of the photochemical kinetic resolution of the enecarbamates; for example, in the photooxygenation at -70 degrees C in methanol, MDB may be obtained in methanol.     

The effect of solvent on tetracene oxidation by singlet molecular oxygen (Δg): aspects of specific solvation

It is evident that the solvent effect on tetracene oxidation by singlet molecular oxygen is predominantly conditioned by a specific solvation of the reaction intermediate in aprotic media that is assumed to be a state with a stressed anionic center.     

The kinetics and mechanism of the substitution reactions oftrans-tetracyanodioxotungstate(IV) ions with monodentate ligands

Summary The kinetics of the substitution reactions of the protonated forms oftrans-tetracyanodioxotungstate(IV) ions with pyridine and thiocyanate ions were studied. The kinetic results were compared with those of the reactions with N ₃ ^– and F^– ions and a linear free energy relationship was obtained which was interpreted as additional evidence for a dissociative mechanism. Infrared data for this type of complex are reported and discussed in terms of thetrans influence of various monodentate ligands.     

New insights on the Lewis acidity of diorganolead(IV) compounds: Diphenyllead(IV) complexes with N,O,S-chelating ligands

The reactions of PbPh₂(OAc)₂ with alkylglyoxylate thiosemicarbazones (HRGTSC, R = Et, Bu) afforded complexes of the type [PbPh₂(GTSC)] · H₂O, [PbPh₂(RGTSC)₂] and [PbPh₂Cl(BuGTSC)]. The structures of HRGTSC (R = Me, Et, Bu), [PbPh₂(OAc)(RGTSC)](R = Me, Et, Bu), [PbPh₂Cl(BuGTSC)] and [PbPh₂(GTSC)] · H₂O have been studied by X-ray diffraction. [PbPh₂(OAc)(RGTSC)] and [PbPh₂(GTSC)] · H₂O have [PbC₂NO₃S] kernels and the coordination sphere of the metal is pentagonal bipyramidal. [PbPh₂Cl(BuGTSC)] has a [PbC₂NOSCl] kernel and the coordination geometry around lead is pentagonal bipyramidal with one vacant site. Analysis of the bond distances in [PbPh₂(GTSC)] · H₂O suggests a significant affinity between diphenyllead(IV) and carboxylate donor groups, supporting a borderline acidic character for this organometallic cation. ¹H and ¹³C NMR spectra in DMSO-d₆ suggest the partial dissociation of the acetate in [PbPh₂(OAc)(RGTSC)] solutions and indicate some differences in the coordination mode of the two RGTSC⁻ ligands in [PbPh₂(RGTSC)₂] complexes.     

Stereoelectronic and solvent effects on the allylic oxyfunctionalization of alkenes with singlet oxygen

The factors that control the stereochemistry of sensitized photooxygenation of alkenes via singlet oxygen (ene reaction) are selectively reported. We also introduce our most recent stereoelectronic effects on the singlet oxygen-ene reaction. The origin of site selectivity and solvent-dependent stereoselectivity in this classical ene reaction with simple as well as functionalized alkenes is highlighted. These studies and other similar studies have enhanced substantially the utility of singlet oxygen in the synthesis of natural and non-natural products.     

Merbromin (mercurochrome)--a photosensitizer for singlet oxygen reactions.

Merbromin, produced in many countries and used world wide as an antiseptic under the trademark "mercurochrome", is shown to be an efficient sensitizer for type II (singlet oxygen) photo-oxygenations by using 2-methyl-2-butene, (+)-limonene, (+)-alpha-pinene, alpha,alpha'-dimethylstilbenes and (--)-L-methionine as oxygen acceptors. Type I photo-oxygenations are negligible. An estimate of the quantum yield of singlet oxygen formation by merbromin in methanol gives a value of about 0.1.     

A comparison of hydrogen bonding solvent effects on the singlet oxygen reactions of allyl and vinyl sulfides, sulfoxides, and sulfones

The singlet oxygen (¹Δ_g) photooxidations of 2-methyl-3-phenylthio-2-butene (1a), 1-[(4-nitrophenyl)thio]-2,3-dimethyl-2-butene (2c), 2-methyl-3-phenylsulfinyl-2-butene (3), 2-methyl-3-phenylsulfonyl-2-butene (6), and 1-[(4-nitrophenyl)sulfonyl]-2,3-dimethyl-2-butene (7c) were conducted in the following deuterated solvents: acetonitrile, benzene, chloroform, methanol, or methanol/water mixture. In each case the ene allylic hydroperoxide products and/or the [2+2] cycloaddition products were quantified and inspected for possible hydrogen bonding induced differences in product selectivity and regiochemistry. After comparison to literature values for related substrates, the results indicate that only photooxidations of vinyl sulfides are susceptible to hydrogen bonding solvent effects.     

Kinetics of the reactions of singlet molecular oxygen (O21Δg) with organic compounds in the gas phase

The reactions of singlet molecular oxygen (O₂¹Δ_g) with a series of organic compounds have been studied in the gas phase at 298°K. The concentration of singlet molecular oxygen was determined by titration with 2,5-dimethylfuran. The titration technique was checked using a photoionization technique. Absolute rate constants were measured on the basis of the loss of organic reactant and, in some cases, of singlet molecular oxygen. It was found that the usual method of producing singlet molecular oxygen in the gas phase can also, under some conditions, allow reactive species other than singlet molecular oxygen to enter the reactor, leading to serious errors in the determination of rate constants. This problem was eliminated by carrying out the rate measurements in the presence of a small amount of nitrogen dioxide a radical scavenger.     

Reactions of singlet oxygen with polyisoprene and model compounds

Results from the study of singlet oxygen reaction with cis-1,4-polyisoprene and with various models are compared. The nature of the products and their reactivity are discussed for two kinds of models, namely oligodienes and trisubstituted mono-olefins with various substituent length. The reactivity of the double bond decreases with lengthening of the alkyl substituents but the reactivity of 4-methyl-4-octene is close to that of the polymer. The hydroperoxidation of a unit in an oligodiene does not deactivate significantly the adjacent units towards singlet oxygen addition. The reactivity of the allylic hydrogen a, b, and c in the olefin

depends on both the position and the length of the substituents and follows the sequences K _c > k _b > k _a. The principle of syn-ene addition can be extrapolated to polyisoprene. The rate constant determined for the cis-1,4-polyisoprene ¹O₂ reaction allows the conclusion that the probability of reaction of ¹O₂ with the solid polymer may be high. Similarity in the behaviour of polymer and models in this reaction allows for the estimation of the reactivity of other unsaturated or reactive polymers towards ¹O₂.     

Electroenzymatic reactions with oxygen on laccase-modified electrodes in anhydrous (pure) organic solvent

The electroenzymatic reactions of Trametes hirsuta laccase in the pure organic solvent dimethyl sulfoxide (DMSO) have been investigated within the framework for potential use as a catalytic reaction scheme for oxygen reduction. The bioelectrochemical characteristics of laccase were investigated in two different ways: (i) by studying the electroreduction of oxygen in anhydrous DMSO via a direct electron transfer mechanism without proton donors and (ii) by doing the same experiments in the presence of laccase substrates, which display in pure organic solvents both the properties of electron donors as well as the properties of weak acids. The results obtained with laccase in anhydrous DMSO were compared with those obtained previously in aqueous buffer. It was shown that in the absence of proton donors under oxygenated conditions, formation of superoxide anion radicals is prevented at bare glassy carbon and graphite electrodes with adsorbed laccase. The influence of the time for drying the laccase solution at the electrode surface on the electroreduction of oxygen was studied. Investigating the electroenzymatic oxidation reaction of catechol and hydroquinone in DMSO reveals the formation of various intermediates of the substrates with different electrochemical activity under oxygenated conditions. The influence of the content of aqueous buffer in the organic solvent on the electrochemical behaviour of hydroquinone/1,4-benzoquinone couple was also studied.     

Chemiluminescence associated with singlet oxygen reactions with amino acids, peptides and proteins

Low level chemiluminescence (CL) is observed after protein oxidation mediated by singlet oxygen produced in Rose Bengal (RB) irradiation. This CL lasts for several minutes after the end of the photolysis. In this work, the mechanism of the process was assessed from the spectral characteristics of the CL and the effect of antioxidants (Trolox or ascorbate), Ebselen (a compound with peroxidase-like activity), azide (a singlet oxygen scavenger) and D2O, added prior to or after RB irradiation. It is concluded that most of the light emission is due to formation of excited states generated in the decomposition of peroxides and/or hydroperoxides accumulated during the photolysis. Experiments carried out in the presence of several amino acids (Cys, Met, His, Tyr and Trp) and di- and tripeptides suggest that peroxides (and/or hydroperoxides) of Trp residues are mainly responsible for the CL observed after singlet oxygen-mediated protein oxidation. The much weaker CL observed after the oxidation of proteins without Trp residues supports this conclusion. A comparison of the results obtained employing free Trp, Ala-Trp and Trp-Ala dipeptides, Ala-Trp-Ala tripeptide and Trp-containing proteins supports the conclusion that blocking the amino group of the Trp moiety strongly increases the efficiency of the chemiluminescent process, producing approximately 2.5x10(-8) photons per oxidized Trp group in Ala-Trp. A mechanism comprising two chemiluminescent oxidation pathways of Trp residues is proposed to explain the results.     

Catalysis of reactions of allyltin compounds and organotin phenoxides by lithium perchlorate

The effect of increase of polarity of the solvent binary mixture methanol-benzene and acetonitrile-chloroform on the selectivity and the rate of metalloene reaction of different allyltin compounds with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD), diethyl azodicarboxylate (DEAD) and singlet oxygen was studied. The more polar solvent favored the production of the M-ene product. Analogous comparative studies were carried out in Et₂O and 4 mol dm⁻³ solutions of LiClO₄ in diethyl ether. All studied reactions were strongly catalysed by LiClO₄. Physicochemical studies were carried out in purpose to explain the catalytic effect of LiClO₄ on the aforementioned reactions. In case of singlet oxygen and diethyl azodicarboxylate it was presumably a result of facilitation of the formation of the polar intermediate by the ionic medium. Whereas, in case of PTAD the mentioned previously effect could be associated with lowering its LUMO by association with lithium. The analogous catalytic effect of LiClO₄ was also observed for reactions of organotin phenoxides with DEAD and bis(trichloroethyl) azodicarboxylate leading to corresponding ring-aminated phenols in excellent yield, and with diethyl acetylenedicarboxylate giving a mixture of corresponding vinyl ethers and ring ethenylated phenols. Organotin phenoxides were distinctly more active than the corresponding phenols.     

Solvent effect and temperature dependence in the interaction of singlet oxygen with α-phenyl-N-tert-butyl nitrone

The solvent effect on the quenching of singlet oxygen by -phenyl-N-tert-butyl-nitrone /PBN/ has been investigated by laser flash photolysis technique registrating luminescence kinetics of¹O₂. The values of the rate constant /k_q/ of the quenching were at 293 K: /9.0±0.4/×10⁶, /4.4±0.3/×10⁶ and /18.3±0.5/×10^{6 –}M^–1 s^–1 in toluene, chloroform and acetonitrile, respectively. The rate constant for the chemical interaction between¹O₂ and PBN, was k_r<1×10⁵ M^–1 s^–1k_q independently of the solvent. At temperatures between 223 and 293 K in toluene E_q=0.4±0.4 kJ mol^–1.     

Heterogeneous reactions of singlet molecular oxygen with solid polymers

The effect of gas-phase singlet molecular oxygen (¹ΔO₂) upon several solid polymers was investigated by using electron paramagnetic resonance, infrared spectroscopy, and chemical detection techniques. The study was performed by use of ¹ΔO₂ produced by microwave discharge. The application of this method to polymer studies was closely examined. The saturated-chain polymers polystyrene, polyurethane, and polyethylene were found to be inert within the experimental conditions to reaction with ¹ΔO₂, while the unsaturated polymers cis-polybutadiene, trans-polybutadiene, and trans-polyisoprene were found to react quite readily in an apparently surface or near-surface limited reaction to produce hydroperoxide and/or peroxide groups. The introduction by homogeneous mixing of some known metal-chelate ¹ΔO₂ quenchers into the polymer trans-polyisoprene appeared to significantly decrease the rate of oxidation observed.     

Theoretical study of the Diels-Alder reactions between singlet (1Delta g) oxygen and acenes

The G3(MP2) method has been employed to study the 1,4-addition reactions between singlet oxygen and five acenes, including benzene, naphthalene, anthrecene, tetracene, and pentacene. In all, nine pathways between O(2) and the five acenes have been investigated. Our calculated results indicate that all nine pathways are concerted and exothermic and that the most reactive sites on the acenes are the center ring's meso-carbons. In addition, reactivity increases along the series benzene < naphthalene < anthrecene < tetracene < pentacene. This trend is identical to that of aromaticity for the five acenes. A correlation between reactivity and aromaticity is briefly rationalized with natural bond orbital (NBO) analysis and frontier molecular orbital (FMO) analysis. Furthermore, some experimental kinetics data from the literature supporting the calculated results are cited.     

Combined effect of solvent content, temperature and pH on the chromatographic behaviour of ionisable compounds

The organic solvent content and the pH in the mobile phase are the usual main factors in reversed-phase liquid chromatographic separations, owing to their strong effects on retention and/or selectivity. Temperature is often neglected. However, even in cases where the impact of this factor on selectivity is minor, the reduction in analysis time is still an interesting reason to consider it. In addition, ionisable compounds may exhibit selectivity changes, owing to the interaction of organic solvent and/or temperature with pH. The separation of ionisable compounds (nine diuretics: bendroflumethiazide, benzthiazide, bumetanide, chlorthalidone, furosemide, piretanide, probenecid, trichloromethiazide and xipamide, and two beta-blockers: oxprenolol and propranolol) exhibiting different acid-base behaviour was studied. The compounds were tested in a Zorbax SB C18 column under a wide range of conditions: 25-45% (v/v) acetonitrile, pH 3-7 and 20-50 degrees C. Models considering two factors (organic solvent/pH and temperature/pH), and three factors (organic solvent/temperature/pH) were developed from a previously reported equation, which considers the polarity contributions of solute, stationary and mobile phases. This allowed a comprehensive method to predict the retention of the 11 compounds, the modification of their acid-base behaviour (i.e. determination of protonation constants and shifts of the retention versus pH curves), and the selectivity changes within the studied factor ranges.