Competitive intramolecular carbenoid reactions of pyrrole derivatives

Rhodium(II) acetate-catalyzed decomposition of 1-diazo-3-phenyl-4-(pyrrol-1-yl)-butan-2-one and its p-methoxy derivative resulted in their intramolecular cyclization to form the 6-phenyl-5,6-dihydroindolizin-7(8H)-ones and 1-(pyrrol-1-yl)methylindan-3-ones as major and minor products respectively in high yields (77–89%). The p-nitrophenyldiazo compound cyclized exclusively to the 5,6-dihydroindolizin-7(8H)-one in 76% yield.     

Competitive reactions of organophosphorus radicals on coke surfaces

The efficacy of organophosphorus radicals as anticoking agents was subjected to a computational study in which a representative set of radicals derived from industrially relevant organophosphorus additives was used to explore competitive reaction pathways on the graphene-like coke surface formed during thermal cracking. The aim was to investigate the nature of the competing reactions of different organophosphorus radicals on coke surfaces, and elucidate their mode of attack and inhibiting effect on the forming coke layer by use of contemporary computational methods. Density functional calculations on benzene and a larger polyaromatic hydrocarbon, namely, ovalene, showed that organophosphorus radicals have a high propensity to add to the periphery of the coke surface, inhibiting methyl radical induced hydrogen abstraction, which is known to be a key step in coke growth. Low addition barriers reported for a phosphatidyl radical suggest competitive aptitude against coke formation. Moreover, organophosphorus additives bearing aromatic substituents, which were shown to interact with the coke surface through dispersive π-π stacking interactions, are suggested to play a nontrivial role in hindering further stacking among coke surfaces. This may be the underlying rationale behind experimental observation of softer coke in the presence of organophosphorus radicals. The ultimate goal is to provide information that will be useful in building single-event microkinetic models. This study presents pertinent information on potential reactions that could be taken up in these models.     

Competitive reactions in solutions of DNA-containing polyelecrtolyte complexes

The reaction of competitive binding of two polyanions - DNA and synthetic fluorescence-tagged poly(methacrylate) (PMA^*) - with the quencher polycation poly(N-ethyl-4-vinylpyridinium) (PEVP) was studied by fluorescence quenching technique. It is found that ability of DNA to displace PMA^* from water-soluble nonstoichiometric interpolyelectrolyte complex (NPEC) formed by PMA^* and PEVP - NPEC(PMA^*-PEVP) - and to form water-soluble NPEC(DNA-PEVP) can be determined by both a parameter Ψ = P_PMA+/P_pevp, where P_pma+ and P_pevp are the degrees of polymerization of PMA^* and PEVP, respectively, and by the nature and concentration of alkali and alkali earth metal cations. According to their ability to shift the equilibrium to the right the cations can be arranged in the series Ca²⁺ > Mg²⁺ >> Na⁺ > K⁺ > Li⁺. The data obtained are in accordance with the differences in the selectivity of alkali and alkali earth cations binding with competitive polyanions.     

Mechanism of photodecomposition of spiropyrans

Spectrophotometric and ESR measurements have been made in a study of photodecomposition and thermodecomposition reactions of spiropyrans (SPs) of the indoline and oxaindan series of solutions and in the crystalline state, at various temperatures. From an analysis of the experimental data, involving certain concepts developed previously regarding the mechanisms of photochemical reactions of SPs, it has been concluded that the most probable reactions of photodecomposition and thermodecomposition of SPs are -rupture of a bond and the formation of a chemically active biradical, as well as irreversible reactions of SP ion radicals that arise as a consequence of phototransfer and dark transfer of electrons in associates of various species of the SP.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 5, pp. 539–545, September–October, 1989.     

Competitive intermolecular pericyclic reactions of free and complexed cyclopentyne

Intermolecular competition for cyclopentyne by different alkenes supports the hypothesis that organolithium-promoted decomposition of precursors to cyclopentyne affords one or more lithium-ion-complexed species. Competition reactions with mixtures of 2,3-dihydropyran do not clearly differentiate between complexed and unencumbered (free) forms of cyclopentyne, but those involving spirodiene 2 and cyclohexene do. Remarkably, the cycloaddition reactions of free cyclopentyne are not diffusion-controlled despite its high reactivity.     

A 193 nm laser photofragmentation time-of-flight mass spectrometric study of chloroiodomethane

The photodissociation dynamics of chloroiodomethane (CH2ICl) at 193 nm has been investigated by employing the photofragment time-of-flight (TOF) mass spectrometric method. Using tunable vacuum ultraviolet undulator synchrotron radiation for photoionization sampling of nascent photofragments, we have identified four primary dissociation product channels: CH2Cl + I(2P(1/2))/I(2P(3/2)), CH2I + Cl(2P(1/2))/Cl(2P(3/2)), CHI + HCl, and CH2 + ICl. The state-selective detection of I(2P(3/2)) and I(2P(1/2)) has allowed the estimation of the branching ratio for I(2P(1/2)):I(2P(3/2)) to be 0.73:0.27. Theoretical calculations based on the time-dependent density-functional theory have been also made to investigate excited electronic potential-energy surfaces, plausible intermediates, and transition structures involved in these photodissociation reactions. The translation energy distributions derived from the TOF measurements suggest that at least two dissociation mechanisms are operative for these photodissociation processes. One involves the direct dissociation from the 2 1A' state initially formed by 193 nm excitation, leading to significant kinetic-energy releases. For the I-atom and Cl-atom elimination channels, the fragment kinetic-energy releases observed via this direct dissociation mechanism are consistent with those predicted by the impulsive dissociation models. Other mechanisms are likely predissociative or statistical in nature from the lower 1 1A' and 1 1A' states and/or the ground X 1A' state populated by internal conversion from the 2 1A' state. On the basis of the maximum kinetic-energy release for the formation of CH2Cl + I(2P(1/2)), we have obtained a value of 53+/-2 kcal/mol for the 0 K bond dissociation energy of I-CH2Cl. The intermediates and transition structures for the CHI + HCl and CH2 + ICl product channels have been also investigated by ab initio quantum calculations at the MP2(full)/6-311G(d) and B3LYP(full)/6-11G(d) levels of theory. The maximum kinetic-energy releases observed for the CHI + HCl and CH2 + ICl channels are consistent with the interpretation that the formation of CHI and CH2 in their ground triplet states is not favored.     

Competitive reactivity of vinylchalcogenyl radicals in their reactions with acetylene

The reactivities of vinythinyl and vinulselenyl radicals generated high temperature gasphase reactions of acetylene with diethyl disulfide and diethyl selenide, respectively, were compared. The co-pyrolysis of u mixture of these reagents at 410–510 °C results in a mixture of thionphene and selenophene. The vinylselenyl radical is more reactive; in addition, it appears to stabilize vinylthiyl radicals. Owing to this, co-thermolysis of diethyl disulfide, diethylselenide, and acetylene at 510 °C allows one to obtain simultaneously thiophene and selenophene in 91.0 and 92.5 % yields, respectively.     

Competitive Diels-Alder reactions: cyclopentadiene and phospholes with butadiene

Diene-dienophile competing Diels-Alder reaction pathways of cyclopentadiene, 1H-, 2H- and 3H-phospholes with butadiene were explored at the B3LYP level using 6-31G(d) and 6-311+G(d,p) basis sets, and at the CCSD(T)/6-31G(d)//B3LYP/6-31G(d) level. Activation barriers show that cyclopentadiene favors a diene rather than a dienophile conformation. Pathways 1 and 2 (A and B) corresponding to butadiene as the diene and dienophile are predicted to be highly competitive in the case of 1H-phosphole. Secondary orbital interactions and the preferable bispericyclic nature of transition states are responsible for the stability of endo transition states. The study indicates that some of the transition states are bispericyclic and most of them are highly asynchronous. The reactions require a lower activation energy when the conversion of weak C=P to C-P occurs in the case of 2H- and 3H-phospholes. The high stability of the products resulting via path 1 can be attributed to the less strain in the bicyclo[4.3.0]nonadiene skeleton compared to the norbornene derivatives obtained from path 2. Activation and reaction energy values for these Diels-Alder reaction pathways are compared with the values reported for the [4+2] cyclodimerizations of each of the reactants to examine the likelihood of cyclodimerizations along these pathways.     

Competitive nucleophilic substitution reactions of methylsulfonyl and nitro derivatives of polychloropyridines

2,6-Di(methylsulfonyl)-3,4,5-trichloropyridine (II) and 4-nitro-2,6-di(methyl-sulfonyl)-3, 5-dichloropyridine (IX) were synthesized, and their reactions with nucleophilic reagents were studied. It was found that the methylsulfonyl groups are replaced on reaction with alkali and sodium alkoxides; on reaction with amines, the chlorine atoms are replaced in the case of II and the nitro group is replaced in the case of IX. The nitro and methylsulfonyl groups are simultaneously replaced in the reaction of excess sodium methoxide with IX.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 802–806, June, 1975.     

Competitive reactions in solutions of the complex of chitosan and DNA

The interaction of water-soluble modified chitosan with a polymethacrylate anion and DNA in aqueous and water-salt solutions is studied via the methods of fluorescence quenching with the use of a pyrenyl-labeled polyacid and the intercalated dye ethidium bromide. After passage from neutral to weakly acidic solutions, the stability of the polysaccharide complex with the polymethacrylate anion significantly increases, but, in the case of the DNA-containing complex, stability remains almost invariable. In spite of a much higher stability of complexes against salt in weakly ionized poly(methacrylic acid), the equilibrium of the competitive binding of chitosan shifts toward complexation with DNA, a result that is indicative of the dominant role of electrostatic interactions in complex formation. The capability for sharp and reversible variation in the state of the system in weakly acidic media may become the starting point for development of biotechnological systems for separation of biological mixtures and creation of vectors for delivery of genetic material that are based on cationic biodegradable polysaccharides.     

Gas and solution phase chloroiodomethane short-time photodissociation dynamics in the B-band absorption

Resonance Raman spectra were obtained within and to the red of the B-band absorption spectrum of gas phase chloroiodomethane and chloroiodomethane in cyclohexane solvent. The spectra show the fundamental and overtones of the nominal C---I stretch (nν₅) and combination bands of the CH₂ wag (ν₃), I---C---Cl bend (ν₆), and the CH₂ scissor (ν₂) fundamentals with the C---I stretch bands (nν₅). The chloroiodomethane B-band short-time photodissociation dynamics have significant substituent effects relative to the B-band of iodomethane due to the presence of the C---Cl chromophore n(X) → σ^* (C---X) transitions ≈170 nm that are close to the B-band absorption of chloroiodomethane but absent in iodomethane.     

Competitive reactions of coordinated carbon monoxide and methyl isocyanide with amines

The relative reactivities of CO and CNR ligands with CH₃NH₂ were investigated in complexes which contained both ligands. Like (C₅H₅Fe(CO)₃⁺; the (C₅H₅)Fe(CO)₂(CNCH₃)⁺ complex reacts with CH₃NH₂ to give the carbamoyl complex (C₅)Fe(CO)(CNCH₃)(CONHCH₃); this is a readily reversible reaction. In contrast, (C₅H₅)Fe(CO)(CNCH₃)₂⁺ reacts with CH₃NH₂ to give the amidinium or carbene complex, (C₅H₅)Fe(CO)(CNCH₃)[C(NHCH₃)₂]⁺]. In a slow reaction, (C₅H₅)Fe(PPh₃)(CO)(CNCH₃)⁺ forms the amidinium complex, (C₅H₅)Fe(PPh₃)(CO)[C(NHCH₃)₂]⁺. Factors that affect the site of CH₃NH₂ reaction are discussed. The complexes have been characterized by IR and NMR spectroscopy; a variable temperature NMR study of (C₅H₅)Fe(CO)(CNCH₃)[C(NHCH₃)₂]⁺ indicates restricted rotation around the CN bonds of the amidinium ligand.     

Vapor-phase photodecomposition of chloropicrin (trichloronitromethane)

Under simulated environmental (atmospheric) conditions, chloropicrin vapor was readily converted in light to phosgene, which remained stable, and nitrosyl chloride which was subsequently photolyzed to nitric oxide and chlorine; a portion of the nitric oxide was oxidized to nitrogen dioxide and dinitrogen tetroxide. The unexpected dependence of the reaction upon oxygen, the initial photoproducts, and the incorporation of ¹⁸O when the photolysis occurred in the presence of ¹⁸O₂ suggest an intramolecular rearrangement involving a trioxazole N-oxide.     

Nifedipine: Differential pulse polarography and photodecomposition

A method for the differential-pulse polarographic determination of nifedipine has been developed, based on the electrochemistry of the aromatic nitro group in the drug. Polarography has also been used in studies of the photodegradation of nifedipine, which is highly light-sensitive under ultraviolet light and artificial daylight.     

BrONO2光解反应的理论研究

采用密度泛函方法,研究了大气臭氧层主要破坏物BrONO2的光解反应机理,在UB3LYP/6-311++G**水平上优化了反应物、产物、中间体和过渡态的几何构型,并在UQCISD(T)/6-311++G**水平上计算了单点能量,为了确证过渡态的真实性,在UB3LYP/6-311++G**水平上进行了内禀反应坐标(IRC)计算和频率分析.研究结果表明,BrONO2的光解反应有两条反应通道,其中生成BrO+NO2的反应活化能较小(14.89 kJ·mol-1),较易发生.     

Competitive reactions model for the pyrolysis of lignocellulose: A critical study

The “two competitive reactions” model for the pyrolysis of lignocellulosic materials finds support in the dependence of charcoal yield on thermal conditions, particularly the reaction temperature. This paper reports a critical study of the model on the basis of our experimental data on the pyrolysis of cellulose and wood. The theoretical bases of the model are briefly reviewed. Experimental results and model predictions — using kinetic parameters calculated from our own data — are compared over a wide range of thermal conditions. The implications for reactor design purposes are briefly discussed.