Chemical Composition and Reaction Mechanisms for Aged p‐Xylene Secondary Organic Aerosol in the Presence of Ammonia

A laboratory study was carried out to investigate the chemical composition of aged aromatic secondary organic aerosol (SOA) formed from the photoxidation of p‐xylene in the presence of ammonia (NH₃). The experiments were conducted by irradiating p‐xylene/CH₃ONO/NH₃ air mixtures without and with NO in a home‐made smog chamber. The particulate products of aged p‐ xylene SOA in the presence of NH₃ were measured by UV–vis spectrophotometry, attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy, and aerosol laser time‐of‐flight mass spectrometry (ALTOFMS) coupled with the fuzzy C‐means (FCM) clustering algorithm. The experimental results show that NH₃ does not alter the gas–particle partitioning in the photoxidation of p‐xylene without NO and that 2,5‐dimethylphenol is the predominant NH₃‐aged p‐xylene SOA without NO. However, NH₃ has a significant promotional effect on the formation of organonitrogen compounds in the OH‐initiated oxidation of p‐xylene with NO. Organic ammonium salts such as ammonium glyoxylate and p‐methyl ammonium benzoate, which are formed from NH₃ reactions with gaseous organic acids, were detected as the major particulate organonitrogen products of NH₃‐aged p‐xylene SOA with NO. 1H‐Imidazole, 4‐methyl‐1H‐imidazole, and other imidazole products of the heterogeneous reactions between NH₃ and dialdehydes of p‐xylene SOA were newly measured. The possible reaction mechanisms leading to these organonitrogen products are also discussed and proposed. The formation of imidazole products suggests that some ambient particles containing organonitrogen compounds may be the result of this mechanism. The results of this study may provide valuable information for discussing anthropogenic SOA aging mechanisms.     

Formation and Decomposition of Diazo Ethers under Acidic Conditions. Effects of Ethanol Concentration,Acidity, and Temperature on the Ethanolysis of 4‐Methylbenzenediazonium Ions

We investigated the effects of solvent composition, acidity, and temperature on the dediazoniation of 4‐methylbenzenediazonium (4MBD) ions in EtOH/H₂O mixtures by employing a combination of spectrometric and chromatographic techniques. First‐order behavior is found in all solvent composition ranges. HPLC Analyses of the reaction mixtures indicate that three main dediazoniation products are formed depending on the particular experimental conditions. These are 4‐cresol (ArOH), 4‐phenetole (ArOEt), and toluene (ArH). At acidities (defined as ?log [HCl])<2, the main dediazoniation products are the substitution products ArOH and ArOEt but upon decreasing the acidity, the reduction product ArH becomes predominant at the expense of ArOH and ArOEt, indicating that a turnover in the reaction mechanism takes place under acidic conditions. At any given EtOH content, the plot of k_obs values against acidity is S‐shaped, the inflexion point depending upon the EtOH concentration and the temperature. Similar S‐shaped variations are found when plotting the dediazoniation–product distribution against the acidity. The acid dependence of the switch between the homolytic and heterolytic mechanisms suggests that the homolytic dediazoniation proceeds via transient diazo ethers, and this complex kinetic behavior can be rationalized by assuming two competitive mechanisms: i) the spontaneous heterolytic dediazoniation of 4MBD, and ii) an O‐coupling mechanism in which the EtOH molecules capture ArN$\rm{{_{2}^{+}}}$ to yield a highly unstable (Z)‐adduct which undergoes homolytic fragmentation initiating a radical process (Scheme). Analyses of the effects of temperature on the equilibrium constant for the formation of the diazo ether and on the rate of splitting of the diazo ether allowed the estimation of relevant thermodynamic parameters for the formation of diazo ethers derived from methylbenzenediazonium ions under acidic conditions.     

Atmospheric chemistry of isopropyl formate and tert‐butyl formate

Formates are produced in the atmosphere as a result of the oxidation of a number of species, notably dialkyl ethers and vinyl ethers. This work describes experiments to define the oxidation mechanisms of isopropyl formate, HC(O)OCH(CH₃)₂, and tert‐butyl formate, HC(O)OC(CH₃)₃. Product distributions are reported from both Cl‐ and OH‐initiated oxidation, and reaction mechanisms are proposed to account for the observed products. The proposed mechanisms include examples of the α‐ester rearrangement reaction, novel isomerization pathways, and chemically activated intermediates. The atmospheric oxidation of isopropyl formate by OH radicals gives the following products (molar yields): acetic formic anhydride (43%), acetone (43%), and HCOOH (15–20%). The OH radical initiated oxidation of tert‐butyl formate gives acetone, formaldehyde, and CO₂ as major products. IR absorption cross sections were derived for two acylperoxy nitrates derived from the title compounds. Rate coefficients are derived for the kinetics of the reactions of isopropyl formate with OH (2.4 ± 0.6) × 10^?12, and with Cl (1.75 ± 0.35) × 10^?11, and for tert‐butyl formate with Cl (1.45 ± 0.30) × 10^?11 cm³ molecule^?1 s^?1. Simple group additivity rules fail to explain the observed distribution of sites of H‐atom abstraction for simple formates. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 479–498, 2010     

Rapid Synthesis of 2‐Alkanol‐substituted Pyrrolo[2,3‐b]quinoxalines from Propargylic Alcohols via Copper‐free Sonogashira Coupling Reaction at Room Temperature

A practical and efficient procedure is established for the synthesis of 2‐alkanol‐substituted pyrrolo[2,3‐b]quinoxalines by the reaction of N‐alkyl‐3‐chloroquinoxaline‐2‐amines with propargylic alcohols. The reaction is carried out in the absence of any copper salt but in the presence of a catalytic amount of Pd(PPh₃)₂Cl₂ at room temperature. The Sonogashira coupling reaction step in this procedure is fast, producing clean products with high yields without contamination by unwanted homocoupling Glaser reaction products. The synthesized pyrroloquinoxaline derivatives are also screened against the three bacterial strains Micrococcus luteus, pseudomonas aeruginosa, and Bacillus subtilis.     

One‐Pot Cascade Transformation of Glucal into Structurally Diverse Drug‐Like Scaffolds

A diversity‐oriented synthesis strategy to produce three types of structurally drug‐like N‐heterocyclic‐fused rings has been developed from abundant biomass‐derived d ‐glucal, aniline and water in a stereoselective manner. The overall transformation which entails a cascade of Ferrier reaction and 4π conrotatory imino‐Nazarov cyclization was performed in one‐pot allowing convenient preparation of scaffolds of high molecular complexity from relatively simple starting materials. While indoline‐fused products were readily accessible using ortho‐unsubstituted secondary anilines as substrates, reactions with ortho‐hydroxyl‐anilines furnished fused 1,4‐benzoxazines instead. In both cases, InBr₃ acted as the Lewis acid catalyst. By altering InBr₃ to Ln(OTf)₃, the indoline‐fused products could be further converted into tetrahydroquinoline‐fused cyclopentenones via ensuing retro‐ene rearrangement.     

Unusual Tritylation Reactions of Tricyclic Analogues of Acyclovir and an Attempt to Elucidate Their Mechanism

In reference to our earlier observation that the 3,9‐dihydro‐3‐[(2‐hydroxyethoxy)methyl]‐6‐methyl‐9‐oxo‐5H‐imidazo[1,2‐a]purine (6‐Me‐TACV) tricyclic antiviral agent derived from acyclovir undergoes unusual C‐tritylation to 7‐trityl and 7‐[4‐(benzhydryl)phenyl] derivatives enforced by a 6‐Me substituent, we studied tritylation of 6‐Ph ( 1a ) and 6‐(4‐MeOPh) ( 1b ) TACV derivatives. The treatment of 1a and 1b with TrCl in K₂CO₃/DMF resulted exclusively in the formation of 7‐[4‐(benzhydryl)phenyl] derivatives 2a , 2b , 3a , 3b , and 4a . Inhibition experiments with radical scavengers DNB and DBNO indicated a single‐electron‐transfer (SET) mechanism for this reaction. Analogous experiments with unsubstituted TACV and 6‐Me‐TACV suggest that the nature of the substituent at C(6) determines the reaction mechanism. The presence of a 6‐aryl substituent results in the exclusive formation of 4‐(benzhydryl)phenyl derivatives via a SET mechanism. On the contrary, when C(6) is unsubstituted, trityl derivatives are the only products of the S_n reaction. In the case of 6‐Me‐TACV, concomitant SET and S_n mechanisms direct the reaction towards 4‐(benzhydryl)phenyl and trityl products.     

Highly Efficient One‐Pot Three‐Component Synthesis and Antimicrobial Activity of 2‐Amino‐4H‐chromene Derivatives

A remarkably efficient three‐component reaction to synthesize 2‐amino‐4H‐chromenes derivatives from malononitrile, various aromatic aldehydes, and orcinol was described at room temperature in CH₂Cl₂ in the presence of catalytic amount of triethylamine. In a facile one‐pot procedure, excellent yields of products were achieved in less than 1 h. Some of the synthesized 2‐amino‐4H‐chromenes derivatives demonstrated potent antibacterial activities against Gram‐positive bacteria including Staphylococcus aureus and Bacillus anthracis, indicated by disk method, minimum inhibitory concentration, and minimum bactericidal concentration approaches. However, none of the tested compounds expressed any antibacterial activities against Enterococcus faecalis and Gram‐negative bacteria.     

Xantphos Doped POPs‐PPh3 as Heterogeneous Ligand for Cobalt‐Catalyzed Highly Regio‐ and Stereoselective Hydrosilylation of Alkynes

A Co(acac)₂/POL‐Xantphos@10PPh₃‐catalyzed hydrosilylation of unsymmetrical internal alkynes with Ph₂SiH₂ has been developed for the synthesis of highly selective syn‐α‐vinylsilane products. Furthermore, terminal alkynes were also used and gave the products with excellent regioselectivity and a wide functional group tolerance. Because this porous organic polymer combines the selectivity and activity merits of Xantphos with the stability advantage derived from the high concentration of PPh₃, the Co(acac)₂/POL‐Xantphos@10PPh₃ can be recycled multiple times without loss of activity and selectivity. This heterogeneous catalyst is expected to find promising applications in industrial synthesis.     

Kinetics of oxidation of pantothenic acid by chloramine‐T in perchloric acid and in alkaline medium catalyzed by OsO4: A mechanistic approach

Kinetics of oxidation of pantothenic acid (PA) by sodium N‐chloro‐p‐toluenesulfonamide or chloramine‐T (CAT) in the presence of HClO₄ and NaOH (catalyzed by OsO₄) has been investigated at 313 K. The stoichiometry and oxidation products are same in both media; however, their kinetic patterns were found to be different. In acid medium, the rate shows first‐order dependence on [CAT]_o, fractional‐order dependence on [PA]_o, and inverse fractional‐order on [H⁺]. In alkaline medium, the rate shows first‐order dependence each on [CAT]_o and [PA]_o and fractional‐order dependence on each of [OH^?] and [OsO₄]. Effects of added p‐toluenesulfonamide and halide ions, varying ionic strength, and dielectric constant of medium as well as solvent isotope on the rate of reaction have been investigated. Activation parameters were evaluated, and the reaction constants involved in the mechanisms have been computed. The proposed mechanisms and the derived rate laws are consistent with the observed kinetics. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 201–210, 2005     

Thermal decomposition kinetics of palladium(II) 1‐allylimidazole complexes

Thermal dehydration and decomposition processes of a Pd(II) coordination compound, [PdL₄]Cl₂·3H₂O ( 1 ), (where L is 1‐allylimidazole) were studied by simultaneous TG/DSC techniques under constant heating rates condition. The released gas products were analyzed by online coupling a FTIR spectrometer to the TG equipment. The so obtained evolved gas analysis confirmed that only two ligand molecules were released and that a new 1‐allylimidazole Pd(II) complex, trans‐[PdL₂Cl₂] ( 2a ), was obtained. The same coordination compound was also prepared by heating 1 at 413.15 K in air atmosphere until a constant weight was reached 2b . Thermal decomposition mechanisms for the 2a and 2b complexes examined were proposed according to the three mass loss steps derived by TG data. Based on the model‐free isoconversional method described by Flynn–Wall–Ozawa (FWO), the dependencies of activation energy on the degree of conversion were determined. A model‐free “single point” method was also applied using the Kissinger equation, and derived results were compared to those of the former method. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 667–674, 2005     

Reaction mechanisms of a cyclic ether intermediate: Ethyloxirane

Oxiranes are a class of cyclic ethers formed in abundance during low‐temperature combustion of hydrocarbons and biofuels, either via chain‐propagating steps that occur from unimolecular decomposition of β‐hydroperoxyalkyl radicals (β‐?QOOH) or from reactions of HO? with alkenes. Ethyloxirane is one of four alkyl‐substituted cyclic ether isomers produced as an intermediate from n‐butane oxidation. While rate coefficients for β‐?QOOH → ethyloxirane + ?H are reported extensively, subsequent reaction mechanisms of the cyclic ether are not. As a result, chemical kinetics mechanisms commonly adopt simplified chemistry to describe ethyloxirane consumption by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error—uncertainty derived from missing or incomplete chemistry. The present work provides fundamental insight on reaction mechanisms of ethyloxirane in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred from the detection of products during chlorine atom‐initiated oxidation experiments using multiplexed photoionization mass spectrometry conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, calculations of stationary point energies were conducted using the ccCA‐PS3 composite method on ?R + O₂ potential energy surfaces for the four ethyloxiranyl radical isomers, which produced barrier heights for 24 reaction pathways. In addition to products from ?QOOH → cyclic ether + ?H and ?R + O₂ → conjugate alkene + HO?, both of which were significant pathways and are prototypical to alkane oxidation, other species were identified from ring‐opening of both ethyloxiranyl and ?QOOH radicals. The latter occurs when the unpaired electron is localized on the ether group, causing the initial ?QOOH structure to ring‐open and form a resonance‐stabilized ketohydroperoxide‐type radical. The present work provides the first analysis of ethyloxirane oxidation chemistry, which reveals that consumption pathways are complex and may require an expansion of submechanisms to increase the fidelity of chemical kinetics mechanisms.     

AgNO3 Catalyzed Regio‐Selective Synthesis of 3‐Alkyl/Aryl‐idene‐3,4‐dihydro‐4‐tosyl‐2H‐1,4‐Benzoxazine: Novel Anti‐Tubercular Scaffolds

A facile and efficient method for the construction of 3‐alkyl/aryl substituted 1,4‐benzoxazine and benzoxazepine via AgNO₃ catalyzed cyclization of propargyloxy sulfonamides and their anti‐tubercular activity against Mycobacterium tuberculosis H₃₇R_V is described. This cyclization proceeds through 6‐exo‐dig manner to generate the products in moderate to good yields.     

Study of the end‐group contribution to ToF‐SIMS and G‐SIMS spectra of poly (lactic acid) using deuterium labelling

In this study, polymeric (MW 50 000) and oligomeric (MW 2000) poly (lactic acid) (PLA), both with and without end‐group deuterium exchange, were analysed using static secondary ion mass spectrometry (SSIMS) to investigate the contribution of end‐group‐derived secondary ions to the SSIMS spectra. By monitoring the SSIMS intensities between the non‐deuterated and deuterated PLA, it is evident that the only significant end‐group‐derived secondary ions are [nM + H]⁺ (n > 1) and C₄H₉O₂⁺. The gentle‐SIMS (G‐SIMS) methodology was employed to establish that deuterated fragments were produced through low energy processes and were not the result of substantial rearrangements. It was noted that end‐group‐derived secondary ions had higher G‐SIMS intensities for oligomeric PLA than polymeric PLA, showing that these secondary ions are simple fragment products that are not the result of rearrangement or degraded product ions. Copyright © 2007 John Wiley & Sons, Ltd.     

Aromatic‐Amide‐Derived Olefins as a Springboard: Isomerization‐Initiated Palladium‐Catalyzed Hydrogenation of Olefins and Reductive Decarbonylation of Acyl Chlorides with Hydrosilane

A highly efficient catalytic protocol for the isomerization of substituted amide‐derived olefins is presented that successfully uses a hydride palladium catalyst system generated from [PdCl₂(PPh₃)₂] and HSi(OEt)₃. The Z to E isomerization was carried out smoothly and resulted in geometrically pure substituted olefins. Apart from the cis–trans isomerization of double bonds, the selective reduction of terminal olefins and activated alkenes was performed with excellent functional group tolerance in the presence of an amide‐derived olefin ligand, and the products were obtained in high isolated yields (up to >99 %). Furthermore, the palladium/hydrosilane system was able to promote the reductive decarbonylation of benzoyl chloride when a (Z)‐olefin with an aromatic amide moiety was used as a ligand.     

Synthesis of 2,3‐diaryl‐3,4‐dihydro‐2H‐1,3‐benzoxazines and their fungicidal activities

A series of novel 2,3‐diaryl‐3,4‐dihydro‐2H‐1,3‐benzoxazines have been prepared in high yields from o‐arylaminomethylphenols and aromatic aldehydes in the presence of SnCl₄ for the first time, and their fungicidal activities were investigated too. Some of the products showed good fungicidal activities against Rhizoctonia solani justified by 100% activity of compound 1b. J. Heterocyclic Chem., (2010)     

Diastereo‐ and Regioselective Addition of Thioamide Dianions to Imines and Aziridines: Synthesis of N‐Thioacyl‐1,2‐diamines and N‐Thioacyl‐1,3‐diamines

Addition reactions of thioamide dianions that were derived from N‐arylmethyl thioamides to imines and aziridines were carried out. The reactions of imines gave the addition products of N‐thioacyl‐1,2‐diamines in a highly diastereoselective manner in good‐to‐excellent yields. The diastereomeric purity of these N‐thioacyl‐1,2‐diamines could be enriched by simple recrystallization. The reduction of N‐thioacyl‐1,2‐diamines with LiAlH₄ gave their corresponding 1,2‐diamines in moderate‐to‐good yields with retention of their stereochemistry. The oxidative‐desulfurization/cyclization of an N‐thioacyl‐1,2‐diamine in CuCl₂/O₂ and I₂/pyridine systems gave the cyclized product in moderate yield and the trans isomer was obtained as the sole product. On the other hand, a similar cyclization reaction with antiformin (aq. NaClO) as an oxidant gave the cis isomer as the major product. The reactions of N‐tosylaziridines gave the addition products of N‐thioacyl‐1,3‐diamines with low diastereoselectivity but high regioselectivity and in good‐to‐excellent yields. The use of AlMe₃ as an additive improved the efficiency and regioselectivity of the reaction. The stereochemistry of the obtained products was determined by X‐ray diffraction.     

Synthesis of the epimer of pericosine B from (-)-quinic Acid

Synthesis of the epimer of pericosine B from (-)-quinic acid was achieved. This synthesis involves some regioselective and stereoselective processes. The desired product showed lower cytotoxic activity in comparison with natural pericosine B against the P388 leukemia cell line. The result implies that the stereocenter of C-6 in natural pericosine B plays an important role in this activity.     

The Electronic Structure and Photochemistry of Group 6 Bimetallic (Fischer) Carbene Complexes: Beyond the Photocarbonylation Reaction

The UV spectra of Group 6 metal carbene complexes bearing a CpM(CO)₃ (Cp=cyclopentadienyl) moiety bonded to the carbene carbon atom exhibit a redshift of the absorption maxima at higher wavelengths with respect to the parent monometallic complexes. This redshift is partly due to a higher occupation on the p_z atomic orbital of the carbene carbon atom. Time‐dependent DFT calculations accurately assign this band to a metal‐to‐ligand charge‐transfer transition, thus showing that the presence of a second metal center does not affect the nature of the transition. However, the photochemical reactivity of Group 6 metal carbene complexes bearing a CpM(CO)₃ moiety strongly depends on the nature of this metal fragment. A new photoslippage reaction leading to fulvenes occurs when Mn‐derived products 11 a , 11 b , and 12 a are irradiated (both Cr and W derivatives), whereas Re‐derived product 11 c behaves like standard Fischer complexes and yields the usual photocarbonylation products. A new photoreduction process occurring in the metallacyclopropanone intermediate is also observed for these complexes. Both computational and deuteration experiments support this unprecedented photoslippage process. The key to this differential photoreactivity seems to be the M–Cp back‐donation, which hampers the slippage process for Re derivatives and favors the carbonylation reaction.     

The Reaction of Peroxynitrite with Morpholine (Secondary Amines) Revisited: The Overlooked Hydroxylamine Formation

The reaction of peroxynitrite/peroxynitrous acid with morpholine as a model compound for secondary amines is reinvestigated in the absence and presence of carbon dioxide. The concentration‐ and pH‐dependent formation of N‐nitrosomorpholine and N‐nitromorpholine as reported in three previous papers ([25] [26] [14]) is basically confirmed. However, ¹³C‐NMR spectroscopic product analysis shows that, in the absence of CO₂, N‐hydroxymorpholine is, at pH ≥ 7, the major product of this reaction, even under anaerobic conditions. The formation of N‐hydroxymorpholine has been overlooked in the three cited papers. Additional (ring‐opened) oxidation products of morpholine are also detected. The data account for radical pathways for the formation of these products via intermediate morpholine‐derived aminyl and α‐aminoalkyl radicals. This is further supported by EPR‐spectrometric detection of morpholine‐derived nitroxide radicals, i.e., morpholin‐4‐yloxy radicals. N‐Nitrosomorpholine, however, is very likely formed by electrophilic attack of peroxynitrite‐derived N₂O₄. ¹⁵N‐CIDNP Experiments establish that, in the presence of CO₂, N‐nitro‐ and C‐nitromorpholine are generated by radical recombination. The present results are in full accord with a fractional (28 ± 2%) homolytic decay of peroxynitrite/peroxynitrous acid with release of free hydroxyl and nitrogen dioxide radicals.     

P2O5 Mediated an Efficient Synthesis and Biological Evaluation of Heterocyclic‐fused Pyrimidine Derivatives as an Antitubercular Agent

An efficient P₂O₅ mediated protocol for the synthesis of pyrido[2,3‐d:6,5‐d]dipyrimidines has been developed through one‐pot three‐component condensation of aromatic aldehydes, barbituric acid/thiobarbituric acid, and 1,3‐dimethyl‐6‐amino‐uracil in ethanol. All the synthesized products were screened for their in vitro antitubercular activity, and results reveal that most of the compounds exhibited moderate to good activity against Mycobacterium tuberculosis H₃₇R_V strain.