Investigation of helium addition for laser-induced plasma spectroscopy of pure gas phase systems: Analyte interactions and signal enhancement

The role of helium addition on the analyte signal enhancement in laser-induced breakdown spectroscopy for analysis of pure gaseous systems was examined using carbon and hydrogen atomic emission lines. Increased analyte response, as measured by peak-to-base and signal-to-noise ratios, was observed with increasing helium addition, with maximum enhancement approaching a factor of 7. Additional measurements revealed a significant decrease in plasma electron density with increasing helium addition. To explore the mechanisms of analyte signal enhancement, the helium emission lines were also examined and found to be effectively quenched with nitrogen addition. In consideration of the data, it is concluded that the role of metastable helium is not as important as the overall changes in plasma properties, namely electron density and laser-plasma coupling. Helium addition is concluded to affect the electron density via Penning ionization, as well as to play a role in the initial plasma breakdown processes.     

Reactivity of (eta(6)-arene)tricarbonylchromium complexes toward additions of anions, cations, and radicals.

A computational and experimental study of additions of electrophiles, nucleophiles, and radicals to tricarbonylchromium-complexed arenes is reported. Competition between addition to a complexed arene and addition to a noncomplexed arene was tested using 1,1-dideuterio-1-iodo-2-((phenyl)tricarbonylchromium)-2-phenylethane. Reactions under anionic and cationic conditions give exclusive formation of 1,1-dideuterio-1-((phenyl)tricarbonylchromium)-2-phenylethane arising from addition to the complexed arene. Radical conditions (SmI(2)) afford two isomeric products, reflecting a 2:1 preference for radical addition to the noncomplexed arene. In contrast, intermolecular radical addition competition experiments employing ketyl radical addition to benzene and (benzene)tricarbonylchromium show that addition to the complexed aromatic ring is faster than attack on the noncomplexed species by a factor of at least 100,000. Density functional theory calculations using the B3LYP method, employing a LANL2DZ basis set for geometry optimizations and a DZVP2+ basis set for energy calculations, for all three reactive intermediates showed that tricarbonylchromium stabilizes all three types of intermediates. The computational results for anionic addition agree well with established chemistry and provide structural and energetic details as reference points for comparison with the other reactive intermediates. Intermolecular radical addition leads to exclusive reaction on the complexed arene ring as predicted by the computations. The intramolecular radical reaction involves initial addition to the complexed arene ring followed by an equilibrium leading to the observed product distribution due to a high-energy barrier for homolytic cleavage of an exo bond in the intermediate cyclohexadienyl radical complex. Mechanisms are explored for electrophilic addition to complexed arenes. The calculations strongly favor a pathway in which the cation initially adds to the metal center rather than to the arene ring.     

Radiation-Induced Addition Reaction of Ethyl Mercaptan and Carbon Tetrachloride to 1,2-Polybutadiene

γ-Ray-induced addition reactions of syndiotactic 1,2-polybuta-diene film with various compounds were carried out at room temperature. The weight of the film markedly increased when ethyl mercaptan was used. In the reaction with ethyl mercaptan, only addition took place without crosslinking. The addition of ethyl mercaptan to the vinyl group of syndiotactic 1,2-poly-butadiene followed anti-Markownikoff rule, and gave a 1:1 addition product. The rate of addition increased as the crys-tallinity of syndiotactic 1,2-polybutadiene used became lower. A similar relation between the crystallinity and the rate of addition was also observed in the γ-ray-induced addition of carbon tetrachloride to syndiotactic 1,2-polybutadiene, but at the same time gelation was pronounced. When liquid 1,2-polybutadiene was used instead of syndiotactic-1,2-polybutadiene, gelation which made the polymer insoluble in carbon tetrachloride did not take place, although a crosslinking reaction was noted. The appearance of the product in this case changed from a viscous liquid to a white powder as the reaction proceeded. The addition of carbon tetrachloride to the vinyl group of liquid 1,2-polybutadiene was also of the anti-Markownikoff type. This addition was accompanied by unexpectedly large vinyl consumption. The total decrease in vinyl group was found to be much larger than the decrease in vinyl group which was brought about by the addition of carbon tetrachloride. This discrepancy was attributed to cyclization and crosslinking reactions which were ascribed to the vinyl group bound by the main chain.     

New method for increasing of electrophilicity of weak electrophiles in addition reactions : Wagner-Meerwein rearrangement in a reaction of 2,4-dinitrobenzenesulfenyl chloride with norbornene

A new method for increasing of the effective electrophilicity of weak electrophyles in addition reactions has been developed, using as a model reaction the addition of 2,4-dinitrobenzenesulfenyl chloride to norbornene. The features of this reaction and some general problems of an electrophilic addition are discussed.     

Iron chloride enhancement of dimethylzinc-mediated radical conjugate addition of ethers and an amine to alkylidenemalonates

The addition of FeCl₃ and the use of DMSO as a solvent enabled the radical conjugate addition of a cyclic acetal and a cyclic amine to alkylidenemalonates using a reagent amount (12.5 equiv) of the radical precursors to give Michael addition products in up to 84% yield. This represents a great improvement over the 5% yield obtained under the previously reported conditions.     

Introduction of functionalized C1, C2, and C3 units to imines through the dimethylzinc-air-initiated radical addition

Introduction of functionalized C1, C2, and C3 units to imines was achieved by using the dimethylzinc-air-initiated alpha-alkoxyalkyl radical addition as a key reaction. The addition to a C[double bond]N double bond chemoselectively occurred in the presence of a C[double bond]O double bond, which is one of the advantages of this radical addition reaction over ionic addition reactions.     

3-叔丁氧甲酰基氨基吡咯烷催化酮和硝基芳基乙烯的不对称Michael加成

用3-叔丁氧甲酰基氨基吡咯烷催化酮和硝基芳基乙烯的不对称Michael加成,并考察了反应物结构对加成的影响,得到了较高的收率和较好非对映选择性.     

Factors affecting the rates of addition of free radicals to alkenes-determination of absolute rate coefficients using the persistent aminoxyl method

The rate of coupling of alkyl radicals with the persistent aminoxyl radical 1,1,3,3-tetramethylisoindolin-N-oxyl (1) has been used as a kinetic probe to determine absolute rate coefficients for the addition of alkyl radicals to methyl acrylate. The results are discussed in terms of the role of the structure and functionalization of the attacking radical on the rates of addition, particularly as they affect steric, polar, and enthalpic factors. The aminoxyl method is assessed against other methods for determining free radical addition rate coefficients.     

Silver(I)-catalyzed tandem 1,3-acyloxy migration/Mannich-type addition/elimination of the sulfonyl group of N-sulfonylhydrazone-propargylic esters to 5,6-dihydropyridazin-4-one derivatives

The addition of nucleophiles to C=N bonds offers a highly efficient synthetic strategy for accessing nitrogen-containing molecules.1 Among the well-developed addition reactions, such as the highly efficient Mannich reaction, various C-H bond-activated compounds including carboxylic acid derivatives, nitroalkanes, and terminal alkynes have been applied as nucleophiles to achieve different classes of amines.2 However, employing new nucleophiles without activated C-H bonds, such as internal alkynes and allenic esters are limited when using metal catalysts.3 Herein, we wish to report a new addition of allenic esters to C=N bonds initiated by a silver-catalyzed 1,3-migration of propargylic esters.     

Orthoplatinated triarylphosphite as a highly efficient catalyst for addition reactions of arylboronic acids with aldehydes: low catalyst loading catalysis and a new tandem reaction sequence

Readily available, air/moisture-stable orthoplatinated triarylphosphite catalyzes the addition reactions of arylboronic acids with aldehydes with the catalyst loading as low as 0.01%. It also cataylzes a new tandem reaction of arylboronic acids with alpha,beta-unsaturated aldehydes to form 1,3-diaryl-1-propanols. Our study provides a new paradigm for the application of orthoplatinated triarylphosphites, and may pave the road to develop other Pt(II) catalysts for such addition reactions and other tandem reactions with such addition reactions as part of the reaction sequence.     

Stereoselective synthesis of trisubstituted E-iodoalkenes by indium-catalyzed syn-addition of 1,3-dicarbonyl compounds to 1-iodoalkynes

Indium-catalyzed addition of 1,3-dicarbonyl compounds to 1-iodo-1-alkynes takes place exclusively in a syn-fashion to produce E-iodoalkenes. The iodine atom serves both as an activating group and as a group that controls the regioselectivity of the addition. The E-alkenyl iodide product can be further derivatized using either a one-pot or a two-pot procedure into trisubstituted olefins in high overall yield with retention of the stereochemistry.     

Photoinduced mechanism of formation and growth of polycyclic aromatic hydrocarbons in low-temperature environments via successive ethynyl radical additions

A novel ethynyl addition mechanism (EAM) has been established computationally as a practicable alternative to high-temperature hydrogen-abstraction-C2H2-addition (HACA) sequences to form polycyclic aromatic hydrocarbon (PAH) -like species under low-temperature conditions in the interstellar medium and in hydrocarbon-rich atmospheres of planets and their moons. Initiated by an addition of the ethynyl radical (C2H) to the ortho-carbon atom of the phenylacetylene (C6H5C2H) molecule, the reactive intermediate loses rapidly a hydrogen atom, forming 1,2-diethynylbenzene. The latter can react with a second ethynyl molecule via addition to a carbon atom of one of the ethynyl side chains. A consecutive ring closure of the intermediate leads to an ethynyl-substituted naphthalene core. Under single-collision conditions as present in the interstellar medium, this core loses a hydrogen atom to form ethynyl-substituted 1,2-didehydronaphthalene. However, under higher pressures as present, for example, in the atmosphere of Saturn's moon Titan, three-body reactions can lead to a stabilization of this naphthalene-core intermediate. We anticipate this mechanism to be of great importance to form PAH-like structures in the interstellar medium and also in hydrocarbon-rich, low-temperature atmospheres of planets and their moons such as Titan. If the final ethynyl addition to 1,2-diethynylbenzene is substituted by a barrierless addition of a cyano (CN) radical, this newly proposed mechanism can even lead to the formation of cyano-substituted naphthalene cores in the interstellar medium and in planetary atmospheres.     

New trimethylaluminum-induced mannich-type reaction of hydrazones

Trimethylaluminum addition to N-monoalkyl or N-monoaryl hydrazones followed by aldehyde addition leads to the formation of N-alkylated hydrazones in a new formal Mannich-type process. Addition compounds were also obtained in moderate yields with ketones. The mechanism as well as possible intermediates involved in the reaction are discussed.     

Heteroatom influence on the pi-facial selectivity of Diels-Alder cycloadditions to 1-oxa-4-thia-6-vinylspiro[4.5]dec-6-ene, 3-methoxy-3-methyl-2-vinylcyclohexene, and 3-methoxy-2-vinylcyclohexene

The facial selectivities of the Diels-Alder cycloadditions of several dienophiles to the title substrates were studied. The observed selectivities are interpreted as a consequence of the relative steric interactions offered by the substituents. The addition of dimethylacetylene dicarboxylate (DMAD) is influenced by the electrostatic repulsion arising from the interaction of an electron pair orbital on the acetal oxygen and the orthogonal pi-orbital of the acetylene unit in DMAD in the syn-to-oxygen addition of the latter. This repulsion is offset on coordination of Li+ to the said oxygen electron pair orbital, and the addition thus proceeds syn to oxygen. The enhanced and accelerated syn-to-oxygen addition in lithium perchlorate in nitromethane is interpreted as a consequence of the coordination of Li+ to both the acetal oxygen and a heteroatom in the dienophile that brings them in close proximity to facilitate a reaction. The Li+-oxygen combination, however, also exerts some steric effect that results in reduced syn-to-oxygen addition of dienophiles having large substituents such as N-phenylmaleimide.     

Stability of carbon-centered radicals: effect of functional groups on the energetics of addition of molecular oxygen

In this paper we examine a series of hydrocarbons with structural features which cause a weakening of the C-H bond. We use theoretical calculations to explore whether the carbon-centered radicals R(*) which are created after breaking the bond can be stabilized enough so that they resist the addition of molecular oxygen, i.e. where the reaction R(*) + O(2) --> ROO(*) becomes energetically unfavorable. Calculations using a B3LYP-based method provide accurate bond dissociation enthalpies (BDEs) for R-H and R-OO(*) bonds, as well as Gibbs free energy changes for the addition reaction. The data show strong correlations between R-OO(*) and R-H BDEs for a wide variety of structures. They also show an equally strong correlation between the R-OO(*) BDE and the unpaired spin density at the site of addition. Using these data we examine the major functional group categories proposed in several experimental studies, and assess their relative importance. Finally, we combine effects to try to optimize resistance to the addition of molecular oxygen, an important factor in designing carbon-based antioxidants.     

Regioselective organocadmium alkylations of substituted quinones.

A series of substituted quinones was alkylated with diethylcadmium. Regiochemistry of addition shifted from quinol formation to conjugate addition as a function of the steric and electronic effects of the substituents.     

Synthesis and reactivity of 5-methylenehydantoins

5-Methylenehydantoin, as well as the N-mono- and N,N-di-protected derivatives, can be obtained by different synthetic routes. These compounds can undergo a large variety of reactions, such as Diels-Alder, epoxidation, methanol addition and conjugate addition reactions of different types of nucleophiles, including carbon (cyanide), nitrogen (piperidine) and sulfur (thiols, thioacetate) nucleophiles. The reactivity with electrophilic reagents, such as m-CPBA or methanol in acidic medium, and the need for Lewis acids to promote the conjugate addition reactions indicate that hydantoin is a poor electron-withdrawing group.     

Anionic Polymerization Chemistry of Epoxides: Electron-Transfer Processes

The reaction of poly(styryl)lithium (PSLi) with styrene oxide (SO) in benzene solution has been investigated. In addition to the functionalized polymer (83.5 wt %) the product mixture consisted of unfunctionalized polymer (9 wt %) and a dimeric product (5.6 wt %). The structure of the dimeric product was determined to be the head-to-head dimer. The regiochemistry of the addition reaction was unselective: 53 mol % addition to the methylene carbon and 47 mol % corresponding to addition to the hindered, methine carbon. Based on the formation of the dimer and the lack of regioselectivity, a pathway involving electron transfer from PSLi to SO was proposed. To further investigate the propensity to react via this electron-transfer mechanism, the SO functionalization of the adduct of PSLi with 1,1-diphenylethylene was investigated as well as the functionalization of PSLi with 1,1-diphenylethylene oxide.     

Orientation of addition of hydrogen atoms to propylene,butene-1, and isobutene

The orientation of addition of H atoms to the asymmetric olefins propylene, butene-1, and isobutene has been determined as a function of atom concentration, olefin concentration, hydrogen pressure, total pressure, and olefin conversion. Conditions have been determined for which complicating secondary processes are believed to be unimportant. The percentage of nonterminal addition is 5.7, 5.7, and 0.48 for propylene, butene-1, and isobutene, respectively. From these data activation energy differences between nonterminal and terminal addition of 1.7 and 3.2 kcal/mole for the linear olefins and isobutene, respectively, may be calculated. For D atom addition to propylene, a small isotope effect was observed, nonterminal addition being 5.4%. These observations are consistent with a predominantly free radical or electroneutral character for H atoms.     

Ceric ammonium nitrate (CAN) as a green and highly efficient promoter for the 1,4-addition of thiols and benzeneselenol to α,β-unsaturated ketones

A mild and efficient process for the 1,4-addition of thiols and benzeneselenols to various α,β-unsaturated ketones using a catalytic amount of CAN with excellent product yields is described. This inexpensive, nontoxic, and readily available catalytic ceric(IV) ammonium nitrate system efficiently catalyzes conjugate addition reactions between thiol derivatives and various α,β-unsaturated ketones under solvent-free conditions. A plausible mechanism for the role of CAN, both as a promoter in free radical chain addition reactions as well as a catalyst for the conjugate addition process is proposed.