In Grignard reagent formation from cyclopropyl bromide in diethyl ether, trapping by DCPH is consistent with diffusing cyclopropyl radical intermediates

[reaction: see text] For reactions of magnesium with cyclopropyl bromide in diethyl ether, added DCPH decreases the yield of cyclopropylmagnesium bromide by as much as 75%, while solvent-derived products disappear and cyclopropylcyclohexylphosphine and tetracyclohexyldiphosphine appear. These changes reflect trapping of diffusing intermediate cyclopropyl radicals.     

Highly efficient Friedel-Crafts-type benzylation via benzyl cations generated in multiple spacer-molecule separated ion-pairs

Deamination, as a facile route to highly reactive carbocations, is of interest to several areas of organic chemistry, industry, and medicine. The reactivity and utility of these carbocations derives from the presence of a lone nitrogenous entity (N(2) or N(2)O) interposed between the ion-pair. We report here the synthesis of a new deamination precursor whose nitrosation and subsequent decomposition constitutes a novel deamination method. In this novel approach, multiple spacer-molecules are generated in the inter-ion space. The resultant cations are exceedingly reactive but are longer-lived than carbocations derived from standard deaminations. The result is nearly quantitative yields of solvent-derived products from even poorly nucleophilic solvents such as benzene, toluene, and mesitylene.     

Photodecomposition of platinum and palladium alkyls: A cidnp study

Several alkyl-palladium and -platinum compounds of the general formulae L₂MR₂ and L₂MRX have been subjected to UV irradiation. A reaction occurs in chlorinated solvents such as chloroform to yield a metal chloride and a radical pair consisting of the alkyl stemming from the organometallic compound and a solvent-derived radical. The radical pair reaction is revealed by CIDNP. The decomposition products containing one more carbon atom than expectedaccording to this mechanism can be accounted for by a novel type of rearrangement, viz. an intramolecular carbene migration. In hydrocarbon solvents alkylplatinums undergo very little reaction, but alkylpalladiums decompose homolytically.     

Electro-nuclear double resonance spectroscopic evidence for a hydroxo-bridge nucleophile involved in catalysis by a dinuclear hydrolase

Despite the current availability of several crystal structures of purple acid phosphatases, to date there is no direct evidence for solvent-derived ligands occupying terminal positions in the active enzyme. This is of central importance, because catalysis has been shown to proceed through the direct attack on a metal-bound phosphate ester by a metal-activated solvent-derived moiety, which has been proposed to be either (i) a hydroxide ligand terminally bound to the ferric center or (ii) a bridging hydroxide. In this work we use (2)H Q-band (35 GHz) pulsed electron-nuclear double resonance (ENDOR) spectroscopy to identify solvent molecules coordinated to the active mixed-valence (Fe(3+)Fe(2+)) form of the dimetal center of uteroferrin (Uf), as well as to its complexes with the anions MoO(4), AsO(4), and PO(4). The solvent-derived coordination of the dinuclear center of Uf as deduced from ENDOR data includes a bridging hydroxide and a terminal water/hydroxide bound to Fe(2+) but no terminal water/hydroxide bound to Fe(3+). The terminal water is lost upon anion binding while the hydroxyl bridge remains. These results are not compatible with a hydrolysis mechanism involving a terminal Fe(3+)-bound nucleophile, but they are consistent with a mechanism that relies on the bridging hydroxide as the nucleophile.     

Photolysis of methyl(triphenylphospine)gold(I)

Methyl(triphenylphosphine)gold(I) reacts with chloroform under UV irradiation to give methyl and solvent-derived radicals in triplet pairs showing CIDNP. An S_H2 substitution by methyl radicals at the gold centre is observed.     

Density functional studies of oxidized and reduced methane monooxygenase. Optimized geometries and exchange coupling of active site clusters

The conflicting protein crystallography data for the oxidized form (MMOH(ox)) of methane monooxygenase present a dilemma regarding the identity of the solvent-derived bridging ligands within the active site: do they comprise a diiron unit bridged by 1H2O and 1OH(-) as postulated for Methylococcus capsulatus or 2OH(-) ligands as suggested for Methylosinus trichosporium? Using models derived explicitly from the M. capsulatus and M. trichosporium protein data, spin-unrestricted density functional methods have been used to study two structurally characterized forms of the hydroxylase component of methane monooxygenase. The active site geometries of the oxidized (MMOH(ox)) and two-electron-reduced (MMOH(red)) states have been geometry optimized using several quantum cluster models which take into account the antiferromagnetic (AF) and ferromagnetic (F) coupling of electron spins. Trends in cluster geometries, energetics, and Heisenberg J values have been evaluated. For the majority of models, calculated geometries are in good agreement with the X-ray analyses and appear relatively insensitive to the F or AF alignment of electron spins on adjacent Fe sites. Discrepancies between calculation and experiment appear in the orientation of the coordinated His and Glu amino acid side chains for both MMOH(ox) and MMOH(red) and also in unexpected intramolecular proton transfer in the MMOH(ox) cluster models. There is additional dispersion between (and among) calculated and experimental Fe(3+)-OH(-) distances with relevance to the correct protonation state of the solvent-derived ligands. In an accompanying paper (Lovell, T.; Li, J.; Noodleman, L. Inorg. Chem. 2001, 40, 5267), a comparison of the related energetics of the active site models examined herein is further evaluated in the full protein and solvent environment.     

Catalyzed oxidation of anthracene with oxygen and tert-butylhydroperoxide

Oxidation of anthracene to anthraquinone by oxygen/RhCl(PPh₃)₃ proceeds via solvent-derived hydroperoxides. tert-Butylhydroperoxide/RhCl(PPh₃)₃ effects the same transformation in up to 96% yield. Possible mechanisms for the TBHP-catalyzed reaction are discussed.     

The mechanism of the reaction catalyzed by ADP-beta-L-glycero-D-manno-heptose 6-epimerase

ADP-l-glycero-d-manno-heptose 6-epimerase (AGME, RfaD) is a bacterial enzyme that is involved in lipopolysaccharide biosynthesis and interconverts ADP-beta-l-glycero-d-manno-heptose (ADP-l,d-Hep) with ADP-beta-d-glycero-d-manno-heptose (ADP-d,d-Hep). AGME is known to require a tightly bound NADP+ cofactor for activity and presumably employs a mechanism involving transient oxidation of the substrate. Four mechanistic possibilities are considered that involve transient oxidation at either C-7' ', C-6' ', or C-4' ' of the heptose nucleotide. In this contribution, the use of solvent isotope incorporation studies and alternate substrates provides strong evidence for a mechanism involving nonstereospecific oxidation/reduction directly at C-6' '. It was found that the epimerization proceeds without any detectable incorporation of solvent-derived deuterium or 18O-isotope into the product. This argues against mechanisms involving either proton transfers at carbon or dehydration/rehydration events. In addition, the deoxygenated analogues, 7' '-deoxy-ADP-l,d-Hep and 4' '-deoxy-ADP-l,d-Hep, were both found to serve as substrates for the enzyme, indicating that oxidation at either C-7' ' or C-4' ' is not required for catalysis.     

Protective effects of a freeze-dried extract of vegetables and fruits on the hydroxyl radical-mediated oxidative damage of DNA and decrease of erythrocytes deformability

The protective effects of a freeze-dried extracts of vegetables and fruits (BauYuan; BY) on the hydroxyl radical-mediated DNA strand breakages and the structural integrity of human red blood cells (RBCs) were investigated. First, the supercoiled plasmid (pEGFP-C1) DNA was subjected to oxidation damage by an ascorbate-fortified Fenton reaction and the protective effects were analyzed by agarose gel electrophoresis. In the absence of BY extracts, exposure of the high-throughput ·OH-generating system (Fe²⁺ concentration >1.0 μM) caused a complete fragmentation of DNA. Supplementation of BY extract (1 mg/mL) to the plasmid DNA prior to the exposure could prevent it significantly. In contrast, as the plasmid exposed to a low-grade ·OH-generating system (Fe²⁺<0.1 μM), the BY extract (1 mg/mL) provided an almost complete protection. Next, the cell deformabilities were measured to assess the protection effects of various BY extracts on human erythrocytes exposed to the oxidative insults. We found that both the aqueous extract and the organic solvent-derived extracts could strongly protect human RBCs from the reactive oxygen species (ROS)-mediated decrease in the deformability indices. The results implicated that the BY extracts could effectively protect the cell membrane integrity via scavenging ROS which enabling RBCs to maintain a balance of water content and surface area to prevent the drop of cell deformability.     

Sialic acid biosynthesis: stereochemistry and mechanism of the reaction catalyzed by the mammalian UDP-N-acetylglucosamine 2-epimerase

The bifunctional enzyme, UDP-N-acetylglucosamine 2-epimerase/ManNAc kinase, catalyzes the first two steps in the biosynthesis of the sialic acids in mammals. The epimerase domain converts UDP-GlcNAc into ManNAc and UDP. This paper demonstrates that alpha-ManNAc is the first formed anomer and therefore the reaction proceeds with a net retention of configuration at C-1. Studies in deuterated buffer show that solvent-derived deuterium is quantitatively incorporated into the C-2 position of the product during catalysis, but it is not incorporated into the remaining pool of substrate. This indicates that the inversion of stereochemistry is ultimately brought about by the removal and replacement of a proton at C-2 and is consistent with a two-base mechanism. Studies with (18)O-labeled UDP-GlcNAc show that the anomeric oxygen of the glycosyl phosphate bond departs with the UDP product and therefore the net hydrolysis reaction involves C-O bond cleavage. Incubation of the putative intermediate, 2-acetamidoglucal, with the enzyme resulted in a slow hydration reaction to give the product, ManNAc. Additional kinetic isotope effect and positional isotope exchange (PIX) experiments address the nature of the rate-determining step of the reaction and show that C-H bond cleavage is not rate limiting. Overall, these results support a reaction mechanism involving an anti-elimination of UDP to give 2-acetamidoglucal, followed by a syn-addition of water.     

Automatic method for identifying reaction coordinates in complex systems

To interpret simulations of a complex system to determine the physical mechanism of a dynamical process, it is necessary to identify the small number of coordinates that distinguish the stable states from the transition states. We develop an automatic method for identifying these degrees of freedom from a database of candidate physical variables. In the method neural networks are used to determine the functional dependence of the probability of committing to a stable state (committor) on a set of coordinates, and a genetic algorithm selects the combination of inputs that yields the best fit. The method enables us to obtain the first set of coordinates that is demonstrably sufficient to specify the transition state of the C(7eq)--> alpha(R) isomerization of the alanine dipeptide in the presence of explicit water molecules. It is revealed that the solute-solvent coupling can be described by a solvent-derived electrostatic torque around one of the main-chain bonds, and the collective, long-ranged nature of this interaction accounts for previous failures to characterize this reaction.     

Two-photon chemistry in the laser jet: generation of radical intermidiates and their photochemical transformations

The novel laser jet technique provides sufficiently high photon densities to permit the observation of the photochemistry of photochemically generated radicals (two-photon chemistry) in the liquid phase. Four recent applications of this novel photochemically useful method are presented: these include the photochemistry of hydroxydiphenylmethyl, 9-hydroxyxanthenyl, diphenylmethyl, and benzoyl radicals under laser jet and normal photolysis conditions.

The regioselectivity of cross-coupling reactions of hydroxydiphenylmethyl or 9-hydroxyxanthenyl radicals with solvent-derived radicals changes when these species are electronically excited,i.e. under the high intensity conditions of the laser jet, cross-coupling at the para position (head-to-tail combination) is significantly enhanced relative to the normal coupling mode at the hydroxy-bearing radical site (head-to-head combination). Semiempirical calculations of the spin density distributions for the ground and first excited states of the radicals confirm the change in spin density from the hydroxy-bearing carbon atom to the conjugating benzene rings in these radical species on photoexcitation.

For the diphenylmethyl radical, two reaction pathways have been observed under the high photon densities of the laser jet: the electronically excited diphenylmethyl radical can either abstract a chlorine atom from carbon tetrachloride through an electron transfer process or can be photoionized on further photoexcitation (multiphoton chemistry). The resulting benzhydryl cation was trapped by methanol as the corresponding ether product, which unequivocally demonstrates that carbene formation by photoejection of a hydrogen atom does not take place under laser jet photolysis conditions.

An advantage of the high photon densities produced in laser jet photolysis is the high steady state concentration of short-lived transients that are generated, which enable unprecedented intermolecular reactions to be observed. Thus, about a millimolar concentration of tert-butoxy radicals can be obtained in the laser jet photocleavage of tert-butyl peroxide. When the tert-butoxy radicals are produced in the presence of benzaldehyde, the main product is tert-butyl benzoate. If carbon tetrachloride is also present, chlorobenzene can be detected. This is rationalized as the product derived from chlorine abstraction by phenyl radicals, which are presumably produced by the photodecarbonylation of benzoyl radicals.

An alternative method of obtaining benzoyl radicals is the two-photon cleavage of benzil. The laser jet photolysis of benzil in tert-butyl peroxide yields mainly tert-butyl benzoate, whereas in carbon tetrachloride, benzoyl chloride, chlorobenzene and ,,-trichloroacetophenone are observed. The first two products result from chlorine atom abstraction by the photochemically generated benzoyl and phenyl radicals, and the last product from in-cage cross-coupling between benzoyl and trichloromethyl radicals.

Such product studies provide detailed mechanistic information on the photochemical behaviour of electronically excited, short-lived transients which complements nicely the kinetic and spectral data of time-resolved laser flash studies. Consequently, the laser jet technique constitutes a valuable tool for determining the mechanism of two- photon reactions.     

取代芳烃电解氧化产物的GC-MS分析（I）

采用GC-MS方法分析了两种取代芳烃直接电解氧化的产物；从对伞花烃电解氧化产物中鉴定出10种主要成分，从对二甲苯电解氧化产物中鉴定出8种主要成分；根据成分鉴定和含量测定结果，提出了以获取目标产物枯茗醛和对甲基苯甲醛为目的的电解氧化反应要求。     

Novel reactivity of ruthenium alkylidenes in protic solvents: degenerate alkylidene proton exchange.

A novel organometallic transformation is reported in which the alkylidene protons of water-soluble ruthenium alkylidenes 1 and 2 undergo nondestructive, degenerate exchange with solvent-derived deuterons in perdeuterated protic solvents such as D(2)O and CD(3)OD. Deuterated alkylidene complex (1-D) was isolated from a solution of alkylidene 1 in D(2)O, and the new alkylidene was fully characterized by (1)H, (2)H, (13)C, and (31)P NMR spectroscopy and fast-atom bombardment mass spectroscopy (FAB-MS). The rate of alkylidene proton exchange for this transformation was found to correlate with the bulk dielectric constant of the solvent or solvent mixtures employed. The data support a mechanism for proton exchange involving the dissociation of a chloride ion from the ruthenium metal center. The rate of alkylidene H/D exchange for alkylidene 2 was faster than the rate of exchange for alkylidene 1, demonstrating that relative rates of exchange are influenced by the electron densities at the metal centers of these complexes. Several additional ruthenium alkylidenes were found to undergo analogous alkylidene H/D exchange reactions, including parent alkylidene (Cy(3)P)(2)Cl(2)Ru=CHPh (3) in CD(2)Cl(2)/CD(3)OD mixtures. These data suggest that this novel reactivity may be general for an entire class of ruthenium alkylidenes provided that protic species are available in solution and that the dielectric constant of the reaction medium is sufficiently high to ionize the halide ligands.     

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS-XIV. THE SPIN TRAPPING OF FREE RADICALS FORMED DURING THE PHOTOLYSIS OF HALOGENATED SALICYLANILIDE ANTIBACTERIAL AGENTS

Several antibacterial halogenated salicylanilides, including 3,3',4',5-tetrachlorosalicylanilide (TCSA) and 3,4',5-tribromosalicylanilide (TBSA) are known to cause photoallergy. We have carried out photochemical and spin trapping studies to determine whether free radicals may be involved in the photoallergic response. Irradiation (lambda greater than 300 nm) of TCSA in buffered (pH 7.4) 50% ethanol resulted in the rapid loss of the 3-chloro atom, followed by the much slower release of 5- and then the 4'-chloro atoms to give 3'-chlorosalicylanilide as a stable photoproduct. Under the same conditions TBSA successively lost the 3-, 5- and 4'-bromine atoms to give salicylanilide. When TCSA or TBSA were irradiated (lambda = 356 nm) in buffered (pH 7.4) 50% ethanol containing 2-methyl-2-nitrosopropane (MNP) only solvent-derived free radicals were detected. However, irradiation (lambda = 356 nm) of TCSA and MNP in 0.1 N NaOH generated an ESR spectrum consisting of a broad triplet (aN = 15.6 G). This spectrum was attributed to the adduct formed by the reaction of MNP with the aryl radical generated by the loss of a chlorine atom from the sterically hindered 3-(or 4'-)-position. Under the same conditions TBSA initially generated a broad triplet (aN = 15.5 G) similar to that observed for TCSA. However, upon further irradiation a 21-line spectrum (aN = 14.4 G, a2H = 2.0 G and a2H = 0.9 G) appeared.(ABSTRACT TRUNCATED AT 250 WORDS)     

Syntheses of diazadithiacrown ethers containing appended coumarin or 1‐aminonaphthalene sidearms

Six crown ethers containing two coumarin ( 11, 13,14,16,17 and 19 ) or two 1‐aminonaphthalene ( 15 and 20 ) fragments as sidearms and two crown ethers bearing one coumarin ( 12 and 18 ) arm were synthesized by a nucleophilic substitution of the secondary macrocyclic amine function on the alkyl halide. The major products of these reactions were the diazadithiacrown ethers containing two sidearms. In some cases, one‐armed diazadithiacrown ethers were separated as minor products, although more than 2.5 mmoles of alkyl halide were used for 1.0 mmole of macrocyclic diamine.     

Macrocycles from simple building blocks by a multifold Heck-type coupling reaction

Diiodobenzenes and bisallylic alcohols—two equivalents of each—were coupled under palladium catalysis to give oxofunctionalized macrocycles as 2:2 products. The 1:1 products as medium sized rings were disfavoured and not observed.     

The Origin of Magnetic Field Dependent Recombination in Alkylcobalamin Radical Pairs

Magnetic field effect studies of alkylcobalamin photolysis provide evidence for the formation of a reactive radical pair that is born in the singlet spin state. The radical pair recombination process that is responsible for the magnetic field dependence of the continuous-wave (CW) quantum yield is limited to the diffusive radical pair. Although the geminate radical pair of adenosylcob(III)alamin also undergoes magnetic field dependent recombination (A. M. Chagovetz and C. B. Grissom, J. Am. Chem. Soc. 115, 12152–12157, 1993), this process does not account for the magnetic field dependence of the CW quantum yield that is only observed in viscous solvents. Glycerol and ethylene glycol increase the microviscosity of the solution and thereby increase the lifetime of the spin-correlated diffusive radical pair. This enables magnetic field dependent recombination among spin-correlated diffusive radical pairs in the solvent cage. Magnetic field dependent recombination is not observed in the presence of nonviscosigenic alcohols such as isopropanol, thereby indicating the importance of the increased microviscosity of the medium. Paramagnetic radical scavengers that trap alkyl radicals that escape the solvent cage do not diminish the magnetic field effect on the CW quantum yield, thereby ruling out radical pair recombination among randomly diffusing radical pairs, as well as excluding the involvement of solvent-derived radicals. Magnetic field dependent recombination among alkylcobalamin radical pairs has been simulated by a semiclassical model of radical pair dynamics and recombination. These calculations support the existence of a singlet radical pair precursor.     

Quality assessment of raw and baked Aucklandia lappa Decne. by color measurement and fingerprint analysis

Aucklandia lappa Decne. has been used as a traditional Chinese herb for thousands of years in treating various kinds of disorders. According to the Chinese Pharmacopoeia, there are two kinds of processed products, raw and baked Aucklandia lappa Decne., which have different therapeutic effect in clinical application. In this study, based on color measurement and fingerprint analysis, the method to assess the quality of these two processed products was established. In color measurement, the reference ranges of color parameters (L^*, a^*, and b^*), standard color difference values, and mathematical prediction functions of these two processed products were obtain after the color was measured by a spectrophotometer. Meanwhile, high‐performance liquid chromatography fingerprints of these two processed products were established, where there were 12 peaks recognized as the common peaks in both processed products, in which two peaks were identified as costunolide and dehydrocostus lactone, and these two processed products were classified with chemometrics analysis subsequently. Furthermore, the correlation between color parameters and sample compositions was explored and the contents of costunolide and dehydrocostus lactone were determined simultaneously by high‐performance liquid chromatography. Consequently, an integral method including color measurement, high‐performance liquid chromatography fingerprint with chemometrics analysis, and quantitative determination was established.