Dynamics of activationless reactions in solution

Recent picosecond and subpicosecond laser spectroscopy experiments have revealed several chemically and biologically important reactions in solution in which the reaction potential surface does not present a barrier to the motion along the reaction coordinate.The dynamics of these reactions display diverse and interesting behavior. They include the dependence of relaxation rate on the solvent viscosity, the solvent polarity, the temperature, and the wavelength of the exciting light. In this article we review the recent developments in the theoretical description of activationless processes in solution and compare them with the available experimental results

Chemical toughening of epoxies. I. Structural modification of epoxy resins by reaction with hydroxy-terminated poly(butadiene-co-acrylonitrile)

In an attempt to toughen the epoxy resin matrix for fiber-reinforced composite applications, a chemical modification procedure of a commercially available bisphenol-A-based epoxy resin using reactive liquid rubber HTBN [hydroxy-terminated poly(butadiene-co-acrylonitrile)] and TDI (tolylene diisocyanate) is described. The progress of the reaction and the structural changes during modification process are studied using IR spectroscopy, viscosity data, and chemical analysis (epoxy value determination). The studies support the proposition that TDI acts as a coupling agent between the epoxy and HTBN, forming a urethane linkage with the former and an oxazolidone ring with the latter. The chemical reactions that possibly take place during the modification are discussed.

Bile-Acids in Asymmetric-Synthesis.2. Cholic-Acid Derived Novel Chiral Auxiliaries For Asymmetric Diels-Alder Reactions

Cholic acid-based chiral acrylate 5 yields a Diels-Alder adduct with cyclopent

Novel photocrosslinkable liquid-crystalline polymers: poly[bis(benzylidene)] esters

A new class of photo-cross-linkable main-chain liquid crystalline polymers (PMCLCPs) containing bis(benzylidene)cycloallranone groups have been synthesized and studied for their liquid crystalline and photochemical properties. The bis(benzylidene)cycloalkanone group in the chain functions both as a mesogen and as a photoreactive center. All of the polymers exhibit a nematic mesophase. Two kinds of photoreactions, namely, photoisomerization and photo-cross-linking, operate in these polymers. Above Tu at the initial stages of irradiation, photoisomerization predominates the cross-linking, which resulta in the disruption of the chromophore aggregates. Below T8, because of the restricted mobility of the chains, only cross-linking takes place. Studies on the model compound, bis(benzylidene)cyclopentanone, confii the above observations and demonstrate further that the cross-linking proceeds by the 2r + 2r cycloaddition reaction of the bis(benzylidene)cycloallranone moieties. The cross-linking rate decreases with increase in the size of the cycloalkanone ring. Heating the solution cast polymer fii results in the ordered aggregation of the chromophores just above TI and also at the crystal to crystal transition temperature, which facilitates the phobcross-linking reactions. In the isotropic phase, the random orientation of the chromophores drastically curtails the cross-linking rata

Organometallic chemistry of diphosphazanes *1: VIII. Synthesis, spectroscopic studies and crystal structure of fac-[Mo(CO)3(MeCN)Ph2PN(iPr)P(Ph)(DMP)], and its reactions with tertiary phosphines

The reaction of fac-[Mo(CO)3(MeCN)3] with the unsymmetrical diphosphazane Ph2PN(iPr)P(Ph)(DMP) (L) gives the complex fac-[Mo(CO)3(MeCN)(L)] (2) in almost quantitative yield. The structure of the complex has been determined by an X-ray diffraction study. The compound reacts with PR3 (where R = Ph, OPh) to give fac-[Mo(CO)3(PR3)(L)] (3a, 4a), which undergoes an intramolecular isomerization to afford mer-[Mo(CO)3(PR3)(L)] (3b, 4b). Synthesis of cis-[Mo(CO)4(L)] (1) and fac-[MO(CO)3L] (2a) and their spectroscopic data are also reported.

Palladium(II) chloride complexes of bis(pyrazolyl)cyclotriphosphazenes

Reactions of the bis(3,5-dimethylpyrazolyl)cyclotriphosphazene derivatives gem-N3P3(MeNCH(2)CH(2)O)(2)(dmp)(2) (1) and nongeminal cis-N3P3(OPh)(4)(dmp)(2) (2) with PdCl2 afford complexes of the type [PdCl2.(L)] (L = 1 or 2). In these complexes, the phosphazenes act as bidentate NN-donor ligands with the two pyrazolyl pyridinic nitrogen atoms bonded to the metal, thus forming a six- and an eight-membered chelate ring, respectively. The structures of 2 and [PdCl2.(2)] (4) have been confirmed by single-crystal X-ray diffraction. Crystal data for 2: a = 16.759(2) Angstrom, b = 10.788(3) Angstrom, c = 19.635(9) Angstrom, beta = 101.61(3)degrees, P2(1/c), Z = 4, R = 0.038 for 4688 reflections with F > 5 sigma(F). Crystal data for 4: a = 9.701(3) Angstrom, b = 24.853(4) Angstrom, c = 15.794(4) Angstrom, beta = 101.46(2)degrees, P2(1/n), Z = 4, R = 0.030 for 5416 reflections with F > 5 sigma(F).

Reactions of copper(I) aryloxides with phenyl isothiocyanate: steric and ligand control in the oligomerization of [N-phenylimino(aryloxy)methanethiolato]copper(I) complexes

The preparation of five different copper(I) complexes [CuSC(=NPh)(OAr)}L(n)]m (1-5) formed by the insertion of PhNCS into the Cu-OAr bond and the crystal structure analyses of three of them have been carried out. A monomeric species 1 (OAr = 2,6-dimethylphenoxide) is formed in the presence of excess PPh3 (n = 2, m = 1) and crystallizes as triclinic crystals with a = 12.419(4) angstrom, b = 13.298(7) angstrom, c = 15.936(3) angstrom, alpha = 67.09(3)-degrees, beta = 81.63(2)-degrees, gamma = 66.54(3)-degrees, V = 2224(2) angstrom3, and Z = 2. The structure was refined by the least-squares method to final R and R(w) values of 0.038 and 0.044, respectively, for 7186 unique reflections. Copper(I) 2,5-di-tert-butyl-4-methylphenoxide results in the formation of a dimeric species 2 in the presence of P(OMe)3 (n = 1, m = 2), where the coordination around Cu is trigonal. Crystals of 2 were found to be orthorhombic with a = 15.691(2) angstrom, b = 18.216(3) angstrom, c = 39.198(5) angstrom, v = 11204(3) angstrom3, and Z = 8. Least-squares refinement gave final residuals of R = 0.05 and R(w) = 0.057 with 6866 unique reflections. A tetrameric species 3 results when PPh3 is replaced by P(OMe)3 in the coordination sphere of copper(I) 2,6-dimethylphenoxide. It crystallizes in the space group P1BAR with a = 11.681 (1) angstrom, b = 13.373(2) angstrom, c = 20.127(1) angstrom, a = 88.55(l)-degrees, beta = 89.65(l)-degrees, gamma = 69.28(1)-degrees, V = 2940(l) angstrom3, and Z = 2. Least-squares refinement of the structure gave final values of 0.043 and 0.05 for R and R(w) respectively using 12214 unique reflections. In addition, a dimeric species 4 is formed when 1 equiv of PPh3 is added to the copper(I) 4-methylphenoxide, while with an excess of PPh3 a monomeric species 5 is isolated. Some interconversions among these complexes are also reported.

Enantiospecific approach to AB-ring system of the diterpenes fusicoccanes

Enantiospecific synthesis of the AB ring system of 5-8-5 tricyclic diterpenes fusicoccanes has been accomplished, starting from the readily available monoterpene (R)-limonene employing an reaction as the key step.

An Approach Toward Homocalystegines and Silyl-homocalystegines. Acid-Mediated Migrations of Acetates in Seven-Membered Ring Systems

A short access to homocalystegine analogues silylated at C7 is described. The synthesis involves the desymmetrization of a (phenyldimethylsilyl)methylcycloheptatriene using osmium-mediated dihydroxylation, followed by the diol protection and a cycloaddition involving the remaining diene moiety and an acylnitroso reagent. Additions of the osmium and acylnitroso reagents were shown, through X-ray diffraction studies of the resulting major isomers, to occur anti and syn, respectively, relative to the SiCH2 substituent. N-O bond cleavage on the resulting cycloadduct then produces the aminopolyol having a silylmethyl substituent. Oxidation of the C-Si bond also afforded an access to unusual amino-heptitols having five contiguous stereogenic centers. In the course of this work, we finally observed a unusual rearrangement taking place on cycloheptanone 18 substituted by two acetyl groups and a neighboring Boc-protected amine. A profound reorganization of the substituents on the seven-membered ring effectively took place under acidic conditions (TFA) leading to the thermodynamically more stable homocalystegine-type compound., DFT calculations of the conformational energy of isomeric silyl homocalystegines indicated that the product observed upon the acid-mediated rearrangement was the most stable of a series of analogues with various distributions of substituents along the seven-membered ring backbone. A tentative mechanism is proposed to rationalize the acetate migrations and inversions of the stereochemistry at various stereocenters.

Ring Resonators and Micro Fluidics: Novel Design for Drug/Disease Detection

MEMS systems are technologically developed from integrated circuit industry to create miniature sensors and actuators. Originally these semiconductor processes and materials were used to build electrical and mechanical systems, but expanded to include biological, optical fluidic magnetic and other systems 12]. Here a novel approach is suggested where in two different fields are integrated via moems, micro fluidics and ring resonators. It is well known at any preliminary stage of disease onset, many physiological changes occur in the body fluids like saliva, blood, urine etc. The drawback till now was that current calibrations are not sensitive enough to detect the minor physiological changes. This is overcome using optical detector techniques 1]. The basic concepts of ring resonators, with slight variations can be used for optical detection of these minute disease markers. A well known fact of ring resonators is that a change in refractive index will trigger a shift in the resonant wavelength 5]. The trigger for the wavelength shift in the case discussed will be the presence of disease agents. To trap the disease agents specific antibody has to be used (e. g. BSA).

1H-4,1,2-Benzothiadiazines from 1,2,3-Benzothiadiazole via Alkylbenzothiadiazolium Salts: A Novel Heterocyclic Ring Expansion Possibly via Nitrogen Ylides.

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Competing peroxidase and oxidase reactions in scopoletin-dependent H2O2-initiated oxidation of NADH by horseradish peroxidase

Addition of NADH inhibited the peroxidative loss of scopoletin in presence of horseradish peroxidase and H2O2 and decreased the ratio of scopoletin (consumed):H2O2 (added). Concomitantly NADH was oxidized and oxygen was consumed with a stoichiometry of NADH: O-2 of 2:1. On step-wise addition of a small concentration of H2O2 a high rate of NADH oxidation was obtained for a progressively decreasing time period followed by termination of the reaction with NADH:H2O2 ratio decreasing from about 40 to 10. The rate of NADH oxidation increased linearly with increase in scopoletin concentration. Other phenolic compounds including p-coumarate also supported this reaction to a variable degree. A 418-nm absorbing compound;d accumulated during oxidation of NADH. The effectiveness of a small concentration of H2O2 in supporting NADH oxidation increased in presence of SOD and decreased in presence of cytochrome c, but the reaction terminated even in their presence. The results indicate that the peroxidase is not continuously generating H2O2 during scopolerin-mediated NADH oxidation and that both peroxidase and oxidase reactions occur simultaneously competing for an active form of the enzyme.

Effects of Ultrafast Solvation on the Rate of Adiabatic Outer-Sphere Electron Transfer Reactions

Recent studies have demonstrated that solvation dynamics in many common dipolar liquids contain an initial, ultrafast Gaussian component which may contribute even more than 60% to the total solvation energy. It is also known that adiabatic electron transfer reactions often probe the high-frequency components of the relevant solvent friction (Hynes, J. T. J. Phys. Chem. 1986, 90, 3701). In this paper, we present a theoretical study of the effects of the ultrafast solvent polar modes on the adiabatic electron transfer reactions by using the formalism of Hynes. Calculations have been carried out for a model system and also for water and acetonitrile. It is found that, in general, the ultrafast modes can greatly enhance the rate of electron transfer, even by more than an order of magnitude, over the rate obtained by using only the slow overdamped modes usually considered. For water, this acceleration of the rate can be attributed to the high-frequency intermolecular vibrational and librational modes. For a weakly adiabatic reaction, the rate is virtually indistinguishable from the rate predicted by the Marcus transition state theory. Another important result is that even in this case of ultrafast underdamped solvation, energy diffusion appears to be efficient so that electron transfer reaction in water is controlled essentially by the barrier crossing dynamics. This is because the reactant well frequency is-directly proportional to the rate of the initial Gaussian decay of the solvation time correlation function. As a result, the value of the friction at the reactant well frequency rarely falls below the value required for the Kramers turnover except when the polarizability of the water molecules may be neglected. On the other hand, in acetonitrile, the rate of electron transfer reaction is found to be controlled by the energy diffusion dynamics, although a significant contribution to the rate comes also from the barrier crossing rate. Therefore, the present study calls for a need to understand the relaxation of the high-frequency modes in dipolar liquids.

Evidence for the involvement of multiple pathways in the biodegradation of 1- and 2-methylnaphthalene by pseudomonas putida CSV86

Pseudomonas putida CSV86, a soil bacterium, grows on 1- and 2-methylnaphthalene as the sole source of carbon and energy. In order to deduce the pathways for the biodegradation of 1- and 2-methylnaphthalene, metabolites were isolated from the spent medium and purified by thin layer chromatography. Emphasis has been placed on the structural characterisation of isolated intermediates by CC-MS, demonstration of enzyme activities in the cell free extracts and measurement of oxygen uptake by whole cells in the presence of various probable metabolic intermediates. The data obtained from such a study suggest the possibility of occurrence of multiple pathways in the degradation of 1- and 2-methylnaphthalene. We propose that, in one of the pathways, the aromatic ring adjacent to the one bearing the methyl moiety is oxidized leading to the formation of methylsalicylates and methylcatechols. In another pathway the methyl side chain is hydroxylated to -CH2-OH which is further converted to -CHO and -COOH resulting in the formation of naphthoic acid as the end product. In addition to this, 2-hydroxymethylnaphthalene formed by the hydroxylation of the methyl group of 2-methylnaphthalene undergoes aromatic ring hydroxylation. The resultant dihydrodiol is further oxidised by a series of enzyme catalysed reactions to form 4-hydroxymethyl catechol as the end product of the pathway.