Stereocontrol at the steady state in radical cyclizations of acyclic dihalides

The first examples of manipulating stereocontrol solely by reaction topography in radical cyclizations starting from acyclic precursors are reported. The kinetic model for acyclic compound stereoselection is verified experimentally by conducting a series of radical cyclizations of 1,3-dihalo-2-(1-phenyl-3-butynyl)propanes with triphenyltin hydride and measuring the ratios of the products. Monohalide intermediates are observed for the first time, and evidence that bromide- and iodide-substituted radicals have different cyclization rate constants is provided.     

Analysis of a microencapsulated-urease fluidized-bed reactor under steady and transient state operating conditions

The feasibility of containing microencapsulated urease within a fludized-bed reactor, eliminating problems of membrane rupture, was demonstrated. Hydrolysis of urea under conditions simulating that of an artificial kidney device was measured as a function of reactor residence time, microcapsule diameter, volume of microcapsules, urea concentration in the feed, and enzyme activity. Empirical correlations were developed based on dimensional analysis, which may be used to predict urea conversion within the range of experimental operating conditions. Results under transient state conditions better represent the operation of the reactor in treatment of uremic patients.     

Theoretical explanation of nonexponential OH decay in reactions with benzene and toluene under pseudo-first-order conditions

OH radical reactions with benzene and toluene have been studied in the 200-600 K temperature range via the CBS-QB3 quantum chemistry method and conventional transition-state theory. Our study takes into account all possible hydrogen abstraction and OH-addition channels, including ipso addition. Reaction rates have been obtained under pseudo-first-order conditions, with aromatic concentrations in large excess compared to OH concentrations, which is the case in the reported experiments as well as in the atmosphere. The reported results are in excellent agreement with the experimental data and reproduce the discontinuity in the Arrhenius plots in the 300 K < T < 400 K temperature range. They support the suggestion that the observed nonexponential OH decay is caused by the existence of competing addition and abstraction channels and by the decomposition of thermalized OH-aromatic adducts back to reactants. We also find that the low-temperature onset of the nonexponential decay depends on the concentration of the aromatic compounds and that the lower the concentration, the lower the temperature onset. Under atmospheric conditions, nonexponential decay was found to occur in the 275-325 K range, which corresponds to temperatures of importance in tropospheric chemistry. Branching ratios for the different reaction channels are reported. We find that for T > or = 400 K the reaction occurs exclusively by H abstraction. At 298 K, ipso addition contributes 13.0% to the overall OH + toluene reaction, while the major products correspond to ortho addition, which represents 43% of all possible channels.     

Synthesis of proton‐ionizable acyclic,macrocyclic and macrobicyclic compounds containing one or two triazole groups

New acyclic, macrocyclic and macrobicyclic compounds containing one or two proton‐ionizable triazole groups are prepared and characterized. The series includes six podands, a macrocycle with one triazole and one pyridine unit in the ring, a bis‐triazolo macrocycle with four pentafluorobenzyl substitutents, and two bis(crown ethers) with a triazolo group connecting the two polyether rings. The solid‐state structure and solubility in supercritical carbon dioxide are determined for the bis‐triazolo macrocycle with pendant pentafluorobenzyl groups.     

Generation and trapping of alkene radical cations under nonoxidizing conditions: formation of six-membered rings by exo- and endo-mode cyclizations

[reaction: see text] It is demonstrated that alkene radical cations generated by the radical ionic fragmentation of beta-(phosphatoxy)alkyl radicals undergo efficient nucleophilic capture by amines in either the 6-exo or 6-endo modes, leading to six-membered nitrogen heterocycles. Suitable placement of an alkene enables the juxtaposition of a radical cyclization resulting in the formation of both the indolizidine and 1-azabicyclo[3.2.1]octane skeleta.     

Exploiting the radical reactivity of diazaphosphinanes in hydrodehalogenations and cascade cyclizations

The remarkable reducibility of diazaphosphinanes has been extensively applied in various hydrogenations, based on and yet limited by their well-known hydridic reactivity. Here we exploited their unprecedented radical reactivity to implement hydrodehalogenations and cascade cyclizations originally inaccessible by hydride transfer. These reactions feature a broad substrate scope, high efficiency and simplicity of manipulation. Mechanistic studies suggested a radical chain process in which a phosphinyl radical is generated in a catalytic cycle via hydrogen-atom transfer from diazaphosphinanes. The radical reactivity of diazaphosphinanes disclosed here differs from their well-established hydridic reactivity, and hence, opens a new avenue for diazaphosphinane applications in organic syntheses.

Unprecedented radical reactivity of diazaphosphinanes was explored to implement efficient hydrodehalogenations and cascade cyclizations.     

Excited state two photon absorption of a charge transfer radical dimer in the near infrared

Nonlinear transmission measurements of a solution of radical dimers of tetramethyl-tetrathiafulvalene, (TMTTF+)2, recorded with 9 ns laser pulses at 1064 nm are reported and interpreted on the basis of a multiphoton absorption process. One finds that the process can be interpreted with a sequence of three photon absorption, the first being a one photon absorption related to the intermolecular charge transfer process characteristic of the dimers and the second a two photon absorption from the excited state created with the first process. A model calculation allows one to obtain the value of the two photon absorption cross section which is found to be several orders of magnitude larger than those usually found for two photon absorbing systems excited from the ground state. These results show the importance of an excited-state population for obtaining large nonlinear optical responses.     

Generation of a C-3'-thymidinyl radical in single-stranded oligonucleotides under anaerobic conditions

[reaction: see text] A C-3'-thymidinyl radical has been photochemically generated site-specifically in DNA oligonucleotides. A nucleoside H-phosphonate bearing a C-3' acetyl group was incorporated into DNA oligomers using a hand-coupling technique. When nucleotides containing the modified monomer were photolyzed (> or =320 nm) in the presence of a hydrogen atom donor, reduction products were detected by RP-HPLC and MALDI-ToF MS analysis.     

Modified reaction conditions to achieve high regioselectivity in the two component synthesis of 1,5-diarylpyrazoles

The factors affecting regioselectivity during the formation of 1,5-diarylpyrazoles from aryl hydrazines and 1,3-diketones are identified and the regioisomers were characterized by 1D NOESY, LC-NMR and X-ray analyses. A simple alteration in the usual reaction conditions is reported, which allows the exclusive formation of 1,5-diarylpyrazoles.     

Electrochemical properties and ion transport behavior of a new family of acyclic polyethers containing two cluster moieties as terminal groups

The electrochemical properties of eight acyclic cluster polyethers [⁵-C₅H₄CH₂(CH₂OCH₂) _n CH₂C₅H₄-⁵][MFeCoE(CO)₈]₂[(1a–f): E = S, M = Mo, n = 2, 3, 4; E = S, M = W, n = 2, 3; E = Se, M = Mo, n = 2], (⁵-MeCOC₅H₄)[(⁵-C₅H₄CH₂CH₂)₂O] [Mo₂FeS(CO)₇][MoFeCoS(CO)₈] (2) and (⁵-MeCOC₅H₄)₂[(⁵-C₅H₄CH₂- CH₂)₂O] [Mo₂FeS(CO)₇]₂ (3) have been investigated in CH₂Cl₂ using cyclic voltammetry with n-Bu₄NPF₆ as the supporting electrolyte. The transport of alkali metal cations through a liquid membrane with the double cluster (3) as carrier has been examined.     

Mild and efficient method for the cleavage of cyclic and acyclic ethers by iodine under solvent-free conditions

Ethers undergo smooth cleavage with acyl chlorides in the presence of a catalytic amount of elemental iodine under extremely mild conditions to give the corresponding halo esters. This new procedure offers significant advantages such as high conversions, short reaction times and enhanced selectivity together with mild reaction conditions, which makes it an attractive strategy.     

Addressing dynamic photovoltaic processes at electrode:active layer and donor:acceptor interfaces in organic solar cells under device-operating conditions

This article presents our experimental studies to unravel the dynamic photovoltaic processes occurring at donor:acceptor(D:A)and electrode:active layer(E:A)interfaces under device-operating conditions by using two unique magneto-optical measurements,namely photo-induced capacitance and magnetic field effect measurement.First,we have found that a higher surface polarization of dielectric thin film can decrease the surface charge accumulation at E:A interface.The photo-induced capacitance results indicate that dielectric thin film plays a crucial role in the charge collection in generating photocurrent in organic solar cells.Second,our experimental results from magnetic field effect show that the binding energies of charge transfer(CT)states at D:A interface can be evaluated by using the critical bias required to completely dissociate the CT states.This is the first experimental demonstration that the binding energies of CT states can be measured under deviceoperating conditions.Furthermore,we use our measurement of magnetic field effect to investigate the most popular organic photovoltaic solar cells,organometal halide perovskite photovoltaic devices.The results of magneto-photoluminescence show that the photogenerated electrons and holes are inevitably recombined into electron–hole pairs through a spin-dependent process in the perovskites.Therefore,using spin polarizations can present a new design to control the photovoltaic loss in perovskites-based photovoltaic devices.Also,we found that introducing D:A interface can largely affect the bulk charge dissociation and recombination in perovskite solar cells.This indicates that the interfacial and bulk photovoltaic processes are internally coupled in developing photovoltaic actions in perovskite devices.Clearly,these magneto-optical measurements show a great potential to unravel the deeper photovoltaic processes occurring at D:A and E:A interfaces in both organic bulk-heterojunction and perovskite solar cells under device-operating conditions.     

Comparison of photoelectron transfer in solution and the solid state for two intervalence radical cations

[structure: see text] The optical diffuse reflectance and solution spectra of two bis-hydrazine radical cationic intervalence compounds have been compared. The results are consistent with an ion-pairing increase and an "effective polarity" in these crystals that is not far from that of acetonitrile or other polar solvents.     

A laboratory comparison of two methods used to estimate the isotopic composition of soil delta13CO2 efflux at steady state

The stable isotopic composition of soil (13)CO(2) flux is important for monitoring soil biological and physical processes. While several methods exist to measure the isotopic composition of soil flux, we do not know how effective each method is at achieving this goal. To provide clear evidence of the accuracy of current measurement techniques we created a column filled with quartz sand through which a gas of known isotopic composition (-34.2 per thousand) and concentration (3,000 ppm) diffused for 7 h. We used a static chamber at equilibrium and a soil probe technique to test whether they could identify the isotopic signature of the known gas source. The static chamber is designed to identify the source gas isotopic composition when in equilibrium with the soil gas, and the soil probe method relies on a mixing model of samples withdrawn from three gas wells at different depths to identify the gas source. We sampled from ports installed along the side of the sand column to describe the isotopic and concentration gradient as well as to serve as a control for the soil probe. The soil probe produced similar isotopic and concentration values as the control ports, as well as Keeling intercepts. The static chamber at equilibrium did not identify the source gas but, when applied in a two end-member mixing model, did produce a similar Keeling intercept produced from the control ports. Neither of the methods was able to identify the source gas via the Keeling plot method probably because CO(2) profiles did not reach isotopic steady state. Our results showed that the static chamber at equilibrium should be used only with a Keeling plot approach and that the soil probe is able to provide estimates of uncertainty for the isotopic composition of soil gas as well as information pertinent to the soil profile.     

X-ray diffraction investigation of siloxanes. I. Effects of organic substituents at the silicon atoms on the structure of acyclic trisiloxane-1,5-diols and on formation of different hydrogen bond systems

The molecular and crystal structure of four acyclic trisiloxane compounds, which differ in substituents at the silicon atoms (Ph-phenyl, mPh-methoxyphenyl, 2mPh-dimethoxyphenyl), was investigated by X-ray diffraction analysis. Due to intermolecular hydrogen bonding between the oxygen atoms of the diol fragments, the crystal structure of 1,1,5,5-tetramethyl-3,3-diphenyl-1,3,5-trisiloxane-1,5-diol (C₁₆H₂₄O₄Si₃) (I) is a double chain architecture with hydrogen-bonded dimeric motifs of C(8)R ₄ ⁴ (12) type in graph set representation. In 1,1,5,5-tetramethyl-3,3-(2-methoxybenzo)-1,3,5-trisiloxane-1,5-diol (C₁₈H₂₈O₆Si₃) (II) and 1,1,5,5-tetramethyl-3,3-(2,6-dimethoxybenzo)-1,3,5-trisiloxane-1,5-diol (C₂₀H₃₂O₈Si₃) (III), a double chain structure with a graph set R ₃ ³ (8)D ₃ ³ (10) is formed. In contrast to I–III, 1,1,3,3,5,5-hexaphenyl-1,3,5-trisiloxane-1,5-diol (C₃₆H₃₂O₄Si₃) (IV) has an intramolecular hydrogen bond S(8). The independent molecules are joined by O-H...O intermolecular hydrogen bonds into centrosymmetrical dimers; the system of hydrogen bonds in general may be described as S(8)R ₄ ⁴ (8).     

Statistical mechanical theory for the structure of steady state systems: application to a Lennard-Jones fluid with applied temperature gradient

The constrained entropy and probability distribution are given for the structure that develops in response to an applied thermodynamic gradient, as occurs in driven steady state systems. The theory is linear but is applicable to gradients with arbitrary spatial variation. The phase space probability distribution is also given, and it is surprisingly simple with a straightforward physical interpretation. With it, all of the known methods of equilibrium statistical mechanics for inhomogeneous systems may now be applied to determining the structure of nonequilibrium steady state systems. The theory is illustrated by performing Monte Carlo simulations on a Lennard-Jones fluid with externally imposed temperature and chemical potential gradients. The induced energy and density moments are obtained, as well as the moment susceptibilities that give the rate of change of these with imposed gradient and which also give the fluctuations in the moments. It is shown that these moment susceptibilities can be written in terms of bulk susceptibilities and also that the Soret coefficient can be expressed in terms of them.     

Superexchange and sequential mechanisms in charge transfer with a mediating state between the donor and acceptor

The rate of intramolecular charge transfer from biphenyl to naphthalene was determined for the radical anions and radical cations of molecules with the general structure: (2-naphthyl)-(steroid spacer)-(4-biphenylyl). Varied degrees of unsaturation (one double bond, NSenB; two double bonds, NSen(2)B; and the b-ring completely aromatized, NSarB) were incorporated into the steroid spacer to examine the effect it would have on the charge transfer rate. The charge transfer rate, as inferred from the decay of the biphenyl radical ion absorption, increased in all cases relative to the completely saturated 3-(2-naphthyl)-16-(4-biphenylyl)-5alpha-androstane (NSB) reference molecule. For the anion charge transfer, the decay rates increased by factors of 1.4, 4.2, and 5.1, respectively, and for the cation, the decay rates increased by factors of 5, 276, and 470. To explain the results, the charge-transfer process was viewed as a combination of two independent mechanisms: a single-step, superexchange mechanism, and a two-step, sequential charge transfer. Using a low level of theory, simple models of the superexchange and two-step mechanisms were developed to elucidate the nature and differences between the two mechanisms. The critical variable for this analysis is the free energy of formation (DeltaG(I) degrees ) of the intermediate state: (2-naphthyl)-[spacer](1)+/--(4-biphenylyl). The conclusion from this treatment is that superexchange is the dominant mechanism when DeltaG(I) degrees is large, but at small DeltaG(I) degrees , the sequential mechanism will dominate. This is because the superexchange rate is shown to have a weak dependence on DeltaG(I) degrees , changing 10-fold for a change in DeltaG(I) degrees of 2 eV, compared to the sequential mechanism in which the rate can change over 10(3) for 0.5 V.     

Formation and spectroscopy of a tryptophan radical containing peptide in the gas phase

A doubly deprotonated tryptophan containing peptide was electrosprayed and isolated in an ion trap. UV excitation on this peptide leads to electron detachment and to the formation of an indolyl radical. The photogenerated radical was fragmented by a second laser. The visible spectrum of the gas-phase neutral tryptophan radical containing peptide has been recorded and constitutes a benchmark for calculations and optical measurements.     

Stereoselective introduction of chiral centres in acylic precursors: a probe into the transition state for V-catalyzed t-butylhydroperoxide (TBHP) epoxidation of acyclic allylic alcohols and its synthetic implications.

Utility of the (Me)₃Si-group for manipulating the relative energies of the diastereomeric transition states for V⁵⁺-catalyzed TBHP epoxidations of acyclic allylic alcohols is presented.