Photochemical Initiation Research Articles

Hydrosilylation of C−C Multiple Bonds Using (Me3Si)3SiH in Water. Comparative Study of the Radical Initiation Step

Fil: Postigo, Jose Alberto. Universidad de Belgrano; Argentina. Consiglio Nazionale delle Ricerche; Italia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Houssay; Argentina

ChemInform Abstract: Allenyl Azide Cycloaddition Chemistry. Photochemical Initiation and CuI Mediation Leads to Improved Regioselectivity.

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Allenyl Azide Cycloaddition Chemistry. Photochemical Initiation and CuI Mediation Leads to Improved Regioselectivity

Irradiation of 2-(3-alkenyl)allenylphenyl azides in the presence of excess CuI furnished functionalized 2,3-cyclopentenylindoles in good yield with only trace amounts of competitive C-N-bonded regioisomeric products. These results represent a significant departure from the modest-to-nonexistent regioselectivity that attended thermal cyclization of these allenyl azide substrates.

Electrochemical oxidation of phenols

Electrochemical oxidation of chlorinated phenols was studied. DOI: 10.1134/S1070427207080198 Chlorinated phenols are superecotoxicants exerting mutagenic, cancerogenic, and bioaccumulating effect on humans [1]. Their cancerogenic properties become stronger with an increase in the number of chlorine atoms in the molecules. Along with exhibiting intrinsic high toxicity, chlorophenols are direct precursors of substances that are still more hazardous for human health: polychlorinated dibenzodioxins, which are formed by condensation of two chlorophenol molecules of any structure. The main sources of chlorophenols in the environment are pulp-and-paper plants where these compounds are formed in the step of cellulose bleaching and breakdown of lignin; also, chlorinated phenols are intermediates in the production of wood preservatives, dyes, pesticides, herbicides, plastics, etc. There are two ways to reduce the environmental pollution by chlorophenols: implementation of new processes that do not involve formation of these superecotoxicants, which is not always acceptable because of the possible loss of the product quality, and improvement of wastewater treatment processes. The most promising processes for solving this problem are oxidation processes, such as ozonation, catalytic decomposition, and photolysis. Today, growing attention is paid to the development of new combined oxidation methods (AOP, Advanced Oxidation Processes) for treating wastewater containing difficultly oxidizable organic compounds. These methods are based on liquid-phase chain oxidation of impurities by the generated highly reactive activated oxygen species, primarily OH . radicals. The combined methods are potentially cleaner environmentally, because they allow, in principle, complete mineralization of difficultly oxidizable pollutants [2 6]. Previously, we have demonstrated the efficiency of the electrocoagulation method based on the reaction between two or several substances having different electrochemical potentials on passing water and air through their mixture. When a system contains hydrogen peroxide, Fenton’s reagent can be formed, which promotes deep degradation of phenols [7]. It has been found that the extent and rate of phenol oxidation depend on many factors, including the presence and nature of substituents [8]. In this study, we examined the electrochemical oxidation of difficultly oxidizable organic compounds in aqueous solutions (with chlorinated phenols as an example) and considered the possibility of photochemical initiation of radical-chain reactions with simultaneous electrochemical oxidation of iron.

New applications of dithiocarbamates in organic synthesis

AbstractThe application of dithiocarbamates in group transfer radical cyclization reactions is described. Carbamoyl diethyldithiocarbamates are synthesized in two high‐yielding steps from secondary amines and act as sources of carbamoyl radicals through chemical or photochemical initiation. Group transfer radical cyclization reactions lead to dithiocarbamate‐functionalized lactams. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:568–571, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20336

Magnetic field effects on the free radical solution polymerization of acrylamide

Systematic polymerization experiments quantified magnetic field (MF) influences on the free radical solution polymerization of acrylamide for various polymerization conditions. The type of initiation, the initiator concentration, the monomer concentration, and the viscosity of the aqueous medium were the subject of variation. The initiator efficiency (Φ) increased up to 60% and the initiator exponent of the overall rate expression raised from 0.1 to 0.28 when a weak MF of 0.1T was applied to photochemical initiation. However, no appropriate effect was observed for thermal initiation. Kinetic analysis proved a reduction of the termination rate coefficient (kt) up to 40%. Photochemically initiated polymerizations in media of enhanced viscosity revealed the highest increment of the initiator efficiency ratio ΦMF/Φ and the most pronounced reduction of ktMF/kt. On the contrary, the propagation rate coefficient (kp) and the monomer exponent were not influenced by MF. Despite considerable increase of the polymerization rate in MF, no reduction of the molar mass was found. Compensation of increased ΦMF and decreased ktMF are suggested as explanation. The singlet–triplet intersystem crossing mechanism for radical pairs served to explain the MF effects.

Stability and repeatability of capillary columns based on porous monoliths of poly(butyl methacrylate-co-ethylene dimethacrylate)

Monolithic poly(butyl methacrylate-co-ethylene dimethacrylate) capillary columns have been prepared via either thermally or photochemically initiated polymerization of the corresponding monomers and the repeatability of their preparation has been explored. Three separate batches of 5 columns each were prepared using thermal and photochemical initiation for a total of 30 columns. All 30 capillary columns were tested in liquid chromatography–electrospray ionisation mass spectrometry mode for the separation of a model mixture of three proteins—ribonuclease A, cytochrome c and myoglobin. Excellent repeatability of retention times was observed for the proteins as evidenced by relative standard deviation (RSD) values of less than 1.5%. Somewhat broader variations with RSD values of up to 10% were observed for the pressure drop in the columns. The stability of retention times was also monitored using a single monolithic column and no significant shifts in either retention times or back pressure was observed in a series of almost 2200 consecutive protein separations.

Fabrication of Chemically Microstructured Polymer Brushes

In this paper, a new and simple pathway to fabricate polymer brush layers with lateral control over the chemical composition is described. The process combines two subsequent free radical grafting from steps: in the first step, a micropatterned polymer brush is grown by photochemical initiation of the polymer growth from the surface through a mask in direct contact. The uncoated areas are then backfilled with a second polymer brush by using the unreacted surface-bound initiator molecules to thermally trigger a second polymerization. As an example for the overall process, the co-assembly of a micropatterned, soft, water-swellable layer consisting of the two-brush system poly(methacrylic acid) (PMAA)-poly(hydroxyethyl methacrylate) (PHEMA) is demonstrated.

Reaction of Alkenes with Hydrogen-Terminated and Photooxidized Silicon Surfaces. A Comparison of Thermal and Photochemical Processes

Reagentless micropatterning of hydrogen-terminated Si(111) via UV irradiation through a photomask has proven to be a convenient strategy for the preparation of ordered bicomponent monolayers. The success of this technique relies upon the differential rate of reaction of an alkene with the hydrogen-terminated and photooxidized regions of the surface. Monolayer formation can be accomplished under either thermal or photochemical conditions. It was observed that, after 3 h, reaction in neat alkene solution irradiation (Rayonet, 300 nm) afforded the expected patterned surface, while thermal conditions (150 degrees C) resulted in a partial loss of pattern fidelity. Monolayer properties and formation were studied on oxidized and hydrogen-terminated silicon under thermal and photochemical initiation, by contact angle, ellipsometry, Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy, and X-ray photoelectron spectroscopy. Results show that alkenes add to silanol groups on the silica surface in a manner consistent with acid catalysis: once attached to the surface, the silica oxidized the hydrocarbon.

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The polymerisation of aqueous sodium undecenoate mesophases

Sodium 10-undecenoate is a polymerisable surfactant having a double bond in the ω position of its chain. This surfactant has been polymerised by McGrath, both in lamellar and hexagonal mesophases by thermal and photochemical initiation, and she obtained relatively low conversions. She could not obtain the polymerisation kinetics. We have polymerised both liquid crystals by irradiation by γ-rays, and we obtained higher conversions: 91.6±0.7% in 50 wt% of surfactant in water (hexagonal mesophase), while McGrath obtained 56% by thermal and 20.6% by photochemical polymerisation; and a maximum of 59.5% in 75 wt% of surfactant in water (lamellar liquid crystal). McGrath obtained conversion values of 24% (thermal initiation) and 19.2% (photochemical initiation) for this mesophase. We could not obtain the polymerisation kinetics of the 50 wt% sample. Even at the lowest irradiation dose the conversion rendered the maximum value. The 75% sample showed a measurable polymerisation increase with γ-radiation dose until 60 kGy, but a large decomposition was observed at 80 kGy. This decomposition, caused by an excess of radiation, is not exceptional. The Fourier transform IR study of the different kinds of water in the system (bulklike water, water related to surfaces and water molecules trapped in the interstices of the microstructures) indicates that there was no significant transformation of the structure during the irradiation; the values of the nonirradiated liquid crystal remained almost unchanged.

An Aliphatic Bifunctional Free Radical Initiator. Synthesis of a Block Copolymer from an Azoperester by Sequential Thermal and Photochemical Initiation

The azoalkaneperester tert-butyl 7-methyl-7-(tert-butylazo)peroxyoctanoate (7) has been found to be a viable bifunctional free radical initiator. Unlike the radicals from previously studied compounds 4−6, those from 7 neither fragment nor cyclize, presumably on account of the medium ring that would be required. Thermolysis of 7 with styrene affords macroinitiator 21 while photolysis affords 22. Photolysis of 21 with MMA yields block copolymer 23 while thermolysis of 22 with MMA affords block copolymer 24. One advantage of 7 is that the photolysis−thermolysis sequence can be reversed, as can the order of addition of monomers.

The photochemical formation of a titan haze analog. Structural analysis by x-ray photoelectron and infrared spectroscopy

The photochemical flow reactor (D.W. Clarke et al., 2000, Icarus 147, 282–291) has been modified to minimize the incorporation of oxygen and other impurities in the photoproducts. A mixture of gases that approximate their mixing ratios on Titan (N 2, CH 4, H 2, C 2H 2, C 2H 4, and HC 3N) (0.98, 0.018, 0.002, 3.5 × 10 −4, 3 × 10 −4, 1.7 × 10 −5, respectively) was irradiated in the flow photochemical reactor using a 185-nm source to give a Titan haze analog as a solid product. X-ray photoelectron spectroscopy (XPS) gave a composition of 93.3% C, 5.3% N, and 1.4% O. Of the 93.3% carbon, high-resolution XPS revealed that 81.2% was present as CH, CC, and CC groups, 12.1% may be CO, CN, CN, CN, and/or CN groups, 5.3% as a CN group. The peak for N was symmetrical and was assigned to the CN while that for oxygen was assigned to the CO and/or the CO group. Some of these assignments were confirmed by FTIR spectroscopy. The polymeric product had a C:N ratio of 17.6, which is significantly greater than that for Titan haze analogs prepared in discharge reactions. When the polymer was exposed to air for seven days the oxygen content increased by 6% along with an increase in the infrared absorption at 1710 cm −1 assigned to the CO group of a ketone. The oxidation is attributed to the reaction of oxygen with free radicals trapped in the polymer matrix. It is proposed that the photochemical initiation of Titan haze formation from compounds formed from starting materials formed high in Titan’s atmosphere is a more plausible model than haze formed in reactions initiated by solely by discharges. These data will be helpful in the interpretation of the data returned from the Huygens probe of the Cassini mission.

Diffusion in porous silicon: effects on the reactivity of alkenes and electrochemistry of alkylated porous silicon

Alkenes are known to react with hydrogen-terminated silicon surfaces to produce robust organic monolayers that are attached to the surface via covalent SiC bonds. In this report we investigate the dependence of the rate of alkylation of porous silicon samples on the reaction time using photochemical initiation. The kinetics of the photochemical alkylation of hydrogen-terminated porous silicon by undec-1-ene in toluene were observed to be pseudo first order, however the apparent rate constant decreased as the concentration of undec-1-ene increased. This behaviour is opposite to what would be expected if the rate-limiting process was an elementary chemical reaction step involving the alkene. Instead, it suggests that transport of the alkene to reactive sites and in the correct orientation is the rate-limiting step. Comparison of the rates of alkylation of porous silicon by undec-1-ene and dimethoxytrityl (DMT)–undecenol is consistent with such an interpretation as the bulky DMT headgroup gives a lower rate of alkylation. The diffusion of some simple redox-active probe molecules in porous silicon was investigated using a scanning electrochemical microscope (SECM). The probe molecules are converted at diffusion-controlled rate at an inlaid disk ultramicroelectrode (UME) consisting of the cross-section of a microwire sealed in glass. If the microelectrode is placed a short distance above the porous silicon, the microelectrode current depends on kinetics of the electrochemical reactions at the porous silicon and the mass transport properties within the open thin layer cell formed by the microelectrode and the alkylated porous silicon. In order to differentiate the effects of finite heterogeneous kinetics at silicon from diffusion limitations, current–distance curves were fitted over a wide range of applied potentials (on the Si) and it was observed that the diffusion coefficient in the porous layer was strongly anisotropic. The measured diffusion rates are comparable to those in bulk water along the pores, but with negligible diffusion between pores. This indicates that few pore–pore interconnections exist in the porous silicon.

On the laser ignition and initiation of explosives

Energetic materials are conventionally ignited by the application of heat to some part of an explosive target. This is most often provided by a flame or by electrical heating using a resistive wire. The material responds by thermally heating and starting a burning zone, which spreads out from the ignition point generating gas. In some cases this zone can accelerate (due to the effect of this gas) into a reactive shock wave, termed detonation. Lasers are used in a variety of applications to thermal heat a range of materials, and thus seem an obvious candidate to trigger chemical reaction in energetic ones. A light pulse offers several advantages over an electrical one, since it may be delivered down a path both immune to electrical effects and chemically stable. Thus, the triggering of safety apparatus (such as the firing of bolts on aircraft exits) represents a major thrust in the development of laser–triggered explosive devices. The energy of the pulse may be used in one of two ways to achieve these effects. In the first, the laser is shone directly upon the chemical medium, which absorbs the light at discrete wavelengths. In the second, it is used to vaporize a metallic flyer that is launched to impact on a target creating a high–pressure zone. Either mechanism starts the required reaction. However, when the pulse is delivered directly to the material, detonation is found to proceed immediately with no transition via burning. This makes the process inexplicable using present concepts. This review will address a range of the experiments conducted and theories developed for this novel field. However, it should be emphasized that the fundamental mechanisms remain to be fully explained making the application both academically stimulating as well as industrially important.

Polymerized Ionic Amphiphiles: Synthesis and Effects in the Enantioselective Hydrogenation of an Amino Acid Precursor

A number of polymerizable ionic amphiphiles has been polymerized in water at various concentrations above their critical micelle concentration (cmc). Oligomeric and polymeric species are formed upon photochemical initiation. Depending on the position of the double bond within the monomer and polymerization conditions (temperature, concentration, use of radical starters) polymeric assemblies were obtained of different size and hence water-solubility. After separation of low-molecular components by dialysis water-soluble polymers have been characterized by their molecular weights (membrane and vapor pressure osmometry) and transmission electron microscopy (TEM). They showed different efficiency as promotors in the rhodium complex-catalyzed asymmetric hydrogenation of (Z)-methyl α-acetamidocinnamate in water and have been compared with their monomeric counterparts. As was demonstrated in one case, associates formed by intramicellar polymerization display an enhanced catalytic efficiency whereas clustered assemblies are inactive.

Gastroinestinal Pathology, Abstract 254–262, Symposium

Alkenes are known to react with hydrogen-terminated silicon surfaces to produce robust organic monolayers that are attached to the surface via covalent SiC bonds. In this report we investigate the dependence of the rate of alkylation of porous silicon samples on the reaction time using photochemical initiation. The kinetics of the photochemical alkylation of hydrogen-terminated porous silicon by undec-1-ene in toluene were observed to be pseudo first order, however the apparent rate constant decreased as the concentration of undec-1-ene increased. This behaviour is opposite to what would be expected if the rate-limiting process was an elementary chemical reaction step involving the alkene. Instead, it suggests that transport of the alkene to reactive sites and in the correct orientation is the rate-limiting step. Comparison of the rates of alkylation of porous silicon by undec-1-ene and dimethoxytrityl (DMT)–undecenol is consistent with such an interpretation as the bulky DMT headgroup gives a lower rate of alkylation. The diffusion of some simple redox-active probe molecules in porous silicon was investigated using a scanning electrochemical microscope (SECM). The probe molecules are converted at diffusion-controlled rate at an inlaid disk ultramicroelectrode (UME) consisting of the cross-section of a microwire sealed in glass. If the microelectrode is placed a short distance above the porous silicon, the microelectrode current depends on kinetics of the electrochemical reactions at the porous silicon and the mass transport properties within the open thin layer cell formed by the microelectrode and the alkylated porous silicon. In order to differentiate the effects of finite heterogeneous kinetics at silicon from diffusion limitations, current–distance curves were fitted over a wide range of applied potentials (on the Si) and it was observed that the diffusion coefficient in the porous layer was strongly anisotropic. The measured diffusion rates are comparable to those in bulk water along the pores, but with negligible diffusion between pores. This indicates that few pore–pore interconnections exist in the porous silicon.

Hydrocarbon Activation with Cerium(IV) Ammonium Nitrate: Free Radical versus Oxidative Pathways

The reactivity of propellane C−C bonds towards cerium(IV) ammonium nitrate (CAN) was studied utilizing photochemical initiation in acetonitrile. Synthetic as well as computational (B3LYP/6-311+G** and MP2/6-31G*) data strongly suggest that the activation of the C−C bonds in cyclopropane and cyclobutane derivatives involves NO3 radical attack on the hydrocarbon ring and does not proceed through single electron transfer. The product structures are not consistent with the intermediate formation of propellane radical cations. These propellane systems could be generated independently by oxidation with charged electrophiles, by anodic oxidation, and through photo-oxidation with 1,2,4,5-tetracyanobenzene; the observed chemical behavior of radical cations is clearly different.

Synthesis of Cross-Linkable Telechelic Poly(butenylene)s Derived from Ring-Opening Metathesis Polymerization

The synthesis of ring-opening metathesis polymerization (ROMP) chain transfer agents (CTA)s bearing methacrylate or epoxide end functionality was accomplished. In the presence of these CTAs, the ROMP of cyclooctadiene (COD) initiated with (PCy3)2Cl2RuCHPh afforded the respective telechelic poly(butenylene)s with either methacrylate or epoxide end groups. Control over the polymer molecular weight was demonstrated by varying the COD/CTA ratio. Successful cross-linking by thermal or photochemical initiation of the bis(methacrylate)-functionalized telechelic poly(butenylene)s or through acid catalysis of the bis(epoxide)-functionalized telechelic poly(butenylene)s was accomplished. While cross-linking was found to reduce solubility of the polymers, it greatly enhanced their thermal stability.

Photochemical alkylation of ketene dithioacetal S,S-dioxides. An example of captodative olefin functionalization

Radical alkylation of some ketene dithioacetal S,S-dioxides failed through the tin hydride promoted chain process but was successfully performed through stoichiometric photochemical initiation, either by electron transfer or hydrogen abstraction. In the first case, alkyl radicals were produced from tetralkylstannanes (t-Bu-, i-Pr-, n-Bu-SnR(3)) via radical cation fragmentation, while in the second case these were produced from alkanes (cyclohexane, adamantane) by benzophenone triplet. When bulky radicals (t-Bu, adamantyl) were involved, the addition occurred with complete diastereoselectivity.