Photochemical Irradiation Research Articles

A silyl-mediated [3+2] photochemical cycloaddition

The generation of larger rings from intermediate cyclobutanes via a two-step [2+2] photocycloaddition – ring expansion, known as the de Mayo reaction, has been widely applied in synthesis. Herein a one-step synthesis of cyclopentanoids has been developed based on the photochemical irradiation of an enone in the presence of an allylsilane. The ability of the silyl moiety to stabilize the intermediate biradical is believed to be responsible for this unique transformation where one normally expects to see the [2+2] cyclobutane adduct.Key words: [2+2] photocycloaddition - ring expansion, allylsilanes, cyclopentanoids, photochemical.

Donor Stabilized Borylnitrene: A Highly Reactive BN Analogue of Vinylidene

A donor atom stabilized borylnitrene, 2-nitreno-1,3,2-benzodioxaborole 4c, is characterized by matrix isolation IR, UV, and ESR spectroscopy as well as multiconfiguration SCF and CI computations. UV irradiation (lambda = 254 nm) of the corresponding azide 6c, isolated in solid argon at 10 K, produces 4c in high yield. The oxygen donor atoms in 4c result in a triplet ground state (|D/hc| = 1.492 cm(-)(1), |E/hc| = 0.004 cm(-)(1)) for the borylnitrene. The lowest energy singlet state ((1)A(1)) is 33 kcal mol(-)(1) higher in energy and closely related to the ground state of vinylidene. Under the conditions of matrix isolation, triplet 4c is photochemically and thermally stable toward rearrangement to the corresponding cyclic iminoborane. Photochemical irradiation (lambda > 550 nm) of 4c rather causes an efficient reaction with molecular nitrogen, lying in matrix sites nearby, to give 6c. Similarly, photochemical, but not thermal, trapping of 4c with CO is possible and results in the corresponding isocycanate 9c. Thermal reaction of 4c with O(2) in doped argon matrixes at 35 K could be observed by IR spectroscopy to result in borylnitroso-O-oxide 17c as shown by (18)O(2) labeling experiments and DFT computations. The diradical 17c is very photolabile and quickly rearranges to the nitritoborane 16c upon irradiation.

Photochemical Ring-Contraction of a Tetrasilaferracyclohexane of the (η5-C5H4)Fe(CO)2 System to Trisilaferracyclopentanes

Treatment of (η 5 -C5H5)Fe(CO)2(SiMe2) n Cl (n = 3, 4) with lithium diisopropylamide leads directly to new trisila- (1) and tetrasilaferracycles (2) [(η 5 -C5H4)Fe(CO) 2 (SiMe 2 ) n ]. The trisilaferracycle (1) exhibits a degree of ring strain as noted by 29 Si NMR spectroscopy and single-crystal X-ray analysis but is stable with respect to limited attempts at ring-opening polymerization and photochemical transformation. In contrast, 2, with no ring strain, readily transforms via photochemical irradiation by way of iron silyl-(silylenes) to a trimethylsilyl-substituted trisilaferracycle 4, which in turn transforms to 1 upon prolonged irradiation.

Prediction of CL-20 chemical degradation pathways, theoretical and experimental evidence for dependence on competing modes of reaction

Highest occupied and lowest unoccupied molecular orbital energies, formation energies, bond lengths and FTIR spectra all suggest competing CL-20 degradation mechanisms. This second of two studies investigates recalcitrant, toxic, aromatic CL-20 intermediates that absorb from 370 to 430 nm. Our earlier study (Struct. Chem., 15, 2004) revealed that these intermediates were formed at high OH− concentrations via the chemically preferred pathway of breaking the C–C bond between the two cyclopentanes, thereby eliminating nitro groups, forming conjugated π bonds, and resulting in a pyrazine three-ring aromatic intermediate. In attempting to find and make dominant a more benign CL-20 transformation pathway, this current research validates hydroxylation results from both studies and examines CL-20 transformations via photo-induced free radical reactions. This article discusses CL-20 competing modes of degradation revealed through: computational calculation; UV/VIS and SF spectroscopy following alkaline hydrolysis; and photochemical irradiation to degrade CL-20 and its byproducts at their respective wavelengths of maximum absorption.

Thermal and photochemical isomerization of exocyclic 1,3-dienes: 1,1-diphenyl-3,4-bis(trimethylsilylmethylene)-1-silacyclopentane s- cis(E,E)

This paper describes the photochemical and the thermal isomerization of s-cis(E,E) 1,1-diphenly-3,4-bis(trimethylsilylmethylene)-1-silacyclopentane ( 1a). Under thermal conditions a 1,3-sigmatropic of the methylene hydrogen occurs, yielding the s-trans isomer ( 1b). The photochemical irradiation of ( 1a) at 300 nm for 1 h in deoxygenated benzene gives the corresponding s-cis( E, Z) isomer ( 1c) and then the s-cis( Z, Z) isomer ( 1d) after prolonged irradiation (3 h). There was no evidence for the formation of the corresponding cyclobutene resulting from the ring closure of the exocylic diene.

Synthesis and Properties of Hybrid Organic−Inorganic Materials Containing Covalently Bonded Luminescent Polygermanes

Covalent binding of arylmethylpolygermanes into hybrid silica materials leads to substantial increases in thermal stability without degrading Ge−Ge σ-conjugation, as determined by UV−vis absorption and fluorescent emission spectroscopy. Poly(methyltrimethoxysilylphenylgermane) was prepared by ruthenium-catalyzed demethanative coupling of dimethyl(trimethoxysilylphenyl)germane, and then hydrolyzed and cured in the presence of either tetraethoxysilane (TEOS) or β-acetoxyethylsilsesquioxane (BAESSQ) to yield hybrid materials containing the polygermane linked to the matrix at every repeat unit. Thermal stability of the bulk hybrid materials was examined by TGA, and films were examined by UV−vis spectroscopy following thermal treatment and photochemical irradiation. Conjugation of the Ge−Ge chains is maintained to 380 °C under nitrogen in the case of the BAESSQ-derived hybrids, but only 200 °C in air. The photolytic stability of the hybrid films is greater than the parent poly(methylphenylgermane), but the gains are less dramatic. A model is proposed in which anchoring each germanium in the polymer chain to the silica matrix reduces mobility of germyl radicals produced by both thermolysis and photolysis, leading to efficient recombination to restore the Ge−Ge chains under nitrogen. On the other hand, germylene intermediates generated only during photolysis are more reactive and can combine irreversibly with Si−O−Si bonds or residual SiOH groups in the silica matrix, leading to faster photolytic degradation of the conjugated Ge−Ge chains, even under nitrogen.

Enantiospecific photochemical carbon skeletal rearrangement of Morita–Baylis–Hillman products in water

Asymmetric carbon skeletal rearrangements of Morita–Baylis–Hillman products, α-hydroxymethylenones, under photochemical irradiation in water are described, wherein the asymmetric induction mechanism is discussed in detail.

PHOTOCHEMICAL DEGRADATION OF POLYMERS CONTAINING URETHANE GROUPS IN THE SOLID PHASE: INFLUENCE OF S BLOCK CATIONS ON THE DEGRADATION

Modification of the physico-mechanical properties of films made of biologically compatible polymers, with urethane groups, complexed with metallic ions from the s-block (Soft Block), have been investigated according to cation nature and photochemical irradiation time. Strain at the break, remnant elongation, elasticity modulus and resistance at the break were the main physico-mechanical properties measured. Experiments were carried out by irradiation with ultraviolet light, on solid film, under condition of accentuated degradation. Ions of both alkaline metals and alkaline-earth metals in the first step of irradiation improve the physico-mechanical properties. But in the middle of the investigated range they strongly decrease, concomitantly with the appearance of some deep photochemical induced structural modifications in the structure of the investigated polyesterurethane.

Enantiospecific synthesis of vicinal stereogenic tertiary and quaternary centers by combination of configurationally-trapped radical pairs in crystalline solids.

[reaction: see text] Photochemical irradiation of crystalline (2R,4S)-2-carbomethoxy-4-cyano-2,4-diphenyl-3-butanone 1 led to highly efficient decarbonylation reactions. Experiments with optically pure and racemic crystals showed that the intermediate radical pairs undergo a highly diastereo- and enantiospecific radical-radical combination that leads to the formation of two adjacent stereogenic centers in good chemical yield and with high chemical control. Reactions with chiral crystals occurred with quantitative enantiomeric yields and >95% diastereomeric yields.

Dithiocarbamate derivatives of μ-thiocarbyne complexes: synthesis and X-ray molecular structure of [Fe 2(μ-CS)(μ-CSMe)(μ-S 2CNMe 2)Cp 2

Reaction of [Fe 2(μ-CS)(μ-CSMe)Cp 2(CO) 2] + ( 1) with sodium N, N-dimethyldithiocarbamate [Me 2dtc]Na affords a mixture of the dithiocarbene [Fe Fe(μ-CS){μ-C(SMe)S C(S)NMe 2}Cp(CO)] ( 2) and the thiocarbyne [Fe 2(μ-CS)(μ-CSMe)(μ-S 2CNMe 2)Cp 2] ( 3). Complex 2 is quantitatively converted into 3 by photochemical irradiation. The X-ray molecular structure of 3 demonstrates the presence of three bridging ligands and exhibits the shortest FeFe interaction found in similar systems [2.453(1) Å]. Complex 3 can also be obtained by reacting [Me 2dtc] − with the di-solvento thiocarbyne [Fe 2(μ-CS)(μ-CSMe)(NCMe) 2Cp 2] + ( 1a). By contrast no stable addition product has been isolated in analogous reactions involving the thiocarbyne [Fe 2(μ-CO)(μ-CSMe)Cp 2(CO) 2] + ( 1b). The [Me 2dtc] − nucleophilic addition at the μ-C to form [Fe 2(μ-CO){μ-C(CN)SC(S)NMe 2}Cp 2(CO) 2] ( 4a) is obtained starting from [Fe 2(μ-CO){μ-C(CN)(SMe 2)}Cp 2(CO) 2]SO 3CF 3 ( 1d). Photochemical reaction of the cyanocarbene 4a causes intramolecular ring closure affording [Fe Fe(μ-CO){μ-C(CN)S C(S)NMe 2}Cp 2(CO)] ( 5a).

Molecular Crystal Architecture and Optical Properties of a Thiohelicenes Series Containing 5, 7, 9, and 11 Rings Prepared via Photochemical Synthesis

Helicenes are intrinsically chiral helical molecules structurally characterized by ortho-fusion of aromatic rings. A series of racemic heterohelicenes composed of thiophene and benzene rings alternated in the helical chain were prepared via photochemical irradiation. The availability of large systems containing 9 and 11 condensed rings allows a better understanding of both optical properties and solid-state organization of thiohelicene molecules. The conformation and the crystal architecture of thiohelicenes, containing up to 11 rings show ring distortions smaller than in carbohelicenes. All the crystals obtained are racemates, and heterochiral assembling is dominant also for the larger systems.

Photochemistry of bridged disilyldiiron complexes (SiMe 2)[(η 5-C 5H 4)Fe(CO) 2SiMe 2SiMe 2R] 2, R=Me, Ph

Photochemical irradiation of the new bimetallic disilyliron complexes (SiMe 2)[(η 5-C 5H 4)Fe(CO) 2SiMe 2SiMe 2R] 2, R=Me, Ph, results in silylene elimination chemistry, as observed for the mono-metallic analogs, resulting in the formation of complexes (SiMe 2)[(η 5-C 5H 4)Fe(CO) 2SiMe 2R] 2. The intermediate silyl(silylene) complexes can be intercepted with HMPA. Apart from a very reactive minor product that we have been unable to identify at this time, the proximity of the two transition metal centers has little impact upon the observed chemistry.

Photochemical and photophysical behaviour of porphyrins and phthalocyanines irradiated with violet or ultraviolet light

Photochemical irradiation of porphyrins or phthalocyanines with violet or ultraviolet light absorbed by their second or higher absorption bands populates their higher excited states. These higher excited states generally dissipate very rapidly, but, competing with such rapid deactivation, often show unique behaviour different from the lowest excited states populated under visible light irradiation of the first absorption band, namely wavelength-dependent reactions or emission. The higher excited states often exhibit characteristic emission and enhanced reactivity in electron transfer, energy transfer and so on, competing with rapid deactivation. This paper attempts to summarize our work, perfomed particularly by means of femtosecond spectroscopy, and recent progress in this area.

Studies on energetic compounds: Part 16. Chemistry and decomposition mechanisms of 5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-one (NTO)

The present review deals with the chemistry and thermolysis of NTO and plausible decomposition pathways have been described. The decomposition of 5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-one (NTO) induced by X-ray, UV, laser, photochemical irradiation has also been discussed. High-speed photographic studies of the impact responses of NTO are also included. The thermal decomposition of labelled NTO has also been described here. Methods of detection as well as safe disposal of NTO have also been mentioned.

Reactivity of the Base-Stabilized Bis(silylene)iron Complex (η5-C5H5)Fe(CO)(η2-SiMe2-OtBu-SiMe2): Elevated Temperature Trapping of SiMe2 by R3EH (R = Me3Si, E = Si, Ge) and Elimination of Me2(OtBu)SiSiMe2H by n-Bu3SnH

The base-stabilized silylene complex (η5-C5H5)Fe(CO)(η2-SiMe2-OtBu-SiMe2) is unreactive toward (Me3Si)3EH (E = Si, Ge) under photochemical irradiation or at room temperature. However, at 80 °C it reacts, presumably via the equilibrium concentration of base-free complex (η5-C5H5)Fe(CO)(SiMe2OtBu)(SiMe2), to transfer the silylene group and form (Me3Si)3ESiMe2H. Attempts to transfer the SiMe2 group to tributyltin hydride led to formation of bis(tributylstannyl)iron complexes.

Sonochemical and photochemical oxidation of organic matter

Recent developments in sonochemistry have led us to study its use to treat water and wastewater. The effects of ultrasound wave in hydrophilic chemical oxidations are mainly due to hydroxyl radical production during the cavitation-induced water decomposition. Currently, the sonochemical destruction of aromatic compounds in water solution is obtained with low rates. The aim of this work is to evaluate the efficiency of the sonochemical effect in conjunction with a photochemical irradiation. Taking phenol as an example, the combined action of sonochemistry and photochemistry has been considered in a ‘sonuv’ reactor. An important enhancement of the degradation rate of phenol has been observed. It may be the result of three different oxidative processes: direct photochemical action, high frequency sonochemistry and reaction with ozone (produced by UV irradiation of air). The process has been successfully tested to lower the chemical oxygen demand of a municipal wastewater.

Rhodium complexes with hydrotris(3- p-anisylpyrazol-1-yl)borate ligand Tp pAn. Intramolecular CH bond activation and dehydro-chlorination processes

Solution studies by 1H- and 13C-NMR, and IR spectroscopy of rhodium(I) complexes [Rh(η 4-nbd)(Tp pAn)] ( 1), [Rh(η 4-cod)(Tp pAn)] ( 2) and [Rh(CO) 2(Tp pAn)] ( 3) (nbd=2,5-norbornadiene; cod=cycloocta-1,5-diene) have been performed. In all cases square planar complexes containing the Tp pAn ligand in a bidentate η 2-bonded form were observed. Compounds 2 and 3 exist as mixtures of two isomers with the third uncoordinated pyrazolyl ring occupying an equatorial position (form A) or an axial position (form B), but in complex 1 only form B is present. X-ray crystallography proved that complexes 1 and 3 are also tetracoordinated in solid state: 1 (monoclinic, space group P2 1/ c) and 3 (monoclinic, space group P2 1/ c), whose structures correspond to two different B forms, their difference lying in the disposition of the axial pyrazolyl group. Photochemical irradiation of [Rh(CO) 2(Tp pAn)] ( 3) in a variety of solvents afforded the aryl hydride [R h(H)(CO){HB(C 3H 2N 2C 6H 3OCH 3)(C 3H 2N 2C 6H 4OCH 3) 2}] ( 4) by intramolecular cyclometallation involving an ortho CH bond of one p-anisyl substituent. Functionalization of the hydride 4 by chlorinated solvents resulted in the chloro complex [R h(Cl)(CO) { HB(C 3H 2N 2C 6H 3OCH 3)(C 3H 2N 2C 6H 4OCH 3) 2}] ( 5), which maintains the intramolecular ortho Cmetal bond. Evolution of the hydride 4 to the chloro complex 5 in CDCl 3 occurs through a hydrodechlorination process as deduced by monitoring the NMR spectra. Analysis of 2D NMR data ( 1H– 1H COSY, 1H– 13C HMQC and 1H– 13C HMBC) allowed the full identification of 4 and 5.

DIFFÉRENCE DE COMPORTEMENT ENTRE LES RÉACTIFS ELECTROPHILES DIÉTHYLTRICHLOROMÉTHYLPHOS- PHONATE ET TÉTRACHLORURE DE CARBONE OPPOSÉS AU TRIÉTHYLPHOSPHITE

The reaction of triethylphosphite 1 with diethyltrichloromethylphosphonate 2b in the absence of solvents and catalysts yields mainly diethyl(1,1-dichloropropyl)phosphonate 4 and diethyldichloromethylphosphonate 7. These main products are accompanied by a variety of by-products which differ considerably from those obtained in the reaction between triethyl phosphite and CCl4. Both reactions would begin by a halophilic substitution of triethylphosphite on the positive halogen. For diethyltrichloromethylphosphonate 2b the formed carbanion would then attack preferentially the carbon β to P+ and the hydrogen γ to P+ on heterophosphonium cation. In contrast, the less sterically hindered trichloromethyl carbanion formed by the halophilic attack of triethylphosphite on carbon tetrachloride would rather attack mainly the positive P+ of this cation. The tetraethyl(dichloromethylene)bisphosphonate 8 previously reported by Kukhar and Sagina to be the main product formed in the thermal reaction between triethylphosphite and diethyltrichloromethylphosphonate was not found under our conditions. We observed its formation only under photochemical irradiation.

Photochemical transformations of matrix-isolated cycloalkyl nitrites †

Photochemical transformations of cycloalkyl nitrites isolated in argon matrices have been studied. In the case of cyclohexyl, cycloheptyl and cyclooctyl nitrites complexes of HNO with the corresponding cycloalkyl ketones were formed. Photochemical irradiation of cyclobutyl nitrite resulted in the formation of 4-nitrosobutanal as the major product. Under similar conditions irradiation of cyclopentyl nitrite gave a mixture of 5-nitrosocyclopentanal and the complex of cyclopentanone and HNO.

Dynamic headspace analysis of the light-induced controlled release of perfumery aldehydes and ketones from ?-keto esters in bodycare and household applications

The controlled release of citronellal and menthone from α-keto esters was tested in different household and bodycare applications by photochemical irradiation of these esters with artificial light sources or natural sunlight. Quantitative dynamic headspace analyses carried out with different application bases, such as all-purpose cleaners and hair conditioners, as well as investigations on different supports, such as glass surfaces, hair or cotton, clearly showed that a sufficient amount of perfume was released in all cases, thus allowing a very broad use of this principle as an efficient delivery system for the controlled release of perfumery chemicals. In addition, the quantitative dynamic headspace analyses carried out in these studies help to elucidate the kinetics of perfume release tested in the different applications for a large variety of compounds. Copyright © 2000 John Wiley & Sons, Ltd.