Irradiation-promoted oxidations

Alcohol Oxidation for Hydrogen Storage and Production Irradiation-Promoted Oxidations... 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

These instruments employ a continuous flow of persulfate solution to promote oxidation prior to ultraviolet irradiation, and have a low system blank and low detection limit. Since all reactions take place in the liquid phase, problems suffered by combustion techniques, such as catalyst poisoning, reactor corrosion, and high-temperature element burnouts, are obviated. However, the ultraviolet-promoted chemical oxidation technique is not designed to handle particulate-containing samples, and tends to give incomplete oxidation for certain types of compounds such as cyanuric acid. 

Microwave irradiation promotes the 1,3-dipolar activity of nitrile oxides generated from hydroximoyl chlorides. They interacted in situ over alumina with alkenes and alkynes (150). The effect was demonstrated in reactions of... 

Figure 12.21 shows the effect in an n-type semiconductor of promotion of electrons by incident photons and subsequent electrode reactions. This figure should be compared with Fig. 6.9 for an n-type semiconductor without incident radiation. Irradiation facilitates oxidation, a significant overpotential being unnecessary. Figure 12.22 compares schematically what is obtained at semiconductor electrodes with and without incident light. As is to be expected there is no photoeffect (except in rare cases) for potentials more negative than Uft,. 

A number of other photo-oxidations involving aromatic substrates have been reported, some of which again have been carried out in non-traditional reaction environments. A light-promoted oxidative cleavage of the olefinic bond in aromatic alkenes, which takes place in mesoporous Si02 (FSM 16) and involves a catalytic amount of I2, has been reported (Scheme 23). The photochemical involvement appears to extend no further than the formation of the iodine atoms. The suggestion that an a-iodoketone is an intermediate in the process is based on NMR evidence from the reaction where R = Bu, but it is not clear how this intermediate is converted to the final product. A similar transformation of aromatic alkenes has been carried out in zeolite NaY. Irradiation of styrene,... 

The irradiation environment plays an important role in the evolution of polymer stability. While unsaturated hydrocarbon like acetylene [61] or divinyl benzene [62] is present in the material surrounding and provides radicals for the formation of intermolecular bridges, oxidative atmosphere, oxygen or air, promotes oxidation as the result of diffusion inside the polymer matrix. The distribution profile for carbonyl products that generated during irradiation takes a parabolic form [63]. The source of radicals may be one of the components of blends, which presents a lower stability. This case can be illustrated by various blends, EPDM/PP [64], EPDN-NR [65]. These polymer mixture show the maximum level of crosslinking at about 120-150 kGy. 

In the medical praxis, UHMWPE have been used successfully as one half of the bearing couple (against metallic alloys or ceramics) in total hip and total knee joint replacements crosslinked under irradiation [82]. Even y-irradiated UHMWPE at doses higher than 100 kGy presents the incipient ffacmre development [83], the alkyl macroradicals are involved in crosslinking and in a smaller proportion they promote oxidation [84, 85]. The prosthesis manufactures by the irradiation of UHMWPE have long durability, because the application of radiation treatment induces an increased crystallinity and promotes sterilization in the whole volume of material. 

In a related transformation, the same group described a three-component domino reaction of 2-hydroxy-l,4-naphthoquinone, acting as a P-dicarbonyl surrogate, aromatic aldehydes and 1-(2-oxo-2-phenylethyl)pyridinium bromides in the presence of ammonium acetate, in water under microwave irradiation, that affords naphtho[2,3-Z)]furan-4,9-diones 31 [22]. Mechanistically, this domino reaction probably follows the same Knoevenagel-Michael intramolecular S 2 pathway described for the preparation of compounds 28, with ammonium acetate acting in this case as the base, to give intermediates 33. These compounds, which can be considered as tautomers of a hydroquinone species, would be finally transformed into the observed quinones by air-promoted oxidation, yielding the final products 31 (Scheme 1.16). 

The solvent-free potassium permanganate promoted oxidation of alcohols into the corresponding carbonyl derivatives has been examined by Luu et al. (2008). Secondary alcohols were oxidized very efficiently to the corresponding ketones at ambient temperature by KMnO absorbed on a fourfold molar amount of copper (II) sulfate pentahydrate. The reaction rate was enhanced considerably by ultrasonic irradiation also, but drastically in the presence of microwave irradiation, may be due... 

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

Water-soluble sdanols such as (1) were found to undergo successive oxidative demethylations with tropospheric ultraviolet irradiation in the presence of suitable chromophores, such as nitrogen oxides (516). The water-soluble methylated sdicones did not promote diatom (Nap cu/apelliculosd) growth but the demethylated photo products did. The sequence of sod-induced degradation of sdicones to water-soluble species such as (1), followed by light-induced conversion to sdicate, suggests a pathway, conceptually at least, for the mineralization of sdicones. 

Luche and coworkers [34] investigated the mechanistic aspects of Diels-Alder reactions of anthracene with either 1,4-benzoquinone or maleic anhydride. The cycloaddition of anthracene with maleic anhydride in DCM is slow under US irradiation in the presence or absence of 5% tris (p-bromophenyl) aminium hexachloroantimonate (the classical Bauld monoelectronic oxidant, TBPA), whereas the Diels Alder reaction of 1,4-benzoquinone with anthracene in DCM under US irradiation at 80 °C is slow in the absence of 5 % TBPA but proceeds very quickly and with high yield at 25 °C in the presence of TBPA. This last cycloaddition is also strongly accelerated when carried out under stirring solely at 0°C with 1% FeCh. The US-promoted Diels Alder reaction in the presence of TBPA has been justified by hypothesizing a mechanism via radical-cation of diene, which is operative if the electronic affinity of dienophile is not too weak. 

Irradiation of 7 in methanol gives a quantitative yield of a-methoxyphosphine oxide 18a il,14) however, when the same carbene (16a) is generated by thermolysis of 7 in ethanol then rearrangement to 17 a also takes place 15). In view of the behavior of 16a, para substituents in the phenyl groups of the phosphoryl group influence the rearrangement only if they exert a promoting effect. Thus it is understandable that... 

The reaction mechanism in the irradiated flue gas is probably quite complex, but basically the EB excites the gas molecules and promotes reactions that convert the oxides to acids. These then react with ammonia or calcium compounds to give solid products that are removed by the filter. The initiation reaction is believed to be brought about by radical formation, such as OH,... 

Microwave irradiation has been extensively employed to generate nitrile oxides and to promote 1,3-dipolar cycloadditions of the previously prepared dipole. 

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