Photochemical oxidation, alkenes

Khenkin, A.M. and Neumann, R. (2000). Aerobic photochemical oxidation in meso-porous Ti-MCM-41 epoxidation of alkenes and oxidation of sulfides. Catal. Lett. 68(1-2), 109-111... 

Photochemical oxidation of electron-rich alkenes with the simultaneous reduction of the initially formed peroxide with tetra-n-butylammonium borohydride to the hydroxy compound has been reported, but the procedure has not been shown to be generally useful [16]. 

In this chapter we will examine the atmospheric degradation mechanisms of the following important classes of anthropogenic molecules alkanes, alkenes, aromatics, nitrogen oxides, S()2, CFCs and Halons, and finally HFCs and HCFCs. Our intent is not to give an exhaustive account of the photochemical oxidation of every man-made chemical species but rather to present examples of the degradation mechanisms of a few representative members of each class of pollutant. First, we need to consider the general features of atmospheric chemistry. 

Examples of photoreactions may be found among nearly all classes of organic compounds. From a synthetic point of view a classification by chromo-phore into the photochemistry of carbonyl compounds, enones, alkenes, aromatic compounds, etc., or by reaction type into photochemical oxidations and reductions, eliminations, additions, substitutions, etc., might be useful. However, photoreactions of quite different compounds can be based on a common reaction mechanism, and often the same theoretical model can be used to describe different reactions. Thus, theoretical arguments may imply a rather different classification, based, for instance, on the type of excited-state minimum responsible for the reaction, on the number and arrangement of centers in the reaction complex, or on the number of active orbitals per center. (Cf. Michl and BonaCid-Kouteck, 1990.)... 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

Other alternatives for the oxidant for stoichiometric oxidations include the use of a selenoxide [99], including a photochemical oxidation of catalytic selenium [100], iodine [101], sodium chlorite [102], hypochlorite [103], and electrochemical methods [101,104]. Even air can be used as the oxidant [99,100], but care has to be taken with regard to the choice of solvent as cleavage of the product 1,2-diol can occur, especially when the alkene has an aryl substituent [53, 105, 106]. 

A new reaction system has been reported in which molecular oxygen oxidizes alkenes to epoxides both thermally and photochemically, in the presence of SO2, under ambient conditions. Irradiation of a mixture of pro-pene and SO2 in acetonitrile at 0°C caused absorption of O2, to yield propene oxide as the sole volatile product. A similar reaction occurred at 25 °C in the dark, in the presence of potassium nitrite. 

We came to this area quite by chance. Our interest in nucleophilic functionalization of aromatics led us to consider photochemical reactions for this purpose. In several cases, such reactions involve ionization of the substrate. Furthermore, we were impressed by the work of Arnold and his co-workers showing that SET often occurs upon photoexcitation yielding an ion radical pair. In view of this fact, one of the experiments we carried out involved irradiation of the photochemical oxidant 1,4-naphthalenedicarbonitrile (DCN) in the presence of toluene and cyanide in deareated acetonitrile. Arnold s work had shown that cation radicals of alkenes add nucleophiles under this condition, and we wanted to test whether a similar reaction with... 

As an example, this apply to enols or tautomeric enols such as maleic acid derivatives. While with a chemical reagent (cerium ammonium nitrate) the only process occurring is oxidative dimerization, when aromatic nitriles are used as the photochemical oxidant, selective trapping of the radicals by an electrophilic alkenes or by the nitrile itself occurs. Under these conditions, both the alkylation of alkenes and the oxidative alkylation/dimerization of dienes have been smoothly obtained (see Scheme 8) and side processes such as double alkylation or polymerization often occurring with other methods have been avoided. A three-component (Nucleophile-Olefin Combination, Aromatic Substitution) process is also possible. ... 

Photochemical oxidations of various substrates such as amines [763], alkenes [769,770,772,775,778-780], fiiran [764,765], sulfide [766,773], phytol [767], bi-adamantylidene [768], thiophenolate [771], catechol [774], allylsilane [776], and imines [777], etc., through reactions with singlet oxygen sensitized by porphyrins have been extensively studied. Photochemical atrop isomerization of picket-fence-type porphyrin should be noted here as another interesting type of photoreaction [793]. 

The photochemical cyclisation of p.y-unsaturated ketoximes to 2-isoxazolines, e.g., 16—>17, has been reported <95RTC514>. 2-Isoxazolines are obtained from alkenes and primary nitroalkanes in the presence of ammonium cerium nitrate and formic acid <95MI399>. Treatment of certain 1,3-diketones with a nitrating mixture generates acyl nitrile oxides, which can be trapped in situ as dipolar cycloadducts (see Scheme 3) <96SC3401>. 

There are a variety of photochemical reactions that non-conjugated dienes can undergo. One of these that is currently of considerable interest is the reactivity brought about by electron-accepting sensitizers such as the cyanoarenes. The photoreactivity of these systems involves the photochemical excitation of the sensitizer to an excited state7. Thereafter, the reactivity is dependent on the ease of oxidation of the alkene or diene. With the transfer of an electron from the diene to the photoexcited sensitizer a radical cation is formed. It is this intermediate that brings about the various processes which occur within the diene systems under investigation. 

Vinylindote radical-cations, for example that derived from 64, take part in a Diels-Alder reaction with alkenes. Subsequent oxidation of the initial product with loss of two protons and dimethylamine gives the pyrido[l,2a]indoie. Reaction is achieved either by direct electrochemical oxidation or by photochemical electron... 

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