Aromatic compounds photochemical methods

Okada, K., Okubo, K., and Oda, M., A simple and convenient photo-decarboxylation method of intact carboxyhc acids in the presence of aza aromatic compounds, /. Photochem. Photobiol. A Chem., 57, 265, 1991. 

The biological activity of several halogenated herbicides in water is destroyed by ultraviolet irradiation (18). Irradiation seems to be a promising method for decontaminating small quantities of pesticides. The chemical similarity between the chlorinated dioxins and other chlo-rinted aromatic compounds suggested that if there were parallels in their photochemical behavior, sunlight might destroy dioxins in the environment. 

PET reactions [2] can be considered as versatile methods for generating radical cations from electron-rich olefins and aromatic compounds [3], which then can undergo an intramolecular cationic cyclization. Niwa and coworkers [4] reported on a photochemical reaction of l,l-diphenyl-l, -alkadienes in the presence of phenanthrene (Phen) and 1,4-dicyanobenzene (DCNB) as sensitizer and electron acceptor to construct 5/6/6- and 6/6/6-fused ring systems with high stereoselectivity. 

Zitko [20] has described a confirmatory method in which the chloroparaffins in sediments are reduced to normal hydrocarbons which are then analysed by gas chromatography. This method lacks sufficient sensitivity for trace (sub-ppm) analysis and the confirmatory method may be difficult to apply. Friedman and Lombardo [21] have described a gas chromatographic method applicable to chloroparaffins that are slightly volatile the method is based on microcoulometric detection and photochemical elimination of chlorinated aromatic compounds that otherwise interfere. 

Radical substitution plays a part in the thermal chemistry of aromatic compounds, but not in the photochemistry, except in so far as many radicals that attack aromatic compounds are generated by photochemical methods from other addends. The reason for this is that reactive radicals exist only in low concentrations, and electronically excited states similarly are formed only in low concentrations the rate of bimolecular reaction between two such reactive species is generally much lower than the rates of alternative processes such as attack of the radical on ground-state aromatic compounds. 

With the development of the cross coupling methodology, many 6-C-substituted purines have been prepared in the past decade. Thus, 6 halopurine derivatives react with arylmagnesium halides,25 alkyl(aryl)zinc or tin reagents,26 trialkylaluminum,27 or alkylcuprates28 to give the 6-alkylpurine derivatives. Also a reverse approach based on the reaction of purine-6-zinc iodide with aryl or vinyl halides has recently been described.29 For the synthesis of 6-arylpurines, an alternative approach makes use of radical photochemical reactions of adenine derivatives with aromatic compounds,30 but this method is very unselective and for substituted benzenes, mixtures of ortho-, meta-, and para substituted derivatives were obtained. 

Applied research on indirect electrolytic methods has experienced a spectacular evolution in the last decades. More efficient, faster, and/or cheaper processes than classical anodic oxidation are now operative or in pilot trials. Redox mediators such as Ag(I)/Ag(II) and Co(II)/Co(III), and on-site production of strong oxidants like ozone at the anode and EFR at the cathode, are notable examples. These methods have been specially focused on aromatic compounds, probably because they are easily oxidiz-able, through hydroxyderivatives and quinones, to aliphatic acids. A further photochemical treatment of these products in the presence of Fe(III) allows their mineralization. 

In general, the electrochemical oxidation potentials of carbonyl compounds are very high (Table 6) and oxidative activation of aliphatic or aromatic carbonyl compounds is problematic whether by anodic oxidation or by photochemical methods. Oxidation at very high positive potentials is circumvented by at least three chemical modifications of the carbonyl group which enable the subsequent oxidation by chemical, electrochemical, or photoinduced electron-transfer processes. 

Photoionization provides an alternative photochemical method for generating radical cations in laser experiments. Laser excitation of a wide range of easily oxidized substrates (usually aromatic compounds) has been demonstrated to result in direct electron ejection to give a radical cation and the solvated elec-tron. - In most cases photoionization occurs from an upper excited state produced either directly by absorption of a single... 

A variety of methods for the direct amination of nitrobenzene, that do not require halogenated aromatic compounds, have been reported, including both vicarious (eliminative) [16, 17,19,42] and oxidative versions of nucleophilic substitution of hydrogen. Recently, we have described the aminatimi of 1,3-dinitrobenzene promoted by fluoride ions through photochemical activation [43], Also, the use of KMn04 as oxidant is of great practical value [3,44]. 

Classical methods rely on electrocyclization reactions promoted by heat or, especially, by photochemical means, usually followed by an oxidation to deliver the aromatic compound. One of the most prominent routes is also known as the Mallory reaction, first reported in 1964 [84, 85]. Since then it has been further optimized [86] and applied, especially in the synthesis of helicenes [87], but also in a number of syntheses of graphene-type structures. Zhang et al. recently combined the Mallory reaction with a Scholl cyclization to access tetrabenzocoronene 135 (Scheme 33) [88]. A similar approach has been reported for the preparation of hexabenzocoronene [62]. 

A lot of photochemical reactions of aromatic compounds have been described in the literature. In this context, photocycloadditions are typical examples [6, 9]. In such reactions involving nn excitation of the chromophore, the aromatic character is not reestablished in the final products as is typical for ground state reactions. Very efficient methods based on this reactivity have been developed for the construction of polycyclic compounds. Thus, molecular complexity is generated from simple and easily available starting compounds in only one step. For these reasons, photochemical... 

Heterocyclic compounds are important targets in organic synthesis. They possess numerous biological activities and can be used as intermediates for further transformations. Starting from readily accessible starting materials, various photochemical cyclization reactions provide convenient methods to build up to a large variety of heterocycles several reviews deal with them. In this context, the cyclization of aromatic compounds plays an important role. In these reactions, as in many ground state reactions of aromatic compounds, the aromatic character is momentarily suppressed. In this chapter, some of these reactions are discussed. 

The most reliable method for generating nitrenes is the thermal or photochemical elimination of nitrogen from azides. An alternative method which is useful for indole and carbazole synthesis is the deoxygenation of aromatic nitro compounds with trivalent phosphorus compounds. Triethyl phosphite is the most commonly used reagent, though more reactive compounds may be useful in special cases (B-79MI30600). 

Photochemical dehydrogenation of azomethines 43 42 (Scheme 20) dihydro compounds are initially formed and aromatization is effected with either oxygen or iodine. Phenanthridine formation by this method is improved by the presence of BF3 <1988TL5213>. 

Photochemical electron transfer reactions of electron donor-acceptor pairs in polar solvents provide a convenient and effective method for the generation of radical cations which can be trapped by complex metal hydrides. One of the most effective systems is based on irradiation of a solution of substrate, sodium borohydride and 1,4- or 1,3-dicyanobenzene. A range of bi- and poly-cyclic aromatic hydrocarbons has been converted into the dihydro derivatives in this way. An especially important aspect of this route to dihydroaromatic compounds is that it may give access to products which are regioisomeric with the standard Birch reduction products. Thus, o-xylene is converted into the 1,4-dihydro product (229) rather than the normal 3,6-dihydro isomer (228). The m- and p-xylenes are similarly reduced to (230) and (231), respectively. ... 

A method for the synthesis of highly substituted aromatic and heteroaromatic hydroxy compounds is the photochemical Wolff rearrangement of an unsaturated a-diazo ketone in the presence of an alkyne. The product, an alkenylcyclobutenone, undergoes ringopening and recyclization to a phenol (equation 60). Three examples of the reaction are the formation of the naphthols 587 and 588 and that of the hydroxybenzofuran 589. Complexes 590 (R = alkyl or aryl r-alkyl or Me3Si), produced from THF, alkynes and... 

---