Cycloaddition-ortho electron transfer

Hydrogenated isoxazole derivatives were obtained by single electron transfer (SET) cyclization of /9,y-unsaturated oximes , by thermal [4- -2] cycloaddition of aldox-imes or ketoximes to conventional dienophiles or isomerization/cyclization of an ortho halogeno or nitro-substituted amidoximes. Preparation of 1,4-disubstituted 3-hydroximino-2-nitro-l-butenes and their oxidative cyclization to 4-nitroisoxazoles are reported " . Synthesis of fluorine-containing substituted isoxazolidines as well as isoxazoles by ultrasonic methods has been also described. 

A correlation between free enthalpy of electron transfer and mode of the photoreaction was also constructed for addition of alkenes to benzonitrile. Four areas could be differentiated Full electron transfer, leading to substitution, is only observed if AG < 0 eV cycloaddition to the cyano group occurs if 0 < AG < 0.4 eV. All olefins for which AG > 0.4 eV preferentially undergo cycloaddition to the aromatic ring, ortho cycloaddition if AG < 1.7 eV and meta cycloaddition if AG > 1.7 eV. 

At the singlet excited state, ortho and meta photocycloadditions are often competitive processes and physicochemical investigations were carried out to rationalize the modes of cycloaddition of arenes with alkenes. In the context of the study of photochemical electron transfer reactions, it has been proposed that the difference of the redox potentials of the reaction partners might play an important role in this competition [10]. Such a discussion involves the intervention of an exciplex as intermediate. The Rehm-Weller equation [11] was used to quantify the relationship. When an electron transfer process is strongly endergonic (AG>1.5eV), the meta cycloaddition should be favored. When such a process is less endergonic (1 < AG< 1.5 eY), the ortho addition dominate [12]. This means that the... 

The simultaneous formation of two or more a-bonds has always attracted synthetic chemists since complex molecules can be built up in one single step. Therefore it is not surprising that highly developed syntheses of natural and artificial products often made use of cycloaddition procedures. The meta photocycloaddition of aromatic compounds to alkenes certainly belongs to this category and has reached its summit of application in the admirable work of Wender s group. Hence this chapter describes photocycloadditions of various types covering mainly meta [3+2], ortho [2+2] and para [4+2] ones. Even an unusual example of an [6+6] cycloaddition is presented. The fact that only one substitution reaction is described may indicate that synthetic studies of electron transfer activation only started recently. 

Photoirradiation of benzene and its derivatives with alkenes give ortho-, meta-, para-cycloaddition products. In most cases, either meta- or orr/io-adducts are obtained as major products [5]. Bryce-Smith and Gilbert suggested a prefulvene type diradical intermediate mechanism (Path A) for the weta-adduct [6], whereas Morrison and Srinivasan groups [7, 8] proposed the exciplex mechanism (Path B) for the formation of these adducts (Scheme 9.1). The exciplex intermediates undergo photo-induced electron transfer processes between donor (D) and acceptor (A) to produce radical ion pairs as intermediates, stabilized by coulombic interactions to give adducts [9]. 

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