Exciplex singlet arenes

As is the case for reactions with conjugated dienes (see Section V), 3-t appears to be more reactive and form more polar, stable exciplexes with triethylamine than do singlet arenes of... 

VII. REACTIONS OF SINGLET ARENES WITH STILBENES A. Singlet Quenching and Exciplex Formation... 

Excitation of the arene and reaction of the singlet excited arene with the alkene, either directly or via an exciplex (Sec. V)... 

Section V describes ortho photocycloadditions of mostly simple alkenes that do not form ground-state complexes with arenes, do not absorb light in the presence of the arenes, and add to the arene in its singlet excited state. Mechanistic investigations, including the search for intermediates (ground-state complexes, excited states, exciplexes, zwitterions), the formulation of empirical rules, and theoretical descriptions of the reaction, have mostly been concerned with this type of ortho addition. Therefore, this section is divided into a number of subsections, each describing a particular aspect of ortho photocycloaddition. 

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 trans stereoselectivity may easily be rationalized as a result of sterics, although direct irradiation of the 1-phenylcyclohexene gives a similar ratio of trans cis stereoselectivity. The singlet reaction presumably does not involve a biradical intermediate of significant lifetime. Thus, the trans selectivity may be enhanced by sterics in the case of the triplet reaction. The preference for trans isomer may be inherent to the approach of the alkenes or, perhaps, it is a function of selective reversibility of biradical intermediates (see Scheme 1). Caldwell has published a formula for predicting the potential for photocycloaddition of alkenes and arenes in the singlet excited state [42], His analysis implicates an exciplex. 

Arenes will undergo photoaddition to olefins, giving bicyclo[4,2,0]octadienes, and considerable attention has been focused on the role of exciplexes in these reactions. The current consensus of opinion is that a singlet exciplex intermediate is generally... 

Photoexcited aromatic hydrocarbons undergo hydrogen abstraction and 1,4-addition reactions with cycloalkanes and hydroxylic compounds. These reactions are believed to take place through an excited singlet state of arene to form an exciplex or ion pair as an intermediate, which on back electron transfer dissociates into triplet diradical and undergoes proton abstraction from solvent or amines, followed by addition of an alkyl or aryl unit to give the products [37]. The following examples of this reaction are illustrative ... 

Like amines, dienes quench many arene singlet and triplet states, a process attributed to the formation of diene/arene exciplexes these have a much smaller degree of electron transfer than the corresponding arene-amine exdplexes. Ambiguity arises because dienes are commonly used as triplet quenchers and because simple arenes such as benzenes, naphthalenes, and biphenyls absorb at the same wavelengths as dienes. ... 

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