Exciplex reactions, aryl halides

Intermolecular photoreaction of an aryl halide with another aromatic compound may lead to the formation of biaryls. In this section several examples of such reactions will be discussed. In some cases, information concerning the reaction mechanism is available but the depth to which mechanisms have been investigated varies greatly. In many cases aryl radicals formed by homolysis of the carbon-halogen bond are the reactive species. Such radicals may also be produced via electron transfer, followed by departure of halide anion. In some cases aryl cations have been proposed as intermediates. Intermolecular bond formation may also be preceded by charge transfer within an exciplex or by formation of radical ion pairs. 

An excited aryl halide may also transfer charge to oxygen and thereby become activated towards attack by nucleophiles. The photoamination of 1-amino-2,4-dibromoan-thraquinone by rc-butylamine, in which the 4-Br atom is replaced, is supposed to proceed via direct interaction between the triplet state and the amine when the reaction is performed under nitrogen. In air atmosphere, however, it proceeds via both the T7, and SY states. From the singlet excited state and oxygen, an exciplex or collisional complex [AQ ()/ ] is formed which undergoes the animation639. 

Acceleration by KI in the substitution reaction of aryl halides with potassium diethyl phosphite or with the 2-naphthoxide ion has also been explained on the basis of an ET through the exciplex of the charge-transfer complex formed between the aryl halide and the iodide ions34a. It has also been reported that iodide ions catalysed the photostimulated reaction of bromoarenes with diethyl phosphite ion34b. 

The excited states of dinuclear platinum, rhodium, and iridium complexes with a variety of bridging ligands exhibit unusually diverse reactivity. These types of compound in their lowest triplet state engage in oxidative and reductive electron transfer reactions, and exciplex formation [56], They can also engage in atom transfer reactions i.e. they can abstract hydrogen atoms from a wide range of substrates as well as halogen atoms from alkyl and aryl halides. 

The direct photolysis of alkyl or aryl halides in solution to form carbon-centered radicals is rarely used in organic synthesis." Alkyl iodides usually afford mixtures of radical and ionic products, while alkyl bromides can produce radical-derived products but in low yield. A notable exception is the photocycliza-tion of haloarenes, which has been shown to produce carbon-centered radicals that can add to aromatic rings. A similar reaction has recently been observed on irradiation of iodoheterocycles, with substituted benzenes or electron-poor alkenes, to form arylated or alkylated heterocycles in good yield. Related reactions have also been reported on irradiation of 4-chloroanilines in the presence of (electron-rich) alkenes, although in this case, the alkylations appear to involve the formation of a phenyl cation. An alternative approach to form carbon-centered radicals is to irradiate the alkyl iodide or bromide in the presence of triethylamine this is proposed to form an amine-haHde exciplex, which cleanly breaks down to give a carbon-centered radical and a halide anion. Cossy and co-workers have shown this to be a fast, convenient, and chemoselective method of radical generation, which has recently been used to prepare the bicyclic core of ( )-bisabolangelone (Scheme 1). ... 

In the presence of an electron rich donor molecule an alternative to direct fission or reaction via an excimer is the formation of an exciplex or radical anion/radical cation ion pair (Eq. 2). The radical anion has been viewed as the key intermediate which undergoes fission to aryl radical and halide ion (Eq. 3). With polyhaloarenes there is an additional option. A polychloroarene radical anion, for example, has two possible modes for bond fission (a) fission to produce aryl radical and chloride ion or (b) fission to form an aryl carbanion and chlorine atom (Scheme 6). The options for fragmentation of a haloarene radical anion... 

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