Aryl halides Atom transfer reactions

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 reaction can be carried out efficiently using aryl diazonium tetrafluoroborates with crown ethers, polyethers, or phase transfer catalysts.103 In solvents that can act as halogen atom donors, the radicals react to give aryl halides. Bromotrichloromethane gives aryl bromides, whereas methyl iodide and diiodomethane give iodides.104 The diazonium ions can also be generated by in situ methods. Under these conditions bromoform and bromotrichloromethane have been used as bromine donors and carbon tetrachloride is the best chlorine donor.105 This method was used successfully for a challenging chlorodeamination in the vancomycin system. 

At this point, it can be concluded that the direct and indirect electrochemical approach of the reaction in the case of aryl halides has provided a quantitative kinetic demonstration of the mechanism and the establishment of the nature of the side-reactions (termination steps in the chain process). In poor H-atom donor solvents, the latter involve electron-transfer reduction of the aryl radical. 

As to the next step, namely, the reaction of aryl radicals with nucleophiles, we should take into account the fact that air molecular orbital, which initially accommodates the incoming electron, is available in the aryl halide. The electron is subsequently transferred in-tramolecularly from the it to the o molecular orbital of the carbon-halogen bond. Aryl radicals effectively scavenge H atoms. Therefore, an abstraction of a hydrogen atom from the solvent may occur. However, in the case of nucleophiles that can act as effective traps of aryl radicals, the addition of a nucleophile to the phenyl radical takes place. At this point, let us focus on the step of addition of the nucleophile (Y ) to the intermediate radical (Ph). When a new a bond begins to form between the sp3 carbon-centered radical (H5C6) and... 

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. 

The methylation of 7-phenyl-2,3,4,5-tetrahydro-l/f-l,4-diazepin-5-one (82) with dimethyl sulfate under phase-transfer conditions gave the 4-methyl analogue in modest yield. Activated aryl halides (l-fluoro-2,4-dinitrobenzene) furnished the 4-aryl analogues, in neither case was there any reaction at N-1 <93BSB89>. Hexahydro-l,4-diazepine is arylated on both nitrogen atoms with activated haloheterocycles such as 2-chloropyridine on reaction at 100°C under high pressure in the presence of triethylamine <90H(31)1217>. 

The photo- RNl reaction of enolate anions with aryl halides has been shown to be useful for both inter- and intra-molecular annelation reactions, but has now been found to suffer one major drawback in its general applicability. When a hydrogen atom is present at the /8-carbon of the enolate anion, hydrogen atom transfer to the transient phenyl radical is the preferred reaction pathway. 

The cupric halide-catalyzed reaction of an aryl diazonium salt with an olefin-known as the Meerwein arylation-is a powerful and well-developed method for the arylation of olefins (136, 137). Good results have been obtained for olefins bearing electron-withdrawing groups such as a carbonyl, a cyano, an aryl or an alkenyl group. Depending on the nature of the olefin and the diazonium salt, the reaction can lead either to the oxidation product 88 or to the Cl-atom transfer product 89 (Equation 13.10) [138]. 

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