Halide anions, arylation

Alkyl halide Alkyl aryl Halide anion... 

Last, the nickel catalyzed halogen exchange for haloarenes, actually the arylation of halide anions, has been investigated. The equilibrated exchange between the phenyl bromide (60 %) and the phenyliodide (40 %) allows with some... 

The palladium(O) complex undergoes first an oxydative addition of the aryl halide. Then a substitution reaction of the halide anion by the amine occurs at the metal. The resulting amino-complex would lose the imine with simultaneous formation of an hydropalladium. A reductive elimination from this 18-electrons complex would give the aromatic hydrocarbon and regenerate at the same time the initial catalyst. 

It is the metal catalyzed halogen exchange for haloarenes, that is to say the arylation of halides anions (chloride, bromide, iodide), acting as nucleophiles (Fig. 21). 

As to the first of these factors, insofar as the aryl halide anion radical formed initially is planar, a symmetry interdiction opposes the transfer of an electron from the n orbital where it sits initially to the a orbital of the cleaving bond. This symmetry constraint may be circumvented by bending... 

Proton nmr halide anion titrations reveal that the ethyl- [79], propyl-[80] and butyl- [81] linked derivatives (Fig. 43) form complexes of 1 1 stoichiometry in acetonitrile solution. Stability constant determinations suggest that the ethyl derivative [79] exhibits selectivity for the chloride anion in preference to bromide or iodide. As the chain length increases, so the selectivity for chloride decreases and also the magnitude of the stability constant which is evidence for an anionic chelate effect with the chloride anion. Receptors containing larger aryl [81], [83], [84] and alkylamino spacers [85] (Fig. 43) form complexes of 2 1 halide anion receptor stoichiometry. 

One final example worth mentioning is the reductive alkylation/arylation with lithium and alkyl/aryl halides in liquid ammonia. This is a two-step process in which negatively charged nanotubes are formed via electron transfer from the metal. This step is relatively easy and fast due to the CNTs electron sink properties, and it enables exfoliation of the tubes through electrostatic repulsion in the second stage, the alkyl/aryl halides react with the charged tubes to form a radical anion which can dissociate into the alkyl radical and the halide anion, with the former species undergoing addition to the CNT sidewalls [42]. 

The simplest way of generating and observing aryl halide anion radicals is to use an electrochemical technique such as cyclic voltammetry. With conventional microelectrodes (diameter in the millimetre range), the anion radical can be observed by means of its reoxidation wave down to lifetimes of 10" s. Under these conditions, it is possible to convert, upon raising the scan rate, the irreversible wave observed at low scan rates into a one-electron chemically reversible wave as shown schematically in Fig. 9. Although this does not provide any structural information about RX , besides the standard potential at which it is formed, it does constitute an unambiguous proof of its existence. Under these conditions, the standard potential of the RX/RX " couple as well as the kinetics of the decay of RX-" can be derived from the electrochemical data. Peak potential shifts (Fig. 9) can also be used... 

Fig. 11 Forward electron transfer (90) rate constant, k, versus the standard potential, F /q, of a series of aromatic anion radicals for rapidly cleaved aryl halide anion radicals (DMF, 20°C). kjy is the bimolecular diffusion limit. (Adapted from Andrieux et al., 1979.)...

Iodonium salts readily transfer one of their aryl groups to a heteroatom substrates successfully arylated range from simple halide anions to complex natural products. The plethora of such reactions leaves no doubt that the use of iodonium salts is the best choice for arylations. 

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. 

In the S l Ar and the S l Ar mechanisms, the aryl halide becomes reactive towards nucleophiles by loss of an electron or a halide anion. Another means of activation is protonation, followed by excitation. This mechanistic pathway has been proposed for the photohydroxydehalogenation of 1-chloroanthraquinones in 97% sulphuric acid734, which leads to high yields (60-70%) of 1-hydroxyanthraquinones. The yields of the photochemical replacement of chlorine by pyridine in 1- and 2-chloroanthraquinone are much lower (3-6%)735. The reactions were performed with pyridine as solvent and the products were converted into aminoanthraquinones by treatment with alkali. Pyridine as nucleophilic... 

The X-ray-determined structure of the complex of 16 and 19 with quaternary salt 15 revealed that the primary discriminative forces leading to an efficient resolution are the formation of directional hydrogen bonds of hydroxy groups of cinchonidine and BINOL with the halide anion as well as aryl-aryl interaction between the naphthyl and the quinoline rings [40]. 

As a matter of fact the photochemically generation of the triplet phenyl cation is synthetically equivalent to activation by metal catalysis that likewise produces a sort of phenyl cation complexes to the metal by oxidative addition of the aryl halide and elimination of the halide anion (see above). In fact, a photochemical parallel has been found for most type of metal catalyzed arylations, via reactions that occur from the same or similar reagents. 

One way of carrying out nucleophilic aromatic substitution reactions under mild conditions is the Ar RNl process, which is initiated by (usually, but not necessarily, photoinduced) electron transfer to an aryl halide, e.g., from an enolate Cleavage of the resulting aryl radical anion with loss of a halide anion gives an aryl radical that combines with the enolate, thus forming the desired aryl-carbon bond. 

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