Aromatic rings electron transfer mechanisms

Solid-supported FeCl3 is shown to act as an electron-transfer oxidant, providing a convenient method for coupling aromatic rings (80JOC749). The mechanism for the formation... 

These copper-mediated reactions very often involve dinuclear intermediates, but detailed mechanistic studies on stoichiometric systems are relatively few. The key features are the formation of p-peroxo or p-superoxo complexes by electron transfer from cop-per(i) to dioxygen. The co-ordinated oxygen may then act as an electrophile to the aromatic ring. A possible mechanism for the ortho-hydroxylation of phenol by dioxygen in the presence of copper catalysts is shown in Fig. 9-29. 

Hydrolytic cleavage of the C-O bond of bicyclic, tetracyclic, and steroidal enolates with HF-SbFs induces their isomerization to the corresponding ketones (Scheme 14.41) [104]. Rearrangement of dienones to aromatic compounds is also promoted by HF-SbFs (Scheme 14.42) [105]. Ring expansion of methyl penicilli-nates is achieved by SbCls to give thiazepine derivatives [106]. 1,3-Dithianes derived from ketones and aldehydes are deprotected with SbCls by means of a single-electron-transfer mechanism [107]. 

In addition, there is another type of reaction which does not change the aromatic character involving either a substitution at the ring or a substitution at the side group. As in the case of acyclic addition an electron transfer mechanism operates. 

Finally there are a number of papers dealing with photoreactions of diaryl-cyclopropenes and arylindenes, which formally involve migration of a carbon group from the side-chain to the ortho-position of an aromatic ring. An example of the basic reaction is the formation of indenes when 1,3-diphenylcyclopropenes (199) are irradiated directly. Now it is shown that isomeric indenes are formed when the reaction is carried out in the presence of 9,10-dicyanoanthracene, and an electron-transfer mechanism is invoked to account for the change of product. In a separate paper concerned largely with intramolecular photocycloaddition reactions, cyclopropenes (199 R = allyl) are reported to give both types of indene on direct irradiation, via the normally postulated vinylcarbene intermediate. The... 

Another effective dehydrating agent is anhydrous acetic add. In addition to an electrophilic attack of the aromatic ring structure, radical mechanisms are also well known, involving a single electron-transfer reaction. A popular example is the nitration of phenol using nitric add [11]. 

The addition followed a radical chain mechanism initiated by photoinitiated electron transfer from the tertiary amine to the excited aromatic ketone and occurred with complete facial selectivity on the furanone ring (99TL3169). The yields increased and best results were obtained with sensitizers (4-methoxyacetophenone,... 

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

HT (hole transfer) processes, and by what mechanism would the charge transfer take place, superexchange or by a conduction-like mechanism with the electron or hole hopping between aromatic rings ... 

Oxidative Polymerization Reactions. Clays can initiate polymerization of unsaturated compounds through free radical mechanisms. A free radical R", which may be formed by loss of a proton and electron transfer from the organic compound to the Lewis acid site of the clay or, alternatively, a free radical cation, R+, which may be formed by electron transfer of an electron from the organic compound to the Lewis acid site of the clay, can attack a double bond or an aromatic ring in the same manner as an electrophile. The intermediate formed is relatively stable because of resonance, but can react with another aromatic ring to form a larger, but chemically very similar, species. Repetition of the process can produce oligomers (dimers, trimers) and, eventually, polymers. 

In contrast with aliphatic nucleophilic substitution, nucleophilic displacement reactions on aromatic rings are relatively slow and require activation at the point of attack by electron-withdrawing substituents or heteroatoms, in the case of heteroaromatic systems. With non-activated aromatic systems, the reaction generally involves an elimination-addition mechanism. The addition of phase-transfer catalysts generally enhances the rate of these reactions. 

---