Aryl cation proposal

An alternative mechanism featuring aryl cation intermediates, rather than aryl radicals, (the aryl cation proposal , Scheme 3) was considered shortly after the discovery of the reaction. This would invoke a completely non-radical mechanism to explain the observations. Loss of dinitrogen from an arenediazonium salt would afford an aryl cation which then cyclizes to cation 4 this could be (reversibly) intercepted by TTF acting as a nucleophile to give rise to salts 2, solvolysis of which would afford alcohols 3. However, this proposal can be dismissed. Firstly, these... 

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. 

The mechanism of the Meerwein arylation is not completely understood. In his seminal paper, Meerwein proposed the involvement of aryl cations, however, this hypothesis was soon eliminated when J.K. Kochi suggested that aryl... 

The mechanism of the Sandmeyer reaction is not completely understood. For a long time it was believed to proceed via aryl cations, but later W.A. Waters and then later J.K. Kochi proposed a radical mechanism which was catalytic for... 

ArPb(OzC-CF3)2 Ar+ + Pb(02C CF3)2]. The aryl cations have been trapped with aromatic compounds to give biaryls [with certain substrates, notably poly-methylbenzenes, high yields (up to 88 %) are obtained], but with reactive aromatic substrates aryl cations are not the precursors to the biaryls and in these cases it is proposed that reaction proceeds via preliminary complex formation between the substrate and a species which contains an aryl-lead bond. Oxidative coupling of methyl-substituted benzenes by the reagent Pb(0Ac)4-CFs C02H to give biaryls and diarylmethane is also considered to involve formation of a radical cation in the primary step. A study has also been made of the plumbylation of monohalogeno-benzenes with Pb(OAc)4-CF3 COsH. ... 

Although it has been reported that divalent copper is involved in this reaction, and a mechanism has been proposed to rationalize the reaction,it is doubtful that the reaction involves the free aryl cation, Cu and Cu + ions. Thus an alternative mechanism is proposed below. 

Noyori et al. recently used ESI-MS to characterize species present in catalytically active solutions during the hydrogenation of aryl-alkyl ketones using their base-free catalyst precursors trans-[Ru((R)-tol-BINAP)((R, RJ-dpenJfHXf/ -BH ] (tol-BI-NAP = 2,2 -bis(ditolylphosphino) -1, T-binaphthyl dpen = 1,2-diphenylethylenedia-mine) in 2-propanol [9b]. Based upon ESI-MS observations, deuterium-labeling studies, kinetics, NMR observations, and other results, the authors proposed that the cationic dihydrogen complex trans-[Ru((R)-tol-BINAP)((R, R)-dpen)(H)( 2-H2)]+ is an intermediate in hydrogenations carried out in the absence of base. 

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

Several arylations involving reactive alkenes such as norbomene or allenes have been reported. Togni and coworkers have shown that norbomene is selectively added to the ortho positions of phenols to produce a mixture of 30 and 31 in 69% and 13% yield, respectively, after 72 hours at 100°C (22) [108, 109]. 1,1-dimethylallene also reacts with aromatic carboxamides (33) to produce prenylation products (34) in the presence of cationic iridium complexes (23) [110]. In both cases, initial ortho C-H bond activation in arenes directed by coordinating groups followed by olefin insertion has been proposed. 

The thermodynamic stabilities of phenonium ions have been determined based on bromide-transfer equilibria in the gas phase and, depending on the substituents, the bridged species (1) has been proposed as an intermediate or transition state on the potential-energy surface for the 1,2-aryl rearrangement of triarylvinyl cations (see Scheme 1). Phenonium ion (3) has been presented as an intermediate to account for the fact that lactonization of methyl 4-aryl-5-tosyloxy hexanoate (2) produces y-lactone (4) selectively under thermodynamic conditions, but affords 5-lactone (5) preferentially under kinetic conditions. It has been shown that anodic oxidation of frany-stilbene in alcohols in the presence of KF or BU4NBF4 is accompanied by its electro-oxidative rearrangement into diphenylacetaldehyde acetals. The mechanism outlined in Scheme 2 has been proposed" for the transformation. 

The ratios of nucleophilic substitution versus [3,3]-sigmatropic rearrangement for the collapse of allenyl(aryl)iodine(III), generated from the reaction of aryliodanes with propargylsilanes in the presence of Bp3.0Et2 in alcohols, have been determined. The mechanism proposed by the authors ° involves the generation of propargyl cations from the allenyliodine (III) via a unimolecular pathway. 

Li+ cation (457) may be postulated. The effect of the aryl substituents is diminished in DME and increased in Et20 solvents, corresponding to increased and diminished solvation of the cation concurrent with tt-complex formation, in further support of the proposed explanation. The structure of the products can be elucidated by H NMR spectroscopy . 

A more detailed evaluation of the diverse structures proposed for the secondary species goes beyond the scope of this review. We mwely emphasize that the ESR results provide detailed evidence for the nature of the radical center, but fail to elucidate the cationic site. The identity of this center is left to secondary considerations or speculation. We also note that any alternative structure has the virtue of not contradicting the ab irutio calculations the potential c ture of chloride ion has precedent in the nucleophilic substitution at a cyclopropane carbon (see Section 7). Another type of ring-opened structure has been postulated as an intermediate in the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes (see Section 5). 

---