Ring systems anion trapping

Conversion of the trimer (80) to the seven-membered ring system (81) occurs readily with cesium fluoride [3,75], whereas in the presence of TAS fluoride, the di-anion (82) is trapped. These observations lead to the most likely mechanism for rearrangement as that in Scheme 37 [3]. The cyclisation step shown in Scheme 37 is made more easily accepted by the fact that the diene (83) (Scheme 38), undergoes rapid rearrangement in the presence of fluoride ion, giving the cyclic system (86) [77]. The ready cyclisation of a crowded anion (84) to give what appears to be a sterically unfavourable intermediate (85) would not be easily predictable ... 

Nucleophilic addition to organic radical-cations is one of the most common pathways to produce radicals. Irradiation of a wide variety of A-(2-alkenyl)- and N-(3-alkenyl)phthalimides and A-alkenylphthalimides [26] with a remote alkenyl double bond [27] afford new 5-, 6- and medium-size ring systems. Irradiation in methanol-acetonitrile triggers intramolecular electron transfer from the olefin double bond to the excited phthalimide carbonyl group. Because of the highly nucleophilic character of methanol, the olefin radical-cation can be trapped. The derived radical combines with the radical-anion to yield coupling products (Scheme 15). 

Anionic cascade cyclisations can be stereoselective (just like their simpler counterparts see below). The cyclisation of 289, for example, gives solely the trans ring-junction product 292 (stereochemistry is determined in the first cyclisation to 290) despite the fact that the cis 5,5-fused system is nearly 30 kJ mol-1 more stable.149 This argues against the reversibility of the cyclisation. Because the cyclisation of 290 is rather slow, trapping the product organolithium 291 with electrophiles other than a proton can pose problems, and considerable amounts of material protonated by the ether solvent are obtained. This can be overcome by decreasing the proportion of ether from 40% to 10% of the solvent volume.150... 

Hindered pyridines can act in three different ways [150,293-295]. The first is to trap protonic impurities and prevent adventitious initiation by water. This improves the control of molecular weights. The second role is similar to that of salts with common anions. The pyridinium salts formed in the system are accompanied by complex anions which may scavenge free ions in a similar manner as tetrabutylammonium salts. Hindered pyridines may also act as nucleophiles (or donors) and interact with some Lewis acids. These interactions will be directed toward the aromatic ring rather than the nitrogen atom which is protected by bulky tert-butyl groups in ortho position [293]. 

Generation of substituted aryl radical cations in the presence of nucleophiles can lead to products of side-chain substitution (processes such as anodic benzylic substitution of toluenes, which are dealt with in a separate chapter) or to products of addition to the aromatic ring itself. Nuclear addition products in /j /m-substituted systems have been proposed to form in essentially one of two ways, depending on substitution pattern and reaction conditions. Radical cations formed by electrochemical reaction (E) may be trapped by chemical reaction (C) with a nucleophile (or its anion). Repeating this sequence leads to nuclear addition products (LXV), formed by what is referred to as the ECEC mechanism [Eq. (31)] [74]. An analogous pattern may be inferred for or / (9-substituted systems. 

Exocyclic allylic cations react to give predominantly fraw-decalins26. If there are two cationic positions leading to six-membered rings, the least alkylated terminus of the system is attacked by the nucleophile in a 6-endo process27. The resulting carbocation is selectively trapped by an external anion in a supposedly concerted mechanism. 

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