Bicyclic stabilized cations

The first example of an epoxide-induced pentacyclization has been describedlos. The process is separated into a tri- and a consecutive bicyclization. Termination of the initial tricyclization leads to a fluorine-stabilized cation, which simultaneously induces the final bicyclization. The whole process is terminated by a 6-exo reaction of a propargyl silane group. Additionally, some mono- and bicyclic byproducts are isolated105. Further investigations with the well-known... 

The initiator can be a radical, an acid, or a base. Historically, as we saw in Section 7.10, radical polymerization was the most common method because it can be carried out with practically any vinyl monomer. Acid-catalyzed (cationic) polymerization, by contrast, is effective only with vinyl monomers that contain an electron-donating group (EDG) capable of stabilizing the chain-carrying carbocation intermediate. Thus, isobutylene (2-methyl-propene) polymerizes rapidly under cationic conditions, but ethylene, vinyl chloride, and acrylonitrile do not. Isobutylene polymerization is carried out commercially at -80 °C, using BF3 and a small amount of water to generate BF3OH- H+ catalyst. The product is used in the manufacture of truck and bicycle inner tubes. 

Super-Stabilization of -Conjugated Cations by Annelation to Bicyclic Frameworks... 

Tri-thiapentalenes 61 and the tetrathia cationic analogue 105 are aromatic compounds146 147 (Schemes 30 and 47). In the bicyclic canonical structures 61b, the central tetravalent sulfur makes an important contribution to the aromatic character. The nonclas-sical bicyclic thiophene 61c, with 10 jr-electrons, is fully stabilized by aromatic delocalization148 (Scheme 47). Tetrathio derivative 106 presents evidence for S-S bonding along the whole range of sulfur at-... 

When the nucleophile bears an appropriately unsaturated chain, these enantioselective cyclizations can be used to advantage in tandem processes leading to bicyclic systems (Scheme 42) [131,141,142], The greater enantios-electivity observed with the mesylate group in this example may be due to the lower degree of stabilization of the alkene radical cation in the looser ion pair, which leads to more rapid cyclization. 

A bicyclic system, 5-azaindole, may be mentioned here because it owes its relatively high basic strength (p/sTg = 8 3) to a resonance stabilization of the cation [71] which is analogous to that of 4-aminopyridinium ion (Adler and Albert, 1960). Nmr spectra of the cations of 5-azaindole and its 1-phenyl derivatives have recently been reported (Dvoryantseva et al., 1973). 

Macropolycyclic ligands containing intramolecular cavities of a three-dimensional nature are referred to as cryptands. The bicyclic cryptands (73) exist in three conformations with respect to the terminal nitrogen atoms, exo-exo, endo-exo and endo-endo 6 these forms can rapidly interconvert via nitrogen inversion but only the endo-endo form has been found in the crystal structures of a variety of complexes372 and for the free ligand ([2.2.2], 73, m = n = / = l).449 In their complexes with alkali and alkaline earth cations, the cryptands exhibit an enhanced stability over the crown ethers and coronands dufe to the macrobicyclic, or cryptate, effect.33 202... 

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