Oxonium ions arenes

The Lewis acid ZnCl2 effects formation of an oxonium ion which is reactive in electrophilic aromatic substitution. The intermediate zinc alkoxide reacts with the arene to form the chloromethylated product and zinc oxides ... 

The two stypes of stabilization are not equivalent the cation and the bromonium ion are different molecules with different shapes, while the two representations of the oxonium ion are just that—they aren t different molecules. This stabilization of an adjacent cationic centre by a heteroatom... 

Under Lewis acidic conditions, cyclic acetals such as 72a-b form oxonium ion intermediates which cyclize via an intramolecular Friedel-Crafts alkylation onto the tethered arene to form polycyclic benzylic ethers, Eq. 57 [85]. 

Similarly, chromium-complexed benzylic cations are also stabilized and organic reactions based on the benzylic cation species have been developed. For example, planar chiral o-substituted benzaldehyde dimethylacetal chromium complexes 4 were treated with 3-buten-l-ol in the presence of TiCl4 to give tet-rahydropyran derivatives with high diastereoselectivity (Eq. 5) [5]. The chromium-complexed benzylic oxonium ion 6 would be also generated and subsequent intramolecular cyclization afforded the cyclization product 7. Furthermore, the chromium-complexed benzyl alcohol derivative having electron-rich arene ring at the side chain produced tetrahydroisoquinoline skeleton by treatment with Lewis acid with stereochemical retention at the benzylic position (Eq. 6) [6]. 

The two types of stabilization are not eguivalent the cation and the bromonium ion are different molecules with different shapes, while the two representations of the oxonium ion are just that—they aren t different molecules. This stabilization of an adjacent cationic centre by a heteroatom with at least one lone pair to form a three-membered ring intermediate is not restricted to bromine or the other halogens, but is also an important aspect of the chemistry of compounds containing oxygen, sulfur, or selenium, as you will see in Chapter 27. 

Japanese herb Angelica keiskd [13]. Thus, treatment of resorcinol derivative 23 with BF3 OEt2 initially led to oxonium ion 24, which underwent loss of a CHjOCHj cation. The adjacent Lewis basic arene rapidly trapped the electrophile, affording the 7t-complex 25 in an intramolecular process as supported by crossover experiments. The short-Hved nature of the cationic species, i.e., CHjOCHj, resulted in exclusive... 

The substituent effects for the dehydration of arene hydrates have been studied." The kinetics of dehydration were studied with a series of substituted arene-hydrates (84) in dilute HCIO4 solutions. The results are consistent with a rate-limiting step involving formation of the arenium ion (86) from the oxonium ion intermediate (85). Rapid deprotonation then provides the dehydration product (87). The Hammett plot of second-order... 

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