Ring-like atomizers

Other forms of tautomerism are much less commonly encountered in heterocyclic six-membered rings. Like protons, acyl groups can occupy alternative positions, on a ring atom or a substituent (e.g. 92 93), and their migration from one position to another is frequently... 

Because of the electron-attracting properties of the ring nitrogen atoms, methyl groups undergo aldol-like condensations. For example, 3- and 4-methylpyridazine react with chloral to give 3- or 4-(2-hydroxy-3,3,3-trichloropropyl)pyridazine, and 4-methylpyridazine reacts with anisaldehyde to yield 4-(p-methoxystyryl)pyridazine. 

S-Alkylthiiranium salts, e.g. (46), may be desulfurized by fluoride, chloride, bromide or iodide ions (Scheme 62) (78CC630). With chloride and bromide ion considerable dealkylation of (46) occurs. In salts less hindered than (46) nucleophilic attack on a ring carbon atom is common. When (46) is treated with bromide ion, only an 18% yield of alkene is obtained (compared to 100% with iodide ion), but the yield is quantitative if the methanesulfenyl bromide is removed by reaction with cyclohexene. Iodide ion has been used most generally. Sulfuranes may be intermediates, although in only one case was NMR evidence observed. Theoretical calculations favor a sulfurane structure (e.g. 17) in the gas phase, but polar solvents are likely to favor the thiiranium salt structure. 

Note that special atom types are defined for carbon atoms involved in small rings, like cyclopropane and cyclobutane. The reason for this will be discussed in Section 2.2.2. 

In spite of the fact that the vast majority of quaternizations of amino-heterocyclic compounds are reported as occurring on the ring nitrogen atom only, it seems quite likely that salt formation may also take place on the exocyclic nitrogen in other cases but that it has been overlooked in the absence of a test such as was available for 4. 

In accordance with the electropositive nature of the bridgehead atoms, all di(pyridyl) substituted anions behave like amides with the electron density accumulated at the ring nitrogen atoms rather than carbanions, phosphides or arsenides. The divalent bridging atoms (N, P, As) in the related complexes should in principle be able to coordinate either one or even two further Lewis acidic metals to form heterobimetallic derivatives. According to the mesomeric structures, (Scheme 7), it can act as a 2e- or even a 4e-donor. However, theoretical calculations, supported by experiments, have shown that while in the amides (E = N) the amido nitrogen does function as... 

An Sn 1 -like reaction, on the other hand, is much more favorable, because the glycosyl cation intermediate is stabilized by charge distribution between C-1 and the ring-oxygen atom. The unfavorable formation of an ion pair on bond cleavage can be avoided by protonation of the glycosidic oxygen atom and thus the requirement for acid catalysis (see Scheme 1). 

It seems likely that benzene forms a n complex (12) with, for example, Br2 (cf. p. 131), and that the Lewis acid then interacts with this. The catalyst probably polarises Br—Br, assists in the formation of a a bond between the bromine molecule s now electrophilic end and a ring carbon atom, and finally helps to remove the incipient bromide ion so as to form a [Pg.138]

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