Nitrogen analogues, carbonic

Chemical shifts for aromatic azoles are recorded in Tables 17-20. Fast tautomerism renders two of the C-13 chemical shifts equivalent for the NH derivatives just as in the proton spectra (Table 17). However, data for the A-methyl derivatives (Table 18) clearly indicate that the carbon adjacent to a pyridine-like nitrogen shows a chemical shift at lower field than that adjacent to a pyrrole-like N-methyl group (in contrast to the H chemical shift behavior). In azoles containing oxygen (Table 19) and sulfur (Table 20), the chemical shifts are generally at lower field than those for the wholly nitrogenous analogues, but the precise positions vary. 

Six membered heterocyclics can be synthesized in various ways. Nitrogen analogues of stilbene, like the anilides of benzaldehyde or acetophenone sometimes form azaphenanthrenes 19). Similarly the plasma reaction of azobenzene yields a small amount of diazaphenanthrene 19>. Better results are observed in the cyclization of hydroxy, amino or nitro compounds. Suitable starting materials are molecules with two phenyl groups linked by a carbon or hetero atom. 

Fluorinated -alkenes and -cycloalkenes have a special relationship with their hydrocarbon analogues, usually exhibiting a chemistry that is complementary. For example, the fluorinated systems are frequently susceptible to nucleophilic attack, in some cases dramatically so, and therefore reactions of nucleophiles with fluorinated alkenes often reveal unique new chemistry. This chapter covers electrochemical reduction, principles governing orientation and reactivity of fluorinated alkenes towards nucleophiles, fluoride ion as a nucleophile and the mirror-image relationship of this chemistry with that of proton-induced reactions, reactions with nitrogen-, oxygen-, carbon- centred nucleophiles etc., and, finally, chemistry of some oligomers of fluorinated -alkenes and -cycloalkenes. 

Hydroxyacetals of structure (85, X = O) undergo a 7-exo-tet cyclisation in the presence of diphenylphosphine and carbon tetrachloride, for which the mechanism in Scheme 11 has been proposed <95SLiii9>. The cyclisation has also been extended to the sulphur and nitrogen analogues (85, X = S or N). 

The intermediate is an aziridinium ion (aziridines are three-membered rings containing nitrogen—the nitrogen analogues of epoxides). The hydroxide ion chooses to attack only the less hindered terminal carbon 1, and a rearrangement results—the amine has migrated from carbon 1 to carbon 2. 

The boron and nitrogen atoms are all in trigonal (sp ) valence state, the 2p lone pairs on the three nitrogens providing 6 n electrons as in benzene. The boron-nitrogen and carbon-carbon analogues are isoelectronic and compounds of this kind show certain similarities, but because the boron-nitrogen... 

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