Containing One Heteroatom

In these compounds the two benzenoid rings tend to behave independently, so that substitution in one ring has little effect upon substitution in the other. Hence symmetrical disubstitution is very easy, and careful control of conditions is needed to avoid this. Conjugation is possible between positions b (or d) and e (or g) only through the intermediacy of 

Attempts to predict the positional reactivity orders in these compounds by MO calculations have had only limited success. The problem is that the molecules can be considered as substituted biphenyls (8.122), in which case positions b and d would be the most reactive (as they are in fluorene). Alternatively, they can be considered as derivatives of diphenyl ether, etc., as in 8.123, in which case positions a and c, which are, 

Calculated Positional Reactivity Order for Dibenzofuran, Dibenzothiophene, and Carbazole 

Dibenzoselenophene is nitrated (nitric acid/acetic acid) 48% in the expected 2-position, but also 25% in the 4-position (55JA1061), implying that the positional reactivity order may be different from that in the oxygen and sulfur analogues. 

Nitration in acetic anhydride of 9-(p-tosyl)carbazole gave the 1-, 2-, and 3-isomers in 28, 19, and 53% yields, respectively, and for 9-acetyl- and 9-nitrosocarbazole these became, correspondingly, 10, 48, and 42%, and 24, 0, and 66% (84T1857). Nitration of carbazole itself under those conditions gave partial rate factors of/, = 32,100 /2 = 1100 and/3 = 77,600 (58JCS3079). 

---

As shown in Scheme 2, two heteroatom-carbon bonds are constructed in such a way that one component provides both heteroatoms for the resultant heterocycle. By variation of X and Z entry is readily obtained into thiazoles, oxazoles, imidazoles, etc. and by the use of the appropriate oxidation level in the carbonyl-containing component, further oxidized derivatives of these ring systems result. These processes are analogous to those utilized in the formation of five-membered heterocycles containing one heteroatom, involving cyclocondensation utilizing enols, enamines, etc. 

CENTRAL RING CONTAINING ONE HETEROATOM Reaction of 2-bromobenzoic acid (1) with chloroSulfonic acid proceeds to afford the sulfonyl chloride 2 treatment with dimethylamine leads to the corresponding sulfonamide (3). Condensation of bromoacid 3 with the anion from thiophenol in the presence of copper powder results in displacement of halogen by sulfur... 

The same "heuristic principles" which are applied to carbocyclic compounds also hold true for simple heterocyclic compounds containing one heteroatom. However, in the case of bridged heterocyclic molecules a modified strategic bond selection must be applied. Besides the strategic bonds which meet Corey s six rules, the bonds directly attached to nucleophilic heteroatoms -such as O, S and N-are also strategic Cf. heuristic principle HP-7), provided that they satisfy rules 2B, 4, 5 and 6. For instance, in compound 31a besides the five strategic bonds determined by rules 1-6 (cf. compound 26), the sixth darkened C-N bond in 31b is also a strategic bond. 

The nomenclature of simple heterocyclic ring systems containing one heteroatom is indicated overleaf. These form a useful reference, but there is Mttle to be gained in committing them to memory. 

A. Heterocyclic Compounds Containing One Heteroatom with the Same Number... 

Heterocycles, Containing One Heteroatom, v-Isoelectronic with the Parent Conjugated Hydrocarbon Ions... 

Five-membered Heterocycles containing One Heteroatom Pyrrole, Furan and Thiophene... 

From five-membered heterocycles containing one heteroatom 91... 

Simple heterocyclic compounds Three-membered heterocycles containing one heteroatom Oxiranes (epoxides), thiiranes (episulfides), aziridines Three-membered heterocycles containing more than one heteroatom Oxaziridines, dioxiranes, diazirines Four-membered heterocycles containing one heteroatom Oxetanes, thietanes, azetidines... 

Five-membered non-aromatic heterocycles containing one heteroatom Pyrrolidine... 

Five-membered non-aromatic heterocycles containing more than one heteroatom 1-Pyrazolines, 1,3-dioxolanes, 2-pyrazolines Five-membered aromatic heterocycles containing one heteroatom Pyrrole, furan, thiophene... 

---