Unsaturated Sulfur-Nitrogen Ring Systems

Perhaps the most notable difference between S-N and N-O compounds is the existence of a wide range of cyclic compounds for the former. As indicated by the examples illustrated below, these range from four- to ten-membered ring systems and include cations and anions as well as neutral systems (1.14-1.18) (Sections 5.2-5.4). Interestingly, the most stable systems conform to the well known Htickel (4n -1- 2) r-electron rule. By using a simple electron-counting procedure (each S atom contributes two electrons and each N atom provides one electron to the r-system in these planar rings) it can be seen that stable entities include species with n = 1, 2 and 3. 

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Tt-Electron-rich compound a compound in which the number of tt-electrons exceeds the number of atoms Heterocyclothiazene an unsaturated sulfur-nitrogen ring system containing another element... 

This section treats seven-membered ring systems with maximum unsaturation containing within the ring two or more oxygen, sulfur or nitrogen atoms, or a combination of these.1... 

Chromatography has also been used to separate the unsaturated phos-phorus(V)-nitrogen-sulfur ring systems 3.4—3.6. " The six-membered ring... 

When a benzene is fused to a nonaromatic heterocycle, the numbers that indicate the position of the heteroatom(s) in the fused-ring system often precede the name as in 2H-3,l,4-benzothiadiazocine (Figure 6.22). In this example, we have a benzene fused to an eight-membered (-ocine) ring. Once the fusion has occurred, the name indicates that the first heteroatom (thia) be inserted into the 3-position with two nitrogens (diaz) placed at the 1 - and 4-positions, respectively. Here we have sulfur in an even-sized ring which requires that there must be one other saturated position, indicated by the 2H. All other positions will be unsaturated. 

Figure 12 Six-membered monocyclic systems containing multiple nitrogen, oxygen, and/or sulfur atoms (names shown correspond to rings with maximum unsaturation).

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