Structures Having Unsaturated Rings

A polymeric structure is exhibited by beryllium dimethyl, which is actually [Be(CH3)2] (see the structure of (BeCl2) shown earlier), and LiCH3 exists as a tetramer, (LiCH3)4. The structure of the tet-ramer involves a tetrahedron of Li atoms with a methyl group residing above each face of the tetrahedron. An orbital on the CH3 group forms multicentered bonds to four Li atoms. There are numerous compounds for which the electron-deficient nature of the molecules leads to aggregation. 

In addition to the types of structures shown thus far, there are several others that are both interesting and important. One such type of structure contains unsaturated rings. Because R-C=N is called a nitrile, compounds containing the -P=N group were originally called phosphonitriles. An unstable molecule having the formula N-PH2 is known as phosphazine. Although this molecule is unstable, 

Several materials having the formula (NPC12) are known, but the most extensively studied of them is the cyclic trimer, (NPC12)3, which has the structure 

The chemistry of these compounds is discussed in greater detail in Chapter 14. 

Therefore, the borazine molecule is considered to be aromatic. Borazine has many properties that are similar to those of benzene, although it is more reactive as a result of the B-N bonds being somewhat polar rather than purely covalent. 

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Eighteen y-lactones having unsaturated rings have been identified in flavors. These compounds are principally of three general structural types a, 3-unsaturated (58), (3,y-unsaturated (59) and a, 3,y,8-unsaturated (60). 

Cyclopolymerizations yielding more complex ring structures have also been reported [Butler, 19896, 1989]. For example, 1,4-dienes such as divinyl ether yield uncrosslinked products with little or no unsaturation and possessing different bicylic structures. The formation of one of the bicyclic structures is shown in Eq. 6-107 [Tsukino and Kunitake, 1979]. 

A good variety of compounds have been prepared and characterized allowing us to obtain reliable information on the chemical shifts of saturated and unsaturated ring protons. A selection of different structures are shown in Table 4. The thiazine derivatives have been organized in groups for comparison. The effects of saturation (54, 105, 106), substituents of variable electronegativity (107, 108), increasing oxidation state of sulfur (121, 122, 27-29), and deprotonation (109-111) can be observed. 

Many pyrrolizidine alkaloids are known to produce pronounced hepatic toxicity and there are many recorded cases of livestock poisoning. Potentially toxic structures have 1,2-unsaturation in the pyrrolizidine ring and an ester function on the side-chain. Although themselves non-toxic, these alkaloids are transformed by mammalian liver... 

The types of monomers that can be polymerized by a chain mechanism are illustrated in Table 3-1. These either have unsaturated (double or triple) bonds or are cyclic. The letters X and Y in some of these structures are used to represent various types of atoms or functional groups (e.g., -CH3, -Cl, a benzene ring, etc.)... 

X-Ray determinations of structure have included the oxides (1) and (2), the latter of which is shown to have a planar heterocyclic ring. The pAa values of a series of phosphine oxides (3) have been measured in diglyme. The mass spectra of 10-phenylphenoxaphosphine oxides (4a) and sulphides (4b), of cxjS-unsaturated phosphine oxides, and of... 

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