Types of heteroatoms

Techniques are available for quantitative identification of the six principal types of heteroatom compounds in asphalt carboxyUc acids, 2-quinolones, phenols, pyrroles, amides, andpyridines (67). 

Tables 1-7 give representative examples, reported since 1995, of the major classes compounds containing a spiro phosphorus atom. The subdivision of classes of compounds is that used in CHEC-II(1996) the compounds are grouped according to the number and type of heteroatoms bonded to the spiro phosphorus atom. The labels k, l, m, and n represent the sizes of the spiro rings.

Nitrogen substituted porphyrazines were the second type of heteroatom-deriva-tized pz macrocycle reported and were prepared from the readily derivatized diami-nomalconitrilc (DAMN) (7). Octakis(dimethylamino)porphyrazines are extremely electron-rich systems and have been used to prepare charge-transfer complexes with Cgo, as well as to peripherally chelate metals or convert to crown appended systems (38, 39). The unsymmetrical dimethylaminoporphyrazine analogues have also been reported (29), as well as the first example of the desymmetrized seco-pz from the dimethylaminoporphyrazine (8, 40). The nitrogen substituted porphyrazines are discussed in Section V. 

Interestingly, homolytic substitution at boron does not proceed with carbon centered radicals [8]. However, many different types of heteroatom centered radicals, for example alkoxyl radicals, react efficiently with the organoboranes (Scheme 2). This difference in reactivity is caused by the Lewis base character of the heteroatom centered radicals. Indeed, the first step of the homolytic substitution is the formation of a Lewis acid-Lewis base complex between the borane and the radical. This complex can then undergo a -fragmentation leading to the alkyl radical. This process is of particular interest for the development of radical chain reactions. 

The last comparison for piperazine involves a 1,4-diheterocyclohexane that has two different types of heteroatoms, morpholine (28, X = NH). We find the following disproportionation reaction ... 

This methodology has proven particularly useful for preparing non-heteroatom-stabilized carbene complexes (Section 3.1.2), but is also suitable for certain types of heteroatom-substituted carbene complex [177] (Table 2.9). 

Clearly, in view of a diversity in the types of heterocyclic compounds, one may hardly expect that all the manifestations of their aromaticity (antiaromaticity) could be rationalized in terms of some simple regularities. We shall therefore attempt to trace certain characteristic trends in the dependence of the aromaticity on the type of heteroatoms, their number and positions in the molecular structure. Our reasoning will be based on the nature of the aromaticity criteria and of the electron count rules. Then turning to individual compounds, we shall add details to the picture. 

Before turning to manifestations of aromaticity or antiaromaticity, relative stability, geometry, and other characteristics of the molecules represented in Scheme 3, we wish to treat in some detail certain regularities in changes of aromaticity depending on the type of heteroatom as well as the relationship between the geometry of a given molecule and its aromatic (or antiaromatic) character. 

This chapter reviews the systems where account is taken of the size of the central carbocyclic ring, the relative orientation of the fused five-membered heterocyclic rings, the types of heteroatoms, their number, and, in rings with more than one heteroatom, their relative situations. 

The organization of this chapter is based on the number and type of heteroatoms only N, O and S are included. Within each heterocyclic system, compounds with reduced rings are discussed before those with higher degrees of unsaturation. 

Transition from the tropylium ion to its neutral heteroaromatic counterparts is possible by replacement of a CH+ group by a heteroatom with a vacant p-orbital. The latter effectively accepts tt-electrons, thus providing ring electron delocalization. A typical example is the boron atom in a seven-membered borepine (8) (92AG1267). Correspondingly, this type of heteroatom can be referred to as borepine-like . Other little-known representatives of this family are alumopine (8, Z = A1H) or gallepine (8, Z = GaH). 

These three fundamental types of heteroatoms are also found in small and large heterocycles. 

Scheme 1 Three possible types of heteroatoms and the relationship between carbocyclic and heterocyclic aromatic systems...

The types of heteroatom present in a ring are indicated by prefixes oxa , thia and aza denote oxygen, sulfur and nitrogen, respectively (the final a is elided before a vowel). Two or more identical heteroatoms are indicated by dioxa , triaza , etc., and different heteroatoms by combining the above prefixes in order of preference, i.e. O, S and N. 

Magnetic criteria have received wide application mainly as a qualitative test for aromaticity and antiaromaticity. The values of the exaltation of diamagnetic susceptibility (in 10-6A cm-3 mol-1), and therefore aromaticity, decrease in the sequence thiazole (17.0) > pyrazole (15.5) > sydnone (14.1). The relative aromaticity of heterocycles with a similar type of heteroatom can be judged from values of the chemical shifts of ring protons. The latter reveals paramagnetic shifts when Tr-electron delocalization is weakened. For example, in the series of isomeric naphthoimidazoles aromaticity decreases in the sequence naphthof 1,2-djimidazole (8 = 7.7-8.7 ppm) > naphtho[2,3- perimidine (8 = 6.1-7.2 ppm). This sequence agrees with other estimates, in particular with energetic criteria. 

The examples we have discussed have all been hydrocarbons, and all the group enthalpy values have been obtained from Table 2.6. However, by using Tables 2.7-2.10, AHf for a wide variety of compounds containing N, O, S, and the halogens can be calculated. The procedure is just the same as for the hydrocarbons all necessary correction factors are given in the tables. In the reference from which Tables 2.6-2.10 are taken, there are tables giving group enthalpy and entropy values for still other types of heteroatoms. There the reader can also find a very... 

The above definition excludes a number of types of heteroatom cyclofunctionalization reactions from discussion in this chapter. Examples include neighboring group participation in which the cyclic structure is only a transient intermediate cyclization reactions where the C—C tr-bond is internally activated... 

The surface chemistry of carbon is rather complex. At a single adsorption site several chemically inequivalent types of heteroatom bonds may form. Strong interactions between surface functional groups further complicate the list of surface chemical structures as derived for the most relevant carbon-oxygen system An additional dimension of complexity is presented by the large variety of substrate structures of carbon which arise from anisotropic covalent bonding rather than by a isotropic metallic interaction. 

Type of heteroatom, according to the Hantzsch-Widman system (O, S, N, P). 

The reactivity of polycyclic heterocyclic compounds is related to the type of heteroatom and to the size of the ring. 

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