Compounds that Contain Two Heteroatoms

There are an increasing number of compounds being prepared that contain two heteroatoms attached to a CF2 group, in particular halodi-... 

Sulfur containing heterocycles are also common Compounds m which sulfur is the heteroatom m three four five and six membered rings as well as larger rings are all well known Two interesting heterocyclic compounds that contain sulfur-sulfur bonds are hpoic acid and lenthiomne... 

In Sect. 11.4, we examined hydrolytic attack at sp3 C-atoms rendered more electrophilic by the presence of one or more halogen substituents. In a logical follow-up, the present section presents the hydrolysis of Mannich bases [78], L e., compounds that contain the X-CH2-N moiety, where an sp3 C-atom is made more electrophilic by the presence of two flanking heteroatoms, one of which is an N-atom. 

The structural diversity associated with bicyclic 5-5 fused heterocyclic systems containing two heteroatoms in each ring has been noted previously in both CHEC(1984) <1984CHEC(6)1027> and CHEC-II(1996) <1996CHEC-11(7)115>. In recent years this diversity has also been reflected in the number of applications that have been found for these compounds which include organic metals, molecular clips and receptors and molecular magnets. These fields will be examined in due course through this chapter. 

Imidazole, the heterocyclic ring of histidine, is the first heterocyclic compound we have encountered that has two heteroatoms. Imidazole is an aromatic compound because it is cyclic and planar, every carbon in the ring has a p orbital, and the tt cloud contains three pairs of tt electrons (Section 15.1). The electrons drawn as lone-pair electrons on N-1 (see p. 896) are part of the tt cloud because they are in a p orbital, whereas the lone-pair electrons on N-3 are not part of the tt cloud because they are in an sp orbital, perpendicular to the p orbitals. 

The characterization of heavy fractions of a coal liquid is especially important and complex because of two factors (1) with increasing boiling point, there is an increase in the structural diversity of hydrocarbons and (2) heteroatomic compounds that contain oxygen, nitrogen, sulfur, or trace elements (Wilson et al., 1983) and are usually concentrated in the higher boiling fractions. 

Although in a strict sense the term metallacycle applies to cyclic complexes containing two Ni-G <7-bonds, in this section we use a broader definition of the term, to comprise also those compounds that contain an Ni-G and an Ni-heteroatom bond where the heteroatom has a formal negative charge. The two types of metallacycles can be distinguished by the terms carbometallacycles and heterometallacycles, but their synthesis and reactivity are closely related, and will be treated together (Table 9). Those complexes displaying cyclic structures closed by one... 

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