Hetero-atom Compounds

Figure 6. Hetero-atom compounds found in crude oil...

Some amplification of the classification of heterocychc compounds may now be given. The two important hetero atoms are oxygen and nitrogen in the former class are included S, Se and Te and in the latter class P, As, etc. The sub-division (hetero-classes) of Division III is as follows ... 

The significance of Dewar s results for a series of polynuclear hydrocarbons, as well as for various compounds containing hetero atoms, has been discussed ( 5.3.2). Though the differences are not often important, we have not in all cases been able to reproduce the values for the partial rate factors reported by these authors, by recalculation from their reported results in table 5.3 the figures in parentheses are some examples of our recalculations. 

The reactions described so far can be considered as alkylation, alkenylation, or alkynylation reactions. In principle all polar reactions in syntheses, which produce monofunctional carbon compounds, proceed in the same way a carbanion reacts with an electropositive carbon atom, and the activating groups (e.g. metals, boron, phosphorus) of the carbanion are lost in the work-up procedures. We now turn to reactions, in which the hetero atoms of both the acceptor and donor synthons are kept in a difunctional reaction produa. 

Before we start with a systematic discussion of the syntheses of difunctional molecules, we have to point out a formal difficulty. A carbonmultiple bond is, of course, considered as one functional group. With these groups, however, it is not clear, which of the two carbon atoms has to be named as the functional one. A 1,3-diene, for example, could be considered as a 1,2-, 1,3-, or 1,4-difunctional compound. An a, -unsaturated ketone has a 1.2- as well as a 1,3-difunctional structure. We adhere to useful, although arbitrary conventions. Dienes and polyenes are separated out as a special case. a, -Unsaturated alcohols, ketones, etc. are considered as 1,3-difunctional. We call a carbon compound 1,2-difunctional only, if two neighbouring carbon atoms bear hetero atoms. 

The most general methods for the syntheses of 1,2-difunctional molecules are based on the oxidation of carbon-carbon multiple bonds (p. 117) and the opening of oxiranes by hetero atoms (p. 123fl.). There exist, however, also a few useful reactions in which an a - and a d -synthon or two r -synthons are combined. The classical polar reaction is the addition of cyanide anion to carbonyl groups, which leads to a-hydroxynitriles (cyanohydrins). It is used, for example, in Strecker s synthesis of amino acids and in the homologization of monosaccharides. The ff-hydroxy group of a nitrile can be easily substituted by various nucleophiles, the nitrile can be solvolyzed or reduced. Therefore a large variety of terminal difunctional molecules with one additional carbon atom can be made. Equally versatile are a-methylsulfinyl ketones (H.G. Hauthal, 1971 T. Durst, 1979 O. DeLucchi, 1991), which are available from acid chlorides or esters and the dimsyl anion. Carbanions of these compounds can also be used for the synthesis of 1,4-dicarbonyl compounds (p. 65f.). 

Low-valent nitrogen and phosphorus compounds are used to remove hetero atoms from organic compounds. Important examples are the Wolff-Kishner type reduction of ketones to hydrocarbons (R.L. Augustine, 1968 D. Todd, 1948 R.O. Hutchins, 1973B) and Barton s olefin synthesis (p. 35) both using hydrazine derivatives. 

Reactions of aromatic and heteroaromatic rings are usually only found with highly reactive compounds containing strongly electron donating substituents or hetero atoms (e.g. phenols, anilines, pyrroles, indoles). Such molecules can be substituted by weak electrophiles, and the reagent of choice in nature as well as in the laboratory is usually a Mannich reagent or... 

The concept of mesohydric tautomerism was advanced by Hunter and his associates in a series of papers which appeared between 1940 and 1950 (e.g., references 15 and 16). This concept was based on the fact that in all cases where the mobile hydrogen atom would be bonded to oxygen, sulfur, or nitrogen atoms in both possible tautomers, the individual forms had not been isolated. It was further established that many of these compounds were associated both in the liquid state and in solution, and it was concluded that the individual tautomers did not exist. The actual molecules were thought to be intermolec-ularly hydrogen-bonded, the mobile hydrogen atom being bonded equally to both of the hetero atoms. This concept has been useful and has led to clarification of the tautomerism which occurs in solids and... 

Interconversion between two tautomeric structures can occur via discrete cationic or anionic intermediates (scheme 24, where T is an anion capable of reacting with a proton at a minimum of two distinct sites). Alternatively, interconversion can occur by simultaneous loss and gain of different protons (scheme 25, w here T has the same definition as in scheme 24). These mechanisms are well established for acyclic compounds, but they have been much less thoroughly investigated for heteroaromatic systems. The rate of interconversion of two tautomers is greatest when both of the alternative atoms to which the mobile proton can be attached arc hetero atoms, and isolation of the separate isomers is usually impossible in this case. If one of the alternative atoms involved in the tautomerization is carbon, the rate of interconversion is somewhat slower, but still fast. When both of the atoms in question are carbon, however, interconversion is... 

The a and y are used to denote the position of a substituent relative to a cyclic hetero atom thus 4-hydroxy isoquinoline is a /3-hydroxy compound. 

The chemical reactions of 3-aminopyridines, 3-aminoquinoIines, etc. in which the amino group is beta to the hetero atom or is attached to a benzenoid ring are similar to those of aniline (cf. references 282, 290), and it has been generally accepted that these compounds exist overwhelmingly in the amino form. 2-Aminoacridine was not considered to exist as 231 (R = H), because 231 (R Me) shows reac-... 

Prototropic Tautomerism of Heteroaromatic Compounds III. Five-Membered Rings and One Hetero Atom A. R. Katritzky and J. M. Lagowski... 

Prior to 1960 little work had been done on reactions of heterocyclic compounds involving hetarynes, i.e. intermediates with a triple bond in the nucleus containing the hetero atom. Since then interest in hetarynes has grown and investigations in this area are developing rapidly using information available from carbocyclic aryne chemistry. Therefore, a short survey of the chemistry of arynes is presented before summarizing typical problems encountered in hetaryne chemistry. 

Prototropic Tautomerism of Heterparomatic Compounds IV. Five-Membered Rings with Two or More Hetero Atoms... 

Systems 2 and 3 are cross-conjugated, but 5 is not, and it might have been expected that a-substituted compounds would be more prone to exist in the aromatic form 1 than the -compounds to exist as 4. From the limited evidence available, the reverse appears to be the case. When the hetero atom is not a very strong electron donor, i.e., Z — S or O, structure 3 would be expected to be relatively more stable than 2, and this is supported by the evidence available. When Z = NR, the difference in the stabilities of 2 and 3 could be smaller. 

E. Potential Hydroxy Compounds with a Ring System Containing Three or Four Hetero Atoms. ... 

All the available evidence suggests that five-membered heterocyclic compounds containing a potential hydroxyl group between the two hetero atoms in the 1- and 3-positions exist predominantly in the oxo form (cf. 89 90). 

Apparently no data are available for mercapto compounds with hetero atoms-1,2 or for 4-(or 5)-mercapto compounds with hetero atoms-1,3. From the data available, the tautomerism of potential... 

Three-membered rings with one hetero atom were known in the 19th century. Today they are of great interest, both scientifically and technically. Because of ring strain, they are energy-rich compounds comparable with cyclopropane. 

Thus it is not surprising that three-membered rings with two hetero atoms were mentioned in the literature at an early stage. For example, at the turn of the century, nitrones, hydrazones, and aliphatic diazo compounds were all formulated with three-membered rings (I, 2, 3). Later the three-membered ring structures for these compounds became questionable. The structure of the aliphatic diazo compounds was studied very intensively. For diazomethane no clas-... 

In the last 10 years many three-membered rings with two hetero atoms have been synthesized. Their formation occurs surprisingly smoothly with an ease comparable to that of the three-membered rings with one hetero atom. The oxaziranes (I), diaziridines (2), and diazirincs (3) have become, in a very short time, classes of compounds with an extensive literature. 

Selenazole is the selenium-containing compound in the series of heterocyclic 5-membered ring azoles with two different hetero atoms, of which the first two members are oxazole and thiazole. The numbering of the ring system is according to the scheme given (1). 

The versatility of the Diels-Alder reaction becomes especially obvious, when considering the hetero-variants. One or more of the carbon centers involved can be replaced by hetero atoms like nitrogen, oxygen and sulfur. An illustrating example is the formation of the bicyclic compound 31, by an intramolecular hetero-Diels-Alder reaction ... 

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