Substitution heteroatom-substituted

Figure 7-7. Equations for the calculation of proton affinities (PA) of simple alkyl amines and of heteroatom-substituted alkyl amines.

Various other heteroatom-substituted earbocations were also found to be activated by superacids. a-Nitro and a-cyanocarbenium ions, R2C N02 or R2C CN, for example, undergo O- or N-protonation, respectively, to dicationic species, decreasing neighboring nitrogen participation, which greatly enhances the electrophilicity of their carbo-... 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Substitutive Nomenclature. The first step is to determine the kind of characteristic (functional) group for use as the principal group of the parent compound. A characteristic group is a recognized combination of atoms that confers characteristic chemical properties on the molecule in which it occurs. Carbon-to-carbon unsaturation and heteroatoms in rings are considered nonfunctional for nomenclature purposes. 

The vacuum residua or vacuum bottoms is the most complex fraction. Vacuum residua are used as asphalt and coker feed. In the bottoms, few molecules are free of heteroatoms molecular weights range from 400 to >2000, so high that characteri2ation of individual species is virtually impossible. Separations by group type become blurred by the sheer mass of substitution around a core stmcture and by the presence of multiple functionahties in a single molecules. Simultaneously, the traditional gc and ms techniques require the very volatiUty that this fraction lacks. 

Several types of nitrogen substituents occur in known dye stmetures. The most useful are the acid-substituted alkyl N-substituents such as sulfopropyl, which provide desirable solubiUty and adsorption characteristics for practical cyanine and merocyanine sensitizers. Patents in this area are numerous. Other types of substituents include N-aryl groups, heterocycHc substituents, and complexes of dye bases with metal ions (iridium, platinum, zinc, copper, nickel). Heteroatom substituents directly bonded to nitrogen (N—O, N—NR2, N—OR) provide photochemically reactive dyes. 

The benzene ring has a fully symmetrical (Deh) planar hexagonal carbon framework. Heteroatom substitution upsets this symmetry, but except in certain special cases, as for instance the thiabenzenes , the planarity of the ring is preserved. Although it is known... 

Surprisingly, no fully detailed syntheses of pyridopyridazines by joining of two heteroatoms have been recorded, although a recent patent claimed the preparation of pyrido-[2,3-c]-, -[3,2-c]-, -[3,4-c]- and -[4,3-c]cinnolines by reduction of 2,2 -dinitro substituted phenylpyridines with a variety of reagents (80GEP2939259). 

The reactivity of five-membered rings with one heteroatom to electrophilic reagents has been quantitatively compared in a variety of substitution reactions. Table 2 shows the rates of substitution compared to thiophene for formylation by phosgene and iV,AT-dimethylfor-mamide, acetylation by acetic anhydride and tin(IV) chloride, and trifluoroacetylation with trifluoroacetic anhydride (71AHC(13)235). 

In general, five-membered heteroaromatic ring systems with one heteroatom all undergo preferential a rather than /3 electrophilic substitution. This is rationalized in terms of the more effective delocalization of charge in the intermediate (36) leading to a substitution than in the intermediate (37) leading to /3 substitution. 

In view of the overall increased reactivity of furan compared with thiophene it would be anticipated that furan would be less regioselective in its reactions with electrophiles than thiophene. Possible reasons for the high regioselectivity of furan in electrophilic substitution reactions include complex formation between substrates and reagents and the ability of heteroatoms to assist in the stabilization of cationic intermediates (80CHE1195). 

Alkyl substituents supplement the a-directing effect of the heteroatom and direct the incoming substituent into the 2-position. However, steric effects can result in an increased proportion of 5-substitution. 

Substituted isoxazoles, pyrazoles and isothiazoles can exist in two tautomeric forms (139, 140 Z = 0, N or S Table 37). Amino compounds exist as such as expected, and so do the hydroxy compounds under most conditions. The stability of the OH forms of these 3-hydroxy-l,2-azoles is explained by the weakened basicity of the ring nitrogen atom in the 2-position due to the adjacent heteroatom at the 1-position and the oxygen substituent at the 3-position. This concentration of electron-withdrawing groups near the basic nitrogen atom causes these compounds to exist mainly in the OH form. 

Table 37 Tautomerism of 3-Substituted Azoles with Heteroatoms-1,2 ...

Complex tautomerism for azoles with heteroatoms in the 1,2-positions occurs for pyrazoles which are not substituted on nitrogen. Scheme 10 shows the four important tautomeric structures (148)-(151) for 3-methylpyrazolin-5-one, and (152) and (153) as examples of other possible structures. A detailed investigation of this system disclosed that in aqueous solution (polar medium) the importance of the tautomers is (149) > (151) (150) or (148), whereas in cyclohexane solution (non-polar medium) (151) > (148) (149) or (150). 

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