Pyridines 1-oxides

For oximes, the word oxime is placed after the name of the aldehyde or ketone. If the carbonyl group is not the principal group, use the prefix hydroxyimino-. Compounds with the group Z = N—OR are named by a prefix alkyloxyimino- as oxime O-ethers or as O-substituted oximes. Compounds with the group r C=N(0)R are named by adding A-oxide after the name of the alkylideneaminc compound. For amine oxides, add the word oxide after the name of the base, with locants. For example, C5H5N—O is named pyridine A-oxide or pyridine 1-oxide. 

Phosphoryl chloride Carbon disulflde, A,A-dimethylformamide, 2,5-dimethylpyrrole, 2,6-dimethyl-pyridine 1-oxide, dimethylsulfoxide, water, zinc... 

Interesting structures can be formed by combinations of ring and side-chain substituents in special relative orientations. As indicated above, structures (28) contain the elements of azomethine or carbonyl ylides, which are 1,3-dipoles. Charge-separated species formed by attachment of an anionic group to an azonia-nitrogen also are 1,3-dipoles pyridine 1-oxide (32) is perhaps the simplest example of these the ylide (33) is another. More complex combinations lead to 1,4-dipoles , for instance the pyrimidine derivative (34), and the cross-conjugated ylide (35). Compounds of this type have been reviewed by Ramsden (80AHCl26)l). 

Pyridine 1-oxide, 4-amino-kinetie data, 2, 197 (73JCS(P2)1065)... 

Pyridine 1-oxide, 2,6-dimethoxy-rate eonstants, 2, 190 (75JCS(P2)1600)... 

Methylpyridine-l-oxide (3-picoline-l-oxide) may be prepared by a method similar to that employed for pyridine-1-oxide. To a mixture of 600-610 ml. of glacial acetic acid and... 

Z values are obtained from Eq. (8-76) for solvents having Z in the approximate range 63-86. In more polar solvents the CT band is obscured by the pyridinium ion ring absorption, and in nonpolar solvents l-ethyl-4-carbomethoxy-pyridinium iodide is insoluble. By using the more soluble pyridine-1-oxide as a secondary standard and obtaining an empirical equation between Z and the transition energy for pyridine-1-oxide, it is possible to measure the Z values of nonpolar solvents. The value for water must be estimated indirectly from correlations with other quantities. Table 8-15 gives Z values for numerous solvents. 

The infrared spectra of 2-, 3-, and 4-acetamido- and 2- and 3-benzamido-pyridine 1-oxides show bands characteristic of the 2-, 3-, or 4-substituted pyridine 1-oxide and of the —NHCOCH3... 

Reaction Constants for the Reactivity of Substituted Pybidines and Pyridine 1-Oxides... 

Fig. 3. The N—O stretching frequencies of 4-substituted pyridine 1-oxides in chloroform cf. ref. 61, Solid circles if a = a = a crossed circles, a = a barred circle, a ( a) open circle, a = a cross, a " ( a).

Shindo studied the hydrogen-bonding ability of a fairly long series of substituted pyridine 1-oxides with methanol in chloroform solution and found that the OH frequency of the hydrogen-bonded OH group in methanol is well correlated with the <7-values. For four compounds, the intensity of the same band is also well correlated. In a similar study the OH frequencies of phenol vary monotonically with the CT-values, but not in a linear fashion. 

Pyridine 1-oxide, like pyridine, can act as a ligand in transition metal complexes, but unfortunately good stability constants are not known. However, Shupack and Orchin have found that the C===C stretching frequency of the ethylene ligand in trans-ethylene pyridine 1-oxide dichloroplatinum(II) varies linearly with the pA and hence with the C7-value (ct+ or a, respectively) of substituents in the pyridine oxide. The data for the above reaction series are included in Table V. 

The methods outlined, of course, are readily applicable to a wide variety of substituted heterocycles like the carboxyl, hydroxy and mercapto derivatives of pyridines, pyridine 1-oxides, pyrroles, etc. The application to amines and to diaza compounds such as pyrimidine, where the two centers are basic, is obvious except that now 23 takes the role of the neutral compound, 21 and 22 the roles of the tautomeric first conjugate bases, and 20 the role of the second conjugate base. Extensions to molecules with more than two acidic or basic centers, such as aminonicotinic acid, pyrimidinecarboxylic acids, etc., are obvious although they tend to become algebraically cumbersome, involving (for three centers) three measurable Kg s, four Ay s, and fifteen ideal dissociation constants (A ), a total of twenty-two constants of which seven are independent. 

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