Pyridine JV-oxide

Pyridine jV-Methyl- pyridinium 2-Pyridone 3-Hydroxy-pyridine 4-Pyridone Pyridine jV-oxide Unknown(s)... 

Hydroxylamine converts pyran-2-ones into pyridine IV-oxides by attack at the 1,2-bond for example, the pyran-2-one (278), obtained by heating 3-acetyI-4-hydroxy-6-methyl-pyran-2-one (dehydroacetic acid) and DMF dimethyl acetal in xylene, reacts with two moles of hydroxylamine under mild conditions to give the pyrano[2,3-6]pyridine JV-oxide (279) (77JHC931). Hydrazine converts pyran-2-ones into l-amino-2-pyridones (78JHC759). 

Complexes with pyridine JV-oxide of zinc thiocyanate and selenocyanate of the stoichiometry ZnLsX2 (X = SCN, SeCN) have been reported.1066 IR evidence shows that they are best formulated as [ZnL6]2+[ZnX4f. ... 

The proposed non-planar ligand conformation agrees with the structure of oxo(salen)chro-mium(V) complex characterized by Srinivasan and Kochi, who isolated an oxo(salen)chro-mium(V) complex as well as its pyridine jV-oxide adduct, and unambiguously demonstrated that the salen ligands of these two complexes took a non-planar stepped conformation on the basis of X-ray crystallographic analyses [49]. The hypothesis that the ligand of oxo-Mn-salen... 

However, Jacobsen was able to show that, after addition of a pyridine JV-oxide derivative, trisubstituted alkenes are in fact excellent substrates. The dissymmetry of the chiral salen ligand can effectively orient the radical selectivity ... 

Pyridine jV-oxides undergo isomerism to the 2-pyridone when heated under reflux in acetic anhydride. In a recent application of this methodology, pyridine iV-oxide 113 was treated with acetic anhydride followed by sodium methoxide to give 6-hydroxypyridine 114 in 60% yield <2002T6951> (Equation 79). 3-Acetylpyridine Ar-oxide can be converted to the 2-pyridone under similar conditions in 28% yield <2001T8841>. 

Synthesis of pyridine jV-oxides has been reported by a direct cyclization in a [3+2+1] annulation reaction (Equation 156) <20010L209>. A one-step synthesis of fused isoxazolo[4,5-/(]pyridine Ar-oxides was reported. Deoxygenation to the pyridine was achieved using PCI3 (Scheme 102) <2003IJB1742>. 

Spectrophotometric data for the stability constants of complexes of Me3SnCl with substituted pyridine JV-oxides in acetonitrile show (76) that these constants correlate linearly with the [Pg.81]

Since sulfonation of pyridine iV-oxide is about as difficult as is that of pyridine itself and takes place at the 3-position,17 it has been assumed18 that, in fuming sulfuric acid, pyridine iV-oxide reacts only in the salt form (3), when the prediction is that substitution at C-3 would take place. It is, however, difficult to account for the fact that bromination, even at 110° in the presence of iron powder, does not occur.17 Bromination in chloroform solution in the presence of acetic anhydride and sodium acetate (when the O-acetate is the the probable substrate) take place readily, however, to give 3,5-dibromopyridine JV-oxide.19 The predicted order of nucleophilic reactivity, on the basis of both atom localization energies and ground-state v-electron density calculations, is 4 > 2 > 3. The same order is predicted for the nucleophilic substitution reactions of the salts of pyridine JV-oxide. In actual practice, iV-alkoxypyridinium derivatives undergo nucleophilic attack preferentially at C-2.20-23 The reaction of some pyridine iV-oxides with phosphorus pentachloride may involve the formation... 

In order to increase the yield and/or the enantioselectivity of the reaction, the reaction temperature and additives were examined. Although aziridination was found to proceed smoothly at 0 °C, the product was not obtained at lower temperatures. Katsuki and co-workers have reported that pyridine /V-oxide is an effective additive for the asymmetric epoxidation catalyzed by salen-manganese(IH) complexes [24], and applied these findings to the asymmetric aziridination of olefins with Phi = NTs [9f]. Thus, the addition of pyridine /V-oxide at 0°C improved the enantioselectivity and allowed the reaction to proceed even at -20 °C (Table 6.1). Other additives, such as 4-phenylpyridine IV-oxide, 4-methylmorphorine N-oxide and 1-methylimidazole were used in the place of pyridine JV-oxide, but positive effects were not observed. 

Acid-catalyzed exchange has not been established in pyridine JV-oxide itself. The exchange of a number of derivatives has been studied [64CI(L) 1576]. 3,5-Dimethylpyridine N-oxide (9.42) reacts as the free base at the 2(6)- and 4-positions giving a zero slope of the rate-acidity profile [67JCS(B) 1222] the 2- and 6-positions are slightly more reactive than the 4-position, as expected. In 2,4,6-trimethylpyridine N-oxide... 

There is even less variety in the known chemistry of the chloro compounds because of their lower redox stability.523 Although MnCl is stable only below — 40°C, apparently decomposing through a disproportionation reaction,623 the oxidation state is stabilized with other ligands. Thus MnCl3 is more robust in donor solvents, undoubtedly as species like [MnCl3L3], and compounds of this type are well established for both N and O donors (Sections 41.4.2 and 41.4.3.6). There is also a series of pyridine-JV-oxide compounds (Section 41.4.3.6) MnCl3L2 which may well be five-coordinate. 

Reaction of [RuBr2(CO)4] with L (L = pyridine JV-oxide, OPPh3) in CHC13 yields the complexes [RuBr2(CO)3L).2152 The products [RuC13L3] (L = 2,6-lutidine V-oxide,2153 phosphole AT-oxide derivatives,2154 8-hydroxyquinoline-A-oxide21552156) and O bonded [RuC13L2] (L = thiomorpholin-3-one)679 have been prepared. 

Table 5 Comparison of Pyridine jV-oxide Dimensions (A, degrees)...

PhIO, cat chiral Mn salen complex, pyridine JV-oxide (enantioselective)... 

Aminopyridine can be prepared by the reaction of 4-chloro-l,3-benzoxazines (47) with pyridine JV-oxides. Here it is proposed that an ion or radical pair is formed initially, which through displacement of hydrogen chloride and rearrangement leads to an IV-substituted benzoxazine (48). Finally, acid hydrolysis gives 2-aminopyridine and salicylic acid (49 Scheme 14) (80CPB465). 

Both pyridinium salts and pyridine JV-oxides are of increased interest as chiral catalysts in organic reactions. Connon and Yamada independently designed and examined pyridinium salts as chiral catalysts in the acylation of secondary alcohols <06OBC2785 06JOC6872>. These two catalysts can be used for kinetic resolution of various sec-alcohols and c /-diols in good to moderate enantiomeric excess. 

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