Azidopyridines photolysis

Whereas photolysis of 2-unsubstituted and 2,4-disubstituted 3-azidopyridincs in the presence of sodium methoxide gives 5//-l,3-diazepines (see Section 4.1.3.2.1.2.), irradiation of 2-sub-stituted 3-azidopyridines 4 under these conditions yields 2//-l,4-diazepines 5.160,183... 

Photolysis of 4-azidopyridines 5 in the presence of sodium methoxide gives 5-methoxy-6//-l,4-diazepines 6.187... 

The pyridyl substituted 1,4-diazepinone derivative 91 was prepared by photolysis of 2,6-bispyridyl-4-azidopyridine, and it represents a new class of ligand for metal ion complexing <00HCA384>. 

Azidopyridines under photolysis or thermolysis can be converted into derivatives of 6H-1,4-diazepine (84CPB4694) (c/. Section 3.2.3.6.4). 

H- 1,2-Oxazines are stabilized as 4,4-disubstituted derivatives but 4H- and 6if-l,2-thiazines are almost unknown. Possibly the paucity of these compounds simply reflects a lack of interest, for 6if-l,2-oxazines are comparatively common, and 3,5-diphenyl-6if- 1,2-oxazine, for example, is a stable crystalline solid. Similarly, cyano-1,2-oxazines (6 R=alkyl) are formed by the photolysis of azidopyridine oxides (5). However in the case where R3 = H, these products rearrange to pyrroles (7) (81CC36). 

Photolysis of 4-azidopyridine-l-oxide in acetone under nitrogen gave a 37% yield of 4,4 -azopyridine-l,r-dioxide. When the photolysis was carried out in the presence of oxygen 4,4 -azoxypyri-dine-ljT-dioxide was obtained in 27% yield. 

The paper which reported the photo-induced ring opening of 3-pyridylcar-bene (17) (see Section 3.1) also described a study of the matrix photolysis of 3-azidopyridine (45) (Scheme 9). Irradiation of (45) at 222 nm in Ar at 7 K gave a product with IR and UV absorptions which indicated that it was the ring-opened ylide (47) two rotamers were identified. The ylide also underwent a 1,7-H shift to give the ketenimine (48) in two rotameric forms. 

The photolysis of 4-azidopyridine in a mixture of methanol and dioxane containing 1.5 equiv. of sodium methoxide gave the relatively unstable 5-methoxy-6//-l,4-diazepine (167), as the sole product (Scheme 32) <84CPB4694>. By analogy with earlier work on the photolysis of phenyl azides, the reaction is presumed to proceed through a heteronorcaradiene intermediate. The thermolysis of 4-azidopyridine in methanol at 200°C for 8 min also yielded 5-methoxy-6//-l,4-diazepine (167) (50% crude yield). The reaction of the 2-methyl-4-azidopyridine under the same thermal conditions gave a 2 1 mixture of 5-methoxy-7-methyl-67/-l,4-diazepine (168) and 5-methoxy-2-methyl-67/-... 

Diazepines could be obtained from 3-azidopyridines only if the 2-position was substituted. For example, 4-azido-2-methylpyridine on photolysis in the presence of sodium methoxide furnished 3-methoxy-2-methyl-2FT-l,4-diazepine. Photolysis of the unsubstituted or 2,4-disubstituted derivatives only gave 4-methoxy-5FT-l,3-diazepines <87CPB1179>. 

Azidopyridines are the simplest and obviously the most investigated heteroaromatic azides. In earlier investigations main attention was paid to identification of various reaction products. Recently, the quantitative structure-reactivity relationship was studied based on comparison of the experimental and quantum-chemical data. Many high-spin nitrenes were obtained by photolysis of poly azidopyridines. 

Photolysis of azidopyridines in the presence of nucleophiles (sodium methoxide, diethylamine, etc.) proceeded with ring expansion and produced different derivatives of diazepines, (Scheme 2). 2-Azidopyridines gave derivatives of IH-1,3-diazepines and 5H-l,3-diazepines [14,15], 3-azidopyridine -derivatives of 2H-l,4-diazepines and 5H-l,3-diazepines[16,17], and 4-azidopyridine - derivatives of 6H-l,4-diazepines [17,18]. 

Scheme 2. Photolysis of azidopyridines with ring expansion.

Photolysis of N-oxides of 4-azidopyridine and 4-azidoquinoline gave corresponding azocompoimds [22], or hydrogen-abstraction products [23], No ring-expansion products were detected [23], It is interesting, that no cross-over azocompound was obtained during photolysis of the mixture of two N-oxides of the azides [22],... 

Upon laser flash photolysis of 4-azidopyridine-1-oxide at room temperature in 3-methylpentane at 266 or 308 nm two temporally distinct featmes were... 

Both 4-azidopyridine and 4-azidoquinoline in their neutral and eationic forms deeompose rapidly upon UV irradiation within the absorption bands. Figure 2 and Figure 3 show spectral changes observed upon irradiation of 4-azidopyridinium hydrochloride and 1-methyl-4-azidoquinolinium methylsulphate, respectively, Isosbestic points at the initial period of photolysis testify to selective reaction... 

Low-temperature (matrix) photolysis (and also laser flash photolysis), eombined with high-level quantum-chemical calculations, are powerful tools for investigation of reaction mechanisms. A lot of information has been obtained by this method relative to the structure and further thermal and photochemical transformations of arylnitrenes, primary products of aiylazides, their rearranged intermediates (azacycloheptatetraenes). Some recent results on photolysis of azidopyridines and related heterocyclic azides at cryogenic temperatures are discussed below. 

Heteroaromatic azides often show photochemistry that is very similar to the photochemistry of simple homoaromatic azides. Ring expansion to cycHc ketenimines is observed, if the photolysis is performed at ambient temperature, and triplet nitrenes are formed upon low-temperature irradiation. If a nitrogen atom is placed adjacent to the nitrene center, such as in 2-pyridylnitrene 100, addition generally takes place away from the nitrogen atom. In the case of the photolysis (or pyrolysis) of the 2-azidopyridine 99, the reaction leads to the cycHc carbodiimide 101. A comphcation arises from the fact that derivatives of 2-azidopyridine and related heterocycles frequently undergo a thermal isomerization to tetrazolopyridines, which also undergo photochemical dediazotation, albeit with a low quantum yield. ... 

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