Pyridines hexadienes

Halogenation of conjugated dienes proceeds chiefly by 1,4-addition with molecular halogens (equation 3). 1,2-Addition is favored in the presence of pyridine-halogen complexes and amine tribromide salts (equation 4)9. The stereochemistry of 1,4-bromine addition with 2,4-hexadienes and cyclopentadiene is primarily anti in the presence of amine, but syn with molecular halogen in the absence of amine. 

Bicyclic azetines tend to be highly unstable species. The more strained the bicyclic system the greater the reactivity. Azetines of the 2-azabicyclo[2.2.0]hexadiene structure have been suggested as transient intermediates in photolytic conversion of pyridines (77JCS(P2)1148, 78JOC944) and of pyrimidines (81JCS(P1)943). 

Under thermal conditions bicyclic azetines undergo facile valence bond tautomerization. Typically, l-azabicyclo[2.2.0]hexadienes give pyridines (equation 16) (76CPB2219), 1,2-diazabicyclo[3.2.0]heptadiene (126) gives diazepine (127) (77CJC56) and 2-... 

Pyridine 20 has been prepared by the silver- or acid-catalyzed isomerization of 1-aza-bicyclo[2.2.0]hexadiene 22, which in turn was obtained in a [2 + 2]-cycloaddition reaction of tris(to7-butyl)azete with hexafluorobut-2-yne. Irradiation of Dewar pyridine 22 gives a high yield of azaprismane 23, which isomerizes thermally to rearranged pyridine 24. Finally, pyridine 24 yields 1-aza Dewar pyridine 25 photolytically.34... 

H2O Water 100 CsIisN Pyridine 115.5 C7II10 Methylcyclo- hexadiene Minimum b.p. 

Pyridine previously dried over powdered KOH is used 2.39 13-Hexadien-5-yne... 

Somewhat less useful is the aluminum hydride reduction of quaternary pyridinium salts. Reduction of the salts may be more conveniently performed by the use of sodium borohydride (see Section II, B, 6). Moreover, the aluminum hydride reductions of some dialkyl-pyridinium salts are accompanied by reductive cleavage of the pyridine ring,77 For example, methiodides of 2,5-dimethylpyridine,77 2-methyl-5-ethylpyridine,77 and 2-ethyl-5-methylpyridine61 afford mixtures of the corresponding tetrahydro and hexahydro bases along with a secondary amine, viz., 5-methylaminomethyl-2,4-hexadiene, 5-methylaminomethyl-2,4-heptadiene, and 7-methylamino-6-methyl-2,4-heptadiene, respectively. 

HYDROGENATION CATALYSTS Arene-chromium tricarbonyls. (1,5-Cycloocta-diene)(pyridine)(tricyclohexyl-phosphine)iridium(I) hexafluorophosphate. Di- x-chlorobis( 1,5-hexadiene)dirhodium. Lindlar catalyst. Palladuim(II) acetate-So-dium hydride-f-Amyl alkoxide. Rhodium catalysts. 

The structures of these stable azaprismane derivatives were established by their rearomatization into pyridine derivatives. To account for the structures of the azaprismanes obtained, rearrangement of an initially produced 2-azabicyclo[2.2.0]hexadiene to the 1-azaprismanes was suggested. ... 

Ultraviolet irradiation of pyridines can prodnce highly strained species that can lead to isomerised pyridines or can be trapped. The three picolines and the three cyano-substituted pyridines constitute photochemical triads irradiation of any isomer, in the vaponr phase at 254 nm, results in the formation of all three isomers. From pyridines and from 2-pyridones 2-azabicyclo[2.2.0]-hexadienes and -hexenones can be obtained in the case of pyridines these are nsnally nnstable and revert thermally to the aromatic heterocycle. Pyridone-derived bicycles are relatively stable, 4-alkoxy- and -acyloxy-pyridones are converted in particnlarly good yields. Irradiation of iV-methyl-2-pyridone in aqueous solution prodnces a mixture of regio- and stereoisomeric 4n pins 4n photo-dimers. ... 

Reactions. Further studies on the valence-bond isomerizations of perfluoro-alkyl-pyridines, reported last year, have shown that photolysis of the perfluoroalkylated derivative [28 R = CF(CF3)2, R = CF2CF3, R = Cp3] in CF2CICFCI2 yields a mixture of the l-azabicyclo[2.2.0]hexadiene (29) and the... 

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