Pyridine cyclodimerization

The sterically hindered base 2,6-bis(tert-butyl)pyridine does not inhibit cyclization triaryl-amine retards this reaction photosensibilized one-electron oxidation of a diene leads to the same products, which are formed in the presence of ammoniumyl salt. As shown, in majority of cases, only the cation-radical chain mechanism of the diene-diene cyclization is feasible (Bauld et al. 1987). Meanwhile, cyclodimerizations of 2,4-dimethylpenta-l,3-diene (Gassman and Singleton 1984) and l,4-dimethylcyclohexa-l,3- or -1,4-diene (Davies et al. 1985) proceed through both mechanisms. 

Sanders and coworkers have shown that the interaction between pyridine and Zn-porphyrins can be used for controlling the outcome of the Glaser coupling of 62. They were able to show that ethynylphenyl-substituted Zn-porphyrins can be cyclodimerized or cyclotrimerized, depending on the specific pyridyl template present in the reaction medium [28], With 4,4 -bipyridene (63), the cyclodimer 64 was obtained in 70% yield, whereas the presence of 1,3,5 -tris (4-pyridyl) -triazine (65) supports the formation of the cyclotrimer 66 (50%) (Scheme 6.10). 

Similar types of cyclodimerizations were also pointed out with vinyl pyridines and vinyl quinoleines [213]. Lastly, selective cyclodimerization reactions of 1,3-diolefins catalyzed by electrogenerated Fe(NO)2 —from athodic reduction of FeCl3 in the presence of NO—allowed the conversion of butadienes into vinylcyclohexenes in good yields [214] (Scheme 40). 

The sigma-complexes of cyclobutadienes and aluminium halides react with activated nitriles (e.g. EtOjCCN) to yield substituted pyridines. The sigma-complexes are themselves formed by aluminium-bromide-promoted cyclodimerization of alkynes (Scheme 3). In an extension of this work, Dewar-... 

Six-membered heterocycles Nitrogen heterocycles. Alkylidenemalononitriles have been cyclodimerized in methanolic sodium methanolate to yield azabicyclooctadienes 208, which underwent retro-Diels-Alder reaction in xylene or acetic acid to yield the substituted pyridine 209. ... 

Likewise when two alkyne molecules coordinate to a transition metal such as Co(I) with subsequent coupling of the C-C bond, oxidative cyclization takes place to give a metallacyclopentadiene. Further reaction of another alkyne molecule with the metallacyclopentadiene followed by reductive elimination liberates benzene derivatives. Thus cyclotrimerization of three alkyne molecules catalyzed by a cobalt complex [40,41] can be performed. If a nitrile is used as the second component, pyridine derivatives are obtained catalytically as shown in Scheme 1.13 [42]. The catalytic cyclotrimerization and cyclodimerization of alkynes and conjugated enynes have found extensive applications in synthesis of complex cyclic compounds such as steroid derivatives [43]. 

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