Molecular complexes bromine-pyridine

Subsequent kinetic and product distribution data on the reactions of 1,3-butadiene with molecular bromine, pyridine-bromine complex and tetra-n-butylammonium tribromide in chlorinated solvents have shown that pyridine-Br2 and tribromide ion act as independent electrophiles, rather than as sources of molecular bromine75. 

Pyridine Complexes. The addition of a halogen to pyridine affords a molecular complex The bromine complex Is capable of the halogen— atlon of olefins as reported by Lloyd and Durocher and Zupan et al (61-63). Anti stereoselectivity Is observed and a nucleophilic solvent can replace the second halogen, thus Implicating a bromonlum Ion. A similar complex with polyvinyl pyrldlne-N-oxlde acts In a similar fashion. The polymer-bound pyrldlnlum hydrobromide perbrom-Ide has also been formed by Frechet, Farrall and Nuyens. It reacts with olefins In the same fashion but has also been shown to halogenate ketones (64). 

The product distribution can be shifted to favor the 1 -product by use of such milder brominating agents as the pyridine-bromine complex or the tribromide ion, Br3. It is believed that molecular bromine reacts through a cationic intermediate, whereas the less reactive brominating agents involve a process more like the AdgS and-addition mechanism. 

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. 

The structure of ra-complexes has been examined by the X-ray crystallographic studies of solid samples and by the spectral measurement of heterocycle-halogen equilibria in solution. By the former approach the structure of the solid 1 2 pyridine-molecular iodine complex has been shown to consist principally of two pyridine molecules collinearly bonded to an iodine atom and of linear triiodide units. Moreover, in the 1 1 complex of dioxane and bromine, the heterocyclic oxygen-bromine-bromine linkages are also collinear.4 In the latter type of study, interest in the well-known phenomenon of brown iodine solutions has occasioned the measurement of the stability constants for many complexes of halogen and heterocycles.25, 29 Information concerning their structure in solution comes from a consideration of the relative size of such constants as a function of heterocycle structure. Thus, the fact that bromine complexes of both 8-bromo- and 8-methylquinolines possess stability constants (K = 1.1 and 4.8 liters/mole) much smaller than that of quinoline itself... 

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