Bromine molecular complexes

X-ray diffraction analysis of crystalline poly(schiff base)s and their low molecular models shows that the formation of molecular complexes is accompanied by an increase in interplanar distances and, in a number of cases, by complete amor-phization. Molecular complexes of poly(schiff base)s with Br2 decompose with time, because of the bromination of the donor components, forming C—Br bonds. Substitution of hydrogen by bromine in phenyl groups occurs only in cases in which these groups are not included into the main polymeric chain. 

First of all, the reaction pathways shown in Scheme 1 involve the formation of charge transfer complexes (CTC) between olefin and Br2- The formation of molecular complexes during olefin bromination had been hypothesized often (ref. 2), but until 1985, when we published a work on this subject (ref. 3), complexes of this type had been observed only in a very limited number of circumstances, all of which have in common a highly reduced reactivity of the olefm-halogen system, i.e. strongly deactivated olefins (ref. 4), or completely apolar solvents (ref. 5) or very low temperatures (ref 6). 

Although the involvement of molecular bromine-olefin complexes as active intermediates in bromination had been widely assumed, this was demonstrated... 

Indeed, more intermediates are involved in alkenes bromination than were previously considered. The first intermediates formed in the early steps are the alkene-halogen molecular complexes, whose ionization gives the corresponding bromonium or p-bromocarbenium bromide (tribromide) ion pairs. (2) The reversibility of the ionization step has been widely discussed in the last years... 

Borowiak, T., Kubicki, M., Wysocka, W. and Przybyl, A. 1998. Regioselective bromination of multiflorine and structures of 3-bromomultiflorine and its molecular complex with succinimide. Journal of Molecular Structure, 442 103-113. 

Similar geometric optimization has been reported for bicyclo[3.2.2]nona-6,8-diene (BND). The double bond situated in the opposite direction to the methylene group was found to be more exo-pyramidalized than the other double bond and the electron density (qi, HOMO) of the former double bond in HOMO of the molecule higher than that of the latter double bond. The exo and endo faces of exo-pyramidalized double bonds proved not to be equal and the electron density was found to be higher on the endo faces. The endo molecular complexes with bromine have been found by the HF/321G method to be more stable than their exo congeners this was attributed to electronic and steric factors. As a result, endo-facial stereoselectivity of bromination ( ) predominates.21 A related theoretical study of facial selectivity and regioselectivity of the electrophilic addition of chlorine to exo-tricyclo[4.2.1.02,5]nona-3,7-diene (exo-TND) has also been reported.22... 

Reaction of selenoxanthene 9 with bromine affords the selenium addition product 72 (Equation 26) however, selenoxanthone 10 reacts with bromine to form a molecular complex 73 (Equation 27). The explanation of this difference in reactivity is based on the differences in ionization potentials of selenium in these molecules which were determined using penning ionization electron spectroscopy <1998JOC8373>. These results are consistent with ab initio calculations of the electronic states of the precursors (see Section 7.11.2). 

Whereas in the MMA photoinitiated polymerization by quinoline-bromine CT complex the formation of radicals is preceded by an instantaneous complexation reaction between the CT complex initiator and the monomer [68], no evidence of this occurrence is observed in the case of the poly(NVC)-Br2 CT complex, probably due to the steric hindrance provided by the polymeric chain. The behaviour of the above system should however be compared with that of the corresponding low-molecular-weight A-alkyl carbazole-Bra CT complex in order to clarify this point. 

Benzo[c]cinnoline forms molecular complexes with halogens in organic solvents thus attempts to brominate it with molecular bromine have been unsuccessful. Using a source of positive bromine, bromine/silver sulfate in sulfuric acid, Corbett and Holt found that reaction occurred at room temperature to give 27% of a monobromo and 4% of a dibromo derivative. These compounds were at first identified as 1-bromo- and l,4-dibromo-(or l,7-dibromo-)benzo[c]cinnolines, but the former was subsequently shown to be the 4 isomer. A reinvestigation of the reaction has shown that both 1- and 4-bromobenzo[c]cinnolines are primary products, formed in the ratio of ca. 2.3 1 at room temperature. The lower isomer ratio as compared with nitration in sulfuric acid probably reflects the greater steric demand of the attacking species. The dibromo compound formed is the 1,4-isomer. The formation of the octachloro derivative by chlorination of benzo[c]cinnoline in the presence of aluminum chloride has been mentioned,but no details are available. 

Two crystalline molecular complexes of selenanthrene with bromine with a ratio of selenanthrene to bromine of 4 5 have been isolated <2004CC140, 2000JOM178>. A key feature of these partial structures 24 and 25 is the linear nature of the bromine atoms in these nonionic complexes. 

Detailed NMR studies have been carried out on l-selena-4-oxane and the adducts formed in the presence of iodine monochloride, iodine monobromide, chlorine, bromine, and iodine <1999JOC2630>. In deuterated chloroform solution the l-selena-4-oxane molecular complex 29 affords equimolar amounts of bipyramidal adduct 30 and the molecular complex 31 (Equation 1). No such mixture was observed in the solid state. When NMR studies were carried out on the corresponding bromine complex 32, an equilibrium between the complex 32 and adducts 33 and 31 was observed (Equation 2). 

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 formation of a complex prior to reaction is well supported in the case of bromination. Molecular bromine forms charge transfer complexes with benzene even in the absence of Lewis acid catalysts. The complexes can be detected spectroscopically and can even be crystallized for structure determination. Complexes of Br with the aromatic compound may also be formed. For a summary of experimental data in this area, see the discussion in reference 178. 

Halogenation (Section 22.1 A) The electrophile is a halonium ion formed as an ion pair by interaction of chlorine or bromine with a Lewis acid. The mechanism involves an initial reaction between Clj and FeClj to generate a molecular complex that can rearrange to give a C1+, FeCl " ion pair. The C1+ reacts as a very strong electrophile with the weakly nucleophilic aromatic 77 cloud to form a resonance-stabilized cation intermediate that loses a proton to give the aryl chloride product. 

EtMcaSiCl. Bromine and iodine eire believed to form molecular complexes with R4Sn. ... 

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