Halogens, complexation with pyridines

These results have been rationalized74 by Heasley and coworkers by assuming that the primary function of the complexes is to limit the concentration of free halogen. In the reaction of free bromine where the reaction is second order in bromine, two or more molecules of halogen participate in the transition state while the halogen complexes with pyridine or amines impose a first-order mechanism by limiting the availability of free halogen (equation 40). 

Positive halogen complexes with pyridine bases are known as versatile halogenating reagents. Bis(sym-collidine)iodine(I) tetrafluoroborate (59) in dimethyl sulfoxide is a potential reagent for the direct conversion of alkenes to a-iodo carbonyl compounds (Scheme 6). The oxidation involves the... 

Heating the o-bromophenyl derivatives of imines 61 in the presence of cuprous(l) iodide in pyridine at reflux afforded 2-cyanobenzimidazoles 57 in good yields (Equation 9) <1998J(P1)3925>. Conducting this reaction in a focused microwave reactor resulted in reduced reaction time with no loss in yields. The electrocyclization and fragmentation process previously suggested may be facilitated by halogen complexation with copper salt. 

Direct halogenation of sucrose has also been achieved using a combination of DMF—methanesulfonyl chloride (88), sulfuryl chloride—pyridine (89), carbon tetrachloride—triphenylphosphine—pyridine (90), and thionyl chloride—pyridine—1,1,2-trichloroethane (91). Treatment of sucrose with carbon tetrachloride—triphenylphosphine—pyridine at 70°C for 2 h gave 6,6 -dichloro-6,6 -dideoxysucrose in 92% yield. The greater reactivity of the 6 and 6 primary hydroxyl groups has been associated with a bulky halogenating complex formed from triphenylphosphine dihaUde ((CgH )2P=CX2) and pyridine (90). 

Heterocychc compounds range from those, such as furan which is readily halogenated and tends to give polyhalogenated products, to pyridine which forms a complex with aluminum chloride that can only be brominated to 50% reaction (23). 

Cyclic ylide complexes are readily oxidized not only by aggressive reagents like elemental halogens but also by milder oxidants like disulfides, or even mercaptanes in the presence of molecular oxygen, as shown for a reaction with pyridine-2-thiol in air (Equation (34)).182 A similar reaction is known to take place with thiuramdi-dulfides.2,180... 

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. 

Electrophilic iodine reagents have also been employed in iodocyclization. Several salts of pyridine complexes with I+ such as bis(pyiidinium)iodonium tetrafluoroborate and bis(collidine)iodonium hexafluorophosphate have proven especially effective.61 y-Hydroxy- and d-hydroxyalkcncs can be cyclized to tetrahydrofuran and tetrahydropyran derivatives, respectively, by positive halogen reagents.62 (see entries 6 and 8 in Scheme... 

A very large number of complexes of pyridines and quinolines with all the halogens and interhalogen compounds are known, and as they have been enumerated elsewhere (74HQ14-S2)407,77HC(32-1)319) just a few examples are illustrated (Scheme 14). Treatment of pyridine with chlorine or bromine in the presence of aluminum chloride yields 4-pyridylpyridinium salts (equation 29), but rather curiously the action of iodine chloride on pyridinium hydrochloride at 250 °C produces the 2-isomer (51 equation 30). 

A similar chiral environment is given by inclusion to cyclodextrins (CDs), cyclic oligosaccharides (3). The outside of the host molecule is hydrophilic and the inside hydrophobic. The diameters of the cavities are approximately 6 (a), 7-8 (j3), and 9-10 A (7), respectively. Reduction of some prochiral ketone-j3-CD complexes with sodium boro-hydride in water gives the alcoholic products in modest ee (Scheme 2) (4). On the other hand, uncomplexed ketones are reduced with a crystalline CD complex of borane-pyridine complex dispersed in water to form the secondary alcohols in up to 90% ee, but in moderate chemical yields. Fair to excellent enantioselection has been achieved in gaseous hydrohalogenation or halogenation of a- or /3-CD complexes of crotonic or methacrylic acid. These reactions may seem attractive but currently require the use of stoichiometric amounts of the host CD molecules. 

X-Ray studies of crystals of pyridine complexes with trimethylsilyl bromide and iodide (264) and of N-methylimidazole adducts to trimethyl-chlorosilane (265) were performed. The tetracoordinate silicon structure of these complexes was proved. The distances between the halogen and silicon atoms in pyridine adducts are 4.359 and 4.559 A for bromine and iodine, respectively, which is approximately 2 A greater than the sum of the covalent radii and significantly longer (0.5 A) than the sum of the van der Waals radii. This result is consistent with the ionic structures of the complexes in the solid state. The distance from silicon to nitrogen (1.86 A) is evidently larger than the length of typical Si—N bonds (1.75 A), which... 

---