Other Carbon Dihalides

There is on the other hand a great deal of evidence showing that the electrochemical reduction of 1,2-dihalides to olefins can occur via a concerted pathway, i.e., via a transition state (39) in which both carbon-halogen bonds are partially broken and the carbon-carbon double bond is partially formed. An important, indeed critical, point of evidence supporting the conclusion that reduction is concerted lies in the remarkable ease with which vicinal dihalides are reduced. For example, the half-wave potentials of ethyl bromide and 1,2-dibromoethane are -2.08 V and -1.52 V (vs. s.c.e.), respectively 15 >46) those of ethyl iodide and /J-chloroethyl iodide are -1.6 V and -0.9 V, respectively 47). These very large differences must reflect the lower energy of delocalized transition state 39 relative to the transition state for reduction of an alkyl monohalide. 

Both Cl2 and Br2 add to carbon-carbon double bonds to produce dihalides as illustrated in the following examples. The other halogens are not commonly used—F2 because it is too reactive and I2 because it is not reactive enough. These reactions are usually run in an inert solvent such as CC14, CHC13, or CH2C12. 

Phenyl tellurium trihalides react with linear olefins and cycloalkenes in methanol or other alcohols. In contrast to reactions in inert organic solvents, in alcoholic media the intermediate adduct between the olefin and the positive phenyldihalotelluro group is attacked by the alcohol and not by the halide ion. Therefore, the product is a 2-alkoxyalkyl phenyl tellurium dihalide. The reactions are regiospecific (tellurium bonding to the less hindered carbon atom) and highly stereospecific (anti-addition). 

Haloamines and other precursors to aziridines can be generated by various polar additions . Three important groups of polar processes leading to aziridines are shown in Scheme 22. In the aza-Darzens route , the imine acts as an electrophile at carbon and later as a nucleophile at nitrogen, while the -haloenolate acts initially as a nucleophile at carbon and later as an electrophile at the same carbon. The roles of the two components are reversed for the polar aziridination route, which is related to the epoxidation reaction. In the -haloenone route, the 1,2-dihalide or -haloenone acts formally as a bis-electrophile while the amine acts as a bis-nucleophile. 

Organic dihalides, aliphatic or aromatic, with >4 carbon atoms between the two halogens do not present any difficulties other than those encountered with their monohalo analogues. To prevent coupling, which in this case may be favored by intramolecular ring closure, the addition of the dihalide should be slow Et20 is used but THE is preferable . With several cyclic dihalides, elimination or fragmentation occurs, e.g. ° ... 

Except for the possible existence of Rela, the only simple dihalides of this group that are known (so far) are those of manganese. They are paie-pink salts obtained by simply dissolving the metal or carbonate in aqueous HX. MnFi is insoluble in water and forms no hydrate, but the others form a variety of very water-soluble hydrates of which the tetrahydrates are the most common. 

The approach used depends on the selective dehydration of hydroxy-isopropyldihydrofuroquinolines to produce terminal olefins. Acid-catalyzed dehydration resulted in the exclusive formation of the more stable isopropylfuroquinolines, e.g. 95. Although other conventional procedures were also largely unsuccessful, reaction of the hydroxy-isopropyldihydrofuroquinoline 93 with thionyl chloride and pyridine gave the terminal olefin 94, which was converted into O-methylptelefolonium salt (84). Triphenylphosphite dihalides in the presence of base, however, proved to be effective reagents. Thus, the phosphite dibromide and potassium carbonate reacted with platydesmine to give olefin 97 predominantly, which was converted into dubinidine (87) by treatment with osmium tetroxide. Ptelefolone was the only product formed when the 6,8-dimethoxy-4-quinolone 92 was treated successively with triphenylphosphite dichloride and with pyridine. 

Although 1,2-dihalides undergo elimination reactions on treatment with magnesium (Mg), it has been reported that if the halogens are different from each other and at least three carbon atoms apart, selective conversion to Grignard reagents is possible. 

As in the other polycondensation reactions, competition between linear polymerization and ring formation also occurs here and it is strongly dependent on the structure of both the dihalide and inor nic polysulfide. CycUc monosulfides are favoured when the dihalide monomer has four or five carbon atoms between the halide terminals [18]. 

---