Dihalides ethers, cyclic

Zinc chloride hydro gen chloride Dihalides from cyclic ethers... 

Replacement of oxygen by chlorine—Dihalides from cyclic ethers s. 18, 599... 

PARHAM Cyclic ether syruhesis Benzoheterocycle synthesis from dihalides. 

The method is readily adapted for the preparation of dibromides from diols. Typical examples are provided in Expt 5.54. The cyclic ethers tetrahydrofuran and tetrahydropyran are readily cleaved by the hydrobromic acid-sulphuric acid medium, and this provides an alternative and convenient preparation of the corresponding a, co-dihalides. 

Etherification.1 In the presence of BuSnClj, 1,4- and 1,5-diols are converted into acyclic and cyclic ethers. The actual catalyst is an organoalkoxytin dihalide, BuSn(OR)Cl2. This dehydration of l,5-heptadiene-4-ol (1) results in three acyclic ethers with 2 being the major product. 

By using a,co-dihalides, three- to seven-membered cyclic ethers have been prepared, simply by treatment with excess sodium hydroxide in the presence of sodium formate212. The synthesis of epoxides by this simple method in 70% yield is rather surprising, considering that the competing side reactions like elimination are usually fairly facile. 

Formation of the tribenzoarsepin derivative involves thermolysis of a quaternary arsonium salt in a reaction similar to that which generates arsafiuorene derivatives [Eq. (86)]. As for the dibenzheterocycles, an organic dihalide provides the starting point for the synthesis. Reaction of o-bromoiodobenzene with magnesium in ether at 0°C followed by reaction with dimethyliodoarsine provides a disubstituted terphenyl derivative. Addition of 1,3-dibromopropane forms the cyclic quaternary salt (232) ... 

PARHAM Cyclic ether synthesis Benzoheterocycle synthesis Irom dihalides. 

Dihalide Cyclic ether ihalide Cyclic ether... 

The critical difference between addition of halogen (X2) and the many HX additions we saw in Chapter 9 is the presence of the intermediate halonium (bromo-nium or chloronium) ion. This intermediate is demanded by the observed preference for anti addition. However, occasionally the cyclic halonium ion is less stable than the corresponding open ion and the open ion will be favored. One way to stabilize the open ion is through resonance. If the open ion is the intermediate, the products of both cis and trans addition will be observed. When a protic nucleophilic solvent is used in the reaction, the solvent as well as the halide will add to the intermediate. In such cases halohydrins or halogenated ethers are formed along with dihalides. 

---