Benzene free radical chain chlorination

The majw work to date on synthetic applications of remote functionalization has involved free radical chlorination. The earliest studies involved the direct attachment of aryliodine dichloride units to the steroid substrates, then intramolecular free radical chain chlorination in benzene or chlorobenzene solution (Scheme 14). Yields were only in the 50% region, but fairly good selecdvities were observed compound (6) afforded chiefly the 9-chloro derivative, while compound (7) produced the 14-chloro steroid. The yields and selectivities were considerably improved when it was realized that aromatic solvents promote intermolecular random processes by forming complexes with C1-, and when the radical relay method was developed. 

Consider the possible free radical chain chlorination of benzene ... 

Under certain conditions, benzene can react with halogens by addition rather than by substitution. In the presence of sunlight, a free-radicaL reaction takes place with chlorine that leads to addition products in which the aromatic character has been lost. The final product is hexa-chlorocyclohexane (benzene hexachloride), which can exist in eight possible stereoisomeric forms. The process starts with the photolytic dissociation of chlorine. Free-radical addition to the 7i-electron system of the aromatic ring follows and a chain reaction ensues (Scheme 9.1). 

Juenge3 has now found that the reagent is effective for nuclear or side-chain halogenation of aromatic systems, the former occurring under ionic conditions, the latter under free-radical conditions. Thus benzene and naphthalene are chlorinated under Lewis acid catalysis 1 -chloronaphthalene can be prepared in 58% yield. Toluene under similar conditions gives a mixture of 2- and 4-chloro-toluene in 66% yield. In the presence of benzoyl peroxide, benzyl chloride is obtained in 44% yield. 

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