Thiourea-Mediated Halogenation of Alcohols.

The halogenation of alcohols under mild conditions expedited by the presence of sub-stoichiometric amounts of thiourea additives is presented. The amount of thiourea added dictates the pathway of the reaction, which may diverge from the desired halogenation reaction towards oxidation of the alcohol, in the absence of thiourea, or towards starting material recovery when excess thiourea is used. Both brominations and chlorinations were highly efficient for primary, secondary, tertiary and benzyl alcohols and tolerate a broad range of functional groups. Detailed EPR studies, isotopic labeling and other control experiments suggest a radical based mechanism. The fact that the reaction is carried out at ambient conditions, uses ubiquitous and inexpensive reagents, boasts a wide scope, and can be made highly atom economic, makes this new methodology a very appealing option for this archetypical organic reaction.