Selectivity is a major goal in modern synthetic chemistry . This goal is relatively difficult to achieve in some electrophilic aromatic substitutions especially in the halogenation of phenols. Mixtures of ortho-and para-substitution products are usually formed which are often difficult to separate. We now report on surprisingly clean chlorinations and brominations with interesting para-position selectivity in the reactions of phenols with tert-butyl hydroperoxide (TBHP) in the presence of halogenated transitionmetal alkoxides such as TiX(OiPr) 3 (X = Cl or Br; for a review on TBHP oxidations see ). The halogenating power of the TiX n (OiPr) m /TBHP system and the selectivities involved have not yet been systematically investigated. Pertrifluoroacetic acid is known to generate hypochlorite ions in the reaction with TiCl 4 that chlorinate phenol (1) with moderate ortho-selectivity (56:22) . A reverse, but also very poor para-selectivity (67.5:25) is obtained when meta-chloroperbenzoic acid is used in presence of HCl .
Our preliminary chlorination experiments of 2-methylphenol (2) in dichloromethane at room temperature with TiCl(OiPr) 3 /TBHP gave low yields but an interesting paraselectivity encouraging further studies. The reaction was finally (see Experimental for details) conducted in THF at -30 ° C with 0.1 molar solutions of the phenols 1-3. Increasing the chlorine content of the titanium catalyst or the addition of lithium or magnesium chloride did not improve Transition Metal-Catalyzed Oxidations.