Titanium catalyzed reduction of aromatic halides by sodium borohydride

The reduction of aryl halides by sodium borohydride is catalyzed by titanium complexes; di(cyclopentadienyl)titanium dichloride (titanocene dichloride) is highly effective. The reaction scope and mechanism are solvent dependent. In dimethylformamide (DMF), an adduct of DMF and sodium borohydride is formed which reduces simple aryl halides by a non-radical, likely nucleophilic route. Dimethylamino- substituted products are formed, as are simple dechlorinated species. In dimethylacetamide or in ethers, a radical-based reaction involving activated titanocene borohydride takes place, and only dechlorinated products result.