Palladium catalyzed ortho-halogenation of 2-arylbenzothiazole and 2,3-diarylquinoxaline

A palladium catalysed ortho-halogenation strategy has been developed using benzothiazoles and quinoxalines as the directing substrates. This method provides mono-o-halogenated product at the other available ortho site of a mono-ortho substituted 2-arylbenzothiazole. However, orthounsubstituted 2-arylbenzothiazole afforded di-ortho halogenated product exclusively. The preformed (or installed) ortho-group is towards the sulphur side of the benzothiazole from energy minimised calculation. Thus the selective formation of di-ortho-halogenated products is due to favourable exposure of the second ortho site for subsequent halogenation. However the phenyl ring in 2,3-diarylquinoxalines can be selectively mono ortho halogenated. A plausible reaction mechanism has been proposed for this halogenation process.