The reaction of primary amines with boron halides. Part II. Arylamines. The effect of ortho-substitution: formation of di-B-halogenoborazoles

The course of the thermal or base-promoted dehydrohalogenation of the 1 : 1 adducts of primary arylamines and boron trichloride or boron tribromide is profoundly affected by ortho-substitution in the benzene nucleus. On heating in solution the 1 : 1 adducts of boron trichloride and aniline or o-toluidine give excellent yields of the corresponding tri-B-chloroborazole, but the evolution of hydrogen chloride follows different rate laws in the two cases.On treatment with a suitable tertiary base aniline–boron trichloride is converted into the tri-B-chloroborazole, whereas o-toluidine–boron trichloride gives, besides the borazole, a compound C14H14B2Cl2N2 which has been identified as 2,4-dichloro-3-o-tolyl-8-methyl-2,4-dibora-1,3-diazaronaphthalene.The 1 : 1-adducts of 2,6-disubstituted anilines and boron trichloride readily lose one mole of hydrogen chloride on heating, with formation of the borazene, but thereafter reaction is slow, and the tri-B-chloroborazole is not formed. When the borazenes are treated with a suitable tertiary base the product is the dichloroborazole, i.e., the product in which one B–Cl linkage has been reduced to B–H. The mechanism of this unusual reaction, and the reactions of the difunctional borazoles to which it gives rise, have been studied.