Heterocycles by Ring Closure of Ortho-Substituted t-Anilines (The t-Amino Effect)

It is the purpose of this chapter to collate all the available material relating to the “t-amino effect” and to attempt a rationalization of its operative influence in a mechanistic manner. The cyclizations of ortho-substituted t-anilines are described in two main sections. The first discusses ring closures between the ortho substituent and the t-nitrogen. Varieties of seemingly unrelated reactions have been reported in which a t-aniline with an ortho substituent of the type N-C-Y is converted into a benzimidazole with loss of a group from the t-nitrogen. All these cyclizations can be rationalized by the intermediacy of an imine bearing a good leaving group. While the second deals with those ring formations which involve the a-methylene groups attached to the nitrogen. The reactions which fall into this category conform to a basic mechanism for an unsaturated ortho substituent (A=B) capable of mesomerism. The polarized mesomer is set up for further reaction since the hydrogens of the a-methylene group are rendered labile by the charged nitrogen, and are consequently abstracted by the nucleophilic center (B-). Here, the discussion will be divided into sections according to the functiona1 group (o-nitroso and o-nitro groups, o-amino and o-acylamino groups, o-azo group, o-azomethine group (=n=c), and o-carbonyl (c=o) and o-imine (c=n) groups) situated at the ortho position.