General Aspects of Organosuperbases

In the field of organic chemistry, a base is generally defined as a reagent capable of abstracting proton to yield a carbanion species. At a basic textbook level, organobases are normally limited to amines, which are categorized as very weak bases according to the above definition. The introduction of an imine function (1⁄4NH) to the a-carbon of amines affords more basic amine species, amidines, which correspond structurally to amine equivalents of carboxylic esters (carboxylic acid imidates). Guanidines, which carry three nitrogen functions (one amine and two imines) and correspond to amine equivalents of ortho esters (carbonimidic diamides), show the strongest Brønsted basicity among these amine derivatives [1]. Thus, basicity is proportional to the number of the substituted nitrogen functions at the same carbon atom; representative examples are shown in Figure 1.1. The basicity of guanidine is comparable to the hydroxyl ion (OH ) [2]. Basic amino acids, lysine and arginine, have amino and guanidine groups, respectively, at the side chains as additional functional groups and can act as base catalysts responsible for important biological actions, such as enzymatic reactions in living organisms, through hydrogen bonding networks caused by these basic characters [3]. On the other hand, histidine belongings to an acidic amino acid in spite of carrying an imidazole ring involving an amidine function as a partial structure [4]. The basicity of these amine derivatives is due to the construction of highly effective conjugation system after protonation under reversible conditions; primitively, it is a reflection of the number of canonical forms, especially isoelectronic forms, in the resonance system (Figure 1.2). This is one of the reasons why guanidines are stronger bases than amidines [5].