Chemistry Reference
In-Depth Information
1
General Aspects of Organosuperbases
Tsutomu Ishikawa
Graduate School of Pharmaceutical Sciences, Chiba University, 1-33 Yayoi,
Inage, Chiba 263-8522, Japan
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 (
-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].
¼
NH) to the
a
