Chemistry Reference
In-Depth Information
groups are generally requested as reaction tools with wide ranges of applicability in
chemically related organic reactions. Thus, in the design of more intelligent molecules
as synthetic tools it is necessary to overcome antipathy between
specific
for functionality
recognition and
for reactivity in organic synthesis. Computer-aided molecular
recognition between ligand and pharmacophore has been progressed in drug discovery
research [4]. This concept may give a clue for the design of the new generation.
Interestingly, a number of novel nitrogen-containing superbase backbones have been
identified through the extensive computational work of Maksic
common
s group; the representatives
are shown in Figure 11.1.
Molecules (e.g. 1) possessing imino structural and electronic motif have been recognized
as important building blocks for the construction of potent superbases with extended
p
-systems [5]. The principles which make systems thermodynamically stable and highly
basic are explained by a large increase in the
-delocalization energy of the corresponding
conjugate acids, thus leading to appreciable stabilization of protonated species.
Another important structural motif for construction of the strong organosuperbases are
cyclopropeneimines (e.g. 2) [6]. They exhibit high basicity due to the significant aromatic
stabilization of the three-membered ring upon protonation and the basicity can be increased
by amine substitutions at the double bond. Amino groups stimulate aromatization of the
cyclopropene fragment and also release some of their lone pair electron density, thus
contributing to a uniform distribution of the positive charge over the entire molecular
system. Further increase in the basicity of cyclopropeneimines could be achieved by
intramolecular hydrogen bonding (IMHB) such as depicted in 2 [7].
Quinonimine (e.g. 3) exhibits a very high basicity, which can be ascribed to significant
aromatization of the semiquinoid structure upon protonation by resonance [8]. The amine
substitutents increase the conjugation of the planar systems thus enhancing the relaxation
effect. The amino group is capable of accommodating the positive charge, thus increasing
the double bond character in the iminium fragment resulting from protonation. A further
increase of the basicity could be achieved by a domino effect in the extended
p
-system
p
involving two quinoid fragments.
Extended polycyclic
-systems (e.g. 4) possessing a carbonyl oxygen terminus serve as a
basic proton scavenger [9]. Carbonyl polyenes (e.g. 5) are also calculated to exhibit high
basicities, even belonging to the lower part of superbasicity scale [10]. These open chain
and zig-zag extended
p
-systems involve polyenes and a carbonyl functional group at the
molecular terminus. Structurally related to carbonyl polyenes are iminopolyenes (e.g. 6),
being extended
p
-systems and a new class of highly basic compounds [11]. The explanation
of iminopolyene basicity is the increase in stabilization triggered by protonation, and amino
substitution is crucial for their superbasicity by amplifying the resonance effect.
Some of the extended
p
-systems (e.g. 7) possessing imino nitrogen atoms as the most
basic sites, which are parts of the [3]iminoradialene or quinonimine structure, are neutral
organosuperbases [12]. The diaminophosphono [
p
¼
P(NR
2
)
2
] and diaminomethylene
[
C(NR
2
)
2
] ends (and 1,3-diamino-2-methylene cyclopropene ring) enable efficient
cationic resonance across the extended linear
¼
-system, contributing to enlarged basicity.
p
The
P(NR
2
)
3
end gives the largest basicity.
Poly-2,5-dihydropyrroles (e.g. 8) represent another class of extended
¼
-system with
pronounced basicity [13]. The aromatization of their protonated bases amplifies the
susceptibility toward the proton attack. Aromatization of the five-membered ring and
p
