Chemistry Reference
In-Depth Information
of bases, and starts with the proton sponges, then gradually moves to stronger groups. In
addition, superbases predicted by calculation have been included, with a separate section on
quantum chemical methods used for basicity estimation and the study of superbases.
2.2 Proton Sponges
2.2.1
'Classical' Proton Sponges
Proton sponges (PS) are organic diamines with unusually high basicity. The exceptional
basicity of the very first proton sponge, DMAN (1) was reported by Alder in 1968 [2]. This
compound has a basicity about 10 million times higher (pK
a
¼
12.1 in water) than other
similar organic amines (its experimentally measured proton affinity (PA) in the gas phase is
246.2 [3], while the calculated value [4] is 246.5 and 258.7 kcal mol
1
, using 6-31G*//6-
31G
ZPE and 6-31G methods). Proton sponges and their complexes have attracted
considerable interest from chemists, giving rise to over 70 structural and 100 spectroscopic
papers (Table 2.1) [5]. The name proton sponge is given because of the high thermodynamic
basicity combined with a kinetic inactivity to deprotonation that resembles the affinity of a
sponge for water.
The general feature of all proton sponges is the presence of two basic nitrogen centres in
the molecule, which have an orientation that allows the uptake of one proton to yield a
stabilized intramolecular hydrogen bond (IMHB). A dramatic increase in basicity of
aromatic proton sponge can be achieved on account of: destabilization of the base as a
consequence of strong repulsion of unshared electron pairs; formation of an IMHB in the
protonated form; and relief from steric strain upon protonation [6]. Two general concepts to
raise the thermodynamic basicity or PA are established. One is to replace the naphthalene
skeleton by other aromatic spacers, thus influencing the basicity by varying the nonbonding
N
þ
N distances of the proton-acceptor pairs. The other concept focuses on the variation of
basic nitrogen centres or its adjacent environment (
...
).
The trend that proton sponges with high thermodynamic basicity typically have a low
kinetic basicity (kinetic activity in proton exchange reactions) is a serious limitation of
proton sponges: the captured proton does not usually take part in rapid proton exchange
reactions, which would allow such neutral superbases to serve as catalysts in base-catalysed
reactions. Their further limitations are moderate solubility in aprotic nonpolar solvents and
stability towards auto-oxidation.
From a physical organic point of view, there is continuing debate about whether the
enhanced basicity in proton sponges is due mainly to strain relief on protonation, or to the
special properties of the hydrogen bonds in their monoprotonated ions.
Experimental and theoretical studies have shed light on the structural factors that
influence the high basicity of proton sponges. Their abnormally high basicity is accepted
to be produced by various contributions: the effective PA for one of the amine groups
(assuming asymmetric protonation at one nitrogen); the relief of strain (possibly also
accompanied by an increase in aromatic stability) caused by loss of destabilizing lone pair-
lone pair repulsion on protonation; the formation of a hydrogen bond which stabilizes the
protonated species; the difference in solvation energies of the base and protonated cation in
solution.
buttressing effect
