Chemistry Reference
In-Depth Information
It was shown that the IMHB and the solvation energies in the protonated forms could each
increase the basicity by 2-4 pK
a
units. The lone pair repulsion was found to be able to
increase the pK
a
values by up to six pK
a
units. Protonation and IMHB reduce steric
deformation to the neutral molecule due to repulsion of lone electron pairs of nitrogen atoms
of the nearby alkyl groups. Structures particularly favourable for forming an almost linear
and, therefore, strong IMHB between nitrogen atoms on protonation (N
H
þ
...
N around
175
) are more stabilized. The structural parameters of the IMHB obtained for twenty
proton sponges show: the N
N distances are 2.717-2.792 A
...
[7], while protonated proton
N, N-H
þ
and N
H
þ
distances of 2.541-2.881, 0.86-0.92 and
sponges generally have N
...
...
1.376-1.580 A
, respectively, and N-H
þ
...
N angles between 150 and 180
. The strength of
the IMHB calculated for proton sponges is in the range of 16-21.5 kcal mol
1
[9].
Detailed study by Pozharskii [12] has shown that the so-called
represents the complex combination of various interactions of ortho substituents with
dimethylamino groups in corresponding bases and cations. Basicity is determined by
interplay of several factors among which are: the polar effect of ortho substituents
(electron-releasing substituents increase basicity); electrostatic repulsion between lone
electron pairs of the dimethylamino (N(CH
3
)
2
) groups (considered as one of the leading
in increasing the basicity); reduction of conjugation between peri-N(CH
3
)
2
groups and the
naphthalene system (may enhance the basicity by 1.2 pK
a
); increase of the p-character of the
nitrogen lone pairs (enhances the basicity of nitrogen atom); increase of the IMHB strength in
the cation; decrease of steric strain in the cation (strain decrease on protonation increases
basicity); p,d-interaction of theN(CH
3
)
2
groupswith d-elements of such ortho-substituents as
SCH
3
, Si(CH
3
)
3
and bromine (slightly reduces basicity); and changes in solvation caused by
substituents (hydrophobic groups inhibit solvation of the cation and thus exert some reduction
in basicity, while hydrophilic groups favour solvation and bring some increase in pK
a
).
Several authors have pointed out the importance of the extent of the p-character of the
lone electron pairs of nitrogen and of the distortion of the n,
buttressing effect
-conjugation (Korzhenevskaya
et al.). Flattening of the nearby NR
2
groups (planarization caused by sp
3
p
sp
2
rehybri-
dization) increases their mutual repulsion, which is supplemented by the Coulomb
repulsion due to high charge density on the nitrogen atoms, so that the neutral molecule
of the proton sponge is structurally destabilized. The consequence of considerable twisting
of the N(CH
3
)
2
groups out of the ring plane is loss of conjugation between the NR
2
and
naphthalene
!
-system, which is initially reduced bythe larger sp
2
character of the nitrogen
(N) atom. Both effects provide an essential increase in basicity [14]. The protonation of the
proton sponge leads to sp
2
p
sp
3
rehybridization of the nitrogen atomic orbitals (AOs).
Repulsion becomes considerably weaker, the N
!
N distance shortens, while strong
attractive interaction appear, which is favoured by formation of the N
H
þ
...
N hydrogen
bond due to the high p-character of lone electron pair of the unprotonated nitrogen atom. At
the same time, sp
2
...
sp
3
rehybridization enhances
!
p
-conjugation and stabilization of the
protonated system.
2.2.2
Proton Sponges with Other Aromatic Backbones
After the discovery of DMAN (1), the search started for other, more basic proton sponges
possessing aromatic backbones with optimal, even shorter N
...
N distances to form
H
þ
...
N
N bridges with the optimal linear geometry. Staab has found that fluorene series
