Chemistry Reference
In-Depth Information
naphthalene proton sponges, 18 lacks hydrophobic shielding of the basic centres and thus of
the hydrogen bond in the monoprotonated cation, which was responsible for the low rate of
proton transfer. Therefore, this kinetically active base is stronger than quinoline by 8 pK
a
units. A quinoline derivative of 16, benzo[1,2-h:4,3-h
0
]diquinoline [20] (19), has a helical
deviation of the planar arrangement of aromatic rings, but the N
N distance of 2.705 A
is
almost identical to DMAN (1) (2.728 A
). Due to lack of conformational flexibility,
protonation exchange experiments showed lower basicity than 16.
Superbasic properties have been predicted by Bucher [21] for a series of bases possessing
the syn-tris-8-quinolylborane framework. Density functional theory (DFT) calculations
employing the B3LYP/6-311
...
G(d,p)//B3LYP/6-31G(d) and IPCM/B3LYP/6-31G(d)
methods indicate a dramatic increase in the basicity of 20-22 possessing three quinoline
nitrogen lone pairs, with additional bridging forcing the nitrogen lone pairs into close
proximity. In particular, 21 is predicted to showa basicity approaching that of themost basic
known neutral nitrogen bases [PA 268.1 kcal mol
1
,pK
a
30.2 (MeCN)], while a smaller PA
was calculated for 22 due to the lower flexibility of the benzoquinolines (Figure 2.1).
þ
2.2.3
Polycyclic Proton Sponges
Further extension of the proton sponge concept by anchoring two nitrogen atoms closely in
a rigid framework and functionalizing them with substituents has led to the design of a
variety of molecular frameworks, which differ from the classical proton sponge framework
consisting of condensed phenyl rings. For instance, superbasic properties have been found
for bispidines (3,7-diazabicyclo[3.3.1]nonanes) 23-25 by Toom [22] (Figure 2.2). Their
respective, spectrophotometrically determined pK
a
values in acetonitrile are 21.25, 21.38
and 21.66 units. Previously a considerably lower pK
a
of 17.50 was determined for (
)-
sparteine in acetonitrile, which might be explained by the presence of moisture that would
generate a levelling effect. Measurements in water gave pK
a
¼
11.96 for 25, while its gas
phase determined GB is 243.4 kcal mol
1
[23]. Calculations indicate that the dominant
basicity-increasing factor in bispidines 23-25 is the nature and strength of the IMHB.
Structurally related, superbasic proton sponges have been designed by Estrada [24]. A
minimal framework was provided by the structures of 3,6,7,8-tetraazatricyclo[3.1.1.1
2,4
]
octane (26) and 4,8,9,10-tetraazatricyclo[5.1.1.1
3,5
]decane (30). These molecules, posses-
sing two pairs of nitrogen atoms fixed in a configuration where two nitrogen atoms are in
B
N
N
B
B
N
N
N
N
N
N
N
N
20
21
22
Figure 2.1
Structures of C3-symmetric organic base quinolines 20-22
