Chemistry Reference
In-Depth Information
guanidines
amines
amidines
H
NH
NH
R
NH
2
R
NH
2
RHN
NH
2
H
(ex)
NH
NH
12.40
13.6
MeNH
2
10.624
Me
NH
2
H
2
N
NH
2
methylamine
actamidine
guanidine
NH
N
11.82
(in MeCN)
Me
2
NH
10.732
13.6
Me
2
N
NMe
2
dimethylamine
N
TMG
DBU
CO
2
-
CO
2
-
CO
2
-
NH
N
H
(CH
2
)
4
NH
2
H
CH
2
H
(CH
2
)
3
NH
10.54
6.04
12.48
N
+
H
3
N
+
H
3
N
N
+
H
3
NH
2
H
lysine
arginine
histidine
Figure 1.1 Structures of amine derivatives and their representative examples (pKa of the
conjugated acids in H
2
O): DBU
¼
1,5-diazabicyclo[5.4.0]undec-5-ene; TMG
¼
1,1,3,3-
tetramethylguanidine
Thus, a pentacyclic amidine (vinamidine) [6] and biguanide [7] with a vinylogous
conjugation system show very strong basicity [8], as expected by the above account
(Figure 1.3).
An alternative stabilization effect on the protonation to these two bases leading to their
highly potential basicity is through bidentate-type hydrogen bond formation as shown in
Figure 1.4. Alder also discussed the effects of molecular strain on the Brønsted basicity of
amines [9].
In 1985, Schwesinger [10] introduced phosphazenes (triaminoiminophosphorane ske-
letons), which contain a phosphorus atom [P(V)] bonded to four nitrogen functions of three
amine and one imine substituents, as organobases containing a phosphorus atom. They are
classified as P
n
bases, based on the number (n) of phosphorus atoms in the molecule [11].
NH
NH
2
N
+
H
2
H
+
N
+
H
2
R
NH
2
R
R
NH
2
N
+
H
2
NH
NH
2
NH
2
H
+
RHN
NH
2
RHN
NH
2
RHN
+
N
+
H
2
NH
2
RHN
Figure 1.2
Conjugation of amidinium and guanidinium ions
