Chemistry Reference
In-Depth Information
of magnitude more basic than DMAN (1). Its high basicity is ascribed to the high energy
content of the base in its initial neutral state and additional stabilization of conjugate acid by
strong intramolecular hydrogen bonding. This cooperative proton chelating effect renders
the bisphosphazene more basic than Schwesinger
s P1 phosphazene bases.
2.6 Guanidinophosphazenes
As described in the previous section, replacement of phosphazene alkyl substituents with
aminosubstituents, and especially with the phosphazo substitutents, leads to a significant
enhancement in the gas phase basicity. However, Schwesinger has shown that the
progressive increase of the basicity of phosphazene bases with increasing number of
phosphorus atoms reaches a limit at n
ΒΌ
5. The further increase in basicity of phosphazenes
(and also guanidines) has been accomplished by Koppel et al. [77] by the introduction of
guanidino or substituted guanidino (cyclic or acyclic) fragments into the phosphazene
structure. In such away, a new family of super strong, uncharged bases of the guanidinopho-
sphazene type has been synthesized, and their basic strengths in THF have been measured
(Table 2.12).
The pattern of basicity change has been observed in THF solution by incorporation of
tetramethylguanidino (tmg) groups in PhP
1
(dma) phosphazene. In THF, changing all three
dma groups in
t
Bu-P
1
(dma) and P
1
(dma) phosphazenes to tmg groups increases the basicity
enormously: by 10.2, 9.1 and 8.9 powers of 10, respectively. The consecutive introduction
of tmg fragments, instead of dma substituents, contributes equally to the total basicity
increase of 9.1 pK
a
units in 131, by an increment of 3.0 pK
a
units per tmg group. The
observed additive basicity increase in THF is in contrast to the findings in the gas phase,
Table 2.12
Guanidinophosphazenes: experimental pK
a
values in THF
NMe
2
NMe
2
Me
2
N
NMe
2
Me
2
N
N
N
Me
2
N
Me
2
N
P
N
NPh
Me
2
N
P
N
NPh
N
P
N
NPh
Me
2
N
Me
2
N
Me
2
N
NMe
2
Me
2
N
NMe
2
NMe
2
133
132
131
Guanidinophosphazenes
GB
calc
p
K
a
(THF)
127
: EtP
1
(tmg)
29.7
t
Bu-P
1
tmg)
128
:
29.1
129
:P
1
(tmg)
276.1
28.6
t
Bu-P
1
(tmg)
2
(NEt
2
)
130
:
26.8
131
: PhP
1
(tmg)
24.3
132
: PhP
1
(tmg)
2
(dma)
21.5
133
: PhP
1
(tmg)(dma)
2
18.4
