Chemistry Reference
In-Depth Information
where the consecutive introduction of guanidino and tmg fragments seems to be nonaddi-
tive. Further increase going from 131 was achieved by replacement of the phenyl imino
group with hydrogen and alkyl substituents (
t
Bu and Et) in 129, 128 and 127 by 4.3, 4.8 and
5.4 pK
a
units, respectively. The most basic guanidinophosphazene experimentally mea-
sured so far is 127with a pK
a
value of 29.7 in THF. There is an increase of about 2.3 pK
a
units
for sterically more demanding tert-butyl groups than phenyl on the imino nitrogen.
As seen from the data collected in Table 2.11, the differences in measured pK
a
values of
phosphazenes in acetonitrile and THF,
[pK
a
(MeCN-THF)] are in the range 7-10 pK
a
units, with an average being eight. This allows an estimate to be made of the acetonitrile
basicities of guanidinophosphazenes (Table 2.12) and compared with those of phospha-
zenes (Table 2.11). The estimates for 127 and 128 in acetonitrile indicate their extreme
basicity comparable to these of P3 and P4 phosphazenes. Furthermore, Raab et al. [70]
found that in acetonitrile solution the basicity of DMAN (1) increased by about 6.9 pK
a
units
when the dma substituents were replaced with tmg groups to form TMGN. In THF solution
a similar change of the substituents in the naphthalene ring results in a somewhat more
modest increase in the basicity of TMGN (by 5.8 pK
a
units). These results allowa prediction
of a further increase of 1 pK
a
unit going from THF to MeCN.
Furthermore, DFT calculations have been used for quantum-chemical studies of gas
phase basicities of some guanidinophosphazene bases (B3LYP/6-311
D
G** method).
Their extremely high basicity has been rationalized by electronic reasons. The strongly
electron-donating tmg groups as the building units lead to a significant delocalization of the
positive charge over the tmg groups, due to the conjugation of guanidino moieties with
either a phosphorus atom of guanidinophosphazene or a phosphonium centre. These
electronic effects reduce the electrophilic character of phosphorus and provide the
respective bases with increased thermodynamic stability.
The calculated gas phas basicities indicate that the protonation at the
þ
NH group of
guanidinophosphazene is the most favourable site in the molecule (Figure 2.13). The
replacement of amino groups by the cyclic guanidino fragment (1,3-dimethylimidazolidin-
¼
H
2
N
NH
2
NH
2
H
2
N
NH
2
N
Me
NH
2
N
N
Me
NH
2
N
H
2
N
NH
2
N
N
N
H
2
N
Me
N
H
2
N
N
N
N
P
N
P
N
H
N
P
H
N
P
NH
N
Me
H
2
N
N
N
N
M
e
H
2
N
N
H
2
N
N
NH
2
NH
2
H
2
N
NH
2
Me
N
N
N
H
2
N
NH
2
135
134
NH
2
H
2
N
136
Figure 2.13
Structures of guanidinophosphazene bases 134-136
