Chemistry Reference
In-Depth Information
Gas phase basicities of guanidine derivatives (kcal mol
1
)
Table 2.10
Guanidine base
GB(B)
PA [18]
TMG
234.9 [18]
243.1
¼
(Et
2
N)
2
C
N
Me
246.1
ITBD
248.3
248.3
ETBD
247.5
247.5
MTBD
246.95; 243.3 [18]
250.9
TBD
244.7; 241.2 [18]
249.3
(Me
2
N)
2
C
¼
N
Me (PMG)
242.8
(Me
2
N)
2
C
¼
N
Ph (TMPhG)
240.7
(Me
2
N)
2
C
¼
N
(CH
2
)
2
OMe
246.6
246.6
(Me
2
N)
2
C
¼
N
(CH
2
)
2
NMe
2
248.4
248.4
(Me
2
N)
2
C
¼
N
(CH
2
)
3
OMe
248.75
248.75
(Me
2
N)
2
C
¼
N
(CH
2
)
3
NMe
2
250.65
250.65
solution pK
a
measurements (Table 2.7). Heteroalkyl groups [(CH
2
)
n
OCH
3
and (CH
2
)
n
N
(CH
3
)
2
, n
¼
2 or 3] at the imino nitrogen induce the strongest increase in basicity. The order
of this increase is: (CH
2
)
2
OCH
3
>
(CH
2
)
3
N(CH
3
)
2
. In-
corporation of the guanidine moiety into cyclic structures such as six-membered rings in
TBD increases basicity comparing to acyclic guanidine, the gas phase basicity values are
further increased by alkylation in MTBD, ETBD and ITBD.
(CH
2
)
2
N(CH
3
)
2
>
(CH
2
)
3
OCH
3
>
2.5 Phosphazenes
The exceptional basicity of phosphazenes (iminophosphoranes) has been discovered by
Schwesinger [83]. The phosphazene derivatives have been proved to be chemically very
stable, kinetically active and highly versatile, and the large number of these bases has
been synthesized. The gas phase and solution equilibrium basicity measurements for a large
number of phosphazenes are conducted by Kaljurand et al. [84], and their PA and pK
a
values are published in various papers. These measurements show that phosphazenes
surpass in their basicity the derivatives of acyclic or bicyclic guanidines, amidines and
vinamidines (Table 2.11). Extraordinary basicity of phosphazenes was theoretically
rationalized byMaksi
c et al. [85], in terms of effective resonance stabilization of protonated
molecules [86].
Condensed phase measurements have been conducted in several solvents. In acetonitrile,
for most of the alkylphosphazenes the pK
a
values are within the range 26-47 pK
a
units. The
analysis of the pK
a
values in acetonitrile (Table 2.11) shows some general trends. The most
significant is that substitution of the hydrogen atoms of the aniline (amine) amino group by
the
P(pyrr)
3
group increases the basicity approximately by 10-12 pK
a
units [69].
Throughout the text Schwesinger
s nomenclature will be used (here P1, P2, etc. numbers
denote number of P
¼
N- units). There is an increase in basicity with increasing number of
phosphorus atoms. Hence, P2 phenyliminophosphoranes are stronger bases than the
corresponding P1 phenyliminophosphoranes by about 4-5 pK
a
units and so on, where for
¼
