Chemistry Reference
In-Depth Information
Theoretical basicity values for phosphorus imines, ylides and phosphines
a
Table 2.14
Z
R
R
P
N
N
R
N
N
PA
b
[106]
p
K
a
(MeCN)
d
[86]
PA
c
Base
R
Z
GB
255.0
e
139
Me
—
274.6
145
Me
¼
CH
2
296.6
144
Me
¼
NH
281.8
147
Me
O
249.4
138
H
—
267.3
244.6
148
H
¼
CH
2
304.7
273.3
149
H
¼
NH
281.8
258.9
150
H
O
252.2
265.2
f
151
Me
¼
NMe
29.0
272.4
f
152
Me
phosphines [78]
tmg
31.0
275.0
c
153
[(NH
2
)
3
P
¼
N]
3
P
283.3
267.1
c
154
[(dma)
2
C
¼
N]
3
P
276.7
258.9
c
155
¼
[(NH
2
)
2
C
N]
3
P
263.7
a
PA and GB values are given in kcal mol
1
.
b
6-31G*.
c
B3LYP/6-311
G**.
d
IPCM/B3LYP/6-311
þ
þ
G**//B3LYP/6-31G*.
e
B3LYP/6-311
þ
G**.
f
B3LYP/6-311
þ
G(2df,p)//B3LYP/6-31G*.
which contain only one phosphorus atom, are significantly stronger bases than the
respective open chain derivatives of phosphines, phosphorus imines and phosphorus ylides,
which also contain only one phosphorus atom. However, P2 and P3 iminophosphoranes and
phosphoranes, which contain several phosphorus atoms, are stronger bases than 138, 149 or
148. Although theory indicated that 146 would be more basic than 139, experiments by
Verkade in acetonitrile showed the opposite [105]. This was rationalized by themore distant
polarizing proton from the lone pair on axial nitrogen in 143H
þ
than in 139H
þ
which is
fully transannulated. Solvation effects probably play a significant role. The same authors
confirmed experimentally that 145 is more basic than 139.
Triguanidinophosphines 154 and 155 are expected by calculations to be rather strong
superbases. The modification from 155 to 154 results in a much stronger base; the gas phase
basicity of 154 is expected to exceed that of the landmark cyclic phosphatrane superbase
139 by 12 kcal mol
1
. Even stronger phosphine superbases are expected to be designed by
replacement of the tmg groups in 154 by (dma)
3
P
N]
3
P. The
basicity of its simple NH
2
analogue 153 is calculated to be about equal to the predicted
basicity of the respective imino base [(H
2
N)
3
P
¼
N
groups to give [(dma)
3
P
¼
273.2 kcal mol
1
).
¼
N]
3
P
¼
NH (GB
¼
N]
3
P at around 280 kcal mol
1
,
Calculations estimate the gas phase basicity of [(dma)
3
P
¼
which is close to the gas phase basicity value for 153.
