Chemistry Reference
In-Depth Information
six- and seven-membered rings, which is associated with smaller ring strain. The strongest
base is DBU, having values close to guanidines. Furthermore, it was found that in water
dihydrodiazepine 64 has high basicity, the pK
a
value range is 13-14 units, due to the
possession of a stable vinamidinium moiety in the conjugate acid [48]. X-ray crystallo-
graphic study showed the extensive
-electron delocalization in the vinamidinium portion
of the molecule, with stabilization energy of about 19 kcal mol
1
.
Schwesinger found a dramatic increase in pK
a
by fusion of a five-membered cyclic
amidine with a vinamidine moiety, which was additionally attached to a 1,3-imidazolidine
ring in 65 [50]. The cation of 65 is planar and has an effective conjugation, which is crucial
for effective aromatic stabilization. Joining together two 65 moieties, tricyclic 2,4-
diaminovinamidine proton sponges 68 and 69 of extraordinary basicity have been prepared
[their pK
a
(MeCN) are 30.03 and 29.51, respectively] [51]. In such a molecular system,
destabilizing lone pair interactions of the chelate forming nitrogen atoms are diminished by
protonation, nitrogen atoms come closer in the protonated form (the N
...
N distance of
2.54 A
indicates strong IMHB), and pronounced conjugation. Vinamidine proton sponge 67
has slightly less pronounced basicity. The introduction of an additional double bond in 68
gives the most basic vinamidine 70 [52]. Calculations of pK
a
(MeCN) values for 66, 68, and
70 by IPCM-B3LYP/6-311
p
รพ
G**//HF/6-31G* method by Kova
cevi
c and Maksi
c [49]
have revealed the importance of enlargement of the
p
-system and aromatic stabilization
upon protonation.
Comparisons of solution and gas phase proton transfer thermodynamic data by
Raczy
nska shed some light on the role played by solvation on amidine basicity. The gas
phase basicity and proton affinity values collected in Table 2.6 indicate identical basicity
order, as established previously by solution basicity measurements (Tables 2.3-2.5). The
following order of gas phase basicities could be established: GB
P1phosphazene
>
GB
VIN
>
GB
AAMs
>
GB
EAMs
.
Vinamidines 66 and 65 are by far the strongest bases containing imidine functionality.
As observed, an increase in the number of amino groups in the molecule has an
exceptionally strong influence on the gas phase basicity. The large set of investigated
compounds allows some additional comparisons to be made. For simple alkyl amidine
derivatives, the basicity in series increases with the polarizability of alkyl groups [56].
Alkylation of the imino nitrogen by increasingly larger groups increases their basicity
(1-adamantyl group being the largest, compound 72). This alkylation effect at the imino
nitrogen is estimated to be
GB
Guanidine
>
GB
CAM
>
GB
DBU
>
GB
AMs
>
GB
FAMs
>
GB
BAMs
>
8.4 kcal mol
1
[57].
The influence of the aryl substitution for derivatives bearing the aryl group at the
functional carbon depends on the electronic nature of the aryl substituent. Smaller basicity
is measured than for acetamidines. Bicyclic amidines show larger basicity, compared to the
most of simple alkyl derivatives, with the gas phase basicities in the following order:
DBU
>
74. DBU has the largest gas phase basicity value of
243.4 kcal mol
1
. Five-membered ring amidines display smaller basicity than six- and
seven-membered cyclic amidines.
Respectable strength has been observed by gas phase basicity and PA measurements for
acyclic derivatives containing the dimethylaminopropyl, dimethylaminoethyl and meth-
oxyethyl groups at the imino nitrogen; however, these compounds exhibit rather slow
proton transfer rates. The PAvalue of 71 is 256.7 kcal mol
1
, indicating that 71 is a stronger
base than TBD and DBU. These groups induce a strong increase in basicity in the following
>
DBD
>
PMDBD
>
DBN
