Chemistry Reference
In-Depth Information
24-27 kcal mol
1
) and the strength of the IMHB, which is enhanced upon protonation. In
the conjugate acid BH
þ
the protonated imino nitrogen of guanidine acts as a proton donor,
whereas the side chain amino group is a proton acceptor. A cooperative IMHB effect in
95H
þ
realized by three N(sp
3
)-H
N(sp
3
) hydrogen bridges (multiple corona effect)
contributes 18.3 kcal mol
1
to the APA of 95. Guanidine 92 assumes an even higher PA due
to an additional relaxation effect caused by the methyl groups. The multiple two-centre
corona effect was later found experimentally by Glasovac in the X-ray structure of 92 [79].
The incorporation of stronger hydrogen bonding acceptor functionalities in the propyl
chain, such as imidazole in molecule 96, is predicted to further enhance guanidine basicity
(275.1 kcal mol
1
) [80]. However, it should be noted that IMHB [81] are almost exclusive to
the gas phase, and their relevance to the reactivity in condensed media is much smaller. This
is so because the hydrogen bond donor of an IMHB prefers to form IMHBs with the solvent
molecules, mainly when they are good hydrogen bond acceptors, rather than being involved
in the IMHB itself.
Extended
...
-electron systems with guanidine and cyclopropenimine structural subunits
[82] devised by Maksi
p
c offer another structural variation of guanidines capable of effective
stabilization of the system upon protonation. It was calculated that the guanidino-cyclo-
propenimines possess higher PAs than their polyguanide counterparts (PA(HF
SC
):
97 : 251.0; 98 : 262.8; 99 : 267.9, and 100 : 280.1 kcal mol
1
). The origin of the increased
basicity lies in a large resonance effect triggered by the protonation via aromatization of the
three-membered rings in the conjugate acid form. Resonance stabilization of guanidine is
within the range of 24-27 kcal mol
1
, while those of guanidino-cyclopropenimines con-
tribute to PAs by 41-44 kcal mol
1
. Alkyl groups further stabilize protonated forms by an
inductive effect through
-hyperconjugation mechanisms (Figure 2.11).
Gas phase basicities of a number of guanidine derivatives with PAs larger than 239 kcal
mol
1
(1000 kJ mol
1
) have been experimentally evaluated by Raczynska et al. [53,54]; the
results are summarized in Table 2.10. Some trends for various classes of compounds could
be identified.
Examination of the gas phase basicity values indicates that for acyclic alkyl derivatives,
the basicity increases with the polarizability of alkyl groups in the following order:
TEGs
s
- and pseudo-
p
>
TMGs
>
ITBD
>
ETBD
>
MTBD
>
TBD, (where TEG is the (Et
2
N)
2
C
¼
N
and
TMG the ((CH
3
)
2
N)
2
C
group). Guanidine derivatives bearing the aryl group at the
imino nitrogen have smaller gas phase basicity values than alkyl groups, hence: TMG-
CH
3
>
¼
N
TMG-Ph. The relative order follows the same substitution trends obtained by
R
1
NR
2
N
R
X
N
N
N
NR
2
N
R
2
N
Y
R
2
N
NR
2
97
: X = R = H, Y = NH
2
98
: X = Y= NMe
2
, R = Me
99
:R = R
1
= Me
100
: R = R
1
= Me
Figure 2.11
Structures of guanidino-cyclopropenimines 97-100
