Chemistry Reference
In-Depth Information
Margeti
c and Ishikawa [74]. The backbone change from benzene to naphthalene and
phenanthrene rings has a minor effect on the APA of 86. Substitution of an aromatic ring in
trans-86 with electron-withdrawing halogen atoms leads to decreased APAs in the order
3,4,5,6-tetrafluoro
<
86. These estimations are in good accor-
dance with literature data showing that aromatic substitution and electron withdrawing
groups decrease the APA of amines. The decrease of basicity going from 86 to 85 is
expected, as earlier discussed for the DMEGN/TMEGN basicity difference (PA(MP2)
86
250.3, PA(HF
SC
)
86
¼
<
3-6-dibromo
<
3-bromo
252.7 kcal mol
1
,pK
a
(MeCN)
86
¼
20.7). A comparison with the
APA of DMAN (1) indicates that o-bisguanidinobenzenes 86 are of similar basicity or
slightly more basic than DMAN (1), thus at the lowest part of the superbasicity ladder,
however still being strong hydrogen acceptors [75].
The increase in the guanidine basicity scale, going from mono- to polyguanidines has
been theoretically studied by Maksic and Kovacevic. Replacement of one of the amino
groups in guanidine by the N
¼
C(NH
2
)
2
structural fragment forms polyguanidines [76]. It
was shown that the increase of APA(HF
SC
) could be as much as 50 kcal mol
1
, compared to
guanidine itself. The origin of their very highly intrinsic basicity is traced to a large increase
in the resonance interaction of the corresponding conjugate bases (in the range of 24-
27 kcal mol
1
): that is, to the high stability of the corresponding protonated cations. The
guanidine group of biguanidine linked by its imino nitrogen to the amidine group has a
strong electron-donating character, and the amidine group linked by its functional carbon to
the guanidine group has a strong electron-accepting character. Therefore, the n-
conjuga-
tion of the guanidine groupmay be transmitted to the imino nitrogen in the amidine group. It
was calculated that the linear chain polyguanides exhibit increased basicity as a function of
the number of guanide subunits. The increase of APA is around 7 kcal mol
1
per guanidine
unit, reaching a limit for n
p
5 (APAvalue of 254 kcal mol
1
). The APA increase going from
mono to biguanide can be explained in terms of
¼
-electron conjugation induced in the
conjugate acid (larger number of resonance structures), indicating greater stability of
protonated species. At the same time, there is a relaxation effect due to reorganization of the
electron density upon protonation. Positive charge is delocalized over the conjugate acid via
mobile
p
-conjugation mechanism. Branched polyguanides have
higher APAs than their linear counterparts, possessing an additional stabilization by IMHB.
The largest PA was found in a doubly bifurcated heptaguanide 89, being as high as
285 kcal mol
1
, due to strong resonance effect, partially enhanced by IMHB intramolecular
multiple corona effect (on six N sites). N-Alkylation further increases APA (for instance,
going from 90 to its heptamethyl derivative, or from 89 to 91), where 91 has the largest
calculated APA of the all studied polyguanides (290.0 kcal mol
1
) (Figure 2.10).
These predictions are further corroborated by pK
a
estimations using the IPCM-B3LYP/6-
311
p
-electrons through the
p
G**//HF/6-31G* method (heptaguanidine 86 (33.5), triguanidine (31.3), hepta-
methyl derivative of biguanide 90 (28.1)) [49], and by the B3LYP/6-31G* calculations of
polyguanidines recently reported by Chamorro et al. [55]. Similarly, Kolomeitsev [77] have
calculated very high gas based basicity (PA) values for triguanidine [(H
2
N)
2
C
þ
¼
N]
2
C
¼
NH
and its tetramethyl derivative [((CH
3
)
2
N)
2
C
¼
N]
2
C
¼
NH at the B3LYP/6-311
þ
G** level -
248.4 (255.1) and 268.4 (276.2) kcal mol
1
, respectively.
The importance of the IMHB for achieving larger intrinsic PAs in organosuperbases has
been mentioned in several papers and discussed in more details by Kova
c et al. in the
case of N,N
0
,N
00
-tris(3-dimethylaminopropyl)guanidine (92) [78]. The aminopropyl chain
cevi
