6434 J. Am. Chem. Soc., Vol. 120, No. 26, 1998
Table 1. Crystal Data of Diphenylguanidines (1-6)
Tanatani et al.
1 (chiral)
1 (racemic)
2
3
4a
4b
5
6
formula
C13H13N3
C13H13N3
C14H15N3
C14H15N3
C15H17N3‚1/2H2O‚ C15H18N3Br C16H19N3
1/4C6H14
C17H22N3I‚H2O
recrystn solvent
crystal system
space group
a, Å
b, Å
c, Å
EtOH
EtOH
CH2Cl2
AcOEt/n-C6H14 n-C6H14
CHCl3
triclinic
P1h
n-C6H14
CHCl3/AcOEt
orthorhombic monoclinic orthorhombic monoclinic
P212121
12.653(5)
20.54(2)
8.944(5)
triclinic
P1h
monoclinic orthorhombic
C2/c P212121
18.532(2) 12.055(3)
7.736(2) 14.776(3)
P21/c
P212121
Cc
8.906(2)
12.342(1)
21.335(2)
5.666(1)
10.126(3)
21.17(2)
11.212(2)
12.757(2)
9.281(1)
11.590(5)
16.44(2)
8.320(4)
97.21(7)
90.73(3)
77.50(7)
1535(2)
1.167
13.034(3)
13.216(4)
9.711(2)
91.85(2)
94.99(2)
65.14(1)
1512.1(6)
1.407
20.462(2) 10.466(4)
R, deg
â, deg
96.66(1)
112.21(1)
104.273(9)
γ, deg
V, Å3
2324(2)
1.207
8a
2329.1(6)
1.205
8a
1214(3)
1.232
4
1229.1(4)
1.217
4
2842(1)
1.184
8
1864.3(7)
1.472
4
d
calc, g cm-3
Z
4a
4a
radiation
temp, K
Mo KR
Cu KR
Mo KR
Cu KR
Mo KR
Cu KR
Cu KR
Cu KR
296
296
173
296
173
296
296
296
no. unique reflctns 3231
0.085
3650
0.048
819
0.075
1135
0.062
4425
0.097
4493
0.079
2304
0.069
1938
0.065
R
a Two independent molecules exist in the asymmetric unit.
guanidines by X-ray crystallographic analysis, owing to the
strong basic properties of these molecules. Most neutral
guanidines so far examined form dimeric or oligomeric struc-
tures through hydrogen-bond networks in the crystals.13
The C-N partial double-bond character often has interesting
effects on molecular structure and physicochemical properties.
As a continuation of our previous studies on the conformations
of aromatic amides with unique intramolecular spatial arrange-
ments and aromatic interactions,14 we were interested in the
stereochemical behaviors of aromatic guanidines. In this
connection, diarylguanidines have recently attracted much
attention in the fields of materials chemistry and medicinal
chemistry.8 Here, we describe the systematic structural analyses
of N,N′-diphenylguanidine and its N-methylated derivatives,
focusing on (i) the properties of aromatic-substituted guanidino
bonds, (ii) the molecular conformational preference as compared
with that of the aromatic anilides, (iii) the existence of chiral
propeller-type conformations due to twisting of the guanidino
bonds, and (iv) the application of the Z-conformational prefer-
ence of N,N′-dimethylated guanidine to construct unique, water-
soluble oligomers with multilayered aromatic structures.
Figure 1. Tautomerization of N,N′-diphenylguanidine (1, E,E-form).
The structures of various guanidines or guanidinium ions have
been investigated in order to clarify their bonding and electronic
properties. A highly symmetrical, planar structure of the
unsubstituted guanidinium ion, C+(NH2)3, which allows sub-
stantial electron delocalization, was elucidated empirically by
X-ray analyses9 and IR and Raman spectroscopies and also was
treated by theoretical calculations.2,10 Mono- or disubstituted
neutral guanidines can exist in two tautomeric forms, conven-
tionally named the imino form and the amino form, as illustrated
for the case of N,N′-diphenylguanidine (1) in Figure 1.
Spectroscopic studies show that the imino form is favored in
monoalkylguanidines such as L-arginine in solution,11 while
monoarylguanidines12 or guanidines substituted with an electron-
withdrawing group (nitro, cyano, and so on)1 favor the amino
form. However, there are only a few examples of the deter-
mination of the tautomeric preference of neutral substituted
Results and Discussion
Five diphenylguanidines and two guanidinium salts (1-6,
Figure 2) were synthesized by standard procedures as described
in the Supporting Information, and their crystal structures were
elucidated by X-ray analyses. The crystal data are summarized
in Table 1. To simplify the discussion, we adopted the
following system for the structures and numbering of guanidino
groups. Tautomers of diphenylguanidines are named as the
imino form or amino form regardless of methyl substituents
(Figure 1). In both tautomers, the phenyl-bearing nitrogen atom
with the shorter C-N bond length is numbered N(1), the
nitrogen with the longer C-N bond length is numbered N(2),
and the nitrogen without a phenyl substituent is numbered N(3),
as shown in Figure 2. Thus, the C(1)-N(1) bond always
possesses much more double-bond character than the C(1)-
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