Tautomerism in Schiff bases. the cases of 2-hydroxy-1-naphthaldehyde and 1-hydroxy-2-naphthaldehyde investigated in solution and the solid state

Article abstract of DOI:10.1039/c1ob06073b

Schiff bases derived from hydroxyl naphthaldehydes and o-substituted anilines have been prepared and their tautomerism assessed by spectroscopic, crystallographic, and computational methods. Tautomeric equilibria have also been studied and reveal in most

Full text of DOI:10.1039/c1ob06073b

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PAPER
Tautomerism in Schiff bases. The cases of 2-hydroxy-1-naphthaldehyde and
1-hydroxy-2-naphthaldehyde investigated in solution and the solid state†
a
a
a
a
a
b
´
R. Fernando Mart´ınez,* Mart´ın Avalos, Reyes Babiano, Pedro Cintas, Jose´ L. Jime´nez, Mark E. Light
and Juan C. Palaciosa
Received 3rd July 2011, Accepted 5th September 2011
DOI: 10.1039/c1ob06073b
Schiff bases derived from hydroxyl naphthaldehydes and o-substituted anilines have been prepared and
their tautomerism assessed by spectroscopic, crystallographic, and computational methods. Tautomeric
equilibria have also been studied and reveal in most cases a slight preference of imine tautomers in
solution; a fact supported by DFT calculations in the gas phase as well as incorporating solvent effects
through the SMD model. To simulate the effect exerted by the crystal lattice on tautomer stability,
we have developed a computational protocol in the case of 1-tert-butyl-2-(2-hydroxy-1-naphthyl-
methylene)aminobenzene whose data have been obtained experimentally at 120 K. Although a rapid
imine-enamine interconversion may be occurring in the solid state, the imine tautomer becomes the most
stable form and the energy difference should be related to the difference in the packing of the molecules.
propylamine, which was previously described as imine,9 shows
to be enamine.10 An intriguing case is exemplified by 15 because
both tautomers co-exist in the same structure,11 while there are two
molecules with different structure, imine (16) and enamine (17),
in the unit cell of the Schiff base derived from 2-aminopyridine
at room temperature.12 Several Schiff bases with zwitterionic
structures have been reported as well (18,13 19,14 2015). Most of the
less common zwitterionic structures come from aliphatic amines.16
Introduction
Schiff bases derived from o-hydroxybenzaldehyde (salicylalde-
hyde) have attracted a great interest not only for their biological
and photophysical properties (thermo- and photochromism, and
non-linear optical behavior),1 but also as model compounds for
assessing the nature of the hydrogen bonding.2 In this context
and due to intramolecular hydrogen bonding, Schiff bases from 2-
hydroxy-1-naphthaldehyde can exhibit phenolimine-ketoenamine
(or zwitterionic) tautomeric structures (1–3) both in solution3 and
the crystalline state.4
However, most crystal data for Schiff bases derived from 2-
hydroxy-1-naphthaldehyde and arylamines (4,5a 5,5b 6,5c 7,5d 8,5e
9,5f 105g), alkylamines (11,6 126), and heterocyclic amines (13,7 148)
are consistent with enamine tautomers, even though one benzene
ring loses its aromatic character.
Moreover, a further re-investigation on the structure of
the adduct arising from 2-hydroxy-1-naphthaldehyde and n-
These compounds show invariably an enamine structure
in solution. For example, the Schiff base from 2-hydroxy-1-
naphthaldehyde and 3-chloroaniline is an enamine tautomer in
DMSO-d6 solution, though it is imine in the solid state at
200 K.17 Spectroscopic data also point to enamine structures
for the condensation products derived from 1,4-diformyl-2,3-
dihydroxynaphthalene in solution18 (21, 22) and, likewise X-ray
diffractometry confirms the dienamine structure for both in the
solid state at -100 ◦C. Similarly, the macrocycle 23, generated
aDepartamento de Qu´ımica Orga´nica
e Inorga´nica, QUOREX Re-
search Group, Facultad de Ciencias-UEX, E-06006, Badajoz, Spain.
E-mail: rmarvaz@unex.es
bDepartment of Chemistry, The University of Southampton, Highfield,
Southampton, U.K, SO17 1BJ
† Electronic supplementary information (ESI) available: Figures S1–S3,
Tables S1–S6, and computational data. CCDC reference numbers 805572–
805574 and 810937. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1ob06073b
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by condensation of an ortho-phenylendiamine derivative, shows
Results and discussion
sixfold enaminone tautomers.18
Syntheses and solid-state structures
The experimental protocol involves the condensation of equimo-
lar amounts of 2-hydroxy-1-naphthaldehyde (or 1-hydroxy-2-
naphthaldehyde) with 2-tert-butylaniline and 2,6-dimethylaniline
in ethanol at room temperature. The resulting Schiff bases 25–28
could thus be obtained in a few minutes with moderate to good
yields (72–88%), although such syntheses were not optimized.
Predictions about solid-state structures are quite difficult with-
out the assistance of X-ray diffraction data. IR spectroscopy
for instance does not enable a clear-cut assignment of imine or
enamine structures, because the stretching vibrations for the C
N
(imine) and C O (enamine) bonds usually overlap.22 However,
the intensity of the latter is much stronger (it uses to be the most
intense band of the spectrum) than that of the less polar C
N
bond. Nevertheless, a comparison of IR intensities for structural
elucidation should be taken with caution.
FT-IR spectra of compounds 25–28 are similar (Figure S1,
ESI†). The most relevant absorption appears at ~1620 cm-1 in the
case of 2-hydroxy-1-naphthaldehyde derivatives, while it is found at
~1605 cm-1 for those of 1-hydroxy-2-naphthaldehyde. This band,
together with the presence of absorptions for the phenol C–O
bond at ~1330 cm-1, allowed us to conclude that compounds 25–
28 most likely exhibit imine structures in the solid state at room
temperature. As we shall see, this conclusion agrees with their
X-ray diffraction analyses.
Thus, crystal data for compounds 25–28 recorded at 120 K are
shown in Fig. 1.23
Motivated by the above-mentioned results and the comprehen-
sive literature of Schiff bases tautomerism, often reporting contra-
dictory data, we recently embarked on the structural elucidation of
Schiff bases derived from tris(hydroxymethyl)aminomethane, usu-
ally abbreviated as TRIS,19,20 and found an enamine structure for
24 in both the solid state and solution. Herein, we extend this study
to Schiff bases generated from 2-hydroxy-1-naphthaldehyde and
1-hydroxy-2-naphthaldehyde and some ortho-substituted anilines.
These amines were chosen in the hope that the steric effects will
disrupt the coplanar arrangement between the imine bond and the
aromatic ring, thereby enabling the delocalization of the nitrogen
lone pair through the latter ring. As a result both the availability
of the lone pair, and hence the basicity, would decrease, thus
inhibiting hydrogen transfer from the phenol group and favoring
an imine structure both in solution and the solid state.
Fig. 1 X-Ray structure of 25–28 at 120 K (thermal ellipsoids drawn at
the 35% probability level).
Tables S1–S4 (see ESI†) collect some bond lengths and bond
angles for such compounds, which have been determined both ex-
perimentally and calculated at two DFT levels of theory and taken
into account imine and enamine tautomers in each case (see later).
The assignment of imine structures for 25–28 in the solid state
is supported by the following facts: long C–O bond lengths (1.355,
Since equilibria between enamine and imine tautomers have
been observed for 25–28 in the crystal lattice (measured at 120 K)
and in solution, this behavior has further been explored through
computational calculations at B3LYP/6-31G** and M06-2X/6-
311++G** levels.21
˚
1.351, 1.315, and 1.337 A, respectively) accompanied by short
C
N bonds corresponding to an imine tautomer (1.291, 1.290,
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Table 1 Hydrogen bonds [A and ◦] for compounds 25–28
˚
˚
1.305, and 1.289 A, respectively), as well as the refined position
of the hydrogen atom linked to the oxygen. Furthermore, the
experimentally-measured C–O and C N bond lengths match
those calculated at both the B3LYP/6-31G** and M06-2X/6-
Comp.
D–H ◊ ◊ ◊ A
d(D–H) d(H ◊ ◊ ◊ A) d(D ◊ ◊ ◊ A) ∠(DHA)
25
26
27
O1–H1 ◊ ◊ ◊ N1 1.044
O1–H1 ◊ ◊ ◊ N1 0.904
O1–H1 ◊ ◊ ◊ N1 0.840
N1–H2 ◊ ◊ ◊ O1 0.880
O1–H1 ◊ ◊ ◊ N1 1.071
1.587
1.723
1.843
1.847
1.569
2.558(2)
2.547(2)
2.552(2)
2.552(2)
2.556(2)
152.2
150.2
141.0
135.6
150.3
˚
311++G** levels (~1.33 and ~1.29 A for C–O and C N,
respectively) (see later). However, the structure of 24 was found
to be enamine in the solid state at 120 K,20 as C–O and C
N
28
˚
bond lengths (1.283 and 1.313 A, respectively) become shorter
and larger than those in 25–28.
46.80◦, respectively); this value reflecting a balance situation
because the orthogonal disposition between such rings would also
minimize the electronic delocalization and steric effects.
The C–O and C N bonds in 27 are significantly shorter
and longer, respectively, than those of 25, 26, and 28. Such
intermediate values are consistent with average values between
the corresponding bond lengths in imine and enamine forms.
Fig. 2 shows the electron density difference Fourier maps for
the four refinements without including the hydrogen in the model.
These clearly show that the hydrogen atom is much localized on
the oxygen in 25 and 26, but in both cases there is a hint of some
density towards the nitrogen. In the case of 27 and 28 the proton
is rather delocalized between the two positions, i.e. the hydrogen
partly bound to oxygen and nitrogen atoms. This suggests the co-
existence of imine and enamine tautomers in a fast equilibrium
within the crystal lattice.
Structures in solution
An imine tautomer can easily be identified by the presence of two
singlets in the 1H NMR spectrum: one deshielded signal (~13–15
ppm) corresponding to the phenol proton bound intramolecularly,
and the other attributed to the iminic proton (~8.5–9.5 ppm).
On the other hand, an enamine structure shows two coupled
doublets: one corresponding to the NH proton involved in the
intramolecular bond as well, which is significantly deshielded
(~14–16 ppm), and the ethylenic proton of enamine fragment
(~8.5–10 ppm). Clearly the multiplicity of such signals is the key
to distinguish them as chemical shifts are quite similar in both
tautomers.12,24,25 Like in the solid state, the structure in solution
may be altered by temperature changes.26,27 Table 2 collects some
relevant NMR data for compounds 25–28 (Figure S2, see ESI†).
Significant differences between imine and enamine structures
can instead be extracted from 13C NMR spectra through diagnostic
signals, because it has been reported that the phenol carbon of
imines is usually more shielded than the corresponding carbonyl
carbon of enamines (~160 ppm versus ~180 ppm).25 This trend can
also be detected in their 15N NMR spectra as the iminic nitrogen
is much more shielded than the nitrogen atom in enamines (~ -63
ppm versus ~ -247 ppm).26,28–32
However, the C-2 signal in 25–28, which resonates between the
phenol carbon of imines and the carbonyl signal of enamines (~168
ppm), together with small coupling constants (JNH,H = ~3–4 Hz) is
evidence again of a rapid imine-enamine equilibrium in DMSO-d6
solution at room temperature (Figure S3, see ESI†).
Tautomeric equilibria
As mentioned above, there is a fast equilibrium in solution
between both tautomers. The tautomerization constant KT, has
been previously determined through 13C shifts of the phenol
carbon or 15N resonances of the iminic nitrogen, as well as by
means of coupling constants, namely 15N–H, 13C–13C and 13C–15N
values.26,29,33,34
Fig. 2 Electron density difference maps for 25 (a), 26 (b), 27 (c), and 28
(d).
Since both tautomers contain a strong intermolecular hydrogen
bond involving the iminic nitrogen and the vicinal OH group
(N ◊ ◊ ◊ H–O), the characteristic data of such bonds have been
gathered in Table 1.
Remarkably, the steric hindrance caused by ortho-substitution
at the aniline ring leads to the lack of coplanarity in the solid-
state structures of 25–28. Thus, the angles between aniline and
naphthalene rings are close to 35–45◦ (44.43, 40.31, 36.35, and
Table 2 Selected spectroscopic dataa for 25–28
Compound
OH/NH
15.34s
15.29d
14.78d
14.67d
JNH,H
0.0
2.4
3.5
4.4
N–CH
9.46s
9.44d
8.77d
8.61d
N–CH
159.0
163.5
161.4
165.5
C-2
25
26
27
28
166.5
167.0
167.1
168.5
a
d in ppm and J in Hz.
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The chemical shifts observed experimentally (dexp) are an average
of those of imine and enamine forms (di and de, respectively).
Accordingly, dexp = nidi + nede, where ni and ne denote the molec-
ular populations of imine and enamine structures, respectively
(obviously, ni + ne = 1). The same reasoning applies to coupling
constants, which can be represented by the equation: Jexp = niJi
+ neJe. These simple equations can therefore be used to estimate
tautomeric populations, assuming that di/de and Ji/Je values are
known. Thus:
with complete geometry optimization have been undertaken to
ascertain the relative stability of imine and enamine structures
and their propensity to interconvert. In addition, the role of
the solvent effect has also been considered by using the SMD
model38 (DMSO as solvent, e = 46.8). Further calculations of
vibrational frequencies have also been performed in solution to
estimate the tautomerization constants in terms of free energies.
Selected geometrical parameters calculated for 25–28 in solution
are collected in Table S5 (see ESI†).
The steric effect caused by two methyl groups at ortho positions
in 26 and 28 is greater than that caused by the bulkier tert-butyl
group in 25 and 27. The effect manifests itself in larger dihedral
angles C11–N1–C12–C13 (Tables S1–S4, see ESI†) for 26 and 28.
The relative stabilities of optimized structures for both imine
and enamine structures, as well as those of the corresponding
transition states (Scheme 1) are summarized in Table 3, which
reflect the greater stability of imine tautomers in the cases of 25
and 26 and approximately equal stability between the tautomers
of 27 and 28.
ni = (de – dexp)/(de – di) and ne = (dexp – di)/(de – di)
The tautomerization constant (KT = [enamine]/[imine] = ne/ni)
for imine-enamine equilibria may then be expressed by:
KT = ne/ni = (dexp – di)/(de – dexp) = (Jexp – Ji)/(Je – Jexp
)
(1)
These tautomerization constants can be calculated using repre-
sentative di/de (or Ji/Je) values for pure imine or enamine forms.
The C-2 resonance has been considered in particular as this signal
is very sensitive to the tautomerization process and experiences
the most significant variations. Eqn (2) has been used in previous
studies of Schiff bases derived from TRIS.20,35
KT = (dC2 – 154.45)/(180.18 – dC2
)
(2)
The results obtained by application of such an equation to shifts
taken from 25–28 are collected in Table 3. The population of
imine molecules is approximately 50% for 25–27 at the equilibrium,
whereas the enamine form slightly predominates in the case of 28.
Scheme 1
Computational calculations
As seen in Table 3, the use of the hybrid functional M06-
2X//6-311++G** in gas-phase calculations predicts a greater
stability of imine tautomers than the B3LYP functional does. At
the M06-2X/6-311++G** level with inclusion of solvent effects,
computation reveals that the imine form is scarcely stabilized in
the cases of 25–27 though the opposite is observed for 28. This
matches relatively well the values obtained experimentally for
KT in solution for 26–28, while for compound 25 the calculated
molecular population of imine tautomer in solution is much
higher than the experimental population obtained by using eqn
(2) (Table 3).
Recently, Zhao and Truhlar21 have recommended the use of the
alternative functional, M06-2X, for main-group thermochemistry,
barrier heights, and for the study of noncovalent interactions
such as intramolecular hydrogen bonding. Bearing this premise
in mind, gas-phase theoretical calculations of the energies at
both the B3LYP/6-31G** and M06-2X/6-311++G** levels36,37
Table 3 Relative energiesa and tautomeric data in solution calculated for
25–28
Comp. Level
Imine TS‡
Enamine KT
1.15
Imine (%)
Nevertheless, the low interconversion barrier (<5 kcal mol-1)
suggests a quick equilibrium at room temperature in the gas phase
and in DMSO solution.
25
26
27
28
B3LYPb
0.00
M06-2Xc 0.00
M06-2Xd 0.00
M06-2Xe 0.00
3.42
5.03
3.93
2.99
3.27
4.96
3.66
0.92
3.43
5.13
4.05
1.79
3.27
4.86
4.21
1.59
2.99
1.13
2.84
1.10
2.68
0.45
0.46
0.09
2.20
0.23
0.44
0.12
1.91
0.00
0.00
0.88f 53f
0.01g 99g
Since the interconversion between imine and enamine tautomers
occurs intramolecularly, one can assume that the calculated energy
for the transition states both in the gas phase (Fig. 3) and in
solution (Table 3) will be roughly similar to that of the solid state.
B3LYPb
0.00
M06-2Xc 0.00
M06-2Xd 0.00
M06-2Xe 0.00
0.95f 51f
0.46g 68g
B3LYPb
0.00
M06-2Xc 0.00
M06-2Xd 0.00
M06-2Xe 0.00
0.97f 51f
0.47g 68g
Simulation of crystal packing
Experimental findings support the higher stability of imines in the
solid state. Since there are no intermolecular interactions in the
gas phase, the calculated stability represents the intrinsic stability
of every tautomer. However, as previously pointed out,39,40 the
intermolecular interactions generated in the crystal lattice may
invert the relative stability in the solid state. In solution, the
relative stabilities can also be inverted by interaction with solvent
molecules or by molecular aggregation.39
B3LYPb
0.00
M06-2Xc 0.00
M06-2Xd 0.54
M06-2Xe 0.16
1.20f 45f
1.31g 43g
a In kcal mol-1. b At the B3LYP/6-31G** level in gas phase. c At the
M06-2X/6-311++G** level in gas phase. d At the M06-2X/6-311++G**
level, including the solvent effect (SMD model, DMSO as solvent). e Free
energies at the M06-2X/6-311++G** level in DMSO. f Experimental data
in solution (eqn (2)). g Calculated data from free energies at the M06-2X/6-
311++G** level in DMSO.
To simulate the packing effect and, on the basis of the crystal
data obtained for 25, we have incorporated into the calculation
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The imine structure in the crystalline lattice (cluster) becomes
more stable than the enamine one by 1.25 kcal mol-1. The change
in energy when the structure of one molecule of 25 in the gas phase
is placed within the crystal can include two components: one is
due to the change of the geometry, i.e. variations in bond lengths
and angles (such a component will be destabilizing); and the other
is due to the interaction involving the core molecule with the rest
of the five surrounding ones.
The first component can be estimated by comparing the energy
for isolated molecules in the gas phase with the energy associated
with a geometry that results from removing the five surrounding
molecules of the cluster. Such a comparison shows that one
isolated imine is destabilized by 22.8 kcal mol-1 (591684.60–
591661.79 kcal mol-1), while the enamine form is destabilized by
21.6 kcal mol-1 (591683.45–591661.85 kcal mol-1) at the B3LYP/6-
31G** level. However, at the higher M06-2X/6-311++G** level
of theory, such a destabilization becomes smaller (1.31 and 1.22
kcal mol-1 for imine and enamine, respectively). On the other
hand, isolated imine and enamine structures of the cluster are
coincidental in energy at the B3YP/6-31G** level. However, the
imine tautomer is stabilized by 2.91 kcal mol-1 with respect to the
enamine form at the M06-2X/6-311++G** level.
To calculate the interactions involving the core molecule with
surrounding molecules in the cluster, the former must be removed.
Such a calculation yielded an energy of -2957457.27 kcal mol-1 at
the B3LYP/6-31G** level. The sum of the latter plus the energy of
the isolated imine or enamine core in the cluster (-3549119.06 and
-3549119.12 kcal mol-1, respectively) were then compared with the
global energies obtained for the clusters with imine and enamine
structures (Table 4). Such a comparison shows how the interaction
of peripheral molecules with the core one in the packing stabilizes
the imine tautomer by 152.16 kcal mol-1 (3549271.22–3549119.06
kcal mol-1), while the enamine-based cluster is stabilized by 150.85
kcal mol-1 (3549269.97–3549119.12 kcal mol-1). Overall, a core
molecule possessing an imine structure becomes more stabilized
than its enamine tautomer by 1.31 kcal mol-1.
Fig. 3 Transition structures and their corresponding imaginary frequen-
cies for imine-enamine interconversions in compounds 25–28 (at the
M06-2X/6-311++G** level in the gas phase).
the five surrounding molecules possessing a close atom (as sum
of van der Waals radii) to a central one (core molecule). This
protocol gives rise to a whole system comprising 6 molecules and
264 atoms. The geometrical optimization at the B3LYP/6-31G**
level has been carried out by fixing the X-ray parameters of the five
surrounding molecules, and without any geometrical restriction
for the core molecule. The resulting geometry for the six-molecule
cluster with imine structures is collected in Fig. 4. Alternatively,
the cluster incorporating five imine molecules around a central
enamine tautomer is depicted in Fig. 5.
Conclusions
The structures of Schiff bases 25–28, generated by condensation
of 2-hydroxy-1-naphthaldehyde, or 1-hydroxy-2-naphthaldehyde,
with o-substituted anilines have been elucidated in the solid state
by both IR spectroscopy and low-temperature X-ray diffraction,
and in solution through NMR analysis. Solid-state structures
are consistent with imine tautomers, although electron density
difference maps point to some delocalization of the hydrogen atom
between oxygen and nitrogen atoms, especially in compounds 27
and 28. In solution such bases exhibit rapid equilibria, which are
slightly shifted to the imine tautomer in the cases of 25–27 as
inferred from an estimation of their tautomerization constants.
DFT calculations employing two hybrid functionals (the standard
B3LYP/6-31G** and the higher M06-2X/6-311++G**) also
corroborate the above findings. Finally, in an attempt to shed light
into the preferential occurrence of imine structures in the solid
state, the crystal lattice of 25 has been simulated and computed
at the B3LYP/6-31G** level. This high-cost computational study
takes into account the interaction of an imine or enamine core
molecule with a first sphere of five imine molecules as emerge from
crystal data. By comparing this cluster effect with the stability of
Fig. 4 Molecular cluster for compound 25 incorporating six molecules,
all having an imine structure: frontal and side views (B3LYP/6-31G**).
Fig. 5 Molecular cluster for compound 25 comprising five imine tau-
tomers around an enamine form: frontal and side views (B3LYP/6-31G**).
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Table 4 Calculated total and relative energies (in parenthesis) (kcal mol-1) for imine-enamine tautomers of compound 25
Structure
Isolated
Imine
Enamine
Gas phasea
-591684.60 (0.00)
-591661.79 (22.81)
-591563.41 (0.00)
-591562.10 (1.31)
-3549271.22 (0.00)
-3549119.06 (152.16)
-591683.45 (1.15)
-591661.85 (22.75)
-591560.41 (3.00)
-591559.19 (4.22)
-3549269.97 (1.25)
-3549119.12 (152.10)
Core geometrya
Gas phaseb
Core geometryb
Clustera
Associated
Pheripheral plus core geometrya
a At the B3LYP/6-31G** level. b At the M06-2X/6-311++G** level.
isolated molecules, the greater stabilization of an imine tautomer
(about 1.3 kcal mol-1 relative to its enamine counterpart) is shown
to reflect the differences in molecular packing.
Computational details
Theoretical calculations were carried out using the Gaussian09
package.46 The B3LYP/6-31G** and M06-2X/6-311++G**
density-functional methods36,37 were selected for all the geometry
optimizations and frequency analysis.
Experimental
Materials and methods
General procedure for the synthesis of Schiff bases
All the chemicals and solvents were of analytical grade and used as
received. 2-tert-butylaniline, 2,6-dimethylaniline, and 2-hydroxy-
1-naphthaldehyde were purchased from Aldrich Chemical Co.
Inc. and 1-hydroxy-2-naphthaldehyde from TCI-Europe, and used
without any further purification. FT-IR spectra were recorded in
the range of 4000–600 cm-1 on a THERMO spectrophotometer.
Solid samples were recorded on KBr (Merck) pellets. NMR
spectra were recorded on Bruker 400/500 AC/PC instruments.
Assignments were confirmed by homo- and hetero-nuclear double-
resonance and DEPT (distortionless enhancement by polarization
transfer). TMS was used as the internal standard (d = 0.00 ppm)
and all J values are given in Hz. Microanalyses were determined
on a Leco 932 analyzer.
To a solution of the aniline derivative (2.90 mmol) in ethanol
(10 mL) was slowly added a solution of the corresponding aldehyde
(2.90 mmol) in a small volume of methanol (~5 mL). When the title
compound did not precipitate, the mixture was evaporated under
vacuum and gave rise to a solid on standing or on cooling. The
resulting product was collected by filtration, washed successively
with cold water, ethanol, and diethyl ether, and recrystallized from
ethanol or methanol.
1-tert-Butyl-2-(2-hydroxy-1-naphthylmethylene)aminobenzene
(25)
From 2-hydroxy-1-naphtaldehyde and 2-tert-butylaniline: (0.774
g, 88%), m.p. 103–105 ◦C, IR (n, KBr): 1622 (CH N), 1602,
X-Ray data collection and structural refinement
1
1578, 1561, 1481 cm-1 (arom). H NMR (400 MHz, DMSO-d6):
Cell dimensions and intensity data for 25–28 were recorded
at 120 K, using a Bruker Nonius KappaCCD area detector
diffractometer mounted at the window of a rotating Mo anode
d 15.34 (1H, s, OH), 9.46 (1H, s, CH N), 8.51 (1H, d, J = 8.8
Hz, H-arom), 8.00 (1H, d, J = 9.2 Hz, H-arom), 7.85 (1H, d,
J = 7.6 Hz, H-arom), 7.55 (1H, t, J = 8.0 Hz, H-arom), 7.45
(1H, d, J = 7.6 Hz, H-arom), 7.35 (4H, m, H-arom), 7.15 (1H,
d, J = 9.2 Hz, H-arom), 1.43 (9H, s, CH3). 13C NMR (100 MHz,
DMSO-d6): d 166.5 (C-OH), 159.0 (C N), 146.7, 142.3, 136.6,
133.3, 129.5, 128.6, 128.1, 127.5, 127.0, 126.7, 124.0, 122.9, 121.1,
121.0, 109.8 (C-arom), 35.3, 31.0 (C[CH3]3). Analysis calcd. for
C21H21NO (303.40): C, 83.13; H, 6.98; N, 4.62. Found: C, 83.01;
H, 6.92; N, 4.78.
˚
(l(Mo-Ka) = 0.71073 A). Data collection and processing were
carried out using the programs COLLECT41 and DENZO42 and
an empirical absorption correction was applied using SADABS43
The structures were solved via direct methods44 and refined by
full matrix least squares on F2. Maximum, minimum peaks (e
-3
˚
A ) in the final difference Fourier synthesis were found as 0.20,
-0.25 (25); 0.16, -0.21 (26); 0.20, -0.27 (27), and 0.19, -0.20
(28), respectively. The hydrogen atoms in both structures were
placed in calculated positions and included in the refinement using
a riding model approximation. The hydroxyl hydrogens atoms
were first located in the difference map and then included in the
refinement using a riding model, the torsion angle was allowed
to refine and in all cases matched the position identified in the
difference map. Crystallographic illustrations were prepared using
the CAMERON programs.45 The molecular structures with the
atom-numbering scheme are shown in Fig. 1.23 All the relevant
crystallographic data and structure refinement parameters for
25–28 are summarized in Table S6 (see ESI†). Selected bond
distances and angles are listed in Tables S1–S4 ◦(see ESI†). Inter-
2-(2-Hydroxy-1-naphthylmethylene)amino-1,3-dimethylbenzene
(26)
From 2-hydroxy-1-naphtaldehyde and 2,6-dimethylaniline: (0.639
g, 80%), m.p. 117–119 ◦C, IR (n, KBr): 1622 (CH N), 1606, 1572,
1469 cm-1 (arom). 1H NMR (400 MHz, DMSO-d6): d 15.29 (1H,
d, J = 2.4 Hz, OH), 9.44 (1H, d, J = 2.4 Hz, CH N), 8.38 (1H, d,
J = 8.8 Hz, H-arom), 7.98 (1H, d, J = 9.2 Hz, H-arom), 7.84 (1H,
d, J = 8.0 Hz, H-arom), 7.50 (1H, m, H-arom), 7.36 (1H, t, J = 7.6
Hz, H-arom), 7.11 (4H, m, H-arom), 2.24 (6H, s, CH3). 13C NMR
(100 MHz, DMSO-d6): d 167.0 (C-OH), 163.5 (CH N), 145.9,
136.3, 133.4, 129.6, 129.4, 128.9, 128.6, 127.4, 125.8, 123.9, 121.3,
120.7, 108.9 (C-arom), 18.7 (CH3). Analysis calcd. for C19H17NO
˚
and intramolecular hydrogen bond data [A and ] for compounds
25–28 are listed in Table 1.
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Org. Biomol. Chem., 2011, 9, 8268–8275 | 8273
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View Article Online
(275.34): C, 82.88; H, 6.22; N, 5.09. Found: C, 82.68; H, 6.09; N,
5.17.
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o356.
1-tert-Butyl-2-(1-hydroxy-2-naphthylmethylene)aminobenzene
(27)
From 1-hydroxy-2-naphtaldehyde and 2-tert-butylaniline: (0.721
g, 82%), m.p. 127-129 ◦C, IR (n, KBr): 1603 (CH N), 1156, 1478
cm-1 (arom). 1H NMR (500 MHz, DMSO-d6): d 14.78 (1H, d, J
= 3.5 Hz, OH), 8.77 (1H, d, J = 3.5 Hz, CH N), 8.35 (1H, d, J =
8.0 Hz, H-arom), 7.81 (1H, d, J = 8.0 Hz, H-arom), 7.64 (1H, t, J
= 7.5 Hz, H-arom), 7.52 (2H, m, H-arom), 7.45 (1H, d, J = 8.0 Hz,
H-arom), 7.33 (3H, m, H-arom), 7.22 (1H, t, J = 8.5 Hz, H-arom),
1.46 (9H, s, CH3). 13C NMR (125 MHz, DMSO-d6): d 167.1 (C-
OH), 161.4 (C N), 144.9, 142.0, 136.9, 130.3, 128.9, 128.1, 128.0,
127.1, 126.9, 126.9, 126.0, 124.5, 122.2, 117.3, 112.2 (C-arom),
35.2, 30.9 (C[CH3]3). Analysis calcd. for C21H21NO (303.40): C,
83.13; H, 6.98; N, 4.62. Found: C, 83.25; H, 6.73; N, 4.63.
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2-(1-Hydroxy-2-naphthylmethylene)amino-1,3-dimethylbenzene
(28)
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From 1-hydroxy-2-naphtaldehyde and 2,6-dimethylaniline: (0.575
g, 72%), m.p. 115–117 ◦C, IR (n, KBr): 1607 (CH N), 1588, 1562,
1503, 1467 cm-1 (arom). 1H NMR (400 MHz, DMSO-d6): d 14.67
(1H, d, J = 4.4 Hz, OH), 8.61 (1H, d, J = 4.4 Hz, CH N), 8.34
(1H, d, J = 8.0 Hz, H-arom), 7.80 (1H, d, J = 8.0 Hz, H-arom),
7.64 (1H, t, J = 7.6 Hz, H-arom), 7.52 (1H, t, J = 7.6 Hz, H-arom),
7.43 (1H, d, J = 8.8 Hz, H-arom), 7.13 (4H, m, H-arom), 2.27
(6H, s, CH3). 13C NMR (100 MHz, DMSO-d6): d 168.5 (C-OH),
165.5 (C N), 143.7, 136.9, 130.3, 130.2, 129.0, 128.8, 128.0, 127.4,
126.3, 125.9, 124.5, 116.7, 111.1 (C-arom), 18.7 (CH3). Analysis
calcd. for C19H17NO (275.34): C, 82.88; H, 6.22; N, 5.09. Found:
C, 83.09; H, 6.32; N, 5.25.
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Acknowledgements
23 Crystal data of structures reported in this work have been de-
posited with the Cambridge Crystallographic Data Centre with num-
bers CCDC-805572 (25), CCDC-805573 (26), CCDC- 810937 (27),
and CCDC-805574 (28); they can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif.
This work was supported by the Junta de Extremadura
(PRI08A032) and the Ministerio de Educacio´n y Ciencia and
FEDER (CTQ2010-18938/BQU). The Research & Technological
Innovation and Supercomputing Center of Extremadura (Ce´nitS)
is gratefully acknowledged for permitting the use of the supercom-
puter LUSITANIA.
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