Experimental and theoretical studies on constitutional isomers of 2, 6-dihydroxynaphthalene carbaldehydes. effects of resonance-assisted hydrogen bonding on the electronic absorption spectra

Article abstract of DOI:10.1021/jo802345f

We prepared and characterized a series of mono-and dicarbaldehydes of 2, 6-dihydroxynaphthalene that bear potential resonance-assisted hydrogen bonding (RAHB) unit(s). X-ray crystal structures of selected compounds revealed that each salicylaldehyde moiety forms an intramolecular hydrogen bond and that the introduction of formyl groups into either the α-or β-position causes a considerable difference in geometry, which was interpretative from a conventional scheme of resonance hybrids including ionic state. Analyses on NMR chemical shifts suggested that the compounds in solution are present as an equilibrium mixture between closed and open forms with respect to RAHB units. Ab initio calculations indicated that the formation of an intramolecular hydrogen bond strikingly influences the aromaticity of the individual local six-membered ring of naphthalene. The trend of the change in aromaticity was analyzed in connection with the extra stabilization energy of RAHB. In the UV-vis spectra, the β-formyl derivatives specifically showed a substantial red shift compared to α-formyl derivatives. The absorption features were successfully reproduced by TD-DFT calculation, and those data were consistently explained from the effects of RAHB on electronic state of the naphthalene's π-system. Finally, we pointed out a similarity in the electronic state between RAHB-bearing molecules and cata-condensed aromatic hydrocarbons. Copyright 2009 by the American Chemical Society.

Full text of DOI:10.1021/jo802345f

Experimental and Theoretical Studies on Constitutional Isomers of
2,6-Dihydroxynaphthalene Carbaldehydes. Effects of
Resonance-Assisted Hydrogen Bonding on the Electronic Absorption
Spectra
Hirohiko Houjou,* Takatoshi Motoyama, Seisaku Banno, Isao Yoshikawa, and Koji Araki
Institute of Industrial Science, UniVersity of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan
houjou@iis.u-tokyo.ac.jp
ReceiVed October 28, 2008
We prepared and characterized a series of mono- and dicarbaldehydes of 2,6-dihydroxynaphthalene that
bear potential resonance-assisted hydrogen bonding (RAHB) unit(s). X-ray crystal structures of selected
compounds revealed that each salicylaldehyde moiety forms an intramolecular hydrogen bond and that
the introduction of formyl groups into either the R- or ꢀ-position causes a considerable difference in
geometry, which was interpretative from a conventional scheme of resonance hybrids including ionic
state. Analyses on NMR chemical shifts suggested that the compounds in solution are present as an
equilibrium mixture between closed and open forms with respect to RAHB units. Ab initio calculations
indicated that the formation of an intramolecular hydrogen bond strikingly influences the aromaticity of
the individual local six-membered ring of naphthalene. The trend of the change in aromaticity was analyzed
in connection with the extra stabilization energy of RAHB. In the UV-vis spectra, the ꢀ-formyl derivatives
specifically showed a substantial red shift compared to R-formyl derivatives. The absorption features
were successfully reproduced by TD-DFT calculation, and those data were consistently explained from
the effects of RAHB on electronic state of the naphthalene’s π-system. Finally, we pointed out a similarity
in the electronic state between RAHB-bearing molecules and cata-condensed aromatic hydrocarbons.
Introduction
compounds’ possible constitutional isomers, 2,6-dihydroxynaph-
thalene attracts considerable interest because it can be oxidized
to amphi-naphthoquinone, a compound in which aromaticity is
lost due to drastic reconfiguration of the π system. However,
the physical and chemical characteristics of amphi-naphtho-
quinone have not been researched as thoroughly as have those
of o- and p-naphthoquinones.9-13
Dihydroxynaphthalenes, by virtue of their π-electron-rich
character, are often employed as electron-donor components in
supramolecular systems and as antioxidants or radical scavengers
in drug and therapeutic research areas.1-8 Among these
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10.1021/jo802345f CCC: $40.75 2009 American Chemical Society
Published on Web 12/04/2008

Constitutional Isomers of 2,6-Dihydroxynaphthalene Carbaldehydes
As prepared, 2,6-dihydroxynaphthalene is redox-active, and
the introduction of a formyl functionality ortho to the hydroxyl
group yields a unique subgroup of double-headed salicylalde-
hyde analogues that are extensively studied because they can
be used to create a variety of macrocyclic and polymeric Schiff
bases and their associated transition-metal complexes.14-22 For
salicylaldehyde and related compounds, the ground state is
described as a resonance hybrid of the phenol-aldehyde
(covalent) state and the protonated quinoid-enolate (ionic)
state.23 For this character of the ground state, the intramolecular
hydrogen bond suffers enhanced stabilization by electronic
delocalization along a proton-bridged quasi-six-membered ring
and, hence, is perceived as resonance-assisted hydrogen bonding
(RAHB),24,25 although its coverage is still controversial.26 For
salicylaldehydes, RAHB is also relevant to intramolecular proton
transfer in the excited state.27-29 An RAHB unit affects the
electronic state of its neighboring aromatic rings, on which an
electronic “push and pull” effect of the substituents operates.
When such compounds serve as a ligand in a metal complex
system, the electronic state of the metal may be allowed to
couple with that of the ligand’s π-electron system, affording
such interesting phenomena as valence tautomerism.30-32 Previ-
ously, dioximato complexes of copper and nickel were prepared
from 2,6-dihydroxynaphthalene-1,5-dicarbaldehyde (3), and
these compounds exhibit semiconducting performance for
FIGURE 1. 2,6-Dihydroxynaphthalene carbaldehydes investigated in
this study.
SCHEME 1. Formylation of 2,6-Dihydroxynaphthalenea
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Okaniwa, K.; Mitani, T.; Yamamoto, H.; Murata, I.; Kawamoto, A.; Tanaka, J.
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a Key: (i) NaH, chloromethyl methyl ether/DMF (0 °C); (ii) n-butyl-
lithium/hexane-THF (-20 °C); (iii) DMF (25 °C); (iv) HCl/EtOH (80 °C).
electronic functional materials applications.33 Despite such
promising results, overall the literature contains surprisingly few
studies using 3 and other related analogues of 2,6-dihydrox-
ynaphthalene (Figure 1). This lack of comprehensive charac-
terization data may be partly attributed to difficulties associated
with the synthesis of these compounds, such as poor regiose-
lectivity of substitution.
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Koshizaki, N.; Kanesato, M. AdV. Mater. 2005, 17, 606–610. Houjou, H.;
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2004, 45, 4225–4227. Nabeshima, T.; Miyazaki, H.; Iwasaki, A.; Akine, S.; Saiki,
T.; Ikeda, C. Tetrahedron 2007, 63, 3328–3333.
We herein report the structural and spectroscopic studies of
the mono- and diformyl derivatives of 2,6-dihydroxynaphthalene
carbaldehydes. We have synthesized 1-4 out of those listed in
Figure 1, and the experimental data are analyzed by means of
ab initio quantum chemical calculations to elucidate the effects
of RAHB units on the electronic state of naphthalene ring. This
study will provide insight into the electronic structure of their
derivatives, allowing their optimization for use in the develop-
ment of new drugs and materials.
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Results and Discussion
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Synthesis of Mono- and Dicarbaldehydes. The hydroxyl
groups of 2,6-dihydroxynaphthalene were protected with a
methoxymethyl group, and the compound was double-metalated
with n-butyllithium and then treated with dimethylformamide
and deprotected with hydrochloric acid (Scheme 1).34,35 This
reaction yielded a mixture of 2,6-dihydroxynaphthalene-1-
carbaldehyde (1), 3,7-dihydroxynaphthalene-2-carbaldehyde (2),
105, 3513–3560.
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J. Org. Chem. Vol. 74, No. 2, 2009 521

Houjou et al.
SCHEME 2. Synthesis of 3
2,6-dihydroxynaphthalene-1,5-dicarbaldehyde (3), 3,7-dihydrox-
ynaphthalene-2,6-dicarbaldehyde (4), and bis(2,6-dihydroxy-7-
formylnaphthyl)methane (6), while 2,6-dihydroxynaphthalene-
1,7-dicarbaldehyde (5) could not be identified. The poor
regioselectivity observed here agreed with previous results
reported for the formylation of hydroxynaphthalene under
similar conditions.6,36 After tedious separation, 1-10% yields
of each compound were obtained.
Since the above method gave 3 quite a poor yield and the
separation of 3 from 4 was difficult, we used a second approach
to synthesize 3. In this method (Scheme 2), the condensation
of 2,6-dihydroxynaphthalene, paraformaldehyde and dimethy-
lamine in ethanol gave a Mannich base (7), which was allowed
to react with hexamethylenetetramine.37,38
FIGURE 2. ORTEP drawings of the crystal structure of (a) 3 and (b)
4. Selected bond lengths in angstroms.
Solid-State Structure. Slow evaporation of a solution of 3
in toluene produced a single crystal suitable for X-ray structural
analysis. Similarly, a single crystal of 4 was obtained from its
DMF solution. Parts a and b of Figure 2 show the ORTEP
drawings of 3 and 4, respectively. Since 3 and 4 formally
represent fused salicylaldehydes, their hydroxy groups intrinsi-
cally have a phenol-aldehyde/quinoid-enol tautomeric char-
acter. For 3 and 4, the C-O(H) bond distances were 1.345 and
1.370 Å, respectively, which are more likely to be phenolic C-O
bond lengths than keto CdO bond. Moreover, the C-C(dO)
bond distances were 1.450 and 1.456 Å, respectively, in a range
of typical aromatic aldehyde bond lengths. Therefore, we
concluded that the salicylaldehyde moieties of 3 and 4 adopted
a phenol-aldehyde form in the crystal state. Since the lengths
of these bonds are closely related to the strength of hydrogen
bond,39 we later discuss them in more detail.
On modeling the crystal structure of 3, we assumed the
formation of an intramolecular hydrogen bond between formyl
and hydroxy groups, in view of the substantial planarity of
OdC-C-C-O moiety (dihedral angle of OdC-C-C is
-5.25°), and the proximity of two oxygen atoms (O---O distance
is 2.593 Å). For 4 as well, we assumed the formation of a similar
intramolecular hydrogen bond, based on the substantial planarity
of the OdC-C-C-O moiety (the dihedral angle of OdC-C-C
is 2.92°) and the proximity of two oxygen atoms (O---O distance
is 2.71 Å). These geometrical parameters are similar to those
observed for their analogous compounds that have intramo-
FIGURE 3. Potential curves of 3 and 4 as functions of the torsion
angle φ of the C(CHO)-C-O-H moiety. φ ) 0° indicates the closed
form of the salicylaldehyde unit.
lecular hydrogen bond.40 IR spectra of 3 and 4 gave significantly
broadened peaks at 3432 and 3302 cm-1, respectively, which
are attributable to the stretch vibration of intramolecularly
hydrogen-bonded hydroxy groups.
To support the validity of assumed intramolecular hydrogen
bonds, we performed ab initio molecular orbital calculations.
Figure 3 shows potential curves of 3 and 4 as a function of the
torsion angle φ of C(CHO)-C-O-H framework, where φ is
defined as 0° when the proton-bridged quasi ring is closed. The
curves are plotted against simultaneous and synchronous change
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2003, 46, 3083–3093.
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522 J. Org. Chem. Vol. 74, No. 2, 2009

Constitutional Isomers of 2,6-Dihydroxynaphthalene Carbaldehydes
SCHEME 3. Resonance structures of 3 (A) and 4 (B)
FIGURE 4. Notation of the conformers of salicylaldehyde: (I) closed,
(II) Z-open, and (III) E-open.
groups at different positions on the electronic state of naphtha-
lene’s π-system.
Solution-State Structure. H NMR spectra of 1-5 were
1
measured in DMSO-d6. The -OH protons resonated at about
11.6 ppm when positioned ortho to an R-formyl group and were
in the range of 10.2-10.5 ppm when positioned ortho to a
ꢀ-formyl group. These characteristic downfield shifts suggest
that intramolecular hydrogen bonding was maintained even in
the polar dimethyl sulfoxide solvent. The chemical shift of the
-OH protons of the R- and ꢀ-formyl derivatives differed by
more than 1 ppm, and the chemical shifts (10.2-10.5 ppm) of
-OH protons neighboring ꢀ-formyl groups were rather close
to those (∼9.7 ppm) observed for hydrogen-bond-free -OH
protons. Such a difference in downfield shift implies that an
R-formyl group serves as stronger hydrogen bond acceptor than
does a ꢀ-formyl group.
The potential surface (Figure 3) of 3 and 4 with respect to
the rotation of the C(CHO)-C-O-H moiety suggests that the
main conformers present in solution are types of I-I, I-III,
and III-III, where I and III means the partial conformation of
salicylaldehyde unit denoted in Figure 4. To more quantitatively
discuss the solution-state structure of the compounds, we
performed an ab initio magnetic shielding calculation by using
the GIAO (gauge invariant atomic orbital) method, which is
widely used to interpret the magnetic properties of organic
compounds including π-conjugated systems.41 Figure 7 shows
the 13C chemical shift of the naphthalene carbons subtracted
FIGURE 5. Calculated bond lengths of 3 and 4 as functions of the
torsion angle φ.
in two φ angles. The energy is calculated by reference to that
of the E-open form (III in Figure 4) of the salicylaldehyde
moiety. Namely, the energy difference between two potential
minima at φ ) 0° and φ ) 180° corresponds to the energy
difference for an adiabatic open/close process of the intramo-
lecular hydrogen bond. The figure implies overwhelming
stability of the closed form (I in Figure 4) against the Z-open
form (II). It is thus reasonable to assume intramolecular
hydrogen bonding for these systems. The origin of a sizable
difference (97.7 kJ/mol for 3 and 69.4 kJ/mol for 4) in the
stabilization energy is discussed later.
Parts a and b of Figure 5 plot the calculated interatomic
distances of C-C and C-O bonds in 3 and 4, respectively, as
functions of φ defined above. Figure 5 clearly shows that the
formation of the intramolecular hydrogen bond brings about
considerable changes in bond length all over the naphthalene
ring. For both 3 and 4, the C-CHO bond (denoted as 7) and
the C-OH bond (denoted as 8) shortened on going from open
to closed form. Simultaneously, the bond (denoted as 1 for 3
and as 2 for 4) between carbons substituted by formyl and
hydroxy groups elongated. The trend of those changes is
understandable in view of the resonance hybrid involving the
covalent and ionic structures of each salicylaldehyde moiety
(Scheme 3). As compared to 4 (Scheme 3B), the structure of 3
(Scheme 3A) has a larger number of possible resonance
structures involving ionic structures, responsible for more
prominent change in the bond length. In other words, the cal-
culated bond lengths well reflect the changes in resonance
structure that were induced by the intramolecular hydrogen
bonding. Further, the observed difference in the length of each
bond between 3 and 4 is plotted against the one calculated for
φ ) 0° (Figure 6). There is an excellent correlation between
calculated (∆dcalcd) and observed (∆dobsd) values of the difference
in bond length, suggesting that the present calculations can
qualitatively (at least relatively) reproduce the effects of formyl
FIGURE 6. Correlation between the calculated and observed values
of the bond length difference between 3 and 4.
J. Org. Chem. Vol. 74, No. 2, 2009 523

Houjou et al.
FIGURE 7. Observed (solid symbols) and calculated (open symbols)
of the chemical shifts of naphthalene carbons of (a) 3 and (b) 4 by
reference to those of 2,6-dihydroxynaphthalene.
by that of the corresponding carbons of 2,6-dihydroxynaphtha-
lene so that these values could represent the effect of formyl
substitution in each compound. For convenience of comparison,
the carbons are numbered by regarding 4 as 2,6-dihydroxynaph-
thalene-3,7-dicarbaldehyde. The observed values (solid symbols)
exhibited sizable downfield shift for carbons C1, C2, and C4
of 3 and C1 and C3 of 4. The calculated values (open symbols)
are the equally weighted average of the shielding constants for
the conformers I-I (φ ) 0° and ( 30°) and III-III. The error
bars indicate the standard deviation of the calculated values with
respect to the conformational change. We have also confirmed
that the conformer I-III simply gave an arithmetic average of
I-I and III-III results. The calculation revealed that the
shielding constants of carbons C3 and C4 of 3 vary largely
depending on the conformational change, and the equally
weighted average moderately reproduces the observed value.
The conformation dependence of the calculated shielding
constants of 4 is relatively small, and again, the equally weighted
average well reproduces the observed values except carbon C4.
The attempt of taking into account other conformers, for
example, II-II, did not improve the fitting between observed
and calculated values. Consequently, the overall agreement
between observed and calculated chemical shifts suggests that
in solution 3 and 4, respectively, are present as an equilibrium
mixture of the conformers I-I, I-III, and III-III, which convert
to each other on a relatively flat potential surface.
Effects of RAHB on the Adjoining π-System. In the
previous section, Figure 5 suggested that the rotation of the
C(CHO)-C-O-H moiety considerably affects the equilibrium
distance of C-C and C-O bonds in 3 and 4. The changes in
bond lengths are quite informative in connection with a
geometrical criterion of local aromaticity. The harmonic oscil-
lator model of aromaticity (HOMA) index is defined as a
normalized variance of the bond lengths by reference to a length
optimum for ideal aromatic system. For evaluation of the
HOMA index, we utilized eq 1, where Ropt ) 1.388 Å and R )
257.7, as proposed by Kruszewski and Krygowski.42 Several
studies have shown that the HOMA index finely correlates with
FIGURE 8. HOMA index of the individual local ring denoted in Figure
9 as a function of φ.
other geometry-based and magnetism-based criteria of aromati-
city.25,43-48 For naphthalene system, a special interest is
addressed to position-dependent substituent effects on aromati-
city.49,50
n
R
HOMA ) 1 -
(Ropt - Ri)2
(1)
∑
n i)1
Figure 8 plots HOMA indices of local six-membered rings
of 1-5 as functions of φ. The numbering of rings and the C-O
bond(s) that rotate(s) are given in Figure 9. For 1, the closure
of the proton-bridged quasi-ring induces the decrease of
aromaticity in the formyl-substituted ring (1(1)) and increase
of aromaticity in the other ring (1(2)). This result indicates that
1 in the closed form is preferably drawn as a structure like I in
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524 J. Org. Chem. Vol. 74, No. 2, 2009

Constitutional Isomers of 2,6-Dihydroxynaphthalene Carbaldehydes
SCHEME 4
SCHEME 5
cylaldehyde units can be treated as if they were independent.
Ontheotherhand,for4,wecanonlydraweithercovalent-covalent
or covalent-ionic structures. The contributions of the ionic
structure from each half are exclusive, and hence, two salicy-
laldehyde units are not independent. As a consequence of
interference between the contributions of ionic structure of two
salicylaldehyde units, 3 undergoes a decrease of aromaticity in
the both ring, while 4 undergoes the compensation of aromaticity
in the both ring. Upon comparison to an electric circuit, the
RAHB units of 3 work as switches of two independent circuits,
whereas the RAHB units in 4 work as inputs of XOR logic
gate.
FIGURE 9. Numbering of the local six-membered rings of 1-5.
Arrows indicate the bonds that rotate as φ changes from 0° (tail) to
180° (head).
Based on the above results, we can expect 5 to exhibit an
extreme imbalance of aromaticity over the two local rings.
Indeed, the closing of the quasi ring increases the HOMA index
of the ring 5a(1) up to 0.90 and decreases that of 5a(2) down
to 0.56. Further, we examined two cases illustrated as 5b and
5c. Interestingly, the HOMA indices of the rings 5b(1) and 5c(1)
showed almost the same dependence on φ, and so did the rings
5b(2) and 5c(2). These results suggest that the closure of either
quasi ring attached to the ring 1 or 2 makes the ring 1 reach the
highest level of aromaticity and makes the ring 2 reach the
lowest level of aromaticity. This phenomenon may also cor-
respond to the resonance hybrid formula (Scheme 5) of 5, where
we can see the contributions of ionic structure from two
salicylaldehyde units are exclusive. Consequently, for 5 the
RAHB units work as inputs of OR logic gate.
The above results may give us a possible interpretation for
the composition of the products shown in Scheme 1: 5 was not
obtained and the yield of 2 was poor. For 2, the C5-carbon
assumes a negative charge, because the C5-C6-OH moiety
has an isolated enol-like character, since the formyl-substituted
ring can be represented as a π-sextet with high aromaticity
(Scheme 5). For 5, on the other hand, the formyl group will
show high reactivity due to its isolated enal-like character. These
compounds may have suffered some coupling reactions to give
unidentifiable oligomers or polymers. A minor product 6 appears
given by the cross-coupling of 2 and 5, which occurred
somewhere during the reaction procedure.
FIGURE 10. Hypothetical energy diagram to evaluate the strength of
RAHB.
Figure 10 using Clar’s π-sextet model.51 Interestingly, for 2,
the trend of change in HOMA index is totally opposite to 1;
namely, the closing of the proton-bridged quasi ring induces
the increase of aromaticity in the formyl-substituted ring (2(1))
and decrease of aromaticity in the other ring (2(2)). Similar
results have already reported by Palusiak et al.,44 but the present
case is much more prominent maybe because of the participation
of 6-hydroxy group to the π-electron resonance.
Further, the results of 3 and 4 highlight the interference effect
of the RAHB. For 3, the closing of the proton-bridged quasi
ring induces the decrease of aromaticity in both rings of 3(1)
and 3(2), and their HOMA index is as small as that of 1(1)
ring. On the contrary, for 4, the closure of the quasi ring hardly
changes the aromaticity of both rings of 4(1) and 4(2), and their
HOMA index is similar to that for the Z-open (i.e., II-II) form.
This equalization of aromaticity corresponds to the character
of a migrating π-sextet system. The above results may be also
interpreted based on the resonance hybrid structure shown in
Scheme 3. For 3, we can draw any of the covalent-covalent,
covalent-ionic, and ionic-ionic structures. Namely, two sali-
Since the origin of the stabilization of RAHB is electronic
delocalization along the proton-bridged quasi-six-membered
ring, the naphthalene’s π-system simultaneously undergoes
electronic donation from the hydroxy group and electronic
withdrawing from the formyl group. Namely, the effect of
resonance-assist can be interpreted as a stabilization energy that
is caused by mixing of the ionic state wave function into the
covalent state wave function. Therefore, the stabilization of the
hydrogen bond on the one hand would cause destabilization by
the loss of aromaticity on the other hand. If such an energetic
(51) Randic´, M. Chem. ReV. 2003, 103, 3449–3605.
J. Org. Chem. Vol. 74, No. 2, 2009 525

Houjou et al.
TABLE 1. List of Calculated Energies Related to the Diagram in
as large as the latter. These results are in agreement with a
prediction based on the discussion of the change in aromaticity,
although the difference in the resonance-assisted effect among
1-5 was smaller than expected. Rather, difference in ∆EHB
Figure 10a
I
II
III
∆EO/C
∆EHB
∆ERA
1
2
3
4
5
(0.0)
(0.0)
(0.0)
(0.0)
(0.0)
(0.0)
(0.0)
(0.0)
(0.0)
(0.0)
50.9
58.8
35.2
41.7
97.7
114.7
69.4
82.1
86.4
100.5
36.2
42.7
17.6
28.3
37.1
57.4
33.6
54.5
47.1
67.5
46.8
56.1
39.3
44.4
89.6
109.6
77.5
87.2
86.4
100.5
37.9
44.1
32.8
35.3
70.6
85.8
64.8
69.4
69.9
79.1
8.9
12.0
6.4
could contribute more critically to ∆EO/C
.
UV-vis Spectra. Figure 11 shows the ultraviolet-visible
(UV-vis) absorption spectra of 1-4 and 6. Moderately intense
bands appeared in the regions of 280-330 nm (region I) and
350-550 nm (region II); these bands are attributable to ππ*
transitions, and the bands in the longer wavelength region are
characteristic of carbonyl-bearing naphthalene systems.52-57 The
spectra of the R-formyl derivatives 1 and 3 were similar to each
other, but the values of the molar absorption coefficient ε of 3
(14300 M-1 cm-1 at 319 nm and 7800 M-1 cm-1 at 404 nm)
were roughly twice as large as those of 1 (7900 M-1 cm-1 at
320 nm and 5000 M-1 cm-1 at 388 nm). For 2, a ꢀ-formyl
derivative, the peak observed in region II, at 414 nm, was
significantly broadened and red-shifted compared to the peak
observed for 1, an R-formyl derivative. A peak similar to 2 was
also observed at 425 nm for 6, indicating that such relatively
red-shifted peaks were characteristic of ꢀ-formyl derivatives.
For 4, the ꢀ-diformyl derivative, the region II peak was even
more red-shifted, to 476 nm, and the absorbance of the region
I peak (314 nm) was about twice as intense as that observed
for any of the other derivatives. These spectral features were
similar to those observed for naphthalene monocarbaldehydes.56,57
The absorption maxima of 1 and 2 were red-shifted by ∼30-40
nm with respect to that of 1- and 3-formyl-2-naphthols,
respectively, and this red shift was interpreted as an electron
donating effect of the OH groups in amphi-positions of 1
and 2.
9.1
19.0
23.8
12.7
17.8
16.5
21.4
a Upper and lower rows contain HF/6-311G(d,p) and B3LYP/
6-311G(d,p)//HF/6-311G(d,p) results, respectively.
“tradeoff” results in a net decrease in aromaticity, it suggests
the predominance of the resonance-assisted effect. On comparing
the aromaticity of 3 and 4 in Figure 8, we can expect 3 to have
a larger contribution of resonance-assist to the intramolecular
hydrogen bond.
In Figure 3, we demonstrated the energy differences for 3
and 4 are 97.7 and 64.8 kJ/mol, respectively, suggesting an
overwhelming thermodynamic stability of closed form of 3.
However, the total amount of energy difference between closed
(I-I) form and open (II-II) form is not simply a measure of
the strength of RAHB.43,44 To clarify the contents of the energy
difference, first we defined ∆EO/C as a net energy necessary to
open the closed quasi ring by using a hypothetical diagram
(Figure 10) to eliminate the effects of intrinsic conformational
preference of hydroxy groups. Next, we calculated the energy
of the structure I′ that was built by changing only the dihedral
angles of formyl and hydroxy groups to close a proton-bridged
quasi ring with the other geometry remaining unchanged from
structure II. Then we divided ∆EO/C into (i) net stabilization of
hydrogen bonding (∆EHB) and (ii) resonance-assisted stabiliza-
tion (∆ERA), according to the diagram in Figure 10. Therefore,
the following relation holds.
We focused on the region II absorption band, which was
shifted with varying substitution positions. A considerable
difference in absorption maxima between 3 (404 nm) and 4 (476
nm) is of special interest. Since the spectral feature mentioned
above is also observed for solid-state samples, the difference
in absorption maxima is originating in the molecular structure,
not in other environmental factors like solvent effects.
∆EO⁄C ) ∆EHB + ∆ERA
(2)
In the previous sections, we revealed several effects of the
intramolecular hydrogen bond(s) on the electronic state of the
molecules for 1-5. In addition, analysis on NMR chemical shifts
suggested that the intramolecular hydrogen bond is maintained
in solution state. The intramolecular hydrogen bond induces a
change in electronic state of the naphthalene ring, which is
illustrated as a mixing of the ionic state into the covalent state
to some extent. For these compounds, the ground-state wave
function Ψg is represented as aΨcov + bΨion, where Ψcov and
Ψion are the wave functions of covalent and ionic states and a
The ∆ERA value is reported to show fairly good correlation
with an aromaticity index based on Bader’s theory.45 It should
be noted that the ∆EHB value does not purely reflect the
hydrogen-bonding energy because it includes O---O repulsion
in the open structure. However, in the present case, difference
in the repulsion energy among 1-5 can be neglected in view
of the similarity of their local structures.
Table 1 lists the values of energy of conformers II and III
relative to that of I (for compounds 3, 4, and 5 the notation I
implies I-I and so on). The results from Hartree-Fock (HF)
calculation (upper rows) of 1-5 indicate that ∆EHB occupies
about 80% (78.8-83.6%) of ∆EO/C and that both ∆EHB and
∆ERA of R-formyl derivatives are greater than those of ꢀ-formyl
derivatives. Interestingly, additivity roughly holds among ∆EHB
(and naturally ∆ERA as well) values of the compounds. The
values of ∆EHB per hydrogen bond are 37 kJ/mol for the quasi
rings including R-formyl group and 33 kJ/mol for the quasi rings
including ꢀ-formyl groups, respectively. Similarly, the values
of ∆ERA per hydrogen bond are 9 kJ/mol for the quasi rings
including R-formyl group and 6 kJ/mol for the quasi rings
including ꢀ-formyl groups, respectively. The DFT calculation
(B3LYP, lower rows) gave results similar to those from HF
calculation, although each value of the former is 1.1-1.2 times
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Taylor, P. J. J. Phys. Chem. A 2004, 108, 7603–7612. Antonov, L.; Fabian,
W. M. F.; Nedeltcheva, D.; Kamounah, F. S. J. Chem. Soc. Perkin Trans. 2
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(56) Chowdhury, P.; Chakravorti, S. Chem. Phys. Lett. 2004, 395, 103–108.
Chowdhury, P.; Panja, S.; Chakravorti, S. J. Phys. Chem. A 2003, 107, 83–90.
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2007, 331, 373–384.
526 J. Org. Chem. Vol. 74, No. 2, 2009

Constitutional Isomers of 2,6-Dihydroxynaphthalene Carbaldehydes
TABLE 2. Lowest Energy Absorption Wave Lengths (λ/nm)
Calculated by the TD DFT Method
I
I′
II
III
obsd
1
2
3
4
5
353.6
399.9
360.2
469.5
403.8
353.7
396.4
359.3
461.1
404.4
344.7
362.7
355.8
402.1
374.9
352.5
371.2
354.7
409.0
376.0
388
414
404
476
8) and the resonance assist effect ∆ERA (Table 1), the contribu-
tion of the ionic state to the ground state is greater for 3 than
for 4. This means that 3 has a relatively greater value of the
coefficient b, responsible for the lowering of the ground-state
level as compared to that of 4. This model can give a qualitative
explanation for a substantial difference in absorption maxima
between 3 and 4.
For a more quantitative discussion of the excitation energy,
we performed a TD (time-dependent) DFT calculation using
B3LYP functional.58 The calculated wavelengths of the lowest
energy absorption are summarized in Table 2. The columns I,
II, and III contain a value calculated for the corresponding
structure, i.e., closed, Z-open, and E-open forms (see Figure 4)
(again, for compounds 3, 4, and 5 the notation I implies I-I,
and so on). The structure I′ is the same structure as mentioned
in Figure 10.
Based on the results in Table 2, we can explain the data of
absorption maxima observed for 1-4. Each data column of I,
II, and III is fairly correlated with the observed data (the right
column). Since the solution-state structures of 3 and 4 were
identified as an equilibrium mixture of the structures I-I, I-III,
and III-III, it seems natural to consider a similar equilibrium
for the other compounds. Relatively small differences (1-6 nm)
between columns I and III for 1 and 3 are in agreement with
sharp peaks observed for these compounds. On the other hand,
a sizable difference (30-60 nm) for 2 and 4 account for the
broad absorption bands observed around 414 and 476 nm,
respectively.
Comparison of the columns of I and II suggests that the
formation of an intramolecular hydrogen bond would result in
a significant red shift of the absorption maxima. The columns
of I and I′, both of which have closed form, show only a slight
difference, indicating that the red shift was induced by the
closure of the quasi ring(s), not by the distortion of the naph-
thalene ring. The absorption wavelengths calculated for the
structure III are more or less similar to those for II, which
confirms that the intramolecular hydrogen bond is responsible
for the red shift. Interestingly, the amounts of the red shift on
going from II to I is prominent for 2 (2565 cm-1), 4 (3570
cm-1), and 5 (1909 cm-1), as compared to those of 1 (730 cm-1)
and 3 (343 cm-1). These values show negative correlation with
the ∆ERA value that represents the amount of stabilization energy
by resonance effect of RAHB (see Table 1). Consequently, the
order of the red shift is explained as follows. The intramolecular
hydrogen bond forms a proton-bridged quasi ring, to which
π-electrons of the naphthalene ring delocalize according to the
resonance hybrid scheme (Schemes 3 and 4). The mixing of
the naphthalene’s and the quasi ring’s π-systems leads to the
elevation of HOMO and the lowering of LUMO, resulting in
red shift of ππ* transition. In case the hydrogen bonding in the
FIGURE 11. UV-vis spectra of the 2,6-dihydroxynaphthalene car-
baldehydes: 1 (0), 2 (O), 3 (9), and 4 (b). Solution-state (in CHCl3)
spectra of (a) R-formyl-bearing compounds (1 and 3) and (b) ꢀ-formyl-
bearing compounds (2, 4, and 6). (c) Solid-state spectra of 3 and 4
measured for thin films smeared on a slide glass.
and b are mixing coefficients, and we can straightforwardly find
the excited-state wave function Ψe to be bΨcov - aΨion.
According to this two-state model, the excitation energy directly
reflects the stability of the ground state, since the energies of
the ground and excited states symmetrically vary depending on
the mixing coefficients. In view of the change in HOMA (Figure
(58) Jacquemin, D.; Perpe`te, E. A.; Scalmani, G.; Frisch, M. J.; Kobayashi,
R.; Adamo, C. J. Chem. Phys. 2007, 126, 144105–144112. Perpe`te, E. A.;
Wathelet, V.; Preat, J.; Lambert, C.; Jacquemin, D. J. Chem. Theory Comput.
2006, 2, 434–440.
J. Org. Chem. Vol. 74, No. 2, 2009 527

Houjou et al.
mixture of the closed form and E-open form, and hence, the
intramolecular hydrogen bond is partially maintained in solution
state. Difference in the thermodynamic stability of the intramo-
lecular hydrogen bond among the compounds studied was also
discussed in terms of the resonance-assist mechanism. It was
found that the aromaticity of the individual ring of naphthalene
changes depended on the topology of substitution of formyl and
hydroxy groups, and the trend of the change was again
compatible with resonance hybrid schemes. UV-vis absorption
profiles of the compounds were finely reproduced by TD DFT
calculations, not only in the wavelength but also in the broadness
of peaks. We were able to rationalize the calculated results in
view of the change in aromaticity of naphthalene ring, and the
stabilization energy corresponding to the resonance assist effect
on hydrogen bonding. The calculations clearly showed that the
origin of the red shift is electronic delocalization over the RAHB
unit, rather than the geometrical distortion of naphthalene ring.
Comparison with the absorption maxima of aromatic hydro-
carbons clearly demonstrated that the o-hydroxyaldehyde moi-
eties of these compounds serve as if they were additional
aromatic rings that affect the molecule’s electronic state.
To date, only a few studies on 2,6-dihydroxynaphthalene
carbaldehydes and their Schiff bases have been presented. To
the best of our knowledge, this is the first systematic study of
a series of 2,6-dihydroxynaphalene mono- and dicarbaldehydes.
Closely related to its amphi-naphthoquinone-like resonance
structure, 2- and 6-OH groups of 2,6-dihydroxynaphalene
carbaldehyde appear to determine the stability of intramolecular
hydrogen bonding through a resonance-assisted mechanism.
These compounds are potentially applicable to several types of
polyimines and their metal complexes.46 The present study
provides a useful guide for designing metal-containing materials
that show specific electronic properties. Namely, since a metal
chelate unit in such compounds will also act as an imaginary
aromatic ring that is fused into a core naphthalene ring, the
properties of such compounds could be interpreted by analogy
with their isotopological hydrocarbons. Our ongoing studies are
aimed at clarifying structure-property relationships for a series
of Schiff base complexes.
FIGURE 12. Comparison of the lowest energy absorptions of naph-
thalene carbaldehydes 1-4 plotted against those of their isotopological
cata-condensed aromatic hydrocarbons: observed (closed square),
calculated for form I (open circle), and calculated for form III (open
triangle).
quasi ring is stabilized by the mixing of the ionic state to the
ground (covalent) state, the energy level of π-electron is lowered
according to the degree of the mixing. The degree of the mixing
is determined by tradeoff relation between the stabilization of
hydrogen bond and the destabilization due to loss of aromaticity,
and their optimal balance depends on a topology of the
substituted molecule.
Practically, it might be possible to regard the quasi ring of
an RAHB as an additional aromatic ring in view of its
delocalized π-electron’s nature. Then, the above discussion is
reminiscent of the relationship between the absorption maxima
and connecting topology for a series of cata-condensed aromatic
hydrocarbons.59 Figure 12 plots the observed and calculated
values of the lowest energy absorption of 1-5 as functions of
the lowest energy absorption peaks of their isotopological cata-
condensed hydrocarbons, namely phenanthrene, anthracene,
chrysene, naphthacene, and benz[a]anthracene, in this order. A
good correlation was observed between the absorptions of 1-5
and those of the hydrocarbons, although the experimental value
for 5 is lacking for the moment. This correlation strongly
supports the above interpretation of spectral variation based on
the RAHB concept. Thus, it is helpful to regard the proton-
bridged quasi rings in 1-5 as additional aromatic rings when
we consider the electronic state of these molecules.
Experimental Section
General procedures of syntheses and measurements are given in
the Supporting Information.
Ab initio calculations were performed using the Gaussian03w
program.60 The geometry of each compound was optimized by
means of the HF method using the 6-311G(d,p) basis set. DFT
calculations using B3LYP hybrid functionals for exchange-cor-
relation energy terms were done for the thus-obtained geometry.
Synthesis of 2,6-Dihydroxynaphthalene-1-carbaldehyde (1),
3,7-Dihydroxynaphthalene-2-carbaldehyde (2), 3,7-Dihydrox-
Conclusions
In summary, we prepared five mono- and diformyl derivatives
of 2,6-dihydroxynaphthalene and interpret differences in their
structural and spectroscopic aspects by means of ab initio
calculations. The difference in the crystal structures between 3
and 4 was successfully explained by the substituent effects. Ab
initio calculations revealed that the formation of the intramo-
lecular hydrogen bond considerably influences the bond lengths
of naphthalene ring, and the trend of the change was under-
standable from the conventional schemes of resonance hybrid.
The analysis of the observed and calculated NMR chemical
shifts implied that the compounds are present as an equilibrium
(60) Gaussian 03, ReVision C.02: Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.;
Vreven, T.; Kudin, N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida,
M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox,
J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, F.; Pomelli, C.; Ochterski,
J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg,
J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.;
Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.;
Al-Laham, A. M.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill,
P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian, Inc.: Wallingford, CT, 2004.
(59) Klevens, H. B.; Platt, J. R. J. Chem. Phys. 1949, 17, 470–481.
528 J. Org. Chem. Vol. 74, No. 2, 2009

Constitutional Isomers of 2,6-Dihydroxynaphthalene Carbaldehydes
1
ynaphthalene-2,6-dicarbaldehyde (4), and 2,6-Dihydroxynaph-
thalene-1,7-dicarbaldehyde (5). Sodium hydride (1.1 g, 45.8
mmol) and chloromethylmethyl ether (3.4 mL, 45.2 mmol) were
added to a DMF solution (40 mL) of 2,6-dihydroxynaphthalene
(3.2 g, 20 mmol) at -20 °C. The solution was stirred for 4 h at
room temperature. The solvent was evaporated, and to the residue
was added ca. 50 mL of water. By extraction with dichloromethane
from the water phase and subsequent concentration, 2,6-bis-
(methoxymethoxy)naphthalene was obtained as a colorless solid
(4.36 g, 88%). A diethyl ether solution (80 mL) of the methoxym-
ethyl derivative was cooled to -20 °C, and n-butyllithium (ca. 1
M in hexane, 41 mL) was added to the cooled solution over a period
of 20 min. The reaction solution was stirred for 8 h. Dimethylfor-
mamide (8 mL) was then added to the solution, and the solution
was stirred for an additional 13 h at room temperature. Aqueous
hydrochloric acid (3 N) was added until the pH of the aqueous
layer was 6. The diethyl ether layer was collected and concentrated
to precipitate the yellow crystalline solid of 3,7-bis(methoxymeth-
yl)naphthalene-2,6-dicarbaldehyde (4MOM). The filtrate was further
concentrated, and the resulting crude solid (80%) was separated
by column chromatography (hexane/ethyl acetate ) 6/4, on silica
gel) into a fraction containing 1MOM, 2MOM, 4MOM, and 6MOM and
a fraction containing 1MOM, 2MOM, and 3MOM. Each fraction was
treated with a mixture of aqueous hydrochloric acid and ethanol to
remove the methoxymethyl group, affording the corresponding 2,6-
dihydroxynaphthalenes (>90%). 1, 2, 4, and 6 were purified by
column chromatography (chloroform/methanol ) 10/1, on silica
gel). Isolation of 3 from a mixture of 3 and 4 was not successful.
The products were recrystallized from chlorobenzene, as necessity
arose. Yields (based on 2,6-bis(methoxymethoxy)naphthalene): 1
(42%), 2 (7%), 4 (10%), 3 + 4 (20%), and 6 (3%).
(calcd for C51H58N2O6 + H+ 795.4); H NMR (DMSO-d6) δ 4.67
(s, CH2, 2H), 7.08 (s, ArH, 2H), 7.35 (d (8.9 Hz), ArH, 2H), 7.48
(d (9.0 Hz), ArH, 2H), 8.71 (s, ArH, 2H), 10.00 (s, OH, 2H),
10.18(s, CHO, 2H), 10.20(brs, OH, 2H); 13C NMR (DMSO-d6) δ
20.5, 111.3, 120.9, 122.1, 123.4, 125.9, 127.4, 129.7, 133.0, 150.2,
152.9, 192.6; UV-vis (CHCl3) λ/nm (ε/M-1 cm-1) 308 (17600),
425 (4000).
1H NMR for 8 (CDCl3): δ 4.67 (s, CH2, 2H), 7.08 (s, ArH, 2H),
7.35 (d (8.9 Hz), ArH, 2H), 7.48 (d (9.0 Hz), ArH, 2H), 8.71 (s,
ArH, 2H), 10.00 (s, OH, 2H), 10.18(s, CHO, 2H), 10.20(brs, OH,
2H);
Synthesis of 2,6-Dihydroxynaphthalene-1,5-dicarbaldehyde
(3). 1,5-Bis(dimethylaminomethyl)-2,6-dihydroxynaphthalene (7)
was prepared according to ref 37. Mannich base 7 (6.60 g, 24 mmol)
was dissolved in 148 mL of aqueous acetic acid (81 vol%), and
9.45 g (67.5 mmol) of hexamethylenetetramine was added to the
solution. The mixture was stirred at 130 °C for 4 h. After the
mixture was cooled to room temperature, the precipitate was
collected by filtration and washed with methanol. Aqueous hydro-
chloric acid (72 g, 4.5 N) was added to the filtrate, which was stirred
at 90 °C an additional 1.5 h. The resulting precipitate was collected
by filtration and recrystallized from chloroform to remove impurities.
3: yellow needles (mp 239-240 °C); IR (KBr) 3432 (νO-H), 1639
(νCdO) cm-1; precise MS (EI+) m/z 216.0433 (calcd for 3+
1
216.0423); H NMR (DMSO-d6) δ 7.35 (d (9.4 Hz), ArH, 2H),
9.18 (d (9.4 Hz), ArH, 2H), 10.76 (s, CHO, 2H), 11.59 (br, OH,
2H); 13C NMR (DMSO-d6) δ 113.3, 121.3, 125.8, 132.4, 161.5,
192.6; UV-vis (CHCl3) λ/nm (ε/M-1 cm-1) 308 (13600), 319
(14300), 389 (7300), 404 (7800). Anal. Calcd for C12H8O4: C, 66.67;
H, 3.73. Found: C, 66.44; H, 3.59.
1: yellow needles (mp 193-194 °C); IR (KBr) 3381 (νO-H), 3234
Crystallographic Data. For X-ray diffraction of single crystals,
data were collected on a MacScienceDIPLabo Imaging Plate
diffractometer, λ(Cu KR) ) 1.5418 Å. The structure was solved
by direct methods and expanded using Fourier techniques. All
calculations were performed with the crystallographic software
package SHELX-97.
3: C12H8O4, MW ) 216.18, monoclinic, a ) 4.3980(2) Å, b )
12.2080(6) Å, c ) 8.6590(5) Å, ꢀ ) 102.870(4)°, V ) 453.23(4)
Å3, Dcalcd ) 1.584 g/cm3, T ) 193 K, space group P21/c (#14), Z
) 2, µ(Cu KR) ) 10.5 cm-1, 4150 reflections measured and 747
unique (2θmax ) 146.9°, Rint ) 0.024), which were used in all
calculations. R ) 0.062, Rw ) 0.181.
4: C12H8O4, MW ) 216.18, monoclinic, a ) 4.8840(5) Å, b )
12.39640(7) Å, c ) 15.3700(17) Å, ꢀ ) 92.857(8)°, V ) 447.14(9)
Å3, Dcalcd ) 1.606 g/cm3, T ) 193 K, space group P21/n (#14), Z
) 2, µ(Cu KR) ) 10.29 cm-1, 4324 reflections measured and 623
unique (2θmax ) 146.9°, Rint ) 0.032), which were used in all
calculations. R ) 0.106, Rw ) 0.311.
(νO-H), 1626 (νCdO) cm-1; precise MS (EI+) m/z 188.0470 (calcd
1
for 1+ 188.0473); H NMR (DMSO-d6) δ 7.13 (d (2.2 Hz), ArH,
1H), 7.14 (d (9.0 Hz), ArH, 1H), 7.17 (dd (2.6, 9.2 Hz), ArH, 1H),
7.93 (d (9.0 Hz), ArH, 1H), 8.79 (d (9.1 Hz), ArH, 1H), 9.65 (s,
OH, 1H), 10.75(s, CHO, 1H), 11.60 (br, OH, 1H); 13C NMR
(DMSO-d6) δ 110.6, 112.7, 118.9, 120.8, 123.7, 125.3, 129.2, 136.9,
153.8, 161.7, 192.6; UV-vis (CHCl3) λ/nm (ε/M-1 cm-1) 320
(7900), 388 (5000).
2: yellow needles (mp 185-186 °C); IR (KBr) 3461 (νO-H), 3240
(νO-H), 1659 (νCdO) cm-1; precise MS (EI+) m/z 188.0472 (calcd
1
for 2+ 188.0473); H NMR (DMSO-d6) δ 7.15 (dd (2.5, 8.9 Hz),
ArH,1H), 7.19 (d (2.36 Hz), ArH, 1H), 7.21 (s, ArH, 1H), 7.64 (d
(8.9 Hz), ArH, 1H), 8.13 (s, ArH, 1H), 9.67 (s, ArOH, 1H), 10.21
(s, OH, 1H), 10.36 (s, CHO,1H); 13C NMR (DMSO-d6) δ 109.9,
110.9, 122.5, 124.1, 127.5, 128.1, 129.9, 132.1, 153.4, 153.6, 192.5;
UV-vis (CHCl3) λ/nm (ε/M-1 cm-1) 288 (13300), 303 (15400),
414 (2500).
4: red plates (dec around 270 °C); IR (KBr) 3301 (νO-H), 1659
(νCdO) cm-1; precise MS (EI+) m/z 216.0434 (calcd for 4+
216.0423); 1H NMR (DMSO-d6) δ 7.41 (s, ArH, 1H), 8.20 (s, ArH,
1H), 10.42 (s, CHO, 1H), 10.46 (s, OH, 1H); 13C NMR (DMSO-
d6) δ 112.4, 126.7, 129.2, 131.0, 153.9, 192.0; UV-vis (CHCl3)
λ/nm (ε/M-1 cm-1) 303 (22800, shoulder), 314 (33000), 476 (2900).
6: brownish orange needles (dec around 320 °C); IR (KBr) 3275
(νO-H), 1655 (νCdO) cm-1; precise MS (EI+) no obvious peak
detected. Instead, the condensation product (8, see the Supporting
Information) with p-octyloxyaniline gave FAB MS(+) m/z 795.3
1
Supporting Information Available: The assignment of H
and 13C NMR chemical shifts, including the spectra of new
compounds (2, 4, 6, and 8), crystallographic information format
(CIF) files for 3 and 4, and numerical data for ab initio
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
JO802345F
J. Org. Chem. Vol. 74, No. 2, 2009 529