Revision of the structure and stereochemistry of the 2-naphthol-acetaldehyde 2:2 condensation product. A study by X-ray crystallography and the AM1 method

Article abstract of DOI:10.1002/poc.355

The structure of the 2:2 condensation product of 2-naphthol and acetaldehyde, 1-(1-methyl-2,3-dihydro-1H-benzo[f]chromen-3-yl)-2-naphthol, revised by X-ray crystallography, is presented. A moderate intramolecular hydrogen bond is found between the phenolic OH and chromene oxygen atom both in the solid state and in solution. In the crystal structure, the naphthyl groups are oriented in parallel with a π-π stacking distance of 354 pm. AM1 studies are used to explain the formation of one of the four possible stereoisomers for the title compound. Comparison of the experimental and calculated coupling constants indicates that the molecule adopts exclusively the solid-state structure in solution. Copyright

Full text of DOI:10.1002/poc.355

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2001; 14: 210–216
Revision of the structure and stereochemistry of the
2
-naphthol±acetaldehyde 2:2 condensation product.
A study by X-ray crystallography and the AM1 method
1
1
2
ParvizRashidi-Ranjbar, * Ali Mohajeri and Amir Hossein Mahmoudkhani
1
Department of Chemistry, Faculty of Science, Tehran University, Tehran, Iran
Department of Inorganic Chemistry, G oÈ teborg University, S-41296 G oÈ teborg, Sweden
2
Received 20 June 2000; revised 20 October 2000; accepted 23 November 2000
ABSTRACT: The structure of the 2:2 condensation product of 2-naphthol and acetaldehyde, 1-(1-methyl-2,3-
dihydro-1H-benzo[ f ]chromen-3-yl)-2-naphthol, revised by X-ray crystallography, is presented. A moderate
intramolecular hydrogen bond is found between the phenolic OH and chromene oxygen atom both in the solid
state and in solution. In the crystal structure, the naphthyl groups are oriented in parallel with a p–p stacking distance
of 354 pm. AM1 studies are used to explain the formation of one of the four possible stereoisomers for the title
compound. Comparison of the experimental and calculated coupling constants indicates that the molecule adopts
exclusively the solid-state structure in solution. Copyright  2001 John Wiley & Sons, Ltd.
KEYWORDS: 2-naphthol; acetaldehyde; phenols; X-ray crystallography; AM1 calculations; intramolecular
hydrogen bonding; p–p stacking; condensation
INTRODUCTION
obtained by using either paraldehyde or acetaldehyde
diethyl acetal.
1
0,11
The reaction of substituted phenols and aldehydes under
controlled conditions has been used to build up supramol-
ecular compounds, the most important ones being
We have become interested in the 2-naphthol–
acetaldehyde 2:2 condensation product as a possible
building block for constructing some types of supramol-
1–5
12,13
calixarenes.
2-Naphthol reacts with aromatic alde-
ecular compounds such as tweezers.
Careful exam-
hydes to produce 14-aryl-14H-dibenzo[a,j]xanthenes,
which could be used as anti-inflamatory agents. One of
ination of the spectroscopic data of the condensation
product, especially the sharp band at 3300 cm in the
6
À1
the first reports on the reaction of 2-naphthol with
Fourier transform (FT) IR spectrum, indicated an intra-
molecular hydrogen bond, difficult to be present in
structure 2 as the model indicates. To solve the problem
with certainty, a single crystal of the condensation
product was prepared and used for X-ray crystallographic
determination of the structure; it turned out that the
structure is 4 (Fig. 1) and not 2. The results of the studies
are reported in this paper. The stereochemistry of the
formation of the condensation product is taken into the
consideration and addressed by AM1 calculations.
aliphatic aldehydes was on the product of the reaction of
7
2
-naphthol with acetaldehyde by Claisen. He reported
that 2-naphthol and paraldehyde react to form a
compound to which he attributed the structure 1, the 1-
[1-(2-hydroxy-1-naphthyl)ethyl]-2-naphthol or dinaphthyl
acetal of acetaldehyde (Fig. 1), m.p. 200–201°C. About a
8
century later Van Allen et al. under similar conditions
separated a compound, m.p. 198–200°C, to which they
assigned the structure 2, 1-(3-methyl-2,3-dihydro-1H-
benzo[ f ]chomen-1-yl)-2-naphthol (Fig. 1). It was shown
later that the structure of the condensation product
depends on the ratio of 2-naphthol to acetaldehyde. With
a molar ratio of 1:1.3 (2-naphthol:acetaldehyde), the same
product (2) is obtained, whereas with a ratio of 2:1 a
mixture of 2 and 14-methyl-14H-dibenzo[a,j]xanthene (3)
RESULTS AND DISCUSSION
Crystal structure of 1-01-methyl-2,3-dihydro-1H-
benzo[f ]chromen-3-yl)-2-naphthol 04)
9
is obtained (Fig. 1). The dibenzo[a,j]xanthene 3 has been
Compound 4 crystallizes in the monoclinic system with
space group P2 /c (No. 14). The asymmetric unit cell
1
*
Correspondence to: P. Rashidi-Ranjbar, Department of Chemistry,
consists of one crystallographically independent mol-
ecule (Fig. 2). The angle between the plane of the two
naphthyl rings in 4 is 56.8° and the distance between the
Faculty of Science, Tehran University, Tehran, Iran.
E-mail: ranjbar@khayam.ut.ac.ir
Contract/grant sponsor: Research Council of Tehran University.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 210–216

STRUCTURE OF 2-NAPHTHOL–ACETALDEHYDE CONDENSATION PRODUCT
211
7
8
9
Figure 1. Proposed structures of 1, 2, by-product 3 and 4 )this workꢀ
plane of parallel naphthyl rings of the molecules in
˚
successive layers in the crystal structure is 3.54 A (Fig.
3
1
4
), a value reported to be suitable for p–p stacking.
In the molecular structure of 4, the naphthoyl group at
the 3-position is arranged such that the second aromatic
ring of the 2-naphthoyl group is on the upper side of the
chromene six-membered ring and close to the methyl
group at the 1-position. The shortest HH distance of
…
˚
naphthoyl-Me [H(29) H(41)] is 2.66 A (Fig. 4). The
phenolic OH group is located on the opposite side to the
methyl group in such a way as to make a moderate
15
internal hydrogen bound to the oxygen atom in the
chromene six-membered ring (see Fig. 4). The distances
…
…
of the donor-H, acceptor H, donor acceptor and
…
donor-H acceptor angles are presented in Table 1
together with the AM1-calculated values. The selected
bond lengths and bond angles are listed in Table 2 and
compared with the AM1-calculated values.
STEREOCHEMISTRY
Figure 2. The unit cell in crystal structure of 4
The conformation of the chromene six-membered ring in
4
is half-chair with the methyl and 2-naphthoyl groups in
a trans orientation. Furthermore, the methyl group at the
-position is pseudoaxial and 2-naphthoyl group at the 3-
1
position is pseudoequatorial, henceforth denoted 4a'e'.
Efforts to obtain the cis diastereomer of 4 was not
Figure 3. The p±p stacking of the naphthyl group in the
solid-state structure of 4
Figure 4. X-ray structure of 4; the methyl and naphthoyl
substituents are in a trans orientation
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 210–216

2
12
P. RASHIDI-RANJBAR, A. MOHAJERI AND A. H. MAHMOUDKHANI
Table 1. The experimental )X-rayꢀ hydrogen bond distances
Formation of products similar to 3 and 6 has been
reported before.
To find out the reason behind the observed stereo-
chemistry, the heats of formation and the structure of
Ê
10,11,16
)
Aꢀ and OHO angles )°ꢀ and the values calculated by the AM1
a
method in 4a'e'.
… …
d(O—H) d(OH O) d(O O) OHO angle
Method
4
5
a'e' and the most stable form of the cis diastereomer
e'e' were calculated by the AM1 semiempirical
X-ray
AM1
0.82
0.97
1.95
2.03
2.64
2.73
142.2
127.4
17
method. The ring-inverted conformers of 4a'e' (i.e.
4e'a') and 5e'e' (i.e. 5a'a') were also included in the
AM1 studies (see Fig. 5). The heats of formation and
selected dihedral angles of 4a'e', 4e'a', 5e'e' and 5a'a' are
given in Table 3, together with the values for dihedral
angles from the X-ray structure. As indicated in Table 3,
a
It should be noted that the locations of hydrogen atoms are not
accurately determined by X-ray crystallography.
Ê
Table 2. Selected bond lengths )Aꢀ and bond angles )°ꢀ in
4
lographic data
a'e': comparison of computed )AM1ꢀ with X-ray crystal-
the difference in heats of formation of 4a'e'–5e'e' and
À1
4
a'e'–5a'a' are 1.6 and 3.2 kcal mol , respectively
a
(1 kcal = 4.l84 k J). The difference in the H values of
f
Bond length or angle
O —C
X-ray
AM1
4a'e' and 5e'e' at equilibrium corresponds to a population
1.447
1.519
1.529
1.520
1.373
1.394
1.523
109.4
110.4
109.7
121.4
123.1
115.6
115.5
1.446
1.523
1.523
1.495
1.385
1.383
1.518
4
3
ratio of 9:1, so it is expected that 5e'e' would be formed if
the reaction proceeds under thermodynamic control
conditions. trans-4a'e' and cis-5e'e' may be formed by
an electrophilic aromatic substitution of 2-naphthol by
the oxenium 7 (Fig. 6), as an intermediate (see Scheme
C —C
3
2
C —C
1
2
1
C —C
13
C —C
14
1
3
C —O
4
1
4
C —CH
1
3
1). The formation of an oxenium intermediate such as 7
O —C —C
110.8
4
3
2
from hemiacetals under acidic conditions is established in
C —C —C
111.0
110.6
120.6
124.6
115.2
110.7
3
2
1
18
the litrature. The stereochemistry of 7 should control
the stereochemistry of the s-complex intermediates and
products. The methyl group in 7 could be oriented in
either a pseudoaxial or pseudoequatorial position, but
starting from different geometries ends up with the one
having a methyl group in a pseudoaxial position (Fig. 6).
Two different s-complexes could be formed by the
attack of 7 on 2-naphthol, a trans-8 and a cis-9 complex
C —C —C
2
1
13
C —C —C
14
1
13
C —C —O
1
3
14
4
C —O —C
1
4
4
3
C —C —CH
2
1
3
a
The third digit in the bond lengths calculated by AM1 is included for
comparison with X-ray values.
(
Fig. 6). The relative stabilities of 8 and 9 were calculated
À1
successful; by variation of the reaction conditions a
mixture of 3, 4 and 6 was obtained (see Scheme 1).
by the AM1 method. It is calculated that 8 is 2 kcal mol
more stable than 9 (Table 4). Also, the dipole moment
1
9
Scheme 1
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 210–216

STRUCTURE OF 2-NAPHTHOL–ACETALDEHYDE CONDENSATION PRODUCT
213
Figure 5. All possible stereoisomers of 4 and 5, calculated by the AM1 method
À1
Table 3. Computed )AM1ꢀ heats of formation )H ꢀ )kcal mol ꢀ and comparison of selected dihedral angles )°ꢀ in all possible
f
stereoisomers of 4 and 5 with X-ray structures )for numbering see Fig. 4ꢀ
O —C — C —C — C —C —
C —C —
C —C —
C —O —
O —C —
4 3
4
14
14
C —C
13
13
C —C
1
1
2
2
3
3
4
Compound
X-ray
C —C
C —O
O —C
C —C
C —C
Hf
1
3
1
1
2
2
3
3
4
4
14
14
13
16
17
À3.0
À2.1
10.2
6.2
17.0
17.3
3.5
À45.9
À45.1
À33.6
À25.1
À45.2
63.3
60.3
51.2
56.7
30.3
À49.0
À44.6
À38.0
À57.0
4.7
19.6
16.3
8.5
26.5
À24.6
37.1
40.1
—
À0.68
6.40
0.94
2.53
4a'e'
4e'a'
5e'e'
5a'a'
À11.7
À5.8
41.6
6.6
27.9
À25.4
of 8 is calculated to be one 1 D greater than that of 9
Table 4). It is expected that in a polar solvent such as
ethanol–water, used for running the reaction, 8 is better
solvated than 9 and the difference in their relative
stabilities increases in favor of 8, which might result in
the exclusive formation of 4a'e'. Complex 8 is converted
inverted to the more stable form 5e'e'. Ring inversion in
(
the cyclohexene ring is found in the range 5–
À1 20,21
6 kcal mol .
The question of conformational equilibrium between
two conformers (4a'e' and 4e'a') in solution was
addressed by comparison of the experimental and
computed coupling constants. For this purpose, the
dihedral angles between aliphatic protons in the chro-
to 4a'e' but 9 is converted to 5a'a', which is
À1
3
.2 kcal mol less stable than 4a'e'. 5a'a' could be ring
Figure 6. The structure of oxenium 7 and sigma complexes 8 and 9 computed by the AM1 method
Copyright  2001 John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2001; 14: 210–216

2
14
P. RASHIDI-RANJBAR, A. MOHAJERI AND A. H. MAHMOUDKHANI
Table 4. Calculated heats of formation and dipole moments
of s-complexes
122.07, 122.33, 124.16, 126.46, 126.63, 128.00, 128.59,
28.67, 128.93, 129.67, 132.28, 132.66, 151.44, 152.53.
1
Heat of formation
Dipole moment
(D)
À1
Complex
(kcal mol
)
Crystal structure determination. A colorless crystal with
dimensions 0.40 Â 0.12 Â 0.10 mm was used for data
collection at 297 K using a Siemens SMART CCD
8
9
162.5
164.5
10.7
9.5
˚
diffractometer with Mo K radiation (ꢁ = 0.71073 A,
Table 5. Comparison of the experimental and calculated coupling constants )Hzꢀ between H1a±H2a, H1a±H2b, H2a±H3 and
a
H2b±H3 protons in 4a'e' and 4e'a' conformational stereoisomers
3
3
3
3
Compound ÈH1a–H2a
Jcalc
Jexp ÈH1a–H2b Jcalc
Jexp
ÈH2b–H3
Jcalc
Jexp
ÈH2a–H3
Jcalc
Jexp
4
a'e'
e'a'
75.9
À37.2
1.30
6.92
1.5
—
À42.5
5.76
À153.9 10.36
5.0
—
66.0
À76.7
1.59
0.89
2.1
—
À176.6
11.27
6.91
12.3
—
4
39.8
a
Torsion angle for 4a'e' is derived from the X-ray structure and for 4e'a' from the AM1 calculations. For the assignment of protons, see Fig. 4.
mene six-membered ring of 4a'e' and 4e'a' derived from
the x-ray structure (for 4a'e') and estimated by the AM1
graphite monochromator). A full sphere of reciprocal
lattice was scanned in 0.3° steps in with a crystal-to-
detector distance of 3.97 cm and an exposure time of 60 s
per frame; for details, see Table 6. A preliminary
orientation matrix was obtained from the first frames
using SMART. The collected frames were integrated
using the preliminary orientation matrix which was
updated every 100 frames. Final cell parameters were
1
method (for 4e'a') were used to calculate the H NMR
22
coupling constants by using Hassnoot et al.’s equation
Table 5). The estimated coupling constants indicate that
e'a' should not be present in equilibrium with 4a'e'.
(
4
EXPERIMENTAL
All reagents were obtained from Merck and used without
further purification. Uncorrected melting-points were
determined with a Buchi apparatus. H and C NMR
spectra were recorded on a Bruker 400 or 80 MHz
spectrometer. FT-IR spectra were obtained with a
Shimadzu 4300 spectrophotometer.
Table 6. Crystallographic and re®nement data for 4
1
13
Empirical formula
C H O
24 20 2
Formula weight
340.40
Crystal system, space group
Monoclinic, P2 /c (No. 14)
1
˚
a (A)
7.56720(10)
7.97840(10)
28.80290(10)
92.4410(1)
1737.37(3)
4
˚
b (A)
˚
c (A)
Synthesis of benzo[f]chromene )4ꢀ.
(
A
2.00 g
b (°)
3
˚
13.9 mmol) amount of 2-naphthol was dissolved in
V (A )
Z
1
0 ml of 1:1 ethanol–concentrated HCl and 0.62 g
T (K)
297(2)
(
14 mmol) of acetaldehyde was added. The reaction
À3
Density (calc.) (g cm
)
1.301
mixture was heated in an oil-bath at 70°C for 2h. The
resulting oily brown liquid was separated and triturated
with small amounts of diethyl ether and the precipitate
was filtered off. The white precipitate washed with small
amounts of cold diethyl ether and dried under vacuum. A
mixture of 3, 4 and 6 was obtained, which was
chromatographed on silica gel with light petroleum–
ethyl acetate (80:20) as eluent, to give 4 in 80% yield,
À1
m (mm
)
0.081
F (000)
720
ꢂ range (°)
1.42–28.28
À10 ꢀ h ꢀ 10
À10 ꢀ k ꢀ 10
À38 ꢀl ꢀ 38
23177
Index ranges
Reflections collected
Independent reflections
4312
R(int.)
0.0416
3021
4312/237
1.036
À1
Reflections obs. [I 4 2s(I)]
m.p. 205°C. FT-IR (KBr): ꢀ cm 3350 s; 3062 w, 2960
Data/restraints/parameters
w; 1622 s, 1591 s, 1514 s, 1458 s; 1398 s, 1240 b, 1215 s,
2
1
G
ooF (F
o
)
9
56 b, 800 b, 742 b. H NMR (DMSO-d , 400 MHz):
a
6
R1/wR2 for obs. reflections
R1/wR2 for all data
Weighting scheme x/y
Larg. res. peak/hole (e, A
0.0495/0.1083
0.0800/0.1233
0.0467/0.4993
0.199/À0.174
1
(
ppm) 1.50 (d, J = 6.9 Hz, 3H); 1.88 (d, J = 13.5 Hz, 1H);
.84 (dt, J = 13.5 and 5.2 Hz, 1H); 3.68 (m, 1H); 6.37 (dd,
J = 12.44 and 2.30 Hz, 1H); 8.23–7.00 (m, 12H); 9.97 (s,
b
2
À3
˚
)
13
1
H). C NMR (DMSO-d , 80MHz): (ppm) 22.33, 25.49,
a
2
2 2
2 2
o
2
6
R1 = SjjF j À jF jj/SjF j, wR2 = {S[w(F À F ) ]/S(F ) } .
o
c
o
o
c
b
2
2
2
2
2
3
2.89, 67.56, 116.15, 117.37, 118.59, 119.02, 122.03,
w = 1/[s (F
o
) + (xP) + yP], P = (F
o
+ 2F
c
)/3.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 210–216

STRUCTURE OF 2-NAPHTHOL–ACETALDEHYDE CONDENSATION PRODUCT
215
Figure 7. Computed )AM1ꢀ internal hydrogen bonds with axial-and equatorial -o xygen lone pairs in 4a'e' and 5e'e'
obtained by refinement on the positions of 8192
membered ring was selected and the methyl at the 1-
reflections with I 4 10s(I) after integration of all the
frame data using SAINT. The data were empirically
position and the naphthoyl at the 3-position were put
either in pseudoaxial or pseudoequatorial orientation.
The conformational space for the methyl and naphthoyl
in each orientation was searched by rigid rotation. The
local minima thus found were subjected to AM1
calculations. Two different orientations for the hydrogen
bond were found possible for 4a'e' and 5e'e', one between
the OH and the axial lone pair of oxygen (4a'e'-a and
5e'e'-a) and the other between the OH and the equatorial
lone pair of oxygen (4a'e'-e and 5e'e'-e), the former being
the more stable. The different orientations of hydrogen
bonds in 4a'e' and 5a'e' are given in Fig. 7.
23
corrected for absorption and other effects using SA-
24
DABS (min./max. transmission = 0.9682/0.9919) based
25
on the method of Blessing. The structure was solved by
the direct method and refined by full-matrix least-squares
2
26
analysis on all F data using SHELXTL. The non-H
atoms were refined anisotropically and the hydrogen
atoms isotropically with restrained bond distances.
Computational. Initial estimates of the geometry of
structures, for semiempirical calculations, were obtained
by the MMX molecular mechanics method implemented
in PCMODEL software (Serena Software, Bloomington,
IN, USA). Full minimization was done by using the
SUPPLEMENTARY DATA
17
semiempirical AM1 Hamiltonian, implemented in the
27
MOPAC 6.0 program. All the structures were char-
acterized as stationary points and true minima on the
potential energy surface using the FORCE keyword. A
stationary point is described if the first derivatives of the
energy with respect to changes in the geometry are zero.
The criterion for a minimum is that all eigenvalues of the
Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with
the Cambridge Crystallographic Center as supplementary
publication No. CCDC 143902. Copies of the data can be
obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK. Fax, (44) 1223
336 033; E-mail, depositccdc.cam.ac.uk.
The *.arc files of all the AM1 calculations for the given
structures are available from the corresponding author via
E-mail.
28
Hessian matrix are postive. AM1 was chosen as it has
been shown to produce the geometry of internal hydrogen
29
bonds better than PM3.
A half-chair conformation of the chromene six
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 210–216

2
16
P. RASHIDI-RANJBAR, A. MOHAJERI AND A. H. MAHMOUDKHANI
1
3. Jeong KS, Tjivikua T, Muehidorf A, Deslongchamps G, Famulok
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University Press: New York, 1997.
1
1
Parviz Rashidi-Ranjbar is grateful to the Research
Council of Tehran University for financial support of
this work. We are indebted to the Department of
Inorganic Chemistry, G o¨ teborg University, for providing
X-ray structure determination facilities.
1
1
6. Dziewonski PM. Bull. Int. Acad. Polon. A 1933; 560.
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