Iron(II) and zinc(II) monohelical binaphthyl-salen complexes with overlapping benz[a]anthryl sidearms

Article abstract of DOI:10.1039/b700001d

The synthesis of the polyaromatic aldehyde 1-hydroxybenz[a]anthracene-2- carboxaldehyde is reported via a seven step protocol from 9,10- dihydroanthracene, with an overall yield of 30%. Two equivalents of the aldehyde are condensed with (R)-1,1′-binaphthyl-2,2′-diamine to produce a new binaphthyl-salen ligand, which is subsequently complexed to iron(ii) and zinc(ii) ions. The ligand and complexes are characterized by single-crystal X-ray crystallography. The complexes have distinct helical structures with overlapping benz[a]anthryl sidearms, and only M-helices are observed. The ligand and complexes are further characterized by solution ¹H and ¹³C NMR spectroscopy as well as UV-visible and ECD spectroscopies. These studies indicate that there is a single component in solution, consistent with the solid state characterization. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b700001d

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Iron(II) and zinc(II) monohelical binaphthyl-salen complexes with overlapping
benz[a]anthryl sidearms†
Alexander V. Wiznycia, John Desper and Christopher J. Levy*
Received 2nd January 2007, Accepted 14th February 2007
First published as an Advance Article on the web 12th March 2007
DOI: 10.1039/b700001d
The synthesis of the polyaromatic aldehyde 1-hydroxybenz[a]anthracene-2-carboxaldehyde is reported
via a seven step protocol from 9,10-dihydroanthracene, with an overall yield of 30%. Two equivalents of
the aldehyde are condensed with (R)-1,1ꢀ-binaphthyl-2,2ꢀ-diamine to produce a new binaphthyl-salen
ligand, which is subsequently complexed to iron(II) and zinc(II) ions. The ligand and complexes are
characterized by single-crystal X-ray crystallography. The complexes have distinct helical structures
with overlapping benz[a]anthryl sidearms, and only M-helices are observed. The ligand and complexes
are further characterized by solution 1H and 13C NMR spectroscopy as well as UV-visible and ECD
spectroscopies. These studies indicate that there is a single component in solution, consistent with the
solid state characterization.
Introduction
The helix is one of the most important chiral motifs in natural
systems and there is an increasing interest in the development
of helical transition metal complexes and related supramolecular
helical structures.1 There is an extensive literature concerning
complexes with two or more metal centers, but mononuclear
helical complexes (monohelices) have received considerably less
attention. We are particularly interested in monohelices formed
by wrapping a single multidentate chiral ligand around a metal
center. The high asymmetry of monohelical complexes make them
attractive candidates as asymmetric catalysts, as has been explored
in several studies.2
Fig. 1 Monohelical systems from previous studies.
Monohelices can be difficult to prepare, since multidentate
Results and discussion
ligands often prefer to bridge metal centers and produce helicates.
The preference derives from the exact geometric relationship
between the donor atoms and the flexibility of the spacers between
them.3 If the donors can orient themselves to form strong binding
interactions with a single metal and if the ligand is pliable enough
to allow for wrapping without strong steric repulsions, then a
monohelical complex is likely.
This contribution focuses on employing chiral ligands to
produce monohelices of only one helical form (M or P). We
recently reported monohelical salen complexes constructed from
rigid phenathryl sidearms attached to a helix-directing (R)-
binaphthyl backbone (Fig. 1).4 In addition we have shown
that monohelical salen complexes with significantly overlapping
sidearms can be generated using the (1R,2R)-cyclohexyl backbone
and benz[a]anthracene sidearms (Fig. 1).5 Herein we detail a new
system that combines the rigid 1,1ꢀ-binaphthyl backbone and the
extended benz[a]anthryl sidearms.
Synthesis
In a previous study,4 we synthesized a phenanthryl aldehyde
precursor by a five step procedure that started with Friedel–
Crafts acylation of naphthalene. The analogous route to the
benz[a]anthryl precursor via the initial alkylation of anthracene is
not feasible, since reaction occurs predominantly at the 9-position:
only 11% of the desired 2-substituted product could be obtained
by this route (eqn (1)).
Department of Chemistry, Kansas State University, 111 Willard Hall,
Manhattan, KS, USA 66506. E-mail: clevy@ksu.edu; Fax: (785) 532-6666;
Tel: (785) 532-6688
† Electronic supplementary information (ESI) available: Full crystal-
lographic data (CIF) and NMR spectra of compounds. See DOI:
10.1039/b700001d
In order to circumvent this problem, 9,10-dihydroanthracene,
1, was used as an entry point into the four-ring system (Scheme 1).
Now, substitution is directed to the 2-position, giving 2, while
the 9-position is inert to electrophilic attack. The alkylation is
accompanied by a 3–5% oxidation to give the fully-aromatized
1520 | Dalton Trans., 2007, 1520–1527
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Scheme 2 Synthesis of (R)-9 and its complexes.
both the ligand and its sodium salt, which is formed in situ. The
iron(II) complex, (R)-11, was highly sensitive to air and water,
while the zinc(II) complex, (R)-10, was air stable but hydrolyzed in
wet solvents.
Structural characterization
Crystal data for the structures presented in this article are provided
in Table 1. Selected bond lengths, bond angles and torsion angles
are presented in Table 2.
a
Single crystals of the ligand were grown by diffusion of hexanes
into a methylene chloride solution of (R)-9. The structure consists
of a single, approximately C2 symmetric, molecule (Fig. 2).10 The
naphthyl and benz[a]anthryl groups, which comprise each half
of the structure, are extensively p-delocalized and show near
co-planarity as indicated by the interplanar angles of 7.0◦ and
7.6◦. Further stabilizing this arrangement are hydrogen-bonding
contacts, which occur between the hydroxyl moieties and imine
nitrogen atoms. For each of these interactions the O–H · · · N
Scheme 1 Synthesis of the key precursor 8. Includes 3–5% of the fully
aromatized analogue.bIncludes 10% of the fully aromatized analogue.
cIncludes 15% of the aldehyde 8.
anthracene analog. No attempt was made to separate this side
product since it will ultimately converge with the main product in
a subsequent dehydrogenation step.
With the sidechain installed on the anthracene framework,
it must now be reduced and cyclized. Reduction of the ketone
was accomplished using standard Wolf–Kishner conditions,6
affording 3. At this point, all attempts at cyclization to form the
fourth ring gave primarily the linear ketone due to unfavorable
regioselectivity.7 This necessitated the aromatization of 3 prior
to cyclization. Prior to this step 3 was converted to its methyl
ester, 4, by acid catalyzed condensation. This prevents catalyst
deactivation8 in the subsequent dehydrogenation by refluxing in
a high boiling solvent over Pd/C to give 5. Treatment of this
product in hot methanesulfonic acid produces the cyclized ketone,
6, in quantitative yield. Condensation of 6 with ethyl formate
gives predominantly 7, but also 15% of the final product, 8. The
remainder of 7 can be readily oxidized using triphenylmethanol in
TFA9 to give the desired aldehyde, 8, in a respectable 30% yield
over seven steps.
˚
distance is 2.497(5) A and the N · · · H contact is calculated to
◦
˚
be 1.75 A. The angle between the naphthyl planes is 108.1 , well
within the broad ∼60–130◦ low energy well for binaphthyl torsion
angles.11
Fig. 2 Thermal ellipsoid (50%) plot of (R)-9.
With aldehyde 8 prepared, the ligand (R)-9 could be synthe-
sized by condensation of 8 with (R)-1,1ꢀ-binaphthyl-2,2ꢀ-diamine
(Scheme 2). The ligand was metallated with FeCl2 and ZnCl2,
employing sodium methoxide as the requisite base (Scheme 2).
For the solvent system, a mixture benzene and ethanol was chosen:
the combination was necessary in order to sufficiently solubilize
Bright yellow single crystals of [(R)-10]2·2CH2Cl2·Et2O were
grown by diffusion of diethyl ether into a methylene chloride
solution of the complex. There are two distinct molecules of (R)-
10 in the asymmetric unit (Fig. 3), with the methylene chlorides of
crystallization sandwiched between them. The two molecules are
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Table 1 Crystal data for the ligand (R)-9 and its metal complexes, (R)-10 and (R)-11
Compound
Formula
M
(R)-9·CH2Cl2
C59H38Cl2N2O2
877.81
Orthorhombic
8.8321(14)
11.5748(18)
42.316(7)
90
[(R)-10]2·2CH2Cl2·Et2O
C122H82Cl4N4O5Zn2
1956.46
Monoclinic
12.5693(17)
32.926(4)
12.6967(17)
90
119.512(2)
90
4572.9(11)
P21
2
100(2)
0.705
29596
15636
0.0270
0.0547
0.1475
0.0572
(R)-11·Et2O
C62H44FeN2O3
920.84
Trigonal
12.6954(3)
12.6954(3)
24.3493(10)
90
90
120
3398.68(18)
P32
3
100(2)
0.385
39455
13200
0.0447
0.0524
0.1178
0.0622
0.1231
1.017
Crystal system
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
90
90
3
˚
Unit cell vol./A
Space group
Z
T/K
l/mm−1
N
4325.9(12)
P212121
4
100(2)
0.200
24399
7579
0.0467
0.0928
0.1882
0.0957
0.1896
1.353
Nind
Rint
a
R1 (I > 2r(I))
a
wR2 (I > 2r(I))
R1 (all data)
wR2 (all data)
GoF
0.1504
1.335
Flack parameter
0.04(17)
0.029(11)
0.004(14)
ꢀ
ꢀ
ꢀ
ꢀ
1/2
a R1 = ꢁF0| − Fcꢁ/ |F0| for F0>2r(F0) and wR2 = { [w(F2 − F2c)2]/ [w(F2c)]}
.
o
very similar, both having M helicity and four coordinate zinc in
a distorted tetrahedral environment. The molecules are stacked
and are rotated by ca. 55◦ with respect to each other. Fig. 3
shows the thermal ellipsoid plot of the arrangement and Fig. 4
shows a space-filling representations of the molecule containing
Zn1. There is clearly a helical structure with overlapping ligand
sidearms. The interplanar angle between the naphthyl fragments
is 78.5◦, significantly smaller than in the free ligand, but still
Fig. 4 Two views of the space-filling model (50%) of (R)-10.
within the region where binaphthyl-based steric repulsions are
relatively low.11 The interplanar angles between the sidearms are
34.0◦ and 35.2◦ for the two molecules, somewhat smaller that the
47.3◦ angle seen for the complex with phenanthryl sidearms (and
a five coordinate zinc center).4
The FeII complex, (R)-11, was isolated as an air and moisture
sensitive paramagnetic dark brown powder. Diffusion of diethyl
ether into a methylene chloride solution of (R)-11 gave single
crystals of (R)-11·Et2O suitable for X-ray analysis. The crystal
structure reveals an M helix (Fig. 5) with a structure very similar
to that of (R)-10: the interplanar angle between the naphthyl
groups is 82.7◦ and the angle between the sidearm planes is 34.7◦.
This latter angle is significantly smaller that the 74.5◦ seen for the
analogous FeII complex with phenanthrene-based sidearms.4 This
can be explained by the added steric requirements of the extended
sidearms in (R)-11: in order to avoid edge-to-face repulsions the
planar fragments cannot be severely twisted with respect to one
another and must adopt a more stacked arrangement.
Fig. 3 Thermal ellipsoid plot (50%) of [(R)-10]2·2CH2Cl2·Et2O showing
the two independent complexes in the unit cell.
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◦
◦
˚
Table 2 Selected bond lengths (A), bond angles ( ) and torsion angles ( )
from crystal data
Compound
(R)-9
(R)-10, M = Zn (R)-11, M = Fe
H102–O102
H102–N119
H202–O202
H202–N219
0.84a
1.75a
0.84a
1.75a
—
—
—
—
—
—
—
—
O102–C102
O202–C202
1.318(5)
1.343(5)
1.292(7)
1.288(7)
1.308(4)
1.316(4)
N119–C119
N119–C122
N219–C219
N219–C222
1.289(6)
1.432(6)
1.292(6)
1.412(6)
1.304(8)
1.421(8)
1.262(8)
1.439(7)
1.289(4)
1.426(4)
1.292(4)
1.441(4)
M1–O102
M1–O202
M1–N119
M1–N219
—
—
—
—
1.896(4)
1.894(4)
2.001(5)
2.038(5)
1.897(2)
1.895(2)
2.030(3)
2.040(3)
O102–H102–N119
O202–H202–N219
147.0a
147.0a
—
—
—
—
C119–N119–C122
C219–N219–C222
123.5(4)
124.9(4)
121.8(5)
124.0(5)
121.2(3)
121.3(3)
Fig. 5 Thermal ellipsoid plot (50%) of (R)-11.
N119–M1–N219
N119–M1–O102
N119–M1–O202
N219–M1–O102
N219–M1–O202
O102–M1–O202
—
—
—
—
—
—
93.45(19)
95.21(19)
133.4(2)
126.38(19)
93.31(18)
116.78(18)
94.54(10)
90.21(11)
131.62(12)
132.23(11)
89.78(11)
121.03(10)
significantly opened up from ideal tetrahedral: 116–121◦ for O–
M–O and 126–133◦ for N–M–O.
NMR Characterization
C122–C121–C221–C222 −106.5(5) −69.5(7)
−72.6(4)
−4.2(5)
The ligand (R)-9 and its ZnII complex (R)-10 have well resolved 1H
C102–C103–C119–N119
C202–C203–C219–N219
−3.1(7) −11.1(10)
2.7(7) 0.9(9)
−3.0(5)
and 13C spectra. In each case, spectra are consistent with a single
1
C2 symmetric compound in solution. The H NMR of the FeII
C103–C119–N119–C122 −179.9(4) 178.7(6)
−178.4(3)
179.0(3)
C203–C219–N219–C222
175.5(4) 178.2(5)
complex, (R)-11, has broad signals and a wide chemical shift range,
consistent with a paramagnetic metal center and an extensively
conjugated ligand system. Seventeen resonances are expected for
a C2 complex, and 15 of these are readily identified. It is likely
that the remaining two resonances are not observed due to strong
paramagnetic broadening of protons near the metal center. For
all three compounds, the C2 symmetry indicated by the NMR
studies is consistent with the X-ray structural data, which show
a symmetric ligand and only M helical complexes for (R)-10 and
(R)-11.
Nap–Napb
108.1
63.2
7.6
78.4
36.8
87.9
80.3
82.7
35.8
88.5
84.1
Arm–Armc
Nap1–Arm1d
Nap2–Arm2e
7.0
a Hydrogen at calculated position. b Absolute value of the angle between the
naphthyl planes. c Absolute value of the angle between the benz[a]anthryl
planes. d Absolute value of the angle between the naphthyl plane C120–
C129 and the benz[a]anthryl plane C101–C118. e Absolute value of the
angle between the naphthyl plane C220–C229 and the benz[a]anthryl plane
C201–C218.
1
In the H spectrum of the ligand, (R)-9, the intramolecularly
bonded O–H · · · N protons appear at 15.17 ppm. While tautomer-
ization may be present for this complex, only an averaged spectrum
would be observed due to rapid proton transfer. Other notable
signals in the 1H spectrum are the imine protons at 8.93 ppm and
the signal at 9.96 ppm which is due to the bay region hydrogens
at the 5-positions of the benz[a]anthryl rings. These latter protons
are effected by edge position aromatic deshielding and anisotropic
deshielding from the hydroxyl group at the nearby 4-position.13
The spectrum of the ZnII complex, (R)-10, is similar to that of the
uncomplexed ligand. The most notable differences are the absence
of the phenolic resonance and the significant downfield shift of
the bay region proton to 12.15 ppm. This dramatic shift is mainly
the result of increased anisotropic deshielding by the phenoxide
donor at the 4-position. Previous studies on phenanthrenes have
shown that the heteroatom at the 4-position can have a dramatic
effect on the chemical shift of the proton at the 5-position.14
In both complexes there is significant rotation of the sidearms
relative to the naphthyl units (dihedral angles range from 80.6◦ to
88.6◦) indicating that there is little delocalization of p electrons
between these aromatic segments. A roughly perpendicular rela-
tionship between the naphthyl units and the aromatic sidearms
has been observed in other binaphthyl Schiff base complexes,12
and is necessary for effective coordination of the four donors to
a single metal center. Neither complex has axial ligands in the
solid state, consistent with sterically-congested metal centers and
limited conformational flexibility due to sidearm interactions (vide
supra). The metal centers are best described as having a distorted
tetrahedral geometry. Chelate pairs of donors have bond angles
close to 90◦ with the metal: 90–95◦ for N–M–O units and 93–95◦
for N–M–N. The bond angles between non-adjacent donors is
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Electronic and ECD spectroscopy
broadening and intensity reduction for the paramagnetic iron
complex compared to the dimagnetic zinc complex, especially
at energies below 240 nm. The ECD spectra of the complexes
have pronounced differences to that of the ligand, consistent
with the wrapping of the extended ligand into conformationally
fixed helical complexes. The signs of signals are generally reversed
through the 240–340 nm range and the low energy signal due to the
imine p–p* transition is prominent in the spectra of the complexes.
The low energy signals from 340 to 480 nm show a negative Cotton
effect at long wavelengths, consistent with the R configuration of
the binaphthyl backbone.11,18a
The electronic spectrum of the ligand precursor 8 (Fig. 6) shows a
number of overlapping signals: there is an envelope of absorptions
from 225 to 300 nm, an absorption at 338 nm, and a lower intensity
envelope from 340–430 nm. The low energy envelope matches
up well with the vibronic series of the first electronic transition
(Lb band), centered at 385 nm, reported for benz[a]anthracene.15
The UV spectral features of 8 are similar to those seen for 4-
hydroxybenz[a]anthracene.16 The high energy envelope is believed
to result primarily from two strongly overlapping electronic
transitions, which are centered at 256 nm for benz[a]anthracene.15
For 8 kmax is shifted to 270 nm, consistent with the conjugation
effect of the aldehyde, as was noted for benz[a]anthryl systems.17
Fig. 6 Electronic spectra of 8 (2.5 × 10−5 M) and (R)-9 (1.5 × 10−5 M) in
Fig. 8 ECD spectra of (R)-9 (2.5 × 10−5 M) (R)-10 (2.5 × 10−5 M) and
(R)-11 (1.5 × 10−5 M) in THF.
THF.
The electronic spectrum of the free ligand, (R)-9, in THF
(Fig. 6) contains signals for p to p* transitions from three major
chromophores: the imine p-bonds, the benz[a]anthryl sidearms,
and the binaphthyl backbone. The low energy envelope for 500
to 350 nm results from imine p–p* absorption18 as well as the
sidearm Lb band. The strong signal with kmax at 271 nm has not
shifted significantly compared to the aldehyde 8. The ligand shows
a high energy absorption in the UV that is not present in 8. This
strong signal, with kmax at 216 nm and a low energy shoulder, is
due to the binaphthyl backbone.11 The electronic spectrum of the
free ligand does not display distinct signals for the keto-amine
tautomer, as were observed for the phenanthryl analog.4
Complexes (R)-10 and (R)-11 have similar electronic spectra
(Fig. 7), both of which have the same general features as the
spectrum of the free ligand, with additional absorption in the low
energy 450–500 nm range. The shift to low energy is consistent
with the shift in the imine p–p* transition upon metallation.18
The ECD spectra of the two complexes (Fig. 8) have a general
similarity, with the most notable differences being significant
Conclusions
The binaphthyl-salen ligand with extend polyaromatic benz[a]-
anthryl sidearms produces helical complexes with M helicity and
overlapping sidearms. The twisted binaphthyl backbone directs
the rigid sidearms and causes a strong preference for the M helix.
The preference is not as strong when the more flexible cyclohexyl
backbone is used, and M and P helices are observed in most
cases, even with the extended benz[a]anthryl sidearms.5 Thus, the
binaphthyl backbone appears to have a high fidelity for generating
helices of a predetermined handedness in salen systems. Despite
the overlapping sidearms, there is no significant face-to-face
p–p stacking of the benz[a]anthryl units within a complex. The
geometric constraints of the binaphthyl backbone are transmitted
to the sidearms, and they are directed away from one another,
instead of stacking. On the other hand, the more flexible cyclohexyl
backbone can allow significant p–p staking interactions of the
benz[a]anthryl sidearms.5
Helical complexes have great potential as asymmetric catalysts,
and we are currently examining their utility for the asymmetric
oxidation of alkenes and sulfides.
Experimental
General methods
All reactions were carried out under inert atmospheres unless
otherwise noted. All air and/or moisture sensitive compounds
were manipulated using standard high-vacuum line, Schlenk,
or cannula techniques, or in a glove box under a nitrogen
atmosphere as described previously.19 All solvents were stored
Fig. 7 Electronic spectra of (R)-10 (2.5 × 10−5 M) and (R)-11 (1.5 × 10−5
M) in THF.
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under vacuum over sodium benzophenone ketyl, titanocene, or
calcium hydride prior to use. Solvents used in metallation reactions
were degassed before use and inert gas was purified by passing
and discarded, and the organic solution dried over anhydrous
MgSO4, then filtered. The solvent was removed in vacuo to give
3 (52.02 g, 87% yield) as a light tan solid. 1H NMR (CDCl3, 400
MHz): d 1.99 (quin, 2 H, J = 7.5 Hz, CH2), 2.40 (t, 2 H, J =
7.5 Hz, CH2), 2.69 (t, 2 H, J = 7.5 Hz, CH2), 3.95 (s, 4 H, CH2),
7.05 (dd, 1 H, J = 1.8, 7.6, Hz, CH), 7.15 (d, 1 H, J = 1.8 Hz, CH),
7.19–7.26 (m, 3 H, CH), 7.29–7.35 (m, 2 H, CH), 11.54 (br, s, 1 H,
˚
through 4 A molecular sieves and Engelhard Q5 catalyst. Magnetic
susceptibilities were measured at 296 K using a Magway Mk1
magnetic susceptibility balance. The instrument was calibrated
with Hg[Co(NCS)4] standard. Magnetic susceptibilities of the
iron salen complexes were corrected for metal core electrons
and the diamagnetism of the free ligands. Effective magnetic
moments were calculated from the corrected susceptibilities as
leff = (8vMT)1/2. NMR data were collected on a Varian Unity
400 MHz spectrometer using the residual solvent protons as an
internal standard. Electronic spectra were collected on a Cary 500
spectrometer with a 1 cm quartz cell. ECD spectra were collected
using a Jasco 720 spectropolarimeter with a 1 cm quartz cell in
a nitrogen-purged cavity. Spectroscopic grade THF was used for
these studies. The concentrations of ligands and complexes ranged
from 1.5 to 2.5 × 10−5 M. All the preparations that follow are new,
although some of the procedures follow general methods we have
previously reported.4
1
OH). H NMR analysis indicated the presence of approximately
10% of the fully aromatized analogue of the product. 13C NMR
(CDCl3, 100 MHz): d 26.56, 33.52, 34.87, 35.97, 36.37, 126.27,
126.27, 126.45, 127.60, 127.62, 127.64, 127.73, 134.61, 136.89,
136.96, 137.02, 139.22, 179.93. Anal. calc. for C18H18O2: C 81.17,
H 6.81. Found: C 80.63, H 6.71%.
Methyl 4-(9,10-dihydro-2-anthryl)butanoate (4). A mixture of
3 (51.79 g, 194 mmol) and sulfuric acid (5.2 mL) in methanol
(520 mL) was refluxed for 18 h. After cooling, the reaction
mixture was diluted with diethyl ether (800 mL) and consecutively
washed with 1 M NaHCO3 (800 mL) and H2O (500 mL). The
organic solution was dried over anhydrous MgSO4, filtered, and
concentrated to afford 4 (53.37 g, 98% yield) as a red-brown oil.
1H NMR (CDCl3, 400 MHz): d 1.97 (quin, 2 H, J = 7.5 Hz, CH2),
2.35 (t, 2 H, J = 7.5 Hz, CH2), 2.65 (t, 2 H, J = 7.5 Hz, CH2),
3.68 (s, 3 H, CH3), 3.93 (s, 4 H, CH2), 7.03 (dd, 1 H, J = 1.4,
7.6, Hz, CH), 7.14 (d, 1 H, J = 1.4 Hz, CH), 7.18–7.25 (m, 3
H, CH), 7.28–7.34 (m, 2 H, CH). 1H NMR analysis indicated the
presence of approximately 10% of the fully aromatized analogue of
the product. 13C NMR (CDCl3, 100 MHz): d 26.85, 33.62, 35.00,
35.98, 36.39, 51.67, 126.26, 126.26, 126.45, 127.59, 127.59, 127.59,
127.73, 134.54, 136.91, 136.91, 137.05, 139.41, 174.17. Anal. calc.
for C19H20O2: C 81.40, H 7.19. Found: C 81.60, H 7.02%.
Synthesis of the aldehyde precursor, 8
3-(9,10-Dihydro-2-anthroyl)propionic acid (2). To a solution
of 9,10-dihydroanthracene (1, 74.23 g, 412 mmol) in 1,2-
dichloroethane (325 mL) at 0 ◦C was added a mixture of succinic
anhydride (37.50 g, 374 mmol) and aluminium chloride (99.83 g,
749 mmol) in small portions over 80 min. The reaction mixture
was allowed to warm to room temperature and stirred for a further
2 h. It was then quenched via the addition of H2O (1.25 L),
and diluted with hexanes (1.5 L). The resulting precipitate was
collected, washed with H2O (500 mL), and suspended into 5 M
NaOH (1 L). After stirring for 30 min it was recollected, washed
with 5 M NaOH (3 × 300 mL), and dissolved into 4 : 1 H2O–
ethanol (1 L). The solution was filtered to remove trace insoluble
solids, and acidified to pH 1 with 5 M HCl. The resulting light
yellow solid was collected and washed with H2O (500 mL) to
afford 2 (63.61 g, 61% yield). 1H NMR (CDCl3, 400 MHz): d 2.83
(t, 2 H, J = 6.5 Hz, CH2), 3.33 (t, 2 H, J = 6.5 Hz, CH2), 4.01
(s, 4 H, CH2), 7.20–7.26 (m, 2 H, CH), 7.29–7.35 (m, 2 H, CH),
7.40 (d, 1 H, J = 7.8 Hz, CH), 7.83 (dd, 1 H, J = 1.8, 7.8, CH),
7.93 (d, 1 H, J = 1.8 Hz, CH), 10.65 (br, s, 1 H, OH). 1H NMR
analysis indicated the presence of 3–5% of the fully aromatized
analogue of the product. 13C NMR (CDCl3, 100 MHz): d 28.33,
33.35, 36.26, 36.51, 126.29, 126.59, 126.63, 127.29, 127.62, 127.65,
127.87, 134.84, 135.74, 136.16, 137.44, 142.92, 178.98, 197.83.
Anal. calc. for C18H16O3: C 77.12, H 5.75. Found: C 76.87, H
5.50%.
Methyl 4-(2-anthryl)butanoate (5). A solution of 4 (53.18 g,
190 mmol) in 2-ethoxyethyl ether (200 mL) was refluxed in the
presence of 5% Pd/C catalyst (5.32 g) for 40 h. After cooling,
the reaction mixture was poured into acetone (300 mL), and
the solution filtered to remove palladium catalyst. Dilution of
the filtrate with H2O (750 mL) gave a precipitate that was
collected, and washed with a further portion of H2O (200 mL).
The precipitate was dissolved into methylene chloride (250 mL),
and the aqueous layer separated and discarded. The organic
solution was dried over anhydrous MgSO4, filtered, and diluted
with hexanes (1.25 L). Upon concentration to 1/2 volume a cream
colored precipitate formed, and this was collected and washed with
hexanes (200 mL) to yield 5 (45.90 g, 87% yield). 1H NMR (CDCl3,
400 MHz): d 2.12 (quin, 2 H, J = 7.5 Hz, CH2), 2.42 (t, 2 H, J =
7.5 Hz, CH2), 2.87 (t, 2 H, J = 7.6 Hz, CH2), 3.68 (s, 3 H, CH3),
7.33 (dd, 1 H, J = 1.5, 8.7, Hz, CH), 7.42–7.49 (m, 2 H, CH), 7.77
(s, 1 H, CH), 7.93–8.02 (m, 3 H, CH), 8.35 (s, 1 H, CH), 8.40 (s, 1
H, CH). 13C NMR (CDCl3, 100 MHz): d 26.18, 33.64, 35.65, 51.70,
125.25, 125.51, 125.70, 126.17, 126.40, 127.43, 128.28, 128.38,
128.52, 130.84, 131.61, 132.10, 132.10, 138.44, 174.11. Anal. calc.
for C19H18O2: C 81.99, H 6.52. Found: C 81.51, H 6.76%.
4-(9,10-Dihydro-2-anthryl)butanoic acid (3). To a solution of
2 (63.07 g, 225 mmol) in diethylene glycol (600 mL) at 100 ◦C
was added hydrazine monohydrate (33 mL, 679 mmol). The
reaction mixture was stirred for 1 h, then potassium hydroxide
was added (37.89 g, 675 mmol) and the temperature was raised to
200 ◦C. After stirring for 8 h the reaction mixture was allowed to
cool, and diluted with H2O (2.4 L). The solution was filtered twice
through a Celite plug to remove insoluble solids, and the clear
filtrate acidified to pH 1 with 5 M HCl. The resulting precipitate
was collected, washed with H2O (750 mL), and dissolved into
methylene chloride (700 mL). The aqueous layer was separated
3,4-Dihydrobenz[a]anthracen-1(2H)-one (6). A solution of 5
(45.70 g, 164 mmol) in methanesulfonic acid (450 mL) was heated
to 90 ◦C and stirred for 2 h. The reaction mixture was poured into
H2O (900 mL) and allowed to cool. The resulting aqueous mixture
was extracted with methylene chloride (1 × 300 mL, 4 × 100 mL),
and the extracts combined and washed consecutively with 1 M
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NaHCO3 (300 mL) and H2O (300 mL). The organic solution was
dried over anhydrous MgSO4, filtered, and concentrated to afford
6 (40.31 g, 100% yield) as a yellow-brown solid. 1H NMR (CDCl3,
400 MHz): d 2.23 (quin, 2 H, J = 6.4 Hz, CH2), 2.84 (t, 2 H, J =
6.6 Hz, CH2), 3.13 (t, 2 H, J = 6.1 Hz, CH2), 7.25 (d, 1 H, J =
8.6, Hz, CH), 7.47–7.55 (m, 2 H, CH), 7.95–8.00 (m, 1 H, CH),
8.06 (d, 1 H, J = 8.6, Hz, CH), 8.11–8.16 (m, 1 H, CH), 8.34 (s,
1 H, CH), 10.11 (s, 1 H, CH). 13C NMR (CDCl3, 100 MHz): d
23.13, 32.09, 41.24, 126.11, 126.16, 126.61, 126.95, 127.01, 127.87,
128.63, 129.69, 131.27, 131.27, 133.89, 135.11, 148.01, 200.38 (one
signal not observed). Anal. calc. for C18H14O: C 87.78, H 5.73.
Found: C 87.75, H 5.72%.
130.99, 131.58, 132.97, 133.13, 139.97, 163.50, 196.74. Anal. calc.
for C19H12O2: C 83.81, H 4.44. Found: C 83.52, H 4.53%.
Synthesis of the ligand and its complexes
2,2ꢀ -[(1R)-[1,1ꢀ -Binaphthalene]-2,2ꢀ -diylbis(nitrilomethylidyne)]-
bisbenz[a]anthracen-1-ol, [(R)-9]. A mixture of 8 (3.009 g,
11.1 mmol) and (R)-1,1ꢀ-binaphthyl-2,2ꢀ-diamine (1.571 g,
5.5 mmol) in ethanol (150 mL) was brought to reflux. After 18 h the
resultant suspension was hot filtered, and the precipitate washed
with boiling ethanol (100 mL) to give (R)-9 (4.149 g, 95% yield)
as a bright red solid. 1H NMR (TFA-d, 400 MHz): d 7.47 (d, 2 H,
J = 8.8 Hz, CH), 7.57 (d, 2 H, J = 8.3 Hz, CH), 7.74–7.82 (m, 6 H,
CH), 7.87–7.94 (m, 4 H, CH), 7.99 (d, 2 H, J = 8.8 Hz, CH), 8.08
(t, 2 H, J = 7.5 Hz, CH), 8.12 (d, 2 H, J = 8.4 Hz, CH), 8.21 (d, 2
H, J = 8.2 Hz, CH), 8.39 (s, 2 H, CH), 8.49 (d, 2 H, J = 9.1 Hz,
CH), 8.61 (d, 2 H, J = 8.3 Hz, CH), 8.69 (s, 2 H, CH), 8.84 (d, 2
H, J = 9.1 Hz, CH), 9.37 (s, 2 H, CH). The phenolic hydrogen was
not observed due to deuterium exchange. In CDCl3 the phenolic
hydrogen is observed at 15.17 ppm. 13C NMR (TFA-d, 100 MHz):
d 115.68, 117.13, 122.26, 125.97, 126.10, 126.32, 127.27, 127.32,
127.56, 129.57, 129.89, 129.95, 130.05, 131.40, 131.59, 131.83,
132.47, 132.74, 134.50, 134.73, 135.37, 135.51, 136.09, 136.64,
138.48, 146.25, 161.43, 163.26 (one signal not observed). Anal.
calc. for C58H36N2O2: C 87.86, H 4.58, N 3.53. Found: C 87.84,
H 4.62, N 3.56%. Single crystals suitable for X-ray analysis were
grown by slow diffusion of hexanes into a solution of (R)-9 in
methylene chloride.
3,4-Dihydro-2-(hydroxymethylene)benz[a]anthracen-1(2H)-one
(7). To a solution of ethyl formate (23.49 g, 317 mmol) in benzene
(500 mL) was added sodium methoxide (12.85 g, 238 mmol). To
this vigorous stirring mixture was added a solution of 6 (39.05 g,
158 mmol) in benzene (500 ml) and the reaction mixture then
stirred for 16 h. This was diluted with hexanes (1 L), filtered,
and the collected solid was washed with a further portion of
hexanes (500 mL). The solid was redissolved into methanol
(300 mL), acidified with 2.5 M HCl (700 mL), and the precipitate
extracted into methylene chloride (3 × 300 mL). The extracts
were combined, dried over anhydrous MgSO4, filtered, and the
solvent removed in vacuo. Purification of the crude material was
accomplished by filtration through a silica gel plug (1 : 1 hexanes–
methylene chloride) to give 7 (33.35 g, 77% yield) as a bright orange
1
solid. H NMR (CDCl3, 400 MHz): d 2.57 (t, 2 H, J = 6.8 Hz,
CH2), 3.04 (t, 2 H, J = 6.8 Hz, CH2), 7.27 (d, 1 H, J = 8.6 Hz, CH),
7.48–7.56 (m, 2 H, CH), 7.79 (d, 1 H, J = 8.4, Hz, CH), 7.96–8.01
(m, 1 H, CH), 8.06 (d, 1 H, J = 8.6 Hz, CH), 8.09–8.14 (m, 1 H,
CH), 8.38 (s, 1 H, CH), 9.91 (s, 1 H, CH), 15.11 (d, 1 H, J = 8.8 Hz,
Zn(II) complex of (R)-9, [(R)-10]. Anhydrous zinc chloride
(0.048 g, 0.35 mmol), sodium methoxide (0.052 g, 0.96 mmol)
and (R)-9 (0.252 g, 0.32 mmol) were suspended into a 2 : 1 mixture
of benzene–ethanol (15 mL) and stirred for 12 h. The reaction
mixture was concentrated to a yellow solid that was dissolved into
THF (10 mL), and filtered to remove fine insoluble solids. The
clear filtrate was diluted with ethanol (20 mL), and concentrated
to 2/3 volume resulting in formation of a yellow precipitate. The
precipitate was collected to afford (R)-10 (0.153 g, 56% yield).
1H NMR (CDCl3, 400 MHz): d 6.70 (t, 2 H, J = 7.5 Hz, CH),
6.99 (d, 2 H, J = 8.6 Hz, CH), 7.09 (d, 2 H, J = 8.2 Hz, CH),
7.19 (t, 2 H, J = 7.5 Hz, CH), 7.23–7.29 (m, 4 H, CH), 7.44 (t,
2 H, J = 7.6 Hz, CH), 7.48 (d, 2 H, J = 8.5 Hz, CH), 7.51–7.55
(m, 4 H, CH), 7.84 (d, 2 H, J = 8.3 Hz, CH), 7.91 (d, 2 H, J =
8.2 Hz, CH), 7.95 (d, 2 H, J = 8.8 Hz, CH), 8.02 (d, 2 H, J =
8.6 Hz, CH), 8.32 (s, 2 H, CH), 8.58 (s, 2 H, CH), 11.30 (s, 2 H,
CH). 13C NMR (CDCl3, 100 MHz): d 116.30, 116.47, 122.37,
123.79, 124.94, 125.54, 125.89, 126.48, 126.59, 126.87, 126.87,
127.10, 127.31, 128.36, 128.61, 129.76, 130.32, 131.02, 131.07,
131.40, 132.31, 132.35, 133.13, 133.99, 134.20, 139.55, 145.53,
170.59, 173.65. Anal. calc. for C58H34N2O2Zn: C 81.35, H 4.00,
N 3.27. Found: C 80.53, H 4.35, N 3.05%. Single crystals suitable
for X-ray analysis were grown by slow diffusion of diethyl ether
into a solution of (R)-10 in methylene chloride.
1
OH). H NMR analysis indicated the presence of approximately
15% of the fully aromatized analogue of the product. 13C NMR
(CDCl3, 100 MHz): d 23.43, 31.44, 110.75, 126.17, 126.23, 126.29,
126.50, 127.28, 127.32, 127.93, 128.33, 129.54, 131.36, 131.55,
133.47, 134.94, 145.66, 168.04, 190.45. Anal. calc. for C19H14O2: C
83.19, H 5.14. Found: C 82.97, H 4.76%.
1-Hydroxybenz[a]anthracene-2-carboxaldehyde (8). A mixture
of 7 (33.08 g, 121 mmol) and triphenylmethanol (62.78 g,
241 mmol) in trifluoroacetic acid (500 mL) was refluxed for
2 h. The solution was cooled and diluted with H2O (1 L). The
resulting precipitate was collected, washed with H2O (200 mL),
and suspended into 2 : 1 1 M NaOH–ethanol (600 mL). The
mixture was stirred for 15 min, filtered, and the insoluble material
washed with a further portion of 2 : 1 1 M NaOH–ethanol
(100 mL). The filtrates were combined, acidified to pH 1 with
5 M HCl, and extracted with methylene chloride (1 × 700 mL,
2 × 200 mL). The organic extracts were combined, dried over
anhydrous MgSO4, filtered, and the solvent removed in vacuo. The
resulting solid was recrystallized from ethyl acetate–ethanol to
afford 8 (27.85 g, 85% yield) as a yellow-green powder. 1H NMR
(CDCl3, 400 MHz): d 7.38 (d, 1 H, J = 8.1 Hz, CH), 7.51 (d, 1 H,
J = 8.9 Hz, CH), 7.57–7.63 (m, 3 H, CH), 7.92 (d, 1 H, J = 8.9 Hz,
CH), 8.00–8.06 (m, 1 H, CH), 8.15–8.21 (m, 1 H, CH), 8.33 (s, 1
H, CH), 9.98 (s, 1 H, CH), 10.28 (s, 1 H, CH), 13.44 (s, 1 H, OH).
13C NMR (CDCl3, 100 MHz): d 117.86, 119.52, 120.54, 126.16,
126.51, 126.64, 127.25, 127.60, 128.38, 128.68, 129.66, 130.57,
Fe(II) complex of (R)-9, [(R)-11]. Iron(II) chloride (0.053 g,
0.42 mmol), sodium methoxide (0.061 g, 1.14 mmol) and (R)-
9 (0.300 g, 0.38 mmol) were suspended into a 2 : 1 mixture of
benzene–ethanol (15 mL) and stirred for 12 h. The reaction
mixture was concentrated to a brown-black residue that was
dissolved into THF (15 mL), and filtered to remove fine insoluble
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solids. The clear filtrate was diluted with ethanol (45 mL), and
stirred for 30 min. A brown precipitate of (R)-11 formed gradually,
and this was collected and dried in vacuo (0.244 g, 76% yield). 1H
NMR (CD2Cl2, 400 MHz): d −38.86 (br, s, 2 H, CH), −13.43 (s, 2
H, CH), −8.90 (s, 2 H, CH), −7.52 (s, 4 H, CH), 2.25 (s, 2 H, CH),
5.02 (s, 2 H, CH), 8.18 (s, 2 H, CH), 10.33 (s, 2 H, CH), 11.90 (s,
2 H, CH), 13.05 (s, 2 H, CH), 13.60–14.40 (m, 4 H, CH), 24.12 (s,
2 H, CH), 62.02 (br, s, 2 H, CH). Anal. calc. for C58H34N2O2Fe: C
82.27, H 4.05, N 3.31. Found: C 82.38, H 4.29, N 3.22%. Single
crystals suitable for X-ray analysis were grown by slow diffusion
of diethyl ether into a solution of (R)-11 in methylene chloride.
CCDC reference numbers CCDC 284716–284718.
3 (a) M. Va´zquez, M. R. Bermejo, M. Fondo, A. Garc´ıa-Deibe, J. A.
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´
´
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b700001d
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Acknowledgements
This work is supported by the National Science Foundation under
grant number CHE-0349258 and by Kansas State University.
Thanks to J. Tomich for use of the ECD facilities.
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The Royal Society of Chemistry 2007
Dalton Trans., 2007, 1520–1527 | 1527
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