Preparation of a family of 10-Hydroxybenzo[h]quinoline analogues via a modified sanford reaction and their excited state intramolecular proton transfer properties

Article abstract of DOI:10.1021/jo202072d

We have developed a highly optimized methodology that allows for the oxidative acetoxylation of a sterically and electronically demanding library of analogues of benzo[h]quinoline. The optimal conditions for the insertion of an OAc group were identified a

Full text of DOI:10.1021/jo202072d

Article
pubs.acs.org/joc
Preparation of a Family of 10-Hydroxybenzo[h]quinoline Analogues
via a Modified Sanford Reaction and Their Excited State
Intramolecular Proton Transfer Properties†
Joanna Piechowska and Daniel T. Gryko*
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01−224 Warsaw, Poland
S
* Supporting Information
ABSTRACT: We have developed a highly optimized methodology that allows for the
oxidative acetoxylation of a sterically and electronically demanding library of analogues of
benzo[h]quinoline. The optimal conditions for the insertion of an OAc group were identified
after examining various reaction parameters (solvent, oxidant, catalyst, temperature, time). The
conditions identified (Pd(OAc)2, PhI(OAc)2, MeCN, 150 °C, 16 h), combined with the
hydrolysis of acetates, resulted in the formation of hydroxybenzoquinolines in 27−59% yield,
whereas all previously published procedures were ineffective. This synthesis was compatible
with diverse functionalities (ester, aldehyde, carbon−carbon triple bond) and, most
importantly, worked for sterically hindered analogues as well as for compounds possessing
electron-donating and electron-withdrawing substituents at various positions. All the obtained
compounds demonstrated excited-state intramolecular proton transfer (ESIPT) manifesting as
small fluorescence quantum yields and large Stokes shifts (8300−9660 cm−1). The effect of
structural variations in eight 10-hydroxybenzo[h]quinoline analogues on absorption and
emission properties was studied in detail.
of the rich chemistry of quinolines (and their benzoanalogues)
with this modern tool could provide easy access to an almost
unlimited variety of structural analogues of 10-hydroxybenzo-
[h]quinoline, which would allow studies on the structure−
optical property relationship. The aim of this study was to
explore this strategy to obtain a range of derivatives and
investigate their fundamental optical properties. This in turn
would allow us to address one of the most important issues
regarding the ESIPT system, i.e., the wide tunability of
chromophore absorption as well as proton transfer emission.
INTRODUCTION
■
Excited state intramolecular proton transfer (ESIPT)1 has
emerged in recent years as a most interesting phenomenon that
can be utilized in the design of fluorescent sensors.2
Compounds displaying ESIPT, such as benzoxazoles,3
flavones,4 imidazoles,5 or anthraquinones,6 possess a large
Stokes shift, and hence many important applications have been
found for them (such as laser dyes,7 fluorescence recording,8
ultraviolet stabilizers,9 probes for solvation dynamics,10 probes
for biological environments,11 and recently organic light-
emitting devices12). 10-Hydroxybenzo[h]quinoline (HBQ)13
constitutes one of the fundamental heterocyclic systems in
which ESIPT occurs. Although this molecule has been used for
a long time as a reagent in the preparation of optical filter
agents in photographic emulsions, it was not until fundamental
studies were carried out by Chou that ESIPT was recognized as
the process responsible for its strongly bathochromically shifted
fluorescence.14 Detailed photophysical and theoretical studies
of that molecule15 showed very fast and solvent-independent
ESIPT, but broader studies were hampered by considerable
difficulties with the preparation of its more elaborate
derivatives.16 The recent discovery of coordination-assisted
acetoxylation of derivatives and analogues of 2-phenylpyridine,
made by Sanford and co-workers, opened up new possibil-
ities.17 Acetate derivatives of 10-hydroxybenzo[h]quinoline
prepared in such a way can be easily hydrolyzed to the
corresponding phenol. It is worth mentioning that in 6-, 7-, and
8-hydroxyquinolines, excited state proton transfer also occurs in
intra- or intermolecular fashion.18 These analogues of HBQ are
known to be photoacids.19 We envisioned that a combination
RESULTS AND DISCUSSION
■
As previously outlined, we designed a general approach toward
derivatives and analogues of 10-hydroxybenzo[h]quinoline
comprising a two-step strategy: (a) the synthesis of derivatives
and analogues of benzo[h]quinoline and (b) their C−H
acetoxylation followed by hydrolysis of the initially formed ester
into a phenol. The investigation started from the synthesis of
benzoacridine 3,20 which was performed via the Bernthsen
reaction (Scheme 1).21 We expected that linear fusion of
benzo[h]quinoline with an additional benzene ring would lead
to a bathochromic shift in both absorption and emission in the
final product 4. This compound was subjected to the original
Sanford conditions (PhI(OAc)2, Pd(OAc)2, CH3CN, 75 °C, 16
h), but conversion was very low, and the yield of product 4 was
only 19% (Scheme 1, Table 1, entry 1) compared to 86% yield
Received: October 11, 2011
Published: November 8, 2011
© 2011 American Chemical Society
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investigated the model reaction under new conditions recently
published recently by Yu with copper acetate as the catalyst and
oxygen as the oxidant,22,23 as well as with a catalytic system
based on rhodium.24 Although all these procedures were
reported to work very well for 2-phenylpyridine, it turned out
that they could not facilitate the key C−H activation in the case
of benzoacridine 3. We attempted to use salts of other metals
like manganese or silver in order to induce the desired
transformation with strikingly negative results (Table 1, entries
8 and 9). The replacement of PIDA with PIFA resulted in no
formation of compound 4 (Table 1, entry 10). Finally, we
decided to perform the reaction in CH3CN at 150 °C using
high-pressure glass tubes (Table 1, entry 11). Under these
conditions, the reaction led to 57% yield of the desired product
4. Although we subsequently found that slightly higher yield
could be reached under analogous conditions in dioxane (Table
1, entry 12), we decided to use CH3CN-based conditions
because of their greater generality.
Scheme 1
To test the generality and scope of the newly developed
conditions, a broad variety of derivatives and analogues of
benzo[h]quinoline possessing various functional groups
(possibly influencing the rate of the Pd-catalyzed Sanford
reaction) were designed. While designing these compounds, we
also kept in mind the influence of structural alterations on the
photophysical properties. Since our research was focused on
hydroxy derivatives, the moderate stability of the initially
formed acetates (on both silica and alumina) inclined us to
hydrolyze them to the corresponding phenols without
purification. In the course of this study, we found that the
yield of hydrolysis was 90−95%. We synthesized the analogue
of compound 3 possessing a methyl group in order to study the
influence of replacement of the aromatic unit with an aliphatic
unit at position 7 (Scheme 2). The benzoacridine 5 was
Table 1. Optimisation Studies for the Synthesis of
Compound 4
temp time
a
entry
1
solvent
(°C)
(h)
catalyst, oxidant
yield of 4 (%)
19
CH3CN
75
16
Pd(OAc)2,
PhI(OAc)2
2
3
4
acetone
AcOH
110
120
100
4
20
16
Pd(OAc)2,
PhI(OAc)2
10
<5
<5
Scheme 2
Pd(OAc)2,
PhI(OAc)2
C2H5CN
CH3CN
Pd(OAc)2,
PhI(OAc)2
5
6
130
145
48
24
Cu(OAc)2·H2O, O2 trace
Cu(OAc)2
toluene,
Ac2O
0
7
NMP
130
40
[Rh(cod)Cl]2, CuI, Rh-complex with
PCy3·HBF4
substrate
8
9
CH3CN
CH3CN
100
90
24
24
AgOAc, PhI(OAc)2
0
0
Mn(OAc)3,
PhI(OAc)2
10
11
12
CH3CN
CH3CN
dioxane
75
150
150
16
16
16
Pd(OAc)2,
19
57
64
PhI(OCOCF3)2
Pd(OAc)2,
PhI(OAc)2
Pd(OAc)2,
PhI(OAc)2
a
Isolated yields.
prepared using a known two-step process comprised of the
Ullmann and Friedel−Crafts reactions.25 Acetoxylation of that
compound led to the corresponding ester, which was
transformed into phenol 6 without isolation, in 27% overall
yield.
It is well-known that steric hindrance affects the coordination
properties of various ligands. Since the single act of
coordination of Pd2+ by the pyridine-type nitrogen atom is
undoubtedly the first step of the mechanism17 in the Sanford
reaction, we found it of critical importance to study that effect.
8,9,10,11-Tetrafluorobenzo[c]acridine (9) was designed as a
in the acetoxylation of benzo[h]quinoline itself.17a Apparently,
steric hindrance caused by the presence of hydrogen from an
additional benzene ring has a significant effect on the
coordination abilities of the basic nitrogen atom. Disappointed
with this result, we investigated the acetoxylation reaction in
various solvents (CH3COOH, C2H5CN, and acetone) to
obtain even lower yields of product 4 (Table 1, entries 2−4).
The utilization of other solvents (ethyl acetate and THF) did
not bring any improvement to the reaction. We also
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compound with not only considerable steric hindrance
imparted by the fluorine atom at position 11, but also low
electron density on the nitrogen atom due to the strong
electron-withdrawing effect of the four fluorines (Scheme 3).
Scheme 4
Scheme 3
The elegant synthesis of tetrafluoroquinolines from penta-
fluorobenzaldehyde (8) was developed in 1993 by Tipping and
co-workers.26 The preparation of benzoacridine 9 was obtained
following this general procedure while replacing the aniline
derivatives with 1-aminonaphthalene (7), furnishing the desired
compound in 51% yield. Unfortunately, all attempts to
acetoxylate this molecule under a variety of conditions (stated
in Table 1) failed, and the substrate was quantitatively
recovered (Scheme 3).
2,4-Dimethylbenzo[h]quinoline (11) was another sterically
hindered analogue that was designed for this study. This
compound was synthesized following the classical Combes
reaction (Scheme 4).27 Acetoxylation at 150 °C followed by
hydrolysis afforded the desired compound 12 in 59% yield. It is
noteworthy to add that classical Sanford conditions gave
product 12 in only 15% yield. Methylpyridines are known to be
easily oxidizable to the corresponding aldehydes. Since the
formyl group allows for an almost unlimited number of further
chemical transformations, 2,4-dimethylbenzo[h]quinoline was
oxidized using SeO2, giving a mixture of two aldehydes
(Scheme 4). The dialdehyde 13 (formed in 19% yield) was
easily identified, but identification of the monoaldehyde 14 was
only possible after obtaining X-ray quality crystals (Figure S1 in
the Supporting Information). It was of interest to investigate if
oxidative acetoxylation could be efficient in the presence of the
easily oxidizable formyl group. It turned out that aldehyde 15
possessing the acetyloxy group could be prepared in 20% yield
(Scheme 4).
One of the most difficult regions of benzo[h]quinoline to
functionalize is the middle ring. These positions of the
molecule cannot be easy functionalized via electrophilic
aromatic substitution (in contrast to the phenolic ring).16 We
approached this problem starting from 1-amino-4-bromonaph-
thalene (16) and performed the Skraup reaction with
paraaldehyde (17) (Scheme 5).28 Acetoxylation of the resulting
bromobenzoquinoline 18 was straightforward and led directly
to phenol 19. Bromobenzoquinoline 18 was also subjected to
both Buchwald−Hartwig amination as well as Sonogashira
coupling (Scheme 5). Both reactions proceeded without
problems, and products 20 and 21 were subsequently
acetoxylated (Scheme 5). The reaction with amine 20
proceeded more efficiently in dioxane (see Table 1, entry
12), and in analogy to the previous example, hydrolysis
occurred under acetoxylation conditions to give phenol 22. The
π-expanded analogue 21 was also smoothly acetoxylated under
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Scheme 5
Scheme 6
rings. Following a recently published procedure, we synthesized
the required benzo[c]phenanthridine (27),30 which was
subsequently transformed into the required 4-hydroxybenzo-
[c]phenanthridine (28) in 50% yield (Scheme 7).
Scheme 7
the previously identified high-temperature conditions, and the
initially formed ester spontaneously hydrolyzed into the
corresponding phenol 23 during chromatography. Such
hydrolysis has been previously observed by Sanford and co-
workers (at around 10%).17a
An elegant way to build a benzo[h]quinoline skeleton
decorated with substituents at the pyridine ring proceeds via
the Dobner reaction. Condensation of 1-aminonaphthalene (7)
̈
with pyruvic acid and 4-chlorobenzaldehyde (24) followed by
esterification gave ester 25 in 12% yield (Scheme 6).29
Subjecting this compound to the new conditions gave phenol
26 directly in 32% yield after chromatography. The attempts to
hydrolyze the acetoxy compound under typical reaction
conditions (NaOH, H2O, and THF) resulted in the hydrolysis
of both ester groups present in the molecule, giving an acid
which could not be isolated.
The spectral characteristics of products 4, 6, 12, 19, 22, 23,
26, and 28 were then examined and compared to those of the
parent 10-hydroxybenzo[h]quinoline (Table 2). The most
notable feature was that ESIPT occurred in all compounds
studied, and emission was only observed from the excited state
of the keto form. According to expectations, the bathochromic
It was of particular interest to prepare the regioisomer of 10-
hydroxybenzo[c]acridine with a nonlinear arrangement of the
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solution of dye 4 resulted in hypsochromic shift of emission
combined with sharp increase in Φfl (see the abstract graphic).
In conclusion, we have developed versatile conditions for the
coordination-assisted C−H acetoxylation of benzo[h]quinoline
derivatives, which work for sterically hindered substrates. This
critical development significantly broadens the scope of
substrates that can be functionalized using the Sanford reaction,
and it gives access to a variety of derivatives and analogues of
10-hydroxybenzo[h]quinoline. Both steric hindrance and
electron density were found to be critical factors influencing
the Sanford reaction. Regardless of the type of structural
modification (π-expansion of the chromophore in various ways,
strong electron-donating and electron-withdrawing substitu-
ents), all compounds displayed excited state intramolecular
proton transfer, which was reflected in a large Stokes shift. A
detailed study of the optical properties of the phenolic products
allowed us to observe that substitution of 10-hydroxybenzo-
quinolines led to a decrease in the Stokes shift while
maintaining the same fluorescence quantum yields. Even
smaller Stokes shifts were displayed by the π-expanded
analogues. We found that chemical modifications resulted in
significantly altered spectroscopic properties relative to the
parent 10-hydroxybenzo[h]quinoline, such as red-shifted
absorption and emission maxima. In the future, derivatives of
HBQ can find applications as fluorophores or emitters in
analogy to well-known complexes of 8-hydroxyquinolines.32
Table 2. Spectroscopic Properties of Compounds 4, 6, 12,
19, 22, 23, 26, and 28
a
compound
solvent
λabs/nm λem/nm
Φfl
Stokes shift (cm−1
)
HBQ
4
CH3CN
CH3CN
CH2Cl2
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
CH2Cl2
370
418
414
371
375
379
381
409
382
625
642
632
573
578
575
603
b
0.003
<0.001
0.001
0.004
0.002
0.002
0.001
b
11000
8300
8300
9500
9370
9000
9660
6
12
19
22
23
26
28
b
b
a
Determined in MeCN using quinine sulfate in 0.05 M H2SO4 as a
b
standard. Below the detection limit.
shift of absorption was visible when going from simple
derivatives of benzo[h]quinoline to the π-expanded analogues
(Table 2, Figure 2, λmax (12) = 371 nm, λmax (4) = 418 nm). It
is well-established that angular fusion of a benzene ring to a
given chromophore usually does not lead to significant
bathochromic shift of absorption.31 Accordingly, the compar-
ison of derivatives of benzo[h]quinoline with derivative of
benzo[c]phenantridine (i.e., 12 → 28) reveals almost no
bathochromic shift (Table 2).
Fluorescence quantum yields of products 4, 6, 12, 19, 22,
and 23 were found to be very low (0.1−0.4) in CH3CN. For
compounds 26 and 28, these values were nonmeasurable
(Table 2). Fluorescence spectra were obtained by exciting the
molecules at 480 nm. When compared to the parent HBQ, the
Stokes shift of all its substituted derivatives was lower (11 000
cm−1 for HBQ and 9000−9660 cm−1 for compounds 12, 19,
22, and 23 in CH3CN). The difference had its origin in the
emission values, which were significantly hypsochromically
shifted, whereas absorption remained at the same place, even
for the π-expanded compound 23. For both hydroxybenzoa-
cridines 4 and 6, the Stokes shift was even lower (8300 cm−1).
Absorption of compound 4 was bathochromically shifted versus
HBQ (∼45 nm); however, emission was shifted only slightly
(Table 2). An addition of an excess of strong acid (TFA) to the
EXPERIMENTAL SECTION
■
All chemicals were used as received unless otherwise noted. Reagent-
grade solvents (CH2Cl2, hexanes) were distilled prior to use. All
1
reported H NMR and 13C NMR spectra were collected using 600,
500, 400, or 200 MHz spectrometers. Chemical shifts (δ ppm) were
determined with TMS as the internal reference; J-values are given in
Hz. The UV−vis absorption spectra were recorded in CH2Cl2 or TFA.
The absorption wavelengths are reported in nm with the extinction
coefficient in M−1 cm−1 in brackets. The melting points of compounds
were determined using a capillary-type apparatus. Chromatography
was performed on silica (230−400 mesh) or neutral alumina. Dry
column vacuum chromatography (DCVC)33 was performed on
preparative thin-layer chromatography alumina. The mass spectra
were obtained via field desorption MS (FD-MS), electrospray
ionization (ESI-MS), and electron impact MS (EI-MS). Compounds
Figure 2. Absorption and emission of compounds 4 (solid line), 22 (dashed line), and HBQ (dotted line).
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3,20 5,25 11,27 18,28 25,29 and 2730 were prepared according to the
literature procedures. A spectrophotometer and a spectrofluorimeter
were used to acquire the absorption and emission spectra.
Spectrophotometric-grade solvents were used without further
purification.
6-Bromo-2-methylbenzo[h]quinoline. (18): mp 101−103 °C
(lit.28 99−100 °C); 1H NMR (400 MHz, CDCl3, δ) 2.81 (s, 3H), 7.36
(d, 1H, J = 8.2 Hz), 7.72−7.80 (m, 2H), 7.94 (d, 1H, J = 8.3 Hz), 7.99
(s, 1H), 8.27−8.33 (m, 1H), 9.34−9.39 (m, 1H); 13C NMR (100
MHz, CDCl3, δ) 25.4, 121.2, 122.7, 124.5, 124.6, 127.3, 127.5, 128.6,
128.8, 131.9, 132.3, 135.0, 145.2, 158.2; EI-HR found 270.9994 [M+],
calcd. 270.9997 (C14H10N79Br); λabs (CH2Cl2, ε × 10−3) 352 (4.5),
336 (3.9), 321 (2.0), 302 (10.0), 290 (9.7), 269 (24.2), 246 (37.1)
nm; λem (cyclohexane) 359, 374, 395, 416 nm. Anal. Calcd for
C14H10NBr: C, 61.79%; H, 3.70%; N, 5.15%. Found: C, 61.71%; H,
3.68%; N, 5.12%.
7-Phenylbenzo[c]acridine.20 (3): mp 141−143 °C (lit.20 140
1
°C); H NMR (500 MHz, CDCl3, δ) 7.42−7.48 (m, 4H), 7.53−7.62
(m, 4H), 7.67−7.73 (m, 2H), 7.76−7.83 (m, 3H), 8.43 (d, 1H, J =
9.5), 9.60 (d, 1H, J = 9.6); 13C NMR (125 MHz, CDCl3, δ) 123.1,
124.0, 125.5, 125.8, 125.9, 126.5, 127.2, 127.4, 127.7, 128.2, 128.4,
129.1, 129.3, 129.9, 130.5, 131.6, 133.6, 136.3, 146.1, 147.3, 147.4; EI-
HR found 305.1202 [M+], calcd. 305.1204 (C23H15N); λabs (cyclo-
hexane) 389, 368, 350, 336; λem (cyclohexane) 395, 417 nm; IR (KBr)
3051, 1492, 750 cm−1.
2-Methyl-6-morpholinebenzo[h]quinoline (20). 6-Bromo-2-
methylbenzo[h]quinoline (18) (500 mg, 1.8 mmol), morpholine
(0.39 mL, 4.5 mol), Pd(OAc)2 (50 mg, 0.22 mmol), P(t-Bu)3 (0.9 mL,
0.25 M in dioxane), t-BuONa (0.3 g, 3.1 mmol), and dioxane (14 mL)
were heated under nitrogen in a Schlenk flask at room temperature for
48 h. The residue was filtered through a small pad of Celite, dissolved
in CH2Cl2, evaporated to dryness, and purified by column
chromatography (DCVC, Al2O3, hexanes → CH2Cl2/hexanes 5:95,
1:9, 2:8). The pure product was obtained after crystallization (AcOEt/
hexanes) as white crystals (357 mg, 71%): mp 127−128 °C; 1H NMR
(600 MHz, CDCl3, δ) 2.80 (s, 3H), 3.17 (s, 4H), 4.00 (t, 4H, J = 4.4),
7.16 (s, 1H), 7.34 (d, 1H, J = 7.9), 7.66−7.72 (m, 2H), 7.96 (d, 1H, J
= 6.5), 8.23−8.26 (m, 1H), 9.35 (s, 1H); 13C NMR (150 MHz,
CDCl3, δ) 25.2, 53.4, 67.4, 112.1, 122.3, 123.4, 124.5, 125.0, 126.8,
127.7, 130.2, 132.5, 135.1, 143.8, 147.4, 156.2; EI-HR found 278.1412
[M+], calcd. 278.1419 (C18H18N2O); λabs (CH2Cl2, ε × 10−3) 356
(3.5), 340 (3.8), 309 (8.8), 272 (19.6), 241 (47.3) nm; λem
8,9,10,11-Tetrafluorobenzo[c]acridine (9). A two-neck round-
bottom flask (100 mL) was charged with pentafluorobenzaldehyde (8)
(864 μL, 7 mmol), 1-aminonaphthalene (7) (2.0 g, 14 mmol), and 1,2-
dichlorobenzene (30 mL). The reaction was heated under reflux for
2.5 h under argon. Subsequently, the reaction mixture was allowed to
cool, which led to product crystallization. Yellow crystals were filtered
and rinsed with hot AcOEt. The pure compound was obtained after
heating the crude product in AcOEt, followed by filtration (1.07 g,
1
51%): mp 257−260 °C; H NMR (500 MHz, CF3COOD, δ) 8.00−
8.25 (m, 5H), 9.16 (d, 1H, J = 9.4), 9.81 (s, 1H), 11.50 (s, 1H); 13C
NMR (125 MHz, CF3COOD, δ) 110.8, 112.7, 113.1, 113.5, 114.4,
114.6, 114.9, 115.3, 115.7, 117.6, 120.4, 123.4, 123.5, 126.8, 129.8,
130.2, 132.5, 135.6, 137.4, 141.0, 142.3; 19F NMR (470 MHz,
CF3COOD, δ) −155.46 (t, 1F, J = 15.5), −152.16 (t, 1F, J = 16.2),
−144.48−(−144.36) (m, 1F), −138.22−(−138.10) (m, 1F); EI-HR
found 301.0504 [M+], calcd. 301.0515 (C17H7NF4); λabs (cyclohexane)
390, 370, 278 nm; λem (cyclohexane) 441, 417, 395 nm; IR (KBr)
(cyclohexane) 411 nm; IR (KBr) 3052, 2824, 1596, 1119, 775 cm−1
.
2-Methyl-6-(phenylacetylenyl)benzo[h]quinoline (21). Pd-
(PhCN)2Cl2 (5.2 mg, 0.014 mmol), 6-bromo-2-methylbenzo[h]-
quinoline (18) (100 mg, 0.450 mmol), and CuI (1.7 mg, 0.009
mmol; stored under argon) were added to a dry Schlenk flask, which
was then purged with argon and charged with dioxane (0.9 mL). P(t-
Bu)3 (117 μL of a 0.25 M solution in dioxane; 0.028 mmol), HN(i-
Pr)2 (76 μL, 0.540 mmol), and phenylacetylene (82 μL, 0.750 mmol)
were added via a syringe to the stirred reaction mixture. During the
reaction, the progress of which was followed by TLC, precipitation of
[H2N(i-Pr)2]Br was observed. After 4 h (at which point the aryl
bromide had been consumed), the reaction mixture was diluted with
AcOEt, filtered through a small pad of silica gel (with AcOEt rinsing),
concentrated, and purified by flash chromatography (SiO2, CH2Cl2/
hexanes 1:9). The pure product was obtained after crystallization from
hot cyclohexane (99 mg, 75%): mp 134−136 °C; 1H NMR (600
MHz, CDCl3, δ) 2.84 (s, 3H), 7.36−7.45 (m, 4H), 7.65−7.70 (m,
2H), 7.72−7.82 (m, 2H), 7.98 (s, 1H), 8.02 (d, 1H), 8.46−8.53 (m,
1H), 9.33−9.40 (m, 1H); 13C NMR (100 MHz, CDCl3, δ) 25.5, 87.3,
94.2, 119.5, 122.6, 123.2, 123.5, 124.6, 126.2, 127.2, 128.4, 128.5,
128.5, 130.0, 131.1, 131.7, 132.9, 135.7, 145.8, 158.6; EI-HR found
293.1217 [M+], calcd. 293.1204 (C22H15N); λabs (CH2Cl2, ε × 10−3)
362 (8.1), 345 (16.4), 322 (27.6), 278 (32.1), 246 (39.8) nm; λem
(cyclohexane + a few drops of CH2Cl2) 408, 388, 369 nm; IR (KBr)
3054, 2923, 2201, 1279, 759 cm−1. Anal. Calcd for C22H15N: C,
90.07%; H, 5.15%; N, 4.77%. Found: C, 90.04%; H, 5.01%; N, 4.64%.
General Procedure for Acetoxylation of Benzo[h]quinolines
and Their Analogues. A sealed tube was charged with the
benzo[h]quinoline-type substrate (0.6 mmol), PhI(OAc)2 (397 mg,
1.2 mmol), Pd(OAc)2 (3.7 mg, 0.0115 mmol), and MeCN (4.8 mL).
The mixture was stirred at 150 °C for 16 h. The residue was moved to
a round-bottom flask, evaporated to dryness, dissolved in MeOH/
THF, and filtered through Celite to remove an insoluble black solid.
Subsequently, solid NaOH (1.0 g, 25 mmol) was added to the
transparent solution. After stirring at room temperature for a few
hours, the mixture was acidified with 10% HCl to neutral pH (the
solvent was removed under reduced pressure) and extracted between
CH2Cl2 and water. The organic layer was dried over dry Na2SO4 and
removed under vacuum. The details of purification are given in each
case as follows.
3078, 1592, 1492, 1026, 756 cm−1
.
2,4-Dimethylbenzo[h]quinoline.27 (11): mp 128−130 °C
(lit.27 126 °C); H NMR (500 MHz, CDCl3, δ) 2.69 (s, 3H), 2.77
1
(s, 3H), 7.22 (s, 1H), 7.62−7.71 (m, 2H), 7.75 (d, 1H, J = 9.1 Hz),
7.83−7.89 (m, 2H), 9.35 (d, 1H, J = 8.1 Hz); 13C NMR (125 MHz,
CDCl3, δ) 18.9, 25.3, 121.2, 123.2, 123.7, 124.8, 126.2, 126.6, 127.5,
127.7, 131.7, 133.4, 143.9, 145.7, 157.2; EI-HR found 207.1057 [M+],
calcd. 207.1048 (C15H13N); λabs (CH2Cl2, ε × 10−3) 348 (3.9), 332
(3.3), 317 (1.9), 299 (8.9), 268 (26.5), 245 (38.7) nm; λem
(cyclohexane) 353, 369, 387, 410 nm; (KBr) 3058, 1498, 741 cm−1
.
Anal. Calcd for C15H13N: C, 86.92%; H, 6.32%; N, 6.76%. Found: C,
87.14%; H, 6.34%; N, 6.48%.
Oxidation of 2,4-Dimethylbenzo[h]quinoline using SeO2.
2,4-Dimethylbenzo[h]quinoline (11) (500 mg, 2.4 mmol) and SeO2
(1.2 g) were heated under reflux in 1,4-dioxane (25 mL) for 38 h. The
residue was filtered through Celite, and the solvent was removed under
reduced pressure. The solid thus obtained was dissolved in hot AcOEt,
and then silica gel was added, and the solvent was evaporated to
dryness. The solid was chromatographed (SiO2, CH2Cl2/hexanes 1:4,
2:3, 1:1), resulting in the separation of two products. The pure
products were obtained after crystallization from hot AcOEt:
monoaldehyde 14 (326 mg, 61%) and dialdehyde 13 (67 mg, 19%).
4-Methyl-2-formylbenzo[h]quinoline (14): mp 154−156 °C; 1H
NMR (500 MHz, CDCl3, δ) 2.77 (s, 3H), 7.69−7.81 (m, 2H),
7.85−7.96 (m, 4H), 9.40 (dd, 1H, J1 = 1.0 Hz, J2 = 2.0 Hz), 10.29 (s,
1H); 13C NMR (125 MHz, CDCl3, δ) 19.21, 119.3, 121.0, 124.9,
127.7, 127.8, 128.4, 128.6, 130.2, 131.2, 133.4, 145.3, 146.0, 150.5,
194.4; EI-HR found 221.0848 [M+], calcd. 221.0841 (C15H11NO); λabs
(cyclohexane) 358, 341, 336, 323, 285, 258 nm; IR (KBr) 3065, 2841,
1694, 763 cm−1. 2,4-Diformylbenzo[h]quinoline (13): mp 176−178
1
°C; H NMR (500 MHz, CDCl3, δ) 7.79−7.87 (m, 2H), 7.96−7.80
(m, 1H), 8.12 (d, 1H, J = 9.2), 8.50 (s, 1H), 8.93 (d, 1H, J = 9.2),
9.41−9.44 (m, 1H), 10.40 (s, 1H), 10.60 (s, 1H); 13C NMR (125
MHz, CDCl3, δ) 120.9, 122.2, 125.0, 125.3, 128.0, 128.4, 129.6, 131.1,
133.4, 133.6, 137.5, 147.8, 151.2, 192.4, 192.9; EI-HR found 235.0639
[M+], calcd. 235.0633 (C15H9NO2); IR (KBr) 3055, 2852, 1698, 769
cm−1. Anal. Calcd for C15H9NO2: C, 76.59%, H, 3.86%, N, 5.95%.
Found: C, 76.30%; H, 3.97%; N, 5.91%.
1-Hydroxy-7-phenylbenzo[c]acridine (4). A sealed tube was
charged with 7-phenylbenzo[c]acridine (3) (51 mg, 0.166 mmol),
10225
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Article
PhI(OAc)2 (107 mg, 0.33 mmol), Pd(OAc)2 (3.7 mg, 0.0166 mmol),
and 1,4-dioxane (1.4 mL). The mixture was stirred at 150 °C for 16 h.
The residue was moved into a round-bottom flask, evaporated to
dryness, dissolved in MeOH/THF, and filtered through Celite to
remove an insoluble black solid. Then, a slight excess of NaOH was
added to the clear solution. After stirring at room temperature for a
few hours, the mixture was acidified with 10% HCl to neutral pH (the
solvent was removed under reduced pressure) and extracted between
CH2Cl2 and water. The organic layer was dried over dry Na2SO4 and
removed under vacuum. The solid was chromatographed (DCVC,
Al2O3, hexanes → hexanes/CH2Cl2 99:1, 98:2, 96:4). The pure
product was obtained after crystallization from hot cyclohexane (34
mg, 64%): mp 218−221 °C; 1H NMR (500 MHz, CDCl3, δ) 7.32 (d,
2H, J = 7.6), 7.39 (d, 1H, J = 9.4), 7.43−7.45 (m, 2H), 7.53 (ddd, 1H,
J1 = 1.2, Hz, J2 = 6.7 Hz, J3 = 7.3 Hz), 7.50 (d, 1H, J = 9.4), 7.59−7.67
(m, 4H), 7.70−7.74 (m, 1H), 7.80−7.85 (m, 1H), 8.30 (d, 1H, J =
8.5), 16.10 (br. s, 1H); 13C NMR (125 MHz, CDCl3, δ) 115.1, 115.2,
118.2, 123.2, 123.3, 125.3, 126.1, 126.7, 127.5, 128.6, 128.6, 128.7,
130.2, 130.3, 131.0, 135.0, 135.6, 143.6, 146.9, 149.9, 160.9; EI-HR
found 321.1167 [M+], calcd. 321.1154 (C23H15NO); λabs (CH2Cl2, ε ×
10−3) 420 (10.4), 370 (6.4), 352 (5.9), 269 (45.8) nm; λem
(OAc)2 (142 mg, 0.44 mmol), and Pd(OAc)2 (2.5 mg, 0.011 mmol)
were reacted according to the general procedure. The solid was
chromatographed (DCVC, Al2O3, hexanes → hexanes/AcOEt 98:2).
The pure product was obtained after crystallization from hot
1
cyclohexane (29 mg, 46%): mp 165−167 °C; H NMR (500 MHz,
CDCl3, δ) 2.80 (s, 3H), 7.31 (dd, 1H, J1 = 1.0 Hz, J2 = 8.0 Hz), 7.39
(d, 1H, J = 8.2 Hz), 7.68 (t, 1H, J = 8.0 Hz), 7.80 (dd, 1H, J1 = 1.0 Hz,
J2 = 8.2 Hz), 8.03 (d, 1H, J = 8.1 Hz), 8.42 (dd, 1H, J1 = 1.3 Hz, J2 =
8.3 Hz), 15.41 (s, 1H); 13C NMR (125 MHz, CDCl3, δ) 24.4, 115.0,
116.1, 117.8, 121.8, 123.0, 124.0, 127.9, 130.6, 133.2, 135.9, 146.7,
154.9, 159.7; EI-HR found 286.9933 [M+], calcd. 286.9946
(C14H10NO79Br); λabs (CH2Cl2, ε × 10−3) 378 (7.2), 363 (6.3), 319
(5.0), 249 (41.5), 241 (43.6) nm; λem (acetonitrile) 578 nm; IR (KBr)
3436, 3056, 2922, 2538, 1593, 1419, 1270, 891 cm−1
.
10-Hydroxy-2-methyl-6-morpholinebenzo[h]quinoline
(22). 6-Morpholino-2-methylbenzo[h]quinoline (20) (100 mg, 0.36
mmol), PhI(OAc)2 (162 mg, 0.72 mmol), and Pd(OAc)2 (8 mg, 0.036
mmol) were reacted according to the general procedure but in 1,4-
dioxane (3 mL). The solid was chromatographed (DCVC, Al2O3,
hexanes → hexanes/AcOEt 98:2). The pure product was obtained
after crystallization from hot cyclohexane (23 mg, 22%): mp 176−177
1
(acetonitrile) 642 nm; IR (KBr) 3041,1465, 754 cm−1
.
°C; H NMR (500 MHz, CDCl3, δ) 2.78 (s, 3H), 3.17 (br. s, 4H),
4.00 (t, 4H, J = 4.5 Hz), 7.13 (s, 1H), 7.26 (d, 1H, J = 6.1 Hz), 7.36 (d,
1H, J = 8.2 Hz), 7.61 (t, 1H, J = 8.0 Hz), 7.74 (d, 1H, J = 8.0), 8.06 (d,
1H, J = 8.2), 15.54 (br. s, 1H); 13C NMR (125 MHz, CDCl3, δ) 24.1,
53.3, 67.3, 111.4, 113.7, 114.0, 116.6, 121.4, 124.5, 129.6, 131.8, 135.9,
145.2, 148.8, 152.6, 160.2; ESI-HR found 295.1454 [M + H+], calcd.
295.1441 (C18H19N2O2); λabs (CH2Cl2, ε × 10−3) 381 (7.7), 323 (9.3),
268 (18.2), 248 (41.1) nm; λem (acetonitrile) 574 nm; IR (KBr) 3039,
2956, 2541, 1528, 1116 cm−1.
10-Hydroxy-2-methyl-6-(phenylacetylenyl)benzo[h]-
quinoline (23). A sealed tube was charged with 6-phenylacetylenyl-
2-methylbenzo[h]quinoline (21) (73.5 mg, 0.25 mmol), PhI(OAc)2
(161 mg, 0.5 mmol), Pd(OAc)2 (1.12 mg, 0.005 mmol), and MeCN
(2.1 mL). The mixture was stirred at 150 °C for 4.5 h. The residue was
moved into a round-bottom flask, evaporated to dryness, and dissolved
in CH2Cl2. Aluminum oxide was added to this solution and again
evaporated to dryness. The solid was chromatographed (DCVC,
Al2O3, hexanes → hexanes/CH2Cl2 95:5, 1:9). The pure, bright-yellow
product was obtained after crystallization from hot EtOH (41 mg,
53%): mp 161−163 °C; 1H NMR (500 MHz, CDCl3, δ) 2.82 (s, 3H),
7.33 (d, 1H, J = 7.8 Hz), 7.38−7.43 (m, 4H), 7.64−7.69 (m, 2H), 7.70
(d, 1H, J = 8.0), 7.89 (s, 1H), 7.99 (d, 1H, J = 7.7) 8.10 (d, 1H, J =
8.2), 15.28 (br. s, 1H); 13C NMR (125 MHz, CDCl3, δ) 24.4, 87.2,
94.8, 114.5, 115.2, 116.6, 121.3, 121.7, 123.0, 123.5, 128.5, 128.7,
128.9, 130.4, 131.8, 134.3, 136.8, 146.8, 155.0, 159.6; EI-HR found
309.1148 [M+], calcd. 309.1154 (C22H15NO); λabs (CH2Cl2, ε × 10−3)
594 (0.1), 385 (9.7), 345 (14.8), 280 (30.8), 249 (43.2) nm; λem
(CH2Cl2) 612 nm, (acetonitrile) 602 nm; IR (KBr) 3056, 2918, 2197,
1271, 754 cm−1.
Ethyl 2-(4-Chlorophenyl)-10-hydroxybenzo[h]quinoline-4-
carboxylate (26). Ethyl 2-(4-chlorophenyl)-benzo[h]quinoline-4-
carboxylate (25) (70 mg, 0.19 mmol), PhI(OAc)2 (125 mg, 0.39
mmol), and Pd(OAc)2 (4.3 mg, 0.019 mmol) were reacted according
to the general procedure. The residue was transferred into a round-
bottom flask, evaporated to dryness, and dissolved in CH2Cl2.
Aluminum oxide was added to this solution and again evaporated to
dryness. The solid was chromatographed (DCVC, Al2O3, hexanes →
hexanes/AcOEt 98:2). The pure product 26 was obtained after
crystallization from hot cyclohexane (23 mg, 32%): mp 157−159 °C;
1H NMR (500 MHz, CDCl3, δ) 1.53 (t, 3H, J = 7.2), 4.57 (q, 2H, J =
7.1), 7.28 (dd, 1H, J1 = 1.0 Hz, J2 = 8.0 Hz), 7.41 (d, 1H, J = 7.2), 7.54
(d, 2H, J = 8.4), 7.65 (t, 1H, J = 7.8), 7.86 (d, 1H, J = 9.5), 8.01 (d,
2H, J = 8.4), 8.35 (s, 1H), 8.49 (d, 1H, J = 9.1), 15.06 (s, 1H); 13C
NMR (125 MHz, CDCl3, δ) 14.3, 62.4, 114.6, 115.4, 118.5, 118.9,
121.6, 123.1, 128.4, 129.6, 130.6, 130.8, 124.8, 135.7, 136.5, 136.6,
149.1, 151.5, 159.2, 165.9; EI-HR found 377.0813 [M+], calcd.
377.0819 (C22H16NO335Cl); λabs (CH2Cl2, ε × 10−3) 409 (9.8), 306
(20.4), 266 (28.6), 244 (39.2) nm; IR (KBr) 3063, 2922, 2201, 1279,
771 cm−1.
1-Hydroxy-7-methylbenzo[c]acridine (6). 7-Methylbenzo[c]-
acridine (5) (50 mg, 0.2 mmol), PhI(OAc)2 (129 mg, 0.4 mmol),
and Pd(OAc)2 (4.5 mg, 0.02 mmol) were reacted according to the
general procedure. The solid was chromatographed (DCVC, Al2O3,
hexanes → hexanes/AcOEt 97:3). The product was suspended in a
mixture of MeOH/cyclohexane, then filtered to obtain pure crystals
(15 mg, 27%): mp 211−214 °C; 1H NMR (600 MHz, CDCl3, δ) 3.03
(s, 3H), 7.25−7.30 (m, 2H), 7.58−7.62 (m, 3H), 7.75−7.80 (m, 1H),
7.82 (d, 1H, J = 9.3), 8.31 (d, 1H, J = 8.6), 8.20 (d, 1H, J = 5.6), 16.18
(s, 1H); 13C NMR (150 MHz, CDCl3, δ) 14.0, 115.1, 118.0, 118.2,
121.3, 123.2, 124.3, 125.3, 125.9, 127.7, 128.4, 130.0, 130.8, 134.8,
142.2, 142.8, 149.1, 160.8; EI-HR found 259.0991 [M+], calcd.
259.0997 (C18H13NO); λabs (CH2Cl2, ε × 10−3) 417 (8.4), 367 (5.6),
350 (5.4), 334 (4.6), 268 (44.3), 247 (25.7) nm; λem (acetonitrile) 632
nm; IR (KBr) 3049, 2961, 1603, 1283, 819 cm−1
.
10-Hydroxy-2,4-dimethylbenzo[h]quinoline (12). 2,4-
Dimethylbenzo[h]quinoline (11) (124 mg, 0.6 mmol), PhI(OAc)2
(397 mg, 1.2 mmol), Pd(OAc)2 (3.7 mg, 0.0115 mmol) were reacted
according to the general procedure. The solid was chromatographed
(DCVC, Al2O3, hexanes → hexanes/CH2Cl2 9:1). The pure product
was obtained after crystallization from hot cyclohexane (80 mg, 59%):
1
mp 124−126 °C; H NMR (500 MHz, CDCl3, δ) 2.70 (s, 3H), 2.73
(s, 3H), 7.17−7.22 (m, 2H), 7.36 (dd, 1H, J1 = 1.0 Hz, J2 = 7.8 Hz),
7.57 (t, 1H, J = 7.8), 7.74 (s, 2H), 15.57 (s, 1H); 13C NMR (125 MHz,
CDCl3, δ) 19.2, 24.2, 113.4, 116.1, 117.7, 120.6, 122.4, 123.3, 127.6,
129.5, 135.0, 145.1, 147.3, 153.8, 159.8; EI-HR found 223.0992 [M+],
calcd. 223.0997 (C15H13NO); λabs (CH2Cl2, ε × 10−3) 372 (9.8), 305
(6.8), 267 (24.3), 244 (68.8) nm; λem (acetonitrile) 573 nm; IR (KBr)
3042, 2970, 1597, 1274, 826 cm−1. Anal. Calcd for C15H13NO: C,
80.69%; H, 5.87%; N, 6.27%. Found: C, 80.37%; H, 5.91%; N, 6.23%.
10-Acetoxy-4-methyl-2-formylbenzo[h]quinoline (15). 4-
Methyl-2-formylbenzo[h]quinoline (14) (57 mg, 0.26 mmol), PhI-
(OAc)2 (167 mg, 0.52 mmol), and Pd(OAc)2 (5.8 mg, 0.026 mmol)
were reacted according to the general procedure. The residue was
moved into the round-bottom flask, evaporated to dryness, and
dissolved in CH2Cl2. Aluminum oxide was added to this solution and
again evaporated to dryness. The solid was chromatographed (DCVC,
Al2O3, hexanes → hexanes/AcOEt 95:5, 1:9). The pure product was
obtained after crystallization from hot cyclohexane (12,3 mg, 20%):
1
mp 109−111 °C; H NMR (500 MHz, CDCl3, δ) 2.60 (s, 3H), 2.81
(s, 3H), 7.47 (d, 1H, J1 = 7.5 Hz), 7.47 (t, 1H, J = 7.8 Hz), 7.89 (d,
1H, J = 7.8 Hz), 7.95−8.01 (m, 3H), 10.26 (s, 1H); 13C NMR (125
MHz, CDCl3, δ) 19.7, 22.1, 119.7, 121.9, 123.2, 123.9, 127.1, 128.8,
129.5, 130.7, 135.9, 145.3, 145.5, 148.8, 150.1, 170.3, 193.4; ESI-HR
found 302.0800 [M + Na+], calcd. 302.0788 (C17H13NO3Na); IR
(KBr) 3049, 2924, 2825, 1696, 1206, 754 cm−1
.
6-Bromo-10-hydroxy-2-methylbenzo[h]quinoline (19). 6-
Bromo-2-methylbenzo[h]quinoline (18) (60 mg, 0.22 mmol), PhI-
10226
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4-Hydroxybenzo[c]phenanthridine (28). Benzo[c]-
phenanthridine (27) (100 mg, 0.44 mmol), PhI(OAc)2 (281 mg,
0.88 mmol), and Pd(OAc)2 (9.9 mg, 0.044 mmol) were reacted
according to the general procedure. The residue was moved into a
round-bottom flask, evaporated to dryness, and dissolved in CH2Cl2.
Aluminum oxide was added to this solution and again evaporated to
dryness. The solid was chromatographed (DCVC, Al2O3, hexanes →
hexanes/AcOEt 99:1, 95:5). The pure product was obtained after
crystallization from hot cyclohexane (52 mg, 48%): mp 166−167 °C;
1H NMR (600 MHz, CDCl3, δ) 7.22 (dd, 1H, J1 = 1.0 Hz, J2 = 7.8
Hz), 7.46 (d, 1H, J = 7.5), 7.59 (t, 1H, J = 7.8), 7.70−7.74 (m, 1H),
7.91 (ddd, 1H, J1 = 1.2, Hz, J2 = 6.8 Hz, J3 = 8.5 Hz), 7.98 (d, 1H, J =
9.0), 8.12 (d, 1H, J = 8.0), 8.41 (d, 1H, J = 9.0), 8.63 (d, 1H, J = 8.5),
14.90 (s, 1H); 13C NMR (150 MHz, CDCl3, δ) 113.3, 117.3, 118.0,
119.3, 120.8, 122.2, 125.7, 127.3, 128.5, 129.0, 129.2, 131.6, 132.6,
134.8, 142.6, 148.4, 158.8; EI-HR found 245.0838 [M+], calcd.
245.0841 (C17H11NO); λabs (CH2Cl2, ε × 10−3) 382 (8.0), 259 (54.2)
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nm; IR (KBr) 3038, 2923, 2545, 1272, 748 cm−1
.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR and 13C NMR spectra of compounds 3−4, 6, 9, 11−
15, 18−23, 26, and 28 as well as X-ray structure of compound
14. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: dtgryko@icho.edu.pl.
■
(13) Schenkel-Rudin, H.; Schenkel-Rudin, M. Helv. Chim. Acta 1944,
27, 1456.
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Lett. 1992, 193, 151. (b) Chou, P.-T.; Chen, Y.-C.; Yu, W.-S.; Chou,
Y.-H.; Wei, C.-Y.; Cheng, Y.-M. J. Phys. Chem. A 2001, 105, 1731−
1740.
ACKNOWLEDGMENTS
This work was supported by the Ministry of Science and
Higher Education (Contract N204 124237).
■
(15) (a) Chou, P.-T.; Wei, C. Y. J. Phys. Chem. 1996, 100, 17059.
(b) Takeuchi, S.; Tahara, T. J. Phys. Chem. A 2005, 109, 10199−
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(d) Higashi, M.; Saito, S. J. Phys. Chem. Lett. 2011, 2, 2366−2371.
(16) Chen, K.-Y.; Hsieh, C.-C.; Cheng, Y.-M.; Lai, C.-H.; Chou, P.-T.
Chem. Commun. 2006, 4395.
DEDICATION
■
†Dedicated to Dr. Lucia Flamigni on occasion of her 60th
birthday.
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