A New method of synthesizing phenalen-1-one: Reduction of 3-hydroxyphenalen-1-one using nabh4 and lanthanoid chlorides

Article abstract of DOI:10.1080/00397910902883546

Phenalen-1-one was obtained in considerable yield by reducing 3-hydroxyphenalen-1-one. Most of known preparation methods are not very practical, either because their yields are very poor or because their processes have many steps. This regioselective 1,2-reduction proceeded by the action of NaBH4 and various cations of rare-earth elements and metals. The yields of phenalen-1-one were examined as a function of typical lanthanoids, molar ratios of lanthanoid ions to 3-acetoxy-phenalen-1-one, and differing methods of protecting the hydroxyl group. Lanthanum chloride (LaCl3) gave the greatest yield (45.3%) of phenalen-1-one at molar ratios higher than a third, probably because La3+ ion is a hard acid and coordinates easily to a hard solvent such as methanol. Further, it has the largest ionic radius among all lanthanoid ions. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910902883546

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A New Method of Synthesizing
Phenalen-1-one: Reduction
of 3-Hydroxyphenalen-1-one
Using NaBH4 and Lanthanoid
Chlorides
Tadanobu Sawada a , Hiroyuki Ishii a , Toyotoshi Ueda
a & Junji Aoki b
a Department of Chemistry, Faculty of Science and
Engineering , Meisei University , Hodokubo, Hino,
Tokyo, Japan
b Department of Chemistry, Faculty of Science ,
Toho University , Miyama, Funabashi, Chiba, Japan
Published online: 07 Oct 2009.
To cite this article: Tadanobu Sawada , Hiroyuki Ishii , Toyotoshi Ueda & Junji
Aoki (2009) A New Method of Synthesizing Phenalen-1-one: Reduction of 3-
Hydroxyphenalen-1-one Using NaBH4 and Lanthanoid Chlorides, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 39:21, 3912-3923, DOI: 10.1080/00397910902883546
To link to this article: http://dx.doi.org/10.1080/00397910902883546
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Synthetic Communications1, 39: 3912–3923, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902883546
A New Method of Synthesizing Phenalen-1-one:
Reduction of 3-Hydroxyphenalen-1-one Using
NaBH4 and Lanthanoid Chlorides
Tadanobu Sawada,1 Hiroyuki Ishii,1 Toyotoshi Ueda,1
and Junji Aoki2
1Department of Chemistry, Faculty of Science
and Engineering, Meisei University, Hodokubo, Hino,
Tokyo, Japan
2Department of Chemistry, Faculty of Science,
Toho University, Miyama, Funabashi,
Chiba, Japan
Abstract: Phenalen-1-one was obtained in considerable yield by reducing
3-hydroxyphenalen-1-one. Most of known preparation methods are not very
practical, either because their yields are very poor or because their processes
have many steps. This regioselective 1,2-reduction proceeded by the action
of NaBH4 and various cations of rare-earth elements and metals. The yields
of phenalen-1-one were examined as a function of typical lanthanoids,
molar ratios of lanthanoid ions to 3-acetoxy-phenalen-1-one, and differing
methods of protecting the hydroxyl group. Lanthanum chloride (LaCl3) gave
the greatest yield (45.3%) of phenalen-1-one at molar ratios higher than a
third, probably because La3þ ion is a hard acid and coordinates easily to a
hard solvent such as methanol. Further, it has the largest ionic radius among
all lanthanoid ions.
Keywords: a-Enone, 3-hydroxyphenalen-1-one, lanthanoid salt, NaBH4,
phenalen-1-one, 1,2-reduction
Received October 27, 2008.
Address correspondence to Tadanobu Sawada, Department of Chemistry,
Faculty of Science and Engineering, Meisei University, Hodokubo 2-1-1, Hino,
Tokyo 191-8506, Japan. E-mail: sawada@ge.meisei-u.ac.jp
3912

Synthesis of Phenalen-1-one
3913
Scheme 1. Polycyclic aromatic compound and two important intermediates.
INTRODUCTION
7H-Benz[de]anthracen-7-one (benzanthrone) (1) gives nanocyclic aro-
matic compounds by its self-condensation.[1] Some of them have been
used as excellent vat dyes since the beginning of the 20th century and
since 1950 have also been shown to be prototypes for organic semicon-
ductors[2] and photoconductors.[3]
Phenalen-1-one (2), which is smaller by one benzene ring than 1, as seen
in Scheme 1, also affords interesting condensation products. Among them,
dibenzo[cd,lm]perylene (peropyrene) (3) has a symmetric structure of
approximate D2h symmetry and is a significant compound that is basic in
the study of these conductors. This hydrocarbon can easily be prepared from
2 by a reported method.[4] However, the synthesis of 2 is not so easy. Several
methods have been already reported (Scheme 2),[5,6] but most of these
methods are not very practical because of poor yields or long processes.
Scheme 2. Previously reported synthetic methods of phenalen-1-one.

3914
T. Sawada et al.
We succeeded in preparing this compound in considerable yield by
reducing 3-acetoxy-phenalen-1-one (5), in which the OH group of
3-hydroxyphenalen-1-one (4) was protected, by the action of sodium
borohydride (NaBH4) and lanthanum chloride. As the starting material,
4 was obtained in a good yield by heating a mixture of naphthalic anhy-
dride and diethyl malonate in the presence of zinc chloride.[7,8] This
method may be considered as a convenient new method of synthesizing
2 and thus 3.
In this article, we report the results obtained under various reaction
conditions. The reaction mechanism will be also discussed.
RESULTS AND DISCUSSION
3-Hydroxyphenalen-1-one (4) has a tautomer, 2,3-dihydrophenalen-1,3-
dione (40), but it has been reported to exist predominantly as 4.[8] To
obtain phenalen-1-one (2) from 4, the hydroxyl group of 4 must first
be protected and then the carbonyl group of 5 [an a,b-unsaturated ketone
(a-enone)] must be reduced to the allylic alcohol type (6). Finally, 2 is
produced from 6 by dehydration and deacetylation with an acid catalyst
as shown in Scheme 3.
On the other hand, it has been previously reported that such regiose-
lective 1,2-reductions can proceed using NaBH4 and metallic ions,[9,10]
and so the yields of 2 under various reaction conditions were investigated
in detail. Each operation was carried out in a manner similar to that
described under 2 in the experimental part.
Scheme 3. New synthetic method of phenalen-1-one.

Synthesis of Phenalen-1-one
3915
Figure 1. Effect of molar ratios of cerium ion Ce3þ to 5 on the yield of 2.
Effect of Molar Ratios of CeCl3 to 5
Gemal and Luche[9] reported that the reduction of a,b-unsaturated
ketone (a-enone) proceeded efficiently in the presence of lanthanoid
chlorides, especially cerium chloride (CeCl3) at the molar ratio of Ce3þ
ion > 1:4 to substrate. When isomolar CeCl3 was applied to the mixture
of NaBH4, methanol, and tetrahydrofuran (1:1), 2 was obtained in a yield
of 38%. By-products were small amounts of 3, 2,3-dihydrophenalen-1-
one, and unknown material. As shown in Fig. 1, the yield of 2 remained
around 38% even when the molar ratio of Ce3þ ion to 5 was varied
between 2.0 and 1=3.
Effect of Other Cerium Salts
When other cerium salts were applied, the yields of 2 were very poor in
the cases of sulfate, carbonate, and acetate as shown in Fig. 2. These
observations may be explained as follows. The former two salts were
hardly soluble, and thus their solutes were suspended in the solvent
[CH3OH–tetrahydrofuran (THF)]. Cerium acetate was soluble but the
degree of ionization was low. On the other hand, the nitrate of a strong
electrolyte gave a yield as great as chloride. The necessity of ionization of
the added salt may be supported by the fact that yields of 2 were
decreased more as the content of cerium acetate was increased by the
addition of sodium acetate to the reactant solution containing cerium
chloride (Fig. 3).

3916
T. Sawada et al.
Figure 2. Effect of cerium salts on the yield of 2.
Yields of 2 Using Various Metallic Ions
Various kinds of metal chlorides were examined, using an isomolar ratio
of Mnþ to 5. As shown in Table 1 and Fig. 4, yields of 2 decreased as the
atomic number of trivalent lanthanoid cations increased from La through
Ce, Sm, and Gd to Yb. In addition, the yield of 2 decreased from La
through Y to Sc as the ionic radius of similar rare-earth metallic ions
decreased. It is well known that the trivalent ionic radius of lanthanoid
Figure 3. Effect of the ionization of Ce3þ ion on the yield of 2, caused by the
composition change between cerium chloride and cerium acetate.

Synthesis of Phenalen-1-one
3917
Table 1. Yield of 2 using various metallic ions
Ions
Yield of 2
Sc3þ
Y3þ
6.7%
26.2%
45.3%
38.2%
25.7%
24.6%
27.1%
16.2%
Trace
1.4%
La3þ
Ce3þ
Sm3þ
Gd3þ
Ho3þ
Yb3þ
Ba2þ
Sr2þ
Tl3þ
Trace
Trace
Trace
1.1%
Al3þ
Mn2þ
Fe3þ
Cu2þ
Trace
Note. The term ‘‘trace’’ signifies a yield of <1.0%.
3þ
˚
ions decreases gradually from 1.061 A in the case of La down to
0.858 A with Yb and 0.848 A with Lu3þ. It is evident from Fig. 4 that
the curve has an inflection point between Gd and Ho: the yield of Ho is
more than that of Gd. This feature might be interpreted by the decrease
of coordination number by one between Gd and Ho in the series of
lanthanoid contraction. Lanthanum ion La3þ, having the largest ionic
radius, gave the greatest yield of 2. Alkaline-earth metallic ions such as
Ba2þ and Sr2þ have larger ionic radii than lanthanoid ions but showed
3þ
˚
˚
Figure 4. Effect of increasing atomic number of lanthanoid ions on the yield of 2.

3918
T. Sawada et al.
Scheme 4. Activated complex model for the region-selective 1,2-reduction.
very poor yield of 2. Other kinds of metallic ions such as Mn2þ, Fe3þ
,
Cu2þ, Al3þ, and Tl3þ, which are bound easily to oxygen, had no effect
on the regioselective reduction. Apparently it is necessary for metallic
ions to be hard acids and to easily coordinate with hard solvents such
as methanol. They also need to have enough space to receive such large
ligands as compound 5, as shown in Scheme 4.
Protection of Hydroxyl Group
These examinations were carried out using acetoxy 4 (i.e., 5). In addition,
other ester types were also examined. Table 2 shows the yields of 2
obtained by 1 molar equivalent (equiv.) LaCl3 and 2 molar equiv. NaBH4
to 1 molar equiv. ester.
The maximum yield (45.3%) was obtained with acetyl protection (5)
and lanthanum ion. The other protecting groups were so large that the
complexation with lanthanum ion is probably less favorable than that
of the acetyl group, and hence the yields of 2 were poor, as shown in
Table 2.
Hypothetical Mechanism of Regioselective 1,2-Reduction
Compound 2 can be obtained from 4 by reduction of the carbonyl group
only to the alcoholic hydroxyl group for a-enone [O(1) ¼ C(2)–C(3) ¼
C(4); see also Scheme 3]. A reducing agent such as sodium borohydride
NaBH4 may give all four types of reduced products; those of 1,2-reduction,

Synthesis of Phenalen-1-one
3919
Table 2. Yield of 2 from ester type (R) protection of 4
R
Yield of 2
H (4)
CH3CO (Ac) (5)
Trace
45.3%
11.0%
3.9%
Note. The term ‘‘trace’’ signifies a yield of <1.0%.
3,4-reduction, 1,4-reduction, and complete 1,2,3,4-reduction are HOꢀCHꢀ
=
=
=
C C, O CꢀCHꢀCH, HOꢀC CꢀCH, and HOꢀCHꢀCHꢀCH,
respectively. NaBH4 alone preferentially gave the starting material 4
and only 4% of 2 after the dehydration and deacetylation. It worked
mainly as a deacetylation agent for 5 rather than a reductant. Regioselec-
tive 1,2-reduction might be carried out by the addition of metallic ions,
especially lanthanoid ions.[9,10]
The result described previously suggests that CeCl3 reacted stoichio-
metrically with 5 rather than catalytically. Vigorous hydrogen evolution
and a large amount of precipitates, perhaps cerium trimethoxide, were
observed upon addition of NaBH4 to a solution of CeCl3 in CH3OH–
THF. This demonstrates an important role for the Ce3þ ion in the
CH3OH–BHꢀ4 reaction. Furthermore, it also suggests that the precipita-
tion reflects the formation of the salt Ce(OCH3)3 and that one-third
molar equivalence (equiv.) of CeCl3 equals one molar equiv. of proton
from methanol. Thus one molar equiv. of 5 was reduced stoichiometri-
cally together with one molar equiv. of NaBH(OCH3)3, as shown in
Scheme 4.

3920
T. Sawada et al.
Our assumption may be described as follows. For the first and second
steps, we accept that the reaction proceeds according to the mechanism of
Gemal and Luche.[9]
CH3OH þ Ln3þ ! CH3 ꢀ OðHÞ ꢀ Ln3þ
ꢀBH4ꢀnðOCH3Þn þ CH3OH !ꢀBH4ꢀpðOCH3Þp þ H2
n ¼ 0; 1; 2; p ¼ n þ 1
The reducing power of ꢀBH4ꢀn(OCH3)n may be expected to be greatest
for ꢀBH (OCH3)3, and 75% of Hꢀ is evolved as hydrogen gas.[9]
The lanthanoid ion Ln3þ may form a bulky chelate complex
composed of methanol and substrate 5. We assume that 1,2-reduction
may be induced more easily by an increased number of chelate rings.
The chelate ring ligates Ln3þ ion with the O atom of methanol and either
=
=
the p electron of C C bond or the O atom of acetyl C O group. This
ligand is very bulky, and the central metal necessitates a large ionic
radius.
In the process of 1,2-reduction, methoxide anion CH3Oꢀ approaches
the [CH3O]3BꢀH group, and then the hydride anion Hꢀ attacks the C
=
atom of the C O group, accompanying the acceptance of a proton H
þ
=
from the methanol ligand at the O atom of the C O group. The lantha-
noid ion may form a trimethoxide salt of Ln(OCH3)3 and precipitate out
of the solution.
EXPERIMENTAL
All melting points were uncorrected. Ultraviolet (UV)–visible absorption
spectra were measured in toluene on a Shimadzu UV-2200 spectrophot-
ometer. NMR spectra were recorded on a Jeol JNM A-500 spectrometer
(1H, 500 MHz) at room temperature. All samples were freshly prepared
(ca. 5 mg) and dissolved in dimethylsulfoxide (DMSO-d6) or CDCl3
(0.5 ml). Purifications with column chromatography were carried out
using silica gel and toluene.
Protection of the Hydroxyl Group in 4
Acetylation
Acetic anhydride (2.8 ml, 30 mmol) was added to a suspension of 3.90 g
(20.0 mmol) of 4 in 9.7 ml of pyridine. The mixture was stirred for 1 h

Synthesis of Phenalen-1-one
3921
and added to 200 g of cold water. It was neutralized with 20% sodium
hydroxide solution; the resultant precipitate was filtered off and dried.
It was crystallized from benzene to give 3.58 g (yield 75.1%) of gold
yellow plates of 3-acethoxyphenalen-1-one (5): mp 120–121ꢁC (ref. 7,
124ꢁC); kmax (toluene) ¼ 356 nm. EI-mass calc. for C15H10O3: [M]þ ¼
238.06; found: m=z ¼ 238. 1H NMR (DMSO-d6) d (ppm) ¼ 8.50 (dd,
J ¼ 7.34, 1.28, 1H), 8.49 (dd, J ¼ 8.07, 1.28, 1H), 8.35 (dd, J ¼ 8.25,
0.92, 1H), 8.15 (dd, J ¼ 7.24, 1.01, 1H), 7.92 (dd, J ¼ 7.98, 0.55, 1H),
7.78 (dd, J ¼ 8.25, 0.92, 1H), 6.60 (s, 1H), 2.49 (s, 3H).
The synthesis was also carried out by the method reported by Eistert
et al.[7] Compound 4 (9.80 g) was converted into sodium salt by sodium
methoxide, and acetylation was performed by acetyl chloride. Yield
4.78 g, 43.8%.
Benzoylation
By a method similar to that described in Ref. 7, sodium salt prepared
from 9.80 g of 4 was dispersed into 100 ml of ether, and to this was added
5.8 ml (50 mmol) of benzoyl chloride. The mixture was stirred for 1 h at
room temperature, and the precipitate was filtered off and dried. It was
crystallized from benzene to give 12.06 g (yield 83.5%) of gold yellow
crystals of 3-benzoyloxyphenalen-1-one (7). Calc. for C20H12O3: C,
79.99%; H, 4.03%. Found: C, 79.58%; H, 4.00%. Mp 144–146ꢁC; kmax
(toluene) ¼ 356 nm. EI–mass calc. for C20H12O3: [M]þ ¼ 300.08. Found:
m=z ¼ 300. 1H NMR (DMSO-d6) d (ppm) ¼ 8.54 (dd, J ¼ 7.31, 1.26,
1H), 8.52 (dd, J ¼ 8.09, 1.26, 1H), 8.37 (dd, J ¼ 8.25, 0.83, 1H), 8.28
(dd, J ¼ 8.39, 1.33, 2H), 8.13 (dd, J ¼ 7.31, 0.94, 1H), 7.95 (dd,
J ¼ 8.05, 0.69, 1H), 7.84 (tt, J ¼ 7.45, 1.54, 1H), 7.78 (dd, J ¼ 8.23, 0.87,
1H), 7.69 (t, J ¼ 7.61, 2H), 6.81 (s, 1H).
Tosylation
By the method similar to that described previously, application of 9.53 g of
p-tosyl chloride gave 4.67 g of 3-(p-tosyloxy)phenalen-1-one (7). Calc. for
C20H14O4S: C, 68.56%; H, 4.03%; S, 9.15%. Found: C, 68.53%; H, 3.94%;
S, 9.12%. Orange brown crystals, mp 148–150ꢁC; kmax (toluene) ¼ 358 nm.
EI–mass calc. for C20H14O4S: [M]þ ¼ 350.06. Found: m=z ¼ 350. NMR
(DMSO-d6) d (ppm) ¼ 8.47 (dd, J ¼ 8.12, 1.20, 1H), 8.46 (dd, J ¼ 7.36,
1.31, 1H), 8.34 (dd, J ¼ 8.32, 0.99, 1H), 8.01 (dd, J ¼ 7.38, 0.96, 1H),
8.00 (d, J ¼ 8.53, 2H), 7.90 (t, J ¼ 7.73, 1H), 7.74 (dd, J ¼ 8.25, 0.87,
1H), 7.51 (dd, J ¼ 8.60, 0.57, 2H), 6.41 (s, 1H), 2.41 (s, 3H).

3922
T. Sawada et al.
Phenalen-1-one (2)[11]
A mixture of 1.20 g (5.0 mmol) of 5, 1.86 g (5.0 mmol) of lanthanum
chloride heptahydrate, 25 ml of methanol, and 25 ml of tetrahydrofuran
was stirred for 30 min at room temperature, and to this was added
0.38 g (10 mmol) of sodium tetrahydroborate NaBH4 in several portions.
The reaction mixture was stirred further for 1 h and evaporated to dry-
ness. The residue was stirred in 30 g of conc. sulfuric acid for 1 h. This
solution was diluted with 60 g of ice water, and after being neutralized
with 20% sodium hydroxide solution, 100 ml of toluene were added sev-
eral times to extract the toluene-soluble part. The toluene extract was
dried with anhydrous sodium sulfate and evaporated to dryness. The resi-
due was purified with column chromatography.
The first eluent gave a small amount (yield: 1%) of yellow leaflet crystals
of 3: mp 378–380ꢁC [Ref. 6 374–375ꢁC, Ref. 4 375–376ꢁC, Ref. 12
380–385ꢁC (decomp)]; kmax (toluene)¼ 441, 414 nm. EI–mass calc. for
C26H14: [M]þ ¼ 326.11. Found: m=z ¼ 326. NMR (DMSO-d6)
d
(ppm) ¼ 9.49 (d, J ¼ 9.5, 4H), 8.56 (d, J ¼ 9.2, 4H), 8.51 (d, J ¼ 7.5, 4H),
8.25 (t, J ¼ 7.6, 2H).
The second eluent gave a smaller amount (yield: <1%) of light-yellow
plate crystals of 2,3-dihydrophenalen-1-one: mp 79–80ꢁC (Ref. 13
80–81ꢁC); kmax (toluene) ¼ 320 nm; NMR (CDCl3) d (ppm) ¼ 8.18 (dd,
J ¼ 7.2, 1.2, 1H), 8.07 (dd, J ¼ 8.2, 1.2, 1H), 7.78 (dd, J ¼ 7.2, 1.7, 1H),
7.59 (dd, J ¼ 8.1, 7.2, 1H), 7.48 (t, J ¼ 7.3, 1H), 7.45 (dd, J ¼ 7.7, 1.6,
1H), 3.43 (t, J ¼ 7.1, 2H), 2.97 (t, J ¼ 7.2, 2H).
The third eluent gave a small amount of yellow needle-like crystals of an
unknown material: mp 299–300ꢁC (decomp); kmax (toluene)¼ 411, 351 nm.
The fourth eluent gave 0.204 g (yield: 45.3%) of yellow plate crystals
of 2: mp 155–156ꢁC (Ref. 6 156ꢁC); kmax (toluene) ¼ 382, 357 nm. EI–
mass calc. for C13H8O: [M]þ ¼ 180.06. Found: m=z ¼ 180. NMR
(DMSO-d6 at 90ꢁC) d (ppm) ¼ 8.47 (d, J ¼ 7.3, 1H), 8.39 (d, J ¼ 7.9,
1H), 8.19 (d, J ¼ 7.6, 1H), 7.96 (d, J ¼ 9.8, 1H), 7.96 (d, J ¼ 8.0, 1H),
7.86 (t, J ¼ 7.6, 1H), 7.70 (t, J ¼ 7.8, 1H), 6.65 (d, J ¼ 9.8, 1H).
Results shown in Figs. 1 and 3 were obtained by the method similar
to that described but using CeCl3 instead of LaCl3. The use of other salts
and other esters gave the results shown in Fig. 2 and Table 1, and Fig. 4
and Table 2.
ACKNOWLEDGMENTS
The authors thank Prof. Masatugu Kajitani in Sophia University for
kindly providing the EI–mass and elementary analyses, Prof. Yoshifumi

Synthesis of Phenalen-1-one
3923
Akama of Meisei University for discussions about lanthanoid ions, and
Miss Michiko Nagao for experimental assistance.
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