A different route to the synthesis of 9,10-disubstituted phenanthrenes

Article abstract of DOI:10.1021/jo050646f

We here report the synthesis of 10-aryl-9-hydroxy- and 10-aryl-9- aminophenanthrenes by reaction of the anions of 9-phenanthrol and 9-aminophenanthrene, respectively, with aryl halides (iodobenzene, 4-iodoanisole, 9-bromophenantrene). Good yields of 9,10-disubstituted phenanthrenes were obtained in these reactions (>75% and ～50% for the 9-amino and 9-hydroxyphenanthrene rings, respectively). Extension of the procedure to the reaction of both anions with o-dihalobenzenes leads to the synthesis of the ring closure products (aza- or oxa-indeno[1,2-l]phenanthrene), which bear a pentacyclic aromatic condensed ring system, although in lower overall yields (～35%).

Full text of DOI:10.1021/jo050646f

aryl bonds, one of the most important tools in modern
organic synthesis. Aryl boronic acids (the Suzuki-
Miyaura coupling)10 and arylstannanes (the Stille reac-
tion)11 are reagents usually employed in these reactions.
The radical nucleophilic substitution mechanism (SRN1)
is an alternative route to the synthesis of biaryls,
particularly, since these reactions are generally carried
out under mild conditions and the substrates tolerate
many functional groups.12 This versatility distinguishes
the procedure from other methods used such as Kumada,
Suzuki, or Stille coupling. Different hydroxybiaryls have
been synthesized in good yields by the photoinitiated
substitution of haloarenes with arylalcoxides.13 On the
other hand, despite the fact that arylindoles and arylimi-
dazoles can be obtained by this procedure,14 only one
example has been reported for the synthesis of amino-
biaryls following this methodology.15
A Different Route to the Synthesis of
9,10-Disubstituted Phenanthrenes
Toma´s C. Tempesti, Adriana B. Pierini, and
Mar´ıa T. Baumgartner*
INFIQC, Departamento de Quı´mica Orga´nica, Facultad de
Ciencias Quı´micas, Universidad Nacional de Co´rdoba,
Ciudad Universitaria, 5000 Co´rdoba, Argentina
tere@dqo.fcq.unc.edu.ar
Received April 1, 2005
These nucleophilic substitutions involve electron
transfer steps (ET) and the intermediacy of radical and
radical anions. The radicals originate from dissociation
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Lett. 1987, 28, 4653-4656.
We here report the synthesis of 10-aryl-9-hydroxy- and 10-
aryl-9-aminophenanthrenes by reaction of the anions of
9-phenanthrol and 9-aminophenanthrene, respectively, with
aryl halides (iodobenzene, 4-iodoanisole, 9-bromophenan-
trene). Good yields of 9,10-disubstituted phenanthrenes were
obtained in these reactions (>75% and ∼50% for the 9-amino
and 9-hydroxyphenanthrene rings, respectively). Extension
of the procedure to the reaction of both anions with o-
dihalobenzenes leads to the synthesis of the ring closure
products (aza- or oxa-indeno[1,2-l]phenanthrene), which bear
a pentacyclic aromatic condensed ring system, although in
lower overall yields (∼35%).
The phenanthrene skeleton is found in a number of
biologically active natural products. Phenanthro[9,10,d]
fused heterocycles are some of the derivatives that have
been reported to have interesting pharmacological prop-
erties.1 Other promising applications of this type of
compounds are based on their photoconducting, optoelec-
trical, and electroluminescence properties.2
Many synthetic schemes have been investigated for the
preparation of this aromatic system and derivatives3 such
as photochemical cyclizations of stilbenes,4 o-metalation
followed by catalyzed cyclization,5 intramolecular acyla-
tion,6 base- and light-catalyzed intramolecular cycliza-
tion7 and Lewis acid mediated cyclization.8 Another
approach used is the palladium methodology.9 This
procedure is also widely used in the formation of aryl-
* To whom correspondence should be addressed. Fax: 54-351-
4333030/4334174. Ph: 54-351-4334170/73.
(1) For selected references on compounds with biological and
pharmacological properties, see: Leong, Y. W.; Kang, C. C.; Harrison,
L. J.; Powell, A. D. Phytochemistry 1997, 44, 157-165. Majumder, P.
L.; Banerjee, S.; Sen, S. Phytochemistry 1996, 42, 847-852. Adam, K.
P.; Becker, H. Phytochemistry 1994, 35, 139. Kende, A. S.; Curran, D.
P. J. Am. Chem. Soc. 1979, 101, 1857-1864. Hudson, B. P.; Barton, J.
K. J. Am. Chem. Soc. 1998, 120, 6877-6888. Kumar, S. J. Org. Chem.
1997, 62, 8535-8539. Banik, B. K.; Becker, F.; Banik, I. Bioorg. Med.
Chem. 2004, 12, 2523-2528. Schmidt, J. M.; Mercure, J.; Tremblay,
G. B.; Page, M.; Kalbakji, A.; Feher, M.; Dunn-Dufault, R.; Peter, M,
G.; Redden, P. R. J. Med. Chem. 2003, 46, 1408-1418. Hudson, B. P.;
Barton, J. K. J. Am. Chem. Soc. 1998, 120, 6877-6888. Kumar, S. J.
Org. Chem. 1997, 62, 8535-8539.
10.1021/jo050646f CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/12/2005
6508
J. Org. Chem. 2005, 70, 6508-6511

SCHEME 1
irradiation (λmax. ) 350 nm), the yield of 1 decreased to
68%.16 The photoinitiated reaction of the anion of 1 with
iodobenzene (2a) afforded only the product corresponding
to substitution at C10 of the phenanthrene cycle (3a, 100%
yield) (eq 5) (Table 1, expt 1). The percentage of substitu-
tion obtained with 2a decreases in the presence of radical
or radical anion traps such as di-tert-butylnitroxide
(DTBN) and p-dinitrobenzene (p-DNB), respectively (Table
1, expts 2 and 3). Moreover, no substitution occurs in the
absence of irradiation (Table 1, expt 4). These facts sup-
port the assertion that a nucleophilic substitution with
radical and radical anions as intermediates is in play.
Employing this protocol, the reaction of the anion of 1
was examined with the aryl halides (2b-d). When the
reaction was performed with 4-iodoanisole (2b), the C10-
arylation compound (3b) was the main product formed
(80% after 1 or 3 h of irradiation) accompanied by the
reduction product anisole (15, 17%) (Table 1, expts 5 and
6). High yields of C10-arylation were also obtained by
reaction with 4-iodotoluene (2c) or 9-bromophenanthrene
(2d) (75% and 93%, respectively) (Table 1, expts 7 and
8). Hydrogen atom abstraction from the solvent by the
radical intermediates is a plausible route to the reduction
product (ArH) obtained in these reactions. It is important
to mention that the N-arylated product is not formed in
any of these reactions despite being formed in ap-
proximately 10% when aromatic halides are reacted with
2-naphthylamide anion.15
TABLE 1. Photoinitiated Reactions of Aryl Halides
with the Anion of 9-Aminophenanthrene (1) and
9-Phenanthrol (4) in Liquid Ammoniaa
products (% yields)c
NuH
ArX
t-BuOK
expt M × 103 M × 103 M × 103 X- (%)b ArH substitution
1
1, 6.1
2a, 2.0
2a, 1.8
2a, 1.8
2a, 1.2
2b, 3.5
2b, 3.2
2c, 3.8
2d, 2.2
2a, 5.0
6a, 3.7
13.5
9.8
98
3a, 100
3a, 62
3a, 76
2d 1, 4.4
3e 1, 4.8
4f 1, 3.8
100
80
9.8
7.8
5
5
1, 9.6
19.7
21.8
22.2
14.3
23.0
20.6
91
15 3b, 80
17 3b, 81
12 3c, 75
12 3d, 93
nqg 5, 53
6g 1, 9.3
98
7
8
9
1, 9.2
1, 6.9
4, 10.1
94
95
100
I- ) 77
Br- ) 76
10 1, 10.5
3a, 16; 7, 36
11h 1, 9.4
12 4, 10.1
6a, 2.6
6b, 3.1
20.1
22.8
3a, 21; 7, 31
5, 26; 8, 35
I- ) 100
a Photoinitiated reactions (unless indicated), carried out under
nitrogen at -33 °C. Reaction time (180 min). b Determined po-
tentiometrically on the basis of the ArX concentration. c Deter-
mined by GLC and the internal standard method on the basis of
the ArX concentration. d Di-tert-butylnitroxide (43 mmol %). e p-
Dinitrobenzene (42 mmol %). f Reaction carried out in the dark.
g nq ) not quantified. h Reaction time ) 60 min.
Based on the results obtained with 1, we investigated
the reaction of the anion of 9-phenanthrol 4 with 2a. The
results obtained in this reaction (eq 6) are similar to those
obtained by reaction of 2a with 1, that is, C10 arylation,
although in a lower overall yield (53%) (Table 1, expt 9).
of the radical anions of the aromatic substrates (ArX)
formed by a photoelectron transfer from the nucleophile
(Nu-) (eqs 1 and 2). The radicals thus formed can react
with the nucleophile to give the radical anion of the
substitution product (eq 3), responsible to continue the
propagation chain of the proposed mechanism (eq 4)
(Scheme 1).
In this paper we propose a selective arylation approach,
based on the SRN1 mechanism, for the preparation of
9,10-disubstituted phenanthrenes. The present synthesis
stems from the observation that aryl halides react with
9-phenanthrylamide or 9-phenanthroxide anions under
irradiation to give the corresponding 10-aryl derivatives
in good yields. Finally, the methodology we report here
can be extended to obtain a pentacyclic aromatic ring
structure by reaction of either anion with o-dihaloben-
zenes.
1-Iodo-2-methoxynaphthalene is another substrate stud-
ied with both anions. Unfortunately, this compound failed
to afford arylation with either nucleophile, the main
reaction being its reduction to 2-methoxynaphthalene
(72% with anion from 4 (liquid ammonia or DMSO) and
60% with anion from 1 (liquid ammonia)). Similar yields
of reduction (∼60%) together with nucleophilic substitu-
tion (∼40%) were previously obtained when this sub-
strate was reacted with 2-naphthoxide anion.17
To explain the regiochemistry of the radical-anion
coupling, the minimum energy potential surfaces for the
reaction of the anions from 1 and 4 with phenyl radicals
were calculated with the AM1 procedure (Figures S1 and
S2, Supporting Information).18 According to the energy
profile obtained, corroborated with the B3LYP (6-31G*)
functional, coupling at either position, N or C10 of 1, is
an exothermic step. Despite the fact that N-coupling
affords the most stable radical anion, coupling at C10
The results of the photoinitiated reactions of the anion
of 9-aminophenanthrene (1) with different aryl halides
(2) (eq 5) are presented in Table 1.
(16) Other products (phenanthrene 27% and 10-phenanthryl-9-
aminophenanthrene (3d)) were also formed in this reaction.
(17) Baumgartner, M. T.; Tempesti, T. C.; Pierini, A. B. Arkivoc
2003, part X, 420-433.
(18) All calculations have been performed with the Gaussian 98
package of programs. For details on the computational procedure, see
Supporting Information.
9-Aminophenanthrene (1), the nucleophile precursor,
was prepared (81% yield) by reaction of 9-bromophenan-
threne (2d) with sodium amide in liquid ammonia under
laboratory light. When this reaction is performed under
J. Org. Chem, Vol. 70, No. 16, 2005 6509

SCHEME 2
FIGURE 1. Product relationship obtained by sampling the
reaction of 1 with o-bromoiodobenzene at different irradiation
times: (9) 3a; (b) 7.
TABLE 2. AM1 Calculated Heats of Formation and
C-Br Bond Distances for Monosubstituted Radical
Anions
occurs with lower activation energy. This profile is similar
to the one calculated for the reaction of 2-naphthylamide
anion with phenyl radicals. Similarly, with the anion of
4, the calculation indicates that coupling at C10 requires
lower activation energy than coupling at oxygen. More-
over, with this anion the reaction at C10 affords the most
stable radical anion. As expected for reactions under kin-
etic control, the regiochemistry predicted by the frontier
orbital (FO) theory is in agreement with the experimental
outcome according to which the C10 position is consider-
ably more reactive than the heteroatomic one (see Figure
S3, Supporting Information). Besides, the FO comparison
between the naphthyl and the phenanthryl rings indi-
cates that the C-substitution is more favored in the phen-
anthryl system. This fact is also in agreement with the
experimental results. That is, whereas the 9-phenanthryl-
amide anion gives only C-substitution, the 2-naphthyl-
amide anion gives C- and N-substitution in a 9/1 ratio.
On the basis of the promising results obtained with
the aryl halides tested, we inspected this experimental
strategy as an alternative procedure to the synthesis of
the pentacyclic ring systems 7 and 8 by biaryl coupling
followed by intramolecular heterocyclization. Within this
goal, we studied the reaction of o-dihalobenzenes with
anions from 1 and 4 (eq 7). o-Bromoiodobenzene 6a reacts
with the anion of 1 to afford the reduced substituted
product 3a and the substituted-cyclized compound 7 in
16% and 36% yields, respectively (Table 1, expt 10). On
the other hand, compounds 5 and 8 were obtained by
reaction of the anion of 4 with o-diiodobenzene 6b (26%
and 35% yields, respectively. Table 1, expt 12).
rC-Br (Å)
RAπ RAσ
NH 1.8827 2.0766 72.38 84.52
1.8766 2.0778 23.39 34.36
∆Hf (kcal/mol)
radical
anion (RA)
∆∆Hfσ-π
Z
RAπ RAσ (kcal/mol)
12.14
10.97
O
NH 1.8829 2.0747 60.32 76.36
1.8810 2.0745 4.35 22.37
16.04
18.02
O
possibility was inspected in our system by sampling the
irradiated reaction of 1 with 6a at different times (every
15 min; see Figure 1). The monosubstitution product with
retention of halide was not detected along the reaction
time. Besides, we observed that both compounds, 7 and
3a, are formed simultaneously, the ratio 7/3a being
unchanged with time.
To explain these results, the heat of formation and
electronic properties of radical anions 10 (Z ) NH or O),
formed by coupling of anion from 1 or 4 with an
o-halophenyl radical, respectively, were calculated, and
the results are presented in Table 2.
The most stable radical anions (10) calculated have the
unpaired spin distribution localized in the π system of
the phenanthryl subunit, which is separated from the
o-haloaryl moiety by an sp3 carbon. The σ isomer, with
an elongated C-halogen bond in which the unpaired
electron is located, was also localized on the anionic
potential surface. As seen, these intermediates present
π-σ electronic isomerism (Scheme 2).20
Despite the fact that the existence of a σ minimum has
been attributed to deficiencies of the semiempirical pro-
cedure to adequately reproduce the σ surface,20 the ener-
gy difference between the π and the σ species (∆∆Hσ-π
)
has been shown to be a good indication of the feasibility
of the intramolecular ET between both electronic sys-
tems.20 This ET is responsible for the dissociation of the
intermediate into radicals and the halide anion.
In this system, the monosubstituted compound with
retention of halide (9, Scheme 2) can be an intermediate
in the formation of the cyclic product, as observed in the
reaction of substrates 6 with 2-naphthoxide anions.19 This
(19) Baumgartner, M. T.; Pierini, A. B.; Rossi, R. A. J. Org. Chem.
1993, 58, 2593-2598.
6510 J. Org. Chem., Vol. 70, No. 16, 2005

(1H, dd), 8.74-8.78 (1H, dd); 13C NMR δ 118.2 (q), 121.6, 121.7
(q), 122.4, 123.1, 123.3, 125.1, 126.6, 126.7, 127.7, 129.4, 130.7
(q), 131.2, 136.6 (q), 137.8 (q); m/z 270 (19.7), 269 (100.0), 268
(25.9), 267 (23.8), 239 (9.3), 133.7 (25.3), 126 (16.2), 119 (17.9);
HRMS calcd for C20H15N 269.120450, found 269.120693.
10-(4-Methoxyphenyl)-9-aminophenanthrene (3b). Iso-
lated by column chromatography and eluted with petroleum
ether/acetone (95:5): mp 179-181 °C; 1H NMR δ 3.91(3H, s,
OMe), 4.06 (2H, s, NH2), 7.08-7.15 (2H, m), 7.28-7.46 (5H, m),
7.60-7.73 (2H, m), 7.93-7.98 (1H, m), 8.60-8.65 (1H, m), 8.73-
8.78 (1H, m); 13C NMR δ 55.4, 114.4, 114.9, 118.4 (q), 121.7,
122.4, 123.3, 125.2 (q), 125.3, 126.6, 126.6, 126.7, 129.5 (q), 130.7,
132.3, 133.4 (q), 136.5 (q), 159.2 (q); m/z 300 (28.5), 299 (100.0),
254 (13.1), 239 (11.3), 128 (11.3), 127 (16.9), 126 (10.6), 113.1
(11.4); HRMS calcd for C21H17NO 299.131014, found 299.131087.
10-(4-Tolyl)-9-aminophenanthrene (3c). Isolated as an oil
by column chromatography and eluted with petroleum ether/
acetone (98:2): 1H NMR δ 2.48 (3H, s, Me), 4.05 (2H, s, NH2),
7.27-7.43 (7H, m), 7.60-7.74 (2H, m), 7.92-7.98 (1H, m), 8.61-
8.78 (2H, m); 13C NMR δ 21.3 (CH3), 118.2 (q), 121.6, 122.4, 122.7
(q), 123.1, 123.3, 125.2, 125.8 (q), 126.5, 126.6, 130.1, 130.6 (q),
131.0, 133.3 (q), 136.7 (q), 137.3 (q); m/z 284 (24.4), 283 (100),
282 (18.2), 281 (11.3), 268 (13.1), 267 (27.1), 139 (15.5), 133
(43.9), 119 (12.6); HRMS calcd for C21H17N 283.136100, found
283.137093.
10-(9-Phenanthryl)-9-aminophenanthrene (3d). Isolated
by column chromatography and eluted with petroleum ether/
acetone (95:5): mp 218-219 °C; 1H NMR δ 4.04 (2H, s, NH2),
7.13-7.17 (1H, dd), 7.22-7.30 (1H, td), 7.34-7.46 (2H, m), 7.51-
7.55 (1H, dd), 7.62-7.79 (5H, m), 7.86 (1H, s), 7.90-7.94 (1H,
dd), 7.98-8.03 (1H, m), 8.69 (1H, d), 8.80-8.86 (3H, m); 13C NMR
δ 121.6, 122.4, 122.7, 123.0, 123.2, 123.4, 125.1 (q), 125.4, 126.6,
126.7, 126.8, 126.9, 127.0, 127.1, 127.5 (q), 128.7, 128.9 (q), 130.2,
130.3 (q), 131.1 (q), 132.1 (q), 133.4 (q), 133.8 (q), 137.6 (q); m/z
370 (29), 369 (100), 368 (31), 367 (34), 366 (11), 365 (16), 352
(14), 184 (10), 183 (18), 182 (33), 177 (17), 176 (39), 175 (16),
168 (13); HRMS calcd for C28H19N 369.151750, found 369.150866.
10-Phenyl-9-phenanthrol (5). Isolated by column chroma-
tography and eluted with petroleum ether/acetone (98:2): mp
144-146 °C (lit.22 142-3 °C).
The ∆∆Hσ-π evaluated (Table 2) indicates that in these
radical anions the intra-ET is favored for the intermedi-
ates that bear a phenanthryl unit (∆∆Hσ-π less endo-
thermic than for the 2-naphthyl analogues). From these
results we can conclude that the preferred reaction
pathway followed by intermediates 10 is the intra-ET
accompanied by dissociation as opposed to the 2-naphthyl
analogues for which the inter-ET to afford monosubsti-
tution is in competition.
In conclusion, we have shown that a series of 9,10-
disubstituted phenanthrenes can be regiospecifically
prepared by a straightforward and efficient synthetic
pathway based on electron-transfer reactions. Further-
more, the methodology we report here can be extended
to aryl halides containing a wide variety of organic
functional groups compatible with the SRN1 mechanism,
such as OR, OPh, SAr, COAr, CN, and others. Moreover,
the reaction provides an alternative access to the pen-
tacyclic systems 7 and 8 in moderate yields.
Experimental Section
General Methods. 1H NMR and 13C NMR spectra were
recorded on a nuclear magnetic resonance spectrometer with
CDCl3 as solvent. The position of the aryl substitution on the
phenanthryl ring was assigned by comparison of the recorded
spectra with those calculated with the ACD labs 4.0 program.
All different and possible substitutions on the phenanthryl
system of 1 and 4 were evaluated. The procedure was validated
by comparison with information reported for the known com-
pound 5.
Reaction of 9-Bromophenanthrene with Amide Anion.
To distilled liquid ammonia (100 mL, -33 °C) were added a
catalytic amount of FeCl3 (ca. 5 mg), and then sodium metal
(8.3 mmol) in two or three pieces. Once the sodium amide was
formed (indicated by bleaching of the blue solution of sodium
metal in liquid ammonia), 9-bromophenanthrene (2.5 mmol) was
added. After 180 min the reaction was quenched by adding
ammonium nitrate in excess, and the ammonia was allowed to
evaporate. The residue was dissolved in water (80 mL), and the
mixture was extracted with CH2Cl2 (3 × 50 mL). The organic
extract was dried with MgSO4. The solvent was removed under
reduced pressure. After purification of the crude residue by
column chromatography (petroleum ether/acetone 9:1) 9-ami-
nophenanthrene (1) was obtained: mp 137.5-139 °C (lit.21
136-7 °C).
Photoinitiated Reaction of the Anion of 9-Aminophenan-
threne (1) with Halobenzenes. The following procedure is
representative of these reactions. The reactions were carried out
in a 250-mL three-neck round-bottomed flask equipped with
nitrogen inlet and magnetic stirrer. To distilled ammonia (100
mL) were added potassium tert-butoxide (1.35 mmol) and then
9-aminophenanthrene (0.61 mmol). After 15 min iodobenzene
(0.2 mmol) was added, and the reaction mixture was irradiated
with two 400-W lamps emitting maximally at 350 nm (air and
water refrigerated) for 180 min. The reaction was quenched with
an excess of ammonium nitrate. The ammonia was allowed to
evaporate, and water (50 mL) was added to the residue and
extracted twice with CH2Cl2 (20 mL). The iodide ions in the
aqueous solution were determined potentiometrically. The or-
ganic extract was dried (MgSO4), filtered, and quantified by
GLC. The solvent was removed under reduced pressure. The
residue after column chromatography (petroleum ether/acetone
9:1) gave 10-phenyl-9-aminophenanthrene (3a): 1H NMR δ 4.01
(2H, s, NH2), 7.24-7.69 (10H, m), 7.93-7.97 (1H, m), 8.61-8.65
13H-13-Aza-indeno[1,2-l]phenanthrene (7). Isolated by
column chromatography and eluted with petroleum ether/
acetone (90:10): mp 193-195 °C (lit.23 191-193 °C).
13-Oxa-indeno[1,2-l]phenanthrene (8). Isolated by column
chromatography and eluted with petroleum ether/acetone (98:
2): mp 153-154 °C (lit.24 156 °C); 1H NMR δ 7.43-7.53 (2H,
m), 7.62-7.81 (5H, m), 8.36-8.41 (1H, m), 8.49-8.54 (1H, m),
8.62-8.67 (1H, dd), 8.73-8.81 (2H, m); 13C NMR δ 111.9, 121.7,
121.7, 123.4, 123.4, 123.8, 124.2, 125.1, 125.4 (q), 127.1, 127.2,
127.4, 128.6 (q), 130.6 (q), 151.2 (q), 155.9 (q); m/z 270 (3), 269
(24), 268 (100), 240 (6), 239 (32), 238 (7), 134 (27), 119 (50);
HRMS calcd for C20H12O 268.088815, found 268.088570.
Acknowledgment. This work was supported by the
Agencia Co´rdoba Ciencia, the Consejo Nacional de
Investigaciones Cient´ıficas y Te´cnicas (CONICET), Fun-
dacio´n Antorchas and SECYT, Universidad Nacional de
Co´rdoba, Argentina.
Supporting Information Available: General methods
1
and materials; H NMR and 13C NMR spectra of compounds
3a, 3b, 3c, 3d, and 8; characterization data for known
compounds 1, 5, and 7; energy potential surface for reaction
of anions from 1 and 4 with phenyl radical; HOMO of
nucleophiles from 1 and 4 and second-order perturbation
between frontier MO for the different coupling positions. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO050646F
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J. Org. Chem, Vol. 70, No. 16, 2005 6511