Synthesis of phenanthridines and related compounds by palladium-catalyzed direct coupling Via C-H and N-H bond cleavages

Article abstract of DOI:10.3987/COM-12-S(N)45

The palladium-catalyzed direct annulation of benzophenone imines with o-dihalobenzenes proceeds through C-H and N-H bond cleavages to produce 6-arylphenanthridine derivatives. 2-Phenyl-imidazole, -benzimidazole, and -indole also undergo the annulation to

Full text of DOI:10.3987/COM-12-S(N)45

HETEROCYCLES, Vol. 86, No. 1, 2012
487
HETEROCYCLES, Vol. 86, No. 1, 2012, pp. 487 - 496. © 2012 The Japan Institute of Heterocyclic Chemistry
Received, 20th June, 2012, Accepted, 2nd August, 2012, Published online, 3rd August, 2012
DOI: 10.3987/COM-12-S(N)45
SYNTHESIS OF PHENANTHRIDINES AND RELATED COMPOUNDS
BY PALLADIUM-CATALYZED DIRECT COUPLING VIA C–H AND
N–H BOND CLEAVAGES
Daisuke Takeda, Koji Hirano, Tetsuya Satoh,* and Masahiro Miura*
Department of Applied Chemistry, Faculty of Engineering, Osaka University,
Suita, Osaka 565-0871, Japan; E-mail: satoh@chem.eng.osaka-u.ac.jp;
miura@chem.eng.osaka-u.ac.jp
Abstract – The palladium-catalyzed direct annulation of benzophenone imines
with o-dihalobenzenes proceeds through C–H and N–H bond cleavages to
produce 6-arylphenanthridine derivatives. 2-Phenyl-imidazole, -benzimidazole,
and -indole also undergo the annulation to form the corresponding tetra- and
pentacyclic compounds.
INTRODUCTION
Nitrogen-containing fused heteroaromatic compounds including phenanthridines can be seen in a wide
range of !-conjugated functional materials such as organic semiconductors and luminescent materials as
well as pharmaceuticals and natural products.1 Among them, 6-arylphenanthridine derivatives are of
particular interest due to their applicability as ligands for phosphorescent iridium complexes, which are
promising emitters in pure red region.2 Therefore, their effective construction has attracted much attention
and a number of synthetic approaches through transition-metal-catalyzed cyclization reactions have been
developed (Scheme 1). However, in reported procedures shown as routes a,3 b,4 and c,5 starting materials
have been prepared through complicated multistep processes.
-------------------------------------------------------------------------------------------------------------------------------
Dedicated to Professor Ei-ichi Negishi on the occasion of his 77th birthday.

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HETEROCYCLES, Vol. 86, No. 1, 2012
X
M
N
X
H
N
c
a
R
R
c
Ref. 5
Ref. 3
!
N
!
a
R
b !
b
a & c
X
Ref. 4
This Work
R
N
+
X
H
H
H
N
H
R
Scheme 1
In contrast, the annulation between two aromatic substrates (routes a & c) appears to be attractive because
of the substrate availability and applicability.6 In the course of our recent studies on
transition-metal-catalyzed annulation reactions involving C–H bond cleavage,7 we succeeded in
conducting the one step synthesis of 6-arylphenanthridines through the direct coupling of benzophenone
imines with o-dihalobenzenes under palladium catalysis, accompanied by C–H and N–H bond cleavages.
The catalyst was also found to be applicable to the related annulation of 2-phenyl-imidazoles and -indole.
These new findings are described herein.
RESULTS AND DISCUSSION
In an initial attempt, benzophenone imine (1a, 0.5 mmol) was treated with o-dibromobenzene (2a, 0.4
mmol) in the presence of Pd(OAc)2 (0.02 mmol), PCy3 (0.04 mmol), and K2CO3 (0.9 mmol) as a catalyst,
o
ligand, and base, respectively, in DMF at 160 C for 9 h. As a result, 6-phenylphenanthridine (3a) was
obtained in 53% yield (Table 1, Entry 1). The use of other bases such as Na2CO3, Cs2CO3, KOBut, and
K3PO4 in place of K2CO3 decreased the product yield (Entries 2-5). The reaction proceeded more
efficiently at 170 oC in DMAc to afford 3a in 65% yield (Entry 6). Even at 170 oC, the 3a yield decreased
in NMP, diglyme, or mesitylene (Entries 7-9). Without PCy3, the reaction did not proceed catalytically
(Entry 10). Increasing the amount of PCy3 to 0.08 mmol decreased the product yield (Entry 11). Other
mono- (Entries 12 and 13) and diphosphine ligands (Entries 14-17) were less effective than PCy3. In the
case using xantphos (Entry 17), a significant amount (45%) of monobromide 4 was formed together with
3a.

HETEROCYCLES, Vol. 86, No. 1, 2012
489
Table 1. Reaction of benzophenone imine (1a) with o-dibromobenzene (2a)a
NH
Br
Br
Pd(OAc)2 / ligand
base / solvent
N
+
1a
2a
3a
Temp. /oC
Entry
Ligand (mmol)b
Base
Time /h
Yield of 3a /%c
Solvent
1
PCy3 (0.04)
PCy3 (0.04)
PCy3 (0.04)
PCy3 (0.04)
PCy3 (0.04)
PCy3 (0.04)
PCy3 (0.04)
PCy3 (0.04)
PCy3 (0.04)
–
K2CO3
DMF
160
160
160
160
160
170
170
170
170
170
170
170
170
170
170
170
170
9
9
9
9
9
6
6
9
9
9
9
6
6
6
9
6
6
53
11
23
0
2
Na2CO3 DMF
Cs2CO3 DMF
3
4
KOBut
K3PO4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
DMF
5
DMF
41
65 (64)
51
20
16
4
6
DMAc
NMP
7
8
diglyme
mesitylene
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
9
10
11
12
13
14
15
16
17
PCy3 (0.08)
PCy2(o-biph) (0.04)
PPh3 (0.04)
dppm (0.02)
dppb (0.02)
dppf (0.02)
xantphos (0.02)
40
20
34
15
34
28
16d
a Reaction conditions: 1a (0.5 mmol), 2a (0.4 mmol), Pd(OAc)2 (0.02 mmol), Base (0.9 mmol), Solvent
(2.5 mL), under N2. b PCy3 = tricyclohexylphosphine, PCy2(o-biph) = [1,1'-biphenyl]-2-
yldicyclohexylphosphine. c GC yield based on the amount of 2a used. Value in parentheses indicates yield
after purification. d A monobromide 4 was also formed (45%).
Br
N
4
Under the optimized reaction conditions (Table 1, Entry 6), the reactions of a series of p-substituted
benzophenone imines 1b-d with 2a were next examined (Scheme 2). 4,4’-Dimethyl- and
4,4’-di-tert-butylbenzophenone imines (1b and 1c) coupled with 2a effectively to form the corresponding
phenanthridines 3b and 3c in 51 and 60% yields, respectively. In the reaction of
4,4’-dimethoxybenzophenone imine (1d), only a small amount of coupling product was detected by GC

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HETEROCYCLES, Vol. 86, No. 1, 2012
and GC-MS. The poor result seems to be due, at least in part, to the poorer solubility of 1d and/or the
product.8
NH
Pd(OAc)2 (0.02 mmol)
Br
PCy3 (0.04 mmol)
N
+
K2CO3 (0.9 mmol)
Br
R
R
DMAc (2.5 mL)
170 oC, 9 h
(0.5 mmol)
(0.4 mmol)
R
R
1b (R = Me)
1c (R = But)
1d (R = OMe)
2a
3b (R = Me) (51%)a
3c (R = But) 60% (58%)a
3d (R = OMe)b 7%a
a GC yield based on the amount of 2a used. Value in parentheses indicates yield after purification.
b Detected by �GC and GC-MS.
Scheme 2
Under similar conditions, the reactions of 1a with o-bromochlorobenzene (2b) and o-dichlorobenzene
(2c) gave 3a in low yields (Scheme 3). 1,2-Dibromo-4,5-dimethylbenzene (2d) coupled with 1a
effectively to produce 3e in 52% yield. In contrast, the reaction of an electron-deficient dibromide,
1,2-dibromo-4,5-difluorobenzene (2e), was sluggish.
R
R
NH
Pd(OAc)2 (0.02 mmol)
X
Y
R
R
PCy3 (0.04 mmol)
N
+
K2CO3 (0.9 mmol)
DMAc (2.5 mL)
170 oC, 9 h
(0.5 mmol)
(0.4 mmol)
1a
2b (R = H, X = Cl, Y = Br)
2c (R = H, X = Y = Cl)
2d (R = Me, X = Y = Br)
2e (R = F, X = Y = Br)
3a (R = H) 26%a
3a (R = H) 21%a
3e (R = Me) (52)%a
3f (R = F)b 5%a
a GC yield based on the amount of 2 used. Value in parentheses indicates yield after purification.
b Detected by �GC and GC-MS.
Scheme 3

HETEROCYCLES, Vol. 86, No. 1, 2012
491
A plausible mechanism for the coupling of 1a with 2a is illustrated in Scheme 4, in which neutral ligands
are omitted. Taking account of a fact that the N-arylation of 1a is known to occur under mild conditions at
o
65 C,9 the arylation seems to first occur on the imino nitrogen of 1a to produce a monobromide 4.
Actually, 4 was obtained in the case using xantphos ligand (Table 1, Entry 17), as described above. Then,
the next intramolecular C–H arylation may occur as follows: Oxidative addition of the remaining C–Br
bond in 4 to Pd0 species gives arylpalladium species A. Subsequent cyclopalladation and reductive
elimination take place to form 3a and regenerate Pd0 species.
Br
Br
Br
NH
N
2a
–HBr
1a
4
Pd0
N
3a
BrPd
N
N
Pd
A
B
KBr, KHCO3
K2CO3
Scheme 4
Under similar conditions, 2-phenylimidazole (5) also underwent the annulation with 2a, accompanied by
C–H and N–H bond cleavages to afford a tetracyclic product 6 in 20% yield (Scheme 5). Increase of the
amount of PCy3 ligand to 0.08 mmol improved the product yield up to 57%. Although the exact effect of
increased ligand is not clear at the present stage, the deposition of palladium black was observed to be
slightly retarded. Similarly, 2-phenylbenzimidazole (7) and 2-phenylindole (9) also coupled with 2a
under modified conditions using Cs2CO3 as a base in place of K2CO3 to produce 8 and 10, respectively. In
the latter case, the use of 0.12 mmol of PCy3 gave the best result.

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HETEROCYCLES, Vol. 86, No. 1, 2012
Pd(OAc)2 (0.02 mmol)
PCy3
NH
Br
N
+
N
K2CO3 (0.9 mmol)
N
Br
DMAc (2.5 mL)
170 oC, 9 h
5 (0.5 mmol)
6
2a (0.4 mmol)
PCy3 (mmol)
Yield of 6 /%a
0.04
0.08
0.12
20
57 (55)
54
Pd(OAc)2 (0.02 mmol)
PCy3 (0.08 mmol)
NH
N
Br
N
N
+
Base (0.9 mmol)
DMAc (2.5 mL)
170 oC, 9 h
Br
7 (0.5 mmol)
8
2a (0.4 mmol)
Base
Yield of 8 /%a
K2CO3
19
Cs2CO3
39 (39)
Pd(OAc)2 (0.02 mmol)
PCy3
NH
Br
N
+
Cs2CO3 (0.9 mmol)
DMAc (2.5 mL)
170 oC, 6 h
Br
9 (0.5 mmol)
10
2a (0.4 mmol)
PCy3 (mmol)
Yield of 10 /%a
0.08
0.12
43
62 (61)
a GC yield based on the amount of 2a used. Value in parentheses indicates yield after purification.
Scheme 5
In summary, we have demonstrated that the synthesis of phenanthridine derivatives from the readily
available building blocks, benzophenone imines and o-dihalobenzenes, can be achieved through C–H and
N–H bond cleavages under palladium catalysis. The procedure is also applicable to the construction of
related fused tetra- and pentacyclic heteroarenes.

HETEROCYCLES, Vol. 86, No. 1, 2012
493
EXPERIMENTAL
General. 1H and 13C NMR spectra were recorded at 400 and 100 MHz for CDCl3 solutions. MS data were
obtained by EI. GC analysis was carried out using a silicon OV-17 column (i. d. 2.6 mm x 1.5 m).
GC-MS analysis was carried out using a CBP-1 capillary column (i. d. 0.25 mm x 25 m). The structures
1
13
of all products listed below were unambiguously determined by H and C NMR with the aid of NOE,
COSY, HMQC, and HMBC experiments.
Starting Materials. Imines 1b-d were prepared according to published procedures.10 Other starting
materials were commercially available.
Typical Procedure for the Reactions of Benzophenone Imines 1, 2-Phenylimidazoles 5 and 7, and
2-Phenylindole 9 with o-Dihalobenzene 2. A mixture of nitrogen-containing substrate 1, 5, 7, or 9 (0.5
mmol), o-dihalobenzene 2 (0.4 mmol), Pd(OAc)2 (0.02 mmol, 4.5 mg), PCy3 (0.04-0.12 mmol), base (0.9
mmol), and dibenzyl (ca. 40 mg) as internal standard was stirred in DMAc (2.5 mL) under nitrogen at 170
oC for 6-9 h. GC and GC-MS analyzes of the mixtures confirmed formation of product 3, 6, 8, or 10.
After cooling, the reaction mixture was extracted with ethyl acetate (100 mL). The organic layer was
washed with water (100 mL, three times), and dried over Na2SO4. After evaporation of the solvent under
vacuum, the product 3, 6, 8, or 10 was isolated by column chromatography on silica gel using
hexane-ethyl acetate as eluant.
Characterization Data of Products.
o
1
6-Phenylphenanthridine (3a):6d mp 99-101 C; H NMR (400 MHz, CDCl3) ! 7.51-7.63 (m, 4H),
7.67-7.78 (m, 4H), 7.84-7.88 (m, 1H), 8.11 (d, J = 8.4 Hz, 1H), 8.25 (d, J = 7.8 Hz, 1H), 8.62 (d, J = 8.2
13
Hz, 1H), 8.71 (d, J = 8.4 Hz, 1H); C NMR (100 MHz, CDCl3) ! 121.9, 122.2, 123.7, 125.2, 126.9,
127.1, 128.4, 128.7, 128.8, 128.9, 129.7, 130.3, 130.6, 133.4, 139.8, 143.8, 161.3; HRMS m/z (M+) Calcd
for C19H13N: 255.1048. Found 255.1045.
9-Methyl-6-(4-methylphenyl)phenanthridine (3b):6d mp 70-72 oC; 1H NMR (400 MHz, CDCl3) ! 2.47
(s, 3H), 2.64 (s, 3H), 7.36 (d, J = 7.3 Hz, 2H), 7.42 (d, J = 8.7 Hz, 1H), 7.62-7.66 (m, 3H), 7.69-7.74 (m,
1H), 8.01 (d, J = 8.7 Hz, 1H), 8.21 (d, J = 8.2 Hz, 1H), 8.46 (s, 1H), 8.58 (d, J = 8.3 Hz, 1H); 13C NMR
(100 MHz, CDCl3) ! 21.4, 22.2, 121.8, 121.9, 123.45, 123.54, 126.5, 128.6, 128.7, 128.9, 129.0, 129.7,
130.3, 133.6, 137.1, 138.5, 140.8, 144.1, 161.1; HRMS m/z (M+) Calcd for C21H17N: 283.1361. Found
283.1358.
o
1
9-(tert-Butyl)-6-[4-(tert-butyl)phenyl]phenanthridine (3c): mp 178-179 C; H NMR (400 MHz,
CDCl3) ! 1.41 (s, 9H), 1.50 (s, 9H), 7.56-7.58 (m, 2H), 7.65-7.75 (m, 5H), 8.12 (d, J = 8.7 Hz, 1H), 8.23
(d, J = 8.7 Hz, 1H), 8.64-8.68 (m, 2H); 13C NMR (100 MHz, CDCl3) ! 31.3, 31.4, 34.7, 35.5, 117.7,
121.8, 123.4, 123.9, 125.35, 125.38, 126.5, 128.5, 128.8, 129.4, 130.3, 133.2, 137.0, 144.1, 151.6, 153.7,
161.0; HRMS m/z (M+) Calcd for C27H29N: 367.2300. Found 367.2302.

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HETEROCYCLES, Vol. 86, No. 1, 2012
2,3-Dimethyl-6-phenylphenanthridine (3e):6d mp 92-94 oC; 1H NMR (400 MHz, CDCl3) ! 2.50 (s, 3H),
2.55 (s, 3H), 7.48-7.57 (m, 4H), 7.71-7.73 (m, 2H), 7.79-7.83 (m, 1H), 8.01 (s, 1H), 8.07 (d, J = 8.2 Hz,
13
1H), 8.34 (s, 1H), 8.65 (d, J = 8.2 Hz, 1H); C NMR (100 MHz, CDCl3) ! 20.1, 20.4, 121.7, 121.96,
121.98, 125.0, 126.5, 128.4, 128.5, 128.8, 129.8, 130.23, 130.24, 133.2, 136.4, 138.4, 140.0, 142.6,
160.3; HRMS m/z (M+) Calcd for C21H17N: 283.1361. Found 283.1359.
2-Bromo-N-(diphenylmethylene)aniline (4):11 mp 97-99 oC; 1H NMR (400 MHz, CDCl3) ! 6.53 (d, J =
7.8 Hz, 1H), 6.76-6.80 (m, 1H), 7.00-7.04 (m, 1H), 7.19-7.27 (m, 5H), 7.40-7.51 (m, 4H), 7.82 (d, J = 7.3
13
Hz, 2H); C NMR (100 MHz, CDCl3) ! 115.2, 121.2, 124.2, 127.4, 127.9, 128.2, 128.7, 128.8, 129.6,
131.1, 132.4, 136.0, 138.8, 150.2, 169.4; HRMS m/z (M+) Calcd for C19H14BrN: 335.0310. Found
335.0308.
Imidazo[1,2-f]phenanthridine (6):12 mp 115-124 oC; 1H NMR (400 MHz, CDCl3) ! 7.49 (td, J = 8.2 and
1.4 Hz, 1H), 7.58-7.66 (m, 4H), 7.84 (dd, J = 8.2 and 0.9 Hz, 1H), 7.97 (d, J = 1.4 Hz, 1H), 8.33-8.37 (m,
1H), 8.43 (dd, J = 8.3 and 1.4 Hz, 1H), 8.63-8.97 (m, 1H); 13C NMR (100 MHz, CDCl3) ! 112.0, 115.8,
121.7, 122.3, 123.7, 124.11, 124.12, 125.1, 127.4, 128.55, 128.60, 128.8, 131.5, 131.7, 142.5; HRMS m/z
(M+) Calcd for C15H10N2: 218.0844. Found 218.0842.
o
1
Benzo[4,5]imidazo[1,2-f]phenanthridine (8):13 mp 145-147 C; H NMR (400 MHz, CDCl3) !
7.44-7.54 (m, 3H), 7.64-7.73 (m, 3H), 8.03-8.06 (m, 1H), 8.33 (t, J = 8.3 Hz, 2H), 8.43 (dd, J = 8.0 and
1.2 Hz, 1H), 8.52 (d, J = 7.8 Hz, 1H), 8.85 (dd, J = 7.8 and 1.8 Hz, 1H); 13C NMR (100 MHz, CDCl3) !
113.9, 116.0, 120.3, 121.7, 122.3, 122.9, 123.4, 124.1, 124.2, 124.4, 126.0, 128.6, 129.1, 129.5, 130.4,
131.9, 134.4, 144.5, 147.5; HRMS m/z (M+) Calcd for C19H12N2: 268.1000. Found 268.1003.
o
1
Indolo[1,2-f]phenanthridine (10):14 mp 146-148 C; H NMR (400 MHz, CDCl3) ! 7.28 (s, 1H),
7.33-7.42 (m, 3H), 7.48-7.53 (m, 2H), 7.57-7.61 (m, 1H), 7.85 (d, J = 8.0 Hz, 1H), 8.13-8.17 (m, 1H),
8.22-8.25 (m, 1H), 8.34 (d, J = 7.8 Hz, 1H), 8.40 (d, J = 8.2 Hz, 1H), 8.56 (d, J = 8.7 Hz, 1H); 13C NMR
(100 MHz, CDCl3) ! 96.1, 114.2, 116.3, 121.0, 121.7, 121.95, 122.01, 122.3, 123.0, 123.9, 124.1, 126.1,
126.8, 127.8, 128.1, 128.7, 130.2, 133.8, 135.2, 135.9; HRMS m/z (M+) Calcd for C20H13N: 267.1048.
Found 267.1044.
ACKNOWLEDGEMENTS
This work was partly supported by Grants-in-Aid from MEXT and JSPS, Japan and Kansai Research
Foundation for technology promotion (KRF).
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8. Actually, 1d was sparingly soluble in DMAc at room temperature.

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