Direct arylation of phenanthroline derivatives via oxidative C-H/C-H cross-coupling: Synthesis and discovery of excellent ligands

Article abstract of DOI:10.1039/c2ob27325j

A concise synthetic protocol for aryl functionalized phenanthrolines has been developed. It was demonstrated that 3,8-diphenyl-1,10-phenanthroline (7a) is competent in promoting transition-metal-free direct arylation and 2,3,8,9-tetraphenyl-1,10-phenanthroline (8a) is a highly efficient ligand in the in situ Pd-catalysed Heck reaction.

Full text of DOI:10.1039/c2ob27325j

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Direct arylation of phenanthroline derivatives via oxidative C–H/C–H
cross-coupling: synthesis and discovery of excellent ligands
Bijin Li, Xurong Qin, Jingsong You,* Xuefeng Cong and Jingbo Lan*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
dione) is a key intermediate to synthesize 2,9ꢀdichloroꢀ1,10ꢀ
phenanthroline.12 We speculate that this N,N′ꢀannelated
dinone (1) may be a competent substrate to achieve the direct
50 C–H bond arylation of Phen at 3ꢀposition and/or 8ꢀposition,
and then through chlorination and subsequent dechlorination
or Suzuki coupling at its 2,9ꢀpositions, a range of 3,8ꢀdiarylꢀ
A
concise synthetic protocol for aryl functionalized
phenanthrolines has been developed. It was demonstrated
that 3,8-diphenyl-1,10-phenanthroline (7a) is competent in
promoting the transition-metal-free direct arylation and
10 2,3,8,9-tetraphenyl-1,10-phenanthroline (8a) is
efficient ligand in the in situ Pd-catalysed Heck reaction.
a highly
1,10ꢀphenanthrolines
and
2,3,8,9ꢀtetraarylꢀ1,10ꢀ
phenanthrolines can be conveniently and efficiently obtained
55 (Scheme 1).
1,10ꢀPhenanthroline (Phen) and its derivatives are an
important class of ligands with a long history and have been
extensively
applied
in
transitionꢀmetal
catalysis,
3,8-diarylation
Ar1
2,3,8,9-tetraarylation
15 supramolecular chemistry, and materials science.1 The
incorporation of aryl substituents onto the parent nucleus,
particularly at the 2,9ꢀpositions and 3,8ꢀpositions, has been
shown a profound influence on photoelectric properties and
coordinating abilities of Phens.2 The main methods to forge
20 aryl substituted Phens, such as multiꢀstep Skraup or
Friedländer condensations,3 transitionꢀmetal catalysed various
crossꢀcoupling reactions of haloꢀsubstituted phenanthrolines
with aryl metallic reagents,4 and nucleophilic additions of
arylꢀlithium compounds,5 often involve tedious and wasteful
25 procedures as well as harsh reaction conditions. Moreover,
many of the raw materials could be difficult to access. It is
therefore highly desirable to develop more economical and
practical methods to prepare various aryl substituted Phens for
screening out versatile ligands.
Ar2
Ar1
Ar2
N
N
N
N
N
N
Ar3
8
Ar3
7
(1) 1,3-dibromopropane
(2) [K3Fe(CN)6], NaOH
Pd, NaBH4
Ar1
Ar-B(OH)2
Ar1
Ar2
PCl5
Ar2
N
N
N
N
Ar1-H
2
POCl3
O
O
Cl
Cl
4 or 5
6
Ar2-H
2
N
N
cat. Pd
cat. Pd
O
O
1
Ar1
N
N
O
O
3
Scheme
1 Synthesis routes of 3,8ꢀdiarylꢀ1,10ꢀphenanthrolines and
2,3,8,9ꢀtetraarylꢀ1,10ꢀphenanthrolines.
The oxidative C–H/C–H crossꢀcoupling between N,N′ꢀ
60 annelated dinone (1a) with unfunctionalized arene could be
one of the most ideal strategies to achieve arylation of Phen.
Therefore, in initial experiments, 1a was treated with benzene
(2a) under the conditions employing Pd(OAc)2 as catalyst,
AgOAc as oxidant, and PivOH as additive at 130 °C for 24
65 hours. To our delight, the desired crossꢀcoupling reaction
could occur smoothly, affording monoꢀ and diꢀarylated
products (3a and 4a) in 37% and 16% isolated yields,
respectively (Supporting Information Table S1, entry 1). The
30
Transition metal catalysed direct arylation of C–H bonds
represents one of the most attractive transformations to
introduce aryl substituents onto aromatic scaffolds, and thus
has attracted considerable attentions in recent years.6 Among
of these, the arylation of pyridines has remained a challenge
35 due to their electronꢀpoor characteristics and the possible
nonproductive coordination mode of nitrogen atom with metal
centers.7 Recently, significant progress has been achieved via
conversion to Nꢀoxides or Nꢀiminopyridinium ylides,8 using
electron poor [RhCl(CO)2]2 or Pd(OAc)2/Phen system,9 intrꢀ
40 oducing directing groups, strong electronꢀwithdrawing groups
or blocking groups,10 and nucleophilic addition or radical
addition.7a,11 However, the arylations of 1,10ꢀphenanthrolines
through these strategies do not work because of their powerful
bidentate coordination or difficult access to raw materials.
45 N,N′ꢀannelated phenanthrolinedinone (1a, 6,7ꢀdihydroꢀ3Hꢀ
1,4ꢀdiazepino[1,2,3,4ꢀlmn][1,10]phenanhrolineꢀ3,9(5H)ꢀ
structure identification of 4a was carried out by H NMR, 13C
1
70 NMR, 1Hꢀ1H COSY spectra, mass spectrometry analysis,
which revealed that the arylation occurred at the 3,8ꢀpositions
of Phen (see the Supporting Information). Subsequently,
variations of palladium sources were investigated to improve
the outcome, Pd(TFA)2 turned out to be the best choice (Table
75 S1, entries 2ꢀ7). Further studies showed that prolonging
reaction time and increasing Pd(TFA)2 loading could
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significantly improve yields, affording 3a in 43% yield and 4a
in 40% yield (Table S1, entry 9). Other oxidants were also
evaluated, Ag2CO3 was clearly a more efficient oxidant (Table
the major product (in 57% isolated yield) when extended the
reaction time to 96 h (Scheme 2, entry 1). While employed
Pd(OAc)2 as catalyst and TBAB (tetrabutyl ammonium
bromide) as additive, the coupling of 1a with benzene and oꢀ
S1, entries 10ꢀ14). Although
a significant increase of
5
diarylated yield (4a, 57%) was observed when reaction time 35 xylene gave monoꢀarylated products 3a and 3b in 60% and
was further extended to 96 h, but the total yield of 3a and 4a
slightly decreased perhaps due to the longꢀtime heating
leading to partial decomposition of the products. Thus, the
optimal reaction condition was finalized when 20 mol% of
62% isolated yields, respectively (Scheme 2, entries 1,2). In
addition, 1ꢀmethylꢀ1,10ꢀphenanthrolinꢀ2(1H)ꢀone (1b) was
also found to be a competent substrate, which coupled with
benzene and gave the corresponding arylated product in 63%
10 Pd(TFA)2 was employed in combination with Ag2CO3 (3.0 40 yield (Scheme 2, entry 10).
equiv), PivOH (4.0 equiv) at 130 °C for 48 h under nitrogen
20 %mol Pd(TFA)2
3.0 equiv Ag2CO3
atmosphere.
Ar1
Ar2
Ar1
+
H
Ar2
20 %mol Pd(TFA)2
3.0 equiv Ag2CO3
4.0 equiv PivOH
130 oC, 48 h
(60~120 equiv)
N
N
N
N
2
Ar1 + Ar1
Ar1
+
O
O
H
Ar1
O
O
4.0 equiv PivOH
130 oC, 48 h
N
N
N
N
3
N
N
5
2
1
, Ar = 3,4-dimethylphenyl5a, Ar1 = 3,4-dimethylphenyl, Ar2 = phenyl
68%
67%
O
O
O
O
O
O
3b
1
3
4
3i, Ar1 = 2,5-difluorophenyl
5b, Ar1 = 2,5-difluorophenyl, Ar2 = phenyl
(60~90 equiv)
, Ar1 = 2,5-difluorophenyl 5c, Ar1 = 2,5-difluorophenyl, Ar2 = 3,4-dimethylphenyl 60%
3i
1
3
4
2
Yield (%) ( + )
Entry
1
3f, Ar1 = 3,5-dimethylphenyl 5d
3h, Ar1 = 2,5-dimethylphenyl
, Ar1 = 3,5-dimethylphenyl, Ar2 = phenyl
65%
71%
5e, Ar1 = 2,5-dimethylphenyl, Ar2 = phenyl
87 (40+47)
83 (26+57)a
82 (60+22)b
H
H
Scheme 3 Synthesis of unsymmetrical diarylated Phens (5).
N
N
O
O
1a
CH3
85 (39+46)
83 (62+21)b
The unsymmetric arylꢀsubstituted phenanthrolines are
known to possess relatively high quantum yields and redꢀ
45 shifted fluorescence emissions.1b,2a,4e Systematic variation of
the molecular symmetry through incorporation of different
aryl substituent onto the Phen nucleus would facilitate the
investigation and fine tuning of the photoelectric properties of
the target molecule. However, only a limited number of
50 unsymmetrical analogues, especially at 3,8ꢀpositions, are
known so far.1b,2g Therefore, the further arylation reaction of
monoꢀaryl substituted phenanthroline derivatives with another
arene were explored to obtain various unsymmetric 3,8ꢀdiaryl
phenanthrolines. It was found that with the same condition,
55 the second aryl substituent could readily be introduced onto
aromatic parent nucleus though the oxidative C–H/C–H crossꢀ
coupling of two coupling substrates, affording the
unsymmetrical products in good yields (Scheme 3).
1a
2
3
CH3
H
H
H
1a
72 (40+32)
66 (37+29)
64 (38+26)
F
F
1a
1a
4
5
Cl
Cl
CH3
6
7
1a
1a
74 (36+38)
80 (37+43)
H
H
H
CH3
CH3
Cl
CH3
CH3
1a
1a
8
64 (37+27)
71 (42+29)
63c
H3C
F
ii
Ar1
Ar1
9
H
F
N
N
7
, Ar1 = phenyl
86%
Ar1
Ar1
7a
i
Ar1
Ar1
7b, Ar1 = 3,4-dimethylphenyl 85%
N
N
N
N
10
Cl
Cl
H
O
O
6
N
N
4
6a, Ar1 = phenyl
76%
73%
69%
iii
1b
Ar1
Ar1
O
, Ar1 = 3,4-dimethylphenyl
6b
6c, Ar1
= 2,5-difluorophenyl
N
N
Scheme 2 Reactions were carried out by using 1 (0.25 mmol), Pd(TFA)2
15 (0.2 equiv), Ag2CO3 (3.0 equiv), PivOH (4.0 equiv), in 2 (2.0 mL, 15~23
mmol, 60~90 equiv) under N2. Sum of the isolated yields of 3 and 4,
Ar3
Ar3
92%
8b, Ar1 = 3,4-dimethylphenyl), Ar3 = phenyl 93%
8
8a, Ar1 = Ar3 = phenyl
respectively.a 96 h. Pd(OAc)2 (0.2 equiv), Ag2CO3 (4.0 equiv), TBAB
b
60 Scheme 4 Synthesis of 3,8ꢀdiarylꢀ1,10ꢀphenanthrolines (7) and 2,3,8,9ꢀ
tetraarylꢀ1,10ꢀphenanthrolines (8). (i) POCl3, PCl5, 120 ºC, 12 h; (ii)
PdCl2dppf, NaBH4, TMEDA, THF, 70 ºC, 16 h; (iii) Ar3ꢀB(OH)2,
Pd(PPh)4, K2CO3, PhMe/DMF (1/1), 120 ºC, 20 h.
(0.2 equiv), 30 h. c 100 ºC
With the optimized conditions, the direct arylation of N,N′ꢀ
20 annelated phenanthrolinedinone (1a) was next investigated.
As shown in Scheme 2, a variety of unactivated arenes,
including benzene, 1,2ꢀ, 1,3ꢀ and 1,4ꢀdisubstituted benzenes,
1,2,3ꢀtrisubstituted benzene, and tetrahydronaphthalene, could
couple with 1a through the cleavage of two C–H bonds,
25 affording the desired 3ꢀaryl and 3,8ꢀdiaryl substituted
phenanthroline derivatives in synthetically useful yields. The
monoꢀarylated products 3 and diꢀarylated products 4 can be
easily isolated through column chromatography due to the
large difference of their molecular polarity. The oxidative
30 crossꢀcoupling of 1a with benzene provided diꢀarylated 4a as
After having obtained a range of diarylated N,N′ꢀannelated
65 phenanthrolinedinones, the subsequent chlorination and
dechlorination or Suzuki coupling were carried out (Scheme
4). According to the procedure described by Lewis et al. and
Yamada et al., 2,9ꢀdichloroꢀ3,8ꢀdiarylꢀ1,10ꢀphenanthrolines
(6) were prepared in good yield by reacting 4 with PCl5 and
70 POCl3 at 120 ºC for 12 h.12 The reductive removal of chloroꢀ
group from 6 by means of couple NaBH4–TMEDA under
palladium catalysis gave 3,8ꢀdiarylꢀ1,10ꢀphenanthrolines
(7).13 When 6 coupled with phenylboronic acids in toluene
and DMF, with Pd(PPh3)4 as catalyst (10 mol% per chloro
2
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(7a) is a competent organocatalyst in promoting the transitionꢀ
substituent) and K2CO3 as base, 2,3,8,9ꢀtetraarylꢀ1,10ꢀ
phenanthrolines (8) were obtained in excellent yield after 50 metalꢀfree direct arylation reaction of benzene with aryl halides.
column chromatography.12a
In order to evaluate the role of these aryl functionalized
phenanthroline ligands, transitionꢀmetalꢀfree direct arylations
of benzene with aryl halides were performed in the presence
The palladium catalysed Mizoroki–Heck reaction using Phens as
ligands showed that 2,3,8,9ꢀtetraphenylꢀ1,10ꢀphenanthrolines (8a)
is a highly efficient ligand with both Pd and 8a loading as low as
0.001 mol%. We expect that the synthetic protocol and
5
of
a catalytic amount of Phens (Scheme 5). Recently, 55 phenanthroline ligands obtained here would find other numerous
transitionꢀmetalꢀfree direct arylations emerged as a very
rapidly growing trend in synthetic organic chemistry because
interesting applications.
This work was supported by grants from 973 Program
(2011CB808600), the National NSF of China (Nos 21172155,
21025205 and 21021001), and Sichuan Provincial Foundation
10 of its environmental friendliness.14 Phen and its derivatives,
especially 4,7ꢀdiphenyl phenanthroline, have been shown to
efficiently promote the reaction.14b,14c Hence, in the initial 60 (2012JQ0002).
experiment, the crossꢀcoupling of benzene with iodobenzene
was investigated with KtOBu (3.0 equiv) as base and 3,8ꢀ
Notes and references
15 diphenylꢀ1,10ꢀphenanthroline (7a, 10 mol%) as an
organocatalyst. Gratifyingly, the coupling product was
obtained in 93% yield. While 2,3,8,9ꢀtetraphenylꢀ1,10ꢀ
phenanthrolines (8a, 10 mol%) was employed as an
organocatalyst, only a small amount of the coupling product
20 was obtained perhaps due to steric hindrance of two phenyl at
2,9ꢀpositions. It was worth noting that aryl bromides could
also couple efficiently with benzene to provide the
corresponding direct arylated products in good to excellent
yields.
Key Laboratory of Green Chemistry and Technology of Ministry of
Education, College of Chemistry, and State Key Laboratory of
Biotherapy, West China Medical School, Sichuan University, 29
65 Wangjiang Road, Chengdu 610064, PR China. Fax: (+86) 28ꢀ85412203;
Eꢀmail: jingbolan@scu.edu.cn; jsyou@scu.edu.cn
†Electronic Supplementary Information (ESI) available: Detailed
experimental procedures, analytical data. See DOI: 10.1039/b000000x/
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t
L
, K OBu (3.0 eq.)
R
R
X
+
100 oC, 24 h
9
MeO
OMe
MeO
9a, 93% (X = I)a
9a, 9% (X = I)b
9b, 93% (X = Br)c
9b, 67% (X = Br)a
c
9d, 83% (X = Br)c
80
9c
, 89% (X = Br)
25
Scheme 5. Crossꢀcoupling of aryl halides with benzene promoted by
Phens, a 7a 10 % mol. b 8a 10 % mol. c 7a 20 % mol.
Subsequently, the palladium catalysed Mizoroki–Heck reaction
was investigated with aryl functionalized Phens as the ligand. As
85
3
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30 shown in Scheme 6, the reaction of iodobenzene with nꢀbutyl
acrylate gave nꢀbutylcinnamate in quantitative yield when 0.001
mol% of Pd(OAc)2 was used as the catalyst and 8a (0.001 mol%)
as the ligand. While 7a or Phen as ligands, the yields were
apparently inferior to 8a under the same conditions, indicating 8a
35 is an excellent ligand in the palladium catalysed Mizoroki–Heck
reaction.15
4
90
95
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O
0.001 mol% Pd(OAc)2, 0.001 mol% L,
O
K2CO3 (2.0 equiv), DMF, 130 oC, 10 h
OnBu
I
OnBu
+
L = 7a,
8a,
yield 52%, TON, 52000
yield 99%, TON, 99000
6
L
=
10a
L = Phen, yield 50%, TON, 50000
100
105
110
Scheme 6. Pdꢀcatalysed Mizoroki–Heck reactions of iodobenzene with nꢀ
butyl acrylate using Phens as ligands.
40
In summary, we have developed an efficient method for the
oxidative
crossꢀcoupling
of
the
6,7ꢀdihydroꢀ3Hꢀ1,4ꢀ
and
7
8
diazepino[1,2,3,4ꢀlmn][1,10]phenanhrolineꢀ3,9(5H)ꢀdione
unactivated arenes via a palladiumꢀcatalyzed twofold CꢀH
activation. Various aryl functionalized Phens including 3,8ꢀ
45 diarylꢀ1,10ꢀphenanthrolines
and
2,3,8,9ꢀtetraarylꢀ1,10ꢀ
phenanthrolines were successfully prepared through subsequent
chlorination and dechlorination or Suzuki coupling. We
furthermore demonstrated that 3,8ꢀdiphenylꢀ1,10ꢀphenanthroline
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