Palladium-catalyzed annulation of vic-bis(pinacolatoboryl)alkenes and -phenanthrenes with 2,2′-dibromobiaryls: Facile synthesis of functionalized phenanthrenes and dibenzo[g,p]chrysenes

Article abstract of DOI:10.1002/anie.200803213

(Chemical Equation Presented) Double-cross: An annulation approach to functionalized polycyclic aromatic hydrocarbons involving a palladium-catalyzed double cross-coupling reaction of vic-diborylalkenes and -phenanthrenes with 2,2′-dibromobiaryls led to a variety of phenanthrenes and dibenzo-[g,p]chrysenes, as well as [5]helicenes, dithienobenzenes, and triphenyleno[1,2-b:4,3-b′]dithiophenes (see scheme; B_pin = pinacolatoboryl).

Full text of DOI:10.1002/anie.200803213

Communications
DOI: 10.1002/anie.200803213
Cross-Coupling
Palladium-Catalyzed Annulation of vic-Bis(pinacolatoboryl)alkenes
and -phenanthrenes with 2,2’-Dibromobiaryls:Facile Synthesis of
Functionalized Phenanthrenes and Dibenzo[g,p]chrysenes**
Masaki Shimizu,* Ikuhiro Nagao, Yosuke Tomioka, and Tamejiro Hiyama
Because of the progress in organic electronics, such as light-
emitting diodes, field-effect transistors, and solar cells, there
has been extensive research on polycyclic aromatic hydro-
carbons (PAHs).[1] Efficient syntheses of PAHs bearing the
appropriate functional groups to control optical and elec-
tronic properties is important, and different structural motifs
are essential for the exploration and improvement of
materials derived from PAHs. Conventional synthetic meth-
ods for making functionalized PAHs include electrophilic or
nucleophilic substitutions,[2] cross-coupling reactions,[3] and
directed-metalation,[4] in which the introduction of the func-
tional groups and the substitution patterns are strongly
dependent on the reactivity of the parent PAHs framework.
Alternatively, formation of the aromatic ring by transition-
metal-catalyzed annulation of functionalized precursors pro-
vides a more flexible and versatile synthesis for functionalized
PAHs by overcoming the drawbacks of the conventional
synthetic approach.[5,6]
Since the cross-coupling reaction of organoboron com-
pounds (Suzuki–Miyaura coupling) is useful for stereospecific
Scheme 1. Palladium-catalyzed double cross-coupling reaction of vic-
diborylated alkenes (1) and phenanthrenes (2) with 2,2’-dibromobiaryl
compounds.
À
Csp2 Csp2 bond formation and is compatible with a broad
range of functional groups,[7] we focused our attention on
vic-diborylalkenes[8] as potential precursors for our annula-
tion approach.[9] Various vic-diborylated alkenes have been
converted into multisubstituted acyclic alkenes by stepwise
cross-coupling reactions with two organic halides,[10] whereas
the double cross-coupling reaction of vic-diborylated com-
pounds with aromatic dihalides as a synthetic method for
functionalized PAHs remains unexplored.[11–13] We report
herein a palladium-catalyzed double cross-coupling reaction
of vic-diborylated alkenes (1) and phenanthrenes (2) with
2,2’-dibromobiaryl compounds. The annulation provides a
convenient synthesis of a variety of phenanthrenes and
dibenzo[g,p]chrysenes, as well as dithienobenzenes and
triphenyleno[1,2-b:4,3-b’]dithiophene, in good to excellent
yields (Scheme 1).
Initially, we chose (Z)-1,2-bis(pinacolatoboryl)stilbene
(1a) as a model vic-diborylalkene and 2,2’-dibromobiphenyl
(3a) as a coupling partner [Eq. (1)]. Initial attempts revealed
that protodeborylation of 1a was a major side reaction,
presumably resulting from the steric hindrance of both 1a and
3a. After screening various reaction conditions,[14] the desired
9,10-diphenylphenanthrene (4aa)[15] was isolated in 80%
yield using [Pd(PPh3)4] (5 mol%), K2CO3 (6 equiv), and
H2O (50 equiv) in THF at 608C for 24 hours. The presence
of water was crucial for the reaction.[16] The optimized
conditions were applicable to gram-scale preparation of the
annulated rings,[17] and neither oligomers nor polymers were
detected.
[*] Prof. Dr. M. Shimizu, I. Nagao, Y. Tomioka, Prof. Dr. T. Hiyama
Department ofMaterial Chemistry, Graduate School ofEngineering
Kyoto University, Kyoto University Katsura
Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2445
E-mail: m.shimizu@hs2.ecs.kyoto-u.ac.jp
Homepage: http://npc05.kuic.kyoto-u.ac.jp/e-index.htm
[**] This work was supported by Grants-in-Aid for Creative Scientific
Research, No. 16GS0209, from the Ministry of Education, Culture,
Sports, Science, and Technology (Japan) and The Asahi Glass
Foundation.
The scope of the annulation reaction leading to function-
alized phenanthrenes (4) is summarized in Table 1. The
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803213.
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Table 1: Synthesis ofuf nctionalized phenanthrenes ( 4).[a]
proceeded smoothly under the optimized reaction conditions
to produce octafluorinated phenanthrene 4ad in 65% yield
(Table 1, entry 3). When 2,2’-dibromobinaphthyl (3e) and
bithiophene 3 f were used with 1a, [5]helicene 4ae and
dithieno[l,2-6:4,3-6’]benzene 4af were obtained, respectively
(Table 1, entries 4 and 5). Substituted diborylstilbenes 1b–d
also underwent the annulation to give both symmetrical and
unsymmetrical phenanthrenes 4ba, 4bd, 4ca, and 4da in high
yields (Table 1, entries 6–9). Additionally, diboryldithienyl-
ethene 1e, 1,2-diboryl-1-phenylbut-1-ene 1 f, and 4,5-diboryl-
oct-4-ene 1g reacted with 3 to give annulated products 4ed,
4 fa, 4ga, and 4gg (Table 1, entries 10–13); a slight modifica-
tion to the reaction conditions was necessary in cases of 1 f
and 1g.[18]
Entry
1
Dibromide 3
Product 4 (yield)[b]
Next we examined the annulation of vic-diborylated
arenes such as 2, which were readily prepared by photo-
cyclization of 1 and subsequent in situ oxidation.[14] By using
the optimized conditions determined for the annulation of 1,
the reaction of 2a with 3a gave dibenzo[g,p]chrysene 5aa in
68% yield. After re-screening the reaction conditions, 3m aq.
K3PO4 was found to be the best base for this system,
producing 5aa in a quantitative yield. The scope of the
annulation reaction leading to functionalized dibenzo-
[g,p]chrysenes (5) is summarized in Table 2. Various dibenzo-
[g,p]chrysenes (5) substituted with electron-donating and
electron-withdrawing groups are not easily accessible by
previous methods,[19] but they were isolated in moderate to
good yields by using our annulation chemistry. Notably,
fluorinated dibenzo[g,p]chrysenes 5ad, 5bd, 5 cd, 5dd, and
5 fd were generally produced in high to excellent yields
(Table 2, entries 3, 6, 8, 9, and 11). The annulation was also
1
2
1a
1a
3b (R=Me)
3c (R=OMe)
4ab (85%)
4ac (82%)
3
4
1a
1a
3d
3e
4ad (65%)
4ae (32%)
5
6
7
8
9
1a
1b
1b
1c
1d
3 f
3a
3d
3a
3a
4af (84%)
4ba (92%)
4bd (85%)
4ca (81%)
4da (77%)
Table 2: Synthesis ofuf nctionalized dibenzo[ g,p]chrysenes (5).[a]
10
1e
1 f
1g
3d
3a
3a
4ed (90%)
4 fa (99%)
4ga (81%)
11[c]
12[d]
Entry
2
3
5
Yield [%][b]
1
2a
2a
2a
2a
2b
2b
2c
2c
2d
2e
2 f
3a
3c
3d
3 f
3a
3d
3a
3d
3d
3c
3d
5aa
5ac
5ad
5af
5ba
5bd
5ca
5 cd
5dd
5ec
5 fd
>99
54
63
88
64
94
61
80
82
40
96
13[e]
1g
3g
4gg (57%)
2[c]
3
[a] Reaction conditions: 1 (1.2 mmol), 3 (1.0 mmol), [Pd(PPh3)4] (58 mg,
0.05 mmol), and K2CO3 (0.83 g, 6.0 mmol), H2O (0.90 mL, 50 mmol),
THF (20 mL), 608C, 48 h. [b] Yield ofthe isolated product. [c] 3 m aq
K3PO4 (6 equiv) was used in place ofK 2CO3/H2O. [d] 3m aq NaOH
(6 equiv) was used in place ofK CO3/H2O. [e] [PdCl2(dppf)] (5 mol%)
and 3m aq K3PO4 (6 equiv) was used in place of[Pd(PPh 3)4] and K2CO3/
H2O, respectively. dppf=1,1’-bis(diphenylphosphanyl)ferrocene.
4
5
6
7
8
9
2
10
11[d]
[a] Reaction conditions: 2 (0.06 mmol), 3 (0.05 mmol), [Pd(PPh3)4]
(2.9 mg, 2.5 mmol), and 3m aq K3PO4 (0.1 mL, 0.3 mmol), THF
(1.0 mL), 608C, 48 h. [b] Yield ofisolated product. [c] 12 m aq K3PO4
(0.6 mmol) was used at 808C. [d] K2CO3 (0.3 mmol) and H2O
(2.5 mmol) were used.
tetramethyl- and tetramethoxy-substituted dibromobiphenyls
3b and 3c were coupled with 1a to give 4ab and 4ac,
respectively, in high yields (Table 1, entries 1 and 2). The
reaction of 2,2’-dibromooctafluorobiphenyl (3d) with 1a also
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applicable to the preparation of triphenyleno[1,2-b:4,3-b’]-
dithiophene (5af; Table 2, entry 4).
materials such as luminescent polymers, thin-field effect
transistors, and molecular wires.[21] Helicene and thiophene-
fused PAHs can also be synthesized easily. The results herein
shed new light on the synthetic utility of vic-diborylated
compounds in organic synthesis. The extension of the use of
dihalides and diboron reagents for annulation reactions are in
progress in our laboratory.
To gain mechanistic insight into the annulation reaction,
we carried out the reaction of 1a with o-bromotoluene
(2 equiv) under the optimized conditions (Scheme 2). The
only product obtained was the singly coupled product (6) in
71% yield; the doubly coupled product (7) was not observed.
This result indicated that an second intermolecular trans-
metalation for the coupling of a second equivalent of ortho-
substituted bromobenzene with 1a was unfavorable. Accord-
ingly, the use of 2,2’-dibromobiaryl compounds as the
coupling partner is crucial for the success of the annulation,
as it suppresses oligomerization/polymerization.
Received: July 3, 2008
Published online: September 15, 2008
Keywords: annulation · arenes · boron · palladium · polycycles
.
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