Rhodium-catalyzed dehydrogenative germylation of C-H bonds: New entry to unsymmetrically functionalized 9-germafluorenes

Article abstract of DOI:10.1021/ol503355q

Rhodium-catalyzed dehydrogenative germylation leading to unsymmetrically functionalized 9-germafluorenes via Ge-H and C-H bond activation is described. Despite the significant achievements made in dehydrogenative functionalization of C-H bonds, only a limited number of examples with the fourth-row atom-H bonds have been reported. The current method enabled the synthesis of various 9-germafluorene derivatives, including tetracyclic as well as donor-acceptor substituted germoles, which may be useful for electronic device applications.

Full text of DOI:10.1021/ol503355q

Letter
pubs.acs.org/OrgLett
Rhodium-Catalyzed Dehydrogenative Germylation of C−H Bonds:
New Entry to Unsymmetrically Functionalized 9‑Germafluorenes
Masahito Murai,*,† Koji Matsumoto,† Ryo Okada,† and Kazuhiko Takai*,†,‡
†Division of Chemistry and Biotechnology, Graduate School of Natural Science and Technology, Okayama University, 3-1-1
Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
‡Research Center of New Functional Materials for Energy Production, Storage and Transport, Okayama University, 3-1-1
Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
S
* Supporting Information
ABSTRACT: Rhodium-catalyzed dehydrogenative germylation leading to unsymmetrically
functionalized 9-germafluorenes via Ge−H and C−H bond activation is described. Despite
the significant achievements made in dehydrogenative functionalization of C−H bonds, only a
limited number of examples with the fourth-row atom−H bonds have been reported. The
current method enabled the synthesis of various 9-germafluorene derivatives, including
tetracyclic as well as donor−acceptor substituted germoles, which may be useful for electronic device applications.
atalytic dehydrogenative functionalization of C−H or
heteroatom−H bonds with unactivated C−H bonds is of
the rhodium-catalyzed dehydrogenative silylation of unactivated
C(sp2)−H bonds leading to 9-silafluorenes.5,6 The results
suggested that similar dehydrogenative functionalization of
other group 14 atom-containing substrates would allow new
syntheses of the corresponding 9-heterofluorenes. Germole-
based π-electron systems, especially in 9-germafluorenes, have
received attention as key components for new organic
functional materials, including solar cells, thin-film transistors,
and host materials for electroluminescent devices, due to their
unique photophysical as well as electronic properties.7 It has
been reported that incorporation of a germanium atom
stabilizes the LUMO through interaction of the low lying π*
orbital of the germanium with the π* orbital of the conjugated
π-system, and also enhances the solid-state ordering compared
to the carbon- and silicon-fused analogues, leading to improved
charge transport.8 Therefore, development of new methods for
the facile synthesis and easy modification of their π-frameworks
is highly desirable. Conventional syntheses of 9-germafluorenes
often require stoichiometric organolithium or magnesium
reagents, which limits the choice of functional groups on the
substrates.7 A transition-metal-catalyzed synthesis has been
reported based on the cross-coupling reaction of hydrogermane
with haloaromatics,9a and a reaction via cleavage of the
germanium−carbon bonds.9b However, limitations in substrate
scope remain for these reactions.
C
interest because of its ability to introduce groups with broad
synthetic potential for the construction of complicated
molecules without preactivation of starting compounds.1
Dehydrogenative functionalization involving borylation (B−
H), arylation (C−H), carbonylation (C−H), amination (N−
H), hydroxylation (O−H), silylation (Si−H), and phosphina-
tion (P−H) has been developed using transition metal
complexes as catalysts (Scheme 1).2 In contrast to the well-
Scheme 1. Challenge in the Catalytic Dehydrogenative
Functionalization
studied bonding reactions between second- or third-row atoms
and hydrogens, much less attention has been paid to the
dehydrogenative functionalization of C−H bonds involving bonds
between the fourth-row atom and hydrogens.3 The present study
focused on germanium as a fourth-row atom and demonstrated
the transition-metal-catalyzed synthesis of 9-germafluorenes via
dehydrogenative germylation (Ge−H) of the unactivated C−H
bonds. Although formal dehydrogenative germylation of
styrene derivatives via regioselective hydrogermylation followed
by β-elimination has been reported by Murai and co-workers
using Ru3(CO)12 as a catalyst,3 the present work describes the
direct dehydrogenative germylation of unreactive aromatic C−
H bonds. Interestingly, the reaction proceeded without the
addition of external or internal oxidants,4 which is usually
required in catalytic dehydrogenative functionalization to
induce catalytic turnover.
Treatment of dimethyl((2-phenyl)phenyl)germane (1a) with
a catalytic amount of Wilkinson catalyst, RhCl(PPh3)3, which
was the most effective catalyst for the synthesis of 9-
silafluorenes via the dehydrogenative silylation of C(sp2)−H
bonds,5a in dioxane at 110 °C gave the 9-germafluorene 2a in
71% yield (Table 1, entry 1). The biphenyl 3 was obtained as a
byproduct in 18% yield under these reaction conditions. In
contrast, similar cleavage of the Si−C bond was not observed
Heterocycles are important because of their unique photo-
physical and electronic properties. A recent report described
Received: November 19, 2014
© XXXX American Chemical Society
A
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Letter
without cleavage of the Ge−C bond leading to biphenyl 3
(entry 6). Several ligands promoted the reaction, but none were
superior to PPh3 (entry 7). Using the combination of
[RhCl(cod)]2 with PPh3, 2a was isolated in 92% yield, even
when the reaction temperature was lowered to 80 °C (entry 8).
Note that the reaction proceeded efficiently even in the absence
of hydrogen acceptors and oxidants, which are usually needed
for catalytic dehydrogenative functionalizations.1
With optimized reaction conditions established, the scope of
substrates for the dehydrogenative germylation of the C(sp2)−
H bond leading to unsymmetrical 9-germafluorenes was
examined (Table 3). The reaction of biarylgermanes with an
Table 1. Catalytic Dehydrogenative Cyclization Leading to
9-Germafluorene 2a and 9-Silafluorene 5a
a
entry
M
results
1
2
Ge (1a)
Si (4a)
71% (2a), 18% (3), 10% (1a)
97% (5a), 0% (3)
a
1
Determined by H NMR.
Table 3. Rh-Catalyzed Germylative Cyclization Leading to
Germole-Fused Tri- and Tetracycles 2
when the corresponding biarylhydrosilane 4a was employed as
a precursor instead of 1a (entry 2). In this case, 9-silafluorene
5a was obtained in 97% yield without producing any
byproducts. The Ge−C bond is inherently weaker than the
Si−C bond based on bonding energy (Ge−C 246.87 vs Si−C
292.88 kcal/mol). Thus, the Ge−C bond in 1a is cleaved more
easily than the Si−C to form 3.
These successful results prompted an examination of the
catalytic activity of various transition-metal complexes for the
synthesis of 9-germafluorene 2a (Table 2). Among the catalysts
Table 2. Optimization of Reaction Conditions
a
yield /%
recovery of
b
entry
catalyst/ligand
2a
3
1a /%
1
2
3
4
[RhCl(cod)]2/PPh3
93
51
15
63
6
0
[Rh(OMe)(cod)]2/PPh3
[lr(OMe)(cod)]2/PPh3
12
15
6
6
57
0
[RhCl(cod)]2/
P(4-MeOC6H4)3
a
1
Isolated yields. Yields determined by H NMR are in parentheses.
b
5
[RhCl(cod)]2/
P(4-CF3C6H4)3
74
12
0
3,3-Dimethyl-1-butene (5 equiv) was added.
6
7
[RhCl(cod)]2/PCy3
21
9
0
73
46
0
electron-withdrawing trifluoromethyl group and electron-
neutral methyl group gave the corresponding 9-germafluorenes
2b and 2c in 88% and 85% yields, respectively (entries 1 and
2).12 Even hydrogermane containing an electron-donating
methoxy group was a good substrate for the current
dehydrogenative cyclization and afforded the 2-methoxy-9-
germafluorene 2d in 92% yield (entry 3). It is noteworthy that
reaction with the corresponding hydrosilane containing a
methoxy group was sluggish, and addition of 3,3-dimethyl-1-
butene as a hydrogen acceptor was required to obtain 2-
methoxy-9-silafluorene in good yield, as described in a previous
report.5a A chlorine group, which can be used subsequently in
various cross-coupling reactions, was also tolerated to afford 2e
in 90% yield, which indicates the potential utility of the present
reaction in various functional material syntheses (entry 4). In
contrast, reductive dechlorination occurred to afford 9-
silafluorene 5a as a byproduct when the corresponding
biarylsilane 4b was employed as a precursor under the current
reaction conditions (eq 1).5a In all cases, cleavage of the Ge−C
bonds to afford the corresponding biaryl did not occur under
the optimized reaction conditions. The current method can be
also applied to the construction of a germylene-bridged
[RhCl(cod)]2/P(C6F5)3
30
b
8
[RhCl(cod)]2/PPh3
97 (92)
2 (2)
a
1
Determined by H NMR. The value in parentheses is isolated yield.
At 80 °C for 2 h.
b
screened, the combination of [RhCl(cod)]2 with 3 equiv of
PPh3 was the most effective to furnish 2a in 93% yield (entry
1).10 Under the reaction conditions, the formation of biphenyl
3 was limited to 6% yield. Although other rhodium and iridium
catalysts including [Rh(OMe)(cod)]2, [IrCl(cod)]2, and [Ir-
(OMe)(cod)]2 exhibited marginal catalytic activity (entries 2
and 3), [Rh(OAc)2]2, Re2(CO)10, Ru3(CO)12,3 and Ir4(CO)12
were almost ineffective, with the recovery of 1a in >95% yield.
Examination of the solvents revealed that the best yield was
obtained in dioxane.11 Next, the effect of ligands using
[RhCl(cod)]2 and dioxane as the metal complex and solvent,
respectively, were investigated. Triarylphosphines with elec-
tron-donating and -withdrawing substituents displayed good
activity to afford 9-germafluorene 2a in 63% and 74% yield,
respectively (entries 4 and 5). When PCy3 was used as a ligand,
selective formation of 2a was observed albeit in low yield
B
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Scheme 3. Synthetic Application to Donor−Acceptor
Substituted 9-Germafluorene 6
expanded π-conjugated system.13 2-(2′-Naphthyl)phenyl-
germane 1f containing naphthyl groups cyclized smoothly to
furnish benzo[b]germafluorene 2f in 80% yield (entry 5). The
reaction occurred selectively at the β-position of the
naphthalene ring without producing its regioisomer, benzo[a]-
germafluorene. Diarylgermane 1g containing a benzothienyl
group also underwent dehydrogenative germylation, affording
the benzothiophene-fused tetracyclic germole 2g14 in 80% yield
in the presence of 3,3-dimethyl-1-butene (entry 6). Due to the
competitive decomposition of hydrogermane 1g, the yield
decreased to 17% without the addition of 3,3-dimethyl-1-
butene (entry 7). Therefore, 3,3-dimethyl-1-butene may be
useful not as a hydrogen acceptor, but as the ligand coordinated
to the rhodium center to control its reactivity.15
occurred smoothly to afford donor−acceptor substituted 9-
germafluorene 6 in 56% yield with the germylene moiety
intact.19,20 Reaction occurred mainly at the 7-position,
indicating that nucleophilic substitution of germoles occurs
preferentially at this position.
Preliminary studies to determine the photophysical proper-
ties of unsymmetrical 9-germafluorenes as new molecular
material candidates have been done (Figure 1).21 The electron-
Based on these results, a plausible mechanism for the
formation of 9-germafluorene 2a is illustrated in Scheme 2
Scheme 2. Proposed Reaction Mechanism
Figure 1. UV−vis (solid line) and fluorescence (dashed line) spectra
of unsymmetrically fuctionalized germafluorene derivatives 2a (black),
2d (yellow), 2f (blue), and 6 (red) in CH2Cl2 (8 × 10−6) at 25 °C.
donating methoxy group of 2d at the 2-position of the 9-
germafluorene ring caused a bathochromic shift in the
absorption maximum by 11 nm (2a vs 2d), indicating
pronounced π-conjugation. A set of well-resolved absorption
bands extending to 300 nm for the naphthalene-fused germole
2f reflects its rigid structure and expanded π-conjugation. The
red-shifted absorption band of donor−acceptor substituted 9-
germafluorene 6, in which the absorption tail extended to 443
nm, was also significant. The high intensity of the solar
irradiation spectrum around this range indicates potential
application of this compound as a component for solar cells.
Furthermore, tetracycle 2f and donor−acceptor substituted 6
exhibited fluorescence, in contrast with the absence of
fluorescence for 9-germafluorene 2a.
In conclusion, the rhodium-catalyzed dehydrogenative
germylation of readily available ((2-aryl)phenyl)germanes has
been shown to be a versatile synthetic route toward germylene-
bridged π-conjugated systems via the activation of Ge−H and
C−H bonds. This reaction is more efficient than the
conventional methods for synthesizing 9-germafluorenes,
because the only byproduct produced is hydrogen. Interest-
ingly, addition of an external or internal oxidant, which is
usually required for dehydrogenative functionalization, is not
necessary for most of the transformations. The resulting 9-
germafluorenes can be used as building blocks for the synthesis
of novel unsymmetrically functionalized 9-germafluorenes.20
Further investigations into the scope and reaction mechanisms,
as well as synthetic applications, are currently in progress.
(phosphine ligands are omitted for clarity). The reaction was
initiated by the oxidative addition of hydrogermane 1a to a
RhI−H species to generate germylrhodium complex A.16
Intermediate A then reacted through two possible pathways
(path A or B). In path A, C(sp2)−H bond activation via σ-
complex-assisted metathesis (σ-CAM)17 forms six-membered
rhodacycle intermediate B. Subsequent elimination of hydrogen
followed by the reductive elimination furnishes 9-germafluor-
ene 2a along with the regeneration of the RhI−H species. In
path B, reductive elimination followed by oxidative addition of
the RhI center in C to an aryl C−H bond generates a RhIII−H
intermediate, which then undergoes reductive elimination to
give 2a. Although C−H bond activation without participation
of a heteroatom containing directing group under neutral
conditions is rare,18 the position of the Ge−Rh moiety in
intermediate A or C, which is near the aryl C−H bond, is
thought to be very important for the promotion of this
energetically unfavored step.
Although 9-germafluorenes with expanded π-conjugation are
candidates as new components of light-emitting materials and
solar cells,7,13 reports on the synthesis of unsymmetrical 9-
germafluorenes are limited. The current reaction can provide
such a rare example of unsymmetrical 9-germafluorenes. In
addition, further transformations of the resulting 2-methoxy-9-
germafluorene (2d) were done to examine its synthetic utility
(Scheme 3). For example, nitration of 2d with Cu(NO3)2
C
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(11) Investigation of solvents with the catalyst system of
[RhCl(cod)]2/PPh3 at 110 °C: toluene 86%, ClCH2CH2Cl 79%,
MeCN 88%, DMF 28%.
(12) The reaction of a regioisomer of 1c, 2-(dimethylgermyl)-2′-
methylbiphenyl, has also been tested to afford the corresponding 9,9-
dimethyl-4-methyl-9-germafluorene in 14% yield.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, spectroscopic data for all new
1
compounds, and copies of H and 13C NMR spectra. This
material is available free of charge via the Internet at http://
(13) Reports on the synthesis of π-expanded 9-germafluorenes are
limited. For spirogermabifluorenes, see: (a) Gilman, H.; Gorsich, R. D.
J. Am. Chem. Soc. 1958, 80, 1883. (b) Russell, A. G.; Spencer, N. S.;
Philp, D.; Kariuki, B. M.; Snaith, J. S. Organometallics 2003, 22, 5589.
For a germa[5]helicene, see: (c) Yasuike, S.; Iida, T.; Okajima, S.;
Yamaguchi, K.; Seki, H.; Kurita, J. Tetrahedron 2001, 57, 10047. For a
trigermasumanene, see: (d) Tanikawa, T.; Saito, M.; Guo, J. D.;
Nagase, S.; Minoura, M. Eur. J. Org. Chem. 2012, 7135.
(14) Thiophene-fused germole derivatives have been thoroughly
investigated as key components for photovoltaic devices. For selected
examples, see: (a) Ohshita, J.; Hwang, Y.-M.; Mizumo, T.; Yoshida, H.;
Ooyama, Y.; Harima, Y.; Kunugi, Y. Organometallics 2011, 30, 3233.
(b) Fei, Z.; Ashraf, R. S.; Huang, Z.; Smith, J.; Kline, R. J.; D’Angelo,
P.; Anthopoulos, T. D.; Durrant, J. R.; McCulloch, I.; Heeney, M.
Chem. Commun. 2012, 48, 2955.
(15) The use of other alkenes, such as cyclooctene, cyclohexene,
norbornene, and styrene, was less effective to afford 2g in less than
20% yield. For representative reviews on the effect of unsaturated
hydrocarbons in the catalytic transformation, see: (a) Johnson, J. B.;
Rovis, T. Angew. Chem., Int. Ed. 2008, 47, 840. (b) Jarvis, A. G.;
Fairlamb, I. J. S. Curr. Org. Chem. 2011, 15, 3175. The yield of 2f was
decreased to 66% when 3,3-dimethyl-1-butene was added to the
reaction of 1f (see Table 3, entry 5).
(16) The H−Rh(PPh3)n species, generated via oxidative addition of
Cl−Rh(PPh3)n to H−SiR3 followed by reductive elimination of Cl−
SiR3, is proposed as an active catalytic species in the hydrosilylation of
alkynes. See: (a) Nishihara, Y.; Takemura, M.; Osakada, K.
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: masahito.murai@cc.okayama-u.ac.jp.
*E-mail: ktakai@cc.okayama-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by a Grant-in-Aid (No.
26248030) from MEXT, Japan, and a Grant-in-Aid for
Scientific Research on Priority Areas (No. 25105739), and
the MEXT program for promoting the enhancement of
research universities.
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D
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