Rhodium-Catalyzed Silylative and Germylative Cyclization with Dehydrogenation Leading to 9-Sila- and 9-Germafluorenes: A Combined Experimental and Computational Mechanistic Study

Article abstract of DOI:10.1002/chem.201701579

Stoichiometric amounts of oxidants are widely used as promoters (hydrogen acceptors) in dehydrogenative silylation of C?H bonds. However, the present study demonstrates that silylative and germylative cyclization with dehydrogenation can proceed efficiently, even without hydrogen acceptors. The combination of [RhCl(cod)]₂ and PPh₃ was effective for both transformations, and allowed a reduction in reaction temperature compared with our previous report. Monitoring of the reactions revealed that both transformations had an induction period for the early stage, and that the rate constant of dehydrogenative germylation was greater than that of dehydrogenative silylation. Competitive reactions in the presence of 3,3-dimethyl-1-butene indicated that the ratio of dehydrogenative metalation and hydrometalation was affected by reaction temperature when a hydrosilane or hydrogermane precursor was used. Further mechanistic insights of oxidant-free dehydrogenative silylation, including the origin of these unique reactivities, were obtained by density functional theory studies.

Full text of DOI:10.1002/chem.201701579

A Journal of
Accepted Article
Title: Rhodium-Catalyzed Silylative and Germylative Cyclization with
Dehydrogenation Leading to 9-Sila- and 9-Germafluorenes: A
Combined Experimental and Computational Mechanistic Study
Authors: Masahito Murai, Ryo Okada, Sobi Asako, and Kazuhiko Takai
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Rhodium-Catalyzed Silylative and Germylative Cyclization with
Dehydrogenation Leading to 9-Sila- and 9-Germafluorenes: A
Combined Experimental and Computational Mechanistic Study
Masahito Murai,*[a] Ryo Okada,[a] Sobi Asako,[a] and Kazuhiko Takai*[a]
Abstract: Stoichiometric amounts of oxidants are widely used
as promoters (hydrogen acceptors) in dehydrogenative silylation
of CH bonds. However, the present study demonstrates that
silylative and germylative cyclization with dehydrogenation can
proceed efficiently, even without hydrogen acceptors. The
combination of [RhCl(cod)]2 and PPh3 was effective for both
transformations, and allowed a reduction in reaction temperature
compared with our previous report. Monitoring of the reactions
revealed that both transformations had an induction period for
the early stage, and that the rate constant of dehydrogenative
germylation was greater than that of dehydrogenative silylation.
Competitive reactions in the presence of 3,3-dimethyl-1-butene
indicated that the ratio of dehydrogenative metalation and
hydrometalation was affected by reaction temperature when a
hydrosilane or hydrogermane precursor was used. Further
mechanistic insights of oxidant-free dehydrogenative silylation,
including the origin of these unique reactivities, were obtained by
density functional theory studies.
limited.[7-9] The reported methods usually require high
temperatures (above 150 °C), microwave heating, and/or a large
excess of aromatic substrates (benzene or toluene) as solvents
(Scheme 1a).[7a-e,h,j,l] The exception was reported by Oestreich
and Tatsumi on the ruthenium-catalyzed CH silylation of indole
derivatives, in which the unique electrophilic aromatic
substitution with sulfur-stabilized silicon cation species was
proposed as a reaction mechanism (Scheme 1b).[7f] Using
pyridine-based directing groups, Sato et al. reported that
dehydrogenative silylation occurred without any hydrogen
acceptors under iridium catalysis.[7g,i] More recently, tBuOK-
catalyzed radical silylation of CH bonds of heterocycles was
reported by Stoltz and Grubbs et al. (Scheme 1c).[7k]
Introduction
Catalytic silylation with hydrosilanes provides synthetically
valuable organosilicon compounds with high atom-efficiency.[1]
The resulting CSi bonds are stable, but can be derivatized
easily to a wide variety of functional groups by reactions such as
oxidative halogenation, Tamao oxidation, and Hiyama cross-
coupling
reaction.[2]
Hydrosilylation
with
unsaturated
carboncarbon multiple bonds, such as alkenes and alkynes,
represents one of the most important CSi bond-forming
reactions.[3] In addition, dehydrogenative silylation of unactivated
CH bonds with hydrosilanes has received attention as a
convenient, economical, and environmentally benign approach
in the past decades.[4] Considerable progress has been made in
this field using catalysts, such as scandium, ruthenium, rhodium,
iridium, and platinum complexes, for the formation of CSi bonds
from ubiquitous CH bonds without preactivation of the starting
compounds. However, most of the reported methods require a
stoichiometric amount of the oxidant as a hydrogen acceptor,
including olefins, a Lewis acid,[5] and peroxides,[6] to overcome
the generally thermodynamically unfavorable formation of CSi
bonds with hydrogen elimination. The development of a catalytic
dehydrogenative silylation without any hydrogen acceptors is
Scheme 1. Catalytic dehydrogenative silylation of CH bonds
without
hydrogen
acceptors
(TpMe2
=
hydridotris(3,5-
dimethylpyrazolyl)borate)
Prior to the work of Oestreich, Sato, Stoltz, and Grubbs, we
reported the rhodium-catalyzed synthesis of 9-silafluorenes via
intramolecular dehydrogenative silylation of 2-biphenylsilanes
(Scheme 2).[8] Wilkinson complex, RhCl(PPh3)3, was effective for
this transformation, and the reaction did not require external
oxidants (hydrogen acceptors)[10] or heteroatom-containing
directing groups. Stemming from this seminal work, oxidant-free
dehydrogenative silylation was applied to the synthesis of
various silacycles using a combination of [RhCl(cod)]2 and
phosphines as a catalyst and ligands, respectively.[9] Despite
these rapid experimental advances, the reaction mechanism,
and the reason why the reaction proceeds without oxidant have
not been fully elucidated. Experimental observations that the
choice of oxidant (hydrogen acceptor) was crucial in the
following similar types of dehydrogenative silylation, were
[a]
Dr. M. Murai, R. Okada, Dr. S. Asako, Prof. Dr. K. Takai
Division of Applied Chemistry, Graduate School of Natural Science
and Technology, Okayama University, 3-1-1 Tsushimanaka, Kita-ku,
Okayama 700-8530, Japan
E-mail: masahito.murai@okayama-u.ac.jp
ktakai@cc.okayama-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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especially interesting: (i) iridium-catalyzed intramolecular
silylation of CH bonds,[11] (ii) rhodium-catalyzed intramolecular
silylation of C(sp3)H bonds,[12] and (iii) rhodium- or iridium-
catalyzed intermolecular silylation of CH bonds.[13]
temperatures; however, the yield of 4a decreased due to
competitive decomposition of the precursor 3a (entries 7 and 8).
Table 1. Catalytic dehydrogenative silylation and germylation of CH
bonds
The present study describes experimental and computational
results related to the unique oxidant-free dehydrogenative
silylation of C(sp2)H bonds. Analogous DFT studies on the
intermolecular dehydrogenative silylation of C(sp2)H bonds and
the intramolecular dehydrogenative silylation of C(sp3)H bonds
in the presence of cyclohexene or norbornene as a hydrogen
acceptor have been previously performed by Sunoj and
Hartwig.[13b,14] Their mechanistic studies revealed that the
catalytic cycle comprised by the following three steps: 1)
oxidative addition of hydrosilane to a hydrorhodium species; 2)
dehydrogenation of the resulting metal silyl dihydride complexes
by hydrogen acceptors; and 3) CH bond activation by the metal
silyl species followed by the reductive elimination to form CSi
bond. Hartwig reported that the resting state of the catalyst is
metal silyl dihydride complex, and its dehydrogenation by
hydrogen acceptor is the rate-determining step in the rhodium-
catalyzed intermolecular dehydrogenative silylation of C(sp2)H
bonds. On the other hand, the rate-determining step is
suggested to be SiC forming reductive elimination in the
iridium-catalyzed intramolecular dehydrogenative silylation of
C(sp3)H bonds by Sunoj. The comparative study of rhodium-
and iridium-catalyzed intramolecular silylation of C(sp2)H bonds
in the current study provides several clues for the development
of oxidant-free functionalization of CH bonds with
dehydrogenation.
As mentioned above, dehydrogenative germylation of
unactivated CH bonds is rare. To compare the reaction profiles
of germylation and silylation, reactions of hydrosilane 1a and
hydrogermane 3a were monitored at 80 °C. The time-course of
the formation of 2a and 4a is shown in Figure 1, which suggests
that both reactions have an induction period during the early
stage, but the rate constant for dehydrogenative germylation
was 1.5 times greater than that for silylation (kGe/kSi).
Scheme 2. Rhodium-catalyzed synthesis of 9-silafluorenes via
dehydrogenative silylation of C(sp2)
H bonds
Results and Discussion
We first found that the combination of [RhCl(cod)]2 with PPh3
(Rh:P = 1:3) as a catalyst gave the expected 9-silafluorene 2a in
90% yield, even at 70 °C from reaction with dimethyl[(2-
phenyl)phenyl]silane (1a) (Table 1, entry 1). Compared with
Wilkinson complex,[8] RhCl(PPh3)3, which was the best catalyst
in our previous report, the efficiency of the reaction improved
(entry 2). The yield decreased to 21% when [IrCl(cod)]2 was
used in place of [RhCl(cod)]2 (entry 3). The yield decreased to
63% when the reaction was conducted at 60 °C, but increased
slightly at 80 °C (entries 4 and 5). The combination of
[RhCl(cod)]2 and PPh3 was also effective for the
dehydrogenative germylation of the CH bonds of
hydrogermane 3a to afford 9-germafluorene 4a in 92% yield
Figure 1. Time-conversion curves for reaction of 1a and 3a
(recovery of 1a (red circles) and 3a (blue circles), yield of 2a (red
squares) and 4a (blue squares)).
(entry 6).[15] This is
a rare example of dehydrogenative
functionalization involving bonds between fourth-row atoms and
hydrogen.[16] The reaction proceeded even at lower
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Although dehydrogenative silylation occurred without any
oxidants, reaction of 1a and 3a was next examined in the
presence of olefins. Among olefins, 3,3-dimethyl-1-butene, which
is a widely used hydrogen acceptor, was chosen due to its mild
reactivity considering the result obtained from our previous
examination.[12,13c] The competitive intermolecular hydrosilylation
of 3,3-dimethyl-1-butene leading to 5 suppressed the desired
dehydrogenative silylation of CH bonds, and the yield of 2a
decreased to 63% at 70 °C (Figure 2(a)).[17] Note that the ratio of
dehydrogenative silylation product 2a and hydrosilylation
product 5 was greatly affected by the reaction temperature. 2a
was obtained selectively at high temperature, while the formation
of 5 via intermolecular hydrosilylation occurred preferentially at
germafluorenes 2 and 4 in high yields. Unfortunately, partial
reductive dechlorination occurred during silylative cyclization of
biarylsilane containing a chloro group to produce a mixture of
the 9-silafluorenes 2c and 2a. In contrast, chemoselective
formation of 9-germafluorene 4c occurred preferentially in the
corresponding germylative cyclization with dehydrogenation.
Note that cyclization of hydrosilane containing an electron-
donating methoxy group leading to 2d proceeded smoothly
when 3,3-dimethyl-1-butene was treated as an additive. In the
absence of 3,3-dimethyl-1-butene, the reaction was sluggish
even at 120 °C. This result indicates that 3,3-dimethyl-1-butene
promoted the dehydrogenative silylation as a hydrogen acceptor,
although a similar positive effect was not observed for cyclization
of 1a as shown in Figure 2.[20] Silylative and germylative
cyclization occurred selectively at the -position of the
low temperature.[18]
In contrast, 3,3-dimethyl-1-butene
successfully promoted the germylative cyclization of
hydrogermane 3a, and the yield of 9-germafluorene 4a
increased from 52% to 83% at 60 °C (Table 1, entry 8 vs Figure
2(b)). Competitive intermolecular hydrogermylation to 3,3-
dimethyl-1-butene leading to 6 did not occur efficiently at any
temperature examined (40~80 °C).[19,20]
Table 2. Rhodium-catalyzed silylative cyclization of 1 leading to 9-
silafluorene derivatives 2
Table 3. Rhodium-catalyzed germylative cyclization of 3 leading to
9-germafluorene derivative 4
Figure 2. Competition between intramolecular dehydrogenative
metalation and intermolecular hydrometalation with 3,3-dimethyl-1-
butene. M = Si for (a) and M = Ge for (b).
Based on the above results, the effect of substitution pattern
of hydrosilane 1 and hydrogermane 3 on dehydrogenative
silylation ([RhCl(cod)]2 / PPh3 at 70 °C) and germylation
([RhCl(cod)]2 / PPh3 with 3,3-dimethyl-1-butene at 60 °C) was
investigated (Tables 2 and 3). A variety of (2-biaryl)hydrosilanes
and hydrogermanes possessing electron-withdrawing and -
donating substituents gave the corresponding 9-sila- and 9-
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naphthalene ring to furnish benzo[b]silafluorene 2f and
benzo[b]germafluorene 4f in moderate yields. The current
dehydrogenative cyclization was applicable to 2-(biaryl)silane
and germane containing thienyl or benzothienyl groups to afford
thiophene-fused benzosilole 2g and benzogermole 4g in good
yields.
According to previous reports, reaction is initiated by oxidative
addition of hydrosilane 1a to a hydrorhodium species (HRh in
Figure 3) to generate silylrhodium complex A.[21] Intermediate A
then reacts through two possible pathways (Path a or b). In Path
a, C(sp2)H bond activation via -complex-assisted metathesis
(-CAM)[22] forms six-membered rhodacycle intermediate B.
Subsequent elimination of H2 followed by reductive elimination
furnishes 9-silafluorene 2a along with regeneration of the
hydrorhodium species. In Path b, silylrhodium intermediate A
first undergoes reductive elimination of H2 to become
intermediate C. Oxidative addition of the rhodium center in C to
an aryl CH bond generates intermediate D, which then
undergoes reductive elimination to give 2a. Path b is essentially
the same mechanism proposed by Hartwig for rhodium-
catalyzed intermolecular dehydrogenative silylation of aromatic
compounds in the presence of cyclohexene as a hydrogen
acceptor.[13b]
Reaction of hydrosilane containing a methyl group ortho to the
silylphenyl group afforded a mixture of 9-silafluorene 2h and
9,10-dihydro-9-silaphenanthrene
7 in 8% and 31% yield,
respectively (Scheme 3).[12] Addition of 3,3-dimethyl-1-butene
was again effective for this dehydrogenative silylation. Similar
selectivity was observed for dehydrogenative cyclization of the
corresponding biarylgermane 3h, in which 9,10-dihydro-9-
germaphenanthrene 8 was obtained as a major product along
with 9-germafluorene 4h. These results demonstrate that
dehydrogenative silylation and germylation of the C(sp3)H
bonds occurred preferentially over the aryl C(sp2)H bonds. This
outcome is unique, because dehydrogenative functionalization
generally favors C(sp2)H bonds under neutral conditions, and
selective functionalization of C(sp3)H bonds remains difficult.
Scheme 3. Competitive dehydrogenative germylation of C(sp3)−H
vs C(sp2)−H bonds
Figure 3. Proposed reaction mechanism (phosphine ligands omitted
for clarity)
The proposed reaction mechanism for rhodium-catalyzed
dehydrogenative silylation and germylation is shown in Figure 3.
Figure 4. Energy profiles for rhodium-catalyzed dehydrogenative silylation of 1a (Path a (red), Path b (black), and Path b with 3,3-dimethyl-1-
butene (blue)). Relative Gibbs free energies and enthalpies calculated at the B3LYP-D3/SDD:6-31G(d) level of theory in dioxane at 70 °C are
given in kcal/mol.
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Figure 5. Optimized structures of rhodium complexes involved in Path (b) calculated at the B3LYP-D3/SDD:6-31G(d) level. Color code: gray,
carbon; white, hydrogen; pink, phosphine; blue, silicon; green, rhodium.
To obtain insight into the transformation, DFT calculations
were performed at the B3LYP-D3/SDD:6-31G(d) level of theory
using hydrosilane 1a and HRh(PPh3)2 as active species (Figures
4 and 5, see SI for details).[14] The free energies were calculated
for the reaction in dioxane at 70 °C (the conditions in Table 2).
The results suggested that the reaction proceeds preferentially
via Path b (Figure 4, black line) involving reductive elimination of
H2, and an alternative mechanism following Path a (Figure 4, red
line) via the -CAM is energetically disfavored. Oxidative
addition of 1a to HRh(PPh3)2 is highly exergonic (G = –21.6
kcal/mol) and gives intermediate A. In Path a, one of the two
hydrogen atoms moves from the equatorial position to the axial
position to form A’, which is ready for -CAM through TSA’B
(TSA’B = 20.1 kcal/mol) leading to B.[23] In Path b, which does not
involve hydrogen acceptors, elimination of H2 from A leads to
silylrhodium intermediate C, in which an ortho CH bond of a
benzene ring of 1a weakly coordinates to the rhodium center in
an 2-fashion (Rh–C: 2.83 Å, Rh–H: 2.51 Å). The transition state
for the reductive elimination of H2 could not be located. The
energy monotonously increased as the distance between the
two hydrogen atoms decreased, which indicates that the reverse
reaction, the rebound of H2 and subsequent oxidative addition, is
virtually barrierless and proceeds rapidly once the intramolecular
2-coordination is removed. Oxidative addition of the aryl C–H
bond (C to D) followed by C–Si bond-forming reductive
elimination (D to E) occurs smoothly with a barrier of 11.2 and
13.5 kcal/mol, respectively. The benzene ring of E coordinates to
the rhodium center in an 2-fashion (Rh–C: 2.46 Å, Rh–C: 2.36
Å).[24] Finally, product dissociation from E regenerates the active
rhodium species, completing the catalytic cycle.
Calculations for the reaction using 3,3-dimethyl-1-butene were
performed next. The energy profile is outlined in Figure 4 with
blue line. The system, in which 3,3-dimethyl-1-butene accepts
H2, gains a driving force as much as 22.2 kcal/mol greater than
Figure 6.
intermediate
Possible reaction pathways from HRh(PPh3)2 to
C in the presence of 3,3-dimethyl-1-butene. The
relative Gibbs free energies and enthalpies calculated at the B3LYP-
D3/SDD:6-31G(d) level of theory in dioxane at 70 °C are given in
kcal/mol.
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the acceptor-less system (–16.0 vs –38.2 kcal/mol for the energy
of intermediate C in Figure 4), which suggests that the reaction
is accelerated by 3,3-dimethyl-1-butene. Since 3,3-dimethyl-1-
butene could not be located to TSCD or D, probably because of
strong -donation by a trans-silyl group, the reaction profile is
essentially the same after formation of C. In contrast,
coordination of 3,3-dimethyl-1-butene to C affords olefin adduct
K (see Figure 7) by replacing the 2-coordinated benzene ring,
which inhibits C–H bond cleavage leading to 9-silafluorene 2a,
and causes the undesired side reaction, hydrosilylation of alkene
to produce 5 (vide infra). Detailed reaction pathways for the
formation of intermediate C starting from HRh(PPh3)2 are
depicted in Figure 6. The most energetically favored pathway is
HRh(PPh3)2 → A → H → I → C.[25] An alternative mechanism
starting from insertion of 3,3-dimethyl-1-butene to the RhH
bond followed by oxidative addition of 1a leading to I may also
be plausible.[26]
Intermediate I can potentially undergo two different reactions:
dehydogenative silylation leading to 9-silafluorene 2 and
hydrosilylation leading to 5. Figure 7 shows possible reaction
pathways for hydrosilylation of 1a with 3,3-dimethyl-1-butene.
Intermediate I may reductively eliminate 5 via TSIJ (9.8 kcal/mol).
However, C–H reductive elimination to produce 2,2-
dimethylbutane and C occurs more easily via TSIC (–4.5
kcal/mol), thus excluding the first mechanism. The following
silylrhodation of olefin adduct K proceeds via TSKL (–14.0
kcal/mol) to form alkylrhodium species L, which eventually
affords 5 after reaction with another molecule of 1a. TSKL is 9.4
kcal/mol higher in energy than TSDE (–23.4 kcal/mol, see Figure
4) leading to intermediate E at 70 °C. Calculations at 25 °C
predicts a smaller energy difference between the transition state
of the silylrhodation and that for the desired reaction (G = 7.3
kcal/mol, TSKL/25: –19.1 kcal/mol, TSDE/25: –26.4 kcal/mol). These
results explain the experimental observation demonstrated in
Figure 2 that intramolecular dehydrogenative silylation occured
preferentially over intermolecular alkene hydrosilylation at high
temperatures, and are rationalized by considering the entropy
effect. Intramolecular 2-coordination of the benzene ring in C
followed by the oxidative addition to CH bond is preferred at
high temperature over intermolecular coordination of another
molecule of 3,3-dimethyl-1-butene leading to intermediate K with
an entropic cost. Thus, the hydrogen acceptor functions as a
thermodynamic promoter by making H2 release irreversible and
gaining driving force, but also acts as a kinetic inhibitor by
forming an olefin adduct, such as K, and causing the undesired
side reaction.
Further insight was obtained by comparing the driving force of
intramolecular dehydrogenative silylation with the intermolecular
version, i.e., reaction of PhMe2SiH with benzene (Figure 8).[13]
The results clearly suggested that the intermolecular reaction is
endergonic even at 70 °C (G = +4.2 kcal/mol), and does not
proceed efficiently without the assistance of
a hydrogen
acceptor, while the intramolecular reaction is exergonic (G = –
4.1 kcal/mol). Besides these thermodynamic considerations, the
intramolecular coordination of a benzene ring to a rhodium
center in intermediate C (Figure 4) is thought to be helpful for
prohibiting the reverse reaction, oxidative addition of H2, as well
as facilitating CH bond cleavage. These are the major
advantages for the current rare example of oxidant-free CH
bond functionalization under neutral conditions.
Figure 8. Comparison of Gibbs free energies and enthalpies of
C(sp2)H bonds dehydrogenative silylation in dioxane at 70
°C.
Finally, the energy profile for iridium-catalyzed intramolecular
dehydrogenative silylation was compared with that for the
rhodium-catalyzed system (Figure 9). The IrIII species AIr is
thermodynamically stable compared with the other IrI species,
much more than the corresponding RhIII species are when
compared with the RhI species (see Figure 4). Therefore,
oxidative addition leading to IrIII is energetically favored
compared to the reductive elimination back to IrI. The energy
Figure 7. Possible reaction pathways for formation of hydrosilylation
product 5. The relative Gibbs free energies and enthalpies
calculated at the B3LYP-D3/SDD:6-31G(d) level of theory in dioxane
at 70 °C are given in kcal/mol.
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difference between AIr and CIr in the absence of a hydrogen
acceptor is greater than that of the rhodium-catalyzed system
(14.0 vs 5.6 kcal/mol), and hence the reverse oxidative addition
of intermediate CIr to H2 leading back to AIr occurs more easily.
dehydrogenative silylations require hydrogen acceptors.[11-13]
Iridium catalysis with a hydrogen acceptor may be suitable for
dehydrogenative silylation of CH bonds, in which oxidative
addition is included as a difficult step, but not effective for the
current oxidant-free intramolecular system, as shown by Table 1,
entry 3.
These results provide
a rationale for the experimental
observations that most of the reported iridium-catalyzed
Figure 9. Energy profile for iridium-catalyzed intramolecular dehydrogenative silylation of 1a. The relative Gibbs free energies and enthalpies
calculated at the B3LYP-D3/SDD:6-31G(d) level of theory in dioxane at 70 °C are given in kcal/mol.
underway.
Conclusions
Experimental Section
A rare example of oxidant-free dehydrogenative silylation and
germylation of C(sp2)H bonds leading to 9-sila- and 9-
germafluorenes was demonstrated. The combination of
[RhCl(cod)]2 with PPh3 was effective as a catalyst, enabling the
reaction to proceed at temperatures lower than those reported
previously.[8] Competitive reactions in the presence of 3,3-
dimethyl-1-butene revealed that the ratio of dehydrogenative
metalation and hydrometalation was affected by reaction
temperature and the hydrosilane or hydrogermane precursors
used. The experimentally observed unique reactivity was
supported by results of theoretical calculations, and a plausible
reaction mechanism, including insights into why the current
rhodium-catalyzed dehydrogenative silylation and germylation
did not require hydrogen acceptors was proposed. In addition to
a thermodynamic preference for the dehydrogenative cyclization
(G = –4.1 kcal/mol) and the relatively easy elimination of H2
General Methods. All reactions were carried out in dry solvent under an
argon atmosphere. 1,4-Dioxane was purchased from Wako Pure
Chemical Industries, distilled from benzophenone Na ketyl solution, and
degassed with an argon gas for 20 min before use. [RhCl(cod)]2 was
purchased from Kanto Chemical Co. PPh3 was purchased from Wako
Pure Chemical Industries and recrystallized before use. Unless otherwise
noted, other chemicals obtained from commercial suppliers were used
without further purification. 2-Bromobiaryls were prepared by Suzuki-
Miyaura cross-coupling reaction between aryl iodides (or aryl bromides)
and aryl boronic acids.[28] 1H (400 MHz) and 13C (100 MHz) NMR spectra
were recorded on a JEOL ECS-400 spectrometer. Proton chemical shifts
are reported in ppm based on the solvent resonance resulting from
incomplete deuteration (CDCl3 at 7.26 ppm) as the internal standard. 13
C
NMR was recorded with complete proton decoupling and the chemical
shifts are reported relative to CDCl3 at 77.00 ppm. The following
abbreviations are used; br s: broad singlet, s: singlet, d: doublet, t: triplet,
q: quartet, m: multiplet. IR spectra were recorded on a SHIMADZU
IRAFFINITY-1 100V J. High-resolution mass spectra (HRMS) were
measured with JEOL JMS-700 MStation FAB-MS. Melting points (mp)
were measured with Yanako MP-J3 and uncorrected.
from RhIII species (GAC
= 5.6 kcal/mol), intramolecular
coordination of a benzene ring to a rhodium center appeared to
kinetically promote oxidative cleavage of a C–H bond and block
oxidative addition of the released H2. This work provides
fundamental understanding of the oxidant-free catalytic
functionalization of CH bonds with dehydrogenation.[27] Further
study of the design of substrates as well as ligands to determine
the novel type of dehydrogenative metalation of CH bonds is
Preparation of 2-(dimethylgermyl)-2’-methylbiphenyl (3h). nBuLi (1.6
M in hexane, 2.8 mL, 4.5 mmol) was added to a solution of 2-bromo-2’-
methylbiphenyl (741 mg, 3.0 mmol) in Et2O (10 mL) at -78 °C, and the
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mixture was stirred for 30 min. A solution of dimethyldichlorogermane
(775 mg, 4.5 mmol) in Et2O (2.0 mL) was added dropwise, and the
resulting mixture was gradually warmed to 25 °C. After stirring for 12 h, a
suspension of LiAlH4 (0.23 g, 6.0 mmol) in Et2O (5.0 mL) was added at
0 °C, and the mixture was further stirred at 25 °C for 14 h. The reaction
mixture was quenched with H2O (0.2 mL), aqueous NaOH (15 wt%, 0.2
mL), and H2O (0.6 mL). The mixture was filtered through a Celite pad,
and the solvent was removed under the reduced pressure. The residue
was purified by flash column chromatography on silica gel with n-hexane
and ethyl acetate (v / v = 50 / 1) as eluent to give 2-(dimethylgermyl)-2’-
methylbiphenyl (3h) (700 mg, 2.4 mmol, 80% yield) as a colorless oil. 1H
NMR (400 MHz, CDCl3):  0.11 (d, J = 3.6 Hz, 3H), 0.16 (d, J = 3.6 Hz,
3H), 2.07 (s, 3H), 4.14 (sept, J = 3.6 Hz, 1H), 7.13 (d, J = 7.2 Hz, 1H),
7.15-7.41 (m, 4H), 7.33 (dt, J = 2.0, 7.2 Hz, 1H), 7.38 (dt, J = 2.0, 7.2 Hz,
1H), 7.59 (dd, J = 1.6, 7.2 Hz, 1H). 13C NMR (100 MHz, CDCl3): -3.73, -
3.59, 20.3, 125.1, 126.4, 127.4, 128.3, 129.1, 129.7, 129.9, 134.3, 136.0,
138.5, 142.8, 147.9. IR (neat / cm-1): 3055, 3017, 2976, 2914, 2029, 1464,
1423, 1246, 1117, 835, 756, 729. HRMS (FAB+): calcd for C14H15ClGe
([M]+) 292.0620; found 292.0608.
Research Center for Computational Science, Okazaki National
Research Institute, is gratefully acknowledged.
Keywords: Rhodium • Dehydrogenation • CH Bond Activation •
Silylation • Germylation
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silylation leading to 9-silafluorenes (Table 2). A flame dried 7.0 mL of
sealed tube was charged with [RhCl(cod)]2 (1.5 mg, 3.0 mol), PPh3 (4.7
mg, 18 mol), 1,4-dioxane (0.20 mL), and stirred at 25 °C for 15 mins. 2-
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General procedure for rhodium-catalyzed dehydrogenative
germylation leading to 9-germafluorenes (Table 3). A flame dried 7.0
mL of sealed tube was charged with [RhCl(cod)]2 (1.5 mg, 3.0 mol),
PPh3 (4.7 mg, 18 mol), 1,4-dioxane (0.20 mL), and stirred at 25 °C for
15 mins. 2-(Dimethylgermyl)biaryls 3 (0.20 mmol) and 3,3-dimethyl-1-
butene (50 mg 0.60 mmol) was added, and the resulting mixture was
stirred at 60 °C for 12 h. The solvent was removed under the reduced
pressure, and the residue was subjected to flash column chromatography
on silica gel with n-hexane as eluent to give the corresponding 9-
germafluorenes 4.
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9,9-Dimethyl-4-methyl-9-germafluorene (4h) and 9,9-dimethyl-9,10-
dihydro-9-germaphenanthrene (8). These compounds were obtained
as a mixture of inseparable constitutional isomers. 1H NMR (400 MHz,
CDCl3):  0.35 (s, 6H for 8), 0.58 (s, 6H for 4h), 2.29 (s, 2H for 8), 2.78
(s, 3H for 4h), 7.17-7.24 (m, 4H), 7.26-7.32 (m, 3H), 7.38-7.49 (m, 2H),
7.50-7.54 (m, 3H), 7.62-7.69 (m, 2H), 8.12 (d, J = 8.1 Hz, 1H for 4h). 13
C
NMR (100 MHz, CDCl3): -4.9, -2.7, 20.9, 24.7, 126.1, 126.3, 126.6,
126.7, 126.9, 127.3, 128.1, 128.8, 129.0, 129.1, 129.6, 130.4, 130.7,
132.3, 132.6, 132.8, 133.7, 134.7, 136.9, 137.6, 138.2, 142.8, 144.2,
148.1. IR (neat / cm-1): 3051, 3022, 2970, 2907, 1584, 1466, 1443, 1234,
1123, 1086, 827, 808, 785, 731, 608. HRMS (FAB+): calcd for C15H16Ge
([M]+) 270.0464; found 270.0461.
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Acknowledgements
This work was financially supported by a Grant-in-Aid for
Scientific Research (A) (No. 26248030 to K.T.) and Scientific
Research (C) (No. 16K05778 to M.M.) from MEXT, Japan, and
the Mitsubishi Foundation (Research Grants in the Natural
Sciences). The generous amount of computational time from the
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10.1002/chem.201701579
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during the formation of 2d even in the presence of 3,3-dimehyl-1-
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is also proposed as active catalytic species in the hydrosilylation of
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kcal/mol).
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Table of Contents
FULL PAPER
Masahito Murai,* Ryo Okada, Sobi
Asako, Kazuhiko Takai*
Page No. – Page No.
Rhodium-Catalyzed Silylative and
Germylative Cyclization with
Dehydrogenation Leading to 9-Sila- and
9-Germafluorenes: Experimental and
Computational Mechanistic Study
The combination of [RhCl(cod)]2 and PPh3 catalyzes the oxidant-free
dehydrogenative silylation and germylation of unactivated C(sp2)H bonds under
neutral conditions. The experimentally obtained unique reactivity is supported by
the results of theoretical calculations, and a plausible reaction mechanism including
insights into why the current transformation does not require hydrogen acceptors
and heteroatom-containing directing groups is proposed.
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