Acceleration effects of phosphine ligands on the rhodium-catalyzed dehydrogenative silylation and germylation of unactivated C(sp3)-H bonds

Article abstract of DOI:10.1021/acs.joc.5b00920

The current work describes the marked rate of acceleration caused by phosphine ligands on the rhodium-catalyzed dehydrogenative silylation and germylation of unactivated C(sp³)-H bonds. The reactivity was affected by the steric and electronic n

Full text of DOI:10.1021/acs.joc.5b00920

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Acceleration Effects of Phosphine Ligands on the Rhodium-Catalyzed
Dehydrogenative Silylation and Germylation of Unactivated C(sp3)-H Bonds
Masahito Murai, Hirotaka Takeshima, Haruka Morita, Yoichiro Kuninobu, and Kazuhiko Takai
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b00920 • Publication Date (Web): 11 May 2015
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Acceleration Effects of Phosphine Ligands on the Rhodium-Catalyzed
Dehydrogenative Silylation and Germylation of Unactivated C(sp3)H
Bonds
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,†
†
†
‡
Masahito Murai,* Hirotaka Takeshima, Haruka Morita, Yoichiro Kuninobu, and
,†,§
Kazuhiko Takai*
†
Division of Applied Chemistry, Graduate School of Natural Science and Technology,
Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
Phone: (+81)-86-251-8095; Fax: (+81)- 86-251-8094
E-mail: masahito.murai@okayama-u.ac.jp
ktakai@cc.okayama-u.ac.jp
‡
Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, 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
ABSTRACT: The current work describes the marked rate of acceleration caused by
phosphine ligands on the rhodium-catalyzed dehydrogenative silylation and germylation of
unactivated C(sp3)H bonds. Reactivity was affected by the steric and electronic nature of
the phosphine ligands. The use of the bulky and electron-rich diphosphine ligand,
(R)-DTBM-SEGPHOS, was highly effective to yield the dehydrogenative silylation products
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selectively in the presence of a hydrogen acceptor. An appropriate choice of C2-symmetric
chiral diphosphine ligand enables the asymmetric dehydrogenative silylation via the
enantioselective desymmetrization of the C(sp3)H bond. The unprecedented catalytic
germylation of C(sp3)H bonds with dehydrogenation was also examined with the
combination of the rhodium complex and a wide bite angle diphosphine ligand to provide the
corresponding 2,3-dihydrobenzo[b]germoles in good yield.
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INTRODUCTION
Investigation of new catalyst systems for the improvement of reaction efficiency and
selectivity is one of the most important research topics in synthetic chemistry. Transition
metal-catalyzed dehydrogenative silylation of CH bonds is a straightforward, atom-efficient,
and environmentally friendly method for the synthesis of organosilicon compounds, and has
received intensive interest.1-3 In addition to the unique function of organosilicon compounds
themselves, they can be used as useful intermediates because silyl groups can be easily
converted to various functional groups by Hiyama cross-coupling4 and TamaoFleming
oxidation,5 etc. Although organosilicon compounds can be synthesized via bond forming
reactions with reactive functional groups, molecules containing the proper substituents are not
always readily available and sometimes must be prepared through additional synthetic steps
from commercially available building blocks. Thus, the development of efficient silylation
methods for ubiquitous CH bonds is highly desirable. There are many reports on the
dehydrogenative silylation of aromatic C(sp2)H bonds without any directing groups,1 whereas
silylation of aliphatic C(sp3)H bonds is still limited from the viewpoint of substrate scope.6-9
In most cases, activated C(sp3)H bonds at the benzylic position,7 or located adjacent to boron
or nitrogen atoms,8 are used as substrates. Generally, C(sp3)H bonds are highly unreactive
due to their thermal stability and low polar nature.
Seminal work on the dehydrogenative silylation of unactivated C(sp3)H bonds was reported
by Berry et al.9a They found Ru(p-cymene)(H)2(SiEt3)2 and Cp*Rh(H)2(SiEt3)2 (Cp* =
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5-C5Me5) complexes were effective for the dehydrogenative silylation of the C(sp3)H bond
adjacent to a silicon atom. Since then, Tilley et al. reported that the rare-earth-metal complex,
Cp*2ScH, could catalyze the dehydrogenative silylation of methane gas (150 atm) with
H2SiPh2.9b These works clearly imply that -coordinated six-electron-donor ligands are
highly important to overcome this unfavorable thermodynamic transformation. Recently,
Hartwig et al. disclosed the iridium-catalyzed hydroxyl group-directed dehydrogenative
silylation of C(sp3)H bonds, in which the phenanthroline-based nitrogen ligand,
3,4,7,8-tetramethyl-1,10-phenanthroline was reported to be optimal.9c The utility of nitrogen
ligands can be understood by considering the fact that nitrogen and oxygen containing
heterocycles have been frequently employed as directing groups for dehydrogenative CH
bond silylation,7c,7e,8,9f since the seminal work on CH bond functionalization by Murai et al.10
In contrast, the use of phosphorus atom-based ligands on the dehydrogenative silylation of
C(sp3)H bonds is limited to the reactions with RhCl(CO)(PMe3)2,7a Ni(PEt3)4,7b and
Ru(H)2(CO)(PPh3)3.8a Nevertheless, phosphorus ligands have been well investigated as one of
the most important and effective ligands in modern organic synthesis.11
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In 2013, we reported the rhodium-catalyzed synthesis of 2,3-dihydrobenzo[b]siloles via the
intramolecular dehydrogenative silylation of 2-alkylphenylsilanes (eq 1).9d The combination
of [RhCl(cod)]2 and bidentate phosphines, was found to be effective to construct
five-membered silicon-containing heterocycles. Although the silylation occurred even at less
reactive secondary C(sp3)H bonds,9a,d-f the reaction required high temperature to overcome the
low reactivity of the CH bond, which detracts from its synthetic utility. Recently, Hartwig et
al. reported that rhodium or iridium complexes with C2-symmetric bisphosphines are useful for
hydroarylation of olefins and dehydrogenative silylation of C(sp2)H bonds. These studies
stimulated us to reexamine the reaction conditions of dehydrogenative silylation of C(sp3)H
bonds carefully to improve the reaction efficiency. We envisioned that the precursors,
2-alkylphenylsilanes, are suitable to study the effect of phosphine ligands, since they do not
contain any heteroatoms, such as nitrogen and oxygen, which potentially coordinate with a
metal center.
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The present study describes the acceleration effect of phosphine ligands on the
rhodium-catalyzed dehydrogenative silylation of C(sp3)H bonds. Bulky and electron-rich
C2-symmetric diphosphine ligands were found to be effective, and the reaction temperature was
markedly decreased compared with our previous report.9d The proper choice of phosphine
ligands also enabled the unprecedented enantioselective silylative desymmetrization as well as
the dehydrogenative germylation of C(sp3)H bonds.
RESULTS AND DISCUSSION
The effect of phosphine ligands on the dehydrogenative silylation of
2-(dimethylsilyl)ethylbenzene 1a was first studied with a catalytic amount of [RhCl(cod)]2 in
o
1,4-dioxane at 180 C (Table 1). The monodentate phosphine ligands, such as PPh3, PCy3,
PMePh2, and P(o-Tol)3, were ineffective, and the yields of 2,3-dihydrobenzo[b]silole 2a were
less than 30% (entries 1-4).12 In contrast, bidentate diphosphines were found to be effective
(entries 5-8). Employing dppp and dppbz as ligands, 2a was obtained in 70% yields (entries
6 and 8). On the other hand, nitrogen-based bidentate ligands, including TMEDA and
1,10-phenanthroline, previously reported as effective ligands for the iridium-catalyzed
hydroxyl group-directed dehydrogenative silylation,9c were less reactive, with more than half
of starting hydrosilane 1a recovered (entries 9 and 10). Although the typical C2-symmetric
bisphosphines, BINAP and (R)-SEGPHOS, were less effective, the yield was increased to
72% when electron-rich and wide-bidentate phosphine, (R)-DTBM-SEGPHOS, was used as a
ligand. The catalytic activity of other rhodium and iridium precursors, [Rh(OMe)(cod)]2,
[IrCl(cod)]2, and [Ir(OMe)(cod)]2, with (R)-DTBM-SEGPHOS was also tested and the
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combination of [RhCl(cod)]2 and (R)-DTBM-SEGPHOS was found to be the most
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effective.
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Table 1. Effect of ligands on the dehydrogenative silylation of the C(sp3)H bond
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Based on these results, we chose dppp, dppbz, and (R)-DTBM-SEGPHOS as ligands and
studied other parameters further. We found the efficiency of the reaction was significantly
improved by the addition of hydrogen acceptors, and the reaction temperature could be
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markedly decreased from 180 C to 50 C. For example, 2,3-dihydrobenzo[b]silole 2a was
isolated in 77% yield when 1 equiv of 3,3-dimethyl-1-butene was added under the reaction
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conditions described in Table 1, entry 13 (Table 2, entry 1). On the other hand, the reaction
did not occur at all even with 3,3-dimethyl-1-butene, when dppp or dppbz were employed as
ligands at 50 oC (entries 2 and 3). Although norbornene can be used as a hydrogen acceptor,
the yield was decreased to 40% due to the competitive hydrosilylation of 1a with norbornene
(entry 4). Other hydrogen acceptors, including 1,5-cyclooctadiene and 1,4-cyclohexadiene,
were totally ineffective with most of 1a recovered (entries 5 and 6). The expected
2,3-dihydrobenzo[b]silole 2a was not obtained when the reaction was examined without
adding any hydrogen acceptor (entry 7).
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Table 2. Effect of hydrogen acceptors
Next, several 2-alkylphenylsilanes were subjected to the current optimized reaction
conditions (Table 3).
Diphenylsilane 1b was converted to the corresponding
2,3-dihydrobenzo[b]silole 2b in 76% yield with a slightly higher temperature. Reactions
with 2-ethylarylsilanes 1c and 1d having anisyl or naphthyl groups gave 2c and 2d in good
yields even at 50 oC.
C(sp3)H bonds of 2-tert-butylphenylsilane 1e and
2-isopropylphenylsilane 1f were also silylated effectively, affording the expected silacycles
2e and 2f in 90% and 83% yields, respectively. The effect of the substituents on the silicon
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was also examined to find that diethyl and diphenylsilanes 1g and 1h could also be used as
silicon sources.13
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Table 3. Rhodium-catalyzed dehydrogenative silylation of
2,3-dihydro-1H-benzo[b]siloles 2
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leading to
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To obtain insight into the ligand effects, the reaction of 1a was reexamined in the presence
of several phosphine ligands and 3,3-dimethyl-1-butene at 80 oC. Representative results are
shown in Table 4. All the phosphine ligands except (R)-DTBM-SEGPHOS afforded a
mixture of the desired 2,3-dihydrobenzo[b]silole 2a and the hydrosilylated product 3 (entries
1-4). In fact, the combination of rhodium and phosphine ligands has been previously
reported as an effective catalyst for the hydrosilylation of olefins.14 In sharp contrast,
formation of hydrosilylated product 3 was not observed, furnishing only 2a in 83% yield,
when (R)-DTBM-SEGPHOS was used with 3,3-dimethyl-1-butene (entry 5). This result
clearly indicates that (R)-DTBM-SEGPHOS can selectively accelerate dehydrogenative
silylation of C(sp3)H bonds even in the presence of olefins without producing hydrosilylated
adducts.
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Table 4. Competition between intramolecular dehydrogenative silylation and intermolecular
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hydrosilylation
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Based on these observations, Scheme 1 presents a plausible mechanism for the current
dehydrogenative silylation of unreactive C(sp3)H bonds. First, a rhodium hydride species
is generated via the oxidative addition of hydrosilane 1 to the rhodium precatalyst followed
by the reductive elimination of chlorosilane.15 This rhodium hydride species is subsequently
added to the SiH bond of 1, and the resulting intermediate A then reacts with
3,3-dimethyl-1-butene to form intermediate B.
Reductive elimination of H and
3,3-(dimethyl)butyl groups on the rhodium center affords intermediate C, whereas that of
silyl and 3,3-(dimethyl)butyl groups produces hydrosilylated product 3. Because sterically
bulky silyl and 3,3-(dimethyl)butyl groups tend to keep their distance from each other, the
reductive elimination to form C might be energetically more favorable. In the absence of a
hydrogen acceptor, 3,3-dimethyl-1-butene, intermediate A is directly converted to
intermediate C via the reductive elimination of H2.
Generally, this step is
o
thermodynamically unfavorable, and therefore requires additional heating to 180 C as shown
in Table 1 and our previous report.9d Rhodium silyl species C can potentially react with
3,3-dimethyl-1-butene to proceed hydrosilylation leading to 3. However, the electron-rich
rhodium center with a strongly electron-donating (R)-DTBM-SEGPHOS ligand should favor
the oxidative addition of C(sp3)H bonds to the rhodium center over the hydrosilylation,
which is usually promoted by the electron-deficient metal complex.16 This is consistent with
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the results shown in Table 4. Moreover, the intramolecular oxidative addition of C(sp3)H
bonds to the rhodium center can be also facilitated by the steric effect of the ligand, due to the
bulky (R)-DTBM-SEGPHOS overhanging outside. The subsequent reductive elimination
provides 2,3-dihydrobenzo[b]silole 2 along with the regeneration of the rhodium hydride
species.
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Scheme 1. Proposed reaction mechanism (phosphine ligand is omitted for clarity)
We further examined this unprecedented rhodium-catalyzed enantioselective
desymmetrization of C(sp3)H bonds via dehydrogenative silylation (Table 5).17,18
Treatment of 2-(isopropyl)silylbenzene 1f with (R)-DTBM-SEGPHOS afforded the
corresponding 2,3-dihydrobenzo[b]silole 2f in 81% yield and 21% ee. Because enantiomers
of 2f were not separated by HPLC methods using a chiral stationary phase, the ee was
determined by the HPLC analysis of diol 4, which could be readily converted from 2f by
TamaoFleming oxidation (Scheme 2).5 To increase the ee, the effect of other chiral
diphosphine ligands was tested. Among the phosphines examined, high yield, as well as
better enantioselectivity, was observed in the reaction catalyzed by the rhodium complex with
(R)-DTBM-Garphos. These studies also confirm that ligands having electron-rich biaryl
backbones were much more reactive, as revealed by a comparison of the reactions with
(R)-DTBM-SEGPHOS and (R)-DTBM-Garphos to those with (R)-3,4,5-MeO-BIPHEP.
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Although the enantioselective silylative cyclization of diethyl and diphenylsilanes 1g and 1h
were also examined, ee’s were less than 10% (data not shown).
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diphosphines
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Scheme 2. Transformation of the 2,3-dihydrobenzo[b]silole 2f
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With (R)-DTBM-SEGPHOS as a ligand, dihydrosilane 5 provided 1,1’-spirosilabiindane 6
in 73% yield with 27% ee (Table 6, entry 1). This is a rare example of the catalytic
construction of tetraorganosilicon stereocenters.19 The reaction proceeded via the sequential
twofold dehydrogenative silylation of C(sp3)H bonds.
The chirality of the
spirosilabiindane is thought to be determined at the first dehydrogenative cyclization.
(R)-DTBM-Garphos, which was the best ligand in the enantioselective dehydrogenative
silylation of 1f, produced both lower yield and ee, although the starting dihydrosilane 5 was
consumed completely (entry 2). Further screening of the catalyst revealed that changing the
ligand to (R)-H8-BINAP increased the ee up to 39%, albeit with only moderate yield of 6
(entry 3). Fortunately, the yield was improved to 75% without deterioration of the ee when
the catalyst loading was increased (entry 4).
As mentioned above, strongly
electron-donating (R)-DTBM-SEGPHOS was the most effective for the silylative cyclization
of 2-alkylphenylsilanes 1. This is probably because oxidative addition of C(sp3)H bonds
can be selectively promoted by the electron-rich phosphine ligand compared with the
competitive hydrosilylation with 3,3-dimethyl-1-butene. In the reaction of 5, however,
(R)-H8-BINAP also worked as an efficient promoter (entry 1 vs 3). The difference might be
explained by considering the fact that the bulky 2-tert-butylphenyl group on the silicon atom
prevented intermolecular hydrosilylation, and facilitated intramolecular C(sp3)H bond
silylation selectively. In fact, no hydrosilylated product was observed under any of the
conditions described in Table 6.
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Table 6. Rhodium-catalyzed sequential two-fold dehydrogenative silylation of C(sp3)H
bonds of 5 leading to 1,1’-spirosilabiindane 6
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In contrast to the well-studied bond formation reactions between second- or third-row
elements and hydrogens, much less attention has been paid to the dehydrogenative
functionalization of CH bonds involving bonds between the fourth-row elements and
hydrogens.20 The present successful result on the catalytic dehydrogenative silylation of
C(sp3)H bonds further stimulated us to examine dehydrogenative germylation of unactivated
C(sp3)H bonds. When (R)-DTBM-SEGPHOS was used as a ligand together with
3,3-dimethyl-1-butene at 100 oC, the yield of the expected 2,3-dihydrobenzo[b]germole 8 was
low (15%) with the decomposition of the precursor 2-germyl-tert-butylbenzene 7. After
further screening of the phosphine ligands, (R)-(S)-BPPFA was found to be effective to afford
8 in 65% yield (Scheme 3).21 It should be noteworthy that the reaction does not require a
hydrogen acceptor, 3,3-dimethyl-1-butene.22 This result is in good agreement with the
previous our report on the dehydrogenative germylation of C(sp2)H bonds.20b Under the
same reaction conditions, dihydrogermane 9 afforded 1,1’-spirogermabiindane 10 in 68%
yield via the sequential twofold dehydrogenative germylation of C(sp3)H bonds (Table 7).
In contrast, when (R)-DTBM-SEGPHOS was used in place of (R)-(S)-BPPFA,
dehydrogenative germylation occurred only one time to furnish selectively
2,3-dihydrobenzo[b]germole 11 in 88% yield without forming 1,1’-spirogermabiindane 10.23
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7
Scheme 3. Rhodium-catalyzed synthesis of 2,3-dihydrobenzo[b]germole 8 via the
dehydrogenative germylation of C(sp3)H bond
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Table 7. Rhodium-catalyzed dehydrogenative germylation of C(sp3)H bonds of
dihydrogermane 9
CONCLUSION
The work described herein is the acceleration effect of phosphine ligands for the
dehydrogenative silylation of C(sp3)H bonds. Proper choice of diphosphine ligands and
hydrogen acceptors is highly important. Among the phosphines examined, bulky and
electron-donating (R)-DTBM-SEGPHOS was found to be the most effective to facilitate the
dehydrogenative silylation of C(sp3)H bonds while suppressing the competitive
hydrosilylation of a hydrogen acceptor, 3,3-dimethy-1-butene. By employing bulky and
electron-rich C2-symmetric diphosphine ligands, asymmetric desymmetrizations of
2-(isopropyl)silylbenzene or dihydrosilane via the silylative cyclization were achieved.
Although the ee was low, this is the rare example of the asymmetric dehydrogenative
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functionalization of C(sp3)H bonds. Furthermore, the use of (R)-(S)-BPPFA enabled the
unprecedented catalytic germylation of C(sp3)H bonds with dehydrogenation. Expansion
of these observations to enantioselective C(sp3)H and C(sp2)H bond functionalization is
underway in our laboratory.
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EXPERIMENTAL SECTION
General Methods. All reactions were carried out in dry solvent under an argon atmosphere.
1,4-Dioxane was purchased from Wako Pure Chemical Industries and was dried by the usual
methods, distilled, and degassed with an argon gas for 20 min before use. [RhCl(cod)]2 was
purchased from Kanto Chemical Co.
(R)-DTBM-SEGPHOS, (R)-DTBM-Garphos,
(R)-H8-BINAP were purchased from Sigma-Aldrich. (R)-(S)-BPPFA was purchased from
Tokyo Chemical Industry. Other chemicals obtained from commercial suppliers were used
without further purification. 1H and 13C NMR spectra were recorded on a 400 MHz
13
spectrometer (100 MHz for C NMR) at 25 °C. Proton chemical shifts are reported with a
residual solvent peak (CDCl3 at 7.26 ppm) as an internal standard. Carbon 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, sept: septet, m: multiplet. The mass
analyzer type used for High resolution mass spectrometry (HRMS) was orbitrap. Melting
points were determined with a micromelting point apparatus without corrections. The
analytical data for (2-alkylaryl)silanes 1a-1e and 1h, and 2,3-dihydro-1H-benzo[b]siloles
2a-2e and 2h have been reported previously by our group.9d
Procedure for the Preparation of (2-Alkylphenyl)silanes. To a mixture of magnesium
turnings (243 mg, 10 mmol) and chlorosilane (10 mmol) in THF (10 mL) was added
o
2-alkylbromobenzene (8.0 mmol) at 25 C. The mixture was refluxed for 1 h, and then
quenched with saturated NH4Cl solution and extracted with Et2O for three times (20 mL × 3).
The combined organic layer was dried over MgSO4, and the organic solvent was removed
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under reduced pressure. The residue was purified by flash column chromatography on silica
gel with hexane as the eluent to give the corresponding (2-alkylphenyl)silanes.
8
9
2-(Dimethylsilyl)isopropylbenzene (1f): A colorless oil (87% yield, 1.24 g, 7.0 mmol); 1H
NMR (400 MHz, CDCl3):  0.39 (d, J = 4.0 Hz, 6H), 1.29 (d, J = 6.8 Hz, 6H), 3.19 (sept, J =
6.8 Hz, 1H), 4.58 (sept, J = 4.0 Hz, 1H), 7.20 (t, J = 7.6 Hz, 1H), 7.33 (d, J = 7.6 Hz, 1H),
7.39 (t, J = 7.6 Hz, 1H), 7.49 (d, J = 7.6 Hz, 1H). 13C{1H } NMR (100 MHz, CDCl3):  -2.7,
24.4, 33.6, 124.7, 125.3, 129.8, 134.6, 135.0, 154.9. IR (neat / cm-1): 3057, 3007, 2962,
2927, 2868, 2119, 1589, 1473, 1458, 1382, 1363, 1249, 1120, 1070, 1029, 883, 837, 773, 763,
748, 731, 711, 644. HRMS (FAB+): calcd for C11H18Si ([M]+) 178.1178; found. 178.1177.
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2-(Diethylsilyl)isopropylbenzene (1g): A colorless oil (84% yield, 1.39 g, 6.7 mmol); 1H
NMR (400 MHz, CDCl3):  0.83-0.89 (m, 4H), 1.00 (t, J = 7.4 Hz, 6H), 1.25 (d, J =7.2 Hz,
6H), 3.13 (sept, J = 7.2 Hz, 1H), 4.34 (quint, J = 3.2 Hz, 1H), 7.16 (t, J = 6.2 Hz, 1H),
7.29-7.38 (m, 2H), 7.44 (d, J = 7.6 Hz, 1H). 13C{1H } NMR (100 MHz, CDCl3):  4.2, 8.4,
24.4, 33.7, 124.7, 125.1, 129.7, 133.2, 135.4, 155.2. The analytical data match those
reported in the literature.23
2-(Diphenylsilyl)isopropylbenzene (1h): A colorless oil (80% yield, 1.94 g, 6.4 mmol); 1H
NMR (400 MHz, CDCl3):  1.09 (d, J = 6.8 Hz, 6H), 3.13 (sept, J = 6.8 Hz, 1H), 5.62 (s, 1H),
7.14 (t, J = 6.8 Hz, 1H), 7.31-7.43 (m, 9H), 7.54 (d, J = 6.8 Hz, 4H). 13C{1H } NMR (100
MHz, CDCl3):  24.1, 34.2, 125.1, 125.3, 127.9, 129.6, 130.6, 130.9, 133.8, 135.8, 137.0,
155.8. The analytical data match those reported in the literature.24
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Di(2-tert-butylphenyl)silane (5): A colorless oil (51% yield, 1.21 g, 4.1 mmol); 1H NMR
(400 MHz, CDCl3): 1.48 (s, 18H), 5.52 (s, 2H), 7.12 (dt, J = 1.2, 7.6 Hz, 2H), 7.37 (dt, J =
1.2, 7.6 Hz, 2H), 7.42 (dd, J = 1.2, 7.6 Hz, 2H), 7.54 (dd, J = 1.2, 8.0 Hz, 2H). 13C{1H }
NMR (100 MHz, CDCl3):  32.1, 37.5, 124.9, 125.8, 129.7, 130.8, 139.9, 157.2. IR (neat /
cm-1): 2986, 2965, 2904, 2869, 2176, 2138, 1586, 1472, 1430, 1363, 1247, 1127, 1116, 1057,
967, 880, 869, 764, 739, 613. HRMS (FAB+): calcd for C20H28Si ([M]+) 296.1960; found.
296.1952.
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Preparation of 2-(dimethylgermyl)-tert-butylbenzene 7.
To
a
solution of
n
2-tert-butylbromobenzene (852 mg, 4.0 mmol) in Et2O (5.0 mL) was added BuLi (3.0 mL,
4.8 mmol, 1.6 M in hexane) dropwisely at -78 oC. After stirred for 10 mins,
dichlorodimethylgermane (833 mg, 4.8 mmol) was added, and the mixture was gradually
o
warmed to 25 C. After stirring for overnight, the resultant mixture was added to a
suspension of LiAlH4 (304 mg, 8.0 mmol) in Et2O (20 mL) at 25 oC, and stirred for further 6
h. The mixture was quenched with H2O (3.0 mL), aq. NaOH (15 wt%, 3.0 mL), and H2O
(1.0 mL). The resultant suspension was filtered through celite, and the solvent was removed
under reduced pressure. The residue was purified by flash column chromatography on silica
gel with hexane as the eluent to afford the 2-(dimethylgermyl)-tert-butylbenzene 7 (94% yield,
891 mg, 3.8 mmol) as a colorless oil. 1H NMR (400 MHz, CDCl3):  0.50 (d, J = 3.2 Hz,
6H), 1.44 (s, 9H), 3.19 (sept, J = 3.2 Hz, 1H), 7.19 (dt, J = 0.8, 7.2 Hz, 1H), 7.28 (dt, J = 1.2,
8.0 Hz, 1H), 7.45 (dd, J = 1.2, 8.0 Hz, 1H), 7.49 (d, J = 7.2 Hz, 1H). 13C{1H } NMR (100
MHz, CDCl3):  -1.6, 32.0, 37.1, 125.1, 125.2, 128.2, 135.4, 139.0, 155.5. IR (neat / cm-1):
2966, 2912, 2060, 1469, 1465, 1437, 1424, 1395, 1363, 1249, 1237, 1112, 847, 833, 766, 732,
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712, 597. HRMS (FAB+): calcd for C12H20Ge ([M]+) 238.0777; found. 238.0762.
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9
Di(2-tert-butylphenyl)germane (9): A colorless oil (56% yield, 766 mg, 2.2 mmol); 1H
NMR (400 MHz, CDCl3): 1.46 (s, 18H), 5.77 (s, 2H), 7.12 (dt, J = 0.8, 7.2 Hz, 2H),
7.32-7.38 (m, 4H), 7.55 (dt, J = 1.2, 7.8 Hz, 2H). 13C{1H } NMR (100 MHz, CDCl3): 32.0,
37.3, 125.2, 126.0, 129.0, 133.6, 138.8, 156.1. IR (neat / cm-1): 2989, 2964, 2358, 2084,
2047, 1471, 1363, 889, 792, 762, 732. HRMS (FAB+): calcd for C20H28Ge ([M]+) 342.1403;
found. 342.1420.
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General Procedure for Rhodium-Catalyzed Dehydrogenative Silylation and
Germylation of Unactivated C(sp3)H Bonds. A flame dried sealed tube was charged
with [RhCl(cod)]2 (1.8 mg, 3.8 mol), phosphines (11 mol) and 1,4-dioxane (0.25 mL), and
the resulting mixture was stirred at 25 oC for 30 min.
2-Alkylphenylsilanes or
2-alkylphenylgermane (0.25 mmol), 3,3-dimethyl-1-butene (21.0 mg, 0.25 mmol) was added
o
to the mixture, and stirred at 50 or 100 C for 24 h. The solvent was removed in vacuo and
the residue was subjected to flash column chromatography on silica gel with hexane as an
eluent
to
give
the
corresponding
2,3-dihydro-1H-benzo[b]siloles
or
2,3-dihydro-1H-benzo[b]germole.
1,1,3-Trimethyl-2,3-dihydro-1H-benzo[b]silole (2f): A colorless oil (83% yield, 36.5 mg,
0.21 mmol); 1H NMR (400 MHz, CDCl3):  0.26 (s, 3H), 0.34 (s, 3H), 0.67 (dd, J = 6.0,
14.8 Hz, 1H), 1.28 (dd, J = 6.8, 14.8 Hz, 1H), 1.34 (d, J = 6.8 Hz, 3H), 3.30-3.32 (m, 1H),
7.21 (t, J = 6.8 Hz, 1H), 7.28 (d, J = 6.8 Hz, 1H), 7.34 (t, J = 6.8 Hz, 1H), 7.51 (d, J = 6.8 Hz,
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1H). 13C{1H } NMR (100 MHz, CDCl3):  -1.7, -0.6, 21.6, 25.2, 38.4, 124.6, 125.7, 129.4,
6
7
8
9
131.8, 139.7, 157.8. IR (neat / cm-1): 3055, 2993, 2956, 2897, 1591, 1560, 1452, 1438, 1406,
1367, 1296, 1247, 1195, 1174, 1128, 1078, 1056, 1022, 997, 920, 844, 802, 769, 756, 723,
694, 644. HRMS (FAB+): calcd for C11H16Si ([M]+) 176.1021; found. 176.1029. The ee of
2f was determined by HPLC analysis after the derivatization leading to diol 4 by Tamao
oxidation (see below for the detail).
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1,1-Diethyl-3-methyl-2,3-dihydro-1H-benzo[b]silole (2g): A colorless oil (88% yield, 44.9
mg, 0.22 mmol); 1H NMR (400 MHz, CDCl3):  0.61 (dd, J = 6.0, 14.8 Hz, 1H), 0.74-0.84
(m, 4H), 0.93-1.04 (m, 6H), 1.29 (dd, J = 8.0, 14.8 Hz, 1H), 1.35 (d, J = 6.8 Hz, 3H), 3.29
(sext, J = 6.8 Hz, 1H), 7.19 (t, J = 7.2 Hz, 1H), 7.28-7.36 (m, 2H), 7.51 (d, J = 6.8 Hz, 1H).
13C{1H } NMR (100 MHz, CDCl3):5.0, 5.6, 7.6, 7.7, 17.6, 25.2, 38.3, 124.6, 125.4, 129.3,
132.4, 137.8, 158.4. IR (neat / cm-1): 3055, 2995, 2954, 2910, 2873, 1591, 1560, 1456, 1438,
1413, 1371, 1298, 1255, 1232, 1126, 1085, 1060, 1006, 956, 756, 682, 665, 624. HRMS
(FAB+): calcd for C13H20Si ([M]+) 204.1334; found. 204.1344.
3-Methyl-1,1-diphenyl-2,3-dihydro-1H-benzo[b]silole (2h): A colorless oil (92% yield,
69.1 mg, 0.23 mmol); 1H NMR (400 MHz, CDCl3):  1.17 (dd, J = 6.0, 14.8 Hz, 1H), 1.38
(d, J = 6.8 Hz, 3H), 1.79 (dd, J = 7.6, 14.8 Hz, 1H), 3.46 (sext, J = 6.8 Hz, 1H), 7.26-7.28 (m,
1H), 7.31-7.40 (m, 8H), 7.53-7.55 (m, 2H), 7.60-7.61 (m, 2H), 7.66 (d, J = 7.6 Hz, 1H).
13C{1H } NMR (100 MHz, CDCl3):20.5, 24.9, 38.4, 124.8, 126.1, 127.8, 127.9, 129.5,
130.0, 133.1, 135.1, 135.2, 158.9. The analytical data match those reported in the
literature.12b
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2-(Dimethyl(3,3-dimethylbutyl)silyl)ethylbenzene (3): A colorless oil (50% yield with
dppbz as a ligand (see Table 4, entry 3), 31.0 mg, 0.13 mmol); 1H NMR (400 MHz, CDCl3):
 0.31 (s, 6H), 0.72-0.77 (m, 2H), 0.85 (s, 9H), 1.15-1.19 (m, 2H), 1.25 (t, J = 7.4 Hz, 3H),
2.75 (q, J = 7.4 Hz, 2H), 7.17 (t, J = 7.6 Hz, 1H), 7.24 (d, J = 7.6 Hz, 1H), 7.33 (t, J = 7.6 Hz,
1H), 7.45 (d, J = 7.6 Hz, 1H). 13C{1H } NMR (100 MHz, CDCl3):  -1.5, 10.6, 16.4, 28.8,
28.9, 31.1, 37.9, 124.8, 127.9, 129.2, 134.7, 136.9, 150.1. IR (neat / cm-1): 3057, 2954, 2866,
1589, 1541, 1508, 1390, 1363, 1249, 1219, 1159, 1128, 1083, 1060, 1041, 1006, 929, 885,
837, 819, 775, 754, 731, 682, 632. HRMS (FAB+): calcd for C16H28Si ([M]+) 248.1960;
found. 248.1967.
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3,3-Dimethyl-1-sila-1,1-spirobiindane (6): A colorless oil (75% yield, 54.8 mg, 0.19 mmol);
The ee was determined to 39% on a Daicel CHIRALPAK OD column with hexane as the
eluent (flow rate = 0.50 mL/min). Retention time for the major enantiomer was 16 min, and
that for the minor enantiomer was 13 min. 1H NMR (400 MHz, CDCl3): 1.25 (d, J = 15.2
Hz, 2H), 1.34 (d, J = 15.2 Hz, 2H), 1.41 (s, 6H), 1.51 (s, 6H), 7.18-7.22 (m, 2H), 7.41-7.44
(m, 6H). 13C{1H } NMR (100 MHz, CDCl3):  29.2, 33.65, 33.69, 43.3, 123.3, 126.0, 130.3,
133.0, 135.5, 163.0. IR (neat / cm-1): 3045, 3005, 2996, 2952, 2881, 2860, 1966, 1927, 1589,
1559, 1462, 1439, 1402, 1379, 1360, 1285, 1257, 1188, 1138, 1098, 1064, 1030, 946, 869,
856, 767, 734, 729, 708. HRMS (FAB+): calcd for C20H24Si ([M]+) 292.1647; found.
292.1626.
1,1,3,3-Tetramethyl-2,3-dihydro-1H-benzo[b]germole (8): A colorless oil (65% yield, 38.5
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mg, 0.16 mmol); 1H NMR (400 MHz, CDCl3):  0.47 (s, 6H), 1.17 (s, 2H), 1.34 (s, 6H),
7.19-7.22 (m, 1H), 7.31-7.33 (m, 2H), 7.51 (d, J = 7.6 Hz, 1H). 13C{1H } NMR (100 MHz,
CDCl3):  -0.98, 30.6, 33.6, 44.4, 123.5, 125.8, 128.9, 132.1, 141.1, 159.4. IR (neat / cm-1):
3065, 3053, 2956, 2907, 2862, 1587, 1464, 1439, 1411, 1379, 1360, 1283, 1255, 1235, 1187,
1123, 1056, 1030, 860, 833, 797, 766, 728, 672, 667, 601, 582, 552. HRMS (FAB+): calcd
for C12H19Ge ([M+H]+) 237.0699; found. 237.0694.
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3,3-Dimethyl-1-germa-1,1-spirobiindane (10): A colorless solid (68% yield with
1
(R)-(S)-BPPFA (Table 7, entry 2), 67.8 mg, 0.17 mmol); mp 83.4-83.7 oC. H NMR (400
MHz, CDCl3): 1.41 (s, 12H), 1.42 (d, J = 12.4 Hz, 2H), 1.51 (d, J = 12.4 Hz, 2H), 7.20 (dt,
J = 1.6, 7.0 Hz, 2H), 7.38 (dt, J = 1.2, 8.0 Hz, 2H), 7.41 (d, J = 8.0 Hz, 2H), 7.45 (d, J = 8.0
Hz, 2H). 13C{1H } NMR (100 MHz, CDCl3):30.0, 33.3, 33.7, 43.8, 123.6, 126.0, 129.5,
132.8, 137.6, 160.3. IR (KBr / cm-1): 3054, 2960, 2905, 1584, 1456, 1436, 1380, 1359, 1261,
1251, 1157, 1121, 1050, 1030, 771, 766, 745, 730, 670. HRMS (FAB+): calcd for C20H25Ge
([M+H]+) 339.1168; found. 339.1158.
3,3-Dimethyl-1-(2-tert-butylphenyl)-2,3-dihydro-1H-benzo[b]germole (11): A colorless oil
(88% yield with (R)-DTBM-SEGPHOS (Table 7, entry 1), 74.8 mg, 0.22 mmol); 1H NMR
(400 MHz, CDCl3): 1.22 (s, 6H), 1.35 (d, J = 13.2 Hz, 1H), 1.36 (s, 9H), 1.52 (d, J = 13.2
Hz, 1H), 5.74 (d, J = 4.4 Hz, 1H), 6.92 (t, J = 7.2 Hz, 1H), 7.10-7.18 (m, 3H), 7.26 (d, J = 3.6
Hz, 1H), 7.38 (d, J = 8.4 Hz, 1H), 7.45 (d, J = 7.2 Hz, 1H). 13C{1H } NMR (100 MHz,
CDCl3):  31.3, 32.3, 33.2, 33.5, 37.2, 44.3, 124.0, 125.2, 125.8, 126.2, 128.9, 129.4, 133.7,
134.6, 136.7, 137.0, 156.1, 160.6. IR (neat / cm-1): 3052, 2956, 2864, 2054, 1586, 1464,
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1439, 1395, 1380, 1362, 1282, 1249, 1189, 1171, 1125, 1112, 1052, 1031, 792, 764, 737, 724,
686, 668, 637. HRMS (FAB+): calcd for C20H26Ge ([M]+) 340.1246; found 340.1251.
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Derivatization of 2f for the Determination of the ee. To a solution of t-BuOK (135 mg,
1.2 mmol) in THF (1.4 mL) was added tert-butyl hydroperoxide (0.22 mL, 5.0~6.0 M in
decane) at 0 oC, and stirred for 10 mins. A solution of 2f (35.2 mg, 0.20 mmol) in THF (1.0
o
mL) and TBAF (1.2 mL, 1.0 M in THF) was added, and the mixture was stirred at 70 C for
o
15 h. The resultant mixture was cooled down to 25 C and Na2S2O3·5H2O (ca. 650 mg) in
water (6.0 mL) was added. After stirred for 30 min, the reaction mixture was quenched with
saturated NH4Cl solution and extracted with Et2O for three times (15 mL × 3). The
combined organic layer was washed with 5 wt% of citric acid, and dried over MgSO4. The
organic solvent was removed under reduced pressure, and then the residue was purified by
flash column chromatography on silica gel with hexane as the eluent to give
2-(2-hydroxy-1-methylethyl)phenol 4 (25.6 mg, 0.17 mmol, 84% yield) as a colorless oil.
The ee was determined to 37% on a Daicel CHIRALPAK OD column with hexane /
2-propanol (v / v = 9 / 1) as the eluent (flow rate = 0.5 mL/min). Retention time for the
major enantiomer was 21 min, and that for the minor enantiomer was 23 min. 1H NMR (400
MHz, CDCl3):  1.32 (d, J = 7.2 Hz, 3H), 3.22-3.28 (m, 1H), 3.74 (dd, J = 7.6, 9.6 Hz, 1H),
3.95 (dd, J = 3.6, 9.6 Hz, 1H), 6.88-6.91 (m, 2H), 7.11-7.16 (m, 2H). 13C{1H } NMR (100
MHz, CDCl3):15.6, 36.8, 69.4, 117.1, 120.7, 127.7, 127.8, 130.5, 154.8. The analytical
data match those reported in the literature.12b
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AUTHOR INFORMATION
Corresponding Author
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*E-mail: masahito.murai@okayama-u.ac.jp ktakai@cc.okayama-u.ac.jp
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ACKNOWLEDGEMENT
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 MEXT
program for promoting the enhancement of research universities. The authors gratefully
thank Mr. Naoki Hosokawa and Mr. Shinji Iba (Okayama University) for HRMS
measurements.
Supporting Information Available: H and 13C NMR spectra for new compounds. This
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material is available free of charge via the Internet at http://pubs.acs.org.
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o
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10 and 80% of 11; (R)-SEGPHOS, 6% of 10 and 78% of 11. The ee of 10 and 11 were
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