1． Introduction

The formation of C(sp 2)─ E (E=Si, Ge) bonds is a funda-mental organic reaction, and has attracted much attention in �elds such as medicine, pharmacology, agrochemistry, and material science. 1 Arylsilanes or arylgermanes are generally prepared by reacting halosilanes or halogermanes with corre-sponding organometallic reagents. However, this approach suffers from poor functional group tolerances when the start-ing substrates bear reactive functional groups. In addition, these starting materials are moisture─ sensitive, corrosive, and dif�cult to handle. They are also often accompanied by sub-stantial salt formation and side reactions. Therefore, the deve-lopment of new, easier methods for the synthesis and modi�ca-tion of group 14 compounds is highly desirable.

The activation of Si─ H and Ge─ H bonds with transition metal complexes is a fundamental step in several catalytic transformations of organosilanes and organogermanes. 2 Reduction and hydrosilylation are representative chemical synthesis. Palladium, platinum, and rhodium catalysts are fre-quently used in these reactions. Another type of transforma-tion is still a challenge in the �eld of organometallic chemistry to form C(sp 2)─ E bonds. Transition metal─ catalyzed C(sp 2)─ E formation reactions with group 14 hydrides appears to be highly promising for the synthesis of functionalized group 14 compounds. Recently, silylation of aryl C─ H bonds with hydrosilanes has been reported in the presence of transition metal catalysts. 3 However, there are still problems such as sub-strate scope, need for an excess of aromatic substrates, and high temperature in this reaction.

This review considers the synthesis and functional proper-ties of novel organosilicon and organogermanium compounds based on the unusual reactions of group 14 hydrides toward organic halides in the presence of a catalytic amount of transi-tion metal complex and a stoichiometric amount of organic

base. 4,5 This reaction shows wide scope for organic halides and group 14 hydrides. An area with great potential is synthetically useful compounds such as biologically active silicon ones. 6 Effective synthesis of some organosilicon─ based pharmaceuti-cals and crop protection agents which are already on the mar-ket was described by the use of this transformation. Some group 14 compounds prepared by this reaction have shown high solid─ state �uorescence intensity, and the relationship between �uorescence and structure has been investigated by X─ ray diffraction analysis and theoretical calculations. We have also demonstrated the application of this transformation to the immobilization of aromatic compounds on hydrogen─ terminated silicon and germanium surfaces.

2． Transition Metal─ catalyzed Synthesis of Group 14 Compounds via Coupling Reaction between Aryl Halides and Group 14 Hydrides

Although palladium─ mediated Si─ C bond formation using cross─ coupling between hydrosilanes and organoiodine com-pounds has been reported by Kunai et al., progress in the cata-lytic arylation of hydrosilanes has been limited. 7 Due to the electronegativity (H: 2.20, Si: 1.74), 8 halosilanes were preferen-tially obtained by transition metal catalysis through hydride─ halogen exchange reactions. Murata and Masuda were the �rst to report the use of trialkoxysilanes as precursors to prepare aryltrialkoxysilanes in the presence of transition metal cata-lysts. 9 Similar reactions were subsequently developed by Komuro, DeShong, and Denmark. 10 However, Murata and Masuda mentioned that triethylsilane was not suitable as a silylating reagent because of its strong reducing power in the palladium─ catalyzed silylation of aryl halides. 11

Our group focused on the arylation of trialkylsilanes with aryl halides in the presence of a Pd(0) catalyst containing the bulky alkyl phosphine ligand. 12 The trend of the reactivity is Ar─ I＞Ar─ Br＞＞Ar─ Cl in this reaction. The steric and elec-

Unusual Reactivity of Group 14 Hydrides toward Organic Halides: Synthetic Studies and Application to Functional Materials

Masaki Shimada, Yoshinori Yamanoi, ＊ and Hiroshi Nishihara ＊

Department of Chemistry, School of Science, �e University of Tokyo 7─ 3─ 1 Hongo, Bunkyo─ ku, Tokyo 113─ 0033, Japan

(Received June 26, 2016; E─ mail: yamanoi@chem.s.u-tokyo.ac.jp; nisihara@chem.s.u-tokyo.ac.jp)

Abstract: Extensive investigation of functional organosilicon and organogermanium compounds has identi�ed various valuable applications in organic chemistry and advanced materials. This review summarizes the major developments of metal─ mediated coupling reactions between group 14 hydrides and organic halides during the last decade with an emphasis on our own studies. High reactivity and selectivity have been achieved for C─ Si and C─ Ge bond formations under mild conditions. This transformation shows good functional group compa-tibility, and can serve as a powerful tool for the synthesis of medicinal, pharmaceutical, agrochemical, electri-cal, and photoluminescent compounds. Ground─ and excited─ state properties of �uorescence materials have been investigated by DFT and TD─ DFT calculations, and several important aspects of the experimental observations have been validated. Direct functionalization of H─ terminated Si and Ge surfaces has been demo-nstrated utilizing Pd─ mediated arylation reactions, illustrating the potential for further development of the Pd─ catalyzed reactions for the organic modi�cation of semiconductor surfaces.Key words: C─ Si bond formation, group 14 compounds, cross─ coupling, palladium, rhodium, platinum,

samarium, σ ─ π conjugation, photoluminescence, surface functionalization, self─ assembled mono-layer

（ 70 ） J. Synth. Org. Chem., Jpn.1098

tronic nature of the phosphine ligand greatly affected its reac-tivity. Screening of palladium catalysts indicated that Pd(P(t ─ Bu) 3) 2 was effective for this transformation. To evaluate the utility of the Pd─ catalyzed Si─ C bond formation, a series of aryl iodides and tertiary silanes were tested (Table 1). Aryl iodides bearing electron─ donating groups showed good results in producing the corresponding arylated silanes (1─ 6). Hin-dered aryl iodide substrates in the ortho position gave poor results, probably because of the steric repulsion between the ligand and the ortho substituents. For example, 1─ iodo─ 2,4─ dimethylbenzene and 2─ iodoanisole, both sterically demand-ing substrates, led exclusively to the catalytic deiodination product (7 and 8). Aryl iodide bearing an electron─ withdraw-ing group did not show good result (9). Further improvement was considered to expand the scope and to overcome limita-tions of this method.

The reaction tolerated a wide range of aryl iodides bearing electron─ donating substituents at ortho, meta, and para posi-tions and electron─ withdrawing substituents at meta and para positions and gave satisfactory yields in the presence of Rh catalyst (Table 2, 8, 10 and 11). 13 This method is compatible with a range of functional groups, such as ester, amino, and hydroxyl ones, which is in sharp contrast to the limitations of classical method using organometallic reagents (12─ 17). We used this C─ Si bond formation to synthesize TAC─ 101 (Scheme 1). Although TAC─ 101 is an important biologically active compound used as drug for liver cancer, previous method for the synthesis is limited by uncommon reagents and multistep procedures. 14 The concise synthesis of TAC─ 101 started with commercially available 3,5─ diiodobenzoic acid that was amidated with methyl 4─ aminobenzoate to the corre-sponding intermediate 18 in 81%. This intermediate was reacted with triethylsilane in the presence of rhodium catalyst and base at room temperature to give doubly triethylsilylated product 19 in 68%. After hydrolysis, TAC─ 101 derivative 20 was �nally isolated in three steps from commercially available reagent. Here, by means of Si─ C bond formation developed in this study, the synthesis of TAC─ 101 derivatives was readily achieved in only three steps from available starting material.

We reported that the arylation of secondary or primary silanes with aryl iodides in the presence of Pd(P(t ─ Bu) 3) 2 pro-vided the corresponding arylated products in good to excellent yields (Table 3). 15 The coupling reaction of secondary silanes and aryl iodides gave the corresponding tertiary silanes (single arylated products) and quaternary silanes (double arylated products) by controlling reaction conditions. Similarly, cou-pling reaction of primary silanes and aryl iodides also gave secondary silanes (single arylated products), tertiary silanes (double arylated products), and quaternary silanes (triple ary-lated products). The same reactivity was observed for a series of tertiary, secondary, and primary germanes with aromatic and aliphatic substituents. The important feature of this method is the one─ pot sequential double arylation of seco-ndary silanes or germanes with two different aryl iodides. Two aryl iodides Ar 1─ I and Ar 2─ I were cross─ coupled with seco-ndary or primary group 14 hydrides to produce unsymmetri-cally substituted group 14 compounds (21─ 26) by controlling equivalent of aryl iodides. Pd─ catalyzed double arylation had an application for the synthesis of �usilazole, a widely used antifungal agent.

This methodology allows easy and versatile synthesis of

Table 1.　 Pd(P(t ─ Bu) 3) 2─ catalyzed Si─ C(sp 2) bond formation with hydrosilanes and aryl iodides.

Table 2.　 Scope of Rh─ catalyzed arylation of tertiary silanes with aryl iodides.

Scheme 1.　 Shorter alternative synthetic route to the TAC─ 101 analog 20 using Rh─ catalyzed Si─ C bond formation as a key step.

Vol.74 No.11 2016 （ 71 ） 1099

unsymmetrical disiloxanes with high functional group compa-tibility. Siloxanes, linked by Si─ O─ Si bonds, have been studied for use in liquid crystals or starting materials for Hiyama cou-pling. 16 Symmetrical disiloxanes are obtained from monomeric organoalkoxysilanes by hydrolysis, followed by condensation. However, there are few reports on the synthesis of unsymmetri-cal siloxanes with reactive functional groups. We have deve-loped the stepwise Pd─ catalyzed arylation of siloxane contain-ing Si─ bonded hydrogens (1,1,3,3─ tetramethyldisiloxane) to prepare unsymmetrical siloxanes. The stepwise arylation pro-ceeded in higher yields in less polar solvent (toluene). Cooler reaction conditions also gave higher yields due to the suppres-sion of reduction of aryl iodides and double arylation of siloxane. Under optimized conditions, they were prepared in up to 82% yield with a broad range of functional groups and sub-stitution positions (Table 4, 27─ 30). 17 This reaction could be exploited to perform triple arylation. The stepwise arylation of 3─ ((dimethylsilyl)oxy)─ 1,1,3,5,5─ pentamethyltrisiloxane gave the desired product including three different aryl groups (Scheme 2, 31).

Optically active silicon─ stereogenic organosilanes are

promising for use as chiral auxiliaries, reagents, resolving agents, and drug candidates. 18 Despite much investigation into the preparation of enantioriched organic compounds with a carbon─ stereogenic center, the available strategies are restricted to the desymmetrization of prochiral organosilanes. We reported a palladium/phosphoramidite 32 catalyzed asymmet-ric arylation of prochiral secondary silanes with aryl iodides to give tertiary silanes in up to 76% enantiomeric excess (ee). 19 In this reaction, ortho ─ substituted aryl iodides are required to achieve high enantioselectivity (Table 5, 33─ 38).

A possible mechanism for the reaction is depicted in Figure 1 based on the experimental results. This transforma-tion gave better results using electron─ donating aryl iodide as a substrate, which exhibited a reactivity to aryl iodides different from typical Pd─ catalyzed C─ C bond formation. From the observation of 1 H NMR in THF─ d 8, the metal─ hydride peak was observed at high magnetic �eld region during the course of stoichiometric reaction. Accordingly, hydrosilane (H─ SiR 3)

Table 5.　Catalytic enantioselective arylation of secondary silanes.

Table 3.　 Stepwise arylation of primary and secondary group 14 hydrides. a

Table 4.　 Palladium─ catalyzed stepwise arylation of 1,1,3,3─ tetramethyldisiloxane. a

Scheme 2.　 Palladium─ catalyzed triple─ stepwise arylation of 3─ ((dimethylsilyl)oxy)─ 1,1,3,5,5─ pentamethyltrisiloxane. a

（ 72 ） J. Synth. Org. Chem., Jpn.1100

adds oxidatively to the Pd(0) center to generate hydrido silyl complexes (H─ Pd(II)─ SiR 3) at the initial stage. Then, a path-way through σ ─ bond metathesis between the Pd(II) species and aryl iodide or a pathway through further oxidative addi-tion of Ar─ I to generate Pd(IV) species would lead to arylsi-lanes, although this step remains poorly understood. Finally, reductive elimination gave arylsilanes by the aid of base along with the generation of catalytic species (path I). During the transformation, another cycle that produces the reduced pro-duct is competitive with this route through I─ Pd(II)─ Ar inter-mediate (path II). Steric and electronic factors of phosphine ligand have a signi�cant effect on this step.

Our metal─ mediated system is not limited to using only aryl iodides as coupling partners. Next, we focused on forming Si─ C(sp 3) bonds using iodoalkanes as coupling partners (Table 6). In the presence of Pt(P(t ─ Bu) 3) 2, coupling reactions of tertiary silanes with 1─ iodoalkane provided alkylated qua-ternary silanes in good yield (39─ 43). 20 This reaction worked well for a broad range of functional groups such as ester and cyano moieties (44 and 45). To highlight its practical utility, we used it in the synthesis of sila�uofen (46), an agricultural insec-ticide. The optimized Pt─ catalyzed alkylation of arylsilanes with alkyl iodide gave sila�uofen in 42% yield, demonstrating suitability of this reaction for the simple preparation of bio-chemical components.

Preferential carbene insertion into Ge─ H was compared with other heavier group 14 hydrides. 21 Metal carbenoids are usually prepared from metals (e.g. Zn, Al, Sm, etc.) 22 and 1,1─ dihaloalkanes, and their insertion into heteroatom─ hydrogen bonds is a well─ established process. Although excellent yields can be obtained by carbenoid insertion into Si─ H bonds, che-moselective carbene insertion into group 14 hydrides has not been reported. To compare the reactivity in a competitive environment, an equimolar solution of triphenylsilane, triphe-nylgermane, and triphenylstannane was treated with Sm car-benoids prepared by samarium metal and bromoiodomethane

in situ (Figure 2(a)). Surprisingly, samarium carbenoid was found exclusively to attack triphenylgermane to give triphenyl-methylgermane (47) in 85%, although Zn and Al carbenoids led to a statistical mixture of products. To demonstrate the synthetic utility of this methodology, we selectively inserted a carbene into a compound containing both Si─ H and Ge─ H moieties in the same molecule (48). As expected, the Sm car-benoid exhibited complete Ge─ H selectivity, and the Si─ H functionality remained unaffected during the transformation (49, Figure 2(b)).

3． Application to Fluorescent Materials

The introduction of various heteroatoms into the back-bone of organic compounds is interesting approach for explor-ing new materials because it is expected to provide unusual electronic structures and photophysical characteristics. Espe-cially, bridging an aromatic ring by Si or Ge atoms in�uences the spectroscopic properties due to hyperconjugation which

Figure 1.　 Proposed catalytic cycle for the arylation of group 14 hydrides (E=Si, Ge) with depiction of side reaction.

Figure 2.　 (a) Chemoselective methylene insertion with samarium carbenoid. (b) Selective carbene insertion into the Ge─ H group of 49.

Table 6.　 Substrate scope of the Pt─ catalyzed alkylation of tertiary silanes with 1─ iodoalkanes.

Vol.74 No.11 2016 （ 73 ） 1101

often enhances the �uorescence intensity. To expand the scope of the transformation, we examined the possibility of its appli-cation to group 14 �uorescent materials.

π ─ Conjugated group 14 metallole units have recently been recognized and extensively studied for their potential applica-tions in organic light─ emitting diodes (OLEDs). 23 The σ ＊ orbital of the Si─ C bond can effectively interact with the π ＊ orbital of the butadiene fragment, decreasing the LUMO energy level. The traditional synthesis of these compounds often suffered from low functional group compatibility owing to the use of stoichiometric organometallic reagents, and cata-lytic Si─ C or Ge─ C bond formation has emerged as a new strategy for the synthesis of group 14 metalloles. A series of group 14 dibenzometalloes was synthesized from the corre-sponding dihydrides and 2,2’─ diiodoarenes using Pd─ catalyzed bond formation to investigate their photophysical properties (Table 7, 50─ 61). 24 Compared with the conventional synthetic strategy for preparing dibenzosiloles and dibenzogermoles, the present cyclization has several advantages in terms of func-tional group tolerances (58─ 61). Attaching functional groups to the dibenzosilole ring in�uences the �uorescence quantum yields. The ester substituted dibenzosilole 59 showed red─ shifted �uorescence with higher �uorescence quantum yield in the solid state than in solution (Figure 3(a)). Solid─ state lumi-nescent materials generally suffer from quenching caused by their aggregation (concentration quenching). In the crystal of 59, molecular motions are restricted due to the CH…O=C hydrogen bonds between the ester group of one molecule and the hydrogen atom of an adjacent molecule (Figure 3(b)). The carbonyl groups of neighboring molecules have also been observed to interact with each other. These supramolecular interactions in the crystals prevent free torsion motions around the Si─ Ph groups, and the molecules are tightly stabilized in the crystal lattice. This effect diminishes the intermolecular quenching effects, and intensi�es the light emissions. 25

Structurally similar silicon─ containing molecules with six─

membered rings are less well investigated due to limitations of the synthetic methodology. We obtained silicon─ containing six─ membered ring compounds, 9,10─ dihydro─ 9,10─ disilaan-thracenes, in acceptable yields (up to 43%) via palladium─ cata-lyzed coupling reactions between 1,2─ bis(dimethylsilyl)arenes and 1,2─ diiodoarenes using the simple one─ pot procedures listed in Table 8 (62─ 69). This reaction provides a rare example of unsymmetrical 9,10─ dihydro─ 9,10─ disilaanthracenes. We also investigated the �uorescence at room temperature and phosphorescence at 77 K of these compounds and carbon analog 70. 26 Compound 67, bearing ester groups on aromatic ring, displayed blue phosphorescence with Φ P of 0.06; although the carbon analog 70 did not show any phosphores-cence at all.

Our method could be useful for constructing other types of luminescent material. Organic molecules containing conju-

Table 8.　Synthesis of 9,10─ dihydro─ 9,10─ disilaanthracenes 62─ 69. aTable 7.　 Synthesis of functionalized dibenzosiloles and

dibenzogermoles.

Figure 3.　 (a) Fluorescence spectra and images under UV lamp of 59: (i) in dichloromethane and (ii) in the solid state. (b) Packing pattern of 59 in the crystal-line state.

（ 74 ） J. Synth. Org. Chem., Jpn.1102

gated π ─ electrons linked to an electron donor (D) at one end and an electron acceptor (A) at the other have been recognized as a class of electroluminescence and nonlinear optical materi-als. The replacement of C=C bonds by Si─ Si bonds is espe-cially interesting, because they have similar chemical and physical properties. This feature may give rise to unique �uo-rescence properties of organodisilane compounds. The intro-duction of electron─ donating and ─ withdrawing groups to the appropriate positions of disilane unit can provide D─ Si─ Si─ A molecules. These unsymmetrical substituted disilanes were obtained by coupling 1─ aryl─ 1,1,2,2─ tetramethyldisilanes with aryl iodides in the presence of a Pd catalyst. 27 We studied their photophysical properties and second order nonlinear optical responses (second harmonic generation (SHG)) by a powder method (Figure 4). Compounds 71─ 76 are stable thermally and chemically, and exhibit a strong emission at room tempera-ture. The introduction of strong electron donors and acceptors is highly sensitive to solvent polarity in �uorescence. Among these compounds, 76 showed intense solid─ state blue emission due to the suppression of nonradiative decay from the excited state and the enhancement of emission by the protection of intermolecular π stacking in the solid state. Besides, compound 73 showed a moderate β value of SHG ef�ciency. This value is similar to those of traditional push─ pull π conjugated systems. A single crystal of this compound showed unsymmetrical packing (Pa). These unique properties resulted from the through─ bond interaction of σ ─ π mixing between Si─ Si bonds and aromatic π ─ systems.

π ─ Conjugated D─ A─ D and A─ D─ A triads are established as strong emitters and electron transporters due to their strong intramolecular charge transfer (ICT) effects. However, π ─ con-jugated systems often face several problems such as concentra-tion quenching and low solubility owing to strong π ─ π stack-

ing. Motivated by these results, we focused on synthesizing D─ Si─ Si─ A─ Si─ Si─ D (77─ 81) and A─ Si─ Si─ D─ Si─ Si─ A (82─ 85) molecules to overcome these problems. 28 The disilane─ bridged molecules shown in Figure 5(a) were also prepared by a Pd─ catalyzed method. For 76─ 81, we used benzothiazole and thie-nopyrazine, nitrogen─ and sulfur─ containing heterocyclic compounds which are well─ known for their strong electron─ withdrawing ability. They displayed broad and weak absorp-tion spectra assignable to ICT absorption. Excitation at this band led to weak emission in solution but strong emission in the solid state. This aggregation─ induced emission enhance-ment (AIEE) was �rst reported by Tang et al., 29 and subsequent works have developed organic compounds showing this phe-nomena. 30 Compound 80 exhibited marked AIEE �uorescence upon its increasing concentration in water (Figure 5(b)). A solution of 80 was dissolved in THF and water fraction (f w) was gradually increased. The PL intensity was diminished up to water concentration of ca. f w＜70%, after this point the intensity starts increasing up to f w=99%. The PL intensity at f w=99% is three times higher than the solution in pure THF. The AIEE of 80 has been rationalized to stem from the sup-pression of π ─ π stacking interactions in a similar way to that reported for other systems. Its crystal structure packing shows no π ─ π stacking which prevents quenching due to aggregation. We concluded that the strong emission of disilane─ bridged D─ A─ D/A─ D─ A triads in the solid state was caused by the sup-pression of both non─ radiative relaxation and intermolecular interaction.

Oligothiophenes have been studied for use in organic light─ emitting devices and photovoltaic cells, 31 and tris(trimethylsilyl)-silyl group ((Me 3Si) 3Si) is expected to be useful for lengthening the σ ─ π conjugation system and suppressing intermolecular interaction in the solid state. We designed and synthesized

Figure 4.　 (a) Structures of D─ Si─ Si─ A mole cules prepared by Pd─ catalyzed arylation. (b) ORTEP drawing of the lattice structure of 73. All hydrogen atoms are omitted for clarity. Displacement ellipsoids drawn at the 50% probability level.

Figure 5.　 (a) Structures of D─ Si─ Si─ A─ Si─ Si─ D and A─ Si─ Si─ D─ Si─ Si─ A molecules prepared by Pd─ catalyzed arylation. (b) Fluorescence photographs recorded under UV (365 nm) irradiation for 80 in THF/water with f w from 0 to 99% (f w=V water/(V water+V THF)).

Vol.74 No.11 2016 （ 75 ） 1103

hybridized compounds of oligothiophene and (Me 3Si) 3Si moi-eties and investigated their photophysical properties (Figure 6, 86─ 91). 32 The products were prepared by the Pd─ catalyzed coupling of (Me 3Si) 3Si─ H and diiodooligothiophene in good to moderate yield. Both absorption and emission maxima of the doubly tris(trimethylsilyl)silylated thiophene derivatives (86─ 89) shifted to longer wavelengths relative to those of unmodi�ed thiophenes. Fluorescence quantum yields in both solution and the solid state were improved generally up to 0.74. DFT calculation revealed that the silylation of oligothiophene mainly affected the HOMO energy level owing to the interac-tion between Si─ Si σ orbitals and aryl π orbitals to decrease the HOMO─ LUMO energy gap. The structures of 87, 89, and 91 were determined by X─ ray crystallography. Compounds 87 and 91 showed planar aryl structure to contribute to the effec-tive π ─ conjugation and steric hindrance of tris(trimethylsilyl)-silyl group to suppress the intermolecular π ─ stacking interac-tion, resulting to display the high quantum yield in the solid state. Compound 89 did not display high quantum yields due to the distorted structures between its thiophene rings.

4． Application to Immobilizing Organic or Complex Molecules onto an Electrode Surface

Highly ordered organic monolayers on a crystalline silicon surface are potentially useful for many applications, and can be prepared easily by reacting a hydrogen─ terminated silicon sur-face and an ole�nic substrate. 33 The most commonly used hydrogen─ terminated silicon surfaces are prepared by etching with diluted HF (1─ 5%) or NH 4F aq. (40%). The surface undergoes reconstruction to form the most stable Si(111)─ 7×7 and Si(100)─ 2×1 surfaces, which have structurally different surfaces (Si─ H: dihydride vs monohydride; Figure 7 (a) and (b)). Si(111) substrates are often preferred to Si(100) for the covalent grafting of organic monolayers, because monohy-dride─ terminated Si(111) is atomically �at, whereas dihydride─ terminated Si(100) prepared under wet conditions is rough. Therefore, organic functionalization on Si(111) yields densely packed, ordered monolayers with higher surface coverage.

Functionalization of silicon electrodes with aryl �lms can be achieved via the decomposition of the corresponding diazo-nium salt (Ar─ N 2

+) as shown in Figure 7(c). 34 UV irradiation decomposes the light─ sensitive diazonium group and electro-chemical reduction covalently connects residual aromatic groups to the silicon surface. The attachment of aryl groups to the substrate surface can be assigned to the reaction of an aryl radical formed by a one─ electron reduction of diazonium salt with the release of N 2. That is, the multilayer formation was assigned to the attack of an aryl radical on the grafted layer along an S H homolytic substitution reaction. The structure of the layer should look like a substituted polyphenylene if steri-cally hindered groups at the 3,5─ positions were not introduced into aromatic moieties.

The catalytic system described above was successfully extended to the modi�cation of electrode surface. Thus, we developed the �rst example of versatile and ef�cient methods to form clean organic monolayers with Si─ aryl and Ge─ aryl bonds on hydrogen─ terminated silicon and germanium sur-faces via a Pd─ catalyzed arylation reaction (Figure 8). 35 This palladium─ catalyzed grafting process was effective for 4─ iodo-phenylferrocene, 5─ iodo─ 1H ─ imidazole, and 9─ (4─ iodophe-nyl)anthracene, and gave the desired surface functionalization. Cyclic voltammetry (CV), and X─ ray photoelectron spectros-copy (XPS) characterized the surfaces and estimated the sur-face coverage. To demonstrate the scope of this approach, we attempted to construct redox─ active metal complex oligomer wire on H─ Si(111) surface using subsequent complexation as

Figure 6.　 (a) Structures of prepared bis(trimethylsilyl)silyl-thiophenes. (b) UV─ vis absorption spectra of 86─ 91. (c) Fluorescence spectra and �uorescence images under UV lamp of 86─ 91.

Figure 7.　 Surface structure of hydrogen─ terminated silicon surface: (a) Si(111) and (b) Si(100). (c) Electrochemical grafting mechanism of aryldiazonium cations on a silicon surface.

（ 76 ） J. Synth. Org. Chem., Jpn.1104

described in our previous reports. 36 The preparation of oligo-mer wire was carried out as shown in Figure 8(b). The oligo-mer wire growth was observed by AFM, and Figure 8(c) shows AFM image of 10 layers of π ─ conjugated complex wire pre-pared by step─ by─ step construction method. The height was 17.0±0.5 nm, which was consistent with theoretical height. The result suggested that π ─ conjugated complex wires were grown vertically through stepwise complexation on the Si sur-face, and that 4’─ (4─ iodophenyl)─ 2,2’:6’,2’’─ terpyridine was immobilized on Si surface via Pd─ catalyzed reaction.

Micro patterning of electrode is an important technology for preparing devices such as memories, sensors, and actua-tors. 37 Especially, chemical modi�cation of silicon surface via deposition and patterning of organic layers is currently of great interest. Most of the methods for producing micro sized silicon electrodes are based on lithographic techniques. Elec-tron beam (EB) induced patterning process is one of the most useful method to prepare them. The preparation of micro pat-terned monolayers on hydrogen─ terminated silicon surfaces has been reported. 38 However, the electrochemical evaluation of the monolayer is dif�cult due to the trace amount of cur-rent.

To overcome this problem, a representative aromatic group, 4─ ferrocenylphenyl one, was covalently bound to a micro─ patterned silicon electrode via the arylation of a hydrogen─

terminated Si(111) surface formed selectively on a Si wafer. 39 Starting from a Si(100)─ on─ insulator wafer, the aromatic monolayer was attached sequentially by spin─ coating a resist, electron beam lithography, Cr/Au deposition, lift─ off, aniso-tropic etching with aqueous KOH solution, 40 and Pd─ catalyzed arylation (Figure 9(a)). The micro─ patterned electrode surface was observed by digital microscope and high─ resolution SEM (Figure 9(b)). CV and XPS were used to characterize the cou-pling reaction between the 4─ ferrocenylphenyl groups and the silicon substrate. The data show that this synthetic protocol gives chemically well─ de�ned and robust functionalized mono-layers on silicon semiconducting surfaces with small electrodes.

5． Conclusions

This review provides an overview of recent advances in the transition metal─ based systems for the synthesis of organosili-con compounds from hydrosilanes and organoiodine com-pounds. As compared to the previously reported procedures which use organometallic reagents and chlorosilanes, the reac-tion described here has a broad scope and can be used to pre-pare a variety of group 14 compounds under mild conditions. The reactivity is the opposite to that expected from the polarity of the Si─ H and Ge─ H bonds and their hydride character. The umpolung arylation strategy enabled us to achieve a variety of methodologies for the preparation of pharmaceutical, agricul-

Figure 8.　 (a) Synthetic scheme of the Pd─ catalyzed surface reaction to immobilize aromatic group directly onto hydrogen─ terminated Si(111) or Ge(111) surfaces. (b) Synthetic procedure of π ─ conjugated complex oligomer wire: (i) immobilization of 4’─ phenyl─ 2,2’:6’,2”─ terpyridine group, (ii) complexation with Fe(BF 4) 2, (ii) complexation with 4’,4’’’’─ (1,4─ phenylene)bis(2,2’:6’,2”─ terpyri-dine), and (iv) stepwise complexation. (c) 3D AFM image (1 μm×1 μm) of 10 layers of π ─ conjugated complex oligomer on Si(111) electrode.

Vol.74 No.11 2016 （ 77 ） 1105

tural, and �uorescent compounds. We also developed push─ pull systems based on Si─ Si σ bonds, which showed blue emis-sion with good to high quantum yields. Several compounds displayed SHG and AIEE. The advantages of this method include good substrate generality, exceptional functional group tolerance, inexpensive reagents and experimental ease. Inspired by this chemistry of group 14 hydrides, we also described an alternative strategy to create Si─ C(sp 2) or Ge─ C(sp 2) linkages on hydrogen─ terminated surfaces. CV, XPS, AFM, and SEM studies were performed to assess the immobilization. The results suggest that this method for forming Si─ C or Ge─ C bonds would allow us to design novel materials for applica-tions in optoelectronic devices.

AcknowledgementThe present work was �nancially in part supported by

CREST from JST, Tokyo Kasei Chemical Promotion Founda-tion, Nippon Sheet Glass Foundation for Materials Science and Engineering, Precise Measurement Technology Promotion Foundation, Grant─ in─ Aids for Scienti�c Research (C) (No. 15K05604), and Scienti�c Research on Innovative Areas “Molecular Architectonics: Orchestration of Single Molecules for Novel Functions” (area 2509, Nos. 26110505, 26110506, 16H00957 and 16H00958) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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4） For our preliminary review, see: Yamanoi, Y.; Nishihara, H. J. Synth. Org. Chem., Jpn. 2009, 67, 778.

5） For other recent reviews on the transition metal─ catalyzed coupling reaction of aryl iodides with trialkylsilanes, see: (a) Xu, Z.; Huang, W.─ S.; Zhang, J.; Xu, L.─ W. Synthesis 2015, 47, 3645. (b) Xu, Z.; Xu, L.─ W. ChemSusChem 2015, 8, 2176.

6） For representative reviews, see: (a) Franz, A. K.; Wilson, S. O. J. Med. Chem. 2013, 56, 388. (b) Tacke, R.; Linoh, H. Bioorganosilicon Chemistry. In Organic Silicon Compounds; John Wiley & Sons, Ltd.: New York, 1989; Vol. 2, pp 1143─ 1206.

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11） Murata et al. reported that their catalytic system was effective for the arylation of tri(2─ furyl)silane, diphenylmethylsilane, di(2─ furyl)-methyl silane, and di(2─ thiophenyl)methylsilane. See: Murata, M.; Ohara, H.; Oiwa, R.; Watanabe, S.; Masuda Y. Synthesis 2006, 1771.

12） Yamanoi, Y. J. Org. Chem. 2005, 70, 9607.13） (a) Yamanoi, Y.; Nishihara, H. Tetrahedron Lett. 2006, 47, 7157. (b)

Yamanoi, Y.; Nishihara, H. J. Org. Chem. 2008, 73, 6671.14） (a) Yamakawa, T.; Kagechika, H.; Kawachi, E.; Hashimoto, Y.;

Shudo, K. J. Med. Chem. 1990, 33, 1430. (b) Nagaki, A.; Imai, K.; Kim, H.; Yoshida, J.─ i. RSC Adv. 2011, 1, 758.

15） (a) Yamanoi, Y.; Taira, T.; Sato, J.─ i.; Nakamula, I.; Nishihara, H. Org. Lett. 2007, 9, 4543. (b) Lesbani, A.; Kondo, H.; Yabusaki, Y.; Nakai, M.; Yamanoi, Y.; Nishihara, H. Chem. Eur. J. 2010, 16, 13519.

16） Kurihara, Y.; Yamanoi, Y.; Nishihara, H. Chem. Commun. 2013, 49, 11275.

17） (a) Imrie, C. T.; Henderson, P. A. Chem. Soc. Rev. 2007, 36, 2096. (b) Boehner, C. M.; Frye, E. C.; O’Connell, K. M. G.; Galloway, W. R. J. D.; Sore, H. F.; Dominguez, P. G.; Norton, D.; Hulcoop, D. G.; Owen, M.; Turner, G.; Crawford, C.; Horsley, H.; Spring, D. R. Chem. Eur. J. 2011, 17, 13230. (c) Denmark, S. E.; Butler, C. R. J. Am. Chem. Soc. 2008, 130, 3690.

18） For recent reviews, see: (a) Bauer, J. O.; Strohmann, C. Eur. J. Inorg. Chem. 2016, 2868. (b) Shintani, R. Asian J. Org. Chem. 2015, 4, 510. (c) Xu, L.─ W.; Li, L.; Lai, G.─ O.; Jiang, J.─ X. Chem. Soc. Rev. 2011, 40, 1777. (d) Xu, L.─ W. Angew. Chem. Int. Ed. 2012, 51, 12932.

19） Kurihara, Y.; Nishikawa, M.; Yamanoi, Y.; Nishihara, H. Chem. Commun. 2012, 48, 11564.

20） Inubushi, H.; Kondo, H.; Lesbani, A.; Miyachi, M.; Yamanoi, Y.; Nishihara, H. Chem. Commun. 2013, 49, 134.

21） Kondo, H.; Yamanoi, Y.; Nishihara, H. Chem. Commun. 2011, 47, 6671.

22） (a) Imamoto, T.; Yamanoi, Y. Chem. Lett. 1996, 705. (b) Imamoto, T.; Takiyama, N. Tetrahedron Lett. 1987, 28, 1307. (c) Molander, G. A.; Harring, L. S. J. Org. Chem. 1989, 54, 3525. (d) Molander, G. A.;

Figure 9.　 (a) Process to create the patterned micro─ sized hydrogen─ terminated silicon electrode. Cross─ sectional views of: (i) spin─ coating the resist, (ii) electron beam (EB) lithography, (iii) Cr and Au mask deposition, (iv) lift─ off, and (v) aniso-tropic etching with HF (1%) then KOH (50%). (vi) Immo-bilization of 4─ ferrocenylphenyl groups via a Pd─ catalyzed method. (b) Top view of the patterned micro─ sized elec-trode after KOH anisotropic etching. (c) SEM observation with 5 kV acceleration voltage of the micro─ patterned electrode. (d) Digital microscopic observation view of the micro─ patterned electrode. (e) High─ resolution SEM image with 20 kV acceleration voltage of the micro─ pat-terned electrode. Tilt angle θ=25°.

（ 78 ） J. Synth. Org. Chem., Jpn.1106

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23） For recent reviews, see: (a) Zhang, Q.─ W.; An, K.; He, W. Synlett 2015, 26, 1145. (b) Shimizu, M.; Hiyama, T. Synlett 2012, 23, 973. (c) Shimizu, M. J. Synth. Org. Chem., Jpn. 2013, 71, 307. (d) Corey, Y. J. Adv. Organomet. Chem. 2011, 59, 181. (e) Fukazawa, A.; Yamaguchi, S. Chem. Asian J. 2009, 4, 1386. (f) Yamaguchi, S.; Xu, C.; Okamoto, T. Pure Appl. Chem. 2006, 78, 721. (g) Yamaguchi, S.; Xu, C. J. Synth. Org. Chem., Jpn. 2005, 63, 1115. (h) Yamaguchi, S.; Tamao, K. Chem. Lett. 2005, 34, 2.

24） Yabusaki, Y.; Ohshima, N.; Kondo, H.; Kusamoto, T.; Yamanoi, Y.; Nishihara, H. Chem. Eur. J. 2010, 16, 5581.

25） For a review on organic molecules exhibiting ef�cient emission in the solid state, see: Shimizu, M.; Hiyama, T. Chem. Asian J. 2010, 5, 1516.

26） Nakashima, T.; Shimada, M.; Kurihara, Y.; Tsuchiya, M.; Yamanoi, Y.; Nishibori, E.; Sugimoto, K.; Nishihara, H. J. Organomet. Chem. 2016, 805, 27.

27） Shimada, M.; Yamanoi, Y.; Matsushita, T.; Kondo, T.; Nishibori, E.; Hatakeyama, A.; Sugimoto, K.; Nishihara, H. J. Am. Chem. Soc. 2015, 137, 1024.

28） Shimada, M.; Tsuchiya, M.; Sakamoto, R.; Yamanoi, Y.; Nishibori, E.; Sugimoto, K.; Nishihara, H. Angew. Chem. Int. Ed. 2016, 55, 3022.

29） Luo, J. D.; Xie, Z. L.; Lam, J. W. Y.; Cheng, L. C.; Chen, H. Y.; Qiu, C. F.; Kwok, H. S.; Zhan, X. W.; Liu, Y. Q.; Zhu, D. B.; Tang, B. Z. Chem. Commun. 2001, 1740.

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33） For representative reviews, see: (a) Yamanoi, Y.; Miyachi, M.; Nishihara, H. “Modi�cation of Electrode Interfaces with Nanosized Materials for Electronic Applications”, In Molecular Architectonics ─ Advances in Single Atom and Molecule Machines, Ogawa, T. Ed. Springer, in press. (b) Yamanoi, Y.; Nishihara, H. Chem. Commun. 2007, 3983.

34） For representative reviews, see: (a) Aryl Diazonium Salts: New Cou-pling Agents and Surface Science, Chehimi, M. M. Ed. Wiley, New York, 2012. (b) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429.

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PROFILE

Masaki Shimada was born in Nagano in 1991. He received a B.Sc. in 2013 and a M.Sc. in 2015 from the University of Tokyo under the guidance of Prof. Hiroshi Nishihara and Dr. Yoshinori Yamanoi. Now he is a second─ year Ph.D. student at the University of Tokyo in the same research group. His research activity is focused on the deve-lopment of organosilicon compounds with functional properties such as strong solid─ state emission and nonlinear optics.

Yoshinori Yamanoi was born in Tokyo in 1971. He obtained a B.Sc. in 1994 from Sci-ence University of Tokyo under the guidance of Prof. Shu Kobayashi. He then moved to the Graduate School of Chiba University, and received a D.Sc. in 1999 under the super-vision of Prof. Tsuneo Imamoto. He spent two years as a JSPS postdoctoral research fellow at Nagoya University (Prof. Makoto Fujita: 1999─ 2000) and at MIT (Prof. Gregory C. Fu: 2000─ 2001). He became an assistant professor at IMS in 2001. In October 2002, he joined Prof. Nishihara’s group, where he now works as an associate professor. His research activity is focused on the creation of nanosized materials and the development of ef�cient transition metal─ catalyzed organic transformations.

Hiroshi Nishihara received his B. Sc. degree in 1977, M.Sc. in 1979 and D.Sc. in 1982 from The University of Tokyo. He was appointed research associate of Department of Chemi-stry at Keio University in 1982, and he was promoted lecturer in 1990, and associate professor in 1992. Since 1996, he has been a professor of Department of Chemistry, School of Science at The University of Tokyo. He also worked as a visiting research associate of Department of Chemistry at The University of North Carolina at Chapel Hill (1987─ 1989), and as a researcher of PRESTO, JST (1992─ 1996). He received Docteur Honoris Causa from University of Bordeaux in 2011, Commendation for Sci-ence and Technology by MEXT in 2014, Japan Society of Coordination Chemistry Award in 2015, and Chemical Society of Japan Award in 2016.

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