Synthesis and Functionalization of a 1 , 2-Bis ( trimethylsilyl )-1 , 2-disilacyclohexene That Can Serve as a Unit of cis-1 , 2-Dialkyldisilene

π-Electron compounds that include multiple bonds between silicon atoms have received much attention as novel functional silicon compounds. In the present paper, 1,2-bis(trimethylsilyl)-1,2-disilacyclohexene 1 was successfully synthesized as thermally stable yellow crystals. Disilene 1 was easily converted to the corresponding potassium disilenide 4, which furnished novel functionalized disilenes after the subsequent addition of an electrophile. Interestingly, two trimethylsilyl groups in 1 can be stepwise converted to anthryl groups. The novel disilenes derived from 1 were characterized by a combination of nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), elemental analyses, and X-ray single crystal diffraction analysis. The present study demonstrates that disilene 1 can serve as a unit of cis-1,2-dialkyldisilene.


Introduction
Stable silicon compounds that include double bonds between silicon atoms (disilenes, R 2 Si=SiR 2 ) have received much attention over the last three decades [1][2][3].Because such silicon π-electron systems have an intrinsically higher π-orbital level and a narrower HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) gap compared to those of the corresponding organic π-electron systems, extended π-electron systems that include the Si=Si double bond(s) should be anticipated to be unprecedented functional π-electron materials.In this context, the reactions of disilenide (a disilicon analogue of vinyl anion [R 2 Si=SiR] − ) with electrophiles is one of the promising routes to introduce a functional group into the silicon π-electron systems [4][5][6][7][8][9].Disilenides have been synthesized by reductive dehalogenation of the corresponding trihalodisilane [10] or reductive cleavage of R-Si(sp 2 ) bond on the Si=Si double bond in a stable disilene [11][12][13][14][15].
Very recently, we found a novel route to a disilenide from a stable disilene under milder conditions [16].In this route, the disilenide was generated from the reaction of a trimethylsilyl-substituted disilene and potassium t-butoxide [17] via selective cleavage of the Si(sp 2 )-Si(sp 3 ) bond on the Si=Si double bond.In this reaction, neither an undesired reduction of the Si=Si double bond nor addition of t-BuOK across the Si=Si double bond occurred.During the course of our study, we designed a novel cyclic disilene, 1,2-bis(trimethylsilyl)-substituted 1,2-disilacyclohexene 1. Compound 1 has two trimethylsilyl groups that can be converted stepwise to the corresponding potassium derivatives after treatment of t-BuOK, as well as alkyl groups that cause at least perturbation to the electronic structure of the Si=Si double bond and should be suitable for the investigation of interactions between the Si=Si double bond and the functional group.Herein, we report successful synthesis, molecular structure and functionalization of 1 and its derivatives.Although 1 has six trimethylsilyl groups on the six-membered ring, one trimethylsilyl group on the Si=Si double bond is selectively eliminated to provide the corresponding disilenide after treatment with t-BuOK.Noticeably, two trimethylsilyl groups in 1 were converted stepwise to anthryl groups to furnish the corresponding 1,2-dianthryldisilene in good yield.
Inorganics 2018, 6, x FOR PROOF 2 of 14 Although 1 has six trimethylsilyl groups on the six-membered ring, one trimethylsilyl group on the Si=Si double bond is selectively eliminated to provide the corresponding disilenide after treatment with t-BuOK.Noticeably, two trimethylsilyl groups in 1 were converted stepwise to anthryl groups to furnish the corresponding 1,2-dianthryldisilene in good yield.

Conversion of 1 to Disilenide 4 and Functionalized Disilenes
Disilene 1 can be effectively converted to the corresponding disilenide through a selective cleavage of the Si(sp 2 )-Si(sp 3 ) bond (Scheme 2) [16].When 1 was treated with one equivalent of t-BuOK in THF, the color of solution gradually turned from yellow to orange.The formation of potassium disilenide 4 as a sole product in the resulting reaction mixture was confirmed by a combination of the multinuclear NMR spectra in C6D6 and the results of the reaction with electrophiles (vide infra).In this reaction, 1,2-addition of t-BuOK across the Si=Si double bond of 1 as well as elimination of a trimethylsilyl group on the carbon atom in the disilacyclohexene ring was not observed at all.The 1 H, 13 C, and 29 Si NMR spectra of 4(thf) in C6D6 exhibits the presence of five trimethylsilyl groups.The 29 Si NMR spectra exhibited two signals due to the unsaturated silicon nuclei at 146.9 ppm (=SiSiMe3) and 219.3 ppm (=Si − ).Similar large differences between the chemical shifts of the unsaturated silicon nuclei have been found in those of structurally related disilenides (Me3Si)TipSi=SiTip[K(thf)n] (101.4 (=Si(SiMe3)Tip), 186.6 (=Si − ) in THF-d8; Tip = 2,4,6triisopropylphenyl) [16] and reported disilenides [27].[18], B [10], and C [24], as well as tetrasila-1,3-dienes D [25], E [12], and F [26].Tip Tip [18], B [10], and C [24], as well as tetrasila-1,3-dienes D [25], E [12], and F [26].

Conversion of 1 to Disilenide 4 and Functionalized Disilenes
Disilene 1 can be effectively converted to the corresponding disilenide through a selective cleavage of the Si(sp 2 )-Si(sp 3 ) bond (Scheme 2) [16].When 1 was treated with one equivalent of t-BuOK in THF, the color of solution gradually turned from yellow to orange.The formation of potassium disilenide 4 as a sole product in the resulting reaction mixture was confirmed by a combination of the multinuclear NMR spectra in C6D6 and the results of the reaction with electrophiles (vide infra).In this reaction, 1,2-addition of t-BuOK across the Si=Si double bond of 1 as well as elimination of a trimethylsilyl group on the carbon atom in the disilacyclohexene ring was not observed at all.The 1 H, 13 C, and 29 Si NMR spectra of 4(thf) in C6D6 exhibits the presence of five trimethylsilyl groups.The 29 Si NMR spectra exhibited two signals due to the unsaturated silicon nuclei at 146.9 ppm (=SiSiMe3) and 219.3 ppm (=Si − ).Similar large differences between the chemical shifts of the unsaturated silicon nuclei have been found in those of structurally related disilenides (Me3Si)TipSi=SiTip[K(thf)n] (101.4 (=Si(SiMe3)Tip), 186.6 (=Si − ) in THF-d8; Tip = 2,4,6triisopropylphenyl) [16] and reported disilenides [27].

Conversion of 1 to Disilenide 4 and Functionalized Disilenes
Disilene 1 can be effectively converted to the corresponding disilenide through a selective cleavage of the Si(sp 2 )-Si(sp 3 ) bond (Scheme 2) [16].When 1 was treated with one equivalent of t-BuOK in THF, the color of solution gradually turned from yellow to orange.The formation of potassium disilenide 4 as a sole product in the resulting reaction mixture was confirmed by a combination of the multinuclear NMR spectra in C 6 D 6 and the results of the reaction with electrophiles (vide infra).In this reaction, 1,2-addition of t-BuOK across the Si=Si double bond of 1 as well as elimination of a trimethylsilyl group on the carbon atom in the disilacyclohexene ring was not observed at all.The 1 H, 13 C, and 29 Si NMR spectra of 4(thf) in C 6 D 6 exhibits the presence of five trimethylsilyl groups.The 29 Si NMR spectra exhibited two signals due to the unsaturated silicon nuclei at 146.9 ppm (=SiSiMe 3 ) and 219.3 ppm (=Si − ).Similar large differences between the chemical shifts of the unsaturated silicon nuclei have been found in those of structurally related disilenides (Me 3 Si)TipSi=SiTip[K(thf) n ] (101.4 (=Si(SiMe 3 )Tip), 186.6 (=Si − ) in THF-d 8 ; Tip = 2,4,6-triisopropylphenyl) [16] and reported disilenides [27].Scheme 2. Synthesis of functionalized disilenes through desilylation of 1.
Treatment of 4, which was generated from 1 and t-BuOK in THF, with an electrophile provide various functionalized disilenes (Scheme 2).For instance, reaction of 4 with triethylchlorosilane gave Et3Si-substituted disilene 5 as yellow crystals in 99% yield.The reaction of 4 with one equivalent of 9-bromoanthracene furnished the corresponding anthryldisilene 6 (33% yield).In a similar manner, (10-bromo-9-anthryl)disilene 6 Br was obtained as a major product from the reaction of 4 and one equivalent of 9,10-dibromoanthracene, although it was not obtained in a pure form due to the inseparable byproducts such as anthracene and 9-bromoanthracene.Although the reactions of 4 with less bulky bromoarenes such as bromobenzene or bromomesitylene (2,4,6-trimethylbromobenzene) afforded a mixture which may contain the desired phenyl or mesityl-substituted disilenes, isolation of these disilenes was unsuccessful probably due to the instability of the resulting less bulky aryldisilenes under these reaction conditions.Noticeably, disilene 6 underwent a further desilylation reaction followed by addition of 9-bromoanthracene to furnish 1,2-dianthryldisilene 7 as black purple crystals in 17% yield.The reaction of 4 with 0.5 equivalent of 1,2-dibromoethane afforded the corresponding tetrasiladiene 8 in 86% yield as red-orange crystals similar to the reaction of Tip2Si=SiTipLi and mesityl bromide leading to the corresponding hexaryltetrasila-1,3-diene D (Figure 1) [25].After removal of the volatiles, recrystallization from toluene provided suitable single crystals of (4(toluene)) 2 .In the single crystals, disilenide 4 forms a dimer with a crystallographic inversion center (Figure 3a).Interestingly, the potassium cations are coordinated by one anionic silicon atom in an η 1 -fashion, one toluene molecule in an η 6 -fashion, and the Si=Si double bond in the other disilenide moiety in an η 2 -fashion.The distance of Si1-K1 is 3.4655(5) Å, which falls in typical range of Si1-K1 distance in the reported potassium disilenides (3.33-3.52Å) [10,13,28], while the Si1•••K1 and Si2•••K1 distances are 3.5066(5) and 3.7645(5) Å.Although the dimeric structure of metal disilenides in the solid state have been reported [10,28,29], to the best of our knowledge, such η 2 -coordination of the Si=Si double bond in a disilenide to the metal cation in the solid state is unprecedented.The Si=Si distance in (4(toluene)) 2 (2.2035(5) Å) is substantially elongated compared that of neutral disilene 1 (2.1762(5)Å) and the geometry around the Si=Si double bond is slightly cis-bent (bent angle β: 3.7 • (Si1-K1), 1.5 • (Si2-SiMe 3 ); twist angle τ: 1.5 • ), which may result from the η 2 -coordination of the Si=Si double bond to the K cation.
Treatment of 4, which was generated from 1 and t-BuOK in THF, with an electrophile provide various functionalized disilenes (Scheme 2).For instance, reaction of 4 with triethylchlorosilane gave Et 3 Si-substituted disilene 5 as yellow crystals in 99% yield.The reaction of 4 with one equivalent of 9-bromoanthracene furnished the corresponding anthryldisilene 6 (33% yield).In a similar manner, (10-bromo-9-anthryl)disilene 6 Br was obtained as a major product from the reaction of 4 and one equivalent of 9,10-dibromoanthracene, although it was not obtained in a pure form due to the inseparable byproducts such as anthracene and 9-bromoanthracene.Although the reactions of 4 with less bulky bromoarenes such as bromobenzene or bromomesitylene (2,4,6-trimethylbromobenzene) afforded a mixture which may contain the desired phenyl or mesityl-substituted disilenes, isolation of these disilenes was unsuccessful probably due to the instability of the resulting less bulky aryldisilenes under these reaction conditions.Noticeably, disilene 6 underwent a further desilylation reaction followed by addition of 9-bromoanthracene to furnish 1,2-dianthryldisilene 7 as black purple crystals in 17% yield.The reaction of 4 with 0.5 equivalent of 1,2-dibromoethane afforded the corresponding tetrasiladiene 8 in 86% yield as red-orange crystals similar to the reaction of Tip 2 Si=SiTipLi and mesityl bromide leading to the corresponding hexaryltetrasila-1,3-diene D (Figure 1) [25].
Similar to the reported anthryl-substituted disilenes [10,24,36], 6, 6 Br , and 7 exhibit a weak and broad absorption band I assignable to π(Si=Si)→π*(anthryl) transition in the visible region as well as a structured intense band II that involves π(anthryl)→π*(anthryl) and π(Si=Si)→π*(Si=Si) transitions (300-400 nm).The absorption maximum of band I of 6 in hexane (535 nm (ε 9.3 × 10 2 )) is slightly bathochromically shifted compared to that of trialkylanthryldisilene B (525 nm (ε 4.2 × 10 2 )) (Figure 1) [10], which would be due to the presence of electron-donating silyl-substituents, while it is moderately hypsochromically shifted compared to triarylanthryldisilene C (550 nm (ε 3800)) [24] (Figure 1) probably due to the absence of extra aryl groups.The maximum of band I of 6 Br (578 nm) is considerably bathochromically shifted compared to that of 6.The lower-lying π* orbitals in 10-bromo-9-anthryl group compared to 9-anthryl group may be responsible for the bathochromic shift of band I, which was qualitatively reproduced by density functional theory (DFT) calculations of 6 and 6 Br (see, Figures S47 and S48).The absorption maximum of band I of 1,2-dianthryldisilene 7 (543 nm (1.68 × 10 3 )) (Figure S49) is close to that of 6 and almost twice as large as that of 6, which is consistent with the presence of two anthryl groups on the Si=Si double bond.
bathochromically shifted compared to that of trialkylanthryldisilene B (525 nm (ε 4.2 × 10 2 )) (Figure 1) [10], which would be due to the presence of electron-donating silyl-substituents, while it is moderately hypsochromically shifted compared to triarylanthryldisilene C (550 nm (ε 3800)) [24] (Figure 1) probably due to the absence of extra aryl groups.The maximum of band I of 6 Br (578 nm) is considerably bathochromically shifted compared to that of 6.The lower-lying π* orbitals in 10bromo-9-anthryl group compared to 9-anthryl group may be responsible for the bathochromic shift of band I, which was qualitatively reproduced by density functional theory (DFT) calculations of 6 and 6 Br (see, Figures S47 and S48).The absorption maximum of band I of 1,2-dianthryldisilene 7 (543 nm (1.68 × 10 3 )) (Figure S49) is close to that of 6 and almost twice as large as that of 6, which is consistent with the presence of two anthryl groups on the Si=Si double bond.

General Procedure
All reactions involving air-sensitive compounds were performed under a nitrogen atmosphere using a high-vacuum line and standard Schlenk techniques, or a glove box, as well as dry and oxygen-free solvents.NMR spectra were recorded on a Bruker Avance 500 FT NMR spectrometer (Bruker Japan, Yokohama, Japan).The 1 H and 13 C NMR chemical shifts were referenced to residual 1 H and 13 C signals of the solvents: benzene-d 6 ( 1 H: δ 7.16 and 13 C: δ 128.0).The 29 Si NMR chemical shifts were relative to Me 4 Si (δ 0.00).Sampling of air-sensitive compounds was carried out using a VAC NEXUS 100027 type glove box (Vacuum Atmospheres Co., Hawthorne, CA, USA).UV-Vis spectra were recorded on a JASCO V-660 spectrometer (JASCO, Tokyo, Japan).Melting points were measured on a SRS OptiMelt MPA100 (SRS, Sunnyvale, CA, USA) without correction.

Preparation of 1,4-Bis
In a two-necked flask (100 mL), 2 (1.00 g, 1.81 mmol) and benzene (20 mL) were placed.To the mixture, bromine (1.19 g, 7.44 mmol) was added dropwise at 0 • C and then the mixture was stirred for 30 min at room temperature.The volatiles were removed in vacuo, and washing the residue with hexane afforded pure 3 as colorless crystals (1.09 g, 1.26 mmol) in 70% yield.All operations were carried out in a glove box.In a Schlenk tube (50 mL), 3 (1.26g, 1.45 mmol), KC 8 (825 mg, 6.10 mmol) and THF (70 mL) were placed.After stirring the mixture at room temperature for three hours, the volatiles were removed under reduced pressure.Then dry hexane was added to the residue and the resulting salt was filtered off.Removal of hexane under reduced pressure gave disilene 1 as yellow crystals (789.0 mg, 1.44 mmol) in 99% yield.
1: yellow crystals; m.p. 120  In a Schlenk tube (50 mL) equipped with a magnetic stir bar, disilene 1 (81.4 mg, 0.149 mmol) and t-BuOK (17.6 mg, 0.157 mmol) and THF (9.0 mL) were placed.After stirring for one hour at room temperature, disilenide 4 was formed as a sole product, which was confirmed by NMR spectroscopy.The volatiles including THF and the resulting t-BuOSiMe 3 were removed under reduced pressure and then the resulting residue was washed with hexane to afford 4(thf) as an orange powder (43.0 mg, 7.35 × 10 −2 mmol) in 49% yield.
Then, THF was removed in vacuo.To the residue toluene (10 mL) and chlorotriethylsilane (150.0 mg, 0.995 mmol) were added.After stirring for 15 min at room temperature, the volatiles were removed in vacuo.To the residue dry hexane was added and the resulting salt was filtered off.Removal of hexane from the filtrate gave Et 3 Si-substituted disilene 5 as yellow crystals (481 mg, 0.816 mmol) in 99% yield.

Synthesis of 1,4-Bis(trimethylsilyl)tetrasila-1,3-diene 8
In a Schlenk tube (50 mL) equipped with a magnetic stir bar, were placed disilene 1 (139.9mg, 0.256 mmol), t-BuOK (29.0 mg, 0.258 mmol) and THF (5 mL).After the mixture was stirred for one hour at room temperature, 1 H NMR spectrum of the mixture showed that disilenide 4 was formed quantitatively.Then, to the mixture, 1,2-dibromoethane (24.0 mg, 0.128 mmol) was added at 0 • C.After additional stirring of the mixture for 15 min at 0 • C, the volatiles were removed under reduced pressure.To the residue dry hexane was added and the resulting insoluble materials were filtered off.Removal of hexane from the filtrate and subsequent recrystallization from diethyl ether at −35 • C gave tetrasila-1,3-diene 8 as red crystals (104.0 mg, 0.110 mmol) in 86% yield.

Figure 2 .
Figure 2. ORTEP (Oak Ridge Thermal Ellipsoid Plot) drawing of 1 with thermal ellipsoids set at 50% probability and hydrogen atoms omitted for clarity.

Figure 2 .
Figure 2. ORTEP (Oak Ridge Thermal Ellipsoid Plot) drawing of 1 with thermal ellipsoids set at 50% probability and hydrogen atoms omitted for clarity.

Figure 2 .
Figure 2. ORTEP (Oak Ridge Thermal Ellipsoid Plot) drawing of 1 with thermal ellipsoids set at 50% probability and hydrogen atoms omitted for clarity.

Table 1 .
Selected structural parameters and spectral data of disilenes.

Table 1 .
Selected structural parameters and spectral data of disilenes.

Table 1 .
Selected structural parameters and spectral data of disilenes.