The chemistry of stable silabenzenes

Journal of the Chinese Chemical Society, 2008, 55, 487-507

487

Invited Paper

The Chemistry of Stable Silabenzenes

Norihiro Tokitoh,^a,* Keiji Wakita,^bTakeshi Matsumoto,^aTakahiro Sasamori,^aRenji Okazaki,^b

Nozomi Takagi,^cMasahiro Kimura^cand Shigeru Nagase^c

^aInstitute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

^bDepartment of Chemistry, Graduate School of Science, The University Tokyo, 7-3-1 Hongo, Bunkyo-ku,

Tokyo 113-0033, Japan

^cDepartment of Theoretical Molecular Science, Institute for Molecular Science, Myodaiji,

Okazaki 444-8585, Japan

Stable silabenzenes (1a; R = Tbt, 1b; R = Bbt) were synthesized by taking advantage of extremely

bulky and efficient steric protection groups, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and 2,6-bis-

[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt). The structure of Tbt-substituted

1a was determined by X-ray crystallographic analysis, which demonstrated the complete delocalization of

the p-electrons of the silabenzene ring. It was found that silabenzene 1a reacted with C–C and C–O multi-

ple bond compounds to give the corresponding [4+2]-cycloadducts via 1,4-addition, while 1a underwent

both 1,2- and 1,4-additions by the reaction with methanol. Silabenzene 1a dimerized very gradually to af-

ford its [4+2]-dimer, although 1b showed no change under the same conditions. Photochemical reaction of

1a gave the corresponding silabenzvalene isomer instead of the Dewar silabenzene isomer.

Keywords: Silabenzene; Kinetic stabilization; Silaaromatics; Delocalized p electrons;

Silabenzvalene.

INTRODUCTION

Benzene, a 6p-electron ring system, is one of the most

ported until the start of our research. The meta-stable sila-

benzene described is 1,4-di-tert-butyl-2,6-bis(trimethyl-

silyl)-1-silabenzene, which is reportedly stable below –100

°C and observed by low-temperature NMR measurements.^4f

However, it was stable only below –100 °C in a special sol-

vent (THF/ether/petroleum ether, 4:1:1) and clearly coordi-

nated with the solvent (most likely THF) as judged by the

relatively high field ²⁹Si chemical shift (d_Si= 26.8). Al-

though a more hindered silabenzene, 1-tert-butyl-2,6-bis-

(dimetylisopropylsilyl)-4-trimethylsilyl-1-silabenzene,

was generated under argon flow, it was stable only at –180

°C in argon matrix.^3f

important and fundamental organic molecules.¹Since sili-

con is located just under carbon in the periodic table and is

expected to have similar characters to those of carbon, the

chemistry of silabenzene (silicon analogues of benzene)

has been much focused on and explored in recent decades.²

Although aromatic characters of silabenzene were pre-

dicted by theoretical calculations,^2c,elittle experimental in-

formation has been obtained for silabenzene. Photoelec-

tron, UV and IR spectra of silabenzene were measured in

low-temperature argon matrices or in the gas phase.³Some

trapping reactions which support the formation of silaben-

zene derivatives were also reported.⁴

Although the successful syntheses of anion and dian-

ion species of silacyclopentadienes⁷and cyclic diamino-

silylenes⁸indicated the existence of “silicon-containing ar-

omatic systems”,^2esynthesis of silicon analogues of simple

benzenoid aromatic compounds still remains as an impor-

tant subject in organosilicon chemistry. A highly efficient

steric protection group is essential for the synthesis of sta-

ble silabenzenoid compounds, since only one substituent

can be introduced on their reactive silicon center. The use

of 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (denoted as

Tbt hereafter) and 2,6-bis[bis(trimethylsilyl)methyl]-4-

Silabenzene can be regarded as not only an aromatic

compound but also a low-coordinated organosilicon com-

pound. Non-conjugated doubly bonded compounds con-

taining a silicon atom have already been synthesized as sta-

ble molecules by taking advantage of kinetic stabilization,

namely, by introducing bulky substituents around the reac-

tive double bonds.^5,6Although applications of this method

to stabilize silabenzene have also been reported, no sila-

benzene stable at ambient temperature has ever been re-

488 J. Chin. Chem. Soc., Vol. 55, No. 3, 2008

Tokitoh et al.

[tris(trimethylsilyl)methyl]phenyl (denoted as Bbt hereaf-

ter) groups, which were developed by us⁹and successfully

applied to stabilize a variety of low-coordinated main

group element compounds,¹⁰brought good results. Thus,

Tbt or Bbt-substituted silaaromatic compounds such as

silabenzene,¹¹1- and 2-silanaphthalenes,^12,139-silaanthra-

cene,¹⁴9-silaphenanthrene¹⁵were successfully synthesized

and their detailed properties, especially their aromatic char-

acter, were revealed. Recently, the heavier analogues of

silaaromatics, i. e., germa-¹⁶and stannaaromatics,¹⁷have

also been obtained as stable “heavy aromatics”. While we

have reported the preliminary contributions on the synthe-

sis of a stable silabenzene and its structural determination

by X-ray crystallographic analysis,¹¹we wish to describe

here the details of the synthesis, structure, and reactivity of

stable silabenzenes bearing 2,4,6-tris[bis(trimethylsilyl)-

methyl]phenyl (Tbt) and 2,6-bis[bis(trimethylsilyl)meth-

yl]-4-[tris(trimethylsilyl)methyl]phenyl (denoted as Bbt)

groups.¹⁸

silanes 3 (Scheme I) according to the method similar to that

used for the synthesis of the substituted silacyclohexa-

dienes.²⁰The mixture of 5 and 6 was subjected to further re-

actions without separation since the mixture was insepara-

ble by any chromatographic methods.

Scheme I

The synthesis of silabenzenes 1 was attempted by us-

ing the same method as that for the stable 2-silanaphtha-

lene,¹³namely, the bromination of hydrosilanes followed

by the treatment with t-BuLi (Scheme II). However, the

bromination of 5 and 6 did not take place efficiently and

gave an impure mixture of 7 and 8, the dehydrobromination

of which resulted in the formation of only small amounts of

silabenzenes 1 (~30% as judged by ¹H NMR).

2

3

1

4

R´

Si

R

R´

Tbt; R´ = SiMe , R´´ = H

3

R´

1a; R = Tbt

1b; R = Bbt

1c; R = H

1d; R = Ph

1e; R = Tbtx

Bbt; R´ = R´´ = SiMe

3

R´´

Tbtx; R´ = SiH , R´´ = H

3

Scheme II

RESULTS AND DISCUSSION

Synthesis of Silabenzenes

Many synthetic routes have been developed towards

transient silabenzenes.²Taking the preparative scale syn-

thesis and our successful synthesis of other metallaaro-

matics^11-17into account, we selected halosilacyclohexa-

dienes as the precursors of silabenzenes 1.

Although a mixture of 5 and 6 as well as that of 7 and

8 was inseparable, silanols 9 could be separated and puri-

fied from a crude mixture of the hydrolyzed products of 7

and 8. The hydroxyl groups of 9 were easily converted to

chlorosilanes 10 by the treatment with PCl₅(Scheme III).

R

X

R

X

or

Si

R

Si

1

X = halogen

Scheme III

R = Tbt, Bbt

Although we first attempted the introduction of a Tbt

group onto 1,1-dihydro- or 1,1-dihalo-1-silacyclohexa-

2,4-diene, the substitutions did not occur efficiently.¹⁹Af-

ter the examinations of several routes, we finally succeeded

in the synthesis of the mixture of silacyclohexadienes 5 and

6 starting from stannacyclohexadiene 2 and bulky trihydro-

The Chemistry of Silabenzenes

J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 489

The treatment of 10a with t-BuLi afforded silaben-

zene 1a and a similar amount of a byproduct as indicated by

the ¹H NMR spectrum. The addition of benzophenone to

the reaction mixture followed by chromatographic separa-

tion gave the byproduct 11 (27%) along with 12 (20%), i.e.,

the benzophenone adduct of silabenzene 1a (Scheme IV).

The formation of 11 is most likely interpreted in terms of

electron transfer from t-BuLi to 10 followed by a radical

coupling reaction (Scheme V), although the mechanism is

unclear at present. After examining several conditions, the

formation of 11 was completely suppressed by using lith-

ium diisopropylamide (LDA) as a base. Thus, the reactions

of chlorosilanes 10a,b with LDA at room temperature pro-

ceeded efficiently to give almost pure silabenzenes 1a,b

(96%).

NMR Spectra

²⁹Si, ¹³C and ¹H NMR chemical shifts of silabenzenes

1a,b are shown in Table 1.²¹The silabenzene rings were

symmetric in terms of NMR spectra, which were assigned

by 2D NMR techniques together with decoupling measure-

ments. In Table 1 are also shown the calculated chemical

shifts of the model compounds 1e (the methyl groups of the

Tbt group are replaced by hydrogens) and the real molecule

1a for comparison. The agreement between the observed

and calculated values is fairly good.

As shown in Table 1, ²⁹Si NMR signals of the central

Si atom of silabenzenes 1a,b were observed around 93

ppm. These values are in the low-field area characteristic of

low-coordinate silicon compounds and are also similar to

those of Tbt-substituted silaaromatics (87.3 ppm for 2-

Tbt-2-silanaphthalene, 91.7 ppm for 1-Tbt-1-silanaphtha-

lene, 87.2 ppm for 9-Tbt-9-silaanthracene, 86.9 ppm for

9-Tbt-9-silaphenanthrene). Although d_Sivalues reported

for stable Si–C doubly bonded compounds are widely scat-

tered depending on the substituents, those of 1 are reason-

ably compared with that (d_Si= 77.6) of Mes₂Si=C(H)CH₂(t-

Bu)²²and clearly different from that (d_Si= 26.8) of previ-

ously reported marginally stable silabenzene, 1,4-di-tert-

butyl-2,6-bis(trimethylsilyl)-1-silabenzene.^4f

Scheme IV

We have examined the delocalization of the double

bonds of Tbt-substituted 2-silanaphthalene on the basis of

the Si–C coupling constants (92 and 76 Hz),¹³which con-

siderably exceed those of typical Si–C single bonds (~50

Hz).²³The J_Si–C2values for silabenzenes 1a,b were mea-

sured to be both 83 Hz. These values are consistent with

the order of bond strengths (Si–C3 of 2-Tbt-2-silanaph-

thalene < 1-Tbt-1-silabenzene < Si–C1 of 2-Tbt-2-sila-

naphthalene) which is analogous to that of benzene and

naphthalene. This order of bond strengths was also sup-

ported by the observed and calculated bond lengths.^13b

The J_Si–Cvalues of silabenzene 1 are also similar to that of

(Me₃Si)₂Si=C(R)OSiMe₃(83–85 Hz),²⁴which has a partial

Si–C double bond character.²⁵

Scheme V

The aromatic characters of silaaromatics have been

discussed on the basis of the low-field shifts of the ¹H NMR

signals.¹³Approximately 1 ppm low-field shifts were ob-

served for the ring protons upon the aromatization of the

silicon-containing six-membered ring.¹³In the case of sila-

benzene 1a (Fig. 1), low-field shifts of approx. 1 ppm were

also found upon aromatization. These shifts are similar to

the difference of chemical shifts between cyclohexadiene

(d_H= 5.84) and benzene (d_H= 7.34), supporting the aro-

Although 1,4-di-tert-butyl-1-silabenzene was previ-

ously postulated as an initial product in a similar reaction of

the corresponding chlorosilane with LDA, the isolated

product was its dimer.^4cIn contrast, silabenzenes 1, effec-

tively protected by the extremely bulky Tbt or Bbt groups,

have enough stability for the investigation of their detailed

properties as described below.

490 J. Chin. Chem. Soc., Vol. 55, No. 3, 2008

Tokitoh et al.

Table 1. Observed and calculated NMR chemical shifts (ppm) of silabenzenes

Compound

Si

C2

C3

C4

H2

H3

H4

1a (R = Tbt, obsd)^a

1b (R = Bbt, obsd)^a

1e (R = Tbtx, calcd)^b

1a (R = Tbt, calcd)^b

93.64

92.93

89.88

96.33

122.15 143.55 116.08

121.25 143.55 115.91

126.67 148.52 121.50

127.98 148.13 119.91

7.11

7.03

6.99

6.81

8.00

7.98

8.08

7.99

6.71

6.69

6.78

6.63

^aC₆D₆, room temperature.

^bGIAO-B3LYP/6-311G(d)(6-311G(3d) on Si)//B3LYP/6-31G(d)

R

Si

C4

C2

H4

C3

H3

H2

matic character of the silabenzene ring.

electron ring system similar to that of benzene. A slight dif-

ference between C3–C4 and C4–C5 lengths is probably

due to the packing effect, which causes a close contact

around para-positions of two silabenzene rings (Fig. 3).

Theoretical calculations at the level of B3LYP/6-

311G(d,p) have predicted the Si–C bond length of parent

silabenzene 1c to be 1.771 Å, which is in good agreement

with the observed value of 1a as shown in Table 2. The cal-

culated C–C bond lengths for 1c (1.398 and 1.401 Å) are

also close to those observed for 1a. To evaluate the effect of

bulky substituents, we calculated the geometry of the real

molecule 1a and the model compounds 1c-e. At B3LYP/6-

31G(d) level, the bond lengths of the model compound 1e

Structure

The D_6hsymmetric hexagonal structure is one of the

most important features of benzene. Although all structural

optimization for silabenzenes indicated delocalized p-sys-

tems and aromaticity of silabenzenes,^2c,eno structural anal-

ysis was performed due to their high instability. We have

succeeded in the X-ray crystallographic analysis of the sta-

ble silabenzene 1a for the first time and its structure was

definitely determined (Fig. 2). The sum of the bond angles

around the central Si atom and that of the interior bond an-

gles of the silabenzene ring are 359.8(3)° and 720.0(9)°, re-

spectively, indicating the planar geometry around the cen-

tral silicon atom and the silabenzene ring. The lengths of

two Si–C bonds in the silabenzene ring were found to be es-

sentially equal to each other (1.765(4) and 1.770(4) Å) and

in the middle between those of Si–C double and single

bonds (1.70²⁶and 1.89²⁷Å, respectively). Furthermore, the

C–C bond lengths (1.381(6) ~ 1.399(6) Å) in the silaben-

zene ring are almost equal to each other within the error of

temperature factors, and also similar to the C–C length of

benzene (1.39–1.40 Å).¹Thus, it has experimentally been

demonstrated that the silabenzene has a delocalized 6p-

5.92

Tbt

H

Si

H

6.06

6.71

6.02

7.11

H

8.00

6.72

1

Fig. 2. Molecular structure of silabenzene 1a (ORTEP,

Fig. 1. Chemical shift changes of in H NMR upon

aromatization.

50% probability).

The Chemistry of Silabenzenes

J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 491

Si–C doubly bonded compound,¹³in contrast to the case of

benzene, which is known to be inert toward some organic

reagents such as alcohol, olefins, alkynes, and nitrile ox-

ides. This reactivity can be explained by its structural simi-

larity to naphthalene; the C1–C2 bond of naphthalene has a

stronger double bond character than the C2–C3 bond. On

the contrary, silabenzene 1a has completely delocalized

double bonds like benzene and its detailed reactivity is

interesting.

Table 2. Observed and calculated bond lengths of silabenzenes

Compound

Method

Si-C2 C2-C3 C3-C4

1.765(4) 1.391(6) 1.399(6)

1.770(4) 1.397(6) 1.381(6)

1a (R = Tbt) X-ray^a

1c (R = H)

B3LYP/6-311G(d,p)^b1.771

1.398

1.400

1.401

1.403

1.402

B3LYP/6-31G(d)^a

1.773

1.776

1.777

1.780

1d (R = Ph) B3LYP/6-31G(d)^a

1e (R = Tbtx) B3LYP/6-31G(d)^a

1a (R = Tbt) B3LYP/6-31G(d)^a

^aThis work. ^bRef. 2e.

As mentioned above, silabenzene 1a was trapped by

benzophenone to afford the corresponding adduct 12

(Scheme IV). The structure of 12 was revealed by X-ray

crystallographic analysis (Fig. 4), which clearly indicated

that the silabenzene reacted with benzophenone via [4+2]-

cycloaddition. Furthermore, C–C double and triple bond

compounds were allowed to react with silabenzene 1a to

give the [4+2]-cycloadducts 13–15 as shown in Scheme

VI. The structures of 13–15 were confirmed by X-ray crys-

tallographic analyses (Fig. 5–7, respectively).

(R = Tbtx) are almost the same as those of the parent com-

pound 1c. Thus, it can be concluded that an extremely

bulky Tbt group exerts essentially no perturbation on the

structure of the silabenzene ring.

UV Spectra

Tbt-substituted silabenzene 1a showed absorption

maxima in the UV-vis spectrum at 260 (sh), 301, 323, and

331 nm, which may be attributable to the several types of

p-p* electron transitions. The comparison of the UV-vis

spectra among benzene – silabenzene – 1,4-disilabenzene

series was also reported.²⁸

Scheme VI

Reactivity

Only limited reactivities of silabenzenes were inves-

tigated using transient species. Although the 2-silanaphtha-

lene was concluded to be highly aromatic, it reacted with

various reagents at its 1,2-position like an unconjugated

Although the reaction of the previously reported sila-

benzenes with methanol afforded the corresponding 1,2-

adduct,³the reaction of stable silabenzene 1a with metha-

nol gave both 1,4-adduct 16 and 1,2-adduct 17 in a ratio of

5:3. Mesitonitrile oxide reacted with silabenzene 1a to af-

ford the [3+2]-cycloadduct 18, the structure of which was

determined by X-ray crystallographic analysis (Fig. 8).

The products of these reactions indicate that silaben-

zene 1a can undergo both 1,2- and 1,4-addition reactions.

Fig. 3. Packing diagram of the silabenzene 1a. Only

two of four molecules in a unit cell are shown

for clarity.

492 J. Chin. Chem. Soc., Vol. 55, No. 3, 2008

Tokitoh et al.

These reactivities are quite different from that of the previ-

ously reported Tbt-substituted 2-silanaphthalene¹³and

9-silaanthracene,¹⁴which reacted selectively with some re-

agents only at 1,2-position and 9,10-position giving the

corresponding adducts, respectively. Since these additions

must accompany the destruction of the fused aromatic moi-

ety, it should be energetically unfavorable. This type of en-

ergetic disadvantage for the 1,4-addition is not conceivable

in the case of silabenzene.

actions of a silabenzene. Addition reactions of the parent

silabenzene 1c with acetylene, nitrile oxide, and methanol

Theoretical Investigation on Reactivity

To clarify the reactivity of silabenzene 1a, we theo-

retically investigated the reaction pathways of addition re-

Fig. 6. Molecular structure of tert-butyl vinyl ether

adduct 14 (ORTEP, 50% probability). Hydro-

gen atoms and one of the disordered oxygen at-

oms and t-Bu groups are omitted.

Fig. 4. Molecular structure of benzophenone adduct

12 (ORTEP, 50% probability). Hydrogen at-

oms are omitted for clarity.

Fig. 7. Molecular structure of phenylacetylene adduct

15 (ORTEP, 50% probability). Hydrogen at-

oms are omitted.

Fig. 5. Molecular structure of styrene adduct 13 (ORTEP,

50% probability). Hydrogen atoms, solvent

molecules, and one of the disordered 8-phenyl-

1-silabicyclo[2.2.2]octa-2,5-diene moieties are

omitted for clarity.

Fig. 8. Molecular structure of mesitonitrile oxide

adduct 18 (ORTEP, 50% probability). Hydro-

gen atoms are omitted.

The Chemistry of Silabenzenes

J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 493

were selected as typical examples.

The free energy surface of a cycloaddition reaction of

silabenzene 1c with acetylene is shown in Fig. 9. The re-

sults suggested that the 1,4-addition, i.e., [4+2]-cycloaddi-

tion is more favorable than 1,2-addition, i.e., [2+2]-cyclo-

addition as judged by the lower energy of the transition

state of the former than that of the latter by 15.5 kcal/mol,

though the 1,2-adduct is thermodynamically more stable

than the 1,4-adduct by 2.0 kcal/mol. The result of this cal-

culation agrees with the experiments, where the reaction of

1a with phenylacetylene afforded the corresponding 1,4-

adduct exclusively.

In the case of the reaction of 1c with nitrile oxide

(Fig. 10), the calculations showed that the transition state

for the 1,2-addition should be more favorable than the

1,4-addition, since the transition state for the 1,4-addition

is less stable than that of the 1,2-addition by 12.9 kcal/mol.

In this case, the 1,2-adduct is also more stable than the

1,4-adduct. The calculated free energy surface supports the

experimental result of the exclusive 1,2-addition of 1a with

MesCNO.

Fig. 10. Free energy surface (kcal/mol) in the reaction

of silabenzene 1c with HCNO. Selected bond

lengths (Å) are shown. Calculated at B3LYP/

6-31G(d) level.

On the other hand, the free energy surface for the ad-

dition reactions of 1c with methanol (Fig. 11) seems to be

inconsistent with the experimental result of the reaction of

Fig. 11. Free energy surface (kcal/mol) in the reaction

of silabenzene 1c with a methanol monomer.

Selected bond lengths (Å) are shown. Calcu-

lated at B3LYP/6-31G(d) level.

Fig. 9. Free energy surface (kcal/mol) in the reaction

of silabenzene 1c with acetylene. Selected bond

lengths (Å) are shown. Caluclated at B3LYP/6-

31G(d) level.

494 J. Chin. Chem. Soc., Vol. 55, No. 3, 2008

Tokitoh et al.

1a with methanol. That is, the calculations showed a lower

transition state for the 1,2-addition than that for the 1,4-ad-

dition, though the treatment of silabenzene 1a with MeOD

afforded 1,4-adduct 16 as a major product along with 1,2-

adduct 17 as a minor product (16 : 17 = 5 : 3, vide supra).

Furthermore, we have examined the theoretical calcula-

tions for the free energy surface of the reaction of 1c with

methanol trimer connected with hydrogen bondings as a

model of methanol oligomer, since a methanol generally

exists as an oligomer in solution due to hydrogen bondings

(Fig. 12). Fig. 12 shows that the transition state for the 1,4-

addition (1,4-TS) has a lower energy than that for the 1,2-

addition (1,2-TS) by 4.6 kcal/mol in contrast to the case of

the reaction of 1c with a methanol monomer. That is, the se-

lectivity between 1,2- and 1,4-additions of silabenzene and

methanol should be susceptible to the situation of aggrega-

tion of methanol molecules with hydrogen bondings. The

experimental results suggested that the methanol domi-

nantly reacts with silabenzene 1a as an oligomeric structure

with hydrogen bondings.

oxide are shown in Figs. 13 and 14, respectively. The acti-

vation energies are higher in the reactions of silabenzene,

1,2-TS

31.0

TS

21.4

SiH HC CH

0

IN

–1.5

–1.7

H C SiH

2

HC CH

1,2-adduct –35.4

2

The aromatic character of a silabenzene was also in-

vestigated on the basis of the reaction energies. Compari-

sons of the potential energy surfaces of silabenzene and

silene in the addition reactions with acetylene and nitrile

adduct –66.8

Fig. 13. Potential energy surface of the reactions of

silabenzene and silene with acetylene calcu-

lated at B3LYP/6-31G(d) level.

1,2-TS

3.9

0.0

0

IN

–1.5

–1.9

TS

SiH HCNO

H C SiH

2

HCNO

2

1,2-adduct –35.4

adduct –66.8

Fig. 12. Free energy surface (kcal/mol) in the reaction

of silabenzene 1c with a methanol trimer. Se-

lected bond lengths (Å) are shown. Calculated

at B3LYP/6-31G(d) level.

Fig. 14. Potential energy surface of the reactions of

silabenzene and silene with HCNO calculated

at B3LYP/6-31G(d) level.

The Chemistry of Silabenzenes

J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 495

and there are large differences in the stability of the prod-

ucts. The adducts of silene are more stable than those of

silabenzene, reflecting the loss of aromaticity in silaben-

zene in these addition reactions.

group has one more trimethylsilyl group at its para-posi-

tion, Bbt group may be slightly bulkier than Tbt group

probably due to the buttressing effect.³⁰

Theoretical calculations on the dimerization reaction

of parent silabenzene 1c were performed. The potential en-

ergy surface of dimerization reactions of 1c showed that the

[4+2]-dimerization should be the most favorable among

the [2+2]- and [4+2]-self-dimerization reactions as shown

in Fig. 16, supporting the experimental results. In addition,

calculations for the dimerization reactions of real mole-

cules 1a,b suggested that the activation energy of [4+2]-

Stability

Silabenzene 1a showed no change in solution even at

100 °C. Furthermore, the ²⁹Si NMR chemical shift of 1a

(d_Si= 93.4) in THF/C₆D₆(6:1) did not differ from that mea-

sured in C₆D₆(d_Si= 93.6). As mentioned above, the ²⁹Si

NMR chemical shift of Märkl’s silabenzene measured in

THF/Et₂O/petroleum ether (4:1:1) is at much higher field

(d_Si= 26.8)^4fthan that of 1a, indicating that it is coordinated

with the solvent molecule, most likely with THF, and hence

stabilized only in such a mixed solvent. The absence of

such a complexation with the solvent in 1a suggests the re-

markable effectiveness of Tbt as a steric protection group.

Although we have reported that silabenzene 1a is sta-

ble for many weeks,¹¹1a was found to undergo gradual

dimerization (~50% conversion after 4 months) at room

temperature in benzene to afford the corresponding [4+2]-

dimer 19a (Scheme VII). The structure of 19a was deter-

mined by X-ray analysis (Fig. 15). It is surprising that sila-

benzene 1a dimerized even with an extremely bulky Tbt

group. This fact strongly indicates the extremely high reac-

tivity of silabenzene towards dimerization²⁹and reason-

ably explains why no stable silabenzene had been reported

before 1a was synthesized. Interestingly, dimer 19a was

thermally labile to undergo dissociation into silabenzene

1a on heating at 80 °C for 9 h. This result means that the

dimer 19a should be thermodynamically less stable than

monomer 1a, and the generation of 19a at room tempera-

ture should be explained by the high crystallinity and low

solubility of 19a in benzene. Thus, Tbt group is a “mar-

ginal” substituent where both silabenzene and its dimer can

be obtained. By contrast, Bbt-substituted silabenzene 1b

did not dimerize at all under the same conditions. Since Bbt

Fig. 15. Molecular structure of silabenzene dimer 19a

(ORTEP, 50% probability). Hydrogen atoms

and one of the disordered parts are omitted.

Scheme VII

Fig. 16. Potential energy surface (kcal/mol) in the di-

merization reactions of silabenzene 1c. Se-

lected bond lengths (Å) are shown. Calculated

at B3LYP/6-31G(d) level.

496 J. Chin. Chem. Soc., Vol. 55, No. 3, 2008

Tokitoh et al.

dimerization reaction of Tbt-substituted silabenzene 1a

should be lower than that of Bbt-substituted silabenzene

1b,³¹while [4+2]-dimer 19a (dimer of 1a) is thermody-

namically more stable than 19b (dimer of 1b) (Fig. 17).

not of four-membered ring silicon compounds but of three-

membered ones.³³Several new signals appeared in the ¹H

NMR spectrum (two in the sp³region and two in the sp²re-

gion in 1:2:1:1 ratio). Furthermore, silanol 22 having a

three-membered ring was isolated from the reaction mix-

ture (the structure of 22 was determined by X-ray analysis,

Fig. 18). Thus, the product of the photoirradiation of sila-

benzene 1a is not Dewar silabenzene derivative 21 but

silabenzvalene 20 (Scheme IX). No other isomer was ob-

served.³⁴

Photochemical Reaction

It is well established that photochemical reaction of

benzene affords interesting isomers, such as Dewar ben-

zene or benzvalene. Although the photochemical isome-

rization of the parent silabenzene to Dewar silabenzene has

already been studied in an argon matrix, the characteriza-

tion of the Dewar silabenzene was based only on the Si–H

stretching frequency in its IR spectrum which shifted from

that of Si(sp²)–H to that of Si(sp³)–H (Scheme VIII).^3e

Scheme IX

hn

No

Si Tbt

21

290–350 nm

1a (d_Si93.6)

20 (d_Si–71.6)

Scheme VIII

Tbt

Si

OH

moisture

22

Although no silabenzvalene derivative has been re-

ported to the best of our knowledge, a stable disilabenz-

valene bearing four trimethylsilyl and two phenyl groups

was reported by Ando et al.^{35 29}Si NMR chemical shift cor-

responding to the two Si atoms of the disilabenzvalene was

reported to be –61.9 ppm, which is very similar to that of

silabenzvalene 20. It was indicated by theoretical calcula-

Photoirradiation of the stable silabenzene 1a using a

solution filter which is transparent at 290–350 nm³²re-

sulted in the formation of a new compound, which showed

a ²⁹Si NMR signal of its central silicon atom at –71.6 ppm.

This signal cannot be assignable to that of Dewar silaben-

zene, since it appears in a high field region characteristic

Fig. 18. Molecular structure of 22(ORTEP, 50% prob -

ability). Hydrogen atoms, solvent molecules,

and one of the disordered 2-hydroxy-2-silabi-

cyclo[3.1.0]-hex-3-ene moieties and p-bis(tri-

methylsilyl)methyl groups of Tbt are omitted.

Fig. 17. Potential energy surface (kcal/mol) in the di-

merization reactions of silabenzenes 1a,b. Se-

lected bond lengths (Å) are shown. Calculated

at B3LYP/6-31G(d) level.

The Chemistry of Silabenzenes

J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 497

completely. The high aromaticity and reactivity of silaben-

zenes here disclosed do not conflict with each other but

rather indicate an intrinsic character of silaaromatics.

SiH

78.0

0.0

42.7

72.1

48.0

not

minmum

EXPERIMENTAL SECTION

Fig. 19. Calculated relative energies of silabenzene

isomers (B3LYP/6-31G(d), kcal/mol).

General Procedure

All experiments were performed under an argon at-

mosphere unless otherwise noted. Solvents were dried by

1

tions that the energy difference between the parent disila-

benzvalene and its Dewar disilabenzene isomer should be

small. In contrast, previous theoretical studies on silaben-

zene isomers were concentrated only on Dewar silaben-

zene and the possibility of silabenzvalene was not taken

into consideration.³⁶Although high-level calculations in-

cluding a silaprismane isomer were recently reported, they

still neglected a silabenzvalene isomer.³⁷We performed,

therefore, B3LYP/6-31G(d) calculations on all the con-

ceivable silabenzene isomers. The possibility of the forma-

tion of silabenzvalene was supported by a small energy dif-

ference between Dewar silabenzene and silabenzvalene

(Fig. 19), though further investigation on the transition

states of the photochemical reaction is required to reveal

the origin of the selectivity.

standard methods and freshly distilled prior to use. H

NMR (500, 400, or 300 MHz), ¹³C NMR (126, 101, or 76

MHz), and ²⁹Si NMR (99, 68, or 59 MHz) spectra were

measured in CDCl₃or C₆D₆with a JEOL JNM-A500, JNM-

EX400 or JNM-AL300 spectrometer. Unless otherwise

noted, the spectra were measured at room temperature. In

¹H NMR spectra, signals due to CHCl₃(7.25 ppm) and

C₆D₅H (7.15 ppm) were used as references, and those due

to CDCl₃(77 ppm) and C₆D₆(128 ppm) were used in ¹³

C

NMR spectra. ²⁹Si NMR was measured with NNE or IN-

EPT techniques using TMS as an external standard. Multi-

plicity of signals in ¹³C NMR spectra was determined by

DEPT technique. High- and low-resolution mass spectral

data were obtained on a JEOL JMS-SX102 and Shimadzu

QP-5050A GC/MS spectrometer, respectively. SCC (short

column chromatography) and FCC (flush column chroma-

tography) were performed on Wakogel C-200 and Merck

Silica Gel 60, respectively. GPLC (gel permeation liquid

chromatography) was performed on an LC-908 (Japan An-

alytical Industry Co., Ltd.) equipped with JAIGEL 1H and

2H columns (eluent: chloroform or toluene). All melting

points were determined on a Yanaco micro-melting point

apparatus and were uncorrected. Elemental analyses were

carried out at the Microanalytical Laboratory of the Depart-

ment of Chemistry, Faculty of Science, The University of

Tokyo. 1-Bromo-2,4,6-tris[bis(trimethylsilyl)methyl]ben-

zene (TbtBr),⁹1-bromo-2,6-bis[bis(trimethylsilyl)meth-

yl]-4-[tris(trimethylsilyl)methyl]benzene (BbtBr),³⁸1,1-

dibutyl-1-stannacyclohexa-2,5-diene (2),³⁹Tbt-substituted

trihydrosilane (3a)⁴⁰were prepared according to the re-

ported procedures. Calculations were carried out with the

Gaussian 98 program.⁴¹

The use of stable silabenzene 1a as a substrate en-

abled us to perform the detailed investigation on the photo-

chemical reaction product of this unique class of ring sys-

tems, leading to the evidence for the photochemical va-

lence isomerization of silabenzene into silabenzvalene. In

view of the results here obtained, the previous report on the

formation of Dewar silabenzene^3eshould be reinvestigated

since the possibility of the formation of silabenzvalene was

disregarded. It should be noted, however, that the effect of

the large substituent of 1a might not be negligible.

CONCLUSION

The stable silabenzenes were successfully synthe-

sized by taking advantage of extremely bulky Tbt or Bbt

groups. The synthesis of these fundamental silaaromatic

compounds enabled us to compare carbon and silicon ana-

logues in aromatic systems, and we found the structure of

the silabenzene here revealed is very similar to that of ben-

zene. Although the structural features and spectroscopic

data indicate a similarity between the carbon and silicon

analogues, the reactivities of aromatic and silaaromatic

compounds are very different from each other. Even in the

case substituted by an extremely bulky Tbt group, it is not

sufficient to suppress the dimerization of the silabenzene

Preparation of {2,6-Bis[bis(trimethylsilyl)methyl]-4-

[tris(trimethylsilyl)methyl]phenyl}silane (3b)

A solution of BbtBr (12.0 g, 17.0 mmol) in THF (160

mL) was cooled to –78 °C, and t-BuLi (1.67 M in pentane,

25.5 mL, 42.6 mmol) was added dropwise to the solution.

After the solution was stirred for 30 min, SiF₄gas was bub-

bled into the solution until the color of the solution had al-

498 J. Chin. Chem. Soc., Vol. 55, No. 3, 2008

Tokitoh et al.

most disappeared. After the mixture was warmed to room

temperature, the solvent was evaporated. Hexane (500 mL)

was added to the residue and filtered using Celite^®. Re-

moval of the solvent afforded a crude product containing

BbtSiF₃, which was dissolved in THF (140 mL) and treated

with LiAlH₄(1.94 g, 51.1 mmol). The mixture was refluxed

for 12 h and quenched by the successive addition of AcOEt

(40 mL), H₂O (50 mL), and 2 M HCl (80 mL) at 0 °C. After

separation of the organic layer, the aqueous layer was ex-

tracted with hexane (100 mL, twice). The organic layers

were combined and washed with aq. NaHCO₃(100 mL,

twice) and H₂O (100 mL). After the organic layers were

dried over MgSO₄, the solvent was evaporated and the resi-

due was reprecipitated from CH₂Cl₂/EtOH to afforded 3b

(7.01 g, 10.7 mmol, 63%). 3b: colorless powder (from

CH₂Cl₂/EtOH), mp. 165.5–170.0 °C. ¹H NMR (300 MHz,

CDCl₃) d 0.05 (s, 36H), 0.24 (s, 27H), 2.15 (s, 2H), 4.23 (s,

3H), 6.73 (s, 2H). ¹³C NMR (76 MHz, CDCl₃) d 0.98 (q),

5.33 (q), 22.27 (s), 31.56 (d), 122.75 (s), 126.01 (d), 146.45

(s), 151.50 (s). ²⁹Si NMR (60 MHz, CDCl₃) d –76.16, 0.64,

1.60. Anal. Calcd for C₃₀H₇₀Si₈·H₂O: C, 53.49; H, 10.77%.

Found C, 53.70; H, 10.34%. LRMS (DI): m/z found 654

[M⁺], calcd for C₃₀H₇₀Si₈654.

(s). ²⁹Si NMR (99 MHz, CDCl₃) d –64.29, 1.59, 1.87.

HRMS (FAB): m/z calcd for C₄₄H₉₅Si₇Sn 939.4841, found

939.4813 ([M+H]⁺). Anal. Calcd for C₄₄H₉₄Si₇Sn: C,

56.30; H, 10.09. Found C, 56.00; H, 9.80.

Preparation of Dihydro{2,6-bis[bis(trimethylsilyl)-

methyl]-4-[tris(trimethylsilyl)methyl]phenyl}(5-tribu-

tylstannylpenta-1,4-dienyl)silane (4b)

Compound 4b was synthesized with the same proce-

dure as that for 4a. The use of 2 (2.53 g, 8.46 mmol), n-

BuLi (1.66 M, 5.21 mL, 8.65 mmol) and 3b (5.55 g, 8.47

mmol) afforded 4b (5.03 g, 4.98 mmol, 59%). 4b: colorless

1

powder (from CH₂Cl₂/CH₃CN), mp. 72.0–75.2 °C. H

NMR (300 MHz, CDCl₃) d 0.04 (s, 36H), 0.25 (s, 27H),

0.88–1.05 (m, 15H), 1.25–1.37 (m, 6H), 1.46–1.59 (m,

6H), 2.16 (s, 2H), 3.06 (t, ³J_HH= 7.2 Hz, 2H), 4.81 (d, ³J_HH

= 3.9 Hz, 1H), 5.60 (dt, ³J_HH= 13.6 Hz, ³J_HH= 4.1 Hz, 1H),

5.93 (d, ³J_HH= 12.5 Hz, 1H), 6.36–6.53 (m, 2H), 6.72 (s,

2H). ¹³C NMR (76 MHz, r.t., CDCl₃) d 1.20 (q), 5.35 (q),

10.28 (t), 13.70 (q), 22.23 (s), 27.31 (t), 29.22 (t), 31.08 (d),

40.62 (t), 122.77 (d), 125.87 (s), 125.94 (d), 130.25 (d),

145.66 (d), 146.35 (s), 148.88 (d), 151.46 (s). ²⁹Si NMR

(60 MHz, r.t., CDCl₃) d –63.32, 0.64, 1.60. Anal. Calcd for

C₄₇H₁₀₂Si₈Sn: C, 55.85; H, 10.17. Found C, 55.76; H,

10.24.

Preparation of Dihydro(5-tributylstannylpenta-1,4-

dienyl){2,4,6-tris[bis(trimethylsilyl)methyl]phenyl}sil-

ane (4a)

Preparation of a Mixture of Silacyclohexadienes 5a

and 6a

To a solution of 2 (720 mg, 2.41 mmol) in Et₂O (15

mL) was added n-BuLi (1.65 M in hexane, 1.53 mL, 2.52

mmol) at –50 °C. After the solution was stirred for 2 h, a so-

lution of 3a (1.40 g, 2.41 mmol) in Et₂O (12 mL) was added

to the reaction mixture. The solution was warmed to room

temperature and stirred for 2 d. After evaporation of the

solvent, removal of inorganic salts by SCC (hexane) fol-

lowed by purification with GPLC (chloroform) and repre-

cipitation from CH₂Cl₂/CH₃CN afforded 4a (1.41 g, 1.50

mmol, 60%). 4a: colorless powder (from CH₂Cl₂/CH₃CN),

mp. 64.9–67.6 °C. ¹H NMR (500 MHz, CDCl₃) d 0.02 (s,

36H), 0.03 (s, 18H), 0.88 (t, ³J_HH= 7.3 Hz, 9H), 0.92–0.95

(m, 6H), 1.27–1.35 (m, 7H), 1.47–1.53 (m, 6H), 2.06 (s,

2H), 3.05 (t, ³J_HH= 7.1 Hz, 2H), 4.78 (d, ³J_HH= 4.0 Hz, 1H),

To a solution of 4a (8.32 g, 8.87 mmol) in THF (150

mL) was added n-BuLi (1.67 M in hexane, 10.9 mL, 18.2

mmol) at –78 °C. After the solution was stirred at –78 °C

for 2 h, the reaction was quenched by the addition of 2 M

HCl (5 mL). Warming the mixture to room temperature fol-

lowed by purification with SCC (hexane), GPLC (chloro-

form), and reprecipitation from CH₂Cl₂/CH₃CN afforded a

mixture of 1-{2,4,6-tris[bis(trimethylsilyl)methyl]phen-

yl}-1,1-dihydro-1-silacyclohexa-2,4-diene (5a) and 1-

{2,4,6-tris[bis(trimethylsilyl)methyl]phenyl}-1,1-di-

hydro-1-silacyclohexa-2,5-diene (6a) (4.52 g, 6.98 mmol,

79%, 5a:6a = 1:0.13). 5a: colorless powder (from CH₂Cl₂/

CH₃CN). ¹H NMR (500 MHz, CDCl₃) d 0.01 (s, 36H), 0.03

(s, 18H), 1.30 (s, 1H), 1.69 (m, 1H), 1.88 (m, 1H), 2.22 (br

s, 1H), 2.24 (br s, 1H), 5.05 (t, ³J_HH= 6.3 Hz, 1H), 5.92 (m,

1H), 6.02 (m, 1H), 6.06 (d, ³J_HH= 13.7 Hz, 1H), 6.25 (br s,

1H), 6.37 (br s, 1H), 6.72 (dd, ³J_HH= 13.7 Hz, ³J_HH= 6.0

Hz, 1H). ¹³C NMR (126 MHz, CDCl₃) d 0.60 (q), 0.71 (q),

0.95 (q), 1.04 (q), 11.56 (t), 28.52 (d), 28.83 (d), 30.44 (d),

121.67 (d), 125.38 (d), 126.09 (d+s), 128.71 (d), 141.11

(d), 144.71 (s), 151.59 (s), 151.87 (s). ²⁹Si NMR (99 MHz,

5.59 (dt, ³J_HH= 13.8 Hz, ³J_HH= 4.3 Hz, 1H), 5.91 (d, ³J_HH

=

12.2 Hz, 1H), 6.27 (br s, 1H), 6.35–6.40 (m, 2H), 6.48 (dt,

³J_HH= 12.2 Hz, ³J_HH= 7.0 Hz, 1H). ¹³C NMR (126 MHz,

CDCl₃) d 0.54 (q), 0.68 (q), 0.83 (q), 10.27 (t), 13.70 (q),

27.32 (t), 29.22 (t), 29.74 (d), 30.07 (d), 30.48 (d), 40.49

(t), 121.39 (d), 123.03 (s), 123.29 (d), 126.20 (d), 130.12

(d), 144.77 (s), 145.77 (d), 148.32 (d), 151.57 (s), 151.77

The Chemistry of Silabenzenes

J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 499

CDCl₃) d –45.85, 1.67, 1.78, 1.91. HRMS (EI): m/z calcd

for C₃₂H₆₆Si₇646.3550, found 646.3555 ([M]⁺). The spec-

tral data of 6a were not collected since this compound was

obtained only as a mixture with 5a.

was transferred into a 5 mm NMR tube, and it was degassed

and sealed. Measurement of ²⁹Si NMR spectrum showed a

signal at 92.51 ppm which is assignable to that of silaben-

zene 1a. The yield of 1a was estimated to be 30% by the ¹H

NMR spectrum.

Preparation of a Mixture of Silacyclohexadienes 5b

and 6b

Preparation of 1-Hydroxy-1-{2,4,6-tris[bis(trimethyl-

silyl)methyl]phenyl}-1-silacylohexa-2,4-diene (9a)

A mixture of silacyclohexadienes 5a and 6a (300 mg,

0.463 mmol) and NBS (86.6 mg, 0.487 mmol) in CCl₄(40

mL) was stirred vigorously at 5 °C in a 50 mL round-bot-

tomed flask equipped with a CaCl₂tube for 1 week. Re-

moval of the solvent afforded a crude mixture containing

bromosilanes 7a and 8a, which was dissolved in Et₂O (15

mL) and then treated with saturated aq. NaHCO₃(2 mL).

After the mixture was stirred for 2 h, the solvent and water

were removed under vacuum. Purification by SCC (CHCl₃)

and FCC (hexane/CHCl₃, 5:1) afforded pure 9a (205 mg,

0.309 mmol, 67%). 9a: colorless powder (from CH₂Cl₂/

CH₃CN), mp. 181.8–185.0 °C. ¹H NMR (400 MHz, CDCl₃)

d 0.02 (s, 9H), 0.00 (s, 9H), 0.03 (s, 27H), 0.05 (s, 9H), 1.30

(s, 1H), 1.57 (br s, 1H), 1.82–1.85 (m, 2H), 2.47 (s, 2H),

Silacyclohexadienes 5b and 6b were prepared by the

same method as that for Tbt-substituted 5a and 6a. The use

of 4b (4.50 g, 4.45 mmol) and n-BuLi (1.66 M, 5.56 mL,

9.23 mmol) afforded 1-{2,6-bis[bis(trimethylsilyl)meth-

yl]-4-[tris(trimethylsilyl)methyl]phenyl}-1-silacyclohexa-

2,4-diene (5b) and 1-{2,6-bis[bis(trimethylsilyl)meth-

yl]-4-[tris(trimethylsilyl)methyl]}-1-silacyclohexa-2,5-

diene (6b) (2.68 g, 3.72 mmol, 83%, 5b:6b = 1:0.13). 5b:

1

colorless powder (from CH₂Cl₂/CH₃CN). H NMR (300

MHz, CDCl₃) d 0.03 (s, 18H), 0.06 (s, 18H), 0.24 (s, 27H),

1.70 (m, 1H), 1.91 (m, 1H), 2.36 (s, 2H), 5.07 (t, ³J_HH= 6.5

Hz, 1H), 5.92 (m, 1H), 6.04 (m, 1H), 6.10 (d, ³J_HH= 14.3

Hz, 1H), 6.68 (s, 2H), 6.73 (dd, ³J_HH= 14.3 Hz, ³J_HH= 7.5

Hz, 1H). ¹³C NMR (76 MHz, CDCl₃) d 1.26 (q), 1.44 (q),

5.39 (q), 11.65 (t), 22.14 (s), 29.69 (d), 125.64 (d), 126.13

(d), 126.30 (d), 128.54 (s), 128.67 (d), 140.89 (d), 146.42

(s), 151.47 (s). ²⁹Si NMR (60 MHz, CDCl₃) d –44.61, 0.65,

1.50, 1.58. The spectral data of 6b were not collected since

this compound was obtained only as a mixture with 5b.

Bromination of Silacyclohexadienes 5a and 6a

5.95–5.99 (m, 1H), 6.08–6.13 (m, 1H), 6.23 (br s, 1H), 6.25

3

(d, ³J_HH= 14.2 Hz, 1H), 6.35 (br s, 1H), 6.75 (dd, J_HH

=

14.2 Hz, ³J_HH= 5.9 Hz, 1H). ¹³C NMR (101 MHz, CDCl₃)

d 0.65 (q), 0.74 (q), 1.00 (q), 1.12 (q), 19.85 (t), 27.92 (d),

28.19 (d), 30.33 (d), 122.03 (d), 125.46 (d), 126.82 (s),

127.02 (d), 129.89 (d), 130.47 (d), 140.08 (d), 144.74 (s),

151.30 (s), 151.71 (s). ²⁹Si NMR (79 MHz, CDCl₃) d 1.43,

1.57, 1.77, 2.14. HRMS (EI): m/z calcd for C₃₂H₆₆OSi₇

662.6499, found 662.3487 ([M]⁺). Anal. Calcd for

C₃₂H₆₆OSi₇·0.5H₂O: C, 57.15; H, 10.04. Found C, 56.86;

H, 9.66.

Silacyclohexadienes 5a and 6a (50 mg, 0.077 mmol)

and NBS (14.2 mg, 0.0798 mmol) in CCl₄(10 mL) were

stirred vigorously with a magnetic stirrer at 0 °C in a 50 mL

round-bottomed flask equipped with a CaCl₂tube. After

the disappearance of the starting materials (confirmed by

TLC, after ca. 5 days), the solvent was removed and hexane

(5 mL) was added to the residue. Removal of the succini-

mide by filtration followed by removal of the solvent af-

forded a crude mixture containing 1-bromo-1-{2,4,6-tris-

[bis(trimethylsilyl)methyl]phenyl}-1-silacyclohexa-2,4-

diene (7a) and 1-bromo-1-{2,4,6-tris[bis(trimethylsilyl)-

methyl]phenyl}-1-silacyclohexa-2,5-diene (8a), though

the total content of 7a and 8a was approx. 60% (estimated

by ¹H NMR). Spectral data of 7a and 8a were not obtained

since 7a and 8a decomposed under the purification condi-

tions.

Preparation of 1-Hydroxy-1-{2,6-bis[bis(trimethyl-

silyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl}-1-

silacyclohexa-2,4-diene (9b)

Silanol 9b was prepared by the same method as that

for Tbt-substituted 9a. The use of a mixture of 5b and 6b

(300 mg, 0.417 mmol) and NBS (81.6 mg, 0.459 mmol) af-

forded 9b (135 mg, 0.183 mmol, 44%). 9b: colorless pow-

der (from CH₂Cl₂/CH₃CN), mp. 166.1–172.5 °C (dec). ¹H

NMR (300 MHz, CDCl₃) d 0.01 (s, 18H), 0.09 (s, 18H),

0.25 (s, 27H), 1.67 (br s, 1H), 1.76–1.95 (m, 2H), 2.62 (s,

Reaction of Bromosilanes 7a and 8a with t-BuLi

The crude mixture of bromosilanes 7a and 8a ob-

tained in the previous experiment was dissolved in cyclo-

hexane-d₁₂(0.6 mL) and t-BuLi (0.355 M in hexane, 0.20

mL, 0.071 mmol) was added to the solution. The mixture

2H), 5.95–6.01 (m, 1H), 6.08–6.14 (m, 1H), 6.29 (d, ³J_HH

14.1 Hz, 1H), 6.67 (s, 2H), 6.75 (dd, ³J_HH= 14.1 Hz, ³J_HH

5.9 Hz, 1H). ¹³C NMR (76 MHz, CDCl₃) d 1.39 (q), 1.56

(q), 5.45 (q), 19.91 (t), 22.05 (s), 28.85 (d), 125.49 (d),

126.78 (d), 129.50 (s), 130.05 (d), 130.43 (d), 139.85 (d),

=

500 J. Chin. Chem. Soc., Vol. 55, No. 3, 2008

Tokitoh et al.

146.49 (s), 151.28 (s). ²⁹Si NMR (60 MHz, CDCl₃) d

–13.07, 0.67, 2.04. Anal. Calcd for C₃₅H₇₄OSi₈: C, 57.14;

H, 10.14. Found C, 56.84; H, 10.12.

added to this solution. The solvent was removed by slow

evaporation with standing for 12 h. C₆D₆(0.7 mL) was

added to the residue, and the solution was transferred into a

5 mm NMR tube. After the tube was degassed and sealed, a

signal due to silabenzene 1a was observed at 93.64 ppm in

²⁹Si NMR. The tube was opened in a glovebox and benzo-

phenone (18 mg, 0.099 mmol) was added to the mixture.

After removal of the solvent, separation of the mixture by

SCC (CHCl₃), GPLC (CHCl₃) and FCC (hexane/CHCl₃=

10:1) afforded 5-tert-butyl-1-{2,4,6-tris[bis(trimethylsi-

lyl)methyl]phenyl}-1-silacyclohexa-2,4-diene (11) (9.1

mg, 0.013 mmol, 27%) and 8,8-diphenyl-1-{2,4,6-tris-

[bis(trimethylsilyl)methyl]phenyl}-7-oxa-1-silabicyclo-

[2,2,2]octa-2,5-diene (12) (7.9 mg, 0.0095 mmol, 20%).

11: colorless powder (from CH₂Cl₂/CH₃CN), mp. 175.0–

Chlorination of Silanol 9a

A mixture of silanol 9a (205 mg, 0.309 mmol) and

PCl₅(643 mg, 3.09 mmol) in Et₂O (25 mL) was refluxed

for 15 h. After the removal of the solvent, hexane (40 mL)

was added to the residue and filtered with Celite^®. Removal

of the solvent from the filtrate followed by purification us-

ing GPLC (toluene) afforded almost pure 1-chloro-1-

{2,4,6-tris[bis(trimethylsilyl)methyl]phenyl}-1-silacyclo-

hexa-2,4-diene (10a) (183 mg, 0.268 mmol, 89%). 10a:

white powder. ¹H NMR (300 MHz, C₆D₆) d 0.15 (s, 18H),

0.20 (s, 18H), 0.21 (s, 18H), 1.46 (s, 1H), 2.18–2.34 (m,

2H), 2.62 (s, 2H), 5.80 (m, 1H), 5.91 (m, 1H), 6.41–6.51

(m, 3H), 6.61 (br s, 1H). ¹³C NMR (76 MHz, C₆D₆) d 0.90

(q), 0.95 (q), 1.01 (q), 1.75 (q), 21.76 (t), 29.13 (d), 29.38

(d), 31.04 (d), 122.79 (d), 125.17 (s), 125.95 (d), 127.80

(d), 128.75 (d), 129.17 (d), 139.42 (d), 146.43 (s), 152.43

(s), 152.96 (s). LRMS (DI): m/z found 680 ([M]⁺), calcd for

C₃₂H₆₅ClSi₇680. Chlorosilane 9a was used without further

purifications due to its gradual decomposition under ambi-

ent conditions.

1

181.5 °C (dec.). H NMR (400 MHz, CDCl₃) d –0.01 (s,

18H), 0.03 (s, 36H), 1.08 (s, 9H), 1.29 (s, 1H), 1.58–1.66

(m, 1H), 1.73 (dd, ²J_HH= 18.1 Hz, ³J_HH= 5.4 Hz, 1H), 2.32

(br s, 1H), 2.34 (br s, 1H), 5.01–5.05 (m, 1H), 5.83 (dd,

³J_HH= 6.3 Hz, ⁴J_HH= 2.0 Hz, 1H), 5.93 (d, ³J_HH= 14.2 Hz,

1H), 6.24 (br s, 1H), 6.36 (br s, 1H), 6.74 (dd, ³J_HH= 14.1

Hz, ³J_HH= 6.3 Hz, 1H). ¹³C NMR (101 MHz, CDCl₃) d 0.50

(s), 0.68 (s), 0.85 (s), 1.07 (s), 11.66 (t), 28.72 (d), 29.03

(q), 30.37 (d), 37.46 (s), 117.62 (d), 121.66 (d), 122.65 (d),

125.30 (s), 126.52 (d), 141.85 (d), 144.65 (s), 150.32 (s),

151.71 (s), 151.94 (s). ²⁹Si NMR (79 MHz, CDCl₃) d

–38.47, 1.75, 1.85. HRMS (EI): m/z calcd for C₃₆H₇₄Si₇

702.4176, found 702.4173 ([M]⁺). Anal. Calcd for C₃₆H₇₄Si₇:

C, 61.46; H, 10.06. Found C, 61.03; H, 10.46. 12: white

powder (from CH₂Cl₂/CH₃CN), mp. 196.0–199.0 °C (dec).

¹H NMR (500 MHz, CDCl₃) d 0.07 (s, 36H), 0.08 (s, 18H),

1.39 (s, 1H), 2.44 (s, 1H), 2.95 (s, 1H), 4.80 (tt, ³J_HH= 7.0

Chlorination of Silanol 9b

Chlorination of silanol 9b was performed by the same

procedure as that for Tbt-substituted silanol 9a. The use of

9b (92.4 mg, 0.126 mmol) and PCl₅(131 mg, 0.629 mmol)

afforded 1-chloro-1-{2,6-bis[bis(trimethylsilyl)methyl]-

4-[tris(trimethylsilyl)methyl]phenyl}-1-silacyclohexa-

2,4-diene (10b) (82.4 mg, 0.109 mmol, 87%). 10b: color-

less powder. ¹H NMR (300 MHz, CDCl₃) d –0.06 (s, 18H),

0.11 (s, 18H), 0.25 (s, 27H), 2.09–2.28 (m, 2H), 2.52 (s,

3

4

3

2H), 5.99 (m, 1H), 6.08 (m, 1H), 6.12 (d, J_HH= 5.1 Hz,

Hz, J_HH= 1.3 Hz, 1H), 6.39 (br s, 1H), 6.45 (dd, J_HH=

1H), 6.68 (m, 1H), 6.70 (s, 2H). ¹³C NMR (76 MHz,

CDCl₃) d 1.55 (q), 1.65 (q), 5.48 (q), 21.37 (t), 22.39 (s),

29.55 (d), 125.55 (d), 127.13 (d), 127.19 (s), 128.68 (d),

128.96 (d), 138.68 (d), 147.97 (s), 151.81 (s). ²⁹Si NMR

(60 MHz, CDCl₃) d –0.79, 0.81, 1.63, 2.40. LRMS (DI):

m/z found 752 ([M]⁺), calcd for C₃₅H₇₃ClSi₈. Chlorosilane

9b was used without further purifications due to its gradual

decomposition under ambient conditions.

11.9 Hz, ⁴J_HH= 1.3 Hz, 2H), 6.47 (br s, 1H), 6.99 (dd, ³J_HH

= 11.9 Hz, ³J_HH= 7.0 Hz, 2H), 7.10 (t, ³J_HH= 7.3 Hz, 2H),

7.20 (t, ³J_HH= 7.3 Hz, 4H), 7.50 (d, ³J_HH= 8.2 Hz, 4H). ¹³

C

NMR (126 MHz, CDCl₃) d 0.75 (q), 0.81 (q), 1.17 (q),

27.31 (d), 27.82 (d), 30.76 (d), 48.21 (d), 83.60 (s), 119.72

(s), 122.11 (d), 126.70 (d), 126.80 (d), 127.64 (d), 136.12

(d), 145.96 (s), 147.21 (s), 147.62 (d), 152.43 (s), 152.68

(s) (one of the m-proton of Tbt group was not observed

probably due to an overlapping with other peaks). ²⁹Si

NMR (99 MHz, CDCl₃) d –29.79, 1.85, 2.07. HRMS (FAB):

m/z calcd for C₄₅H₇₅OSi₇827.4203, found 827.4227

([M+H]⁺). Anal. Calcd for C₄₅H₇₄OSi₇·0.5H₂O: C, 64.59;

H, 9.03. Found C, 64.73; H, 8.83.

Synthesis and Trapping of Silabenzene 1a from

Chlorosilane 10a and t-BuLi

In a glovebox filled with argon, chlorosilane 10a

(33.2 mg, 0.0487 mmol) was dissolved in cyclohexane (2

mL), and t-BuLi (0.589 M, 80.0 mL, 0.0471 mmol) was

The Chemistry of Silabenzenes

J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 501

Synthesis of Silabenzene 1a from Chlorosilane 10a

and LDA

luene) afforded 7-phenyl-1-{2,4,6-tris[bis(trimethylsilyl)-

methyl]phenyl}-1-silabicyclo[2.2.2]octa-2,5-diene (13)

(11.6 mg, 0.0155 mmol, 29%). 13: colorless powder (from

CH₂Cl₂/CH₃CN), mp. 147.8–150.0 °C (dec). ¹H NMR (300

MHz, CDCl₃) d 0.03 (s, 36H), 0.06 (s, 18H), 0.98 (dd, ²J_HH

In a glovebox filled with argon, chlorosilane 10a (144

mg, 0.212 mmol) was dissolved in hexane (5 mL), and

LDA (2.0 M in heptane/THF/ethylbenzene, 0.123 mL,

0.246 mmol, Aldrich Chemicals Co.) was added to the so-

lution. The solvent was removed under reduced pressure,

and hexane (5 mL) was added to the residue. The mixture

was left for 2 days and the precipitates were removed by

decantation. Removal of the solvent afforded almost pure

1-{2,4,6-tris[bis(trimethylsilyl)methyl]phenyl}-1-silaben-

zene (1a) (136 mg, 0.211 mmol, 96%). 1a: colorless pow-

= 13.4 Hz, ³J_HH= 5.0 Hz, 1H), 1.35 (s, 1H), 1.43 (dd, ²J_HH

=

13.4 Hz, ³J_HH= 10.0 Hz, 1H), 2.20 (br s, 1H), 2.53 (br s,

1H), 2.98 (m, 1H), 3.70 (m, 1H), 6.31 (br s, 1H), 6.40 (d,

3

³J_HH= 11.9 Hz, 1H), 6.44 (br s, 1H), 6.60 (d, J_HH= 12.1

3

Hz, 1H), 6.81 (dd, J_HH= 12.1 Hz, J_HH= 6.6 Hz, 1H),

7.14–7.27 (m, 6H). ¹³C NMR (76 MHz, CDCl₃) d 0.76 (q),

1.10 (q), 20.26 (t), 28.53 (d), 28.77 (d), 30.49 (d), 43.50

(d), 46.73 (d), 121.89 (d), 122.96 (s), 125.77 (d), 126.83

(d), 127.79 (d), 128.07 (d), 133.04 (d), 135.78 (d), 145.17

(s), 145.82 (d), 147.95 (s), 150.16 (d), 152.21 (s), 152.66

(s). ²⁹Si NMR (60 MHz, CDCl₃) d –37.44, 1.83, 1.96. Anal.

Calcd for C₄₀H₇₂Si₇: C, 64.09; H, 9.68. Found C, 63.86; H,

9.38.

1

der (from hexane), mp. 125.1–140.0 °C (dec). H NMR

(400 MHz, C₆D₆) d 0.13 (s, 36H), 0.15 (s, 18H), 1.50 (s,

1H), 2.55 (br s, 1H), 2.66 (br s, 1H), 6.58 (br s, 1H), 6.71 (t,

³J_HH= 8.3 Hz, 1H), 6.71 (br s, 1H), 7.11 (d, ³J_HH= 12.2 Hz,

2H), 8.00 (dd, ³J_HH= 12.2 Hz, ³J_HH= 8.3 Hz, 2H). ¹³C NMR

(101 MHz, C₆D₆) d 0.61 (q), 0.81 (q), 0.97 (q), 31.38 (d),

36.62 (d), 37.20 (d), 116.08 (d), 121.42 (d), 122.15 (d),

125.95 (d), 126.24 (s), 143.55 (d), 147.75 (s), 152.12 (s),

152.16 (s). ²⁹Si NMR (79 MHz, C₆D₆) d 2.19, 2.54, 93.64.

Anal. Calcd for C₃₂H₆₄Si₇: C, 59.55; H, 9.99. Found C,

58.48; H, 10.12.

Reaction of Silabenzene 1a with tert-Butyl Vinyl

Ether

In a glovebox filled with argon, 1a (26.7 mg, 0.0414

mmol) was dissolved in C₆D₆(0.7 mL) and the solution was

put into a 5 mm NMR tube. After tert-butyl vinyl ether

(0.03 mL) was added to the solution, the NMR tube was de-

gassed and sealed. Although the NMR signals due to 1a

were observed just after the mixing, they disappeared after

standing overnight. Then, the tube was opened and the sol-

vent was evaporated. The hexane solution of the residue

was passed through a column of Florisil^®and concentrated

to give a crude mixture, the separation of which by GPLC

(toluene) afforded 7-tert-butoxy-1-{2,4,6-tris[bis(trimeth-

ylsilyl)methyl]phenyl}-1-silabicyclo[2.2.2]octa-2,5-diene

(14) (12.2 mg, 0.0164 mmol, 40%). 14: colorless powder

Synthesis of Silabenzene 1b

Bbt-substituted silabenzene 1b was synthesized by

the same procedure as that for Tbt-substituted 1a. The use

of 10b (96.9 mg, 0.129 mmol) and LDA (2.0 M, 0.077 mL,

0.15 mmol) afforded almost pure 1-{2,6-bis[bis(trimethyl-

silyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl}-1-sila-

benzene (1b). 1b: colorless powder. mp. 127 °C (dec). ¹H

NMR (400 MHz, C₆D₆) d 0.15 (s, 36H), 0.35 (s, 27H), 2.79

(s, 2H), 6.69 (t, ³J_HH= 8.3 Hz, 1H), 7.03 (s, 2H), 7.03 (d,

³J_HH= 12.7 Hz, 2H), 7.98 (dd, ³J_HH= 12.7 Hz, ³J_HH= 8.3

Hz, 2H). ¹³C NMR (101 MHz, C₆D₆) d 1.17 (q), 5.43 (q),

23.14 (s), 38.94 (d), 115.91 (d), 121.25 (d), 125.88 (d),

129.15 (s), 143.55 (d), 149.31 (s), 151.72 (s). ²⁹Si NMR (79

MHz, C₆D₆) d 1.11, 2.12, 92.93.

1

(from CH₂Cl₂/CH₃CN), mp. 189.0-193.8 °C (dec). H

NMR (300 MHz, CDCl₃) d 0.02 (s, 36H), 0.05 (s, 18H),

0.64 (dd, ²J_HH= 13.0 Hz, ³J_HH= 2.9 Hz, 1H), 1.20 (s, 9H),

1.33 (s, 1H), 1.39 (dd, ²J_HH= 13.0 Hz, ³J_HH= 9.0 Hz, 1H),

2.10 (br s, 1H), 2.14 (br s, 1H), 3.75 (m, 1H), 3.81 (m, 1H),

6.27 (br s, 1H), 6.34 (d, ³J_HH= 11.9 Hz, 1H), 6.40 (br s, 1H),

6.57 (d, ³J_HH= 11.9 Hz, 1H), 6.95–7.02 (m, 2H). ¹³C NMR

(76 MHz, CDCl₃) d 0.72 (q), 0.76 (q), 1.00 (q), 1.09 (q),

24.34 (t), 28.25 (d), 28.51 (d), 28.66 (q), 30.46 (d), 48.07

(d), 70.26 (d), 73.74 (s), 121.82 (d), 122.68 (s), 126.71 (d),

134.70 (d), 135.32 (d), 145.15 (s), 145.21 (d), 147.15 (d),

152.25 (s), 125.63 (s). ²⁹Si NMR (60 MHz, CDCl₃) d

–39.12, 1.80, 1.91, 1.98. Anal. Calcd for C₃₈H₇₆OSi₇: C,

61.21; H, 10.27. Found C, 60.98; H, 10.02.

Reaction of Silabenzene 1a with Styrene

In a glovebox filled with argon, 1a (34.9 mg, 0.0541

mmol) was dissolved in C₆D₆(0.7 mL) and the solution was

put into a 5 mm NMR tube. After styrene (0.03 mL) was

added to the solution, the NMR tube was degassed and

sealed. The disappearance of 1a was confirmed by NMR

measurements. Then, the tube was opened and the solvent

was evaporated. A hexane solution of the residue was

passed through the column of Florisil^®and concentrated to

give a crude mixture, the separation of which by GPLC (to-

502 J. Chin. Chem. Soc., Vol. 55, No. 3, 2008

Tokitoh et al.

Reaction of Silabenzene 1a with Phenylacetylene

122.02 (d), 125.42 (s), 127.02 (d), 128.07 (d), 144.41 (s),

145.85 (d), 151.85 (s), 152.26 (s). ²⁹Si NMR (99 MHz,

CDCl₃) d –28.99, 1.67. HRMS (EI): m/z calcd for

C₃₃H₆₇DOSi₇677.3718, found 677.3734 ([M]⁺). Anal.

Calcd for C₃₃H₆₇DOSi₇: C, 58.42; H, 10.25. Found C, 58.12;

H, 9.96. 17: colorless powder (from CH₂Cl₂/CH₃CN), mp.

160.0–168.2 °C. ¹H NMR (300 MHz, CDCl₃) d –0.02 (s,

18H), 0.03 (s, 36H), 1.29 (s, 1H), 1.69 (br s, 1H), 2.53 (s,

In a glovebox filled with argon, 1a (27.7 mg, 0.0429

mmol) was dissolved in C₆D₆(0.7 mL) and the solution was

put into a 5 mm NMR tube. After phenylacetylene (0.024

mL) was added to the solution, the NMR tube was degassed

and sealed. The NMR signals of 1a were observed just after

the mixing, but they disappeared after standing overnight.

Then, the tube was opened and the solvent was evaporated.

The hexane solution of the residue was passed through a

column of Florisil^®and concentrated to give a crude mix-

ture, the separation of which by GPLC (toluene) afforded

3-phenyl-1-{2,4,6-tris[bis-(trimethylsilyl)methyl]phen-

yl}-1-silabicyclo[2.2.2]octa-2,5,7-triene (15) (6.5 mg,

0.0087 mmol, 20%). 15: colorless powder (from CH₂Cl₂/

CH₃CN), mp. 169.4–174.1 °C. ¹H NMR (300 MHz, CDCl₃)

d 0.05 (s, 36H), 0.08 (s, 18H), 1.38 (s, 1H), 2.35 (br s, 2H),

3

2H), 3.34 (s, 3H), 5.97 (m, 1H), 6.14 (d, J_HH= 14.0 Hz,

1H), 6.14 (m, 1H), 6.21 (br s, 1H), 6.33 (br s, 1H), 6.86 (dd,

³J_HH= 14.0, 6.1 Hz, 1H). ¹³C NMR (60 MHz, CDCl₃) d 0.65

(q), 0.72 (q), 0.75 (q), 1.00 (q), 1.14 (q), 17.02 (dt, ¹J_CD

=

17.8 Hz), 27.70 (d), 27.92 (d), 30.30 (d), 49.63 (q), 122.07

(d), 125.66 (d), 126.30 (s), 127.04 (d), 127.80 (d), 130.88

(d), 141.27 (d), 144.57 (s), 151.57 (s), 151.98 (s). HRMS

(EI): m/z calcd for C₃₃H₆₇DOSi₇677.3718, found 677.3723

([M]⁺). Anal. Calcd for C₃₃H₆₇DOSi₇·0.5H₂O: C, 57.65; H,

10.26. Found C, 57.49; H, 9.61.

5.30 (m, 1H), 6.36 (br s, 1H), 6.49 (br s, 1H), 6.71 (d, ⁴J_HH

=

2.0 Hz, 1H), 6.81 (d, ³J_HH= 10.6 Hz, 1H), 7.15–7.43 (m,

5H), 7.49 (dd, ³J_HH= 10.6 Hz, ³J_HH= 6.9 Hz, 2H). ¹³C NMR

(76 MHz, CDCl₃) d 0.77 (q), 1.03 (q), 28.97 (d), 29.13 (d),

30.62 (d), 51.76 (d), 121.55 (s), 121.95 (d), 124.89 (d),

126.86 (d), 127.29 (d), 128.39 (d), 131.49 (d), 137.32 (d),

141.96 (s), 145.50 (s), 151.07 (d), 152.44 (s), 152.79 (s),

162.56 (s). ²⁹Si NMR (60 MHz, CDCl₃) d 1.49, 1.87, 2.06.

Anal. Calcd for C₄₀H₇₀Si₇·0.5H₂O: C, 63.50; H, 9.46.

Found C, 63.22; H, 9.16.

Reaction of Silabenzene 1a with Mesitonitrile Oxide

In a glovebox filled with argon, 1a (27.7 mg, 0.0429

mmol) and mesitonitrile oxide (20.8 mg, 0.129 mmol) were

dissolved in hexane (3 mL). Filtration of the mixture with

Celite^®followed by separation with GPLC (toluene) af-

forded 3-{2,4,6-tris[bis(trimethylsilyl)methyl]phenyl}-2-

aza-3a,7a-dihydro-1-oxa-7a-silaindene (18) (20.8 mg,

0.0258 mmol, 60%). 18: colorless powder (from CH₂Cl₂/

CH₃CN), mp. 192.9–201.2 °C (dec). ¹H NMR (300 MHz,

CDCl₃) d –0.04 (s, 18H), 0.05 (s, 9H), 0.06 (s, 9H), 0.08 (br

s, 18H), 1.36 (s, 1H), 1.85 (br s, 1H), 1.97 (br s, 1H), 2.14

(s, 6H), 2.27 (s, 3H), 3.35 (d, ³J_HH= 7.7 Hz, 1H), 5.84 (dd,

³J_HH= 10.2 Hz, ³J_HH= 7.7 Hz, 1H), 6.10 (dd, ³J_HH= 6.1 Hz,

³J_HH= 10.2 Hz, 1H), 6.23 (d, ³J_HH= 14.3 Hz, 1H), 6.35 (br

s, 1H), 6.45 (br s, 1H), 6.84 (s, 2H), 6.87 (dd, ³J_HH= 14.3

Hz, ³J_HH= 6.1 Hz, 1H). ¹³C NMR (76 MHz, CDCl₃) d 0.49

(q), 0.67 (q), 0.73 (q), 0.88 (q), 1.10 (q), 1.47 (q), 19.74 (q),

20.11 (q), 21.06 (q), 29.41 (d), 29.75 (d), 30.76 (d), 38.20

(d), 121.67 (d), 124.46 (d), 125.18 (d), 126.74 (d), 127.06

(d), 128.36 (d), 128.52 (d), 128.62 (d), 129.60 (d), 135.45

(s), 137.30 (s), 138.18 (s), 141.90 (d), 146.31 (s), 151.93

(s), 152.13 (s), 164.53 (s). ²⁹Si NMR (60 MHz, CDCl₃) d

1.63, 1.86, 2.79, 6.47. Anal. Calcd for C₄₂H₇₅ONSi₇·

0.5H₂O: C, 61.85; H, 9.39. Found C, 61.95; H, 8.98.

Dimerization of Silabenzene 1a

Reaction of Silabenzene 1a with MeOD

In a glovebox filled with argon, silabenzene 1a (23.2

mg, 0.0359 mmol) was dissolved in THF (3 mL), and

MeOD (0.03 mL) was added to the solution. The mixture

was stirred for 2 h and the solvent was evaporated. Separa-

tion of the mixture by FCC (hexane) afforded 1-methoxy-

5-deuterio-1-{2,4,6-tris[bis(trimethylsilyl)methyl]phen-

yl}-1-silacyclohexa-2,5-diene (16) (4.3 mg, 0.0063 mmol),

1-methoxy-4-deuterio-1-{2,4,6-tris[bis(trimethylsi-

lyl)methyl]phenyl}-1-silacyclohexa-2,4-diene (17) (9.4

mg, 0.014 mmol), and the mixture of 16 and 17. The total

yields of 16 and 17 were 49 and 29%, respectively. The D

content of both 16 and 17 was 65% as judged by ¹H NMR.

16: colorless powder (from CH₂Cl₂/CH₃CN), mp. 187.4-

1

189.0 °C. H NMR (500 MHz, CDCl₃) d –0.01 (s, 18H),

0.00 (s, 18H), 0.03 (s, 18H), 1.28 (s, 1H), 2.61 (br s, 1H),

2.62 (br s, 1H), 2.92 (br s, 1H), 3.29 (s, 3H), 6.10 (dd, ³J_HH

= 14.7 Hz, ⁴J_HH= 2.1 Hz, 1H), 6.20 (br s, 1H), 6.32 (br s,

1H), 6.83 (dd, ³J_HH= 14.7 Hz, ³J_HH= 3.1 Hz, 1H). ¹³C NMR

(126 MHz, CDCl₃) d 0.63 (q), 0.72 (q), 0.98 (q), 27.24 (d),

27.47 (d), 30.27 (d), 33.22 (dt, ¹J_CD= 18.6 Hz), 49.48 (q),

In a glovebox filled with argon, 1a (41.9 mg, 0.0649

mmol) was dissolved in C₆D₆(0.7 mL) and the solution was

put into a 5 mm NMR tube. After the tube was degassed

and sealed, the tube was stored at room temperature and the

The Chemistry of Silabenzenes

J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 503

reaction was monitored by ¹H NMR measurements. After 4

months about half of 1a was converted into dimer 19a. Af-

ter the removal of the solvent, hexane was added to the resi-

due and the mixture was filtered with Celite^®. Removal of

the solvent followed by separation with GPLC (toluene) af-

forded 1,7-bis{2,4,6-tris[bis(trimethylsilyl)methyl]phen-

yl}-1,7-disilatricyclo[3.2.2.0^1,6]dodeca-2,4,8,11-tetraene

(19a) (18.9 mg, 0.0146 mmol, 45%). 19a: colorless powder

tion was added benzophenone (21.9 mg, 0.120 mmol). Af-

ter evaporation of the solvent, hexane was added to the res-

idue, and the solution was filtered with Celite^®. Removal of

the solvent followed by separation of the residue with

GPLC (toluene) afforded 2-hydroxy-2-{2,4,6-tris[bis(tri-

methylsilyl)methyl]phenyl}-2-silabicyclo[3.1.0]hex-3-

ene (22) (8.1 mg, 0.012 mmol, 30%). 20 (complete data

could not be obtained since this compound was obtained

1

(from CH₂Cl₂/CH₃CN), mp. 152.1–154.4 °C (dec). H

only as a mixture with silabenzene 1a): H NMR (300

NMR (300 MHz, CDCl₃) d 0.016 (s, 18H), 0.031 (s, 18H),

0.037 (s, 9H), 0.043 (s, 18H), 0.07 (s, 27H), 0.10 (s, 9H),

0.14 (s, 9H), 1.25 (d, ³J_HH= 1.8 Hz, 1H), 1.29 (s, 1H), 1.34

(s, 1H), 2.04 (s, 1H), 2.13 (s, 1H), 2.33 (s, 1H), 2.55 (s, 1H),

4.14 (t, ³J_HH= 7.2 Hz, 1H), 5.54 (d, ³J_HH= 13.8 Hz, 1H),

5.69 (dd, ³J_HH= 10.3 Hz, ³J_HH= 5.9 Hz, 1H), 6.26–6.47 (m,

8H), 6.84 (dd, ³J_HH= 12.2 Hz, ³J_HH= 7.7 Hz, 1H), 7.47 (dd,

MHz, C₆D₆) d 2.24 (m, 1H), 2.50 (d, ³J_HH= 5.1 Hz, 2H),

5.82 (dd, ³J_HH= 9.3 Hz, ⁴J_HH= 2.0 Hz, 1H), 7.41 (dd, ³J_HH

=

9.3, 3.5 Hz, 1H). ¹³C NMR (126 MHz, C₆D₆) d 22.98 (d),

39.81 (d), 120.89 (d), 161.19 (d). ²⁹Si NMR (79 MHz,

C₆D₆) d –71.60. 22: colorless powder (from CH₂Cl₂/

CH₃CN), mp. 170.6–172.2 °C (dec). ¹H NMR (300 MHz,

CDCl₃) d 0.01 (br s, 18H), 0.04 (s, 18H), 0.05 (br s, 18H),

0.33 (m, 1H), 0.53 (m, 1H), 1.07 (m, 1H), 1.32 (s, 1H), 2.00

3

³J_HH= 12.7 Hz, J_HH= 6.8 Hz, 1H). ¹³C NMR (76 MHz,

CDCl₃) d 0.77 (q), 0.79 (q), 0.90 (q), 1.00 (q), 1.08 (q), 1.10

(q), 1.24 (q), 1.38 (q), 1.55 (q), 1.66 (q), 1.79 (q), 2.12 (q),

14.36 (d), 27.06 (d), 27.45 (d), 29.10 (d), 29.17 (d), 30.36

(d), 30.55 (d), 39.88 (d), 122.42 (d), 122.50 (d), 122.82 (d),

124.05 (d), 127.37 (d), 127.93 (d), 128.22 (d), 129.01 (s),

129.35 (d), 130.26 (d), 135.50 (d), 140.75 (d), 144.07 (s),

144.99 (s), 148.01 (d), 150.68 (d), 151.97 (s), 153.10 (s),

153.24 (s), 153.58 (s). ²⁹Si NMR (60 MHz, CDCl₃) d

–42.64, –24.12, 1.66, 1.72, 1.78, 1.84, 1.87, 1.95, 2.01,

2.08, 2.15, 2.23, 2.48. Anal. Calcd for C₆₄H₁₂₈Si₁₄·H₂O: C,

58.73; H, 10.01. Found C, 58.88; H, 9.62.

(m, 1H), 2.39 (br s, 1H), 2.44 (br s, 1H), 6.65 (d, ³J_HH= 9.6

3

Hz, 1H), 6.25 (br s, 1H), 6.38 (br s, 1H), 7.01 (dd, J_HH

=

9.6 Hz, ³J_HH= 2.6 Hz, 1H). ¹³C NMR (76 MHz, CDCl₃) d

0.49 (q), 0.69 (q), 0.75 (q), 1.09 (q), 2.27 (d), 20.52 (t),

24.12 (d), 28.12 (d), 28.38 (d), 30.43 (d), 121.85 (d),

126.76 (d), 127.41 (s), 128.94 (d), 144.99 (s), 150.88 (s),

151.30 (s), 153.53 (d). ²⁹Si NMR (60 MHz, CDCl₃) d 1.64,

1.72, 1.78, 2.04, 11.85. LRMS (DI): m/z found 662 ([M]⁺).

Anal. Calcd for C₃₂H₆₆OSi₇: C, 57.93; H, 10.03. Found C,

57.32; H, 9.60.

X-ray Crystallographic Analyses

Thermal Dissociation of Silabenzene Dimer 19a

Crystal data of 1a, 12–15, 18, 19a, and 22 are shown

in Table 3. Crystals of all compounds suitable for X-ray

analysis were obtained by slow evaporation of their solu-

tions, and the solvents are also described in Table 3. Only

crystals of 1a were grown in a glovebox. All crystals were

colorless. A crystal of 1a was mounted inside a glass capil-

lary, while the other crystals were mounted on glass fibers.

All intensity data were collected on a Rigaku R-AXIS

RAPID imaging plate area detector with graphite mono-

chromated MoKa radiation (l = 0.71069 Å) at –180 °C to

2q_max= 55º. Empirical absorption corrections (symmetry

related method) were performed with the ABSCOR pro-

gram.⁴²Equivalent reflections were merged. The structures

were solved by direct methods (SIR92 or SHELXS-97) and

refined by full-matrix least-squares procedures on F²for all

reflections (SHELXL-97).⁴³Phenyl or methyne hydrogens

of 1a were refined isotropically; all the other hydrogens

were placed using AFIX instructions. For 13, 14, 19a, and

22 some restraints as shown below were applied to refine

In a glovebox filled with argon, the solution of 19

(11.4 mg, 0.00883 mmol) in C₆D₆(0.7 mL) was put into a 5

mm NMR tube. After the tube was degassed and sealed, the

tube was heated at 80 °C for 9 h during which time the reac-

1

tion was monitored by H NMR measurements. Almost

quantitative conversion of 19 into silabenzene 1a was ob-

served after 9 h.

Photolysis of Silabenzene 1a

In a glovebox filled with argon, the solution of 1a

(25.9 mg, 0.0401 mmol) in C₆D₆(0.8 mL) was put into a 5

mm NMR tube. The tube was degassed, sealed, and irradi-

ated with a 400 W high pressure Hg lamp through a filter

solution which is transparent between 290 and 350 nm (2

M NiSO₄, 1 M CoSO₄, 0.1 M CuSO₄in 5% H₂SO₄). After

irradiation for 30 h, a 1:5 mixture of silabenzene 1a and

2-{2,4,6-tris[bis(trimethylsilyl)methyl]phenyl}-2-silatri-

cyclo[3.1.0.0^2,6]-hex-3-ene (20) was obtained (by ¹H NMR).

The sealed tube was opened in a glovebox, and to the solu-

504 J. Chin. Chem. Soc., Vol. 55, No. 3, 2008

Tokitoh et al.

Table 3. Crystal data of 1a, 12-15, 18, 19a, and 22

1a

C₃₂H₆₄Si₇

12

13

14

empirical formula

formula weight

solvent

C₄₅H₇₄OSi₇

827.67

C₄₀H₇₂Si₇·CH₂Cl₂C₃₈H₇₆OSi₇

645.46

834.53

745.62

hexane

0.4 ´ 0.4 ´ 0.1

monoclinic

P2₁/n

18.1163(4)

12.8263(3)

20.7598(5)

90

hexane

CH₂Cl₂/CH₃CN

crystal size (mm)

crystal system

space group

a (Å)

b (Å)

c (Å)

0.5 ´ 0.5 ´ 0.2

monoclinic

P2₁/a

18.9186(3)

10.9969(2)

20.0757(4)

90

0.4 ´ 0.3 ´ 0.02 0.6 ´ 0.3 ´ 0.2

monoclinic

P2₁/a

18.3020(6)

10.9614(3)

25.8847(7)

90

triclinic

P-1

12.8367(3)

17.5750(7)

12.4173(4)

108.012(1)

98.693(1)

104.485(1)

2498.7(1)

2

a (deg)

b (deg)

99.2711(6)

90

4122.1(1)

4

106.916(2)

90

4968.2(3)

4

99.8824(5)

90

g (deg)

V (Å³)

4752.3(2)

4

1.042

Z value

D_calc

1.040

1.107

1.109

no. of reflections (all) 9446

11339

497

0

0.053

0.124

11333

597

144

0.058

0.177

1.059

10889

471

17

0.083

0.209

1.303

no. of parameters

no. of restraints

R₁(I > 2s(I))

411

0

0.097

0.258

1.043

wR₂(all data)

goodness of fit

0.796

15

18

19a

22

empirical formula

formula weight

solvent

crystal size (mm)

crystal system

space group

a (Å)

b (Å)

c (Å)

a (deg)

b (deg)

C₄₀H₇₀Si₇

747.59

hexane

0.6 ´ 0.08 ´ 0.08

triclinic

P-1

12.316(1)

21.153(3)

9.814(2)

91.418(6)

111.384(4)

78.145(3)

2326.3(6)

2

C₄₂H₇₅ONSi₇

806.66

heptane

0.4 ´ 0.2 ´ 0.1

triclinic

P-1

11.042(1)

26.030(3)

9.315(1)

94.939(2)

113.197(3)

86.247(2)

2450.2(5)

2

C₆₄H₁₂₈Si₁₄

1290.92

CH₂Cl₂/CH₃CN

0.5 ´ 0.1 ´ 0.1

monoclinic

C2/c

34.628(2)

10.3929(5)

22.776(1)

90

C₃₂H₆₆OSi₇·0.5C₆H₁₂

705.56

cyclohexane

0.3 ´ 0.2 ´ 0.1

monoclinic

C2/c

38.407(1)

13.2176(4)

18.4041(6)

90

104.5197(7)

90

95.510(2)

90

8159.0(7)

4

g (deg)

V (Å³)

9044.3(4)

8

1.036

Z value

D_calc

1.067

1.093

1.051

no. of reflections (all) 7847

9035

9233

10263

no. of parameters

no. of restraints

R₁(I > 2s(I))

wR₂(all data)

goodness of fit

443

0

0.125

0.329

1.743

476

0

0.097

0.274

686

498

0.057

0.155

553

105

0.074

0.218

1.446

0.930

1.293

disordered structures adequately.

tions.

Structural Refinement of 14

Structural Refinement of 13

The 8-phenyl-1-silabicyclo[2.2.2]octa-2,5-diene moi-

eties were disordered at two overlapped positions in a

0.718(3):0.282(3) ratio, which were restrained to be identi-

cal to each other using SAME, SIMU, and DELU instruc-

The oxygen atoms and t-Bu groups were disordered

at two positions in 0.591(7):0.409(7) and 0.533(9):0.467(9)

ratios, respectively. The structures of disordered parts were

restrained to be identical to each other using SADI instruc-

The Chemistry of Silabenzenes

J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 505

1998; Chapter 1.

tions.

3. (a) Solouki, B.; Rosmus, P.; Bock, H.; Maier, G. Angew.

Chem., Int. Ed. Engl. 1980, 19, 51. (b) Maier, G.; Mihm, G.;

Reisenauer, H. P. Angew. Chem., Int. Ed. Engl. 1980, 19, 52.

(c) Kreil, C. L.; Chapman, O. L.; Burns, G. T.; Barton, T. J. J.

Am. Chem. Soc. 1980, 102, 841. (d) Maier, G.; Mihm, G.;

Reisenauer, H. P. Chem. Ber. 1982, 115, 801. (e) Maier, G.;

Mihm, G.; Baumgärtner, R. O. W.; Reisenauer, H. P. Chem.

Ber. 1984, 117, 2337. (f) Jutzi, P.; Meyer, M.; Reisenauer, H.

P.; Maier, G. Chem. Ber. 1989, 122, 1227.

Structural Refinement of 19a

The central 1,7-disilatricyclo[3.2.2.0^1,6]dodeca-

2,4,8,11-tetraene moieties and Tbt groups were disordered

and their occupancies were fixed to 0.5. The structure of

disordered Tbt groups were restrained to be identical to

each other using SADI, SAME, SIMU, and DELU instruc-

tions.

Structural Refinement of 22

4. (a) Barton, T. J.; Banasiak, D. S. J. Am. Chem. Soc. 1977, 99,

5199. (b) Barton, T. J.; Burns, G. T. J. Am. Chem. Soc. 1978,

100, 5246. (c) Märkl, G.; Hofmeister, P. Angew. Chem., Int.

Ed. Engl. 1979, 18, 789. (d) Barton, T. J.; Vuper, M. J. Am.

Chem. Soc. 1981, 103, 6788. (e) Sekiguchi, A.; Tanikawa,

H.; Ando, W. Organometallics, 1985, 4, 584. (f) Märkl, G.;

Schlosser, W. Angew. Chem. Int. Ed. Engl. 1988, 27, 963.

5. Recent reviews on low-coordinated organosilicon com-

pounds: (a) Okazaki, R.; West, R. Adv. Organomet. Chem.

1996, 39, 232. (b) Baines, K. M.; Stibbs, W. G. Adv. Orga-

nomet. Chem. 1996, 39, 275. For reviews of Si–C doubly

bonded compounds, see ref. 1.

The central 2-hydroxy-2-silabicyclo[3.1.0]-hex-3-

ene moieties, p-bis(trimethylsilyl) groups of Tbt, and one

atom of solvent cyclohexane molecule were disordered in

0.481(3):0.519(3), 0.249(3):0.751(3), and 0.780(8):0.220(8),

respectively. The structure of disordered parts were re-

strained to be identical to each other using SAME, SIMU,

and DELU instructions.

ACKNOWLEDGMENT

This work was partially supported by Grants-in-Aid

for Creative Scientific Research (No. 17GS0207), Science

Research on Priority Areas (No. 19027024, “Synergy of

Elements”), Young Scientist (B) (Nos. 18750030), the 21st

Century COE Program B14 (Kyoto University Alliance for

Chemistry), and the Global COE Program B09 (Interna-

tional Center for Integrated Research and Advanced Edu-

cation in Materials Science) from the Ministry of Educa-

tion, Culture, Sports, Science and Technology, Japan.

6. A stable tetrasila-1,3-butadiene has conjugated Si–Si double

bonds. Weidenbruch, M.; Willms, S.; Saak, W.; Henkel, G.

Angew. Chem. Int. Ed. Engl. 1997, 36, 2503.

7. Reports on the delocalization of silole anions and dianions:

(a) Freeman, W. P.; Tilley, T. D.; Liable-Sands, L. M.;

Rheingold, A. L. J. Am. Chem. Soc. 1996, 118, 10457. (b)

Goldfuss, B.; Schleyer, P. v. R. Hampel, F. Organometallics,

1996, 15, 1755. (c) Goldfuss, B.; Schleyer, P. v. R.

Organometallics, 1997, 16, 1543. (d) Choi, S.-B.; Boudjouk,

P.; Wei, P. J. Am. Chem. Soc. 1998, 120, 5814. (e) Dysard, J.

M.; Tilley, T. D. J. Am. Chem. Soc. 1998, 120, 8245.

8. (a) Arduengo, A. J., III; Bock, H.; Chen, H.; Denk, M.;

Dixon, D. A.; Green, J. C.; Hermann, W. A.; Jones, N. L.;

Wagner, M.; West, R. J. Am. Chem. Soc. 1994, 116, 6641. (b)

West, R.; Denk, M. Pure. Appl. Chem. 1996, 68, 785. (c)

Heinemann, C.; Müller, T.; Apeloig, Y.; Schwarz, H. J. Am.

Chem. Soc. 1996, 118, 2023. (d) Boehme, C.; Frenking, G. J.

Am. Chem. Soc. 1996, 118, 2039. (e) West, R.; Buffy, J. J.;

Haaf, M.; Müller, T.; Gehrhus, B.; Lappert, M. F.; Apeloig,

Y. J. Am. Chem. Soc. 1998, 120, 1639.

Supporting Information Available

Results of X-ray structural analyses of 1a, 12–15, 18,

19a, and 22 (CIF), and details of the theoretical calcula-

tions (PDF). This material is available free of charge via the

Internet at http://pubs.acs.org.

Received October 18, 2007.

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Article Doi

The chemistry of stable silabenzenes

DOI: 10.1002/jccs.200800073

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