Phanes bridged by group 14 heavy elements

ADVANCES IN INORGANIC CHEMISTRY, VOL.

50

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS HIDEKI SAKURAI Department of Industrial Chemistry, Faculty of Science and Technology, Science University of Tokyo, Noda, Chiba 278-8510, Japan

I. Introduction II. An Introductory Case Study on 3,4,7,8-Tetrasilacycloocta-1,5-diyne III. [2.2]Cyclophanes A. Octamethyltetrasila[2.2]paracyclophane, Metacyclophane, and Orthocyclophane B. Octamethyltetragerma[2.2]paracyclophane C. Tetrastanna[2.2]paracyclophane D. Comparison of Structures of [2.2]Paracyclophanes Bridged by Group 14 Elements E. Through-Space and Through-Bond interactions in [2.2]paracyclophanes Bridged by Group 14 Elements IV. Tetrasila[2.2](2,5)heterophanes A. Octamethyl-1,2,8,9-tetrasila[2.2](2,5)furanophane and Thiophenophane B. Tetrasila[2.2](2,5)pyrrolophane C. NMR Studies and Molecular Structure D. Reactions of Octamethyl-1,2,8,9-tetrasila[2.2](2,5)furanophane V. Hexasila[2.2.2](1,3,5)cyclophane A. Preparation and Properties of 1,2,9,10,17,18-Hexasila[2.2.2](1,3,5)cyclophane B. Molecular Structure of 1,2,9,10,17,18-Hexasila[2.2.2](1,3,5)cyclophane VI. [2n]Paracyclophanes Bridged by Si, Ge, and Sn VII. [n]Cyclophanes Bridged by Si A. Heptasila[7]paracyclophane B. Pentasila[5](2,5)thiophenophane and Hexasila[6](2,5)thiophenophane VIII. Conclusion References

I. Introduction

The chemistry of cyclophanes may be traced back to the work of Pellegrin on [2.2]metacyclophane in 1899 (1). Although the first 359 Copyright  2000 by Academic Press. All rights of reproduction in any form reserved. 0898-8838/00 $35.00

360

HIDEKI SAKURAI

[2.2]paracyclophane was isolated by Brown and Farthing from the polymerization residue of p-xylene in 1949 (2), the chemistry of cyclophanes has been developed greatly by the direct synthesis of a series of paracyclophanes by Cram et al. (3). The name paracyclophane (1) originates from the work of Cram.

Subsequently, over more than four decades, numerous studies have been devoted to exploring this fascinating class of compound (4). In addition to metacyclophane (2), analogues with more than two bridges between rings such as 3 (5) are a part of the family. The limit is superphane (4) (6). Compounds with a single benzenoid skeleton as a part of a large cycle such as 5 and 6 are also synthesized (7). In a strict sense, the name of cyclophane should be limited to analogues with benzene ring(s), since phane comes from phenyl, but similar compounds with other aromatic ring(s) are also included to the family. In these cases, ‘‘cyclo’’ is replaced by the ring name to denote, for example, thiophenophane (7) and furanophane (8) (8). Phanes have been suggested as a general name for all such bridged aromatic com-

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

361

pounds (8). The phane family expanded its scope further to include non-benzenoid aromatic compounds. Thus in the past decades, almost all types of phanes have been prepared and structures and reactions have been investigated in detail. One remaining interest in the chemistry is to replace the C–C bridge(s) of phanes with bonds of other elements, in particular of Group 14 heavy elements such as Si, Ge, and Sn. What is the interest associated with pursuing the chemistry of such compounds? First, the synthetic challenges to the compounds is obvious, because weaker M–M bonds (M ⫽ Si, Ge, Sn) than the C–C bond require special precautions for synthesis. In addition to this, physical properties due to the extensive mixing of ␴ –앟 orbitals are interesting. Higher ␴-orbital energies of these element bonds such as Si–Si, Ge– Ge, and Sn–Sn should enable these bonds to mix with 앟-orbitals of aromatic rings to result in through-bond conjugation. Bond lengths that are longer than those of C–C bonds also create additional strains to phanes. In this chapter, the author tries to summarize the present status of the chemistry of phanes bridged by Group 14 heavy elements.

II. An Introductory Case Study on 3,4,7,8-Tetrasilacycloocta-1,5-diyne

Before going into a detailed account of the chemistry of phanes, the author will touch on 3,4,7,8-tetrasilacycloocta-1,5-diyne briefly, since the compound illustrates the importance of ␴ –앟 mixing. The ionization potential of the Si–Si bond is estimated by photoelectron spectroscopy to be 8.69 eV (9). Thus, the HOMO level of the Si–Si is comparable to most HOMOs of 앟 systems. Consequently, the Si–Si bond can conjugate efficiently with carbon–carbon double and triple bonds, benzene rings, and other 앟 systems. Most Si–Si bonds are stable enough to construct sophisticated structures by themselves and with organic molecules (10). The first UV spectral evidence of the ␴ –앟 mixing between the Si–Si ␴ and the benzene 앟 bonds was reported by three independent groups in 1964 (11). Since then, extensive ␴ –앟 mixing has been shown to occur between the Si–Si bond and various 앟 systems in spectra and reactions (12). In 1983, prior to the studies on phanes, the author tried to construct a new ring system, 3,4,7,8-tetrasilacycloocta-1,5-diyne, composed of two Si–Si ␴ and two CIC 앟 bonds (13). As shown in the

362

HIDEKI SAKURAI

FIG. 1. Qualitative MO diagram of 9.

qualitative MO diagram (Fig. 1), two Si–Si ␴ orbitals can overlap with one of the two 앟 orbitals of each CIC bond to make up new molecular orbitals with through-bond conjugation. This is a unique situation for the system since neither spatial nor through-bond interaction between two CIC bonds was observed for the corresponding cycloocta1,5-diyne, the smallest known cyclic diyne (14). 3,3,4,4,7,7,8,8-Octamethyl-3,4,7,8-tetrasilacycloocta-1,5-diyne (9), prepared first by the ring contraction reaction of 10 together with a seven-membered compound (11), exhibited a large bathochromic shift in UV spectra at 250 nm (␧ ⫽ 13,400) compared with an open chain analogue, Me3Si–CIC–SiMe2SiMe2 –CIC–SiMe3 (␭max ⫽ 230 nm).

(1) This is a good indication of strong mixing between Si–Si ␴ orbitals and CIC 앟 orbitals with through-bond conjugation as expected. The molecular structure itself was not very much distorted, as evidenced by the X-ray crystallographic analysis shown in Fig. 2. The through-bond conjugation with strong ␴ –앟 mixing for tetrasilacycloocta-1,5-diyne is further demonstrated by photoelectron spectroscopic (15) and ab initio (3-21G basis set) studies (16).

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

363

FIG. 2. ORTEP drawing of 9.

III. [2.2]Cyclophanes

A. OCTAMETHYLTETRASILA[2.2]PARACYCLOPHANE, METACYCLOPHANE, AND ORTHOCYCLOPHANE In view of the interesting physical properties of 9 described earlier, similar or even more interesting properties were expected for a [2.2]paracyclophane bridged by two Si2Me4 units. The target compound, 1,1,2,2,9,9,10,10-octamethyl-1,2,9,10-tetrasila[2.2]paracyclophane (12), was prepared according to Scheme 1 in low yield (1.6%) (17). Related compounds 13 and 14 were also prepared. The reaction of 9 with 움-pyrone gave 15, and acid-catalyzed isomerization of 15 afforded 16 (18). The yield of 12 was very low, but in spite of several attempts, such as ring contraction reactions of 13 and 14 by photochemical silylene extrusion, 12 was obtained only by the direct process shown in Scheme 1.

(2) Compound 12 is readily sublimed as colorless crystals. The crystal belongs to the space group P21 /n, the molecular structure being

364

HIDEKI SAKURAI

SCHEME 1

shown in Fig. 3. The compound 12 is highly symmetric with a center ˚ ) deviate only of symmetry. The Si–Si bond lengths of 12 (3.376 A ˚ slightly from the normal values (3.34 A). The 3 and 6 (and 11 and 14) carbon atoms of the aromatic rings are displaced slightly out of the plane of the other four atoms inward. The degree of the displacement is 4.3, which is far smaller than that of the [2.2]paracyclophane (12.6⬚), indicating a smaller degree of distortion of the benzene rings of 12 than those of [2.2]paracyclophane (19). However, the silicon atoms of the bridges are displaced appreciably from the aromatic ring toward the cyclophane cavity. The degree of this displacement is 15.0⬚, larger than that of [2.2]paracyclophane (11.2⬚). The distances ˚ for C3–C14 and 3.456–3.460 A ˚ for between aromatic rings, 3.347 A ˚) C4–C15 and C5–C16, are close to that observed in graphite (3.40 A and longer than the mean intramolecular aromatic ring separation of ˚ . Two benzene rings and methyl [2.2]paracyclophanes around 3.00 A groups eclipse completely. Figure 4 compares the structures of tetrasila[2.2]paracyclophane 12, [2.2]paracyclophane 1, and [3.3]paracyclophane. Reflecting the ˚ ) than the C–C bond (1.562 A ˚ ), distortion longer Si–Si bond (2.376 A of the benzene ring in 12 (4.3⬚) is much less than that of 1 (12.6⬚), being close to the value for [3.3]paracyclophane (6.4⬚). Because of the symmetric structure, 1H NMR spectra of 12 show only two signals assignable to methyl and phenyl protons at 웃 0.50

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

365

FIG. 3. Molecular structure of 12.

and 6.75 ppm, respectively. The latter signal is shifted to the high field by 0.45 ppm from unsubstituted benzene. This is similar to 1 (웃 6.47 ppm) (10), but the degree of shift is smaller. These structural data demonstrate that 12 is a rather less distorted molecule than [2.2]paracyclophane. However, a dramatic effect of the strong ␴ (Si–Si)–앟 interaction was observed in UV spectra as shown in Fig. 5. In the UV spectrum of phenylpentamethyldisilane, an intramolecular ␴(Si–Si)–앟 charge-transfer band appears around 231 nm (11a, 12). Octamethyltetrasila[2.2]ortho- (15) and metacyclophane (16) show similar absorptions, but the band splits into two bands at 223 nm (␧ ⫽ 19,100) and 263 nm (␧ ⫽ 22,500) in 12. This type of red shift in the UV spectra occurs only in 12 among other polysilaparacyclophanes such as 13 and 14.

366

HIDEKI SAKURAI

FIG. 4. Comparison of structures of paracyclophanes.

The photoelectron spectrum of 12 was observed and the measured ionization energies were compared with the orbital energies calculated for the parent tetrasila[2.2]paracyclophane (21). The He I photoelectron spectra of 1 was recorded to have a broad first band at 8.1 eV that was well separated from other bands (22). The first bands of 12 are remarkably different from those of 1. A shoulder at 7.8 eV and a broad band centered at 8.3 eV was observed. A comparison between the band sequence of 12 and 1 based on both empirical and quantum chemical studies shows a smaller through-space and a larger throughbond interaction for 12. The through-bond interaction between the b3u (␴) orbital and the corresponding 앟 orbital yielded a different orbital sequence in 12 as compared to 1 (Fig. 6). Yatabe has developed a slightly modified process for preparing 12 (23). Thus, the reductive coupling reaction of 1,4-bis(chlorodimethylsilyl)benzene with sodium in toluene in the presence of 18-crown-6 pro-

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

367

FIG. 5. UV spectra of 12 and related compounds.

vided the cyclophane 12 (Scheme 2). After removal of the solvent and the resulting salt, silica gel chromatography with hexane followed by recrystallization from ethanol gave 12 in 2.2% yield as colorless crystals. The presence of 18-crown-6 is essential; otherwise the reaction resulted in the formation of intractable polymers. In fact, Ishikawa et al. (24) reported the polycondensation reaction of various 1,4bis(chlorodialkylsilyl)benzenes with sodium in toluene in the absence of the crown ether. They did not succeed in the isolation of the corre-

SCHEME 2

368

HIDEKI SAKURAI

FIG. 6. Molecular orbital sequence for 12.

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

369

sponding [2.2]paracyclophanes. The Wurtz-type coupling reaction is highly affected by the reactivity of the silyl anion. The crown ether would be expected to increase the reactivity of the silyl anions by forming a crown ether–sodium ion complex in the hydrocarbon solvent, resulting in an increase in the intramolecular reaction, although the net yields are still very low. In relation to this work, Elschenbroich reported that dechlorinative coupling of bis[bis(chlorodimethylsilyl)-␩6-benzene]chromium with lithium naphthalenide in dimethoxyethane (DME) led to the formation of (1,2,9,10-tetrasila-␩6[2.2]paracyclophane)chromium as red crystals in 3.5% yield (25).

(3) The reaction of 12 with Cr(CO)6 and Mo(CO)6 gave the corresponding monocoordinated complexes in 54 and 47% yields, respectively (26).

(4) B. OCTAMETHYLTETRAGERMA[2.2]PARACYCLOPHANE Although the one-pot process shown in Scheme 2 did not increase the yield of 12 dramatically, the process can be applied to the synthesis of a higher analogue of 12, octamethyltetragerma[2.2]paracyclophane (17) (27). The precursor, 1,4-bis(chlorodimethylgermyl)benzene (18), was prepared according to Scheme 3. 1,2,9,10-Tetragerma[2.2] paracyclophane was synthesized in 2.6% yield as colorless crystals by reductive condensation of the chlorogermane 18 with molten sodium metal in refluxing toluene in the presence of 18-crown-6. After removal of the solvent and the resulting salt, silica gel chromatography

370

HIDEKI SAKURAI

SCHEME 3

with a 1 : 1 hexane/benzene mixed solvent as an eluent followed by recrystallization from benzene gave 17 as colorless crystals. The [2.2]paracyclophane structure of 17 was confirmed by the following spectroscopic data. The mass spectrum showed the M⫹ cluster ion in the range of m/z 554–572, in agreement with the calculated formula of C20H32Ge4 . The 1H and 13C NMR spectra are fully consistent with the highly symmetrical structure: 1H NMR (CDCl3 , 웃) 0.55 (s, 24 H, Me), 6.74 (s, 8 H, ArH); 13C NMR (CDCl3 , 웃) ⫺4.78 (Me), 133.07 (ortho C), 140.12 (ipso C). In the proton NMR spectra, the aromatic protons of 7, 6, and [2.2]paracyclophane 17 appear at 6.74, 6.79, and 6.48 ppm, respectively. Because of the shielding effect of each facially arrayed benzene ring, these aromatic protons appreciably shift upfield (⌬웃 ⫽ 0.59–0.63 ppm) relative to 1,4-bis(pentamethyldigermanyl)benzene (7.35 ppm), 1,4-bis(pentamethyldisilanyl)benzene (7.38 ppm), and 1,4-diethylbenzene (7.11 ppm). The cyclophane 17 is quite stable and does not change upon heating to 300⬚C. Furthermore, the Ge–Ge bonds are inert to trimethylamine N-oxide or sulfur in refluxing benzene, contrary to the case of 12. The compound 12 can be oxidized easily to disiloxane-bridged paracyclophane by oxidation with trimethylamine N-oxide. The molecular structure of 1,2,9,10-tetragerma[2.2]paracyclophane 17 was determined by the X-ray diffraction study. The single crystals of 17 for X-ray crystallography were obtained from a toluene solution. Similar to 12, the crystal belongs to the space group P21 /n, and the data collection was carried out at 13⬚C. The ORTEP drawing of 17 is shown in Fig. 7. The molecule is highly symmetric with a center of symmetry. The ˚ ) is close to that of length of the bridging Ge–Ge bonds (2.438(1) A normal Ge–Ge bonds. The Ge–Car bond lengths (1.941(10)–1.946(10) ˚ , av. 1.944 A ˚ ) are also normal. The bridged aromatic carbons bend A

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

371

FIG. 7. Molecular structure of 17.

inward from the planes of the other aromatic four carbons by 3.1⬚ (C1) and 5.9⬚ (C4) (av. 4.5⬚). In addition, the germanium atoms at the bridges deviate from benzene rings by 10.4 (Ge1) and 8.8 deg (Ge2) (av. 9.6 deg). As a result, the two benzene rings are distorted slightly into a boat form. The two aromatic rings completely eclipse each other ˚ (C1–C4⬘) and 3.496 A ˚ (av. with interplanar space distances of 3.382 A C2–C5⬘ and C3–C6⬘). Interestingly, the value is very close to those ˚ ), as observed for 12 and is close to that observed in graphite (3.40 A indicated in the case of 12. C. TETRASTANNA[2.2]PARACYCLOPHANE For comparison, it seemed extremely interesting to prepare octamethyltetrastanna[2.2]paracyclophane (19). However, the attempted reductive coupling of 1,4-bis(chlorodimethylstannyl)benzene did not

372

HIDEKI SAKURAI

SCHEME 4

afford the desired product 19 (28). Fine grains of tin metal were liberated during the reflux of toluene. Next, the synthesis of 19 was attempted by dehydrogenative coupling reaction of 1,4-bis(dimethylstannyl)benzene with tetrakis(triphenylphosphine)palladium. However, the reaction afforded only hexastanna[2.2.2]paracyclophane. The methyl group on tin seemed to be too small to protect against oxidative cleavage of the Sn–Sn bond (Scheme 4). Then dehalogenative coupling of chlorostannane 20 and dehydrogenative condensation of hydrostannane 21, both of which contain isopropyl groups as protecting groups, were examined. Dechlorination coupling of 20 gave octa(isopropyl)tetrastanna[2.2]paracyclophane 22 in 0.3% yield as colorless crystals together with dodeca(isopropyl)hexastanna[2.2.2]paracyclophane 23. Alternatively, dehydrogenative condensation of hydrostannane 21 in benzene with a palladium catalyst at 40⬚C afforded the cyclophane 22 in 1.1% yield. Both processes gave 22 only in very low yield, but the first paracyclophane bridged by Sn–Sn bonds has been thus obtained (Scheme 5). The cyclophane 22 is stable toward atmospheric oxygen and moisture, although it decomposes upon heating to 200⬚C. The molecular structure of 1,2,9,10-tetrastanna[2.2]paracyclophane 22 was determined by the X-ray diffraction method. The crystal belongs to the space group P21 /a, and the data collection was carried out at 13⬚C. The ORTEP drawing of 22 is shown in Fig. 8. The molecule is also highly symmetric with a center of symmetry. The two methyl groups of the two isopropyl groups on the tin atoms protect efficiently the Sn–Sn bonds. One of the methyl groups is located above the tin atom as viewed along the Sn–Sn bond. The other

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

SCHEME 5

FIG. 8. Molecular structure of 22.

373

374

HIDEKI SAKURAI

is located around the Sn–Sn bond. The lengths of the bridging Sn–Sn ˚ ) are close to those of the normal Sn–Sn bonds (e.g., bonds (2.808(1) A ˚ 2.780 A in Ph6Sn2) (29). The Sn–C bond lengths (2.160(5)–2.164(5), ˚ ) are also normal. The benzene rings are nearly planar. av. 2.162 A Thus, the bridged aromatic carbons (C1, C4) are displaced only slightly inward out of the plane of the other four aromatic carbons (C2–C3–C5–C6) by 1.9⬚ (C1) and 2.5⬚ (C4) (av. 2.2⬚). In contrast, the C–Sn bonds at the bridges are appreciably inclined from the benzene ring by 8.3⬚ (C1–Sn1) and 9.5⬚ (C2–Sn2) (av. 8.9⬚). Of great interest is the conformation between the aromatic rings and the bridging Sn–Sn bonds. The benzene rings are connected by the bridging Sn–Sn bonds not perpendicularly but in a stepped conformation; surprisingly, the dihedral angle is 67.4⬚! The two benzene rings are completely parallel with an interplanar space distance of ˚ ; again it is very close to that found for graphite (3.4 A ˚ ). 3.48 A The side view of the molecular structure is illustrated in Fig. 9. Interestingly, C1⬘, C5, C6, and C4⬘ atoms are arranged in a plane. The conformation between the two separated rings is apparently similar to that of the layered structure of graphite. The steric congestion among the neighboring alkyl groups is very small because of the relatively long the Sn–Sn and Sn–C bonds. The shortest distance be-

FIG. 9. ORTEP drawing of 22 viewed down the vector C2–C3.

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

375

˚ between H2 (aromatic) and H9 tween the hydrogen atoms is 2.54 A (methyl), indicating that the steric repulsion is also very small. Furthermore, the crystal packing is normal and probably does not influence the conformation between the benzene rings. Therefore, it is reasonable that the stepped conformation in 22 results from the factor similar to that on graphite structure. D. COMPARISON OF STRUCTURES OF [2.2]PARACYCLOPHANES BRIDGED BY GROUP 14 ELEMENTS An interesting systematic comparison of the geometries by X-ray diffraction in a series of [2.2]paracyclophanes, 1, 12, 17, and 22, is given in Table I and Fig. 10. In all the [2.2]paracyclophanes, the bridging bonds (b) are somewhat elongated from the normal bond lengths, and the benzene rings are distorted into boat conformations. The strain angles 움 of the aromatic rings appreciably decrease in the order 12.6 (C) ⬎ 4.3 (Si) ⬎ 4.5 (Ge) ⬎ 2.2 (Sn). The rings of the cyclophane 1 bridged by carbon atoms are considerably deformed into boat TABLE I BOND LENGTHS AND ANGLES IN [2.2]PARACYCLOPHANES BRIDGED BY GROUP 14 E–E BONDS

˚) (A ˚) (A ˚) (A ˚) (A

a b p q 움 (deg) 웁 (deg) 웂 (deg)

C

Si

Ge

Sn

1.511 1.593 2.778 3.093 12.6 11.2 113.7

1.883 2.376 3.347 3.458 4.3 10.6 105.0

1.944 2.438 3.382 3.496 4.5 9.6 104.1

2.163 2.808 3.454 3.482 2.2 8.9 101.0

376

HIDEKI SAKURAI

FIG. 10. Conformations of two benzenes in [2.2]paracyclophanes bridged by Group 14 E–E bonds.

shapes, whereas those of the cyclophane 22 bridged by tin atoms are quite flat. On the other hand, the bending angles 웁 of the bridging atoms slightly decrease in the order 11.2 (C), 10.6 (Si), 9.6 (Ge), and 8.9 (Sn). As a result, the 웁 angles of 12, 17, and 22 are larger than the corresponding 움 angles. In the cyclophane 1, however, the situation is opposite. The distortions in cyclophanes 12, 17, and 22 are accumulated in the E–C (E ⫽ Si, Ge, and Sn) bonds to a larger extent than the benzene rings. In contrast, the distortion of 1 is concentrated on the benzene rings. This can evidently be ascribed to the marked differences in the bond lengths at the bridge. Of particular interest are both the conformations between the separated aromatic rings and the interplanar space distances (q). In the cyclophane 1, a slight twist (3⬚) of the aromatic rings occurs in opposite directions due to the transannular repulsion resulting from the ˚ ) than the van der Waals contact disquite shorter distance (3.09 A ˚ tance (3.4–3.6 A) (30). On the other hand, the two benzene rings of the cyclophanes 12 and 17 eclipse each other completely with the dis˚ for 12; 3.50 A ˚ for 17) just equal to the van der Waals tances (3.46 A contact distance. In sharp contrast, the benzene rings of the cyclophane 22 are connected by the bridging Sn–Sn bonds not perpendicularly, as in 12 and 17, but in a stepped conformation, the dihedral angle being 67.4⬚. The rings of 22 are parallel with an interplanar ˚ , which is almost the same as those of 12 (3.46 space distance 3.48 A ˚ ˚ A) and 17 (3.50 A).

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

377

[2.2]Paracyclophanes bridged by Group 14 heavy elements try to keep interplanar distances constant to the value for graphite, regardless of ˚ for Si–Si, 2.44 A ˚ for any increase in the bridging bond lengths: 2.38 A ˚ Ge–Ge, and 2.81 A for Sn–Sn! Such a motive is impossible for the carbon analogue. E. THROUGH-SPACE AND THROUGH-BOND INTERACTIONS IN [2.2]PARACYCLOPHANES BRIDGED BY GROUP 14 ELEMENTS A systematic comparison of through-space and through-bond interactions in a series of [2.2]paracyclophanes bridged by Group 14 E–E ␴-bond (E ⫽ C, Si, Ge, and Sn) is now possible. The UV spectra of [2.2]paracyclophanes 1, 12, 17, and 22, bridged by E–E bonds (E ⫽ C, Si, Ge, and Sn), are shown in Fig. 11 (23). The cyclophane 12 bridged by two silicon atoms exhibits two absorption bands with maxima at 223 nm (␧ ⫽ 18,400) and 263 nm (␧ ⫽ 22,400). Similar to 12, two absorption bands with maxima at 223 nm (␧ ⫽ 25,000) and 254 nm (␧ ⫽ 23,500) were observed for the cyclophane 17 bridged by two germanium atoms. On the other hand, the cyclophane 22 bridged by two tin atoms shows an absorption band

FIG. 11. UV spectra of [2.2]paracyclophanes, 1, 12, 17, and 22.

378

HIDEKI SAKURAI

with maximum at 265 nm (␧ ⫽ 26,500, sh). In the latter case, the absorption bands observed for 12 and 17 are superimposed on the absorption band attributed to the electronic transition between ␴ and ␴ * orbitals of the Sn–Sn bonds in the 220–240 nm region. Actually, an absorption band with maximum at 215 nm (␧ ⫽ 38,500), tailing into the 280 nm region, was observed for hexaisopropyldistannane. In sharp contrast, an absorption band with maximum at 223 nm (␧ ⫽ 18,200) and a shoulder at 245 nm (␧ ⫽ 2,800) was observed for the cyclophane 1 bridged by the C–C bonds. The compound 1 exhibits no appreciable absorption band in the long wavelength region as found for 12, 17, and 22. These observations are best explained by accounting for the difference of the HOMO levels resulting from through-space and throughbond interactions (Fig. 12). In [2.2]paracyclophane systems, the two facing benzene 앟-systems interact strongly because of the throughspace overlap. As a consequence, the bonding combinations are stabilized (b3u and b2u), whereas the antibonding combinations are destabilized (b2 g and b3 g). From the four linear combinations of the benzene 앟 MOs, the b3u linear combination depends very strongly on the nature of the bridge due to through-bond interaction (31).

FIG. 12. MO energy diagram of [2,2]paracyclophanes bridged by Group 14 E–E bonds.

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

379

In the cyclophane 1, although the overlap between the 앟-system (2p) and the bridging ␴-bonds (2s2p) is most effective, these orbital energy levels match worst, the first ionization potentials being 9.25 eV for benzene and 12.1 eV for ethane. As a result, the HOMOs are the almost pure 앟 MOs with the b2 g and b3 g combinations. Both the PE spectrum and theoretical calculation demonstrate the degeneracy of the two HOMO levels. The absorption bands are attributed to the 앟–앟 * transitions associated with the HOMOs. On the other hand, in the systems with high-lying ␴-orbitals, the through-bond interaction effect can be predominant. In the cyclophane 12 bridged by Si2Me4 units, the energy level of the ␴-bridge is greatly raised compared to the C2H4 bridge of the parent system 1. The first ionization potential of Me3Si–SiMe3 is 8.69 eV. Consequently, the HOMO of 12 is the MO by ␴ –앟 mixing with the b3 g combination resulting from the through-bond interaction. Such an orbital interaction causes considerable destabilization of the HOMO level. The first absorption band in the long wave length region of 12 is attributed to the (␴ –앟)–앟 * transitions associated with the HOMO. The second band is probably attributed to the 앟–앟 * transition between the pure 앟 MOs, judging from the fact that the absorption band appears at the same region as that observed for 12. Like 12, in the cyclophanes 17 and 22 bridged by the Ge–Ge and Sn–Sn bonds, respectively, the HOMOs are formed with the b3 g linear combination and the HOMO levels are similarly destabilized relative to those of MOs with the b2 g and b3 g symmetries. The absorption bands in the long wavelength region for 17 and 22 are the result of the effective through-bond interaction. Among the cyclophanes 12, 17, and 22, the absorption band of 22 appears at the longest position. However, the bathochromic shifts in these cyclophanes should be compared with the corresponding acyclic compounds: 1,4-bis(pentamethyldisilanyl)benzene 23, 1,4-bis(pentamethyldigermanyl)benzene 24, and 1,4-bis(pentaisopropyldistannanyl)benzene 25 (Fig. 13). The degree of bathochromic shift decreases in the following order: (⌬v/cm⫺1) 2500 (12–23) ⬎ 2000 (17–24) ⬎ 1100 (22–25). This is a consequence of the decreasing effect of the orbital interaction due to both the higher lying orbital energy level of the bridging E–E ␴-bond (Me3Si–SiMe3 : 8.69: Me3Ge–GeMe3 8.57: Me3Sn–SnMe3: 8.20 eV) and the less effective overlap of the benzene 앟-system (2p) with the bridging ␴-orbitals (Si: 3s3p; Ge: 4s4p; Sn: 5s5p) as the bridging units are replaced by the heavier atoms.

380

HIDEKI SAKURAI

FIG. 13. UV spectra of acyclic compounds 23, 24, and 25.

IV. Tetrasila[2.2](2,5)heterophanes

Effects of ␴ –앟 interaction are maximized if the interacting orbitals match energetically. Since the HOMOs of heteroaromatic systems such as furan (8.89 eV), thiophene (8.80 eV), and pyrrole (8.20 eV) are higher than that of benzene (9.25 eV), better energy matching with ␴ (Si–Si; 8.69 eV) orbitals is expected. Therefore, tetrasila[2.2](2,5)heterophanes should be interesting synthetic targets. A. OCTAMETHYL-1,2,8,9-TETRASILA[2.2](2,5)FURANOPHANE AND THIOPHENOPHANE Lithiation of furan and thiophene, followed by the reaction with 1,2-dichlorotetramethyldisilane, gave linear compounds 26 and 27 (Scheme 6). The second lithiation and reaction with 1,2-dichlorotetramethyldisilane under the conditions of high dilution afforded the desired furanophane (28) and thiophenophane (29) (32). Figures 14 and Fig. 15 show UV spectra of furanophane (28) and

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

SCHEME 6

FIG. 14. UV spectra of 28 and related compounds.

381

382

HIDEKI SAKURAI

FIG. 15. UV spectra of 29 and related compounds.

thiophenophane (29), respectively. UV spectra of the corresponding derivatives are also included in both figures. As clearly shown, the absorption maxima of 28 and 29 are redshifted considerably as compared with the corresponding acyclic compounds. The C–C bridged parent thiophenophane (7) also shows a redshift in UV spectra. Thus, 7 gave two absorption maxima at 245 nm (␧ ⫽ 7700) and 275 mm (␧ ⫽ 5720), quite different from that of 2,5-dimethylthiophene (␭max ⫽ 238 nm; ␧ ⫽ 7250). This is attributed to the transannular 앟–앟 interaction between two thiophene rings (33). However, furanophane (8) did not show such a redshift and indicated no such 앟-앟 interaction between two furan rings. At room temperature by NMR studies, it is reported that two furan rings in 8 rotate freely,

383

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

TABLE II IONIZATION ENERGIES

AS

DETERMINED

Compound 12

29

PHOTOELECTRON SPECTRA Band (eV)

7.8

8.4

8.2 28

BY

9.3 9.0

—

9.9 9.8

7.4

8.1

8.9

9.4

8.1

8.5

9.3

10.0

7.4

8.0

8.7

8.0

8.6

9.0

9.3

whereas in 7, rotation of two thiophene rings is frozen to force a transannular interaction (34). NMR studies on 28 and 29 indicate that both the thiophene and furan rings rotate freely at room temperature (vide infra) and therefore, anomalies in the UV spectra of 28 and 29 should be attributed to the through-bond interaction between the Si–Si ␴ bonds and aromatic 앟 bonds. This was further confirmed by photoelectron spectral studies. As shown in Table II, the lift of HOMO for 12 relative to the model compound was 0.4 eV, but those for 28 and 29 were 0.7 and 0.6 eV, respectively. Apparently, more effective through-bond interaction occurs for 28 and 29 (21). B. TETRASILA[2.2](2,5)PYRROLOPHANE N, N,1,1,2,2,8,8,9,9-Decamethyl-1,2,8,9-tetrasila[2.2](2,5)pyrrolophane (30) is prepared by a procedure similar to that applied to 28 and 29 through 1,2-bis(N-methylpyrrolyl)disilane 31 with slight modification of the reagents. The lithiation was performed with t-BuLi/ TMED. Under dilute conditions pyrrolophane 30 was obtained in 4% yield from 31 as air-stable colorless crystals besides a polymeric residue (35) (Scheme 7). The UV spectrum of 30 (Fig. 16) exhibits a large bathochromic shift (␭max ⫽ 270 nm) compared to other related compounds such as 2-pentamethyldsilanyl-N-methylpyrrole (␭max ⫽ 248 nm). A similar bathochromic shift is observed in charge-transfer complexes of pyrrolo-

384

HIDEKI SAKURAI

SCHEME 7

phane 30 and related compounds with TCNE (Fig. 17). Thus, the maximum absorption of the CT complex of 30 and TCNE occurs at 718 nm compared with, e.g., 630 nm for the complex of 30 and TCNE. The spectroscopic behavior of 30 reveals the strong interaction between the Si-Si ␴ bonds and the 앟 systems of the pyrrole rings. Such an interaction has been shown to result in a destabilization of HOMO and thus a bathochromic shift in electronic spectra. C. NMR

STUDIES AND

MOLECULAR STRUCTURE

Similar to [2.2]metacyclophane (2), relative conformations of two aromatic rings in [2.2](2,5)heterophanes have been studied in detail

FIG. 16. UV spectra of 30 and related compounds.

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

385

FIG. 17. CT spectra of 30 and related compounds with tetracyanoethylene.

by NMR. Scheme 8 shows equilibration of conformational isomers in heterophanes. The most stable conformation of heterophanes are believed to be anti-A or anti-B. In fact, anti conformations are observed in crystals (36). In solution, furanophane 8 undergoes rapid equilibration between anti-A and anti-B at room temperature. Dynamic NMR studies indicate that the energy barrier between two conformations is ⌬G ⫽ 16.8 kcal/mol for [2.2](2,5)furanophane 8. However, no change was observed for proton signals of [2.2](2,5)thiophenophane 7 even at 200⬚C, indicating a very high rotational barrier for 7. Namely, at

SCHEME 8

386

HIDEKI SAKURAI

room temperature, flipping of rings occurs in furanophane but not in thiophenophane. Tetrasila[2.2](2,5)thiophenophane 29 shows a singlet for SiMe signals at room temperature. Thus, contrary to the parent thiophenophane 7, thiophene rings of 29 undergo free rotation at room temperature. However, at low temperature, the methyl signal starts to split into two signals. The coalescent temperature is 15.0⬚C and inversion barrier is determined to be 14.9 kcal/mol. The greater length of the Si–Si bond is reflected in the smaller barrier to rotation. Contrary, the methyl signal of furanophane 28 retains singlet even at ⫺78⬚C, indicating a very low barrier for rotation. 1 H NMR of pyrrolophane 30 shows two singlets for SiMe groups that do not coalesce in the temperature range of ⫺80 to ⫹110⬚C, in contrast to furanophane 28, which shows rapid anti/syn isomerization down to ⫺78⬚C, and thiophenophane 29, with a coalescence temperature of 15⬚C. The rigidity of 30 is obviously due to the steric bulk of the N-methyl groups. The great steric demand of the methyl group on N was further demonstrated by NMR characteristics of Si–O–Si bridged pyrrolophane 32 prepared by oxidation of 30 with excess Me3N–O in refluxing benzene (74% yield). Even with the considerably longer Si–O–Si bridges, no anti/syn isomerization occurs for 32 between ⫺80 and ⫹110⬚C.

(5) Insertion of oxygen into the Si–Si bond of 30 also blocks ␴ –앟 conjugation effectively. The fact can be seen by the shift toward short wavelength for both absorption of 31 in UV (␭max ⫽ 239 nm) (Fig. 16) and the CT complex with TCNE (␭max 593 nm) as compared to pyrrolophane 30 (Fig. 17). The crystals of thiophenophane 29 belong to the P21 /n group and are monoclinic, with two molecules in a unit cell similar to that for 12. An ORTEP drawing is shown in Fig. 18 with pertinent distances and angles. Figure 19 shows a side view of 29. In crystals, two thiophene rings take an anti conformation. The ˚ , which is slightly longer than the normal Si–Si bond length is 2.361 A

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

387

FIG. 18. Molecular structure of 29.

˚ ) but shorter than that observed for 12 (2.376 A ˚ ). This value (2.34 A indicates smaller distortion for 29 than for 12. The sulfur atoms are not on the plane composed of four carbons but deviate outward by 1.9⬚. This results from avoiding interactions between the lone-pair and 앟 electrons, and the phenomenon is also observed for the carbon analogue 7 (37). Two thiophene rings are staggered with respect to each other in the direction where each sulfur atom directs the center of the five-membered ring.

FIG. 19. Side view of 29.

388

HIDEKI SAKURAI

Suitable single crystals of 30 for X-ray analysis were also obtained by slow crystallization from toluene at 10⬚C. Results for the molecular structure of pyrrolophane 30 are shown in Fig. 20. Pyrrolophane 30 crystallizes in anti configuration, with the N-methyl groups placed directly above the plane of the opposite pyrrole ring. The Si–Si bond ˚ ) is the same length as that found in paracyclophane 12 (2.386 A ˚ (2.376 A) and is inclined against the plane of the pyrrole rings by about 64⬚. The same characteristic is observed in the corresponding tetrasila [2.2](2,5)furanophane 28. In contrast, in the case of thiophenophane 29, the Si–Si bond is nearly perpendicular to the aromatic rings. Table III summarizes important characteristics of these compounds. All tetrasila[2.2](2,5)heterophanes show only a small deviation of the heteroatoms from ideal planar arrangement in the rings. Also the Siring bonds are bend inwards with the biggest deviation occurring in the thiophenophane, which also has the biggest interplanar distance ˚ ). In the case of furanophane 28 the between the heterocycles (3.21 A bridge torsion angle of 47⬚ is relatively close to a staggered arrangement (60⬚) of substituents around the Si-Si bond. In pyrrolophane 30,

FIG. 20. Molecular structure of 30.

389

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

TABLE III STRUCTURAL DATA

FOR

TETRASILA[2.2](2,5)HETEROPHANES

X⫽

O (28)

S (29)

NMe (30)

Envelope-shaped ring (움) (⬚) Bending of bridge atom (웁) (⬚) ˚) Distance between rings (d ) (A ˚) Si–Si bond length (A Bridge torsion (⬚)

1.3 7.0 2.63 2.347 47.0

1.9 13.4 3.21 2.361 30.6

1.1 13.0 2.94 2.363 25.1

however, the bridge is considerably twisted towards eclipsed conformation (torsion angle 25⬚) possibly reflecting the greater steric demand of the methyl group. D. REACTIONS OF OCTAMETHYL-1,2,8,9-TETRASILA[2.2](2,5)FURANOPHANE [2.2](2,5)Furanophane 8 is reactive and undergoes interesting reactions with dienophiles. The reaction of 8 with dimethyl acetylenedicarboxylic acid gave a polycyclic compound, as shown in the following equation. The compound was derived by intramolecular Diels–Alder reaction of the initially formed 1 : 1 adducts (38).

(6) Tetrasila[2.2](2,5)furanophane 28 did not react with dimethyl acetylenedicarboxylic or with 4-phenyl-1,2,4-triazoline-3,5-dione. Low distortion of the furan rings in 28 together with steric hindrance of

390

HIDEKI SAKURAI

FIG. 21. ORTEP drawing of 33.

methyl groups prevented the reaction. However, the reaction of 28 with benzyne did afford 33 in good yield.

(7) Figure 21 shows an ORTEP drawing of 33. Photochemical reaction of 33 gave a new Si–Si bridged benzooxepinophane 34. The latter is a new type of phane. In general, photochemical conversion of 1,4epoxy-1,4-dihydronaphthalene to benzooxepin occurs only in very low yield, but 1,4-bis(trimethylsilyl)- and 1,4-bis(pentamethyldisilanyl)1,4-epoxy-1,4-dihydronaphthalene gave the corresponding benzooxepin in 87 and 59% yield. Apparently steric hindrance of silyl groups protects the unstable oxepin structure.

(8)

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

391

(9)

V. Hexasila[2.2.2](1,3,5)cyclophane

A. PREPARATION AND PROPERTIES OF 1,2,9,10,17,18-HEXASILA [2.2.2](1,3,5)CYCLOPHANE The degree of through-bond interaction in [2.2]paracyclophane 12 bridged by two Si–Si bonds is the greatest among the series of the cyclophanes bridged by Group 14 elements. Therefore, the properties of [2.2.2](1,3,5)cyclophane 35 bridged by three Si2Me4 units are of special interest because of the extremely rigid structure and the expectedly strong ␴ –앟 conjugation. The cyclophane 35 was obtained by a reductive coupling reaction of 1,3,5-tris(chlorodimethylsilyl)benzene with molten sodium metal in refluxing toluene in the presence of 18-crown-6. The GCMS spectrum of the reaction mixture showed the presence of a very small amount of the cyclophane 35 with its parent ion (M⫹ ⫽ 498). After careful isolation, pure samples of 35 were obtained as white solids. This provides the most direct route to 35, but the yield is very low (0.22%) (39) (Scheme 9). The structure of [2.2.2](1,3,5)cyclophane 35 was first confirmed by the mass spectrum, which showed the M⫹ cluster ion in the range of m/z 498–502 in agreement with the calculated formula of C24H42Si6 . The 1H and 13C NMR spectra are fully consistent with the highly symmetrical structure. In the proton NMR spectrum, the aromatic protons of 35 appear at 6.69 ppm and are shifted appreciably upfield (⌬웃 ⫽ 0.79 ppm) compared to 1,3,5-tris(pentamethyldisilanyl)benzene (7.48 ppm). The magnitude of the ⌬웃 value is somewhat larger than that between [2.2]paracyclophane 12 (6.79 ppm) and the acyclic compound (7.38 ppm); the fact points to the shortening of the inter-ring distance. The 29Si NMR (CDCl3) spectrum of 35 revealed a single resonance at ⫺15.08 ppm, which shifts somewhat downfield compared with that observed for 12 (⫺18.36 ppm).

392

HIDEKI SAKURAI

SCHEME 9

The cyclophane 35 is stable and remains unchanged upon heating to 300⬚C. The compound 35 is, however, readily oxidized at the Si–Si units by trimethylamine oxide in refluxing benzene, giving the cyclophane 36 containing three Si–O–Si bonds.

(10) B. MOLECULAR STRUCTURE OF 1,2,9,10,17,18-HEXASILA [2.2.2](1,3,5)CYCLOPHANE The crystals of 1,2,9,10,17,18-hexasila[2.2.2](1,3,5)cyclophane belong to the space group P21 /n. The ORTEP drawing is shown in Fig. 22. The molecule has no crystallographic symmetry. The three Si–Si bonds, Si1–Si2: 2.378(6), Si3–Si4: 2.373(6), and Si5–Si6: ˚ (av. 2.378 A ˚ ), are different in length and somewhat longer 2.384(5) A ˚ ), whereas those in 12, which has than the normal Si–Si bond (2.34 A ˚ ). The benzene rings of a center of symmetry, are identical (2.376 A the cyclophane 35 are almost flat.

FIG. 22. ORTEP drawing of 35.

394

HIDEKI SAKURAI

The aromatic carbons (C1, C3, C5, C7, C9, C11) attached to the ˚ ) outside the mean silicon atoms lie only slightly (0.003–0.029 A planes of the aromatic rings. However, the Si–C(ring) bonds deviate considerably from the planes of the benzene rings (Sil–Cl/Cl–C2– C6: 11.0⬚, Si2–C7/C71–C8–C12: 14.8⬚, Si3–C3/C2–C3–C4: 13.5⬚, Si4–C9/C8–C9–C10: 16.6⬚, Si5–C5/C4–C5–C6: 12.6⬚, Si6–C12/C10– C11–C12: 16.2⬚ av. 14.1⬚). The deviation angles are greater than that of 12 (10.6⬚). The two aromatic rings are so arranged to take the eclipsed position. The average distance between the two aromatic ˚ , somewhat shorter than that in 12 (3.35 and rings of 35 is 3.32 A ˚ 3.46 A). Because the Si–Si bond is 1.5 times longer than the C–C bond, the strain in the cyclophane must be much smaller than that of the carbon analogue. The UV spectra of hexasila[2.2.2](1,3,5)cyclophane 35 and tetrasila [2.2]-paracyclophane 12 are shown in Fig. 23. The cyclophane 35 exhibits two absorption bands with maxima at 224 nm (␧ ⫽ 35,200, sh) and 264 nm (␧ ⫽ 20,900) and a shouldered absorption band at 240 nm (␧ ⫽ 25,000) similar to those for 12 (223 (␧ ⫽ 18,400) and 263 nm (␧ ⫽ 22,400)). Furthermore, the absorption band with maximum at 264 nm for 35 is largely redshifted compared to that for 1,3,5-tris(pentamethyldisilanyl)benzene (␭max ⫽ 233 nm, ␧ ⫽ 30,500). The through-bond interaction between the three Si–Si ␴-bonds and the benzene 앟-systems should be the origin of the bathochromic shift.

FIG. 23. UV spectra of 12 and 35.

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

395

VI. [2n]Paracyclophanes Bridged by Si, Ge, and Sn

The direct coupling reactions of ClMe2M–C6H4 –MMe2Cl with sodium naphthalenide in toluene at ⫺78⬚C afforded [2.2]paracyclophanes (M ⫽ Si, 12, 2.5%; M ⫽ Ge, 17, 1.7%) as described earlier. Careful examination of the products has revealed that several macrocyclic [2n]paracyclophanes such as 37 and 38 (M ⫽ Si) and 39 and 40 (M ⫽ Ge) exist as well in low yield. The corresponding reaction for M ⫽ Sn gave no [2.2]paracyclophane 19 with the Me2SnSnMe2 bridge, but produced dodecamethyl hexastanna[23]paracyclophane 41 in 12% yield.

396

HIDEKI SAKURAI

The possible through-bond interactions in these compounds are schematically indicated in Scheme 10. Through-bond conjugation is possible for [2.2]paracyclophane and [2.2.2.2]paracyclophane because of the Hu¨ckel overlap between the interacting orbitals, but is not possible for [2.2.2]paracyclophane because of the Mo¨bius conjugation. Thus, it is interesting to examine the point by UV spectra. Figures 24 and 25 show UV spectra of silicon and germanium series of paracyclophanes. In both cases, [2.2]paracyclophanes 12 and 17 exhibit absorption at the longest wavelength in the series. [2.2.2]Paracyclophanes, 37 and 39, exhibit absorptions at almost the same position as acyclic reference compounds, 42 and 43, although absorption intensities increase about three times more.

SCHEME 10

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

397

FIG. 24. UV spectra of [2n]paracyclophane bridged by Si.

This indicates that effective through-bond conjugation does not exist in [2.2.2]paracyclophanes. In contrast, [2.2.2.2]paracyclophanes, 38 and 40, show slightly redshifted absorptions, indicating the existence of through conjugation, although the degree of conjugation is smaller

FIG. 25. UV spectra of [2n]paracyclophane bridged by Ge.

398

HIDEKI SAKURAI

SCHEME 11

than in 12 and 17. Hexastanna[2.2.2]paracyclophane 41 shows similar absorption to 37. Yoshida et al. have reported the preparation of trisila[1.1.1]metacyclophane (trisilacalix[3]arene, 44) and tetrasila[1.1.1.1]metacyclophane (tetrasilacalix[4]arene, 45) (40) (Scheme 11) and indicate conformational analyses by using semiempirical MO calculations. A one-pot reaction gave the two different organosilicon macrocyclic compounds concomitantly. The structures of these metacyclophanes,

FIG. 26. Molecular structure of 44.

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

399

FIG. 27. Molecular structure of 45.

44 and 45, were determined by X-ray crystallography. As shown in Figs. 26 and 27, 44 and 45 adopted a saddle structure and a 1,2alternate structure, respectively. The conformations of these silacalixarenes were also analyzed by semiempirical PM3 calculations, which indicate that the conformations of the silacalixarenes in the crystal do not represent an energy minimum. A completely C3 symmetric cone structure of 44 and a 1,3-alternate structure of 45, respectively, were estimated as the most stable conformations. They have also examined the formation of the 앟-complex between the silacalixarenes and silver cation by FAB mass spectrometry. Similar 앟-complex formation with silver cation was observed for hexasila [2,2,2]paracyclophanes 37 (41). Trisila[1.1.1]orthocyclophane (46) was prepared by the reaction of 3,3,6,6,9,9-hexamethyl-3,6,9-trisilacyclonona-1,4,7-triyne and 움pyrone. The structure of 46 determined by X-ray crystallographic analysis (Fig. 28) indicates a twisted saddle conformation (10).

(11)

400

HIDEKI SAKURAI

FIG. 28. Molecular structure of 46.

VII. [n]Cyclophanes Bridged by Si

A. HEPTASILA[7]PARACYCLOPHANE The smallest [n]paracyclophane so far isolated is the [6]paracyclophane, first prepared by Jones et al. in 1974. Bickelhaupt prepared and succeeded in spectroscopic characterization of [5]paracyclophane, which is stable at low temperature in solution, but not isolable. As the first example of [n] paracyclophanes bridged by heteroatom chains, Ando et al. reported preparation and the crystal structure of heptasila[7]paracyclophane (47) (42) (Scheme 12). Reductive coupling of 1-chloro-6-[4-(chlorodimethylsilyl)phenyl]1,1,2,2,3,3,4,4,5,5,6,6-dodecamethylhexasilane with sodium in refluxing toluene in the presence of [18]crown-6 afforded heptasila[7]paracyclophane 47 in 1.2% yield. Compound 47, obtained as colorless crys-

SCHEME 12

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

401

FIG. 29. Molecular structure of 47.

tals (UV (hexane) ⫽ 248 nm (log ␧ ⫽ 4.63)), is stable toward atmospheric oxygen and moisture and does not decompose even when heated to its melting point of 88–88.5⬚C. Similar to the previous cases, the presence of [1.8]crown-6 is also essential; otherwise, only polymeric materials were obtained. In addition to NMR studies, the X-ray structural determination established the molecular structure of 47 as shown in Fig. 29. The molecule had no crystallographic symmetry. Although the Si–Si bond lengths are normal and range between ˚ , an alternation in the bond angles around the 2.336(5) and 2.371(4) A polysilane chain is observed. Similar to other cyclophanes, the benzene ring of 47 deformed from planarity. The strain angles 움 of the aromatic rings are 6.6 and 4.5⬚, while the bending angles 웁 of the bridging atoms are 6.5 and 9.0⬚. These values are comparable with those observed in 12 and are smaller than those of the [7]paracyclophane derivative 43 and the [8]paracyclophane derivative 44. Thus, the deformation in the benzene ring is smaller in 47 than in the analogous compounds. However, it should be noted that an appreciable increase in the deformation in the polysilane chain was observed. C. PENTASILA[5](2,5)THIPHENOPHENE AND HEXASILA[6](2,5)THIOPHENOPHANE Decamethylpentasila[5](2,5)thiophenophane (48) and dodecamethylhexasila[6](2,5)thiophenophane (49) were prepared as shown in

402

HIDEKI SAKURAI

SCHEME 13

Scheme 13 (45). Major by-products are unidentified polymeric materials. Both compounds are stable to air and water. Molecular structures of 48 and 49 are shown in Figs. 30 and 31. The strain angles 움 of the thiophene rings of 48 and 49 are 1.1 and 2.1⬚, while the bending angles 웁 of the bridging atoms are 9.9 and 13.7⬚, respectively. Thiophene rings are almost planar in

FIG. 30. Molecular structure of 48.

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

403

FIG. 31. Molecular structure of 49.

both compounds, but the pentasilane chain in 49 is much more distorted. Figure 33 shows UV spectra of 48 and 49. Since the reference compounds, 1,5-dithienyl-1,1,2,2,3,3,4,4,5,5-decamethylpentasilane and 1,6-dithienyl-1,1,2,2,3,3,4,4,5,5,6,6-dodecamethylpentasilane, have absorption maxima at 258 and 265 nm, respectively, considerable bathochromic shifts are observed for 48 and 49. Ring opening polymerization of hexasila[6](2,5)thiophenophane 49 was examined (46). Anionic ring opening polymerization of 49 with

FIG. 32. Comparison of structures of 48 and 49.

404

HIDEKI SAKURAI

FIG. 33. UV spectra of 48 and 49.

BuLi, Ph2MeSiNa ⫹ cryptand[2.2.1], Ph2MeSiK ⫹ cryptand[2.2.2], and 2-lithiothiophene gave poly[2,5](dodecamethyl-1,6-hexasilanylene)thienylene (50) in 15–76% yield. The molecular weight (MWt) was 5500–9500 and MWt/Mn was 1.5–1.8.

(12)

Interestingly, 1H NMR of 50 shows three sharp signals at 0.05, 0.10, and 0.38 ppm in addition to the signal assigned to the thiophene ring at 7.20 ppm. The 29Si NMR also shows three sharp signals at ⫺43.1, ⫺39.0, and ⫺20 ppm. These facts indicate that the polymer 50 has a perfectly regulated alternate structure. The reaction of 49 with excess RLi (R ⫽ Me and 2-thienyl), followed by quenching with ethanol, gave R-(SiMe2)6-substituted thiophen, exclusively.

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

405

SCHEME 14

(13) Based on this observation, the mechanism of polymerization was proposed as indicated in Scheme 14. It has been reported that anionic polymerization of tetraphenylgermole-spiro-cyclogermatetrasilane proceeds by a similar mechanism involving 2-germolyl anions as propagating species (47). In contrast, ring opening polymerization of similar silole derivatives, tetraphenylsilole-spiro-pentasilane, occurs in a different way involving pentacoordinate silicon species (48).

VIII. Conclusion

To date, almost all types of phanes bridged by silicon have been prepared, whereas those bridged by germanium and tin are fewer. Much remains to be explored. At present, a particular drawback of

406

HIDEKI SAKURAI

the field is the low yield of products; new methods are needed therefore for further development. However, structures and reactions of these compounds are quite interesting and polymerization of the phanes to polymers of ␴ –앟 systems is expected to be important as a contribution to materials science of the future. The author wishes to thank all his co-workers whose names are listed in relevant references.

REFERENCES 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

Pellegrin, M. Rec. Trav. Chim, 1988, 18, 457. Brown, C. J.; Farththing, A. C. Nature (London) 1949, 164, 915. Cram, D. J.; Steinberg, H. J. Am. Chem. Soc. 1951, 73, 5691. For reviews see: (a) Boekelheide, V. Acc. Chem. Res. 1980, 13, 65; (b) Hopf, H. Naturwiss. 1983, 70, 349. (c) The Cyclophanes. Vols. I and II; Keehn, P. M.; Rosenfeld, S. M., Eds.; Academic Press: New York, 1983; (d) ‘‘Cyclophanes;’’ Vo¨gtle, F., Ed.; Springer-Verlag: Berlin, 1983. Nakazaki, M; Yamamoto, K.; Yasuhiro, M. J. Org. Chem. 1978, 43, 1041. (a) Sekine, Y.; Brown, M.; Boekelheide, V. J. Am. Chem. Soc. 1979, 101, 3126. (b) Sekine, Y.; Boekelheide, V. J. Am. Chem. Soc. 1981, 103, 1777. Griffin, R. W., Jr. Chem. Rev. 1963, 63, 45. Vo¨gtle, F. Tetrahedron Lett. 1969, 3193. Pitt, C. G.; Bock, H. J. Chem. Soc. Chem. Commun. 1972, 28. Sakurai, H. Pure Appl. Chem. 1987, 59, 1637. (a) Sakurai, H.; Kumada, M. Bull. Chem. Soc. Jpn. 1964, 37, 1894. (b) Hague, D. N.; Prince, R. H. Chem. Ind. (London) 1964, 1492, (c) Gilman, H.; Atwell, W. H.; Schebke, G. L. J. Organomet. Chem. 1964, 2, 369. Sakurai, H. J. Organomet. Chem. 1982, 200, 261. Sakurai, H.; Nakadaira, Y.; Hosomi, A.; Eriyama, Y.; Kabuto, C. J. Am. Chem. Soc. 1983, 105, 3359. Koster-Jensen, E.; Wirz, J. Angew. Chem. Int. Ed. Engl. 1973, 12, 671. Gleiter, R.; Scha¨fer, W.; Sakurai, H. J. Am. Chem. Soc. 1985, 107, 3046. Morokuma, K. In ‘‘Organosilicon and Bioorganosilicon Chemistry;’’ Sakurai, H., Ed.; Ellis Horwood: Chichester UK, 1984. Sakurai, H.; Hoshi, S.; Kamiya, A.; Hosomi, A.; Kabuto, C. Chem. Lett. 1986, 1781. Sakurai, H.; Nakadaira, Y.; Hosomi A.; Eriyama, Y. Chem. Lett. 1982, 1971. Singler, R. E.; Cram, D. J. J. Am. Chem. Soc. 1971, 93, 4443. Wilson, D. J.; Boekelheide, V.; Griffin, Jr. R. W. J. Am. Chem. Soc. 1960, 82, 6302. Gleiter, R.; Scha¨fer, W.; Krennirch, G.; Sakurai, H. J. Am. Chem. Soc. 1988, 110, 4117. Koenig, T.; Wielesek, R. A.; Snell, W.; Balle, T. J. Am. Chem. Soc. 1975, 97, 3225 and references therein. Yatabe, T. Ph.D. Thesis, 1992, Tohoku University. Ishikawa, M.; Ni, H.; Watanabe, H.; Saheki, Y.; Nate, K. Organometallics 1987, 6, 1673. Elschenbroich, C.; Hurley, J.; Massa, W.; Baum, G. Angew. Chem. Int. Ed. Engl. 1988, 27, 684

PHANES BRIDGED BY GROUP 14 HEAVY ELEMENTS

407

26. Hoshi, S. Unpublished results. 27. Sekiguchi, A.; Yatabe, T.; Kabuto, C.; Sakurai, H. J. Organomet. Chem. 1990, 390, C27. 28. Sakurai, H.; Yatabe, T.; Sekiguchi, A. To be published. 29. (a) Preut, Von H.; Haupt, H.-J.; Huber, F. Z. Anorg. Allg. Chem. 1973, 81, 396. (b) Piana, H.; Kirchgassner, U.; Schubert, U. Chem. Ber. 1991, 124, 743. 30. Bondi, A. J. Phys. Chem. 1964, 68, 443. 31. Gleiter, R. Tetrahedron Lett. 1969, 4453. 32. Sakurai, H.; Hoshi, S.; Kabuto, C. To be published. 33. Winberg, H. E.; Fawcett, F. S.; Mochel, W. E.; Theobald, C. W. J. Am. Chem. Soc. 1960, 82, 1428. 34. Fletcher, J. R.; Sutherland, I. O. Chem. Commun. 1969, 1504. 35. Kobs, U.; Sekiguchi, A.; Kabuto, C.; Sakurai, H. 38th Symposium on Organometallic Chemistry, Kyoto, 1991; abstracts, p. 196. 36. Pahor, N. B.; Calligaris, M.; Randaccio, L. J. Chem. Soc., Perkin II 1978, 42. 37. Theodore, F. J.; Seyferth, D. Inorg. Chem. 1968, 7, 1245. 38. Cram, D. J.; Montgomery, C. S.; Knox, G. R. J. Am. Chem. Soc. 1966, 88, 515. 39. Sekiguchi, A.; Yatabe, T.; Kabuto, C.; Sakurai, H. Angew. Chem., Int. Ed. Engl. 1989, 28, 757. 40. Yoshida, M.; Goto. M.; Nakanishi, F. Organometallics 1999, 18, 1465. 41. Yatabe, T. Unpublished results. 42. Ando, W.; Tsumuraya, T.; Kabe, Y. Angew. Chem., Int. Ed. Engl. 1990, 29, 778. 43. Allinger, N. L.; Walter, T. J.; Newton, M. G. J. Am. Chem. Soc. 1974, 96, 4588. 44. Newton, M. G.; Walter, T. J.; Allinger, N. L. J. Am. Chem. Soc. 1973, 95, 5652. 45. Sakurai, H.; Furuya, M.; Sakamoto, K. Chem. Lett., submitted for publication. 46. Furuya, M. Unpublished results. 47. Sanji, T.; Furuya, M.; Sakurai, H. Chem. Lett. 1999, 547. 48. Sanji, T. Sakai, T.; Kabuto, C.; Sakurai, H. J. Am. Chem. Soc. 1998, 120, 4552.