Dissociation of the Disilatricyclic Diallylic Dianion [(C4Ph4SiMe)2]−2 to the Silole Anion [MeSiC4Ph4]− by Halide Ion Coordination or Halide Ion Nucleophilic Substitution at the Silicon Atom

The reductive cleavage of the Si-Si bond in 1,1-bis(1-methyl-2,3,4,5-tetraphenyl-1-silacyclopentadiene) [(C4Ph4SiMe)2] (1) with either Li or Na in THF gives the silole anion [MeSiC4Ph4]− (2). The head-to-tail dimerization of the silole anion 2 gives crystals of the disilatricyclic diallylic dianion [(C4Ph4SiMe)2]−2 (3). The derivatization of 3 (crystals) with bromoethane (gas) under reduced pressure provides [(MeSiC4Ph4Et)2] (4) quantitatively. The reverse addition of 3 in THF to trimethylsilyl chloride, hydrogen chloride, and bromoethane in THF gives 1-methyl-1-trimethylsilyl-1-silole [Me3SiMeSiC4Ph4] (6), 1-methyl-2,3,4,5-tetraphenyl-1-silacyclo-3-pentenyl-1-methyl-1-silole [C4Ph4H2SiMe-MeSiC4Ph4] (7), and 1-methyl-2,5-diethyl-2,3,4,5-tetraphenyl-1-silacyclo-3-pentenyl-1-methyl-1-silole [C4Ph4Et2SiMe-MeSiC4Ph4] (8), respectively. The reaction products unambiguously suggest that the silole anion [MeSiC4Ph4]− is generated by coordination of the chloride ion at the silicon atom in 3 or by the nucleophilic substitution of either chloride or bromide ion at one of two silicon atoms in 3. The quenching reaction of 3 dissolved in THF with water gives 1,2,3,4-tetraphenyl-2-butene, the disiloxane of 1-methyl-2,3,4,5-tetraphenyl-1-silacyclo-3-pentenyl [O(MeSiC4Ph4)2] (10) and methyl silicate.

Interestingly, the addition of the trimethylchlorosilane to 3 dissolved in THF provides only 1,1bis(1-methylsilole) [(MeSiC 4 Ph 4 ) 2 ] (1) along with hexamethyldisilane (Scheme 2). Moreover the reverse addition of 3b in THF to trimethylchlorosilane in THF for an extended reaction time gives [Me(Me 3 Si)SiC 4 Ph 4 ] (6) in 65% yield (Scheme 2). These results indicate that the disilatricyclic diallylic dianion 3b is dissociated into the silole anions 2 in THF solution. A similar dissociation of the gallole dimer [(C 4 Me 4 Ga-t-Bu) 2 ] in benzene was previously reported [36].    The minor product 4 unambiguously indicates that some of the unique disilatricyclic structure in 3 is sustained during the reaction. However, the major products of the disilanes 7 and 8 clearly suggest that in THF the silole anion 2 and the silyl halide [R 2 Ph 4 C 4 SiMeX] (R=H, X=Cl for 7, R=Et, X=Br for 8) are generated in a ratio of 1:1 from 3, as indicated in Scheme 5: Scheme 5. Suggested mechanism for the formation of disilanes (7 and 8). The coupling reaction of an allylic anion in one of two 5-membered rings with RX releases above the C 4 Si ring the latter's halide ion, which is easily associated with the vicinal silicon atom of the silicon in 3. This association easily induces ring opening in the highly strained 1-3-disilacyclobutane via a pentacoordinated state since the angles of the C-Si-C bonds in both C 4 Si ring and 1,3silacyclobutane ring are nearly 90 degrees [29]. The nucleophilic substitution at the silicon atom produces a [Si=C] double bond in the other 5-membered ring to generate the novel silole anion [C 4 Ph 4 SiMe] − (9) via a pentacordinated anionic intermediate, in which the methyl group and one Si-C bond of the C 4 Si ring occupy pseudo-axial positions and two bonds of X-Si-C and the other Si-C bond of the C 4 Si ring occupy pseudo-equatorial positions. Simultaneously the pushing of the electron pair in the other Si-C bond of 1,3-disilacyclobutane ring to the C α carbon produces an allylic carbanion, which immediately reacts with RX. The just generated silole anion 9 is beneath the silicon atom of the C 4 Si ring, so coordination of the double bond [Si=C] to the silicon atom above of the C 4 Si ring would be an alternative mechanism involving a pseudo-pentacoordinated intermediate to stabilize the [Si=C] moiety of the silole anion 9, such as in (η 5 -C 5 Me 5 )(PR 3 )RuH(η 2 -CH 2 =SiPh 2 ) [37,38] and H 2 (PMe 3 ) 3 Ru(η 2 -CH 2 =SiMe 2 ) [39]. However, the preferred mechanism is a coupling reaction of the silole anion 9 with the halide. Then the nucelophilic substitution at the silicon provides the Si-Si bond of 7 or 8 while the allylic anion is rearranged to form a C=C double bond in the other C 4 Si ring to lead the silole moiety [MeSiC 4 Ph 4 ].
Similar pseudo-pentacoordiated silole intermediates lacking highly electronegative atoms on the silicon atom have been proposed, and are produced by the apical attack of methyl lithium, diphenylmethylsilyl lithium [40,41], sodium bis(trimethylsilyl)amide [42], and potassium hydride [35] on the less hindered SiC 4 ring. Even the neutral pseudo-pentacoordinate silole [Me(Me 2 NNp)SiC 4 Ph 4 ] has been reported [44]. In those pentacordinated anionic intermediates it was suggested that the SiC 4 ring occupies one axial and one equatorial position during its peudorotation and substitution reactions [45].
For some reason the reverse addition of 3 in THF to an excess of trimethylchlorosilane in THF produces chloride ion from the trimethylchlorosilane. Then, the association of the chloride ion to the silicon atom of 3 induces ring opening dissociation in the highly strained 1,3-disilacyclobutane to form [Si=C] bonds in two silole anions 9 via a pentacoordinated state, in which the methyl group and one Si-C bond of the C 4 Si ring occupy pseudo-axial positions and two bonds of X-Si-C and the other Si-C bond of the C 4 Si ring occupy pseudo-equatorial positions. Then the cation coordination enhances the stability by the delocalization in the silole ring [32]. The silole anion 9 is not consumed instantly by the reaction with trimethylchlorosilane to produce [Me(Me 3 Si)SiC 4 Ph 4 ] (6) due to the bulkiness of trimethlsilyl group [35]. Therefore, only from the reverse addition of 3 in THF to an excess of trimethylsilylchloride for the extended reaction time 6 could be obtained (Scheme 6).

Scheme 6.
Suggested mechanism for the formation of 1. (1) when trimethylsilylchloride is added to 3 in THF since the reaction rate of the silole anion 9 with trimethylchlorosilane is much slower than that of the anion 9 with the Si-Si bond of 6. Alternatively, the silole anion 9 is persistent in the solution for a while due to the lower reactivity of the bulky trimethylsilylchloride. A similar result in the form of a radical reaction of 1,1-bis(1-phenylsilole) [(PhSiC 4 Ph 4 ) 2 ] has been reported by Jutzi and Karl [46].
The quenching reaction of 3 dissolved in THF with water gives no 1,2,3,4-tetraphenyl-1,3butadiene, which should be obtained if 1-methyl-2,3,4,5-tetraphenyl- The disiloxane 10 is the condensation product of 1-methyl-1-hydroxy-2,3,4,5-tetraphenyl-1silacyclo-3-pentene [MeHOSiC 4 Ph 4 H 2 ] (11), which is produced from the protonation of the allylic anions and the hydrolysis cleavages of two Si-C bonds in 1,3-disilacyclobutane ring of 3. 1,2,3,4-Tetraphenyl-2-butene and methylsilicate are the hydrolysis products of 10 and/or 11. These reaction products unambiguously indicate that there is no 1-methyl-2,3,4,5-tetraphenyl-1-silacyclopentadienyl moiety, but rather the disilatricyclic diallylic dianion species in the THF solution of 3 and/or the silole anion 9. However, the self dissociation of 3 in THF to the silole anion 9 is not plausible since there is no derivatization at the silicon atom from the reaction of 3 with hydrogen chloride. Therefore it is suggested that the disilatricyclic diallylic dianion of 3 is sustained in THF solutions of 3 without halide ion to give its hydrolysis products (Scheme 8). An alternative mechanism would be involve a bent η 2chloride bridge between two silicon atoms of 1,3-disiacyclobutane in 3 to lead the silole anion 9 [48].  (2). From the reaction products the resonance form A (Scheme 9) can be proposed as a prominent contributor to the silole anion. The other four resonance forms are reduced to two sets of the symmetric resonance forms (B and E, C and D), which have the allyllic carbanion and silaethene moiety. The coordination of the cation, especially lithium, to the silole anion 2 enhances the delocalization in the silole ring [32], then the negative charge or electron density of the silicon atom moves to the carbons in the ring to induce some siliconcarbon double character. Consequently, the silicon atom becomes more electrophilic or less nucleophilic than that of the silyl anion A. It is reasoned that the the silole anion 2 has considerable Si=C double bond character and it dimerizes by head-to-tail [2 + 2] cycloaddition, which is known as silenes [49]. A t-Bu [25] or Si(SiMe 3 ) 3 [26] substituent on the silicon hinders the dimerization, in addition the electronic effect of the substituents on the silicon and on the carbons may be critical for the planarity of the silole anion to induce a stable 2-silaallylic anion in it (Scheme 9). It is noteworthy that the novel silole anion 9 coincides with resonance forms B and D or E and C of the silole anion 2 and it is an analogue of 2H or 3H-silole.

General
All reactions were performed under a nitrogen atmosphere using standard Schlenk techniques. Air sensitive reagents were transferred in a nitrogen-filled glove box. THF and diethyl ether were distilled from sodium benzophenone ketyl under nitrogen. Pentane was stirred over concentrated H 2 SO 4 and distilled from CaH 2 . NMR spectra were recorded on Bruker WP SY and Bruker AM 200 FT-NMR spectrometers. MS data were obtained on a mass spectrometer DMX 300. IR spectra were recorded as KBR pellets on a Shimazu IR 440 and melting points were measured on a Wagner & Meunz Co. capillary type apparatus. Elemental analyses were done using a Yanaco elemental analyzer at the Analytic Center of College of Engineering, Seoul National University.

[(MeSiC 4 Ph 4 ) 2 ]
(1): Stirring of 1-chloro-1-methyl-2,3,4,5-tetraphenyl-1-silacyclopentadiene [MeClSiC 4 Ph 4 ] (4.58 g, 10 mmol) and sodium (0.23 g, 10 mmol) in THF (170 mL) at room temperature for 10 hrs produced a pale green precipitate. After evaporating THF from the mixture, the remaining solid was treated with water and dichloromethane. Removal of dichloromethane under reduced pressure from the organic suspension, gave a pale green solid. The solid was washed with ether for purification. It was identified by the comparison of its analytical data with an authentic sample [50]. Yield: 3.69 g (90%); mp 322-329 °C (lit. [50] mp > 300 °C), 13  [(MeSiC 4 Ph 4 Me) 2 ] (5): Silole anion crystals (1.42 g, 1.7 mmol for the lithium salt, 1.48 g, 1.70 mmol for the sodium salt) was exposed to methyl iodide vapor for 2 hrs under reduced pressure. After evacuating the unreacted methyl iodide, the residual yellow solid was extracted with ether. Concentration and storage at −15 °C for 1 day gave yellow crystals of 4. Yield: 0.85 g (60%) for 3a, 1.22 g (87%) for 3b; mp 334−338 °C (lit. [29] 336−339 °C). The NMR data of 5 agreed with those reported earlier [30].   2 O] (10): Aqueous HCl (0.10N) was added to a THF solution of 3 with stirring at room temperature until the pH of the mixture reached neutrality. Filtration of the hydrolyzed mixture gave a white solid, which was insoluble in organic solvents and water, did not melt when heated to over 300 °C, and showed a broad band at 1000−1100 cm −1 in the IR spectrum. After THF was removed from the filtrate, the residue was extracted with diethyl ether. The concentrated ether solution was kept at −20 °C for 1 day and pale green crystals of 10 were obtained. The mother solution was concentrated and after standing at −20 °C for 1 day it yielded colorless crystals of 1,2,3,4,-tetraphenyl-2-butene (yield: 0.72 g, 40%), whose spectral data agreed with those reported earlier [9]. Yield (10)

Conclusions
The pathway for generation of the silole anion 9 is dependent on the reactivity of the alkyl halide used. Two silole anions 9 are generated from the disilatricyclic diallylic