1,1,3,3-Tetrakis(alkylthio)-1,3-dilithio-2-silapropanes: useful reagents for the synthesis of polysilacycloalkanes via dianionic ring formation

Treatment of 1,1,3,3-tetrakis(alkylthio)-2-silapropanes with t -BuLi in THF at -40 °C generated 1,1,3,3-tetrakis(alkylthio)-1,3-dilithio-2-silapropanes which reacted with various bifunctional chlorosilanes to give the corresponding 4-to 7-membered polysilacycloalkanes in moderate to good yields. Furthermore, double alkylation of the dilithiated silanes with bis(halomethyl)diorganosilanes or dihaloalkanes was found to proceed smoothly giving rise to 1,4-disilacyclohexanes or silacycloalkanes in good yields, respectively, in THF and an aprotic polar co-solvent such as hexamethylphosphoric triamide (HMPA) or 1,1,3,3-tetramethylurea (TMU). The sulfenyl groups in the cyclized products were smoothly removed by radical reduction with tributyltin hydride.


Introduction
Growing interest has been paid on polysilacycloalkanes, cycloalkanes containing one or more silicon atoms in the ring, as target molecules or precursors of polymers, ceramics, and functional materials as well as in the hypervalent silicon chemistry.For example, 1,3-disilacyclobutanes are converted into polycarbosilanes by ring-opening polymerization, 1 and silacyclohexanes are shown to behave as a mesogen of liquid crystals, 2 while 1,1,3,3,5,5-hexafluoro-1,3,5trisilacyclohexane 3 and 1,1,4,4-tetrafluoro-1,4-disilacyclohexane 4 are demonstrated to capture a fluoride ion.Therefore, there have been needs of general synthetic methods for polysilacycloalkanes, applicable to a variety of ring sizes and substitution patterns of silicon atoms. 5We have envisaged that the ring-closure reaction of 1,3-dimetallo-2-silapropanes 1 with bis(electrophile) 2 should be a powerful strategy for the preparation of polysilacycloalkanes 3 (equation 1). 6 Due probably to their instability, however, 1,3-dimetallo-2-silapropanes 1 are rarely used in organic synthesis in contrast with (triorganosilyl)methylmetal reagents.There a few precedents of 1,3-dilithio-2-silapropanes that are classified into bis(lithiomethyl)diorganosilanes (1: Z 1 , Z 2 = H) 7 and bis(lithiophenylmethyl) dimethylsilane (1: Z 1 = H, Z 2 = Ph). 8Those dianionic reagents were prepared by chlorine-lithium exchange of the parent silanes with lithium, or reductive cleavage of C-S bonds with lithium or lithium naphthalenide (Scheme 1), and allowed to react with an electrophile such as chlorotrimethylsilane and tributylchlorostannane to give the corresponding acyclic products in moderate to good yields, whereas ring construction of 1,3dilithio-2-silapropanes was limited to the synthesis of such 4-membered rings as 1,3disilacyclobutanes, 1-germa-3-silacyclobutane, and 1-titana-3-silacyclobutane, and yields of the cyclized products were usually low to moderate at best.

Generation and disilylation of bis[bis(methylthio)lithiomethyl]dimethylsilane
First of all, the synthesis of various-sized polysilacycloalkanes was studied by disilylation of bis[bis(methylthio)lithiomethyl]dimethylsilane (1a) with bifunctional chlorosilanes 2 (Scheme 4).Lithiation of 5a (1 molar amount) with t-BuLi (2.2 molar amounts) in THF at -40 °C followed by reaction with bifunctional chlorosilanes 2 (1.1 molar amounts) in THF at -78 °C to room temperature gave cyclized product 7.The results are summarized in Scheme 4 and Table 1.With dichlorodiorganosilanes 2a-c, four-membered silanes 7a-c were produced in moderate to good yields, respectively (entries 1-3).The yields are generally higher than those obtained from the reaction of 2a with bis(lithiomethyl)dimethylsilane (24%) or bis(lithiomethyl)diphenylsilane (46%) (see, Scheme 1), and thus it is apparent that the sulfenyl groups in 1,3-dianion 1a are the key to the success of the dianionic ring formation.Silylation of 1a with 1,2-dichlorodisilane 2d successfully gave 1,3,4-trisilacyclopentane 7d (entry 4).Six-membered rings 7e-g were also prepared in a similar way (entries 5-7).Silicon-silicon and silicon-oxygen bonds were found to tolerate the basic conditions.The introduction of dimethylsilylene, methylene, or oxygen into the 4-position of 1,3,5-trisilacyclohexane derivatives could be effected simply by changing the bis(electrophile) employed.Low yield of 7g may be attributed to longer bond lengths of both silicon-silicon and silicon-carbon bonds than a carbon-carbon bond making an enthaplic factor closer to 7-membered ring formation rather than 6-membered carbocycle formation.Similarly, 7membered trisilacycle 7h was produced by silylation with 2h in a relatively low yield (entry 8).
For an alkylation reagent, we first used a bis(halomethyl)silane which seems to be relatively reactive due to the silicon accelerating effect. 14hen 1a was treated with bis(bromomethyl)diphenylsilane (6b) in THF or THF/HMPA, alkylation did not take place and 1a was recovered unchanged.As 6b was consumed completely to give unidentified products, the 1,3-dianion 1a appears to have behaved as a base to cause deprotonation at the bromine-attached carbons.In view that 2-lithio-2-triorganosilyl-1,3dithianes undergo alkylation, 15 we reexamined the alkylation with bis[1,3-dithian-2-yl]silane 1b, and the results are summarized in Scheme 5 and Table 2.Although no alkylation took place in THF only, 6b was recovered quantitatively in sharp contrast with 1a (entry 1). 16In view that 2lithio-2-silyl-1,3-dithiane can be usually alkylated in THF, 15 the result of entry 1 shows that the reactivity of 1b is lower than that of 2-lithio-2-silyl-1,3-dithiane.To our delight, addition of such an aprotic polar co-solvent as 1,3-dimethylpropyleneurea (DMPU), 1,1,3,3-tetramethylurea (TMU), and hexamethylphosphoric triamide (HMPA) was effective to afford 1,4disilacyclohexane 8a (entries 2-4).Judging from the fact that the addition of an electron donor solvent often enhances the carbanionic character of alkyllithiums by decreasing aggregation degree, 17 the success of alkylation may be attributed to some structural change of 1b.As for an alkylating reagent 6, bromine turned out to be the best leaving group among the tested ones (entries 4-7).To our surprise, bis(iodomethyl)silane 6c gave no desired product, although iodine is the better leaving group than bromine usually.Using the solvent system consisting of THF and TMU, and bis(bromomethyl)silanes as an alkylating reagent, 1,4-disilacyclohexanes 8b-f having various substituents on silicon were prepared as colorless plates (entries 8-12).These are the first examples of the alkylation of 1,3-dilithio-2-silapropanes.
The alkylation methodology is applicable to the synthesis of different types of silacycloalkanes only by choosing different bifunctional electrophiles (Scheme 6).Silacyclopentane 10a was obtained with 1,2-dibromoethane in a low yield, whereas alkylation of 1b with 1,3-dibromopropane gave silacyclohexane 10b in an acceptable yield.Methylene, cyclohexylidene, and hydroxyl group-substituted silacyclohexanes 10c-e also produced in fair yields.Moreover, with 1,2-bis(bromomethyl)benzene, silacycloheptane 10f was successfully synthesized in 67% yield probably thanks to the geometrical constraint at the bromomethyl groups.

Removal of the sulfenyl groups in polysilacycloalkanes
Desulfurization of the cyclized products was finally examined.Attempted reduction of 7e with Raney Ni (W2) or Benkeser reduction (lithium/primary amine) failed, whereas radicalic reduction with hydrosilane or -stannane in the presence of AIBN was found effective as summarized in Equation 2. In particular, Bu 3 SnH reduced 7e much faster than hydrosilane to give 11 in a yield (NMR) comparable to the silane reduction.To demonstrate generality of the stannane reduction, the conditions were applied to 7c, 7d, 7e, 8a, and 8e, cleanly producing 12-15, respectively, as summarized in Scheme 7. In summary, we have shown that 1,1,3,3-tetrakis(alkylthio)-1,3-dilithio-2-silapropanes are a novel class of dianionic reagent and successfully undergo double silylation and alkylation leading to tetrakis(alkylthio)-substituted polysilacycloalkanes.The sulfenyl groups can be easily removed with Bu 3 SnH.This protocol definitely allows to prepare a diverse range of polysilacycloalkanes that will find applications in silicon-containing materials.

Experimental Section
General Procedures.Melting points were determined using a Yanagimoto Micro Melting Point Apparatus and were not corrected. 1H NMR spectra were measured on a Varian Mercury 200 (200 MHz), 300 (300 MHz), or 400 (400 MHz) spectrometer.The chemical shifts of 1 H NMR are expressed in parts per million downfield relative to the internal tetramethylsilane (δ = 0 ppm) or chloroform (δ = 7.26 ppm).Splitting patterns are indicated as s, singlet; d, doublet; t, triplet; q, quartet; brs, broad singlet. 13C NMR spectra were measured on a Varian Mercury 400 (100 MHz), or JEOL EX-270 (67.8 MHz) spectrometer with tetramethylsilane (δ = 0 ppm) or chloroform-d (δ = 77.0ppm) as an internal standard. 29Si NMR spectra were measured on a Varian Mercury 300 (59.6 MHz) spectrometer with tetramethylsilane as an internal standard (δ = 0 ppm).Chemical shifts are given in parts per million downfield relative to the internal standard.Infrared spectra (IR) were recorded on a Shimadzu FTIR-8400 spectrometer.GC-MS analyses were performed with a JEOL JMS-700 spectrometer by electron ionization at 70 eV unless otherwise indicated.Elemental analyses were carried out with a YANAKO MT2 CHN CORDER machine at Elemental Analysis Centers of Kyoto University or Tokyo Institute of Technology.TLC analyses were performed by means of Merck Kieselgel 60 F 254 ; column chromatography was carried out using Merck Kieselgel 60 (230-400 mesh).All reactions were carried out under an argon atmosphere.Tetrahydrofuran, diethyl ether, and hexane were distilled from sodium benzophenone ketyl right before use.HMPA, DMPU and TMU were purchased from Tokyo Kasei Organic Chemicals or Aldrich Chemical Co. Inc. and distilled from CaH 2 and stored over MS4A.

Table 4 .
Reduction of 7e a Yield was determined by 1 H NMR anaylsis of the crude product using trichloroethylene as an internal standard.Scheme 7. Removal of the sulfenyl groups in 7 and 8.