Tricyclic laddersiloxanes

Synthesis of Tricyclic Laddersiloxanes (6-8-6-Membered Fused Rings) 436... 

SYNTHESIS OF TRICYCLIC LADDERSILOXANES (6-8-6-MEMBERED FUSED RINGS)... 

Tricyclic laddersiloxanes with 8-8-8-membered rings were the first laddersiloxanes those structures were determined. Synthesis of Me,Ph-tricyclic laddersiloxane (Me8Ph4Si80io) was reported hy Brown in 1965 (1), and later the structure was determined to have the syn-form hy Shklover (2). First nnh-tricyclic laddersiloxane was reported hy Shklover in 1981 (3). All these compounds were obtained by the reaction of chlorosilanes and silanols in the presence of base. Our group applied this method to obtain l-Pr,Ph-tricyclic laddersiloxane in 85% yield (4). In this synthesis, using pyridine as a solvent afforded better yield than using stoichiometric amount of pyridine. 

In this section, synthesis of all-phenyl tricyclic laddersiloxane is shown. Because of the slight instability of all-cls-[PhSi 0H)0]4 in the presence of base, the isolated yield was 9%. The result of Si NMR spectra showed two resonances at —43.6 ppm and —76.3 ppm in CDCI3 for terminal and inner silicon, respectively. The syn-structure was confirmed by X-ray crystallographic analysis (5). 

Dichlorotetraphenyldisiloxane (2.9 g, 6.4 mmol) in THF (10 mL) was added dropwise to a solution of all-crs-[PhSi(0H)0]4 (1.7 g, 3.1 mmol) and triethylamine (1.2 g, 12 mmol) in THF (10 mL) at 0°C imder argon atmosphere. The mixture was refluxed for one day. To the reaction mixture, CHCI3 was added and the organic phase was washed with a saturated ammonium chloride aqueous solution and hrine. The organic phase was dried over anhydrous sodium sulfate and concentrated to afford a crude solid, which was washed with methanol and ethanol. The solid was further purified by recrystallization from acetone. Tricyclic laddersiloxane (0.37 g, 9%) was obtained as a white solid. 

For the understanding of stmcture-property relationships in polysilsesquioxanes -particularly those based on tacticity - it is important to study smaller but well-defined and structurally characterized oligo-silsesquioxanes. The controlled synthesis of such compounds was proven by the formation of tricyclic laddersiloxanes pentacyclic ladder siloxanes bi-, tri-, tetra-, and pentacyclic laddersiloxanes with an all-anfi conformation extendible pentacyclic as well as heptacyclic ladder siloxanes and methyl-substituted ladder polysilsesquioxane [36],... 

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