Preparation and Structure of Alkenyl- and Alkynyl-Substituted Polysilamod­ules HC(SiMe₂SiMe₂)₃CR

Abstract

Facile methods for the introduction of alkenyl and alkynyl groups at the bridgehead position of polysilamodule HC(SiMe 2 SiMe 2 ) 3 CH are established. Deprotonation of the module with BuLi/t-BuOK in THF at -40°C followed by alkylation with bromoacetaldehyde diethyl acetal gives the corresponding acetal, which was hydrolyzed with zinc chloride, yielding a formylmethylsubstituted cage compound. Treatment of the aldehyde with triflic anhydride in the presence of 2,6-di-t-butyl-3-methylpyridine produced the corresponding enol triflate consisting of a 4:1 mixture of E and Z isomers. Palladium-catalyzed cross-coupling of the triflate with organometallic reagents afforded alkenyl group-substituted cage molecules in good yields. Meanwhile, lithiation of phenylthiosubstituted cage derivative with lithium 4,4¢-di-tert-butylbiphenylide in THF followed by transmetalation with copper iodide and subsequent coupling with iodoalkynes allowed us to prepare alkynyl-substituted polysilacage compounds. The structures were determined by X-ray diffraction of single crystals.Since s-electrons of Si-Si bonds in polysilanes can delocalize, compounds containing Si-Si bonds show unique electronic and optical properties resulting from the delocalization of s-electrons. 1 Exploration of new siliconbased organic materials has led us to design 2,3,5,6,7,8-hexasilabicyclo[2.2.2]octane 1 which bundles three parallel Si-Si bonds in a three-dimensional manner and to achieve its facile synthesis (Figure 1). 2 Bridgehead-silylated or -stannylated derivative 2 or 3 exhibited a red shift of UV absorption maxima compared to parent compound 1, 2a while s-p electronic interaction between the polysilacage moiety and a phenyl group at the bridgehead position was found operative by UV spectra of arylated derivatives C n H 2n+1 C(SiMe 2 SiMe 2 ) 3 CAr 4. 3 To gain further insights into s-p conjugation 4 between the cage moiety and a pconjugated system at the bridgehead position, it is intriguing to connect a p-conjugated moiety to the polysilacage module via a carbon-carbon unsaturated group. We report herein the facile synthesis of alkenyl-and alkynyl groupsubstituted polysilacage compounds 5 and 6, respectively, and their molecular structures determined by X-ray diffraction. Figure 1 Polysilacage compoundsAt the outset, we studied the introduction of a carbon-carbon double bond into 1. Since nucleophilic substitution at an sp 2 carbon is generally difficult to perform, we designed an indirect strategy for the purpose. Thus, 1 was first deprotonated with BuLi/t-BuOK in THF at -40°C and trapped with bromoacetaldehyde diethyl acetal, giving rise to acetal 7 in 70% yield (Scheme 1). Hydrolysis of 7 with zinc chloride gave formylmethyl-substituted cage compound 8 in 87% yield. Treatment of 8 with triflic anhydride in the presence of 2,6-di-t-butyl-4-methylpyridine produced enol triflate 9 consisting of a 4:1 mixture of E and Z isomers in 74% yield. Since recrystallization of 9 from hexane afforded E-isomer (mp 175.5-176.5°C) as single crystals, the structure w...