Soichiro Kyushin

A series of novel bis(alkenyl)oligosilanes was synthesized via ruthenium‐catalyzed double hydrosilylation of alkynes with dihydrooligosilanes. The structural and photophysical features of the bis(alkenyl)oligosilanes were studied by X‐ray crystallography and UV and fluorescence spectra. The functional groups on the alkenyl moieties significantly affect not only their conformation and arrangement in crystals but also the UV and fluorescence spectra.

Ruthenium‐catalyzed hydrosilylation of aldehydes with hydrooligosilanes is reported. Among the examined ruthenium catalysts, RuH₂(CO)(PPh₃)₃ gave the best results. The reactions proceed with the Si−Si bonds preserved to afford the corresponding alkoxyoligosilanes in good yields. A series of aryl aldehydes with an electron‐withdrawing group is applicable, whereas those with an electron‐donating group are not suitable for the reaction. Mechanistic considerations suggest that the key points for the preservation of the Si−Si bonds are the formation of zero‐valent ruthenium species and the efficient coordination of the aldehyde to the ruthenium center prior to the oxidative addition of the Si−H bond.

Two stereoisomers of bicyclic ladder oligosilanes bearing phenyl and tert‐butyldimethylsilyl groups were synthesized, and their structures and electronic properties are compared with a previously reported perisopropyl‐substituted analog. These novel ladder oligosilanes have highly twisted structures with the twist angles between the two terminal Si−Si bonds of av. 37.7 and 39.8°. The UV spectra of these compounds show bathochromically shifted absorption maxima with the longest wavelength compared with that of the perisopropyl‐substituted bicyclic ladder oligosilane. The electronic properties of the ladder oligosilanes were analyzed by theoretical calculations.

The four phenyl groups of 1,1,3,3‐tetraphenyl‐2,2,4,4‐tetrakis(trimethylsilyl)cyclotetrasilane were partially replaced by chlorine atom(s) by successive treatment with trifluoromethanesulfonic acid and lithium chloride. The replacement of all phenyl groups was carried out with hydrogen chloride in the presence of aluminum chloride. The structural features of the resulting mono‐, di‐, and tetrachlorocyclotetrasilanes were compared by X‐ray crystallography.

Introduction of various types of substituents onto oligosilane chains is important for not only synthesis scheme of functionalized oligosilanes but also modification and tuning of the unique photophysical and electronic properties. To explore further potentials of oligosilanes, development of their synthetic methods is essential. Since transition metal-based methodologies have contributed greatly to the remarkable progress in synthetic organic chemistry, application of similar strategies for the oligosilane synthesis is highly desirable. However, few such attempts have been reported so far, probably due to the intrinsic reactivity of the Si-Si bond toward transition metal species. We are trying to develop the transition metal-catalyzed reactions of oligosilanes without Si-Si bond cleavage. In this paper, titanium-catalyzed selective monoreduction of dichlorooligosilanes, ruthenium-catalyzed alkoxylation of hydrooligosilanes, and ruthenium-catalyzed regio- and stereoselective hydrosilylation reactions using hydrooligosilanes are described. By choosing appropriate catalysts, these reactions successfully provide a series of novel oligosilane derivatives with the Si-Si bond intact.

In 1996, we reported that silyl groups of 9,10-disilylanthracenes significantly affect the UV/Vis and fluorescence spectra. Although the results indicate that the silyl groups have strong electronic effects on anthracene, the details of the mechanisms responsible for this have not yet been clarified. This article describes the analysis of the UV/Vis and fluorescence spectra of 9,10-bis(diisopropylsilyl)anthracene by theoretical calculations. This study reveals that π conjugation of anthracene is extended by cooperation of σ–π and σ*–π* conjugation between the silyl groups and anthracene. This effect increases the transition moment of the π–π* transition of anthracene. As a result, the molecular extinction coefficient of the ¹L_a band and the fluorescence quantum yield are increased.

The fluorescence quantum yield of triphenylene is low because the transition moment between the HOMOs and the LUMOs is negligibly small. To improve the fluorescence quantum yield, break of the symmetrical structure of triphenylene is necessary. We found that the rotational effect of the 4-(trimethylsilyl)phenyl groups of 2,3,6,7,10,11-hexakis[4-(trimethylsilyl)phenyl]triphenylene is effective to break the symmetry in spite of the symmetrical structure, leading to improvement of the fluorescence quantum yield.