Application of the Sila-Friedel-Crafts Reaction to the Synthesis of π-extended Silole Derivatives and Their Properties

The intramolecular sila-Friedel-Crafts reaction was developed as a new method for the construction of a dibenzosilole skeleton. This reaction proceeds under mild conditions to afford the target in relatively good yield, indicating its availability as a versatile synthetic method. This reaction can be applied to the synthesis of π-extended silole derivatives such as ladder-type silafluorene 8 and spiro-type silabifluorene 9. Furthermore, the synthesis of two-dimensionally extended silole derivatives utilizing the sila-Friedel-Crafts reaction as the multiple intramolecular cyclization was achieved, including the first synthesis of trisilasumanene 18. The X-ray crystallographic analysis of trisilasumanene 18 demonstrated the planarity in the main π-framework, in contrast to sumanene and its sulfur analogue, trithiasumanene, bearing the bowl-shaped structures. In the UV-vis absorption spectra, the absorption bands of triphenylenosiloles 18 and 19 were slightly red-shifted compared to that of hexabutoxytriphenylene 22. The weak absorption bands were also observed in the longer-wavelength region in 18 and 19, which is derived from σ*-π* conjugation of the silole skeletons. In addition, 18 and 19 showed the blue fluorescence in dichloromethane and in the solid state.