Synthesis and Characterization of Silyl[n]acediynes (Linearly Fused Benzodehydro[12]annulenes): Utilizing Bulkiness of Silyl Groups to Improve Selectivity.

Abstract

[n]Acediynes have unique properties owing to their highly strained and extended π-electron system. We have previously reported the selective synthesis of a silyl[1]acediyne, i.e., silyldibenzotetradehydro[12]annulene, utilizing the bulkiness of the silyl groups to improve the selectivity of the reaction. Herein, we report the facile synthesis of high-order silyl[n]acediynes via Eglington coupling between silyldiethynylbenzene and silyltetraethynylbenzene. In fact, silyl[2]acediyne as well as silyl[1]acediyne can be isolated from the reaction mixture. Their structures were determined using X-ray crystallography and discussed with reference to our previous report, and their spectroscopic properties, including absorption, fluorescence, Raman, and IR spectra, were fully characterized with DFT calculations. A drastic redshift in their absorption and fluorescence spectra occurred with an increase in the order number n of the [n]acediynes. The antiaromaticities of the dehydro[12]annulene moieties at the center of the annulene ring in these silylacediynes were also evaluated using NICS values. The calculated values were small and positive, which confirmed the weak antiaromatic character of the rings.