Aromatic Compounds Consisting of Heavier Group 14 Elements

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The chemistry of aromatic compound consisting of heavier Group 14 elements has been remarkably developed. In 1997, we prepared tris (tri-tert-butylsilyl) cyclotrigermenyium ion [(^tBu_3SiGe)_3^+], which is the first example of a 2π aromatic system consisting of germanium atoms as well as a free germyl cation. In this research project, we have established the preparation of heavier analog of cyclopropene and also investigated its transformation to aromatic compounds consisting of heavier Group 14 elements.1. The reductive coupling of ^tBu_2MeSi-substituted-dibromosilane and-tribromosilane gave tetrakis [di-tert-butyl (methyl) silyl] cyclotrisilene (1) with an unsaturated three membered ring structure consisting of silicon. The reaction of cyclotrisilene (1) with [Et_3Si (benzene)]^+ TPFPB^-[TPFPB^-=(C_6F_5)_4B^-] produced 1, 2, 3-tris [di-tert-butyl (methyl) silyl]-4, 4-di-tert-butylcyclotetrasilenium TPFPB^-(2^+ TPFPB^-), which can be isolated as yellow crystals. The crystallographic analysis as well as NMR study reveal that cyclotetrasilenium ion (2^+) is a "free" silyl cation with a homoaromatic 2π-electron system in the condensed phase.2. Cyclotetrasilenium ion (2^+) was found to undergo stepwise reduction to give cyclotetrasilenyl radical (2^< >) by one electron reduction and cyclotetrasilenide ion (2^-) by two electron reduction, respectively. We succeeded in synthesizing red crystals of cyclotetrasilenyl radical (2^< >) as the first isolable silyl radical with an allyl type delocalized structure.3. We have also succeeded in synthesizing tetrakis [di-tert-butyl (methyl) silyl]-1-disilagermirene (3) with a Si=Si double bond, which is thermally or photochemically isomerized to tetrakis [di-tert-butyl (methyl) silyl]-2-disilagermirene (4) with a Si=Ge double bond. 2-Disilagermirene (4) reacted with phenylacetylene to give 3-germa-1, 2-disilacyclopent-2, 4-diene derivative (5) with a Si=Ge-C=C conjugated system.