Synthesis of germacyclic compounds by cyclization and annulation reactions utilizing in situ generated germyl cations

Germyl cations are one of the heavier group 14 element analogues of carbocations and have been extensively studied for their structures, spectroscopic features, and reactivities to various molecules. 4,4′-Di-tert-butyl-2-diphenylgermylbiphenyl (1) was reacted with [Ph₃C][B(C₆F₅)₄] in the presence of 2,6-lutidine to produce dibenzogermole 2 in good yield via the Friedel−Crafts-type cyclization of a germyl cation generated in situ at the neighboring benzene ring. This germa-Friedel−Crafts reaction was used to synthesize ladder-type germole 4 and trigermasumanene 6. The emission bands of 2 were red-shifted relative to those of fluorene, which for dibenzosilole are associated with orbital interactions between a heavier group 14 element moiety and a π-conjugated framework, such as σ*−π* conjugation. The following reaction was conducted to demonstrate annulation between an in situ generated germyl cation and alkynes. Benzyldiphenylgermane (7) was treated sequentially with [Ph₃C][B(C₆F₅)₄], 2,6-di-tert-butyl-4-methylpyridine, and alkynes to produce 1,2-dihydro-2,2-diphenyl-2-germanaphthalene derivatives 8, together with alkynylgermane 9 in some cases. We compare the products obtained in this study with those obtained from reactions involving an in situ generated silyl cation, which we reported previously.