Reactions of Germenes with Somepara-Quinones: Formation of a Tricyclic Compound from 1,4-Benzoquinone Undergoing an Unexpected Rearrangement

Germenes Mes 2 Ge═CR 2 1a (Mes = 2,4,6-trimethylphenyl; CR 2 = fluorenylidene) and Mes 2 Ge═CR′ 2 1b (CR′ 2 = 2,7-di- tert -butylfluorenylidene) react with 1,4-benzoquinone and 2,3,5,6-tetramethyl-1,4-benzoquinone to give compounds 3a, 3b, and 5a, containing a 1,4-cyclohexadiene unit as central ring. Upon prolonged storage in diethyl ether or THF solution, compounds 3a and 3b undergo a double 1,3-hydrogen shift, leading to their structural isomers 4a and 4b . Theoretical calculations performed on the model compound H 2 Ge═CH 2 1H postulate that the first steps of its reaction with 1,4-benzoquinone are a double [2+2] cycloaddition between the Ge═C and C═O double bonds, leading to a transient dispiro compound containing two oxagermetane rings, followed by its isomerization to derivative 3H . Between 1a and 9,10-anthraquinone, a double [2+4] cycloaddition is observed involving the oxygens and the ortho -carbon atoms of one adjacent aromatic ring system, to generate the dioxadigermabenzopyrene derivative 2a .