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

Germenes Mes(2)Ge=CR2 1a (Mes = 2,4,6-trimethylphenyl; CR2 = fluorenylidene) and Mes(2)Ge=CR2' 1b (CR2' = 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 4h. Theoretical calculations performed on the model compound H2Ge=CH2 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.