Cycloaddition Reactions of an Acetylene-Linked Bis(germaethene)1,2

Abstract

3,5-Di-tert-butyl-1,2-benzoquinone reacts with the acetylene-linked bis(germaethene) Ar2GeC(R)−C⋮C−C(R)GeAr2, R = C6H5, Ar = 2-tBu-4,5,6-Me3C6H (4), in a [4+2] fashion to furnish the sterically crowded product 5 of 2-fold cycloaddition to the GeC bonds. The reaction of 4 with 1,2-dicyanoethylene proceeds differently, namely, by [2+4] cycloaddition of one of the C⋮N bonds to the GeC and C⋮C bonds, followed by a [2+2] cycloaddition of the remaining GeC bond to the newly formed CC bond to give the bicyclic compound 6. An unprecedented C−H activation of a tert-butyl and an aryl C−H bond leads finally to the hexacyclic compound 8 via the tetracyclic product 7. Cyanogen gas reacts similarly by [2+4] and [2+2] cycloadditions to furnish the bicyclic compound 9. In this case, no C−H addition to the endocyclic CN bond is observed.