Synthesis of bis(germacyclopropa)benzenes and structures of their annelated benzene rings

Extremely hindered bis(germacyclopropa)benzenes (3a,b; the IUPAC name is 4,8-digermatricyclo[5.1.0.0^3,5]octa-1,3(5),6-triene) were synthesized as stable crystalline compounds by the reaction of the corresponding dilithiogermane Tbt(Dip)GeLi₂ (8; Tbt = 2,4,6- tris[bis(trimethylsilyl)methyl]phenyl, Dip = 2,6-diisopropylphenyl) with 1,2,4,5-tetrabromobenzene. The structures of the two stereoisomeric bis(germacyclopropa)benzenes (3a, cis isomer; 3b, trans isomer) were definitively determined by X-ray crystallographic analysis. The central benzene ring of 3a was found to be folded, in contrast to the planar benzene ring of 3b. The X-ray crystallographic analyses of 3a and 3b and the theoretical calculations for some model molecules revealed that the annelated benzene rings have no distinct bond alternation.