Not only steric shielding but also resonance with a delocalized π‐electron system can help to stabilize nonclassical multiple bonds. Thus, despite high steric demands, 15 C atoms and the Ge atom of the title compound 1 lie nearly in the same plane. The experimental finding is in agreement with the result of a MNDO calculation.
There is now a great deal of interest in group 13, 14, and 15 elements in low coordination states.1 Since 1981, several new compounds of the type M=M/ (M and M': group 13, 14, and 15 elements) withbonds have been reported. In organogermanium chemistry, the first stable derivatives with a sp2-hybridized germanium have only been prepared very recently: they are the digermenes R2Ge=GeR22 (R = bis(trimethylsilvl) methyl,23 2,6-dimethylphenyl,2b 2,6-diethylphenyl2c), the germaphosphene R2Ge=PR,3a (R = mesityl, R' = 2,4,6-tri-rert-butylphenyl), and the germaimines R2Ge=NR,3b (R = (Me3Si)2N, R' = N=C-(R,,)(R,,/))• Germenes R2Ge=CR2' have long been speculated as reactive intermediates and could only be characterized by trapping reactions.lc,d'4'5
Phosphagermaallene Mes*PCGe(t-Bu)Tip 1 (Tip = 2,4,6-triisopropylphenyl, Mes* = 2,4,6-tri-tert-butylphenyl) has been obtained by an improved synthesis in relation to the previously reported preparation starting from t-BuGeCl3. Very short GeC (1.761(2) Å) and PC (1.625(2) Å) double bond lengths, trigonal planar geometry around the Ge atom, and GeCP bond angle of 166.57° are indicative of an heteroallenic structure for 1. Its reactions with crotonaldehyde and cinnamaldehyde lead to 1-oxa-2-germacyclobutanes by a [2 + 2] cycloaddition between the GeC and CO double bonds; with methyl vinyl ketone, both a [2 + 4] cycloaddition between the GeC and the OC−CC moieties and an ene-reaction are observed leading to a 1-oxa-2-germacyclohex-5-ene and a germyl(butadienyl)ether, respectively. With acetophenone, an ene-reaction occurs to afford exclusively a germyl(vinyl)ether. DFT calculations have been performed to explain the regiochemistry observed.
Addition of the germene Mes2Ge=CR2 to 1,4-naphthoquinone yields a singular o-quinodimethane which gives Diels-Alder reactions at room temperature and reacts cleanly with oxygen to form an endoperoxide.
Germenes Mes 2 GedCR 2 1a (Mes = 2,4,6-trimethylphenyl; CR 2 = fluorenylidene) and Mes 2 GedCR 0 2 1b (CR 0 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 GedCH 2 1H postulate that the first steps of its reaction with 1,4-benzoquinone are a double [2þ2] cycloaddition between the GedC and CdO double bonds, leading to a transient dispiro compound containing two oxagermetane rings, followed by its isomerization to derivative 3H. Between 1a and 9,10anthraquinone, a double [2þ4] cycloaddition is observed involving the oxygens and the ortho-carbon atoms of one adjacent aromatic ring system, to generate the dioxadigermabenzopyrene derivative 2a.
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