Cycloadditions 2 + 2 , digermenes

Digermenes undergo [2 + 2] cycloaddition with phenylacetylene, as shown in reaction 20125,126. 

Tetrakis(2,6-diethylphenyl)digermene 23 undergoes smooth cycloaddition with PhC CPh to yield a four-membered 1,2-digermacyclobutene ... 

Reaction of the digermene 57 with diazomethane or trimethylsilyldiazomethane yields the azadigermiranes 58 via a [2+l]-cycloaddition (Equation (119)).158 Coupling of a cyclic carbene and a cyclic germylene furnishes the zwitter-ionic species 59 (Equation (120)).159... 

Attempts to trap the digermene 74 in solution with benzil afforded only the cycloadduct 109 because of the [1 + 4] cycloaddition with germylene 10863 [Eq. (26)]. Another proof of the formation of germylene 108 in solution from 74 was given by cryoscopic studies.63... 

Compounds with a Si=Si or Ge=Ge bond (i.e., disilenes and digermenes) can be isolated when the double bond contains bulky substituents. Only a few cycloaddition reactions with diazo compounds are known, and two reaction modes have been observed. One of the paths leads to the formation of disiliranes 184 (242) and digermiranes 185 (243) (Scheme 8.42), probably by ring contraction of an initially formed [3 + 2] cycloaddition product. The other path involves a 1,1-cycloaddition of the diazoalkane to give disilaaziridine 186 (244) and digermaazir-idine 187 (245). This nitrene-like reactivity is rather uncommon although some intramolecular examples are known (see Section 8.6.1). 

A germacyclopropane with electronegative groups at the ring carbon atoms, 33, was obtained by the [1-I-2] cycloaddition reaction of tetrakis(2-/-butyl-4,5,6-trimethylphenyl)digermene with hexafluorobut-2-yne. A short C=C double bond (1.324(5) A) and a sharp C-Ge-C bond angle of 39.93(14)° were determined by X-ray analysis <2002ZFA2821>. 

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