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Reactivity Studies of N-Heterocyclic Carbene Complexes of Germanium(II)

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posted on 08.11.2010, 00:00 by Paul A. Rupar, Viktor N. Staroverov, Kim M. Baines
The chemistry of three N-heterocyclic carbene (NHC) complexes of GeR2, where R = Cl (1), OtBu (2), Mes (3) (Mes = 2,4,6-trimethylphenyl), toward 2,3-dimethylbutadiene (DMB), 3,5-di-tert-butylorthoquinone, methyl iodide, pivalic acid, and benzophenone is reported. Upon heating, 2 and 3 cyclize with DMB to yield a germacyclopentene and free NHC. In contrast, 1 does not react with DMB. PB1PBE/6-311+G(d,p) model chemistry shows that the cycloaddition reactions of NHC-GeX2 (X = F, Cl) with butadiene are not thermodynamically favorable. 3,5-Di-tert-butylorthoquinone reacts rapidly with 13 to form cyclic products; in the case of 1 and 2, the NHC remains coordinated to the germanium, resulting in a hypervalent species. Compounds 13 react with methyl iodide by displacement of I; in each case [NHC-GeR2Me]+ is produced. Only compound 3 reacts in a controlled fashion with pivalic acid; both 1:1 and 1:2 adducts were characterized. Benzophenone failed to react with 1 or 2 but did undergo cycloaddition with 3. In comparison with uncomplexed GeR2 species, the NHC-GeR2 complexes are less reactive. The prospect of using NHC-GeR2 as a synthon for GeR2 appears to be reaction specific.