Free singlet Me2Ge is used as pilot germylene to explore the reactions of this group of heavy carbene analogs. After a short review on the known state, new results from the authors' laboratory are compiled. Insertions into O-H and other acidic X-H bonds depend on the acidity of the proton, but also on the donor quality of R-O-H resp. R-X-H (X = S, N). Acidic C-H bonds don't insert. A reversible complex Me2Ge'D is concluded as reactive intermediate, exhibiting an individual chemistry. It slows down the usual reactions of Me2Ge, but opens, e. g., new cycloadditions to different alkynes including those not reactive toward Me2Ge itself, and nucleophilic 1,4-as well as 1,6additions to open-chained or cyclic en-3-ones or l,3-dien-5-ones of the Michael or super-Michael type. -The formation, structure and reactions of the exotic 1,2,3trigermacyclopent-4-enes are studied, starting from Me2Ge. -Chemical and physical investigations on the photoinduced degradation of 7-dimethylgerma-1,2,3,4-Ph 4norbornadiene reveal a rapid one-step splitting of its (fluorescing) excited singlet into Me2Ge s and ground-state Ph 4 -naphthalene without any further intermediate.
A new type of reaction between dimethylgermylene (Me2Ge) 6-adducts, the new allylgermanes 3a-g, 4a-c, 10a, b, 11. The (l), generated in situ from a 'I-germanobornadiene, and elec-cis configuration of the resulting double bond of the 6 adducts tron-deficient 1,3-dienes RR'C = CH -CH = CH -Z (Z = 4a -c is remarkable. With open-chain dienes cycloaddition of C(O)Me, C02R" 2a-f] or cyclic dienones 8, 9 in the presence 1 without participation of water, yielding germacyclopentenes of water is described. The Michael-type behaviour of the sys-5a, b or germacyclopentanes ?a, b, competes with the Mitem Me2Ge/H,0 is clearly shown by the nucleophilic addition chael-type reaction. A slight variation of the substitution patto the unsaturated systems resulting in the formation of p-and tern of the dienes has a strong influence on the reaction course.The chemistry of organogermylenes R2Ge, explored mainly by means of dimethylgermylene MezGe (1) as a prototype, has taken an impressive development in the last decade'']. The behaviour of carbenes and silylenes as reactive intermediates differs significantly from its heavier analogue, germyleneC2], although Me2Ge (1) undergoes principally carbene analogous reactions, that means i n s e r t i~n [ '~~~] and cycloaddition reaction^",^].1,2-Cycloadditions of free MezGe to alkenes yielding stable germiranes are unknown so far. The rea~tion'~] between a sterically crowded thiacycloheptyne and a 7-germanorbornadiene has turned out to be not a germylene reaction[lal. 1,2-Cycloaddition plays an important role in the carbener6] and silylene chemistry[71.Quantum mechanical calculations of the strain enthalpy of a germirane, the 1,2-addition product of a germylene to an alkene, shows a comparative instability relative to the homologues cyclopropanes and siliranesLgl. It is assumed that germiranes are intermediates in the reaction with styrenes, the only mono-alkenes that react with Me2Ge, leading lastly to germacy~lopentanes~~~. Electron-deficient alkenes do not yield stable products with Me2Ge, but react regiospecifically, when water is present, to digermoxanes, with the germanium being attached to the electrophilic carbon atom of the alkenes[lo1 [see eq. (l)]. Recently, we have reported that this Michael-type reaction is not a hydrogermylation of the unsaturated system by 1,1,3,3-tetramethyldigermoxane. A precomplexation of MezGe by water had to be stated, instead[3b1. But, details of the reaction between alkenes and Me2Ge . OH2 are still lacking.In this paper we report on the reaction between Me2Ge and electron-deficient dienes in the presence of water. The extension of the reaction principle mentioned above to dienes provides an access to unusually substituted allylgermanes 3a-g, 4a-c, 10a, b, 11, and allows new mechanistic insights. The nucleophilic behaviour of MelGe . OH2, which acts as a Michael donor, is clearly shown by the regioselective reaction to the allylgermanes. Stereoselectivity and formation of unexpected products 7a, b gives further insight into reaction mechanisms. Results a...
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