Dimere der Ethene Me2EC(SiMe3)2 (E=Si, Ge, Sn): Auf welchem Wege entstehen sie aus Me2EXCM(SiMe3)2? Wie sind sie strukturiert?

Alkali metal organyls or silyls MR (e.g. LiMe, LinBu, LitBu, LiPh, LiCH(SiMe3)2, LiC(SiMe3)3, NaSitBu3) convert equimolar amounts of bromomethanes Me2EXCBr(SiMe3)2 with E=Si, Ge, Sn and electronegative substituents X (e.g. F, Br, OPh) in organic solvents (e.g. pentane, diethyl ether, tetrahydrofuran) (i) by a very fast Br/M exchange into the ‘metalation products’ Me2EXCM(SiMe3)2, which thermolyze under formation of ‘cyclobutanes’ [Me2EC(SiMe3)2]2, and (ii) to a lesser extent by X/R exchange into ‘substitution products’ Me2ERCBr(SiMe3)2. As shown by trapping experiments, the unsaturated compounds Me2EC(SiMe3)2 play the role of short-lived intermediates in both reactions. They are formed from Me2EXCM(SiMe3)2 by MX elimination and add the present alkalimetal compounds Me2EXCM(SiMe3)2≡MR′ or MR, respectively. The products Me2ER′CM(SiMe3)2 with R′=C(EXMe2)(SiMe3)2, obtained in this way, eliminate MX under formation of the mentioned ‘cyclobutanes’. On the other hand, the compounds Me2ERCM(SiMe3)2 convert unreacted Me2EXCBr(SiMe3)2 in Me2EXCM(SiMe3)2 under formation of Me2ERCBr(SiMe3)2. Relative rates of both the metalation reactions and the salt eliminations are determined. X-ray structure analyses of [Me2EC(SiMe3)2]2 (E=Si, Ge, Sn) prove their 1,3-dielementacyclobutane structure with planar four-membered ECEC rings.