Dimere der Ethene Me 2EC(SiMe 3) 2 (E=Si, Ge, Sn): Auf welchem Wege entstehen sie aus Me 2EXCM(SiMe 3) 2? Wie sind sie strukturiert?

Abstract

Alkali metal organyls or silyls MR (e.g. LiMe, Li n Bu, Li t Bu, LiPh, LiCH(SiMe 3) 2, LiC(SiMe 3) 3, NaSi t Bu 3) convert equimolar amounts of bromomethanes Me 2EXCBr(SiMe 3) 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’ Me 2EXCM(SiMe 3) 2, which thermolyze under formation of ‘cyclobutanes’ [Me 2EC(SiMe 3) 2] 2, and (ii) to a lesser extent by X/R exchange into ‘substitution products’ Me 2ERCBr(SiMe 3) 2. As shown by trapping experiments, the unsaturated compounds Me 2EC(SiMe 3) 2 play the role of short-lived intermediates in both reactions. They are formed from Me 2EXCM(SiMe 3) 2 by MX elimination and add the present alkalimetal compounds Me 2EXCM(SiMe 3) 2≡MR′ or MR, respectively. The products Me 2ER′CM(SiMe 3) 2 with R′=C(EXMe 2)(SiMe 3) 2, obtained in this way, eliminate MX under formation of the mentioned ‘cyclobutanes’. On the other hand, the compounds Me 2ERCM(SiMe 3) 2 convert unreacted Me 2EXCBr(SiMe 3) 2 in Me 2EXCM(SiMe 3) 2 under formation of Me 2ERCBr(SiMe 3) 2. Relative rates of both the metalation reactions and the salt eliminations are determined. X-ray structure analyses of [Me 2EC(SiMe 3) 2] 2 (E=Si, Ge, Sn) prove their 1,3-dielementacyclobutane structure with planar four-membered ECEC rings.