Addukte der Ethene Me 2EC(SiMe 3) 2 (E=Si, Ge, Sn) mit LiR und RN 3: Wie rasch bilden sie sich?

Abstract

Unsaturated compounds Me 2EC(SiMe 3) 2 (E=Si, Ge, Sn) are formed as short-lived intermediates by reaction of Me 2EXCBr(SiMe 3) 2 with LiR via Me 2EXCLi(SiMe 3) 2 (X=electronegative substituent; R=organyl) and — in the absence of trapping reagents — react with Me 2EXCLi(SiMe 3) 2 and LiR (as long as present) under formation of cyclobutanes [Me 2EC(SiMe 3) 2] 2 as well as adducts Me 2ERCLi(SiMe 3) 2 of Me 2EC(SiMe 3) 2 and LiR. In the presence of an excess of organyl or silyl azides RN 3, as well as lithium organyls or silyls LiR, which indeed act as very active trapping reagents for Me 2EC(SiMe 3) 2 with formation of [3+2] cycloadducts Me 2EC(SiMe 3) 2 ·RN 3 and adducts Me 2ERCLi(SiMe 3) 2, the formation of cyclobutanes and adducts is suppressed in the first case, whereas adducts are formed exclusively in the second case. As a result of determination of relative amounts of the products, formed by addition of Me 2SiBrCBr(SiMe 3) 2 to two different trapping reagents LiR and LiR′ or LiR and RN 3 in Et 2O, relative rates of the reactions of LiR or RN 3 with the silene Me 2SiC(SiMe 3) 2 are determined. Hereafter the insertion reactivity of LiR and the [3+2] cycloaddition reactivity of RN 3 decreases when the bulkiness of R increases (decreasing reactivity in the order LiMe>Li n Bu>LiPh>Li t Bu; LiMe>LiCH(SiMe 3) 2>LiC(SiMe 3) 3; Me 3SiN 3> t BuMe 2SiN 3> t Bu 2HSiN 3> t Bu 2MeSiN 3>Ph 3SiN 3> t Bu 3SiN 3). The influences of electronic effects are obviously smaller than those of steric effects (decreasing reactivity in the order LiC(SiClMe 2)(SiMe 3) 2>LiC(SiBrMe 2)(SiMe 3) 2>LiC(SiMe 3) 3; t Bu 2MeSiN 3> t Bu 2ClSiN 3; Me 3CN 3>Me 3SiN 3).