Addukte der Ethene Me2EC(SiMe3)2 (E=Si, Ge, Sn) mit LiR und RN3: Wie rasch bilden sie sich?

Unsaturated compounds Me2EC(SiMe3)2 (E=Si, Ge, Sn) are formed as short-lived intermediates by reaction of Me2EXCBr(SiMe3)2 with LiR via Me2EXCLi(SiMe3)2 (X=electronegative substituent; R=organyl) and — in the absence of trapping reagents — react with Me2EXCLi(SiMe3)2 and LiR (as long as present) under formation of cyclobutanes [Me2EC(SiMe3)2]2 as well as adducts Me2ERCLi(SiMe3)2 of Me2EC(SiMe3)2 and LiR. In the presence of an excess of organyl or silyl azides RN3, as well as lithium organyls or silyls LiR, which indeed act as very active trapping reagents for Me2EC(SiMe3)2 with formation of [3+2] cycloadducts Me2EC(SiMe3)2·RN3 and adducts Me2ERCLi(SiMe3)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 Me2SiBrCBr(SiMe3)2 to two different trapping reagents LiR and LiR′ or LiR and RN3 in Et2O, relative rates of the reactions of LiR or RN3 with the silene Me2SiC(SiMe3)2 are determined. Hereafter the insertion reactivity of LiR and the [3+2] cycloaddition reactivity of RN3 decreases when the bulkiness of R increases (decreasing reactivity in the order LiMe>LinBu>LiPh>LitBu; LiMe>LiCH(SiMe3)2>LiC(SiMe3)3; Me3SiN3>tBuMe2SiN3>tBu2HSiN3>tBu2MeSiN3>Ph3SiN3>tBu3SiN3). The influences of electronic effects are obviously smaller than those of steric effects (decreasing reactivity in the order LiC(SiClMe2)(SiMe3)2>LiC(SiBrMe2)(SiMe3)2>LiC(SiMe3)3; tBu2MeSiN3>tBu2ClSiN3; Me3CN3>Me3SiN3).