Silaheterocyclen, XVII / Silaheterocycles, XVII

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Abstract
The silene 2 is formed by reacting 2-chloro-1,3-bis(trimethylsilyl)-2-vinyl-1,3-diaza-2-silacyclopentane (5) with LiBut in n-pentane in the temperature range from -10 to +10°C. The reaction primarily leads to the corresponding a-lithio adduct 6 by addition of the Li-organyl to the vinyl group of 5; subsequent 1,2-LiCl elimination yields 2 as an intermediate. 2 can be trapped by polar reagents Me3SiX (X = OMe, OSO2CF3) as well as by organic dienes which add across the Si=C bond; in the absence of trapping agents, trisilacyclohexane 7 is isolated. The cycloaddition behaviour of 2 is best described by the two mesomeric formulae (a) and (β), with competitive [4+2] and [2+2] addition routes: While with butadienes and norborna-2,5-diene the [2+2] products are formed mainly in a multiple step reaction via zwitterionic intermediates (1,4 dipoles), the ratio of the [4+2]/[2+2] derivatives from 2 and cyclohexa-1,3-diene becomes nearly equivalent. 2 is thus quite comparable in its reactivity to dichloroneopentylsilene, Cl2Si = CHCH2But.

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