Silaheterocyclen, XXV. (Me3SiO)2SiCHCH2tBu: Ein Siloxy‐funktionalisiertes Syntheseäquivalent für Dichlorneopentylsilaethen

Silaheterocycles, XXV[1]. — (Me3SiO)2SiCHCH2tBu: A Siloxy‐Functionalized Synthetic Equivalent for Dichloroneopentylsilene (Me3SiO)2SiCHCH2tBu (4) is obtained by the reaction of (Me3SiO)2Si(Cl)CHCH2 (2) with LitBu. The reaction proceeds via the α‐lithiated species 3, which can be intercepted by trimethylsilyl triflate to give 5. Silaethene 4 dimerizes to the E/Z‐isomers of disilacyclobutane 7 and adds to Me3SiOMe to give 6. The reactivity of 4 resembles that of Cl2SiCHCH2tBu (1): A [4 + 2] cycloadduct is obtained from the reaction with anthracene, and with 1,3‐cyclohexadiene and 1,3‐cycloheptadiene mixtures of monosilacyclobutanes (10 and 12) and bicyclic Diels‐Alder adducts (9 and 11) are available. Exclusive [2 + 2] cycloaddition reactions take place with styrene (formation of 13), 2,3‐dimethyl‐1,3‐butadiene (→19), and 1,3‐butadiene (→21). With quadricyclane only the product (14) originating from a [2 + 2 + 2] cycloaddition is formed, while with 2,5‐norbornadiene the [2 + 2] and [2 + 2 + 2] cycloadducts (14 and 15) are formed. In addition the ene products (16, 18, 20, 22) are isolated in some cases from side reactions. — The tendency of 1 and 4 to preferably participate in [2 + 2] cycloaddition reactions is ascribed to the polarity of the SiC bond and the additional influence of the π donor ligands at silicon. These factors enable a stepwise [2 + 2] reaction in which the primary zwitterionic intermediates B are cyclized to yield the SiC four‐membered ring compounds.