Singlet silylene, cycloaddition

Silylene 59 also reacts with some dienes to give 1+4 cycloaddition products. With Iransjrans-1,4-diphenylbuladiene, stereospecific addition takes place to give the d.v-pro-duct, as expected for a singlet silylene (equation 112). A second example is the reaction... 

The cycloaddition reaction of singlet GeX2 (X = F, Cl) with formaldehyde was studied employing the HF/6-311+G theory.106 The reaction proceeds in two steps barrierless formation of an intermediate complex followed by rate-determining isomerization to form the product. The results were compared with those from other cycloadditions of germylenes and silylenes. 

It has been theoretically and experimentally well established that silylenes have a singlet ground state [1]. Such species posses a free electron pair in a o-orbital and an empty orbital of Jt-symmetry therefore, they are a priori ambiphilic compounds, which can react either as an electrophile or as a nucleophile towards appropriate substrates. However, most silylenes have revealed a distinctive "electrophilic character". Dimethylsilyene, e g., adds to olefins and alkynes in the gas phase via a rate-controlling step that is accelerated by electron-donating substituents [2] these experimental results are in good agreement with a theoretical study of the reaction of SiH2 with ethylene, which shows that this cycloaddition proceeds via an initial electrophilic phase in which the silylene LUMO interacts with the 7t-electron system of the double bond [3]. Up to now, only some stable silylenes, such as recently described 1 [4] or silicocene 2 [5] have shown nucleophilic reactivity. 

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