Silenes : novel synthetic reagents for olefin functionalisation
Silenes, compounds containing silicon-carbon double bonds, have been subject to much theoretical interest over the past 25 years. However, despite their high reactivity, to date there have been no reports of the attempted use of silenes as synthetic reagents. Silenes have been formed by the thermolysis of acyltris(trimethylsilyl)silanes, which have been found to react with dienes to give Diels-Alder cycloadducts in good yield. Further to this, it has been found that silene dimers, formed by the photolysis of acyltris(trimethylsilyl)silanes, have been found to revert to give silenes at much lower temperatures. These dimers have been found to be excellent low-temperature silene sources. The reactions of acyltris(trmiethylsilyl)silanes with silyl Lewis acids have been found to give novel products in excellent yield, via a series of migration reactions. These migrations are believed to involve the formation of a silyl cation or "silylenium" ion intermediate. Furthermore, the reaction with trimethylsilyl triflate is believed to proceed via the formation of a silene intermediate, the existence of which has been supported by trapping with methanol. Finally, the oxidation of silene cycloadducts has been attempted, using established conditions. The first series of silene cycloadducts obtained were found to be unreactive towards oxidation. However, the reaction of a silene, formed by a Peterson elimination process, with dienes has been found to give a cycloadduct as a single diastereoisomer. It has been shown that this cycloadduct can be oxidised, following established protocols, to give either a homoallylic alcohol or 1,5-diol in reasonable overall yield. This result provides evidence for the utility of silenes as reagents for functionalising dienes.