Trimethylchlorosilane-modified Clemmensen reduction of metallocenyl ketones: trapping and X-ray structures f aliphatic, olefinic, silylated pinacolic, and rearranged pinacolonic products

Abstract

The Clemmensen reduction of carbonyl group to give, in the simplest cases, a methylene unit, is one of the more familiar reactions in organic chemistry. Trimethylchlorosilane as a replacement for the proton allows controlled reductive deoxygenation of carbonyl compounds under formally anhydrous conditions as an alternative to the McMurry dicarbonyl coupling reaction. Aromatic carbonyl compounds with electron-donor substituents are preferred substrates for this reaction to yield symmetrical olefins in good yield. The high donor capacity of metallocenyl groups should especially favor the clean formation of olefins from the corresponding metallocenyl ketones. In this study, a number of ferrocenyl and ruthenocenyl aldehydes and ketones were treated with Zn and trimethylchlorosilane under controlled anhydrous reaction conditions with (a) catalytical and (b) low amounts of H + present. The observed main products include alkanes, alkenes, silylated pinacols, and rearranged pinacolones, which have been characterized by spectroscopic methods and by X-ray analyses. The diverse outcome in these reactions indicates a reaction mechanism which involves electrophilic catalysis with different energetically similar pathways.