(Alkenyl)(amino)carbene Complexes: Potential Starting Materials for the Synthesis of Cyclopropylacetic Acid Derivatives

Abstract

The (δ-hydroxyalkenyl)(pyrrolidino)carbene complexes, prepared by means of aldol additions of the (propenyl)(pyrrolidino)carbene complex to p-nitrobenzaldehyde and 4-pyridinecarbaldehyde, have been treated with methyl N-(triethylammoniosulfonyl)carbamate inner salt (Burgess reagent) to afford the corresponding adducts in almost quantitative yields. Treatment of these adducts with NaOH in methanol gave the corresponding conjugated polyunsaturated (amino)carbene complexes in fair yields. Alternatively, heating the adducts to 55−60 °C led to the isolation of cyclopropylacetic acid derivatives as the main reaction products, in yields of 20−40% depending on the polarity of the solvent. The adduct of the aldol addition product of the (propenyl)(pyrrolidino)carbene complex and p-O2NC6H4−CH=CH−CHO with the Burgess reagent could not be isolated, but directly afforded the cyclopropane derivative as well as the polyunsaturated (amino)carbene complex. We present herein a brief discussion of the key steps of the mechanism of this cyclopropane ring formation.