Synthesis of 2‐Aryl‐N‐(EWG‐methyl)‐N‐methylpyrrolidinium Salts as Precursors of Ylides Entering the [1,2] Stevens Rearrangement

Abstract

N-(EWG-CH2 )-N-methyl-2-phenylpyrrolidinium (EWG -electronwithdrawing group) salts were obtained in two different ways. N-(EWG-CH 2 )-2-phenylpyrrolidines were quaternized with iodomethane, dimethylsulfate or methyl trifluoromethanesulfonate forming pyrrolidinium iodides, methylsulfates and triflates. The counterion exchange in the methylsulfates was carried out yielding respective tetrafluoroborates. The second method was based on the quaternization of N-methyl-2-phenylpyrrolidine with methyl α-halogenoacetate or phenacyl bromide. All listed salts were oily and formed in high yields as mixtures of diastereoisomers, with cis or trans isomer predominating, depending on the method applied. The effect of the counterion present in the obtained salts was analyzed. Ylides generated from these salts in different base/solvent systems afforded products of the [1,2] Stevens rearrangement, 2-EWG-1-methyl-3-phenylpiperidines, with the trans isomers predominating, in moderate to good yields. In case of the [1,2] shift, the yields of respective products depended on EWG present in the substrate (methoxycarbonyl, acetyl, dimethylcarbamoyl and benzoyl).