The reaction of pyrrole with dimethyl carbonate under phosphazene catalysis: N-methoxycarbonylation vsN-methylation

Abstract

Phosphazene (t-butylimino-tris(dimethylamino)phosphorane (P1-t-Bu), t-butylimino-tris(pyrrolidino)phosphorane (BTPP)) and amidine (1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)) superbases have been investigated as catalysts in the direct reaction of pyrrole with dimethyl carbonate (DMC). The system phosphazene/pyrrole/DMC proves to be a flexible synthetic tool, as it offers a solution not only for the direct and selective phosgene-free synthesis of 1-methoxycarbonyl pyrrole (1), but also for the straightforward and selective synthesis of 1-methylpyrrole (2) through a safer way which avoids the use of harmful methylating agents such as methyl halides and (MeO)2SO2. The influence of factors (temperature, catalyst loading, reaction time, etc.) affecting yields and selectivities has been investigated. We have also ascertained that a major reaction pathway to the formation of 2 involves the decarboxylation of the primary product 1. Also this process was catalytically promoted by the phosphazene catalyst. Co-generated CO2 opened a way to the deactivation of phosphazene catalyst, which converted mainly into catalytically inactive OP(NR2)3 (NR2=NMe2, NC4H8).