Green chemistry is an important topic in academic and industrial research,1 because it efficiently utilizes raw materials,eliminates waste and avoids the use of toxic and/or hazardous reagents and solvents in the manufacture and application of chemical products.2 Our coworkers 3 recently discovered an efficient method to synthesize pyrroles through gem-dialkylthio vinylallenes and their alkylthio-regulated reactivity,featuring a metal-free,step-economic domino cyclization process.To develop a greener process,in comparison to the above experiment,we consider more environmental friendly process and even without any additional organic reagent other than the reacting substrates.Herein,we present our theoretical investigation using density functional theory(DFT)calculations and confirmed by experimental study of the domino cyclization of pyrroles aiming at green synthesis.We took thioamide intermediate(Re)and benzylamine as model system to explore the detailed mechanism under three different conditions: DMSO-assisted,trace water-catalyzed,and dehydrated solvent-free.4 Two types of mechanism,i.e.,M1 and M2 were explored by DFT calculations.The difference between M1 and M2 lies in the nucleophilic attacking sites,either allenic carbon of Re or carbonyl carbon of conformational isomer of Re.Our calculations suggest that trace water-catalyzed mechanism M2 is favorable than DMSO-assisted M1.Therefore,organic solvent DMSO is not necessary for this reaction,which is further verified by the experimental outcome.In this domino cyclization reaction of pyrroles by M2,the carbonyl group plays a critical role as either a proton acceptor or a proton-relay stabilizer,and water acts as a proton shuttle.The noncovalent interaction and NBO analysis further confirm our results.Our finding suggests more green chemistry reaction process and provides a new synthetic strategy for pyrroles.