Synthesis of Pyrrolidine Derivatives by a Platinum/Broensted Acid Relay Catalytic Cascade Reaction

A new catalytic reaction for the synthesis of pyrrolidine derivatives is presented. The method implies the coupling of N-Boc-protected alkynamine derivatives and appropriate alkenes or alkynes in a process catalysed by a platinum/triflic acid catalytic binary system. This reaction is believed to proceed through a cascade process implying an initial platinum-catalysed cycloisomerization of the alkynamine derivative followed by a triflic acid promoted nucleophilic addition of the alkene or alkyne and trapping of the cationic species formed by the Boc group. Not only simple alkenes and alkynes were used in this reaction but also allyltrimethylsilane and propargyltrimethylsilane. Particularly, when allyltrimethylsilane is used as the alkene counterpart interesting bicyclic compounds containing a trimethylsilane group are obtained. However, when propargyltrimethylsilane is used in the presence of water we observed the formation of a related bicyclic compound lacking the trimethylsilane group and containing an exocyclic carbon--carbon bond. A team effort: Structurally complex pyrrolidine derivatives are easily available from starting materials as simple as N-Boc-protected alkynamine derivatives and alkenes or alkynes through a cascade process catalysed by a platinum/triflic acid catalytic binary system (see scheme; Boc=tert-butoxycarbonyl).