Rhodium(I)-catalyzed [4+1] cycloaddition reactions of alpha,beta-unsaturated imines with terminal alkynes for the preparation of pyrrole derivatives.

Vinylidene complexes have attracted much attention for being unique reactive intermediate and synthetically useful, and characteristic reactions have been developed using various kinds of transition-metal complexes. One of the main reactions involves the addition of heteroatom nucleophiles such as alcohols, carboxylates, and carbamates to give anti-Markovnikov addition products. However, the addition of the heteroatom of C=X bonds to the vinylidene carbon atom has rarely been achieved in spite of the high synthetic potential of the zwitterionic intermediates that are produced. Herein, we report a rhodium(I)-catalyzed [4+1] cycloaddition reaction between a,b-unsaturated imines and terminal alkynes for the preparation of synthetically useful, substituted pyrrole derivatives through the addition of the imine nitrogen atom to rhodium vinylidene intermediates. When a mixture of b-TMS-substituted a,b-unsaturated imine 1a and 1-octyne (2a) was treated with a catalytic amount of [{RhCl(coe)2}2] and PCy3 in toluene at 110 8C for nine hours, pyrrole 3a (a formal [4+1] cycloaddition product) was obtained in good yield (Scheme 1). In this reaction [{RhCl(coe)2}2], as well as other phosphine ligands such as PPh3, P(iPr)3, 1,4-bis(diphenylphosphanyl)butane (dppb), and 1,1’-bis(diphenylphosphanyl)ferrocene (dppf) showed almost no activity. Also, [{RhOH(cod)}2] (cod= 1,5-cyclooctadiene), [Rh(cod)2]BF4, and [{IrCl(coe)2}2] in the presence of PCy3 did not show any activity either. Meanwhile, [{RhCl(cod)2}2]/PCy3 showed somewhat lower activity compared to [{RhCl(coe)2}2]/PCy3. The reaction is thought to proceed by the nucleophilic addition of the nitrogen atom of the imine 1a to the carbene carbon atom of the rhodium vinylidene complexA, which is generated in situ from the terminal alkyne, to afford a zwitterionic intermediate B (Scheme 2). This intermediate