Rh(II)‐Catalyzed Isomerizations of Cyclopropenes Evidence for Rh(II)‐Complexed Vinylcarbene Intermediates

Abstract

The thermocatalytic rearrangements of cyclopropenes 1‐4 have been investigated in the presence of Rh(IT) perfiuorobutyrate. 1, 2, 3‐Triphenylcyclopropene (1a) undergoes rearrangement lo diphenylindene 5a or, with alkoxycyclopropene derivatives, to α, β‐unsaturated ketone 6. Furan formation occurs with 2, 3‐diphenylcyclo‐propenecarboxylate 2, but the diethyl counterpart 3 rearranges to dienoate 8. 2‐Alkylcyclopropenecarboxylates 4 afforded (E)‐methylidenecyclopentane derivatives 9 as the only isolable product in yields of ca. 35 %. A mechanism involving regio‐ and stereospecific cyclopropene ring opening to a Rh‐complexed vinylcarbene and insertion of the latter into the CH bond to give 9 is proposed. An analogous mechanism should account for the rearrangement products of 1 to 3.