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

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 CH bond to give 9 is proposed. An analogous mechanism should account for the rearrangement products of 1 to 3 .