A cationic rhodium(I) complex, [Rh(cod)2]SbF6, catalyzes the [3 + 2] cycloaddn. of propargyl esters with dialkyl acetylenedicarboxylates in good yields, presumably through carbonyl-stabilized cationic rhodium(I) alkenylcarbene intermediates. The same rhodium(I) complex also catalyzes the [2 + 1] cycloaddn. (cyclopropanation) of propargyl esters with N,N-disubstituted acrylamides in good yields with perfect diastereoselectivity. [on SciFinder(R)]