Mechanism and Origin of Enantioselectivity in the Rh₂(OAc)(DPTI)₃-Catalyzed Cyclopropenation of Alkynes

The mechanism of cyclopropenations of alkynes with ethyl diazoacetate catalyzed by Rh₂(OAc)₄ and Rh₂(OAc)(DPTI)₃ (1) is studied by a combination of kinetic isotope effects and theoretical calculations. With each catalyst, a significant normal ¹³C KIE was observed for the terminal acetylenic carbon, while a very small ¹³C KIE was observed at the internal acetylenic carbon. These isotope effects are predicted well from canonical variational transition structures for cyclopropenations with intact tetrabridged rhodium carbenoids. A viable mechanism based on the recently proposed importance of a [2 + 2] cycloaddition on a tribridged rhodium carbenoid could not be identified. An explanation for the enantioselectivity with DPTI ligands is described.