Mechanism and Origin of Enantioselectivity in the Rh<sub>2</sub>(OAc)(DPTI)<sub>3</sub>-Catalyzed Cyclopropenation of Alkynes

2005-05-04T00:00:00Z (GMT) by Daniel T. Nowlan Daniel A. Singleton
The mechanism of cyclopropenations of alkynes with ethyl diazoacetate catalyzed by Rh<sub>2</sub>(OAc)<sub>4</sub> and Rh<sub>2</sub>(OAc)(DPTI)<sub>3</sub> (<b>1</b>) is studied by a combination of kinetic isotope effects and theoretical calculations. With each catalyst, a significant normal <sup>13</sup>C KIE was observed for the terminal acetylenic carbon, while a very small <sup>13</sup>C 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.