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.
Nowlan
Daniel T
DT
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842, USA.
Singleton
Daniel A
DA
D000480:Alkynes / Q000737:chemistry*; D003500:Cyclization; D003521:Cyclopropanes / Q000138:chemical synthesis* / Q000737:chemistry; D007093:Imidazoles / Q000737:chemistry; D009942:Organometallic Compounds; D012238:Rhodium / Q000737:chemistry; D013237:Stereoisomerism
D000480:Alkynes; D003521:Cyclopropanes; D007093:Imidazoles; D009942:Organometallic Compounds; C021985:cyclopropene; D012238:Rhodium