title: The role of proton donors in SmI2-mediated ketone reduction: new mechanistic insights.
abstract: The effects of proton donors (alcohols and water) on the rate of reduction of acetophenone by SmI2 have been examined utilizing stopped-flow spectrophotometric studies. The rate orders with respect to proton source and the kinetic isotope effects were determined as well. The reaction was first-order in phenol, 2,2,2-trifluoroethanol, methanol, and ethanol and zero-order in 2-propanol and 2-methyl-2-propanol when 25 equiv of proton source were used in the reduction. Methanol, ethanol, 2,2,2-trifluoroethanol, and phenol also showed a direct correlation between the pKa of the alcohol and the rate of reduction. Under the same conditions, water had a fractional rate order of 1.4. Further studies showed that water has a rate order of 1 at lower concentrations (<8 equiv) and a rate order of 2 at higher concentrations (>80 equiv). These results clearly indicate that the nature of the proton donor and its concentration affects the rates of reduction. Water has a high affinity for SmI2 (compared to that of the alcohols), and the onset of coordination at relatively low concentrations channels the reaction through a mechanistically distinct pathway.
authors.affiliation: Department of Chemistry & Biochemistry, Texas Tech University, Box 41061, Lubbock, Texas 79409-1061, USA.
authors.forename: Pramod R
authors.initials: PR
authors.lastname: Chopade
authors.forename: Edamana
authors.initials: E
authors.lastname: Prasad
authors.forename: Robert A
authors.initials: RA
authors.lastname: Flowers