ORGN 264 |
| Preparation of keto acids through the alkylation of meso cyclic carboxylic anhydrides by diorganozinc species is efficiently catalyzed by nickel(0) with either bipyridine (bipy) or isopropylphosphinooxazoline (iPrPHOX) ligands using styrene as an activator. Mechanistic studies suggest significantly different mechanisms. Using Ni/bipy, results indicate that the turnover limiting step changes from transmetalation to reductive elimination with increasing diorganozinc concentration. Styrene appears to have no effect on the initial rate of reaction. Under Ni/iPrPHOX catalysis, oxidative addition of the anhydride limits catalyst turnover, thus making the rate and enantioselectivity of this reaction independent of the zinc reagent. Saturation behavior of the rate with styrene, including a non-zero rate in its absence, suggests two concurrent cycles. Without styrene, catalysis proceeds slowly, providing the keto acid in very low enantioselectivity. Under high concentrations of styrene, catalysis proceeds four times faster, providing the desired product in higher enantioselectivity.
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Metal-Mediated Reactions and Syntheses
8:00 AM-12:00 PM, Monday, 11 September 2006 Moscone Center -- Room 133, Oral
Division of Organic Chemistry |
