ORGN 455 |
| Differentiation of the two halogens in 1,1-dihaloalkenes for selective Negishi monoalkylation with alkylzincs remains a challenging synthetic task. Pd-catalyzed cross-couplings of 1-fluoro-1-haloalkenes 1 with primary alkylzinc bromides were found to give an access to multisubstituted fluoroalkenes 2. The alkylation was trans-selective giving pure Z-fluoroalkenes. The highest yields were obtained with Pd2(dba)3 and PdCl2(dppb) catalysts but the best stereochemical outcome was obtained with less reactive Pd(PPh3)4. The tertiary alkylzincs also produced desired fluoroalkenes 2 in high yields. The β,β-dichlorostyrene reacted [PdCl2(dppf)/THF/65 oC/14 h] with BrZn(CH2)3CO2Et, to give a desired trisubstituted chloroalkene 3 (Z, 65%), via trans selective monoalkylation, in addition to the monocoupled/reduced byproduct 4. Couplings with more reactive β,β-dibromostyrene produced mainly dialkylated product 5.
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Metal-Mediated Reactions and Syntheses
8:00 AM-12:00 PM, Wednesday, 29 March 2006 Georgia World Congress Center -- C302, Oral
Division of Organic Chemistry |
