ORGN 222 |
| N-heterocyclic carbenes (NHCs) have emerged as an important class of potent nucleophilic catalysts. NHCs have been employed in various organic transformations such as benzoin condensations, Stetter reactions, and transesterification reactions. Recently, we have shown that NHCs catalyze the ring-opening polymerization of cyclic esters, providing well-defined polyesters of controlled molecular weights and narrow polydispersities. To gain insight into the reaction mechanism, we have investigated the NHC-catalyzed ring opening of β-lactones by alcohols. Contrary to the previously proposed carbene-base mechanism, mechanistic studies using a variety of β-lactones and alcohols implicate a nucleophilic mechanism in which the carbenes activate β-lactones. We have also characterized crucial intermediates that strongly support the carbene-mediated acyl activation. DFT calculations and experimental studies reveal that substituents on β-lactones greatly influence the reactivity of the intermediates. Furthermore, NHC-catalyzed polymerization of β-lactones enables the formation of structurally interesting polyesters. |
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New Reactions and Methodology
1:00 PM-5:00 PM, Monday, 27 March 2006 Georgia World Congress Center -- C301, Oral
Division of Organic Chemistry |