Thermally initiated cycloadditionreactions of nonsymmetricalallenyl azines 1 with alkynes or other dipolarophiles usually lead to compounds with three fused, five-membered heterocyclic rings. With alkynes with pronounced "push-pull" systems, however, the reaction ends with the formation of substituted pyrrolidino[1,2-b]pyrazoles 4 and, in the case of azines with trifluoromethyl substitution, ring opening
Aza Cope Rearrangement of Propargyl Enammonium Cations Catalyzed By a Self-Assembled “Nanozyme”
作者:Courtney J. Hastings、Dorothea Fiedler、Robert G. Bergman、Kenneth N. Raymond
DOI:10.1021/ja8013055
日期:2008.8.1
The tetrahedral assembly [Ga4L6](12-) [L = N,N-bis(2,3-dihydroxybenzoyl)-1,5-diaminonaphthalene) encapsulates a variety of cations, including propargyl enammonium cations capable of undergoing the aza Cope rearrangement. For propargyl enammonium substrates that are encapsulated in the [Ga4L6](12-) assembly, rate accelerations by factors of up to 184 compared with the background reaction rate were observed. After rearrangement, the product iminium ion is released into solution and hydrolyzed, allowing for catalytic turnover. The activation parameters for the catalyzed and uncatalyzed reaction were determined, revealing that a decrease in the entropy of activation is responsible for the observed rate enhancements. The catalyzed reaction exhibits saturation kinetics: the rate data obeyed the Michaelis-Menten model of enzyme kinetics, and competitive inhibition using a nonreactive guest was demonstrated.