Transition-Structure Recognition in Polyether-Catalyzed Ester Aminolysis Carried Out in Nonpolar Media.

The mechanism of aminolysis of aryl acetates carried out in chlorobenzene involves rate-determining breakdown of the tetrahedral adduct formed by the nucleophilic attack of an amine on the carbonyl carbon of an aryl acetate ester. The breakdown of this tetrahedral adduct is assisted by the intervention of either a second amine moiety or a weakly basic catalyst moiety. The second amine or catalyst hydrogen bonds to an ammonium hydrogen of the zwitterionic tetrahedral adduct, which stabilizes this adduct by dispersing the positive charge on its cationic (ammonium) portion. Stabilization of the adduct occurs at the expense of a weaker 1,3-dipolar-stabilizing interaction that exists between the cationic ammonium region and the oxyanion region of this tetrahedral adduct. Breakup of this interaction by hydrogen-bonding bases destabilizes the oxyanion of the tetrahedral adduct, effectively raising the pK\sb{\rm a} of the oxyanion. This facilitates expulsion of the aryloxide nucleofuge by the oxyanion in the rate-determining step. Aryloxide expulsion yields a hydrogen-bond-stabilized, N-protonated amide and aryloxide ion pair. An ammonium proton is subsequently shuttled from nitrogen to aryloxide in one or more fast steps to yield neutral products. A preassociation mechanism cannot be ruled out on the basis of available data. Preassociation involves attack by hydrogen-bonded amine dimer or amine-catalyst complex on ester to form the hydrogen-bond-stabilized tetrahedral adduct directly. Glymes hydrogen bond their oxygens in pairs, in a bifurcated fashion, to each available ammonium hydrogen in the rate-determining transition structure for the reaction class. Glyme catalysis can be energetically dissected into bifurcation and bridging energies. Bifurcation is worth 1.2-1.4 kcal/mol and bridging between two ammonium hydrogens by catalyst is worth about 5 kcal/mol catalytically. Triglyme binds all four of its oxygens in a bridged, doubly-bifurcated, hydrogen-bonding fashion, to the two available ammonium hydrogens of the rate-determining transition structure in butylaminolysis, in either a lock-and-key or an induced-fit fashion. This work documents the first example of transition-structure recognition by glymes

Clay Science and Technology

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