Mechanisms and consequences of oxygen transfer reactions

Abstract

Abstract:The geometry about oxygen in the transition-state structures for oxygen transfers from a nitrone to phosphorous, from a percarboxylic acid to a carbon-carbon double bond, and from an N-sulfonyl oxaziridine to a carbon-carbon double bond have been evaluated by the endocyclic restriction test. The former can proceed at an oblique angle, while the latter two require a large angle between the entering and leaving groups on oxygen. This information is used to determine the mechanism of the aldehyde-dependent oxygen transfer from molecular oxygen to a carbon-carbon double bond.Many chemical reactions, when broken down to their elementary steps, are atom transfer processes. Proper descriptions of the pathways of these reactions require experimental determinations of the arrangement of atoms in transition-state structures. With this information in hand, an informed choice between alternative pathways may be made, and understanding of the mechanism can be developed.The endocyclic restriction test is based on work by Eschenmoser, who investigated nucleophilic substitution of carbon and Hogg and Vipond who investigated nucleophilic displacements at sulfur [1][2][3]. We have used this approach to evaluate the orientation of atoms in transition-state structures for reactions in which the atom Y is transferred from the leaving group L to a nucleophile N. The test determines whether the transfer of Y from L to N can occur in an endocyclic ring. The size of the ring restricts the bond angles available in a transition-state structure (2) [1]. Experimentally, the approach involves determination of whether reaction (2) is intramolecular or intermolecular.For example, if a nucleophile and leaving group are linked by three methylene groups, the atom transfer could occur intramolecularly or intermolecularly (3). An intramolecular reaction would be observed if N-Y-L bond angle of approximately 120°would be accessible in a transition-state structure. If that or a smaller bond angle were not possible for the transfer of atom Y, the reaction could proceed in an intermolecular mode. In order to obtain a sound mechanistic conclusion, evaluation of a dissociative reaction would be required.