title: Formation, Oxidation, and Fate of the Breslow Intermediate in the N-Heterocyclic Carbene-Catalyzed Aerobic Oxidation of Aldehydes.
abstract: The reaction paths and intermediate structures related to the formation of the Breslow intermediate and its oxidation along the oxidative/oxygenative lanes have been studied from a mechanistic point of view, with the support of gas-phase and computational studies. The results confirm the occurrence of a single-electron transfer from the Breslow intermediate to the molecular oxygen with formation of a radical couple that recombines either as a peroxide anion 7' to afford the aldehyde-to-carboxylic acid product or as a hydroperoxy derivative 7″ that evolved into an electrophilic acyl azolium, opening to the aldehyde-to-ester conversion. Steric factors enter into determining the different reactivity. All of the intermediates of both catalytic paths have been observed and characterized under mass spectrometric conditions. In particular, for the imidazoline catalyst, the (+)ESI-MS/(MS) detection of the genuine Breslow intermediate was made possible in virtue of its limited reactivity. Mechanistic aspects of the N-heterocyclic carbenes catalyzed aerobic oxidation of aldehydes shares important similarities with that one of the recently revisited benzoin condensation.
authors.affiliation: Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara , Via Fossato di Mortara 17-19, 44121 Ferrara, Italy.
authors.forename: Olga
authors.identifier: 0000-0002-8428-2310
authors.initials: O
authors.lastname: Bortolini
authors.affiliation: Dipartimento di Farmacia, Università di Pisa , Via Bonanno 33, 56126 Pisa, Italy.
authors.forename: Cinzia
authors.identifier: 0000-0001-6615-908X
authors.initials: C
authors.lastname: Chiappe
authors.affiliation: Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara , Via Fossato di Mortara 17-19, 44121 Ferrara, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Fogagnolo
authors.affiliation: Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara , Via Fossato di Mortara 17-19, 44121 Ferrara, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Massi
authors.affiliation: Dipartimento di Farmacia, Università di Pisa , Via Bonanno 33, 56126 Pisa, Italy.
authors.forename: Christian Silvio
authors.initials: CS
authors.lastname: Pomelli