title: A Radical Mechanism for the Vanadium-Catalyzed Deoxydehydration of Glycols.
abstract: We propose a novel mechanism for the deoxydehydration (DODH) reaction of glycols catalyzed by a [Bu4N][VO2(dipic)] complex (dipic = pyridine-2,6-dicarboxylate) using triphenylphosphine as a reducing agent. Using density functional theory, we have confirmed that the preferred sequence of reaction steps involves reduction of the V(V) complex by phosphine, followed by condensation of the glycol into a [VO(dipic)(-O-CH2CH2-O-)] V(III) complex (6), which then evolves to the alkene product, with recovery of the catalyst. In contrast to the usually invoked closed-shell mechanism for the latter steps, where 6 suffers a [3+2] retrocycloaddition, we have found that the homolytic cleavage of one of the C-O bonds in 6 is preferred by 12 kcal/mol. The resulting diradical intermediate then collapses to a metallacycle that evolves to the product through an aromatic [2+2] retrocycloaddition. We use this key change in the mechanism to propose ways to design better catalysts for this transformation. The analysis of the mechanisms in both singlet and triplet potential energy surfaces, together with the location of the MECPs between them, showcases this reaction as an interesting example of two-state reactivity.
authors.affiliation: Departamento de Química Orgánica, Facultade de Química, Universidade de Vigo , Campus Lagoas-Marcosende, 36310 Vigo, Spain.
authors.forename: Luis Carlos
authors.initials: LC
authors.lastname: de Vicente Poutás
authors.affiliation: Departamento de Química Orgánica, Facultade de Química, Universidade de Vigo , Campus Lagoas-Marcosende, 36310 Vigo, Spain.
authors.forename: Marta
authors.initials: M
authors.lastname: Castiñeira Reis
authors.affiliation: Departamento de Química, Facultad de Ciencias, Universidad de Burgos , Pza. Misael Bañuelos s/n, 09001 Burgos, Spain.
authors.forename: Roberto
authors.initials: R
authors.lastname: Sanz
authors.affiliation: Departamento de Química Orgánica, Facultade de Química, Universidade de Vigo , Campus Lagoas-Marcosende, 36310 Vigo, Spain.
authors.forename: Carlos Silva
authors.initials: CS
authors.lastname: López
authors.affiliation: Departamento de Química Orgánica, Facultade de Ciencias, Universidade de Vigo , Campus As Lagoas, 32004 Ourense, Spain.
authors.forename: Olalla Nieto
authors.initials: ON
authors.lastname: Faza