A Radical Mechanism for the Vanadium-Catalyzed Deoxydehydration of Glycols

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.