Oxidation of 2-methyl-1-naphthol (MNL) with 3O2 proceeds efficiently under mild reaction conditions (3 atm O2, 60–80 °C) in the absence of any catalyst or sensitizer and produces 2-methyl-1,4-naphthoquinone (MNQ, menadione, or vitamin K3) with selectivity up to 80-85% in non-polar solvents [1]. 1H NMR and 1H,1H-COSY studies revealed the formation of 2-methyl-4-hydroperoxynaphthalene-1(4H)-one (HP) during the reaction course. Several mechanistic hypotheses, including conventional radical autooxidation, electron transfer mechanisms, photooxygenation, and thermal intersystem crossing (ISC), have been evaluated using spectroscopic, mass-spectrometric, spin-trapping,18O2 labeling, kinetic, and computational techniques [2]. The delicate balance among different oxidation mechanisms depends on the dioxygen pressure, the solvent nature, and the presence of additives. The following facts collectively implicate that ISC contributes significantly into MNL oxidation at elevated pressure: (i) the reaction rate is unaffected by light; (ii) C–C-coupling dimers are practically absent and initial MNL concentration has very little effect on MNQ selectivity; (iii) the reaction is first order in both MNL and O2; (iv) the observed activation parameters are similar to those found earlier for the spin-forbidden reaction of helianthrene with3O2 [3]; and (v) the external heavy atom effect (2-fold increase of the reaction rate in iodobenzene) points to spin inversion in the rate-limiting step. The radical autoxidation mechanism can be pushed by the addition of chain initiators, including transition metals, but the product selectivity changes significantly in favor of dimeric byproducts, 4,4’-di(2-methyl-1-naphthol) and 4,4’-di(2-methyl-1-naphthoquinone), which result from C–C coupling of naphthoxyl radicals.
[1] O.A. Kholdeeva, M. Rossi, WO 2,006,104,411 (2006).
[2] O.A. Kholdeeva, I.D. Ivanchikova, O.V. Zalomaeva, A.B. Sorokin, I.Y. Scobelev, E.P. Talsi, J. Phys. Chem. B 2011,115, 11971.
[3] M. Seip, H.-D.Brauer, J. Am. Chem. Soc. 1992, 114, 4486.