title: Mechanistic insights into oxidation of 2-methyl-1-naphthol with dioxygen: autoxidation or a spin-forbidden reaction?
abstract: Oxidation of 2-methyl-1-naphthol (MNL) with molecular oxygen proceeds efficiently under mild reaction conditions (3 atm O(2), 60-80 °C) in the absence of any catalyst or sensitizer and produces 2-methyl-1,4-naphthoquinone (MNQ, menadione, or vitamin K(3)) with selectivity up to 80% in nonpolar solvents. (1)H NMR and (1)H,(1)H-COSY studies revealed the formation of 2-methyl-4-hydroperoxynaphthalene-1(4H)-one (HP) during the reaction course. Several mechanistic hypotheses, including conventional radical autoxidation, electron transfer mechanisms, photooxygenation, and thermal intersystem crossing (ISC), have been evaluated using spectroscopic, mass-spectrometric, spin-trapping, (18)O(2) labeling, kinetic, and computational techniques. Several facts collectively implicate that ISC contributes significantly into MNL oxidation with O(2) at elevated pressure: (i) the reaction rate is unaffected by light; (ii) C-C-coupling dimers are practically absent; (iii) the reaction is first order in both MNL and O(2); (iv) the observed activation parameters (ΔH(‡) = 8.1 kcal mol(-1) and ΔS(‡) = -50 eu) are similar to those found for the spin-forbidden oxidation of helianthrene with (3)O(2) (Seip, M.; Brauer, H.-D. J. Am. Chem. Soc.1992, 114, 4486); 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.
authors.affiliation: Boreskov Institute of Catalysis, Lavrentiev Avenue 5, Novosibirsk, 630090, Russia. khold@catalysis.ru
authors.forename: Oxana A
authors.initials: OA
authors.lastname: Kholdeeva
authors.forename: Irina D
authors.initials: ID
authors.lastname: Ivanchikova
authors.forename: Olga V
authors.initials: OV
authors.lastname: Zalomaeva
authors.forename: Alexander B
authors.initials: AB
authors.lastname: Sorokin
authors.forename: Igor Y
authors.initials: IY
authors.lastname: Skobelev
authors.forename: Eugenii P
authors.initials: EP
authors.lastname: Talsi
mesh_terms: D007700:Kinetics; D009284:Naphthols / Q000737:chemistry*; D009285:Naphthoquinones / Q000138:chemical synthesis / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D013113:Spin Labels
chemical_list: D009284:Naphthols; D009285:Naphthoquinones; D012997:Solvents; D013113:Spin Labels; C569981:2-methyl-1-naphthol; C035342:1,4-naphthoquinone; D010100:Oxygen