Oxidation of 2-methyl-1-naphthol (MNL) with molecular oxygen 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% in nonpolar solvents. 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 autoxidation, electron transfer mechanisms, photooxygenation, and thermal intersystem crossing (ISC), have been evaluated using spectroscopic, mass-spectrometric, spin-trapping, ¹⁸O₂ labeling, kinetic, and computational techniques. Several facts collectively implicate that ISC contributes significantly into MNL oxidation with O₂ 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 O₂; (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 ³O₂ (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.

中文翻译：

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机械氧化对2-甲基-1-萘酚被双氧氧化的机理：自氧化还是自旋禁止反应？

2-甲基-1-萘酚（MNL）用分子氧氧化温和反应条件下有效地进行（3个大气压Ò ₂在不存在任何催化剂或敏化剂的，60-80℃），并产生2-甲基-1,4- -萘醌（MNQ，甲萘醌或维生素K ₃）在非极性溶剂中的选择性高达80％。¹ H NMR和¹ H，¹ H-COSY研究表明，在反应过程中形成了2-甲基-4-氢过氧萘-1（4 H）-一（HP）。已使用光谱，质谱，自旋阱等方法评估了几种机械假设，包括常规的自由基自氧化，电子转移机制，光氧化和热系统间穿越（ISC），[ ^18]O ₂标记，动力学和计算技术。几个事实共同表明，ISC在高压下与O _2一起在MNL氧化中起重要作用：（i）反应速率不受光的影响；（ii）实际上不存在C-C耦合二聚体；（iii）在MNL和O _2中反应都是一阶的；（iv）观察到的活化参数（ΔH ^‡ = 8.1 kcal mol ^–1和ΔS ^‡ = −50 eu）类似于用³ O ₂自旋禁止氧化菊红的参数（Seip，M。； Brauer ，H.-D. J.化学会会志。1992年，114，4486）；（v）外部重原子效应（碘代苯中反应速率提高了2倍）表明在限速步骤中自旋反转。