Activation of Dioxygen in Air by a Phenol/Selectfluor System: An Application in the Oxidation‐Dimerization of Alkynes to 2‐Ene‐1,4‐diones

Abstract

Activation of dioxygen in air by a combined phenol/Selectfluor system has been revealed. This technique has been exploited to develop an approach to 2-ene-1,4-diones via an oxidation-dimerization of alkynes under metal-free conditions.Dioxygen is no doubt the most "green" oxidant as well as the ideal oxygen source due to its natural, inexpensive, and environmentally benign characteristics. [1] However, with its triplet ground state, dioxygen is not kinetically favored for use in the oxidation of organic substrates. [2] Thus, the activation of dioxygen becomes the crucial process in the direct utilization of dioxygen as the terminal oxidant and/or the oxygen source in organic synthesis. [1, 2] Over the past few decades, transition-metal-mediated dioxygen activation has witnessed rapid progress. [1] A range of transition metals including Pd, [1a, b, d-f, i] Cu, [1h, 3a, b] Fe, [2b] Ni, [3c] Mn, [3d-f] and Ag [3g] have been investigated for achieving this purpose. In sharp contrast, relatively little attention has been focused toward the activation of dioxygen by organic molecules, though they serve as more "green" dioxygen activators than transition metals, [4] which is especially important in the pharmaceutical discipline. In this regard, Tan and Wille [4a] reported that dioxygen activation by S-radicals enables the oxidative conversion of C C bonds into a-diketones, Alexanian and co-workers [4b,c] developed an intramolecular and an intermolecular dioxygenation of alkenes involving activation of dioxygen by hydroxamic acid derivatives, and recently, Lei and co-workers [4d] have reported that dioxygen activation by sulfinic acids enables aerobic oxysulfonylation of alkenes leading to secondary and tertiary b-hydroxysulfones. Despite increasing advances, [4] the