A Common Active Intermediate in the Oxidation of Alkenes, Alcohols and Alkanes with H2O2 and a Mn(II)/Pyridin-2-Carboxylato Catalyst

Abstract

The mechanism and the reactive species involved in the oxidation of alkenes, and alcohols with H₂O₂, catalysed by an in situ prepared mixture of a Mn^II salt, pyridine-2-carboxylic acid and a ketone is elucidated using substrate competition experiments, kinetic isotope effect (KIE) measurements, and atom tracking with ¹⁸O labelling. The data indicate that a single reactive species engages in the oxidation of both alkenes and alcohols. The primary KIE in the oxidation of benzyl alcohols is ca. 3.5 and shows the reactive species to be selective despite a zero order dependence on substrate concentration, and the high turnover frequencies (up to 30 s^-1) observed. Selective ¹⁸O labelling identifies the origin of the oxygen atoms transferred to the substrate during oxidation, and is consistent with a highly reactive, e. g., [Mn^V(O)(OH)] or [Mn^V(O)₂], species rather than an alkylperoxy or hydroperoxy species.

Keywords: alkene; isotope labelling; kinetic isotope effect; manganese; oxidation chemistry.

Scheme 1

Oxidation of alkenes and alcohols with the Mn^II pyridine‐2‐carboxylic acid base catalyst prepared in situ.

Scheme 2

Overall mechanism of oxidation of substrates with the catalyst prepared in situ from Mn^II and pyridine‐2‐carboxylic acid. The structures of the resting state and active species are unknown.

Figure 1

(a) Oxidation of a mixture of styrene (0.25 M) and 1‐phenyl ethanol (0.25 M), see Scheme 1 for conditions, (b) concentration of products over time, (c) Raman spectra at selected times showing bands at 1255 and 1690 cm⁻¹ of the epoxide and ketone products. (black) initial spectrum, (red) 3.5 min (blue) 7 min (grey) 22 min after addition of H₂O₂.

Figure 2

(a) O₂ formation detected by headspace Raman spectroscopy (λ_exc 785 nm) before (start) addition of H₂O₂ (at 25 s), during (and at the end (end) of the reaction, (b) ratio of O₂ and N₂ bands over time after addition of H₂O₂ (at 25 s) in the presence (black) and absence (blue) of styrene under standard conditions (scheme 1). The spectra were normalised to the N₂ Raman band. Note that the Mn(II) concentration used was 0.5 mM to increase reaction rate. See Figure 3 for conditions.

Figure 3

Concentrations of ¹⁸O‐butanedione (red), ¹⁶O‐butanedione (black) monitored by Raman spectroscopy (λ_exc 785 nm) and amount of ¹⁶O₂ (blue) formed after 2 additions of H₂O₂ (at ca. 1 and 6 min) monitored by headspace Raman spectroscopy (λ_exc 785 nm) in a reaction mixture without added substrate (0.5 mM Mn^II‐salt, 2.5 mM PCA, 5 mM NaOAc, 0.1 M AcOH, 0.25 M butanedione) and with initially ¹⁸O‐(75 %)labelled butanedione.

Scheme 3

Substrates and conditions used to determine primary KIE in alcohol oxidation.

Scheme 4

Conditions used for ¹⁸O labelling studies.

Scheme 5

Average ¹⁸O incorporation into 5 substrates using (top) H₂ ¹⁸O (middle) H₂ ¹⁸O₂, and (bottom) ¹⁸O labelled butanedione (see Tables S1 and S2, and Figure S8‐S11 for details) corrected for isotope composition of reagents.

Scheme 6

Equilibria between butanedione and H₂O₂ and exchange of H₂O₂ between butanedione molecules.

Scheme 7

Proposed mechanism including oxygen atom tracking with oxygen in H₂O₂ (red) and oxygen in butanedione (blue).