## 4 Microwave-Assisted Catalytic Peroxidative Oxidation of Cyclohexanol to Cyclohexanone  
The oxidation of cyclohexanol to cyclohexanone was chosen as a model alcohol oxidation reaction, aiming to improve its sustainability. In fact, an industrially important step of the polycaprolactone production process involves the non-sustainable oxidation of KA oil (cyclohexanol and cyclohexanone mixture) to pure cyclohexanone catalyzed by Cu and Cr oxides at 250 \({}^{\circ}\)C and high pressure [27]. Thus, complexes 1-4 were tested as catalysts for the microwave (MW)-assisted oxidation of cyclohexanol using aqueous _tert_-butyl hydroperoxide (\({}^{\dagger}\)BuOOH or TBHP, aq. 70%) as the oxidizing agent at 60-80 \({}^{\circ}\)C under low power (10-50 W) MW irradiation (0.5-3 h reaction time) and in the absence of any added solvent and in additive-free medium (Scheme 3). Under the optimized conditions (Table S4), 1 h of irradiation (10 W) at 70 \({}^{\circ}\)C, 91.0%-93.1% yield of cyclohexanone was obtained from the MW-assisted oxidation of cyclohexanol in the presence of 0.1 mol% vs. substrate of 1-4 (TOFs up to \(1.50\times 10^{3}\) h\({}^{-1}\), entry 62, Table S4) with a selectivity of 100%. Indeed, no traces of by-products were detected by GC-MS analysis in the final reaction mixtures for the above conditions. Moreover, control experiments in the absence of the Fe(III) complex led only to 7% of alcohol conversion, highlighting the crucial role of 1-4 to efficiently catalyze the oxidation of cyclohexanol. The very similar catalytic activities of 1-4 (Figure 7) are in accord with their very similar reduction potentials as detected by cyclic voltammetry (see above), which conceivably determine the iron-promoted decomposition of TBHP (see mechanistic considerations below). \begin{table}
\begin{tabular}{c c c} \hline
**Complex** & \({}^{\mathbf{I}}E_{\frac{1}{2}}^{\mathbf{red}}\) & \({}^{\mathbf{II}}E_{\frac{1}{2}}^{\mathbf{red}}\) \\ \hline
1 & 0.079 & \(-\)1.638 \\
2 & 0.076 & \(-\)1.646 \\
3 & 0.074 & \(-\)1.654 \\
4 & 0.076 & \(-\)1.655 \\ \hline \end{tabular} \({}^{a}\) Values in V vs. SCE, in a 0.2 M [Bu\({}_{4}\)N][BF\({}_{4}\)]/DMSO solution, at a Pt disc working electrode, determined by using the [Fe(n\({}^{5}\)-C\({}_{5}\)H\({}_{5}\))\({}_{2}\)]\({}^{0/+}\) redox couple (\(E_{1/2}\)\({}^{\infty}\) = 0.42 V vs. SCE) [32] as internal standard at a scan rate of 100 mVs\({}^{-1}\).  
\end{table} 
Table 1: Cyclic voltammetric data\({}^{a}\) for the Fe(III) complexes 1–4. It is worth mentioning that such a high conversion of cyclohexanol into cyclohexanone is scarcely reported [40; 41; 42], and the present catalysts 1-4 are among the most active and selective ones for this industrially important reaction. For example, Fe(III) complexes derived from \(N\)-acetylpyrazine-2-carbohydrazide, viz., the mononuclear [Fe((kNN'O-HL)Cl\({}_{2}\)] and the binuclear [Fe(kNN'O-HL)Cl(\(\mu\)-OMe)]\({}_{2}\), when used as catalysts for the oxidation of cyclohexanol with TBHP under solvent-free MW irradiation conditions, led to yields up to 36% and 37%, respectively, of cyclohexanone after 1 h at 120 \({}^{\circ}\)C [42]. Moreover, decomposition at the reaction media impaired their reuse in further catalytic cycles. Therefore, this study may be of significance towards the improvement of the sustainability of polycaprolactone manufacture. A very favorable effect of MW irradiation is observed in this study even at the applied low power of 10 W. For example, for 3, the cyclohexanone yield increases from 18% (obtained using an oil bath heating) to 93.1% upon MW irradiation, the other conditions being the same. An increase of the reaction temperature from 60 \({}^{\circ}\)C to 70 \({}^{\circ}\)C enhanced the product yield from 89.2% to 93.1% after 1 h of reaction time, for compound 3 as catalyst (entries 38 and 44, Table S4), whereas a further raise in temperature to 80 \({}^{\circ}\)C resulted in a decrease in reaction yield to 88.2% (entry 51, Table S4, Figure 8A).