João J. R. Frausto da Silva

The new mononuclear Fe^II and Cu^II complexes [FeCl₂(L^1,3)] [L¹ (1), L³ (4)], [Cu(OTf)₂(L^1,2)] [L¹ (2), L² (3)] and [Cu(OTf)(L³)(H₂O)](OTf) (5) were generated by the treatment of iron(II) chloride or copper(II) triflate in THF solution at ambient temperature with the 14‐membered N₂S₂ macrocycles 6,7,13,14,15,16,17,18‐octahydrodibenzo(e,m)‐1,4,8,11‐dithiadiazacyclotetradecine (L¹), 8,11‐dimethyl‐5,6,7,8,9,10,16,17‐octahydrodibenzo(e,m)‐1,4‐dithia‐8,11‐diazacyclotetradecane (L²), and a new nine‐membered NS₂ macrocycle bearing a pendant 2‐methylpyridyl arm 7‐(2‐methylpyridyl)aza‐1,4‐dithiacyclononane (L³). Complexes 1–5 were characterized by IR spectroscopy, ESI‐MS(+), elemental analysis and single‐crystal X‐ray diffraction (for 2, 4 and 5). The oxidation catalytic properties of 1–5 were evaluated in two model reactions: (i) the mild oxidation of cyclohexane to cyclohexanol and cyclohexanone by H₂O₂ in MeCN and (ii) the solvent‐free oxidation of 1‐phenylethanol to acetophenone by tBuOOH under low microwave (MW) irradiation power (10 W). The Fe^II complexes 1 and 4 exhibited the highest efficiency in both homogeneous oxidation systems, leading to maximum product yields (based on substrate) of up to 21 and 92 % in the oxidation of cyclohexane and 1‐phenylethanol, respectively. In the latter transformation, markedly high values of TONs (up to 1200) and TOFs (up to 4200 h^–1) were also attained.

The [2+3] cycloaddition of nitriles (RCN) with 2,2‐dimethyl‐3,4‐dihydro‐2H‐pyrrole 1‐oxide, in the presence of palladium dichloride (PdCl₂) gives the corresponding 2,3‐dihydro[1.2.4]oxadiazole (Δ⁴‐1,2,4‐oxadiazoline) palladium(II) complexes 1–4 in good yields. However, the Pd(II)‐assisted reaction of pentafluorobenzonitrile with the same pyrroline N‐oxide gives a mixture of oxadiazoline 5, ketoimine 6 and pyrrolylbenzamide‐ketoimine 7 Pd(II) complexes, which affords upon heating in refluxing acetone the unprecedented fused tricyclic ketoimine complex 8 as the exclusive product. Under heating, compounds 5 and 7 transform to 6, the latter undergoing intramolecular cyclization by nucleophilic attack of the amino moiety to the ortho carbon of the pentafluorophenyl ring leading ultimately to 8. The compounds were characterized by IR, ¹H and ¹³C NMR, ESI⁺‐MS, elemental analyses and, in the cases of 3, 6, 7 and 8, also by X‐ray diffraction analyses. The catalytic properties of the Pd complexes were evaluated in Suzuki–Miyaura cross‐coupling reactions, using supercritical carbon dioxide (scCO₂) as a green solvent. Cross‐couplings of aryl halides with phenylboronic acid give the desired biaryl products in quantitative yields, in a short reaction time, for substrate‐to‐catalyst molar ratios as high as 4.0⋅10⁴.

Trends between the Hammett's σ_p and related normal , inductive σ_I, resonance σ_R, negative and positive polar conjugation and Taft's σ_p° substituent constants and the distance, δ_NH NMR chemical shift, oxidation potential (E_p/2°x, measured in this study by cyclic voltammetry (CV)) and thermodynamic parameters (pK, ΔG⁰, ΔH⁰ and ΔS⁰) of the dissociation process of unsubstituted 3‐(phenylhydrazo)pentane‐2,4‐dione (HL₁) and its para‐substituted chloro (HL₂), carboxy (HL₃), fluoro (HL₄) and nitro (HL₅) derivatives were recognized. The best fits were found for σ_p and/or in the cases of , δ_NH and E_p/2°x, showing the importance of resonance and conjugation effects in such properties, whereas for the above thermodynamic properties the inductive effects (σ_I) are dominant. HL₂ exists in the hydrazo form in DMSO solution and in the solid state and contains an intramolecular H‐bond with the distance of 2.588(3) Å. It was also established that the dissociation process of HL_1–5 is non‐spontaneous, endothermic and entropically unfavourable, and that the increase in the inductive effect (σ_I) of para‐substitutents (H < Cl < COOH < F < NO₂) leads to the corresponding growth of the distance and decrease of the pK and of the changes of Gibbs free energy, of enthalpy and of entropy for the HL_1–5 acid dissociation process. The electrochemical behaviour of HL_1–5 was interpreted using theoretical calculations at the DFT/HF hybrid level, namely in terms of HOMO and LUMO compositions, and of reactivities induced by anodic and cathodic electron‐transfers. Copyright © 2010 John Wiley & Sons, Ltd.

A facile, efficient and selective solvent-free synthesis of ketones from secondary alcohols with tert-butylhydroperoxide (TBHP) as the oxidant under microwave irradiation is achieved, where the copper(II) 2,4-alkoxy-1,3,5-triazapentadienato complexes are efficient catalysts providing high yields (up to 100%), TONs (up to 890) and TOFs (up to 1780 h⁻¹).