New Fe^II and Cu^II Complexes Bearing Azathia Macrocycles – Catalyst Precursors for Mild Peroxidative Oxidation of Cyclohexane and 1‐Phenylethanol

Abstract

The new mononuclear FeII and CuII complexes [FeCl2(L1,3)] [L1 (1), L3 (4)], [Cu(OTf)2(L1,2)] [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 FeII 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.

Catalytic activity of a benzoyl hydrazone based dimeric dicopper(II) complex in catechol and alcohol oxidation reactions

“…4, entry 1). A similar inhibitory effect was observed for other Cu(II) systems [59,60]. In contrast, heteroaromatic N-based base additives have a beneficial effect on the acetophenone product yield.…”

“…The TEMPO promoted reaction conceivably involves its coordination, as well as of the alcohol substrate, followed by Cu-centered oxidative dehydrogenation of the alcohol upon H-abstraction [26][27][28][29][71][72][73][74][75]. Our group reported some efficient systems involving alkoxy-1,3,5-triazapentadienate Cu(II) complexes [69], bis-and tris-pyridyl amino and imino thioether Cu complexes [52] for the MW-assisted oxidation of secondary alcohols to the corresponding ketones. The above catalytic systems lead to comparable yields and/or TONs to those of this work.…”

“…A plausible mechanism for catecholase like activity promoted by R-Cu 2+ (the nitrate ligand is omitted for the sake of clarity). (Hpic) and 2-pyrazinecarboxylic acid (Hpca), or bases such as pyridine, 3,5-dimethyl-1H-pyrazole, pyridazine, piperidine and triethylamine were tested, since they have been reported [54][55][56][57][58][59][60][61] Fig. 4, entry 1).…”

“…The mechanism may involve the metal-assisted generation of Bu t OO Å and Bu t O Å radicals (upon oxidation and reduction of Bu t OOH by a Cu II or a Cu I center, respectively) [26,[50][51][52][53] the latter behaving as an H-atom abstractor from the alcohol [26,50,52].…”

Efficient catalytic oxidation of hydrocarbons mediated by tricopper clusters under mild conditions

“…In the presence of H 2 O 2 , the copper catalyst can activate H 2 O 2 to give Å OH and Å OOH radicals, which in turn can oxidize the hydrocarbon to form the products [20][21][22]. Given that we are invoking a direct ''O-atom insertion'' mechanism with the [Cu I Cu I Cu I (L)] 1+ tricopper cluster, it is imperative for us to rule out the involvement of radicals here.…”