Oxidation of Monoterpenes Catalysed by a Water‐Soluble Mn^III PEG‐Porphyrin in a Biphasic Medium

Abstract

It is well established that the transformation of abundant and cheap natural products, such as terpenoids, can produce other more valuable compounds. Thymoquinone, which has a commercial value significantly higher than that of its precursors, can be obtained by oxidation of carvacrol and thymol. In this work, a new water‐soluble MnIII PEG‐porphyrin is reported as catalyst in a water/hexane (1:1) biphasic medium for the oxidation of carvacrol and thymol into thymoquinone. The reactions were performed using tert‐butyl hydroperoxide as oxidant in the presence of ammonium acetate as co‐catalyst, reaching 94 and 78 % of conversion after 5 h of reaction for thymol and carvacrol, respectively. Experiments with oregano essential oil as substrate revealed selective transformation of thymol and carvacrol into thymoquinone. The main advantage of this biphasic system based on a water‐soluble catalyst and on substrates and products soluble in hexane, is the straightforward isolation, recovery and recycling of the catalyst by simple phase separation. Recycling studies of the MnIII PEG‐porphyrin using thymol as substrate showed high conversion values throughout four catalytic cycles.

Phase Separation and pH-Dependent Behavior of Four-Arm Star-Shaped Porphyrin-PNIPAM4 Conjugates

“…Star-shaped PNIPAM homopolymers , or block copolymers − with a variety of different architectures (e.g., number of arms, ,, terminal groups, or core–shell structures − ) have been published. Various other four-armed star-shaped porphyrin-centered molecules with arms consisting of polymers other than PNIPAM have also been described including those containing poly­(ethylene oxide), , poly­( N , N -diethylacrylamide), poly­(ε-caprolactone), poly­(ε-caprolactone)- block -poly­(oligo­(ethylene glycol)­methyl ether methacrylate), or poly­(ε-caprolactone)- block -poly­(gluconamidoethyl methacrylate) . Porphyrins covalently linked to polyethylene glycol or PNIPAM in block copolymer formation (i.e., the porphyrin n - block -monomer m ) have also been reported, − although those molecules do not have a porphyrin-centered star-shaped structure.…”

“…Thermoresponsive hydrogels based on PNIPAM with tetrakis­(4-aminophenyl)­porphyrin, protoporphyrin IX, or pheophorbide A photosensitizers used as cross-linkers have also been reported. Many of the abovementioned porphyrin–polymer systems have been investigated as potential drug-release systems, photosensitizers for photodynamic therapy, ,,− antibacterial agents, (photo)­catalysts, ,, magnetic resonance imaging contrast agents, or ultra-sensitive colorimetric thermometers …”

The polymerization of carvacrol catalyzed by Mn-porphyrins: obtaining the desired product guided by the choice of solvent, oxidant, and catalyst

“…Some studies describe carvacrol's oxidation employing metalloporphyrins as catalysts, 10–15 mainly because this class of compounds has shown to be able to mimic cytochrome P450's catalytic properties, thus catalyzing oxidation reactions under mild temperature and pressure conditions. 16–18 The catalytic activity of iron( iii )-porphyrins (FeP) and manganese( iii )-porphyrins (MnP) was evaluated in association with hydrogen peroxide (H 2 O 2 ), KHSO 5 (Oxone®), or tert -butyl hydroperoxide (TBHP), in homogeneous or heterogenized systems.…”

“…16–18 The catalytic activity of iron( iii )-porphyrins (FeP) and manganese( iii )-porphyrins (MnP) was evaluated in association with hydrogen peroxide (H 2 O 2 ), KHSO 5 (Oxone®), or tert -butyl hydroperoxide (TBHP), in homogeneous or heterogenized systems. 10–15 It should be highlighted that, in the majority of the cases, the metalloporphyrins presented catalytic behaviors to produce thymoquinone as the only identified product. 10–14 There are only two reports in which the FeP systems allowed the identification of thymohydroquinone and a polymeric product, and possibly other non-identified products could have been formed besides thymoquinone.…”

Biomimetic Oxidation of Benzofurans with Hydrogen Peroxide Catalyzed by Mn(III) Porphyrins

“…Biomimetic processes are also a convenient alternative for eco-sustainable oxidation processes, since they allow the use of green oxidants, such as H 2 O 2 or O 2 (that afford water as the only by-product), and mild conditions [12][13][14][15][16][17]. This leads to a significant reduction of environmental impact relative to stoichiometric oxidants, such as Cr 2 O 7 2− , MnO 4 − , and dimethyldioxirane (DMD), or milder but expensive oxidants that also generate sub-products, such as m-chloroperoxybenzoic acid (m-CPBA), PhIO, ClO − , HSO 5 − , and IO 4 − .…”