Patricia A. Robles-Dutenhefner

Hydration and acetoxylation of monoterpenes catalyzed by heteropoly acid

The liquid-phase hydration and acetoxylation of limonene (1), β-pinene (2) and α-pinene (3) catalyzed by dissolved or silica-supported heteropoly acid H3PW12O40 (PW) in acetic acid and acetic acid/water solutions have been studied. All three substrates give α-terpineol (4) as the main product along with α-terpenyl acetate (5). The reaction rate increases in the order: limonene<α-pinene<β-pinene. Synthetically useful homogeneous and heterogeneous acetoxylation and hydration of 1, 2 and 3 into 4 and 5 have been developed. At room temperature under optimized conditions, 2 and 3 form a mixture of 4 and 5 with 85% selectivity at 90% substrate conversion. 1 gives 4 and 5 with 85% selectivity at 50% conversion, with the main product being ester 5 in acetic acid (4/5≈30/70) and alcohol 4 in HOAc/H2O (90/10 v/v) solutions (4/5≈85/15). Virtually no oligomerization of monoterpenes occurs under the optimized conditions. The catalyst can be separated without neutralization and may be reused. The PW shows a much higher catalytic activity than conventional acid catalysts such as H2SO4 and Amberlyst-15.

Cobalt catalyzed autoxidation of monoterpenes in acetic acid and acetonitrile solutions

Abstract Oxidation of limonene, α-pinene and β-pinene with dioxygen in acetic acid and acetonitrile solutions containing catalytic amounts of CoCl 2 has been studied. Limonene and α-pinene give both allylic oxidation and epoxidation products in a molar ratio of near 1/1, with chemoselectivities for corresponding products being higher in acetonitrile than those in acetic acid. On the other hand, oxidation of β-pinene leads essentially to allylic products, i.e. highly valuable pinocarveol, pinocarvone, myrtenol and myrtenal. In acetic acid, a combined selectivity for these products does not exceed 40% due to the concomitant substrate isomerization and acetic acid addition, while in acetonitrile, good selectivities of up to 85% at a 40–50% substrate conversion have been achieved.

Palladium-catalyzed oxidation of bicyclic monoterpenes by hydrogen peroxide

Abstract The activity of the PdCl 2 –CuCl 2 combination in the oxidation of camphene, α-pinene, and β-pinene by dioxygen in acetic acid solutions has been studied. The reactions of α-pinene and β-pinene yield a mixture of carveyl acetate (up to 25% on reacted olefin), α-terpenyl acetate, bornyl chloride, and fenchyl chloride. Camphene undergoes a skeletal rearrangement and an acetic acid/water addition resulting in bornyl acetate as a major product, along with borneol and α-pinene. No oxidation products are detected. In an attempt to develop a CuCl 2 -free catalytic system for the selective oxidation of bicyclic monoterpenes, the oxidation of β-pinene and camphene by hydrogen peroxide catalyzed by Pd(OAc) 2 in acetic acid solutions has been studied. β-Pinene gave the allylic oxidation products, i.e., pinocarveol, pinocarveyl acetate and myrtenyl acetate, with selectivity up to 75% at virtually complete conversion, and camphene gave camphene glycol monoacetate with a 90% selectivity at 80% conversion. The oxidation reaction competes with the skeletal rearrangement of monoterpenes accompanied by a nucleophilic addition of hydroxy and acetoxy groups. The introduction of benzoquinone (BQ) in catalytic amounts exerts a beneficial effect on the catalyst stability and selectivity for glycol monoacetate formation. For the Pd(OAc) 2 –BQ–H 2 O 2 system, more than 200 turnover numbers could be achieved in the acetoxylation of camphene.

Synthesis of methyl acetate from methanol catalyzed by [(η5-C5H5)(phosphine)2RuX] and [(η5-C5H5)(phosphine)2Ru(SnX3)] (X=F, Cl, Br): ligand effect

Abstract Monometallic Ru and heterobimetallic complexes containing RuSn bonds, [(η5-C5H5) P2RuX] and [(η5-C5H5)P2Ru(SnX3)], where P=PPh3, PPh2Me, P2=1,2-bis(diphenilphosphine) ethane (dppe), and X=F, Cl, Br, were synthesized and characterized. These complexes were tested as catalysts in a single-step methanol conversion to acetic acid (methyl acetate) in the absence of CO. All complexes showed a high selectivity with their catalytic activity being strongly dependent on the nature of the ligands P and X. The effect of the ligand P showed the order of PPh3>PPh2Me≅dppe and the halogen effect: F>Cl≅Br and SnF3>SnCl3≅SnBr3. Heterobimetallic complexes showed ca. doubled activities compared to their monometallic analogues, indicating the importance of the RuSn bond in a catalytic active specie. The order of catalytic activities followed the increase in the positive charge on the ruthenium atom, which was confirmed by 31P and 119Sn NMR spectroscopy and X-ray diffraction techniques. The obtained data support the mechanistic view of acetic acid formation by dehydrogenation of methanol giving formaldehyde via the rate-determining β-hydrogen abstraction in the Ru(II)–OMe intermediate, followed by formaldehyde dimerization into acetic acid.

Ce1−xSmxO1.9−δ nanoparticles obtained by microwave-assisted hydrothermal processing: an efficient application for catalytic oxidation of α-bisabolol

Heterogeneous catalysts based on Sm-doped ceria were employed for the first time in the liquid-phase oxidation of α-bisabolol. Nanometer-sized catalysts were obtained by microwave-hydrothermal synthesis and were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), Raman spectroscopy and N2-physisorption. The influence of Sm doping, temperature and the solvent used on the catalytic behavior was investigated. Conversions up to 84% and a combined selectivity for the products up to 77% were obtained for Ce0.9Sm0.15O1.85−δ catalysts. The reactions were highly selective for the epoxidation products (only bisabolol oxides A and B were obtained) and shown to be strongly dependent on the temperature and solvent employed. Best results were achieved for higher Sm concentrations, which indicate that changes in the textural properties due to doping produced a significantly more active catalyst.

Characterization and catalytic activity studies of sol–gel Co–SiO2 nanocomposites

Abstract Silica embedded with transition metals exhibits adequate properties for applications in catalysis, sensors and optics. Cobalt–silica (Co–SiO 2 ) nanocomposites were prepared by the sol–gel method and thermally treated at 700, 900, 1100 and 1250 °C. Characterization of the samples was performed by XRD and BET nitrogen adsorption. The performance of the nanocomposites was investigated by catalysis reactions of oxidation. These catalysts were found to be recyclable showing a catalytic activity even after a third recovering. The results indicate that thermal treatment of sol–gel nanocomposites at temperatures higher than 900 °C is essential for the preparation of active heterogeneous catalysts.

Metal complex catalyzed functionalization of naturally occurring monoterpenes: oxidation, hydroformylation, alkoxycarbonylation+

Selective oxidation, hydroformylation and alkoxycarbonylation of monoterpenes, i.e., limonene, α-pinene, β-pinene, and camphene, via homogeneous catalysis by metal complexes were studied. “Pd(II)+reversible co-oxidant (CuCl2 or LiNO3)” systems were applied for the oxidation of monoterpenes with dioxygen. Hydrogen peroxide was also used as a final oxidant in the presence of the catalytic amounts of Pd(OAc)2 or Pd(OAc)2/benzoquinone. The Rh and Pt/Sn systems with various achiral and chiral diphosphines as ligands were applied for the hydroformylation of monoterpenes and the Pd/Sn/phosphine(diphosphine) systems for the alkoxycarbonylation, which provided the carboxylic acid derivatives. It was found that in all systems the starting monoterpenes as well as the primarily formed products can undergo various concurrent transformations, such as isomerization, hydrogenation, cyclization, addition, with the balance between the reaction pathways being delicate. Efforts were made to investigate the effects of the reaction variables and the catalyst composition on the product distribution in order to find the most favorable conditions for selective syntheses. Stereochemistry of the products was also studied.