Elena V. Gusevskaya

Novel highly selective catalytic oxychlorination of phenols

The highly selective oxychlorination of various phenols catalyzed by CuCl2 under mild conditions, in which chloride ions are used as chlorinating agents and dioxygen as a final oxidant, has been developed.

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.

Platinum/tin catalyzed hydroformylation of naturally occurring monoterpenes

Abstract (−)-β-Pinene, R -(+)-limonene, and (−)-camphene have been hydroformylated regiospecifically to give exclusively the linear isomers of corresponding aldehydes. The following systems were used as catalysts: PtCl 2 (PPh 3 ) 2 /SnCl 2 /PPh 3 , and PtCl 2 (diphosphine)/SnCl 2 /PPh 3 whose diphosphines were 1,2-bis(diphenylphosphino)ethane, 1,3-bis(diphenylphosphino)propane and 1,4-bis(diphenylphosphino) butane. The hydroformylation of β-pinene yields trans- 10-formylpinane with a 98% diastereoisomeric excess (d.e.), while limonene and camphene give the diastereoisomers of the corresponding aldehydes in approximately equal amounts (d.e. of ca. 10 and 15%, respectively). Differently from most of the rhodium and cobalt catalysts, the undesirable isomerization of β- to α-pinene is rather slow (1–5% based on reacted β-pinene). The primarily formed aldehyde of limonene undergoes the highly diasteroselective intramolecular cyclization (d.e. of virtually 100%) catalyzed by the platinum/tin active species yielding 4,8-dimethyl-bicyclo[3.3.1]non-7-en-2-ol. The effects of the catalyst composition and ligand nature on the product distribution have been studied. The use of PPh 3 as the only phosphorous-containing ligand, as well as the excess of SnCl 2 (Sn/Pt>1) promote the isomerizations of monoterpenes. The system with 1,3-bis(diphenylphosphino)propane causes excessive hydrogenation of the olefinic double bonds. Under optimized conditions, chemoselectivities for aldehyde formation of near 90% have been attained for all monoterpenes studied.

Palladium(II) catalyzed oxidation of naturally occurring terpenes with dioxygen

Abstract Limonene can be efficiently and selectively oxidized by dioxygen at 60–80°C in glacial acetic acid containing LiCl, in the presence of the PdCl 2 CuCl 2 catalytic combination, giving rise to the formation of trans -carveyl acetate as the major product. Several concurrent transformations of limonene occur in the reaction solutions, i.e., isomerization, acetic acid addition, and allylic oxidation. The effect of the reaction variables on the product distribution and reaction rate has been studied. The most favorable conditions for the carveyl acetate synthesis have been found. The reactions of α-pinene and β-pinene under the same conditions yield a mixture of carveyl acetate, α-terpenyl acetate, bornyl chloride, and fenchyl chloride. The activity of the Pd(OAc) 2 LiNO 3 combination in the oxidation of limonene, α-pinene, and β-pinene has also been examined.

Phosphotungstic Acid as a Versatile Catalyst for the Synthesis of Fragrance Compounds by α‐Pinene Oxide Isomerization: Solvent‐Induced Chemoselectivity

The remarkable effect of the solvent on the catalytic performance of H3PW12O40, the strongest heteropoly acid in the Keggin series, allows direction of the transformations of alpha-pinene oxide (1) to either campholenic aldehyde (2), trans-carveol (3), trans-sobrerol (4 a), or pinol (5). Each of these expensive fragrance compounds was obtained in good to excellent yields by using an appropriate solvent. Solvent polarity and basicity strongly affect the reaction pathways: nonpolar nonbasic solvents favor the formation of aldehyde 2; polar basic solvents favor the formation of alcohol 3; whereas in polar weakly basic solvents, the major products are compounds 4 a and 5. On the other hand, in 1,4-dioxane, which is a nonpolar basic solvent, both aldehyde 2 and alcohol 3 are formed in comparable amounts. The use of very low catalyst loading (0.005-1 mol %) and the possibility of catalyst recovery and recycling without neutralization are significant advantages of this simple, environmentally benign, and low-cost method. This method represents the first example of the synthesis of isomers from alpha-pinene oxide, other than campholenic aldehyde, with a selectivity that is sufficient for practical usage.

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.

Hydroformylation of myrcene: metal and ligand effects in the hydroformylation of conjugated dienes

The hydroformylation of myrcene catalyzed by Rh and Pt/Sn catalysts containing different P-donor ligands leads to the formation of a number of mono- and dialdehydes. Nine major products of the reaction have been characterized, showing that they arise from the n-alkyl and η3-allyl intermediates, formed through the reaction of the metal catalysts with the less substituted CC bond of the substrate. Thus, 4-methylene-8-methyl-7-nonenal is the major aldehyde formed with Pt/Sn catalysts, regardless of the P-donor ligand used. This aldehyde is also the main product of the reaction catalyzed by the Rh/xantphos system (xantphos = 9,9-dimethyl-4,6-bis(diphenylphosphino)xantene). However, with ligands such as bisbi (bisbi = 2,2′-bis((diphenylphosphino)methyl)-1,1′-biphenyl), also with bite angles around 120°, but with more flexible backbones than xantphos, rhodium catalysts yield mainly cis- and trans-3-ethylidene-7-methyl-6-octenal. These two aldehydes are also formed in the reactions catalyzed by Rh and P-donor monodentate ligands or the bidentante ones with bite angles around 90° (dppe, dppp). For the last type of ligands, an increase in the flexibility of the backbone reduces the selectivity for the β,γ-unsaturated aldehydes.

Rhodium catalyzed hydroformylation of β-pinene and camphene: effect of phosphorous ligands and reaction conditions on diastereoselectivity

Abstract The effect of phosphorus ligands on the rhodium catalyzed hydroformylation of β-pinene and camphene has been studied. In unmodified systems, β-pinene undergoes a fast isomerization to α-pinene. At longer reaction times and higher temperatures, the isomerization equilibrium is shifted resulting in the 80% chemoselectivity for β-pinene hydroformylation products (97% trans ). The addition of various diphosphines, phosphines or phosphites improves the chemoselectivity and shifts the hydroformylation towards cis aldehyde 3a . Both the rate and diastereoselectivity of the hydroformylation of β-pinene are largely influenced by the basicity of auxiliary ligands, but surprisingly no correlation between their steric characteristics and the diastereoselectivity of the catalytic system has been revealed for the ligands with cone angles of 128–165°. The systems with more basic ligands show lower activities, higher diastereoselectivities and usually higher chemoselectivities in the β-pinene hydroformylation. Camphene gives linear aldehyde 6, with virtually 100% regio- and chemoselectivity in both modified and unmodified systems. The addition of phosphorus ligands favors the formation of endo isomer 6b : 6a / 6b ≈1/1.5, whereas the ratio is ca. 1/1 in unmodified systems. Neither steric nor electronic parameters of the ligands have been found to influence significantly the diastereoselectivity of the camphene hydroformylation.

Palladium-catalyzed oxidation of monoterpenes: novel tandem oxidative coupling–oxidation of camphene by dioxygen

The Pd-catalyzed stereoselective oxidative coupling of (−)-camphene gives mainly (E,E)-diene, i.e., bis(3,3-dimethyl-2norbornylidene)ethane, which can be oxidized into the corresponding ,-unsaturated ketone, i.e. (3,3-dimethyl-2-norbornyl) (8-camphenyl)ketone, with LiNO3 being used as a reoxidant and dioxygen as a final oxidant; these two reactions are performed in a one port process with up to 90% selectivity for ketone. The applications of the Pd(II)/NO 3 − catalytic system to the oxidation of limonene and -pinene have also been studied.