Eduardo N. dos Santos

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.

Chemical Plants: High-Value Molecules from Essential Oils

As society faces a future of dwindling petrochemical supplies at increasing cost, much attention has been focused on methods to degrade biomass into renewable commodity-chemical building blocks. Reported here is a powerful complementary approach that amplifies the complexity of molecular structures present in plant materials. Essential-oil phenylpropenoids are transformed via acrylate cross-metathesis into potent antioxidants that are widely used in perfumery and cosmetics, and in treating disorders associated with oxidative damage.

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.

Rhodium-catalyzed hydroformylation of allylbenzenes and propenylbenzenes: effect of phosphine and diphosphine ligands on chemo- and regioselectivity

Abstract Various allylbenzenes and propenylbenzenes have been hydroformylated with a 97–99% chemoselectivity using bis[(μ-acetate)(1,5-cyclooctadiene)rhodium(I)] as a catalyst precursor. Regioselectivity of the hydroformylation can be controlled by the nature of phosphorus auxiliary ligands. The Rh-NAPHOS (2,2′-bis[(diphenylphosphino)methyl]-1,1′-binaphthyl) system promotes the hydroformylation of allylbenzenes into linear aldehydes in near 95% selectivity and propenylbenzenes into branched aldehydes with a formyl group in α-position to phenyl ring in near 90% selectivity, while the Rh-dppp (1,3-bis(diphenylphosphino)propane) system gives branched aldehydes with a formyl group in β-position in near 70% selectivity starting from allylbenzenes. The regioselectivity of Rh-diphosphine systems correlates with a ligand bite angle. Both the rate and regioselectivity of the hydroformylation are largely influenced by the basicity of monophosphine auxiliary ligands, however, no correlation between their steric characteristics and the regioselectivity has been observed.

Diastereoselective hydroformylation of camphene catalyzed by platinum/tin complexes

Abstract The hydroformylation of camphene in the presence of platinum(II)/tin(II)/phosphine (diphosphine) catalytic systems with various achiral and chiral P-donor ligands has been studied. The reaction occurs regio-specifically to give exclusively the linear isomer of the corresponding aldehyde. With triphenylphosphine and chelating achiral diphosphines, i.e. 1,2-bis(diphenylphosphino) ethane, 1,3-bis(diphenylphosphino) propane and 1,4-bis(diphenylphosphino) butane, the chemoselectivity for hydroformylation of 90–96% and diastereoisomeric excess (d.e.) of the exo isomeric aldehyde of 16–30% have been achieved. The highest d.e. of 60% has been shown by the platinum/tin/( R )- or ( S )-BINAP system (BINAP—2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) with ca. 90% chemoselectivity for hydroformylation products at ca. 90% camphene conversion. A new aldehyde derived from α-fenchene, which could result from the skeletal isomerization of camphene under the reaction conditions has been detected (up to 30%) at the elevated tin/platinum ratio.

Convenient one-pot synthesis of 4,8-dimethyl-bicyclo[3.3.1]non-7-en-2-ol via platinum/tin catalyzed hydroformylation/cyclization of limonene

Abstract Limonene ( 1 ) was converted in one step into two diasteroisomers of 4,8-dimethyl-bicyclo[3.3.1]non-7-en-2-ol ( 2 ), useful as perfumes, employing PtCl 2 (PPh 3 ) 2 /PPh 3 /SnCl 2 and PtCl 2 (diphosphine)/PPh 3 /SnCl 2 systems as bifunctional catalysts whose diphosphines were 1,3-bis(diphenylphosphino)propane (dppp) and 1,4-bis(diphenylphosphino)butane (dppb). In the presence of the PtCl 2 (dppb)/PPh 3 /SnCl 2 system, which was found to be the most promising combination, the selectivity for 2 reached the value of 82% at 95% conversion of 1 .

Palladium/tin catalyzed alkoxycarbonylation of naturally occurring bicyclic monoterpenes

Abstract The alkoxycarbonylation of camphene and β -pinene has been studied. The following systems have been used as catalysts: PdCl 2 (PPh 3 ) 2 /SnCl 2 /PPh 3 and PdCl 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. It was observed that several concurrent transformations of monoterpenes occur in the reaction solutions. To find the optimum conditions for alkoxycarbonylation the effect of the reaction variables on the product distribution has been investigated. The reaction of β -pinene exclusively yields the products of the Lewis acid catalyzed skeletal rearrangement accompanied by a nucleophilic addition of chloride and methoxy groups. No products of carbon monoxide incorporation were observed. Camphene was converted to the corresponding linear ester composed of approximately equal amounts of exo and endo isomers with a selectivity of 90% and virtually 100% regioselectivity (linear/branched esters) using a PdCl 2 (PPh 3 ) 2 /SnCl 2 /PPh 3 system as a catalyst. Methyl bornyl ether was the only major by-product. SnCl 2 and PdCl 2 (PPh 3 ) 2 exhibited a strong synergetic effect on the camphene alkoxycarbonylation. PdCl 2 (PPh 3 ) 2 alone showed a very low catalytic activity promoting the predominant formation of the thermodynamically more stable exo isomer (the exo/endo ratio is approximately 2:1).

Synthesis of Fragrance Compounds from Biorenewables: Tandem Hydroformylation–Acetalization of Bicyclic Monoterpenes

The Rhodium‐catalyzed tandem hydroformylation–acetalization of the terpenes 3‐carene, 2‐carene, α‐pinene, and β‐pinene was studied in ethanol solutions in the presence of PPh3 or tris(O‐tert‐butylphenyl)phosphate, P(O‐o‐tBuPh)3, ligands. All these terpenes are constituents of turpentine oils obtained commercially from coniferous trees. β‐Pinene gave the corresponding aldehyde and acetal in excellent combined yields in both systems. 3‐Carene, 2‐carene, and α‐pinene, which contain sterically encumbered endocyclic double bonds, showed an extremely low reactivity with PPh3. The use of P(O‐o‐tBuPh)3 not only accelerated the hydroformylation of all four substrates remarkably but also increased the acetalization activity of the catalyst. In the Rh/P(O‐o‐tBuPh)3 system, various fragrance acetals and aldehydes were obtained from these renewable substrates in nearly quantitative combined yields. The process was performed under mild conditions, in environmentally friendly ethanol as a solvent, and in the absence of acid cocatalysts.

Synthesis of Fragrance Ingredients by Tandem Hydroformylation‐Cyclization of Limonene Catalyzed by Rhodium Complexes and Pyridinium p‐Toluenesulphonate

The rhodium‐catalyzed hydroformylation of limonene (1) in the presence of PPh3 or P(O‐o‐tBuPh)3 as auxiliary ligands and pyridinium p‐toluenesulfonate, as an acid co‐catalyst, gave two diasteroisomers of 4,8‐dimethyl‐bicyclo[3.3.1]non‐7‐en‐2‐ol (3) in nearly quantitative yield. Limonene is a cheap natural product obtained commercially from citrus fruits, in particular, as a sub‐product of the orange juice industry; whereas alcohol 3 is an expensive perfume ingredient. Alcohol 3 is formed through the hydroformylation of 1 giving a corresponding aldehyde (2) followed by the intramolecular carbonyl ene reaction resulting in cyclization of the aldehyde. The cyclization step is highly stereoselective with only one diasteroisomer of 3 being formed from each of two diasteroisomers of 2. The use of the P(O‐o‐tBuPh)3 ligand not only remarkably accelerates the hydroformylation step compared to the system with PPh3, but also increases significantly the cyclization activity of the catalytic system.