Oxidation Of Geraniol Research Articles

Remarkable Substituent Effects on the Oxidizing Ability of Tetraarylbismuthonium Tetrafluoroborates in Alcohol Oxidation

Abstract Substituent effects on the oxidizing ability of tetraarylbismuthonium tetrafluoroborates in alcohol oxidation are reported. Intermolecular and intramolecular competition experiments on geraniol oxidation by the combined use of tetraarylbismuthonium tetrafluoroborates and N,N,N′,N′-tetramethylguanidine (TMG) have revealed that the oxidizing ability of the bismuthonium salt increases by the introduction of methyl groups at the ortho position and an electron-withdrawing group at the para position of the aryl ligands. The intermolecular and intramolecular H/D kinetic isotope effects observed for the competitive oxidation of p-bromobenzyl alcohols have shown that the present oxidation reaction consists of fast pre-equilibrium leading to alkoxytetraarylbismuth(V) intermediates (first step) and α-hydrogen abstraction by the aryl ligand attached to the bismuth (second step). The experimental results demonstrate that the electron-deficient aryl ligands enhance the electrophilicity at the bismuth center to put forward the first step and that the bulky ligands destabilize the alkoxybismuth(V) intermediates to accelerate the second step. The newly explored mesityl- and 2,6-xylyltriarylbismuthonium salts have proven to convert primary and secondary alcohols to the corresponding carbonyl compounds with high efficiency under mild conditions. A remarkable steric effect of these oxidants has also been exhibited in the chemoselective oxidation between primary and secondary benzylic alcohols.

Geraniol dehydrogenase, the key enzyme in biosynthesis of the alarm pheromone, from the astigmatid mite Carpoglyphus lactis (Acari: Carpoglyphidae)

Geraniol dehydrogenase (GeDH), which plays an important role in the biosynthesis of neral, an alarm pheromone, was purified from the astigmatid mite Carpoglyphus lactis. The enzyme was obtained in an apparently homogeneous and active form after 1879-fold purification through seven steps of chromatography. Car. lactis GeDH was determined to be a monomer in its active form with a relative molecular mass of 42 800, which is a unique subunit structure in comparison with already established alcohol dehydrogenases. Car. lactis GeDH oxidized geraniol into geranial in the presence of NAD+. NADP+ was ineffective as a cofactor, suggesting that Car. lactis GeDH is an NAD+-dependent alcohol dehydrogenase. The optimal pH and temperature for geraniol oxidation were determined to be pH 9.0 and 25 degrees C, respectively. The Km values for geraniol and NAD+ were 51.0 microm and 59.5 microm, respectively. Car. lactis GeDH was shown to selectively oxidize geraniol, whereas its geometrical isomer, nerol, was inert as a substrate. The high specificity for geraniol suggests that Car. lactis GeDH specializes in the alarm pheromone biosynthesis of Car. lactis. Car. lactis GeDH is composed of 378 amino acids. Structurally, Car. lactis GeDH showed homology with zinc-dependent alcohol dehydrogenases found in mammals and a mosquito (36.6-37.6% identical), and the enzyme was considered to be a member of the medium-chain dehydrogenase/reductase family, in view of the highly conserved sequences of zinc-binding and NAD+-binding sites. Phylogenetic analyses indicate that Car. lactis GeDH could be categorized as a new class, different from other established alcohol dehydrogenases.

Size dependence in solvent-free aerobic oxidation of alcohols catalyzed by zeolite-supported palladium nanoparticles

Abstract The size dependence in the palladium nanoparticle-catalyzed solvent-free aerobic oxidation of alcohols was studied. Palladium nanoparticles with tunable mean sizes in a range of 2.0–10.5 nm were prepared over NaX zeolite by ion exchange of Na + with an ionic Pd precursor followed by calcination and H 2 reduction. The calcination temperature was found to be a crucial factor in determining the mean size of Pd nanoparticles. Pd/NaX catalysts with proper mean sizes of Pd could catalyze the solvent-free aerobic oxidation of various alcohols, and were particularly efficient for the oxidation of benzylic alcohols without substituents in benzene ring. Detailed studies using the Pd/NaX catalysts with different mean sizes of Pd revealed that the solvent-free aerobic oxidation of benzyl alcohol was structure-sensitive, and the intrinsic turnover frequency (TOF) reached a maximum at a medium mean size of Pd (2.8 nm). On the other hand, for the oxidation of geraniol or 2-octanol without delocalized π-ring, the reaction was structure-insensitive, and the intrinsic TOF did not change significantly with the mean size of Pd particles.

Palladium-catalyzed oxidation of geraniol in dense carbon dioxide

The performance and stability of supported Pd catalysts were investigated in the partial oxidation of geraniol with molecular oxygen in dense CO 2. All catalysts deactivated to a certain extent in the continuous-flow reactor; the least stable among them was Pd/Al 2O 3. Beside oxidative dehydrogenation, isomerization and hydrogenation reactions were also catalyzed by Pd. The initial selectivity of Pd to citral formation was only 50–60% but improved up to 90% with time-on-stream. The extensive catalyst deactivation and the shift in product distribution are (partly) attributed to formation of geranic (neric) acid, and the acid-catalyzed dimerization and oligomerization of reactant and products leading to site blocking by the high molecular weight by-products. ATR-IR spectroscopy revealed that no CO was present on the Pd surface during geraniol oxidation though it appeared in the absence of oxygen. A shift from two-phase to single phase conditions diminished the reaction rate, presumably due to accumulation of water co-product on the catalyst surface. Despite of the initial deactivation of Pd/SiO 2, an outstanding TOF of 169 h −1 was achieved on the stabilized catalyst under steady state conditions, at 80 °C and 150 bar.

Utilization of geraniol is dependent on molybdenum in Pseudomonas aeruginosa: evidence for different metabolic routes for oxidation of geraniol and citronellol

Mini-Tn5-induced mutants with defects in utilization of linear terpenes such as citronellol, geraniol, citronellate and/or geranylate were isolated from Pseudomonas aeruginosa. One mutant was unable to utilize geraniol but showed wild-type growth with the three other acyclic terpenes tested. The Tn5 insertion site of the mutant was determined by DNA sequencing. Comparison with the P. aeruginosa genome sequence revealed that PA3028, an ORF with high similarity on the amino acid level to molybdenum cofactor biosynthesis protein A2 (encoded by moeA2), was the target of mini-Tn5 in the mutant. Disruption of moeA2 in P. aeruginosa PAO1 wild-type by insertion mutagenesis resulted in the same geraniol-minus phenotype. The ability to utilize geraniol was restored to the mutant by conjugative transfer of PCR-cloned wild-type moeA2 on a broad-host-range plasmid. Growth of P. aeruginosa PAO1 on geraniol and geranial, but not on citronellol, citronellate or geranylate, was inhibited by the presence of 10 mM tungstate, a molybdenum-specific inhibitor. Inhibition by tungstate was prevented by addition of molybdate. The results indicate that at least one step in the oxidation of geraniol to geranic acid (geranial oxidation) is a molybdenum-dependent reaction in P. aeruginosa and is different from the molybdenum-independent oxidation of citronellol to citronellate.

Remarkable Substituent Effects on the Oxidizing Ability of Triarylbismuth Dichlorides in Alcohol Oxidation.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Remarkable Substituent Effects on the Oxidizing Ability of Triarylbismuth Dichlorides in Alcohol Oxidation

Substituent effects on the oxidizing ability of triarylbismuth dichlorides were examined by intermolecular and intramolecular competition experiments on geraniol oxidation in the presence of DBU. It was found that the oxidizing ability of the dichlorides increases with increasing electron-withdrawing ability of the para substituents, and by introduction of a methyl group at the ortho position of the aryl ligands attached to the bismuth. The intermolecular and intramolecular H/D kinetic isotope effects observed for the competitive oxidation of p-bromobenzyl alcohols indicate that the rate-determining step involves C-H bond cleavage. Several primary and secondary alcohols were oxidized efficiently under mild conditions by the combined use of newly developed organobismuth(V) oxidants and DBU.

Oxidation of geraniol and other substituted olefins with hydrogen peroxide using mesoporous, sol–gel-made tungsten oxide–silica mixed oxide catalysts

The preparation of a series of mesoporous tungsten oxide–silica mixed oxides by sol–gel methods under basic conditions is reported. Surface modification with methyl and 3-chloropropyl groups is possible in an amount between 10 and 40 mol% with respect to the silane precursor. The amount of polar organic functional groups controls the surface area, the porosity, and the catalytic activity of the solids in the oxidation of different substrates with hydrogen peroxide. The oxidation of geraniol is studied in detail. The catalysts are active and produce epoxides in good yields. The latter are influenced by the presence of polar organic groups. The preparation method allows the preparation of catalysts that are resistant to leaching and can be recycled several times without appreciable loss of activity.

Liquid-phase Oppenauer oxidation of primary allylic and benzylic alcohols to corresponding aldehydes by solid zirconia catalysts

Abstract Hydrous zirconia and grafted zirconium 1-propoxide catalysts were found active in the Oppenauer oxidation of cinnamyl alcohol, geraniol and 4- tert -butylcyclohexanol ( cis - and trans -). The most active hydrous zirconia catalysts were formed by calcining at 250–300 °C. Grafted zirconium 1-propoxide on silica gel and MCM-41 were active in the Oppenauer oxidation of geraniol with high selectivity to the desired citral product. However, over an acidic support such as AlMCM, the grafted zirconium 1-propoxide catalysed the dehydration and isomerisation of the alcohol, leading to low yield to citral. Also, furfural was found to be an efficient oxidant for the titled Oppenauer oxidation. Other solid catalysts such as γ-Al 2 O 3 , Na–Al 2 O 3 , zeolite beta and Mg/Al hydrotalcite showed only moderate catalytic activity and selectivity in the Oppenauer oxidation of geraniol. As compared to other solid catalysts, hydrous zirconia solid catalysts used in this work are active and selective towards the formation of desired carbonyl oxidation products; additionally, these solid zirconia catalysts are easy to prepare and recycle, and applicable to different alcohol substrates.

Epoxidation of allylic alcohols with TiO2-SiO2: hydroxy-assisted mechanism and dynamic structural changes during reaction

Epoxidation of allylic alcohols and cyclohexene with TBHP and titania–silica aerogels containing 1 and 5 wt% TiO2 has been studied. For the oxidation of geraniol and cyclohexenol, the regio- and diastereoselectivities and kinetic data indicate an OH-assisted mechanism involving a dative bond between the OH group and the Ti site. This mechanism is disabled in the oxidation of cyclooctenol due to steric hindrance. The moderate regio- and diastereoselectivities of the aerogels, compared with those of TS-1 and the homogeneous model Ti(OSiMe3)4, are attributed to the presence of non-isolated Ti sites and to a “silanol-assisted” mechanism, according to which model the allylic alcohol is anchored to a neighboring SiOH group instead of the Ti-peroxo complex. Kinetic analysis of the initial transient period revealed rapid catalyst restructuring during the first few turnovers. A feasible explanation is the breaking of Si–O–Ti linkages of the carefully predried aerogels by water or TBHP, resulting in active Ti sites with remarkably different catalytic properties.

Oxidation of monoterpenes with hydrogen peroxide catalysed by Keggin-type tungstoborates

The Keggin-type anions [MnIII(H2O)BW11O39]6− and [BW12O40]5− were used for the first time as catalysts in studies of oxidation of monoterpenes by hydrogen peroxide. The oxidation of geraniol 1, nerol 2, (+)-3-carene 3, thymol 4 and carvacrol 5 was examined. The reactions were carried out in acetonitrile, at room temperature (for 1 and 2) or at reflux (for 3, 4 and 5). Compounds 1 and 2 were preferentially epoxidised at the C2–C3 double bond, whereas 3, under controlled reaction conditions, could afford only the α-epoxide. The oxidation of 4 and 5 yielded a mixture of benzoquinones. For tetrabutylammonium salts, the MnIII substituted anion was found to be a more efficient catalyst than [BW12O40]5−. The infrared spectra of the recovered solids at the end of reaction showed that the Keggin anions were still present, but the electronic spectra of the manganese residues indicate that oxidation of MnIII had taken place.

Synthesis and structural characterization of zirconium complexes containing aminodiolate ligands and their use as Lewis acid catalysts

A series of aminodiols RN(CH2CH2C(OH)Rprime2)2 (R, Rprime = Me, Me 4; Me, Ph 5; tert-butyl, Me 6; tert-Bu, Ph 7; (S)-PhCH(Me), Me 8) were prepared by the Michael addition of a primary amine to methyl acrylate followed by reaction of the resulting aminodiester with excess methyl or phenyl lithium. Reaction of two equivalents of the aminodiols 4-8 with tetrabenzylzirconium afforded the zirconium bis(aminodiolates) 10-14 in excellent yield. Complex 11 (R, Rprime = Me, Ph) adopts a cis, fac-octahedral geometry in solution and in the solid state. Complexes 10-14 are fluxional in solution by NMR spectroscopy: small substituents at nitrogen and large substituents at the alkoxide carbons slow the rate of exchange. The chiral complex 14 functions as a Lewis acid catalyst in the nitroaldol (Henry) reaction and the oxidation of geraniol by tert-butyl hydroperoxide with modest enantioselectivities (30 and 46% enantiomeric excess (ee), respectively).Key words: diol, Michael addition, zirconium, synthesis, structure, fluxional, alkoxide, Lewis acid, catalysis.

Chemoselective Oxidation of Alcohols to Aldehydes and Ketones by tert-Butyl Hydroperoxide Catalyzed by a Ruthenium Complex of N,N‘,N‘‘-Trimethyl-1,4,7-triazacyclononane

An operationally simple method based on [Cn*RuIII(CF3CO2)3·H2O] (Cn* = N,N‘,N‘‘-trimethyl-1,4,7-triazacyclononane) catalyst and 1−1.2 equiv of tert-butyl hydroperoxide as terminal oxidant is effective for selective transformation of alcohols to aldehydes and ketones in methylene chloride. The reaction proceeds in high yield and selectivity. Preparation of benzaldehyde (98% yield) from benzyl alcohol on a 200 mmol scale can be performed without modification of the procedure such as slow addition of the oxidant or cooling to 0 °C, and catalyst turnovers of 700 are achieved. Oxidation of geraniol which contains an isolated trisubstituted CC bond leads to geranial selectively without oxidation of the CC bond. Results from Hammett correlation studies (ρ = −0.47) and primary kinetic isotope effect (kH/kD = 4.8) for the catalytic benzyl alcohol oxidations are inconsistent with an oxoruthenium (ORu) based mechanism. A mechanism involving reactive tBuO•/tBuOO• radicals is also excluded based on results from previous works: Cheng, W.-C.; Fung, W.-H.; Che, C.-M. J. Mol. Catal. (A) 1996, 113, 311; absence of di-tert-butyl peroxide; and using cumyl hydroperoxide as a radical probe. A tert-butylperoxoruthenium complex is postulated to be the active intermediate.

New Chiral C3-Symmetric Triols as Ligands for Vanadium and Titanium Complexes

Several new chiral triols 1a−c with C3 symmetry have been prepared in enantiopure form via a route involving the iterative catalytic asymmetric alkylation of aldehydes by organozinc reagents followed by diastereoselective hydroboration. Reactions of these chiral triols with titanates Ti(OR)4 and vanadates V(O)(OR)3 are reported. The structure of the binuclear vanadium triolato(3−) complex [V(O){(OCH-Et)3CH}]2 (12a) has been determined by X-ray analysis. A series of model reactions was conducted to assess the catalytic properties of the chirally modified titanium and vanadium secondary alkoxides. The reactions chosen were the addition of diethylzinc and of trimethylsilyl cyanide to benzaldehyde and the oxidation of geraniol and of ethyl phenyl sulfide with tert-butyl hydroperoxide. Only a moderate asymmetric induction was found in some model reactions catalyzed by titanium complexes 11a,b. On the other hand, a very efficient chemoselective oxidation of sulfides to sulfoxides with tert-butyl hydroperoxide was possible in the presence of vanadium complex 12a.

ChemInform Abstract: The Allylic Oxidation of Geraniol Catalyzed by Cytochrome P‐450Cath., Proceeding with Retention of Configuration.

AbstractIncubation of the chiral geraniol (I) with microsomal cytochrome P‐450Cath.

The Allylic Oxidation of Geraniol Catalyzed by Cytochrome P‐450Cath., Proceeding with retention of configuration

AbstractIncubation of the geraniols (R)‐(8‐2H1)[8‐3H1]‐1 and (S)‐(8‐2H1)[8‐3H1]‐1 with microsomal cytochrome P‐450Cath. from the subtropical plant Catharanthus roseus (L.)G. DON resulted in the formation of the chiral 8‐hydroxygeraniols (S)‐(8‐2H1)[8‐3H1]‐2 and (R)‐(8‐2H1)[8‐3H1]‐2. Their absolute configuration was assigned on the basis of the 1H‐decoupled 3H‐NMR Spectra of the corresponding dicamphanates (S)‐(8‐2H1)[8‐3H1]‐9 and (R)‐(8‐2H1)[8‐3H1]‐9, of which the configurations are established in relation to the synthetic reference samples. The results clearly indicate retention of configuration during the allylic oxidation of 1.

Rhodium carbenoid mediated cyclisations. Part 4. Synthetic approaches to oxepanes related to zoapatanol

Synthetic approaches to the oxepane diterpene zoapatanol are described, in which the key step is the formation of the 7-membered cyclic ether by a rhodium carbenoid cyclisation. The model oxepane (15) was synthesised from (E)-geraniol by selective epoxidation of the allylic alcohol, followed by conversion into the corresponding epoxy iodide (9). Reaction of the iodide with the anion of ethyl acetate gave, after opening of the epoxide, diazo-transfer and acetylation, the substrate (14) for cyclisation. Treatment of (14) with rhodium(II) acetate resulted in cyclisation to the required oxepane (15) in good yield (Scheme 3). The approaches to the functionalised side chain of zoapatanol centred on the allylic alcohols (25), prepared by allylic oxidation of geraniol and functional group transformation. After asymmetric epoxidation, these substrates were converted into the diazo alcohols (30) using a similar sequence to above (Scheme 5). Finally, cyclisation using rhodium(II) acetate gave the unstable optically active oxepanes (31), which were characterised by conversion into the silyl enol ethers (32).

Regioselective epoxidation of geraniol with m-chloroperbenzoic acid in emulsion systems

Oxidation of geraniol with m-chloroperbenzoic acid (m-CPBA) in emulsion systems gave 2,3-epoxygeraniol selectively in high yield.

Oxidation of geraniol by an enzyme system from orange

Enzyme preparations from orange juice vesicles reversibly oxidized geraniol in the presence of NADP. NAD was ineffective for this reaction. The enzyme mediating this reaction—geraniol dehydrogenase (GeDH)—was separated from alcohol dehydrogenase by ammonium sulfate fractionation and gel filtration. GeDH also oxidized nerol, farnesol and citronellol, but at slower rates. Whereas pH9·0 was optimal for geraniol oxidation, geranial was reduced maximally at pH 6·5. Thiol-reacting compounds and metal chelators inhibited GeDH activity. Increasing the ratio of the reduced to oxidized forms of NADP progressively depressed the rate of geraniol oxidation. Thus, in juice cells the redox state of the nucleotide coenzyme may regulate the equilibrium between geraniol and geranial.

Studies on a mevaldic acid reductase of rat liver

A TPNH-specific mevaldic reductase of rat liver has been partially purified. This enzyme fraction catalyzes the reduction of both optical isomers of mevaldic acid to isomeric mevalonic acids. Although the evidence is not unequivocal, most data are in agreement with the conclusion that a single enzyme mediates the reduction of the two isomers of mevaldic acid. The enzyme is strongly inhibited by hydroxylamine, but it is unaffected by iodoacetamide and only weakly inhibited by p-chloromercurisulfonic acid and N-ethyl maleimide. The enzyme exhibits no metal ion requirement and has a rather broad pH optimum (6.5 to 7.7). The biologically active isomer of mevaldic acid is reduced much more rapidly than the other isomer, and, in the reductive process, one mole of mevaldic acid is reduced and one mole of TPNH is oxidized for each mole of mevalonic acid formed. Biological activity of the enzymatic reduction product is demonstrated by its conversion to terpenol pyrophosphates and by its utilization as substrate for mevalonic kinase. Possible metabolic roles for the enzyme and its substrate are discussed. A second aldehyde-reducing activity was detected, which is DPNH specific and has activities similar to alcohol dehydrogenase. This enzyme shows significant activity in the oxidation of geraniol and farnesol in the presence of DPN.