Abstract:An aerobic oxidation of aldehydes towards carboxylic acids in MeCN using 1 atm of pure oxygen or oxygen in air as the oxidant and a catalytic amount of single component catalyst, Fe(NO3)3·9H2O, has been developed. Carboxylic acids with different synthetically useful functional groups were obtained at room temperature. Two mechanistic pathways have been proposed based on isotopic labeling, NMR monitoring, and control experiments. The practicality of this reaction has been demonstrated by conducting several 50 m… Show more
“…Then Au-catalyzed cycloisomerization of ( R a )- 4ja generated γ-butyrolactone bearing a trans C=C bond ( S , E )- 5ja in 95% yield with 91% ee and 98:2 of E / Z selectivity 24 . After sequential hydrogenation of ( S , E )- 5ja and Fe-TEMPO-catalyzed aerobic oxidation 60,61 of the resulting hydrogenated product without purification, ( R )-traumatic lactone was afforded in 67% yield with 98% ee by one time recrystallization.Fig. 4The first enantioselective synthesis of ( R )-traumatic lactone.…”
Metal-catalyzed enantioselective construction of the loosening axial allene chirality spreading over three carbon atoms using a chiral ligand is still a significant challenge. In the literature, steric effect of the substrates is the major strategy applied for such a purpose. Herein, we present a general palladium-catalyzed asymmetrization of readily available racemic 2,3-allenylic carbonates with different types of non-substituted and 2-substituted malonates using (R)-(−)-DTBM-SEGPHOS as the preferred ligand to afford 1,3-disubstituted chiral allenes with 90~96% ee. This protocol has been applied to the first enantioselective synthesis of natural product, (R)-traumatic lactone. Control experiments showed that in addition to the chiral ligand, conducting this transformation via Procedure C, which excludes the extensive prior coordination of the allene unit in the starting allene with Pd forming a species without the influence of the chiral ligand, is crucial for the observed high enantioselectivity.
“…Then Au-catalyzed cycloisomerization of ( R a )- 4ja generated γ-butyrolactone bearing a trans C=C bond ( S , E )- 5ja in 95% yield with 91% ee and 98:2 of E / Z selectivity 24 . After sequential hydrogenation of ( S , E )- 5ja and Fe-TEMPO-catalyzed aerobic oxidation 60,61 of the resulting hydrogenated product without purification, ( R )-traumatic lactone was afforded in 67% yield with 98% ee by one time recrystallization.Fig. 4The first enantioselective synthesis of ( R )-traumatic lactone.…”
Metal-catalyzed enantioselective construction of the loosening axial allene chirality spreading over three carbon atoms using a chiral ligand is still a significant challenge. In the literature, steric effect of the substrates is the major strategy applied for such a purpose. Herein, we present a general palladium-catalyzed asymmetrization of readily available racemic 2,3-allenylic carbonates with different types of non-substituted and 2-substituted malonates using (R)-(−)-DTBM-SEGPHOS as the preferred ligand to afford 1,3-disubstituted chiral allenes with 90~96% ee. This protocol has been applied to the first enantioselective synthesis of natural product, (R)-traumatic lactone. Control experiments showed that in addition to the chiral ligand, conducting this transformation via Procedure C, which excludes the extensive prior coordination of the allene unit in the starting allene with Pd forming a species without the influence of the chiral ligand, is crucial for the observed high enantioselectivity.
“…[12] Ma and co-workers found that a threecomponent catalyst (TEMPO, Fe(NO 3 ) 3 •9H 2 O and KCl) enabled room-temperature aerobic oxidation of primary alcohols to carboxylic acids in 1,2-dichloroethane (Scheme 1d), [13] in which Fe(NO 3 ) 3 •9H 2 O catalyzed further oxidation of aldehydes to carboxylic acids. [14] From the perspective of green and sustainable Laccase coupled with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) is a wellknown catalytic system for the oxidation of alcohols to the carbonyl compounds. In this work, a simple yet effective solvent engineering strategy is developed to enable aromatic alcohols/aldehydes to be efficiently oxidized to carboxylic acids by a laccase-TEMPO system.…”
challenging, because most of the aerobic alcohol oxidations stop at the aldehyde stage even with noble metals. [4] In addition, many methods suffer from such drawbacks as use of environmentally unfriendly catalysts (e.g., metals) and/ or solvents (e.g., aqueous alkaline solutions and chloroalkanes), harsh reaction conditions, and unsatisfactory selectivities. According to Tojo and Fernández, [1a] approximately 40% of the processes toward carboxylic acids were still performed through a two-step oxidation, with the isolation of the aldehyde intermediates; therefore, this technique for direct access to carboxylic acids is still immature.Nitroxyl radicals are versatile organocatalysts for catalytic oxidations, where they, alone or in combination with other catalysts (e.g., transition metals, NO x and laccase), are used in the presence of oxidants. [5] 2,2,6,6-Tetramethylpiperidinyl-1-oxy (TEMPO) and its derivatives, a type of stable nitroxyl radicals, have been widely used for the production of pharmaceuticals, fragrances and flavors, agrochemicals, and specialty chemicals. [6] To date, few studies on catalytic oxidation of primary alcohols to carboxylic acids by nitroxyl radicals have been reported, [7] although the routes to aldehydes have been well established. [8] In Anelli's oxidation, primary alcohols were oxidized to carboxylic acids by TEMPO and KBr in a biphasic CH 2 Cl 2 /water mixture in the presence of a phasetransfer catalyst (Scheme 1a), with chlorine bleach (NaOCl) as an oxidant. [9] Then a series of modified Anelli's protocols were developed by using alternative oxidants (e.g., NaClO 2 , and bis(acetoxy)iodobenzene (BAIB)). [10] Yakura et al. reported a TEMPO-iodobenzene hybrid catalyst for the synthesis of carboxylic acids in acetic acid in the presence of peracetic acid (Scheme 1b). [11] Stahl and co-workers presented a clean direct electrochemical method to oxidize primary alcohols and aldehydes to carboxylic acids in the presence of 4-acetamido-TEMPO (Scheme 1c). [12] Ma and co-workers found that a threecomponent catalyst (TEMPO, Fe(NO 3 ) 3 •9H 2 O and KCl) enabled room-temperature aerobic oxidation of primary alcohols to carboxylic acids in 1,2-dichloroethane (Scheme 1d), [13] in which Fe(NO 3 ) 3 •9H 2 O catalyzed further oxidation of aldehydes to carboxylic acids. [14] From the perspective of green and sustainable Laccase coupled with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) is a wellknown catalytic system for the oxidation of alcohols to the carbonyl compounds. In this work, a simple yet effective solvent engineering strategy is developed to enable aromatic alcohols/aldehydes to be efficiently oxidized to carboxylic acids by a laccase-TEMPO system. Citrate buffer (100-300 mm, pH 6) rather than the widely used acetate buffer (pH 4-5) proves to be optimal for this purpose. The roles of citrate are discussed in laccase-TEMPOcatalyzed synthesis of carboxylic acids. 5-Hydroxymethylfurfural (HMF) of 200 mm is oxidized to 2,5-furandicarboxylic acid (FDCA), a top-value biobased chemical in the...
“…Although aerobic oxidation of aldehydes can be performed without catalysts, numerous homogeneous catalysts have been described for this particular reaction and, in particular, first-row transition metals . Recently, Y.-B.…”
The organocatalyzed aerobic oxidation of aldehydes to
acids was
reproduced from the original report. In- and ex-situ analysis of the reaction mixture as the function
of time reveals that, unlike the claim in the publication, the aerobic
oxidation of aromatic and aliphatic aldehydes leads predominantly
to the formation of peracids. The latter are transformed into the
corresponding carboxylic acids during the workup procedure. The buildup
of peracids in solution poses safety problems that should not be overlooked.
This finding has also an influence on the way new catalysts are investigated
to improve this reaction as well as on aerobic aldehyde-mediated co-oxidation.
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