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Silver(I) as a widely applicable, homogeneous catalyst for aerobic oxidation of aldehydes toward carboxylic acids in water-"silver mirror": From stoichiometric to catalytic.

Liu M, Wang H, Zeng H, Li CJ - Sci Adv (2015)

Bottom Line: The first example of a homogeneous silver(I)-catalyzed aerobic oxidation of aldehydes in water is reported.More than 50 examples of different aliphatic and aromatic aldehydes, including natural products, were tested, and all of them successfully underwent aerobic oxidation to give the corresponding carboxylic acids in extremely high yields.Chromatography is completely unnecessary for purification in most cases.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and FRQNT Centre in Green Chemistry and Catalysis, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 0B8, Canada.

ABSTRACT
The first example of a homogeneous silver(I)-catalyzed aerobic oxidation of aldehydes in water is reported. More than 50 examples of different aliphatic and aromatic aldehydes, including natural products, were tested, and all of them successfully underwent aerobic oxidation to give the corresponding carboxylic acids in extremely high yields. The reaction conditions are very mild and greener, requiring only a very low silver(I) catalyst loading, using atmospheric oxygen as the oxidant and water as the solvent, and allowing gram-scale oxidation with only 2 mg of our catalyst. Chromatography is completely unnecessary for purification in most cases.

No MeSH data available.


Related in: MedlinePlus

Reaction mechanism.(A) Plausible mechanism and (B) ZPE-corrected energies from B3LYP/6-31G(d) and LANL2DZ for Ag. All values are given in kcal/mol and referred to the system AgOH(PMe3) + PhCHO. Gibbs free energy is considered for drawing.
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Figure 4: Reaction mechanism.(A) Plausible mechanism and (B) ZPE-corrected energies from B3LYP/6-31G(d) and LANL2DZ for Ag. All values are given in kcal/mol and referred to the system AgOH(PMe3) + PhCHO. Gibbs free energy is considered for drawing.

Mentions: On the basis of the results of our study, a plausible reaction mechanism that involves two catalytic cycles is proposed in Fig. 3: one of the cycles is responsible for extracting the hydride from the aldehyde, whereas the other is responsible for activating the dioxygen molecule in water. Each cycle consumes one molecule of aldehyde and generates one molecule of carboxylic acid. The two tandem cycles operate cooperatively to give the very efficient oxidation observed. It has previously been reported that the combination of silver(I) oxide and NHC ligand gives an NHC-Ag(I)-Cl complex (40). After the chloride complex is introduced to our aqueous NaOH solution, the Cl− of the complex is substituted with hydroxyl to give the suggested NHC-Ag(I)-OH catalyst species. The catalyst then coordinates to the C=O double bond of the aldehyde and exchanges its coordinated –OH with the –H of the aldehyde, possibly through either a nucleophilic attack of –OH followed by β-hydride elimination (Fig. 4A) or a four-membered ring transition state where the exchange of –OH and –H occurred simultaneously (Fig. 4B). The catalyst then releases the carboxylic acid as the product and a silver(I)-hydride species, whose presence has been suggested by many of our recent studies (41–43) and has also been directly detected recently (44). We also conducted a brief computational study for the proposed mechanism (detailed in the Supplementary Materials). To reduce the complexity of calculation, we used a simplified molecule model that simplifies the complicated NHC ligand into a simple trimethylphosphine ligand because of their electronic similarity. The result has shown that it is possible for the existence of a silver(I)-hydride intermediate when the silver center is coordinated to a strong electron-donating ligand (Fig. 4B). The computational result shows that a four-membered ring transition state is favored; however, considering the simplification of the calculation, there is still no solid proof of such a pathway. We tentatively propose that the silver(I)-hydride is responsible for the activation of oxygen in water, generating a silver(I)-hydroperoxyl intermediate. The hydroperoxide then nucleophilically attacks another carbonyl of the aldehyde. The hydrogen of the aldehyde is then extracted by silver with a similar β-hydride elimination (44). Then, the silver(I)-hydride is oxidized by the coordinating peroxyl acid to give the corresponding carboxylate and to release a water molecule. The remaining carboxylate is then substituted by a hydroxide to release the carboxylate and regenerate the silver(I)-hydroxide catalyst.


Silver(I) as a widely applicable, homogeneous catalyst for aerobic oxidation of aldehydes toward carboxylic acids in water-"silver mirror": From stoichiometric to catalytic.

Liu M, Wang H, Zeng H, Li CJ - Sci Adv (2015)

Reaction mechanism.(A) Plausible mechanism and (B) ZPE-corrected energies from B3LYP/6-31G(d) and LANL2DZ for Ag. All values are given in kcal/mol and referred to the system AgOH(PMe3) + PhCHO. Gibbs free energy is considered for drawing.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4643818&req=5

Figure 4: Reaction mechanism.(A) Plausible mechanism and (B) ZPE-corrected energies from B3LYP/6-31G(d) and LANL2DZ for Ag. All values are given in kcal/mol and referred to the system AgOH(PMe3) + PhCHO. Gibbs free energy is considered for drawing.
Mentions: On the basis of the results of our study, a plausible reaction mechanism that involves two catalytic cycles is proposed in Fig. 3: one of the cycles is responsible for extracting the hydride from the aldehyde, whereas the other is responsible for activating the dioxygen molecule in water. Each cycle consumes one molecule of aldehyde and generates one molecule of carboxylic acid. The two tandem cycles operate cooperatively to give the very efficient oxidation observed. It has previously been reported that the combination of silver(I) oxide and NHC ligand gives an NHC-Ag(I)-Cl complex (40). After the chloride complex is introduced to our aqueous NaOH solution, the Cl− of the complex is substituted with hydroxyl to give the suggested NHC-Ag(I)-OH catalyst species. The catalyst then coordinates to the C=O double bond of the aldehyde and exchanges its coordinated –OH with the –H of the aldehyde, possibly through either a nucleophilic attack of –OH followed by β-hydride elimination (Fig. 4A) or a four-membered ring transition state where the exchange of –OH and –H occurred simultaneously (Fig. 4B). The catalyst then releases the carboxylic acid as the product and a silver(I)-hydride species, whose presence has been suggested by many of our recent studies (41–43) and has also been directly detected recently (44). We also conducted a brief computational study for the proposed mechanism (detailed in the Supplementary Materials). To reduce the complexity of calculation, we used a simplified molecule model that simplifies the complicated NHC ligand into a simple trimethylphosphine ligand because of their electronic similarity. The result has shown that it is possible for the existence of a silver(I)-hydride intermediate when the silver center is coordinated to a strong electron-donating ligand (Fig. 4B). The computational result shows that a four-membered ring transition state is favored; however, considering the simplification of the calculation, there is still no solid proof of such a pathway. We tentatively propose that the silver(I)-hydride is responsible for the activation of oxygen in water, generating a silver(I)-hydroperoxyl intermediate. The hydroperoxide then nucleophilically attacks another carbonyl of the aldehyde. The hydrogen of the aldehyde is then extracted by silver with a similar β-hydride elimination (44). Then, the silver(I)-hydride is oxidized by the coordinating peroxyl acid to give the corresponding carboxylate and to release a water molecule. The remaining carboxylate is then substituted by a hydroxide to release the carboxylate and regenerate the silver(I)-hydroxide catalyst.

Bottom Line: The first example of a homogeneous silver(I)-catalyzed aerobic oxidation of aldehydes in water is reported.More than 50 examples of different aliphatic and aromatic aldehydes, including natural products, were tested, and all of them successfully underwent aerobic oxidation to give the corresponding carboxylic acids in extremely high yields.Chromatography is completely unnecessary for purification in most cases.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and FRQNT Centre in Green Chemistry and Catalysis, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 0B8, Canada.

ABSTRACT
The first example of a homogeneous silver(I)-catalyzed aerobic oxidation of aldehydes in water is reported. More than 50 examples of different aliphatic and aromatic aldehydes, including natural products, were tested, and all of them successfully underwent aerobic oxidation to give the corresponding carboxylic acids in extremely high yields. The reaction conditions are very mild and greener, requiring only a very low silver(I) catalyst loading, using atmospheric oxygen as the oxidant and water as the solvent, and allowing gram-scale oxidation with only 2 mg of our catalyst. Chromatography is completely unnecessary for purification in most cases.

No MeSH data available.


Related in: MedlinePlus