Abstract
Alcohol oxidation to carbonyl compounds is one of the most commonly used reactions in synthetic chemistry. Herein, we report the use of base metal layered double hydroxide (LDH) catalysts for the oxidation of benzylic alcohols in polar solvents. These catalysts are ideal reagents for alcohol oxidations due to their ease of synthesis, tunability, and ease of separation from the reaction medium. LDHs synthesized in this study were fully characterized by means of X-ray diffraction, NH3-temperature programmed desorption (TPD), pulsed CO2 chemisorption, N2 physisorption, electron microscopy, and elemental analysis. LDHs were found to effectively oxidize benzylic alcohols to their corresponding carbonyl compounds in diphenyl ether, using O2 as the terminal oxidant. LDH catalysts were also applied to the oxidation of lignin β-O-4 model compounds. Typically, for all catalysts, only trace amounts of the ketone formed from benzylic alcohol oxidation were observed, the main products comprising benzoic acids and phenols arising from β-aryl ether cleavage. This observation is consistent with the higher reactivity of the ketones, resulting from weakening of the Cβ–O4 bond that was shown to be aerobically cleaved at 180 °C in the absence of a catalyst.
Highlights
The oxidation of alcohols to carbonyl compounds is one of the most widely used reactions in chemistry [1,2]
layered double hydroxide (LDH) are expected to form under basic conditions as long as the metal cations (M2+ and have an ionic radius similar to Mg2+ and the trivalent metal ratio (χ) is between 0.2 and 0.4 as noted above [26]
In this study all catalysts were synthesized with the theoretical χ value within the aforementioned limits
Summary
The oxidation of alcohols to carbonyl compounds is one of the most widely used reactions in chemistry [1,2]. There are a variety of common reagents that are well-known for their ability to perform such transformations at the laboratory scale. The use of stoichiometric reagents for alcohol oxidation typically requires toxic and/or environmentally harmful Cr(VI) [3,4,5,6] species and permanganate salts [7], or unpleasant activated DMSO [8,9,10,11,12,13,14,15]. Catalytic systems exist that are reasonably effective for this transformation. In many cases, these systems utilize commercially available homogeneous catalysts such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) [16] or (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) [17,18,19], which are difficult to separate from products, or expensive noble metal catalysts (such as Pd, Pt, Rh, or Ru) [20]. We describe alcohol oxidations using prepared and inexpensive base metal layered double hydroxide (LDH)
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