Oxidation of Vicinal Diols to α‐Hydroxy Ketones with H₂O₂ and a Simple Manganese Catalyst

Abstract

α‐Hydroxy ketones are valuable synthons in organic chemistry. Here we show that oxidation of vic‐diols to α‐hydroxy ketones with H2O2 can be achieved with an in situ prepared catalyst based on manganese salts and pyridine‐2‐carboxylic acid. Furthermore the same catalyst is effective in alkene epoxidation, and it is shown that alkene oxidation with the MnII catalyst and H2O2 followed by Lewis acid ring opening of the epoxide and subsequent oxidation of the alkene to α‐hydroxy ketones can be achieved under mild (ambient) conditions.

Origins of Catalyst Inhibition in the Manganese‐Catalysed Oxidation of Lignin Model Compounds with H2O2

“…[34] With the in situ-prepared manganese catalyst( Scheme 1), 1-phenyl-1,2ethanediol underwent 60 %c onversion to the corresponding a-hydroxyketone, as reported previously. [39] Surprisingly,o nly minor (< 10 %) conversion of 1 was achieved under these conditions ( Figure S1 in the Supporting Information). The low conversion was not owing to loss of H 2 O 2 because it remained in solution unreacted.…”

“…The oxidation of 1‐phenyl‐1,2‐ethanediol and 1 with H 2 O 2 was studied here by using conditions optimized earlier for the oxidation of secondary aliphatic and benzylic alcohols . With the in situ‐prepared manganese catalyst (Scheme ), 1‐phenyl‐1,2‐ethanediol underwent 60 % conversion to the corresponding α‐hydroxyketone, as reported previously . Surprisingly, only minor (<10 %) conversion of 1 was achieved under these conditions (Figure S1 in the Supporting Information).…”

“…Recently, we reported a manganese(II) catalyst prepared in situ with pyridine‐2‐carboxylic acid (PCA) and sub‐stoichiometric ketones for the oxidation with H 2 O 2 of a broad range of organic compounds such as alkanes, olefins, aliphatic and benzylic alcohols under ambient conditions with high turnover numbers (up to 300 000 for the epoxidation of electron‐rich alkenes) and low catalyst loadings (Scheme ) . The simplicity of the catalyst in preparation and, importantly, its wide solvent scope make it a potential candidate for large‐scale application.…”

“…The simplicity of the catalyst in preparation and, importantly, its wide solvent scope make it a potential candidate for large‐scale application. In particular, the good conversion shown in the oxidation of secondary benzylic alcohols, such as p ‐X‐1‐phenylethanol (X=H, Br, OCH 3 ) and aromatic vicinal diols to ketones, even if the benzylic alcohol positions were protected as ethers, encouraged us to consider the potential of this Mn system in catalytic lignin degradation, in particular for attack at the β‐O‐4 linkage …”

“…Recently,w er eported am anganese(II) catalyst prepared in situ with pyridine-2-carboxylic acid (PCA) and sub-stoichiometric ketonesf or the oxidation with H 2 O 2 of ab road range of organic compounds such as alkanes, olefins, aliphatic and benzylic alcohols under ambient conditions with high turnover numbers( up to 300 000 for the epoxidation of electron-rich alkenes) andlow catalystloadings (Scheme 1). [33][34][35][36][37][38][39] The simplicity of the catalysti np reparation and, importantly,i ts wide solvent scope make it ap otential candidate for large-scale application. In particular,t he good conversion shown in the oxidationo f secondary benzylic alcohols, such as p-X-1-phenylethanol (X = H, Br,O CH 3 )a nd aromatic vicinal diols to ketones, even if the benzylic alcohol positions were protected as ethers, encouraged us to consider the potential of this Mn system in catalytic lignin degradation, in particular for attack at the b-O-4 linkage.…”