Effective Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran by an Acetal Protection Strategy.

An acetal protection strategy for 5-hydroxymethylfurfural (HMF) was used to obtain 2,5-diformyfuran (DFF) using concentrated HMF solutions and a γ-Al2 O3 -supported Ru catalyst (Ru/γ-Al2 O3 ). The HMF-acetal with 1,3-propanediol can be oxidized to DFF-acetal with a yield of 84.0 % at an HMF conversion of 94.2 % from a 50 wt % solution. In contrast, aerobic oxidation of nonprotected HMF using a 10 wt % solution afforded DFF only in a moderate yield (52.3 %). Kinetic studies indicated that the six-membered ring acetal group not only prevents side reactions but also accelerates aerobic oxidation of the -CH2 OH moiety to -CHO under retention of the acetal functionality. Organic deposits formed during the reaction explained the significant decrease in the activity of the Ru/γ-Al2 O3 catalyst, which could be recovered neither by washing in water or organic solvents, nor by a calcination-reduction treatment. Sonication of the used Ru/γ-Al2 O3 catalyst in an aqueous NaOH solution successfully removed the deposits and allowed reuse of the catalyst for at least four times without activity loss.

Dehydration of Glucose to 5-Hydroxymethylfurfural Using Nb-doped Tungstite.

Dehydration of glucose to 5-hydroxymethylfurfural (HMF) remains a significant problem in the context of the valorization of lignocellulosic biomass. Hydrolysis of WCl6 and NbCl5 leads to precipitation of Nb-containing tungstite (WO3 ⋅H2 O) at low Nb content and mixtures of tungstite and niobic acid at higher Nb content. Tungstite is a promising catalyst for the dehydration of glucose to HMF. Compared with Nb2 O5 , fewer by-products are formed because of the low Brønsted acidity of the (mixed) oxides. In water, an optimum yield of HMF was obtained for Nb-W oxides with low Nb content owing to balanced Lewis and Brønsted acidity. In THF/water, the strong Lewis acidity and weak Brønsted acidity caused the reaction to proceed through isomerization to fructose and dehydration of fructose to a partially dehydrated intermediate, which was identified by LC-ESI-MS. The addition of HCl to the reaction mixture resulted in rapid dehydration of this intermediate to HMF. The HMF yield obtained in this way was approximately 56 % for all tungstite catalysts. Density functional theory calculations show that the Lewis acid centers on the tungstite surface can isomerize glucose into fructose. Substitution of W by Nb lowers the overall activation barrier for glucose isomerization by stabilizing the deprotonated glucose adsorbate.

Efficient routes to A3B-type meso-(4-carboxyphenyl) porphyrin derivatives.

A3B-type meso-(4-carboxyphenyl) porphyrins were prepared either by stepwise coupling of aniline substituents to meso-tetrakis(4-carboxyphenyl) porphyrin (TCPP) or by utilizing its partially protected trimethyl ester derivative. We demonstrate the high utility of this building block, which can be synthesized in very good yields by microwave-assisted Me3SnOH hydrolysis.