The concurrent formation of furan-2,5- and furan-2,4-dicarboxylic acid under solvent free conditions via a disproportionation reaction is described. By reacting potassium-2-furoate at 260 °C in the presence of 22 mol% of (Lewis acidic) catalysts like CdI₂ or ZnCl₂, potassium-2-furoate is disproportionated to furan and furandicarboxylic acids. Besides furan and furan-2,5-dicarboxylic acid (2,5-FDCA) as the main products, furan-2,4-dicarboxylic acid (2,4-FDCA) is also formed as a by-product. Experimental evidence has been obtained that, under the reaction conditions applied, 2,5-FDCA and 2,4-FDCA are formed by separate reaction pathways. Selectivity towards the different FDCA isomers is affected by the type of catalyst used. Single-crystal X-ray analysis shows that 2,4-FDCA has a more ‘linear’ character compared to 2,5-FDCA and hence is structurally more comparable to terephthalic acid (TA), making it an interesting monomer for synthetic polyesters.

中文翻译：

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呋喃-2,5-和呋喃-2,4-二羧酸的同时形成：汉高反应的出乎意料的方面†

同时形成呋喃2,5-和 呋喃-2,4-二羧酸描述了在无溶剂条件下通过歧化反应。通过反应2-糠酸钾在260°C下，在22 mol％的（路易斯酸性）催化剂（如CdI ₂或ZnCl ₂）的存在下，2-糠酸钾 不成比例地 呋喃和呋喃二甲酸。除了呋喃 和 呋喃-2,5-二羧酸 （2,5-FDCA）为主要产品， 呋喃-2,4-二羧酸（2,4-FDCA）也形成为副产物。实验证据表明，在所应用的反应条件下，2，5-FDCA和2，4-FDCA是通过分开的反应途径形成的。对不同FDCA异构体的选择性受所用催化剂类型的影响。单晶X射线分析表明，与2,5-FDCA相比，2,4-FDCA具有更“线性”的特征，因此在结构上与对苯二甲酸 （TA），使其成为合成聚酯的有趣单体。