초록
<P>C-H carboxylation of furoic acid (FA) with CO<SUB>2</SUB> is an atom-efficient strategy to produce 2,5-furandicarboxylic acid (2,5-FDCA) from lignocellulose. The existing carbonate-promoted CO<SUB>2</SUB> carboxylation processes rely on the use of large amounts of expensive Cs<SUB>2</SUB>CO<SUB>3</SUB> as a deprotonating reagent and molten salt. Substitution of Cs with other cheap and abundant alkali ions (such as K and Na) can reduce the use of Cs, but it faces the problem of a low yield of 2,5-FDCA. This study found that the addition of catalytic amounts of ZnCl<SUB>2</SUB> as a Lewis acid can increase the yield of 2,5-FDCA in the CO<SUB>2</SUB> carboxylation reaction of Na/K-FA in a molten salt reaction system. <SUP>1</SUP>H NMR analysis and DFT calculations confirmed that ZnCl<SUB>2</SUB> coordinates with the furan ring through electron transfer from the conjugated furan ring to Zn<SUP>2+</SUP>, thereby activating the H at the C<SUB>5</SUB> position of Na/K-FA. This coordination lengthened the C<SUB>5</SUB>-H bond and lowered its heterolytic dissociation energy, making it more susceptible to being deprotonated by CO<SUB>3</SUB><SUP>2−</SUP> and subsequently carboxylated by CO<SUB>2</SUB>. The developed Lewis acid coordination strategy provides a new idea for the efficient construction of C-C bonds between CO<SUB>2</SUB> and aromatics through carbonate-promoted C-H carboxylation.</P><P>Graphic Abstract</P><P>A Lewis acid coordination strategy was proposed to promote C-H carboxylation of furoic acid (FA) with CO<SUB>2</SUB> to produce 2,5-furandicarboxylic acid (2,5-FDCA).<BR/><IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=d4dt01196a'/><BR/></P>