Carbon dioxide utilization via carbonate-promoted C-H carboxylation

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From: Nature(Vol. 531, Issue 7593)
Publisher: Nature Publishing Group
Document Type: Article
Length: 7,507 words
Lexile Measure: 1440L

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Using carbon dioxide (C[O.sub.2]) as a feedstock for commodity synthesis is an attractive means of reducing greenhouse gas emissions and a possible stepping-stone towards renewable synthetic fuels (1,2). A major impediment to synthesizing compounds from C[O.sub.2] is the difficulty of forming carbon-carbon (C-C) bonds efficiently: although C[O.sub.2] reacts readily with carbon-centred nucleophiles, generating these intermediates requires high-energy reagents (such as highly reducing metals or strong organic bases), carbon-heteroatom bonds or relatively acidic carbon-hydrogen (C-H) bonds (3-5). These requirements negate the environmental benefit of using C[O.sub.2] as a substrate and limit the chemistry to low-volume targets. Here we show that intermediate-temperature (200 to 350 degrees Celsius) molten salts containing caesium or potassium cations enable carbonate ions (C[O.sup.3.sub.2-]) to deprotonate very weakly acidic C-H bonds (p[K.sub.a] > 40), generating carbon-centred nucleophiles that react with C[O.sub.2] to form carboxylates. To illustrate a potential application, we use C-H carboxylation followed by protonation to convert 2-furoic acid into furan-2,5-dicarboxylic acid (FDCA)--a highly desirable bio-based feedstock (6) with numerous applications, including the synthesis of polyethylene furandicarboxylate (PEF), which is a potential large-scale substitute for petroleum-derived polyethylene terephthalate (PET) (7,8). Since 2-furoic acid can readily be made from lignocellulose (9), C[O.sub.3.sup.2-]-promoted C-H carboxylation thus reveals a way to transform inedible biomass and C[O.sub.2] into a valuable feedstock chemical. Our results provide a new strategy for using C[O.sub.2] in the synthesis of multi-carbon compounds.

The chemistry described here is inspired by ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which effects C-C bond formation in the Calvin cycle by deprotonating a C-H bond of ribulose-1,5-bisphosphate and exposing the resulting carbon-centred nucleophile to C[O.sub.2] to form a carboxylate (C-C[O.sub.3.sup.2-]) (10). Emulating this strategy synthetically requires deprotonating un-activated C-H bonds using a simple base that does not have a large C[O.sub.2] footprint. To meet these requirements, we envisioned a C[O.sub.3.sup.2-]-promoted C-H carboxylation reaction, wherein C[O.sub.3.sup.2-] reversibly deprotonates a C-H bond to generate HC[O.sub.3.sup.-] and a carbon-centred nucleophile that reacts with C[O.sub.2] to form C-C[O.sub.2.sup.-] (Fig. 1a). HC[O.sub.3.sup.-] decomposition results in a net consumption of one-half equivalents of C[O.sub.3.sup.2-] and C[O.sub.2] per C-C[O.sub.2.sup.-] produced. The cycle could be closed by protonating C-C[O.sub.2.sup.-] with strong acid and using electrodialysis to regenerate the acid and base (11,12), effecting a net transformation of C-H and C[O.sub.2] into C-C[O.sub.2]H without using any other stoichiometric reagents. Alternatively, C[O.sub.2]-promoted esterification could be used to convert the carboxylate into an ester (C-C[O.sub.2]R) and regenerate C[O.sub.3.sup.2-] directly (13). Previously, researchers have shown that [Cs.sub.2]C[O.sub.3] can deprotonate alkynyl (14), allylic (15), and activated heteroaryl C-H bonds with p[K.sub.a] values of up to 27 (ref. 16) in organic solvents at elevated temperature (where p[K.sub.a] is the negative base-10 logarithm of the acid dissociation constant). However, using C[O.sub.3.sup.2-]-promoted C-H carboxylation for commodity synthesis requires deprotonating C-H bonds that are considerably less acidic.

Carbonate-promoted C-H carboxylation is particularly desirable for the synthesis of polymer units from biomass. A longstanding goal of renewable polymer chemistry is a scalable synthesis of FDCA from lignocellulose to replace petroleum-derived terephthalic acid for polyester synthesis (Fig. 1b) (6). In particular, PEF has been...

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Gale Document Number: GALE|A445983394