The Interplay between Kinetics and Thermodynamics in Furan Diels–Alder Chemistry for Sustainable Chemicals Production

Abstract

Biomass‐derived furanic platform molecules have emerged as promising building blocks for renewable chemicals and functional materials. To this aim, the Diels–Alder (DA) cycloaddition stands out as a versatile strategy to convert these renewable resources in highly atom‐efficient ways. Despite nearly a century worth of examples of furan DA chemistry, clear structure–reactivity–stability relationships are still to be established. Detailed understanding of the intricate interplay between kinetics and thermodynamics in these very particular [4+2] cycloadditions is essential to push further development and truly expand the scope beyond the ubiquitous addend combinations of electron‐rich furans and electron‐deficient olefins. Herein, we provide pertinent examples of DA chemistry, taken from various fields, to highlight trends, establish correlations and answer open questions in the field with the aim to support future efforts in the sustainable chemicals and materials production.

Amorphous Silica‐Alumina as Suitable Catalyst for the Diels‐Alder Cycloaddition of 2,5‐Dimethylfuran and Ethylene to Biobased p‐Xylene

“…In general, DAC reaction rates can be enhanced by application of acid catalysts [9,10,22] . For the production of p ‐xylene, solid Brønsted acids are typically regarded the most active, [21–28] whereas Lewis acids provide a higher selectivity towards aromatics [13,30,31] .…”

“…[21] In general, DAC reaction rates can be enhanced by application of acid catalysts. [9,10,22] For the production of pxylene, solid Brønsted acids are typically regarded the most active, [21][22][23][24][25][26][27][28] whereas Lewis acids provide a higher selectivity towards aromatics. [13,30,31] Density functional theory calculations demonstrated that the initial DAC step can proceed uncatalyzed, while the dehydration of the oxanorbornene intermediate is effectively catalyzed by Brønsted acids.…”

Targeting Valuable Chemical Commodities: Hydrazine‐mediated Diels‐Alder Aromatization of Biobased Furfurals

“…However, it is important to note that modulating the electronic properties of the furan 2‐ (and 5‐) substituent(s) also strongly impacts the subsequent dehydration step. In this regard, formyl derivatization with a hydrazine auxiliary stands out as highly beneficial, as the hydrazone group formed at the adduct bridgehead position substantially reduces the barrier of ether bridge fission and dehydration; aromatization typically occurs spontaneously (even at temperatures as low as 25 °C, for example with maleic anhydride as dienophile, and generally below 100 °C, Scheme 1B); this in sharp contrast to most other furan DA adducts [11] …”

“…While formyl furans such as furfural and 5‐HMF are the most readily accessible bio‐based furans, they are notoriously poor dienes in cycloaddition chemistry [11,13] . Diels‐Alder reactivity is enabled by chemical derivatization of the aldehyde group, with reduction reactions typically being pursued; redox‐neutral activation approaches such as acetalization, [31–33] hydration, [34] or hydrazone formation [35–41] are relatively more difficult to implement in the context of renewable aromatics production, and examples thereof generally feature highly specific targets and narrow scope.…”

Bond breaking of furan-maleimide adducts via a diradical sequential mechanism under an external mechanical force

“…We further show that the tension inhibits the bond rupture up to pulling forces of ≈ 3 nN before inducing a preferential activation of the Pendo stereoisomer in agreement with the experimental results of Wang et al 12 On the ground electronic state at room temperature, thermally activated [4+2] retro Diels-Alder (rDA) reactions proceed most of the time via a concerted and stereospecific pathway involving a single concerted transition state at room temperature. [35][36] Activation energies of 25-30 kcal/mol for the concerted rupture of the two scissile bonds at room temperature confer them a dynamic character. Endo adducts are more reactive than exo ones because of their lower activation barriers.…”