Vinylogous hydrazone strategy for the organocatalytic alkylation of heteroaromatic derivatives

N,N-Dimethylhydrazine as a Reversible Derivatization Agent to Promote the Hydroxymethylation of Furfural with Formaldehyde

In this report, the synthesis of 5-hydroxymethylfurfural from concentrated feeds of two low-cost and industrially abundant chemicals: Furfural and formaldehyde is explored. By adjusting the acidity of the solvent, an alternative mechanism is discovered in which the reaction selectivity stops to the hydroxymethylation step, in contrast to previously reported acid-catalyzed pathways leading to the formation of the bisfuranic dimer as a major product. One of the keys of this study relies on the reversible derivation of the -CHO group of furfural with N,N-Dimethylhydrazine which plays a dual role: (1) it restores the nucleophilicity of the furan ring and (2) it reacts with HCHO to form in situ an electrophilic zwiterrionic species stabilized through hydrogen transfer. By means of experimental and theoretical investigations, this reaction is optimized and it is discovered that guaiacol can be used as a bio-based and safe solvent. Under optimized conditions, the hydroxymethylation of the furan ring of furfural occurs with more than 95% selectivity, at only 50 °C and with a stoichiometric amount of HCHO. A concentrated feed of furfural as high as 40 wt% in guaiacol can be employed without impacting the reaction selectivity, leading to an improvement of the reactor productivity to about 25 kg m-3 h-1. The recovery of the reaction products and the recycling of the N,N-dimehylhydrazone are also discussed.

Acid-Catalyzed Activation and Condensation of the =C5H Bond of Furfural on Aldehydes, an Entry Point to Biobased Monomers

Furfural is an industrially relevant biobased chemical platform. Unlike classical furan, or C-alkylated furans, which have been previously described in the current literature, the =C5H bond of furfural is unreactive. As a result, on a large scale, C=C and C=O bond hydrogenation/hydrogenolysis is mainly performed, with furfuryl alcohol and methyl tetrahydrofuran being the two main downstream chemicals. Here, we show that the derivatization of the -CHO group of furfural restores the reactivity of its =C5H bond, thus permitting its double condensation on various alkyl aldehydes. Overcoming the recalcitrance of the =C5H bond of furfural has opened an access to a biobased monomer, whose potential have been investigated in the fabrication of renewably-sourced poly(silylether). By means of a combined theoretical-experimental study, a reactivity scale for furfural and its protected derivatives against carbonylated compounds has been established using an electrophilicity descriptor, a means to predict the molecular diversity and complexity this pathway may support, and also to de-risk any project related to this topic. Finally, by using performance criteria for industrial operations in the field of fuels and commodities, we discussed the industrial potential of this work in terms of cost, E-factor, reactor productivity and catalyst consumption.

Hydrazone Activation in the Aminocatalytic Cascade Reaction for the Synthesis of Tetrahydroindolizines

In this Letter, we demonstrate the usefulness of hydrazone activation for the synthesis of biologically relevant tetrahydroindolizines. A pyrrol-derived hydrazone bearing a Michael acceptor moiety in the N-alkyl side chain has been designed with the aim of participating in the aminocatalytic cascade reaction leading to the annulation of the new six-membered heterocyclic scaffold. The application of (S)-(-)-α,α-diphenyl-2-pyrrolidinemethanol trimethylsilyl ether as the aminocatalyst allows for the iminium ion-enamine-mediated cascade to proceed in a fully stereoselective manner.