In transition-metal catalysed reductive carboxylation reactions utilizing CO2, stoichiometric quantities of metallic reductants (eg. Zn, Mn, and many others.) or natural sacrificial electron-donors (eg. iPr2NEt, BI(OH)H, and many others.) had been often required to regenerate energetic low-valent metals or steel hydrides from steel carboxylates. Nevertheless, these CO2 carboxylation methodologies accompanied era of a major quantity of chemical wastes, which is past the unique intention of inexperienced chemistry. Clearly, the most effective substitute of these reagents is to make use of the cleanest reductant, H2. It ought to be famous that Leitner’s group described a proper hydrocarboxylation of olefins below excessive temperature and excessive strain of CO2/H2 in 2013 (Angew. Chem. Int. Ed. 2013, 52, 12119.), however this response really utilized CO generated by water-gas shift response.
After the long-term research of creating novel discount strategies for carboxylations, comparable to R3Al, formate salts, photoredox-amine, and many others., our group had reached the stage to consider some actually difficult transformations, that’s, “hydrocarboxylation of olefins with CO2 and H2”, a 100% atom-economical response (Fig. 1).
Fig. 1. Hydrocarboxylation of olefinic compounds utilizing CO2 and H2
In fact, the most important problem was find out how to suppress the undesired hydrogenation of olefins, which was often thought of as a fast and straightforward transformation within the presence of steel catalysts. As well as, CO2 is just not very reactive. However it’s completely true that with out trials, nobody can succeed. The truth is, after we initiated our examine based mostly on our earlier analysis on Rh-catalysed hydrocarboxylation reactions (Chem. Commun. 2017, 53, 3098.; Entrance. Chem. 2019, 7, 371.), we certainly obtained a considerable amount of hydrogenation product. We struggled for a very long time with this undesirable aspect response, which nearly introduced us to a useless finish.
The scenario began to alter after we discovered that one of many Buchwald-type ligands, DavePhos, behaved fairly uncommon as in comparison with different ligands. The mix of Rh-catalyst and DavePhos confirmed excessive reactivity for hydrocarboxylation, whereas the hydrogenation was in some way suppressed. After intensive optimization of the circumstances utilizing Davephos, we have now succeeded in creating a suitable system to afford the specified hydrocarboxylation merchandise in good yields. The response was established, however two new questions remained unclarified: (a) Why is DavePhos good? (b) Why is an extra equal of DavePhos needed?
These two mysteries haunted us for nearly two years, till we lastly outlined the construction of a phosphorus-containing species within the response combination to be a phosphonium salt derived from DavePhos. This construction steered the operate of DavePhos on this response system to be not solely as a ligand, but in addition as an electron-donor for the preliminary era of Rh–H species from the cationic Rh precursor (Fig. 2). That is essentially the most rational clarification of the 2 mysteries up to now. Fig. 2. Era of Rh–H and commentary of DavePhos phosphonium salt
On this examine, we have now developed a photocatalytic hydrocarboxylation of styrene derivatives with CO2 and H2, which is a 100% atomic economical response (Fig. 3). We hope that our findings will lengthen the toolbox of extra environment friendly and sustainable reactions for the utilization of CO2. Please discover extra particulars of the unique paper of our work in Nature Communications: “Catalytic direct hydrocarboxylation of styrenes with CO2 and H2” (doi: 10.1038/s41467-022-35293-3).
Fig. 3. Catalytic direct hydrocarboxylation of styrenes with CO2 and H2