To understand the lower activity and selectivity for the carbonylation of AA compared to PL, further analyses were run to check for side-products. After carbonylation runs, propionic acid could be detected in the liquid phase, but in too small amounts (<5%) to make up for the low mass balance recovered when taking only AA and SA into account (see S.I., section 4.1). After carbonylations in toluene, the formation of a purple slurry was also observed, which, after washing and drying under vacuum, rendered a lilac solid, Different levers were tested to achieve higher yields and selectivity for this carbonylation. Applying a higher temperature was counterproductive, and reduced the catalytic activity of the carbonylation (see S.I., section 2.1). In our previous study, the addition of H2 in the gas phase favoured the carbonylation of AA to SA, so catalytic experiments were also run with syngas in the top solvents (Table ). Gratifyingly, under a pressure of 50 bar of CO/H2 (90:10), after 6 h of heating at 90 °C, an excellent yield of SA was obtained in MeCN (91%, Table , entry 1), and even a quantitative yield was measured in acetone (Table , entry 2). In acetone, lowering the total pressure from 50 to 15 bar reduced the selectivity to 77% (Table , entry 3). Lowering the CO partial pressure from 45 to 20 bar, while maintaining the H2 partial pressure constant at 5 bar, was also detrimental to the selectivity toward SA (Table , entry 4). A minimal CO pressure is thus needed, probably either for the carbonylation step to be favoured, or to stabilize the active species. The positive influence of the presence of H2 may be explained by the formation and stabilisation of cobalt hydride intermediates, which can be proposed as active species. 46 i [Co-AA] was isolated from a raw mixture in toluene, while the reaction between AA and [Co2(CO)8] under argon was conducted in hexane. Both solvent are non coordinating solvents. It may be expected that the formation of [Co-AA] would be slowed down (or even hindered), in presence of a coordinating solvent (or a stabilizing ligand, like a diphosphine in our previous study (cf. Nicolas & Cantat, ChemCatChem 2023, 15, e202300720). The best catalytic system for the carbonylation of AA (CO:H2 mixture, Acetone as a solvent) was applied to two related, yet more sterically hindered, compounds: crotonic acid (CA) and methacrylic acid (MA) (Table ). Two products may be expected after the carbonylation of CA: methylsuccinic anhydride (MeSA), and the 6-membered cyclic anhydride, glutaric anhydride (GA). Moderate yield and conversion were obtained in acetone at 90 °C under CO/H2 pressure: 26% of cyclic anhydrides after 6 h, and 47% after 16 h (Table , entries 4 and 5). Increasing the temperature to 110 °C enhanced the activity, with a yield in cyclic anhydrides of 72% after 6 h, while the selectivity is maintained (80%, Table , entry 6, average of two runs). GA is the major product in these conditions (80% of cyclic anhydrides), indicating that in our case isomerization is faster than carbonylation, consistent with previously reported results. Increasing the proportion of H2 in the gas phase from 10 vol% to 20 vol% was detrimental to the selectivity of the carbonylation (Table , entry 8). The carbonylation of MA to MeSA gave poorer results: only 7% of MeSA was detected after 6 h at 90 °C (Table , entry 10). Trying the best set of conditions obtained so far (110 °C, 50 bar CO:H2) we could only enhance the amount of MeSA to 24% (Table , entry 11). Under all conditions presented here, a substantial amount of MA is hydrogenated to butyric acid. The competition between hydrogenation and carbonylation partly explains the poor activity of this substrate.