# Simplified carbonylation catalyst based on [Co_2(CO)_8] and coordinating solvents for the carbonylation of β-lactones and α,β-unsaturated acids to cyclic anhydrides

Marie-Hélène Pietraru, Antonin Homassel, Thibault Cantat, Emmanuel Nicolas

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Marie-Hélène Pietraru, Antonin Homassel, Thibault Cantat, Emmanuel Nicolas. Simplified carbonylation catalyst based on [Co_2(CO)_8] and coordinating solvents for the carbonylation of β-lactones and α,β-unsaturated acids to cyclic anhydrides. ACS Sustainable Chemistry & Engineering, 2025, 13 (31), pp.12564. 10.1021/acssuschemeng.5c04119. cea-05284666

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# Simplified carbonylation catalyst based on [Co₂(CO)₈] and coordinating solvents for the carbonylation of β-lactones and α,β-unsaturated acids to cyclic anhydrides

Marie-Hélène Pietraru†[a], Antonin Homassel†[a], Thibault Cantat*[a], Emmanuel Nicolas,*[a]

[a] Université Paris-Saclay, CEA, CNRS, NIMBE, 91191 Gif-sur-Yvette, France. E-mail: emmanuel.nicolas@cea.fr; thibault.cantat@cea.fr

## Abstract

Cyclic anhydrides are valuable chemical intermediates and monomers that can be synthesized through carbonylation of oxygenated substrates like β-lactones and unsaturated acids. Sophisticated cobalt-based catalysts which require the addition of ligands or pre-synthesis of a metal cation have been extensively used to promote such reactions. Herein we show that the commercial complex [Co₂(CO)₈] is a suitable catalytic precursor for the carbonylative ring-expansion of β-lactones, in a wide range of conditions, and even at low CO pressure. In particular, succinic anhydride is formed quantitatively from β-propiolactone in the presence of only [Co₂(CO)₈] in acetonitrile under a moderate CO pressure. The carbonylation of acrylic acid in these conditions is possible as well, and provides quantitative yields of product when the gas phase includes H₂. This work reinvestigates the potential of [Co₂(CO)₈] as a catalyst precursor: its disproportionation in the presence of a coordinating solvent (including MeCN and acetone) yields active catalytic species and thus paves the way toward the design of simpler catalytic systems for carbonylations.

**Keywords:** carbonylation, cobalt complexes, lactones, coordinating solvents, acrylic acid.

## Introduction

Cyclic anhydrides are useful molecules, used either as intermediates to platform chemicals,¹ or as monomers for the production of polyesters.²⁻⁴ The current industrial production of cyclic anhydrides is based on the oxidation of fossil feedstock, as exemplified by succinic anhydride (SA), which is mainly derived from *n*-butane.⁵ Alternatively, cyclic anhydrides can be obtained after the catalytic carbonylation of oxygenated substrates: epoxides,⁶,⁷ β-lactones,⁸⁻¹¹ and α,β-,¹² β,γ-,¹³,¹⁴ or γ,δ-unsaturated acids.¹⁵ In particular, SA can be obtained from β-propiolactone (PL) or acrylic acid (AA), which have the potential to be bio-sourced through intermediates like glycerol, lactic acid, 3-hydroxypropanoic acid (3-HPA), or ethanol (Scheme 1).¹⁶ Provided this carbonylation takes place with carbon monoxide produced by reduction of carbon dioxide or gasification of biomass,¹⁷,¹⁸ this would mark a step forward towards sourcing the carbon skeleton of SA from renewable feedstocks (i.e. biomass or chemical waste).
<figure><img src="image_2.png" /><figcaption>Scheme 1 – Synthetic route for polyesters synthesis from renewable feedstocks.</figcaption></figure>

After a first attempt by Tsuji in 1969 that afforded a moderate yield in anhydride under harsh conditions (100 bar CO, 150 °C, Scheme 2a), efforts have focused on incorporating a Lewis acid in order to activate the lactone and enable its carbonylation. The group of Coates have designed the state-of-the-art catalyst systems for such reactivity: an ionic pair composed of $[Co(CO)₄]⁻$ and a metallic-based Lewis acidic cation (Scheme 2b). As a fine-tuned catalyst was required to afford a selective carbonylation process, the aluminium- or chromium-based cations were designed by coordination to substituted tetradentate salen or porphyrin ligands which require multiple steps of synthesis and purification. The high activity of the synthesized species comes at the price of its sophistication.

In turn, we envisioned that the *in-situ* combination of $[Co_2(CO)₈]$ with a Lewis base (LB) could lead to its disproportionation, and thus to the generation of an active species for the carbonylation of PL, with the formula: Lewis acidic (di)cation + $[Co(CO)₄]⁻$ (Scheme 2c).¹⁹

<figure><img src="image_3.png" /><figcaption>Scheme 2 – (a) $[Co_2(CO)₈]$ in a non-coordinating solvent shows low reactivity. (b) State-of-the-art catalyst requires the synthesis and purification of a well-defined catalyst. (c) This work: the carbonylation active species would be generated *in-situ* by disproportionation of the commercial precursor $[Co_2(CO)₈]$ by a Lewis basic solvent.</figcaption></figure>
The effect of Lewis basic additives on cobalt carbonyl-catalyzed carbonylations have been studied since the 1950s, with varied results according to their coordination force, their lability, their Brønsted basicity, or their concentration.²⁰⁻²² Although their exact role in the catalysis was not always clear, the authors suggested that these Lewis bases could promote the formation of the active species from the precursor [Co₂(CO)₈], i.e., [HCo(CO)₄] or [Co(CO)₄]⁻,¹⁴,²¹,²³⁻²⁸ and/or stabilize some reaction intermediates.²⁹⁻³¹ The speciation of the complexes resulting from the coordination of the LB on [Co₂(CO)₈] has been established for some LB:¹⁹ for instance, phosphines may yield [Co(CO)₃(LB)₂]⁺+[Co(CO)₄]⁻ (along with the substitution complex [Co(CO)₃(LB)]₂),³² whereas pyridine, N,N-dimethylformamide (DMF) or acetonitrile lead to [Co(LB)₆]²⁺+2[Co(CO)₄]⁻.³³⁻³⁵ It is noteworthy that this does not necessarily inform the nature of the exact active species during the carbonylation, because of the many possible equilibria between cobalt complexes.³⁴ In the past few years, efforts have been made to identify them thanks to *in-situ* IR spectroscopy. In 2021, the group of Dong showed that the active species for the carbonylation of thietane arises from the disproportionation of [Co₂(CO)₈] by the substrate itself, and comprises an unidentified Co²⁺ dication alongside [Co(CO)₄]⁻.²⁴ In 2024, the groups of Gusevskaya and Beller described the catalytic performances of [Co₂(CO)₈]/OPCy₃ for the hydroformylation of epoxides: their IR studies confirmed the promoting effect of OPCy₃ on the formation of the active species [HCo(CO)₄], but they could only hypothesize on the structure of the disproportionation product, [Co(LB)₆][Co(CO)₄]₂.²⁷ Another example was provided by the group of Alexanian, who reported a photochemical version of the hydroaminocarbonylation of alkenes: they proposed that [Co₂(CO)₈] would be disproportionated by the amine substrate, yielding the ionic pair [cation]⁺[Co(CO)₄]⁻, followed by its light-promoted decarbonylation to the active species [Co(CO)₃]⁻.²⁸ These recent works show also a regain interest in studies of the potential of generating active species from the disproportionation of [Co₂(CO)₈].

In 1962, Iwanaga and coworkers published two studies³⁶,³⁷ in which they were able to titrate [Co(CO)₄]⁻ from mixtures of [Co₂(CO)₈] in methanol, ethanol, ethyl acetate, acetone, THF, dioxane, or acetonitrile. This suggests that all of these solvents are able to disproportionate [Co₂(CO)₈] to some extent. We reasoned that a Lewis basic enough solvent would be able to generate [Co(CO)₄]⁻ as well as a cobalt cationic species exhibiting Lewis acidity. We screened multiple solvents, including some classified as 'recommended' as regards their environmental impact,³⁸ in order to tune the Lewis acidity of the *in-situ* formed cobalt (di)cation and applied our catalytic system to the carbonylation of β-lactones (Scheme 2Scheme 2).
<figure><img src="image_4.png" /><figcaption>Scheme 3 – Envisioned mechanism for the carbonylation of PL to SA, in presence of [Co₂(CO)₈] and a Lewis base. LB: Lewis base; LA: Lewis acid.</figcaption></figure>

A screening of ten solvents in the presence of catalytic amounts of the commercial precursor [Co₂(CO)₈] was thus run for the carbonylation of β-propiolactone (PL) to succinic anhydride (SA). Our study includes acrylic acid (AA), a substrate more challenging than PL⁸,³⁹ despite these two substrates being isomers, and whose catalytic cyclizing carbonylation has been recently achieved by our group.¹² The scope was also extended to substituted derivatives of PL and AA: β-butyrolactone (BL), crotonic acid (CA), and methacrylic acid (MA).

## Results and Discussion

The methodology of this study relies on the screening of three sets of CO pressure (5, 15 and 50 bar) and ten common organic solvents: toluene, anisole, acetonitrile (MeCN), dimethyl carbonate (DMC), 1,4-dioxane, dimethoxyethane (DME), ethyl acetate (EtOAc), acetone, tetrahydrofurane (THF), and *N*,*N*-dimethylformamide (DMF). There is an interplay between CO pressure and the nature of the solvent for the generation of a potential active species (Scheme 3). While this step depends of course on the coordination properties of the LB, balance should also be found in terms of CO pressure: CO may be released during the disproportionation, which is then favoured at lower pressure, but concurrently CO pressure is required for the carbonylation itself.
<figure><img src="image_5.png" /><figcaption>Table 1 – Study of the influence of solvents and pressure on the carbonylation of PL to SA.ª</figcaption></figure>

<table><thead><tr><th>Entry</th><th>Solvent</th><th>Pressure (bar)</th><th>PL conversion (%)</th><th>SA yield (%)</th><th>Selectivity towards SA (%)</th></tr></thead><tbody><tr><td>1</td><td rowspan="3">Toluene</td><td>5</td><td>53</td><td>44</td><td>83</td></tr><tr><td>2</td><td>15</td><td>5</td><td>4</td><td>80</td></tr><tr><td>3</td><td>50</td><td>&lt;1</td><td>&lt;1</td><td>/</td></tr><tr><td>4</td><td rowspan="3">Anisole</td><td>5</td><td>40</td><td>38</td><td>95</td></tr><tr><td>5</td><td>15</td><td>9</td><td>13</td><td>&gt;99</td></tr><tr><td>6</td><td>50</td><td>1</td><td>&lt;1</td><td>/</td></tr><tr><td>7</td><td rowspan="3">MeCN</td><td>5</td><td>89</td><td>73</td><td>82</td></tr><tr><td>8</td><td>15</td><td>98</td><td>99</td><td>&gt;99</td></tr><tr><td>9</td><td>50</td><td>49</td><td>38</td><td>77</td></tr><tr><td>10</td><td rowspan="3">DMC</td><td>5</td><td>61</td><td>51</td><td>84</td></tr><tr><td>11</td><td>15</td><td>17</td><td>13</td><td>76</td></tr><tr><td>12</td><td>50</td><td>&lt;1</td><td>&lt;1</td><td>/</td></tr><tr><td>13</td><td rowspan="3">1,4-dioxane</td><td>5</td><td>&gt;99</td><td>76</td><td>76</td></tr><tr><td>14</td><td>15</td><td>44</td><td>38</td><td>86</td></tr><tr><td>15</td><td>50</td><td>&lt;1</td><td>5</td><td>/</td></tr><tr><td>16</td><td rowspan="3">DME</td><td>5</td><td>&gt;99</td><td>74</td><td>74</td></tr><tr><td>17</td><td>15</td><td>90</td><td>76</td><td>86</td></tr><tr><td>18</td><td>50</td><td>62</td><td>71</td><td>&gt;99</td></tr><tr><td>19</td><td rowspan="3">Ethyl acetate</td><td>5</td><td>&gt;99</td><td>76</td><td>76</td></tr><tr><td>20</td><td>15</td><td>42</td><td>37</td><td>88</td></tr><tr><td>21</td><td>50</td><td>9</td><td>16</td><td>&gt;99</td></tr><tr><td>22</td><td rowspan="3">Acetone</td><td>5</td><td>&gt;99</td><td>77</td><td>77</td></tr><tr><td>23</td><td>15</td><td>73</td><td>57</td><td>78</td></tr><tr><td>24</td><td>50</td><td>28</td><td>29</td><td>&gt;99</td></tr><tr><td>25</td><td rowspan="3">THF</td><td>5</td><td>&gt;99</td><td>69</td><td>69</td></tr><tr><td>26</td><td>15</td><td>96</td><td>83</td><td>86</td></tr><tr><td>27</td><td>50</td><td>65</td><td>58</td><td>89</td></tr><tr><td>28</td><td rowspan="3">DMF</td><td>5</td><td>60</td><td>5</td><td>8</td></tr><tr><td>29</td><td>15</td><td>52</td><td>4</td><td>8</td></tr><tr><td>30</td><td>50</td><td>48</td><td>4</td><td>8</td></tr></tbody></table>

The screening is reported in Table 1. Gratifyingly, not only some catalytic activity is observed in virtually all solvents tested, but a quantitative yield of SA is obtained in MeCN under 15 bar of CO (Table 1, entry 8).

Some general trends can be extracted from this screening. In toluene, anisole and DMC, the yield of SA and selectivity increase with decreasing CO pressure: under 5 bar, 38 to 51% of SA is obtained, with a selectivity up to 95% (Table 1, entries 1–6, 10–12). Noteworthy, the activity is completely suppressed at a CO pressure of 50 bar.

<footnote>ª Conditions: PL (0.5 mol·L⁻¹) and mesitylene (internal standard, 10 mol%) with [Co₂(CO)₈] (5 mol%) in the solvent (2 mL), heated for 6 h under CO pressure. PL conversions and SA yields were measured by GC-MS analysis; the selectivity was calculated as the ratio between yield and conversion. Due to the uncertainty of the analysis method, conversions lower than the yields can be obtained, and in that case, the selectivity value was reported as >99%. Selectivity was not reported and noted "/" when the conversion was <1%.</footnote>
In acetone, dioxane and EtOAc, the activity is enhanced at low pressure, but selectivity increases with pressure (Table 1, entries 13–15, 19–24). In DME, yields average around 70–75%, but higher selectivity is achieved at high CO pressure (Table 1, entries 16–18, 28–30). Considering its performances, DMF is not suitable for this reaction (less of 5% of yield, and very low selectivity, Table 1, entries 28–30). In MeCN and THF, the optimal reactivity is obtained under moderate pressure (15 bar, Table 1, entries 7–9, 25–27).

Two additional experiments support the claim that MeCN is a suitable solvent owing to more than just its solvation properties: (i) when 5 vol% of MeCN is added to toluene, 57% of SA with 92% selectivity is obtained, a yield which is significantly higher than the one obtained in toluene alone in similar conditions (4%, Table 1, entry 2); (ii) when adding only two drops of MeCN to $[Co_2CO]_8$, the brown slurry that is formed can be dissolved in toluene and submitted to reaction conditions (20:1 toluene:MeCN solvent system, 5 bar CO): 85% yield in SA is obtained, much higher than the 44% obtained in pure toluene (Table 1, entry 1). The comparison between the results obtained in MeCN and DMF highlights that a solvent with a high Lewis basicity is not necessarily more susceptible to lead to a high catalytic activity. Both solvents are known to disproportionate $[Co_2(CO)₈]$ similarly, and form $[Co(LB)₆]^{2+}+2[Co(CO)₄]⁻,^{34,35}$ but DMF displays a higher Lewis basicity (heat of complexation with BF₃: −60.39 ± 0.46 kJ·mol⁻¹ for MeCN and −110.49 ± 0.18 kJ·mol⁻¹ for DMF)⁴⁰,⁴¹ and a lower kinetic lability than MeCN (kinetic lability for solvent (S) exchange in $[Co(S)₆]^{2+}$: ~10⁵ s⁻¹ for MeCN and 3.9 x 10⁻¹ s⁻¹ for DMF).⁴²⁻⁴⁴ Although the exact active species can only be hypothesized, it seems reasonable to propose that the activation of the lactone requires the prior decoordination of the LB from the cobalt (di)cation, which would be less favoured in DMF than in MeCN.

Toluene and anisole have low Lewis basicities, and are thus not likely to disproportionate $[Co_2(CO)₈]$. Inspired by Dong *et al.*,²⁴ the substrate itself may be proposed as responsible for the generation of the active species, a reasoning which may be extended to all solvents: a suitable active species would be generated by disproportionation of $[Co_2(CO)₈]$ by either the solvent, or the substrate, or both. A high CO pressure disfavours such disproportionation, hence the quasi-systematic lower PL conversion at higher pressure.

THF has a relatively high Lewis basicity, however when dissolving $[Co_2(CO)₈]$ in THF, no CO bubbling is observed, indicating that THF does not induce the disproportionation of the cobalt precursor. As reported by Funaioli *et al.*,⁴⁵,⁴⁶ addition of a low amount of water (2.5 mol%) does induce disproportionation. When the reaction was performed using this solution with an *in-situ* formed catalyst, much better yields are obtained (*cf*. S.I. for details): for PL, quantitative carbonylation is achieved under 15 bar CO (>99% yield). This does improve the already high yield obtained in the absence of water (83%, Table 1, entry 26), at the price of controlling the amount of water inside the reaction medium.

To conclude, the trends observed are consistent with the mechanistic model proposed in Scheme 3, where the Lewis base may either be the solvent or the substrate, and where the disproportionation of the $[Co_2(CO)₈]$ is a key step for the generation of an
active species: no carbonylation activity is observed in the conditions where the disproportionation of the precursor $[Co_2(CO)_8]$ is less favoured, i.e., in a non-coordinating solvent, and at high CO pressure (see Table 1, entries 3 and 6).

Further assessment of the performances of the best catalytic system obtained so far was undertaken, i.e., MeCN/15 bar, starting with the effect of temperature (Table 2). A higher temperature increased the activity of the catalytic system: 83% of SA was obtained after only 1 h at 110 °C (Table 2, entry 4), but after 3 h, the PL was entirely converted, while the yield stagnated at 87% (Table 2, entry 3). The decrease of the selectivity was even more exacerbated at 130 °C, to the point where the yield of SA reached barely 44% after 1 h (Table 2, entry 6), and 54% after 3 h (Table 2, entry 7). The selectivity of the carbonylation of PL to SA is then quite sensitive to the temperature. Interestingly, increasing the CO pressure prevented this selectivity drop: 95% of SA was afforded at 130 °C, after 3 h under 50 bar of CO (Table 2, entry 8). The latter result indicates the interdependency between pressure and temperature. Finally, some catalytic activity was observed at temperatures as low as 70 °C and 50 °C (82% yield of SA after 65 h and 51% yield of SA after 65 h, Table 2, entries 9 and 10).

<figure><img src="image_6.png" /><figcaption>Table 2 – Study of the influence of temperature and reaction time for the carbonylation of PL to SA.ª</figcaption></figure>

<table><thead><tr><th>Entry</th><th>Temperature (°C)</th><th>Time (h)</th><th>PL conversion (%)</th><th>SA yield (%)</th><th>Selectivity towards SA (%)</th></tr></thead><tbody><tr><td>1</td><td rowspan="3">90</td><td>1</td><td>31</td><td>42</td><td>&gt;99</td></tr><tr><td>2</td><td>3</td><td>93</td><td>92</td><td>99</td></tr><tr><td>3</td><td>6</td><td>98</td><td>99</td><td>&gt;99</td></tr><tr><td>4</td><td rowspan="2">110</td><td>1</td><td>80</td><td>83</td><td>&gt;99</td></tr><tr><td>5</td><td>3</td><td>97</td><td>87</td><td>89</td></tr><tr><td>6</td><td rowspan="2">130</td><td>1</td><td>70</td><td>44</td><td>63</td></tr><tr><td>7</td><td>3</td><td>90</td><td>54</td><td>60</td></tr><tr><td>8<sup>b</sup></td><td>130</td><td>3</td><td>&gt;99</td><td>95</td><td>95</td></tr><tr><td>9</td><td>70</td><td>65</td><td>&gt;99</td><td>82</td><td>82</td></tr><tr><td>10</td><td>50</td><td>65</td><td>60</td><td>51</td><td>85</td></tr></tbody></table>

A lower catalyst loading slows down the reaction, but maintains the selectivity towards succinic anhydride (Figure 1). 92% of SA was obtained after 16 h with 2.5 mol% of $[Co_2(CO)_8]$, and 60% after 72 h with 1 mol%, with a high selectivity (for each experiment, selectivity >87%).

<footnote>ª Conditions: PL (0.5 mol·L⁻¹) and mesitylene (internal standard, 10 mol%) with $[Co_2(CO)_8]$ (5 mol%) in MeCN (2 mL), heated for the indicated duration under 15 bar of CO at the indicated temperature. PL conversions and SA yields were measured by GC-MS analysis. ᵇ Under 50 bar of CO.</footnote>
<figure><img src="image_7.png" /><figcaption>Figure 1 – SA yield over time at catalyst loadings of 1, 2.5 and 5 mol%.ᵃ</figcaption></figure>

Scaling-up experiments were successfully conducted in large-volume autoclaves, proving the applicability of this method up to the gram scale. An experiment with 5 mmol (360 mg) of PL afforded 90% of the desired product. When 10 mmol (720 mg) of PL were engaged in catalysis, 83% of SA was obtained after 8 h of reaction. In the case of the 10 mmol scale-up, the product could be isolated in 70% yield (Figure 2). A similar good yield was obtained within a few days when only 1 mol% of [Co₂(CO)₈] was introduced (See S.I. Table S10). High selectivity was achieved in each case.

<figure><img src="image_8.png" /><figcaption>Figure 2 – Scaling-up experiments – reported yields are measured in GC-MS compared to an internal standard, yield in parenthesis is the isolated yield.</figcaption></figure>

The recyclability of the catalytic system was then assessed over multiple runs. Up to
of the four runs of the reaction were performed successively in a single autoclave, by
injecting additional amount of PL between each run (see S.I. for details), without

<footnote>ᵃ Conditions: PL (0.5 mol·L⁻¹) and mesitylene (internal standard, 10 mol%) with [Co₂(CO)₈] (x mol%) in MeCN (2 mL), heated at 90 °C for the indicated duration under 15 bar of CO. SA yields measured by GC-MS analysis. Each point of the graph is a distinct experiment. The selectivity ranges from 87 to >99%.</footnote>
addition of catalyst. We can observe that the conversion and yields remained excellent (up to 95 % for the third run), and only slightly started to decrease on the fourth run (Figure 3). The same recycling experiments were performed at the 5 mmol scale, where again, conversions were found to remain high over 5 runs (See S.I., section 2.7 for details).

<figure><img src="image_9.png" /><figcaption>Figure 3 – Recycling of the catalytic medium – conversion and yield are measured by GC-MS analysis, with an uncertainty of ±5%. Reaction times : run 1: 6h; run 2: 16h; run 3: 6h; run 4: 16h.</figcaption></figure>

To extend the scope of this reaction, the carbonylation of β-butyrolactone (BL) to
methylsuccinic anhydride (MeSA) was conducted (Table 3). Our simple
[Co₂(CO)₈]/MeCN system was unfortunately unable to match the performances of the
well-defined catalysts developed by Coates and co-workers for the carbonylation of
substituted lactones to cyclic anhydrides.⁶,⁹ Our previously optimised conditions for PL
proved inefficient for BL (3% MeSA, Table 3, entry 1). Increasing the temperature to
110 °C improved the yield, but longer reaction times were needed to achieve complete
conversion (Table 3, entries 2–4). The selectivity was however limited, and a maximum
yield of 62% of BL was afforded after 72 h. Heating up to 130 °C decreased the
selectivity further, similar to what was already observed for PL (Table 3, entry 5, and
see Table 2). Increasing the CO pressure improved the selectivity only on the short-
term (Table 3, entries 6–7), going from >99 to 63% after 16 and 72 h respectively.
Combining high pressure and high temperature did not improve the performances of
the system. In particular, contrary to what was observed with PL, a higher pressure did
not compensate the selectivity loss caused by a high reaction temperature (Table 3,
entries 8–9). This may be explained, at least in part, by a side-reaction of
decarboxylation of the BL to form propylene and CO₂, which were observed in the gas
phase by GC when the reaction was run at temperatures higher than 130 °C (see S.I.
for details).
<figure><img src="image_10.png" /><figcaption>Table 3 – Optimization of the reaction conditions for the carbonylation of BL to MeSA.ª</figcaption></figure>

<table><thead><tr><th>Entry</th><th>Pressure (bar)</th><th>Temperature (°C)</th><th>Time (h)</th><th>BL conversion (%)</th><th>MeSA yield (%)</th><th>Selectivity towards MeSA (%)</th></tr></thead><tbody><tr><td>1</td><td>15</td><td>90</td><td>6</td><td>&lt;1</td><td>3</td><td>/</td></tr><tr><td>2<sup>b</sup></td><td>15</td><td>110</td><td>16</td><td>68</td><td>48</td><td>71</td></tr><tr><td>3<sup>b</sup></td><td>15</td><td>110</td><td>39</td><td>87</td><td>67</td><td>77</td></tr><tr><td>4<sup>b</sup></td><td>15</td><td>110</td><td>72</td><td>94</td><td>62</td><td>66</td></tr><tr><td>5</td><td>15</td><td>130</td><td>39</td><td>&gt;99</td><td>38</td><td>38</td></tr><tr><td>6</td><td>50</td><td>110</td><td>16</td><td>29</td><td>31</td><td>&gt;99</td></tr><tr><td>7</td><td>50</td><td>110</td><td>72</td><td>54</td><td>34</td><td>63</td></tr><tr><td>8</td><td>50</td><td>130</td><td>16</td><td>89</td><td>47</td><td>53</td></tr><tr><td>9</td><td>50</td><td>150</td><td>16</td><td>&gt;99</td><td>47</td><td>47</td></tr></tbody></table>

Similarly to what was done with PL, THF with 2.5 mol% H₂O was used as solvent: after 39 h at 110 °C and under 15 bar CO, 97% yield of MeSA are obtained (Figure 4, cf. S.I. for details). Improvement of yield for the carbonylation of BL is achieved at the cost of the simplicity of the catalytic system: water has to be present, but its amount has to be precisely controlled. Previous reports by the group of Funaioli⁴⁶ indicated that too much water prevents subsequent catalytic activity (hydroformylation in their case), even if disproportionation of [Co₂(CO)₈] had occurred.

<figure><img src="image_11.png" /><figcaption>Figure 4 – Carbonylation of BL in THF in the presence of water.</figcaption></figure>

We then turned our attention towards acrylic acid (AA), an isomer of PL and a platform chemical with millions of tons in global production volumes.⁴⁷ Carbonylation of AA has been attempted by Tsuji et al. in their study on PL carbonylation in 1969.⁸ Even though their conditions enabled isomerisation of PL to AA, they showed that AA could not be carbonylated to SA. Similarly, Falbe and co-workers noted during their research on acrylamide carbonylation to succinimide that AA polymerized in the presence of [Co₂(CO)₈] under CO atmosphere (300 bar).³⁹ In a recent study, our group was the first to achieve quantitative cyclizing carbonylation of AA to SA.¹² Capitalizing on our knowledge on this reaction and on the simple system developed herein, the same

<footnote>ª Conditions: BL (0.5 mol·L⁻¹) and mesitylene (internal standard, 10 mol%) with [Co₂(CO)₈] (5 mol%) in MeCN (2 mL), heated under CO pressure. BL conversions and MeSA yields were measured by GC-MS analysis; the selectivity was calculated as the ratio between yield and conversion. Due to the uncertainty of the analysis method, conversions lower than the yields can be obtained, and in that case, the selectivity value was reported as >99%. Selectivity was not reported and noted "/" when the conversion and/or the yield were <1%. ᵇ Traces of glutaric anhydride (GA) detected (1–2%).</footnote>
systematic screening of solvents and pressures already performed for PL was applied to AA and is reported in Table 4. Overall, the conversions, yields, and selectivities were lower with AA than with PL. The yields and selectivities were improved in general at higher pressure, and the best result was obtained in acetone under 50 bar of CO: 65% of SA, with 78% of selectivity (Table 4, entry 24). SA was not detected at all when DMF was used.

<figure><img src="image_12.png" /><figcaption>Table 4 – Study of the influence of solvents and pressure on the carbonylation of AA to SA.ª</figcaption></figure>

<table><thead><tr><th>Entry</th><th>Solvent</th><th>Pressure (bar)</th><th>AA conversion (%)</th><th>SA yield (%)</th><th>Selectivity towards SA (%)</th></tr></thead><tbody><tr><td>1</td><td rowspan="3">Toluene</td><td>5</td><td>46</td><td>4</td><td>9</td></tr><tr><td>2</td><td>15</td><td>53</td><td>26</td><td>49</td></tr><tr><td>3</td><td>50</td><td>63</td><td>20</td><td>32</td></tr><tr><td>4</td><td rowspan="3">Anisole</td><td>5</td><td>36</td><td>8</td><td>24</td></tr><tr><td>5</td><td>15</td><td>59</td><td>25</td><td>44</td></tr><tr><td>6</td><td>50</td><td>59</td><td>30</td><td>51</td></tr><tr><td>7</td><td rowspan="3">MeCN</td><td>5</td><td>n.d.</td><td>20</td><td>n.d.</td></tr><tr><td>8</td><td>15</td><td>84</td><td>57</td><td>68</td></tr><tr><td>9</td><td>50</td><td>67</td><td>52</td><td>78</td></tr><tr><td>10</td><td rowspan="3">DMC</td><td>5</td><td>44</td><td>11</td><td>24</td></tr><tr><td>11</td><td>15</td><td>86</td><td>55</td><td>64</td></tr><tr><td>12</td><td>50</td><td>39</td><td>28</td><td>72</td></tr><tr><td>13</td><td rowspan="3">1,4-dioxane</td><td>5</td><td>60</td><td>17</td><td>28</td></tr><tr><td>14</td><td>15</td><td>n.d.</td><td>30</td><td>n.d.</td></tr><tr><td>15</td><td>50</td><td>45</td><td>14</td><td>31</td></tr><tr><td>16</td><td rowspan="3">DME</td><td>5</td><td>n.d.</td><td>25</td><td>n.d.</td></tr><tr><td>17</td><td>15</td><td>n.d.</td><td>30</td><td>n.d.</td></tr><tr><td>18</td><td>50</td><td>67</td><td>36</td><td>54</td></tr><tr><td>19</td><td rowspan="3">Ethyl acetate</td><td>5</td><td>43</td><td>13</td><td>30</td></tr><tr><td>20</td><td>15</td><td>n.d.</td><td>42</td><td>n.d.</td></tr><tr><td>21</td><td>50</td><td>53</td><td>32</td><td>60</td></tr><tr><td>22</td><td rowspan="3">Acetone</td><td>5</td><td>47</td><td>24</td><td>50</td></tr><tr><td>23</td><td>15</td><td>&gt; 99</td><td>68</td><td>68</td></tr><tr><td>24</td><td>50</td><td>83</td><td>65</td><td>78</td></tr><tr><td>25</td><td rowspan="3">THF</td><td>5</td><td>n.d.</td><td>17</td><td>n.d.</td></tr><tr><td>26</td><td>15</td><td>n.d.</td><td>24</td><td>n.d.</td></tr><tr><td>27</td><td>50</td><td>66</td><td>35</td><td>53</td></tr><tr><td>28</td><td rowspan="3">DMF</td><td>5</td><td>26</td><td>0</td><td>0</td></tr><tr><td>29</td><td>15</td><td>n.d.</td><td>0</td><td>0</td></tr><tr><td>30</td><td>50</td><td>18</td><td>0</td><td>0</td></tr></tbody></table>

<footnote>ª Conditions: AA (0.5 mol·L⁻¹) and mesitylene (internal standard, 10 mol%) with [Co₂(CO)₈] (5 mol%) in the solvent (2 mL), heated for 6 h under CO pressure. AA conversions and SA yields measured by GC-MS analysis; selectivity calculated as the ratio between yield and conversion. n.d.: not determined. Because of overlap with other peaks on the GC trace, the conversion may not always be measured.</footnote>
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, [Co-AA] (see S.I., section 2.2). The IR bands of [Co-AA] suggest that it is a cobalt (II) species, with a structure of the type [Co''(acrylate)₂]. The formation of [Co-AA] could thus consume at least 2 equivalents of AA per Co atom, which would explain the lower selectivity in favour of the carbonylation product (with 5 mol% of [Co₂(CO)₈], around 20% of AA may be lost). In addition, a lilac solid with similar IR bands was synthesized in less than 30 min by reacting an excess of AA with [Co₂(CO)₈] under argon (see S.I., section 2.3): the formation of [Co-AA] is thus quite fast comparatively to the duration of the carbonylation runs, and would be particularly favoured at low CO pressure.ª This is consistent with the poor performances at 5 bar of the catalytic systems reported in Table 4. It is noteworthy that [Co-AA] is not an active complex for the carbonylation of AA (see Table S5 in the S.I.). In other words, the substrate itself is prone to deactivating the precursor [Co₂(CO)₈] (or cobalt carbonyl species formed *in-situ*) by coordinating on the metal centre as an acrylate. This gives hints on the difficulty of the carbonylation of AA to SA.

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). Using the THF-H₂O solvent system did prove useful, as it improved the yield to 73% SA, much higher than the 24% yield obtained without water (Table 4, entry 26 - cf. S.I. for details). In our previous study,¹² 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 5). 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 5, entry 2), and even a quantitative yield was measured in acetone (Table 5, entry 3). In acetone, lowering the total pressure from 50 to 15 bar reduced the selectivity to 77% (Table 5, entry 4). 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 5, entry 5). 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.⁴⁸

<footnote>ª [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).</footnote>
<figure><img src="image_13.png" /><figcaption>Table 5 – Effect of the addition of H₂ in the gas phase for the carbonylation of AA.ª</figcaption></figure>

<table><thead><tr><th>Entry</th><th>Pressure (bar)</th><th>CO/H<sub>2</sub> mix</th><th>Solvent</th><th>AA conversion (%)</th><th>SA yield (%)</th><th>Selectivity (%)</th></tr></thead><tbody><tr><td>1</td><td>50</td><td>100:0</td><td>Acetone</td><td>83</td><td>65</td><td>78</td></tr><tr><td>2</td><td>50</td><td>90:10</td><td>MeCN</td><td>&gt;99</td><td>91</td><td>91</td></tr><tr><td>3</td><td>50</td><td>90:10</td><td>Acetone</td><td>98</td><td>&gt;99</td><td>&gt;99</td></tr><tr><td>4</td><td>15</td><td>90:10</td><td>Acetone</td><td>&gt;99</td><td>77</td><td>77</td></tr><tr><td>5</td><td>25</td><td>80:20</td><td>Acetone</td><td>&gt;99</td><td>70</td><td>70</td></tr></tbody></table>

The catalytic system was applied to two related, yet more sterically hindered, compounds: crotonic acid (CA) and methacrylic acid (MA) (Table 6). After the carbonylation of CA, two products may be expected: methylsuccinic anhydride (MeSA), and the 6-membered cyclic anhydride, glutaric anhydride (GA). Only MeSA is obtained when performing the reaction in MeCN under pure CO, and only in poor yields (Table 6, entries 1-3). In acetone, both anhydrides are formed (Table 6, entry 4-8). 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 6, entries 5 and 6). 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 6, entry 7, average of two runs). GA the major product when acetone is used as solvent, independently of the presence of H₂ in the gas phase (Table 6, entry 4 vs. entries 5-8). This indicates that in acetone the isomerization of cobalt carbonyl intermediates (cobalt-alkyl complexes) is possible, consistent with previously reported results.⁴⁹ Increasing the proportion of H₂ in the gas phase from 10 vol% to 20 vol% was detrimental to the selectivity of the carbonylation, as it probably enhanced hydrogenation of the substrate (Table 6, entry 8). The carbonylation of MA to MeSA gave poorer results: only 7% of MeSA was detected after 6 h at 90 °C, whether under pure CO or CO/H₂ (Table 6, entries 10-11). With 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 6, entry 12). Under all conditions presented here, a substantial amount of MA was hydrogenated to isobutyric acid. MA appears to be more reactive towards hydrogenation, explaining the poor performances of carbonylation of this substrate.

Interestingly, when submitting CA to the best set of conditions (110 °C, 6 h, 50 bar CO/H₂) in acetonitrile, similar conversion of CA is obtained but with reversed selectivity: MeSA is obtained with 53% yield, and GA is obtained with 25% yield (>99% selectivity towards anhydrides, 68:32 MeSA:GA ratio) (Table 6, entry 9). As previously reported,¹⁵ the regioselectivity of the carbonylative cyclization of unsaturated acids is quite sensitive to slight modifications of the catalytic system. In our case, the modulation of the selectivity may be simply achieved by changing the reaction solvent.

<footnote>ª Conditions: AA (0.5 mol·L⁻¹) and mesitylene (internal standard, 10 mol%) with [Co₂(CO)₈] (5 mol%) in the solvent (2 mL), heated for 6 h under pressure of desired gas mix. AA conversions and SA yields measured by GC-MS analysis; selectivity calculated as the ratio between yield and conversion.</footnote>
<figure><img src="image_14.png" /><figcaption>Table 6 – Optimization of the reaction conditions for the carbonylation of CA and MA.ª</figcaption></figure>

<table><thead><tr><th>Entry</th><th>SM<sup>b</sup></th><th>Solvent</th><th>T (°C)</th><th>t (h)</th><th>CO/H<sub>2</sub> ratio (Pressure)</th><th>Cyclic anhydrides yield (%)</th><th>MeSA:GA</th><th>Selectivity toward cyclic anhydrides (%)</th></tr></thead><tbody><tr><td>1</td><td>CA</td><td>MeCN</td><td>90</td><td>6</td><td>100:0 (15 bar)</td><td>13</td><td>100:0</td><td>51</td></tr><tr><td>2</td><td>CA</td><td>MeCN</td><td>90</td><td>6</td><td>100:0 (50 bar)</td><td>2</td><td>100:0</td><td>7</td></tr><tr><td>3</td><td>CA</td><td>MeCN</td><td>110</td><td>6</td><td>100:0 (15 bar)</td><td>15</td><td>100:0</td><td>37</td></tr><tr><td>4</td><td>CA</td><td>Acetone</td><td>110</td><td>16</td><td>100:0 (50 bar)</td><td>23</td><td>39:61</td><td>37</td></tr><tr><td>5</td><td>CA</td><td>Acetone</td><td>90</td><td>6</td><td>90:10 (50 bar)</td><td>26</td><td>31:69</td><td>59</td></tr><tr><td>6</td><td>CA</td><td>Acetone</td><td>90</td><td>16</td><td>90:10 (50 bar)</td><td>47</td><td>32:68</td><td>82</td></tr><tr><td>7</td><td>CA</td><td>Acetone</td><td>110</td><td>6</td><td>90:10 (50 bar)</td><td>72</td><td>29:71</td><td>80</td></tr><tr><td>8</td><td>CA</td><td>Acetone</td><td>110</td><td>6</td><td>80:20 (25 bar)</td><td>40</td><td>38:62</td><td>41</td></tr><tr><td>9</td><td>CA</td><td>MeCN</td><td>110</td><td>6</td><td>90:10 (50 bar)</td><td>78</td><td>68:32</td><td>&gt;99</td></tr><tr><td>10</td><td>MA</td><td>MeCN</td><td>90</td><td>6</td><td>100:0 (15 bar)</td><td>4</td><td>100:0</td><td>n.d.</td></tr><tr><td>11</td><td>MA</td><td>Acetone</td><td>90</td><td>6</td><td>90:10 (50 bar)</td><td>7</td><td>100:0</td><td>12</td></tr><tr><td>12</td><td>MA</td><td>Acetone</td><td>110</td><td>16</td><td>90:10 (50 bar)</td><td>24</td><td>100:0</td><td>24</td></tr></tbody></table>

<footnote>ª Conditions: substrate (0.5 mol·L⁻¹) and mesitylene (internal standard, 10 mol%) with [Co₂(CO)₈] (5 mol%) in acetone (2 mL) (or MeCN (2 mL) for entries 1, 2, and 3), heated for the indicated duration under pressure of gas mix. MA and CA conversions and MeSA and GA yields measured by GC-MS analysis; selectivity calculated as the ratio between yield and conversion. n.d.: not determined. ᵇ SM: Starting Material.</footnote>
## Conclusion

Herein we report the carbonylation of β-propiolactone and acrylic acid using [Co₂(CO)₈] as catalyst. For PL, we achieved quantitative yields and excellent selectivity in acetonitrile, under 15 bar of CO, 90 °C and 6 h reaction time. Scaling-up the reaction to 1 g batches does not undermine reactivity. Despite the relatively high catalyst loading, good yields were obtained over multiple recycling of the catalytic system. Less toxic, recommended solvents are suitable for this reaction. Our catalytic system proves efficient for acrylic acid as well when H₂ is added to the gas phase, in which case quantitative yields are obtained. Methylated variant of acrylic acid proved active under the depicted conditions, despite carbonylation proceeding with poorer performances. Better understanding of the reaction mechanism, of the catalyst deactivation pathways, and *in-situ* analysis of speciation species formed during [Co₂(CO)₈] solvent-induced disproportionation would be valuable to further improve this system and extend it to a wider variety of substrates. Research is currently ongoing in our group to address these topics.

## Experimental Section

*Representative procedure for the carbonylation of β-lactones or α,β unsaturated acids in small-volume autoclaves*

**Preparation of the mixture.** Inside the dry box, in a vial, [Co₂(CO)₈] (17.1 mg, 0.05 mmol, 5 mol%) was dissolved in acetonitrile (2 mL), yielding a brown solution. Immediate bubbling (of CO) was observed. After stirring the solution at room temperature for 30 min, i.e. until the gas evolution stopped, the homogenous solution turned pale pink. Mesitylene (internal standard, 14 µL, 0.1 mmol), and β propiolactone (63 µL, 1 mmol) were added in the mixture. The mixture was homogenized, then taken into a syringe. The needle of the syringe was stuck in the septum of a sealed vial to keep the mixture under inert atmosphere during the transfer outside the dry box.

**Injection in the autoclave and start of the reaction.** The autoclave is sealed, it is purged three times with 5 bar of carbon monoxide. The mixture prepared in the dry box and collected in the syringe was injected quickly in the purged autoclave thanks to the use of a long metallic needle. The autoclave was purged one more time with 5 bar of carbon monoxide. The autoclave was then pressurized at 15 bar, and put in a heating block for 6 h at 90 °C.

**End of the reaction and analysis of the raw mixture.** The reaction was stopped by cooling the autoclave in an ice bath. The autoclave was carefully depressurized until <2 bar: a sample of the gas phase was collected in a gas-tight microsyringe, and injected in a GC apparatus for analysis. After complete depressurization and unlocking of the autoclave, ethyl acetate, a solvent that solubilizes well succinic anhydride, was added in the glass tube (3 mL of solvent per mmol of β propiolactone initially injected). After homogenization, the mixture was filtered on a Celite pad, and a sample was collected for GC MS analysis.
**Supporting Information**

Detailed experimental procedures.

**Notes**

† These authors contributed equally

This article was posted as a preprint on ChemRxiv. DOI: 10.26434/chemrxiv-2024-07h4m-v2

**Acknowledgements**

For financial support of this work, we acknowledge CEA, CNRS, the University Paris-Saclay. This work has benefited from French State aid managed by the Agence Nationale de la Recherche under France 2030 plan, bearing the reference code ANR22-PESP-0010 : Projet ciblé "POWERCO2" within the PEPR project SPLEEN

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For Table of Contents Use Only.

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* ✓ No external ligand

* ✓ Commercial precursor

* ✓ Common organic solvents

Carbonylation of lactones is enabled by the use of cobalt carbonyl and coordinating solvents, to easily form succinic anhydride.