The direct addition of CO2 onto ethylene to give acrylic acid (scheme 1) is industrially unattractive due to thermodynamic limitations (ΔG=34.5 kJ/mol) and the unfavorable equilibrium, which at room temperature is virtually completely to the side of the reactants (K293=7×10−7).

By using a base, it is possible to convert the α,β-ethylenically unsaturated acid to the salt thereof and thus to shift the equilibrium to the side of the products. The reaction, however, is kinetically inhibited and therefore requires a homogeneous or heterogeneous carboxylation catalyst.
According to Aresta et al. (J. Chem. Soc., Dalton Trans. 1977, 7, 708) and Hillhouse et al. (Inorg. Chem. 2010, 49, 10203), a ligand and a homogeneous Ni(0) species such as bis(1,5-cyclooctadiene)nickel (Ni(COD)2) in the presence of CO2 readily form a ligand-Ni—CO2 adduct (scheme 2), which is thermally labile, and one way in which it decomposes is with oxidation of the ligand, even at low temperatures of 80° C. This is disadvantageous since the potential catalyst or precursors thereof are thus degraded.

According to Yamamoto et al. (J. Am. Chem. Soc. 1980, 102, 7448), the equimolar reaction of acrylic acid with Ni(COD)2 in the presence of a tertiary phosphine ligand at temperatures above 0° C. gives rise to the stable five-membered nickelalactone ring A, called the Hoberg complex (scheme 3). At temperatures below 0° C., the same reaction gives an equimolar mixture of the lactone A and of the open-chain π-complex B. The thermal cleavage of A or of the mixture A/B to give free acrylic acid did not succeed. An equilibrium between A and B, which is an important prerequisite for a catalytic transformation, was likewise postulated by Walther et al., but not observed experimentally (Eur. J. Inorg. Chem. 2007, 2257).

Nickelalactones A may bear one or more ligands and arise from the direct and stoichiometric coupling of CO2 and ethylene, as found by Hoberg (J. Organomet. Chem. 1983, C51). The reaction was performed at industrially unfavorable temperatures of down to −70° C. (J. Organomet. Chem. 1982, 236, C28; Angew. Chem. Int. Ed. Engl. 1987, 26, 771). In addition, for example, the nickelalactones which originate from the reaction of the basic 2,2′-bipyridine ligand, an Ni(0) species, alkenes and CO2 are isolable as stable solids (J. Organomet. Chem. 1982, C28), which demonstrates the exceptional stability of these compounds.
The treatment of such stable nickelalactones with aqueous mineral acids gives rise to the saturated acid propionic acid, but not acrylic acid. This suggests that the β-hydride elimination needed to form acrylic acid and derivatives thereof from the complex A is difficult. Accordingly, there has still been no description of a catalytic variant of this reaction.
This suggestion is supported by quantum-mechanical studies by Buntine et al. These show the increase in stability by ˜40 kcal/mol of the intermediate nickelalactone bearing two DBU ligands compared to the desired acrylic acid elimination product (Organometallics 2007, 26, 6784).
Rieger et al. have for the first time successively released an acrylic acid derivative from a nickelalactone by reaction with methyl iodide or with LiI. The transformation gives, as well as methyl propionate, which indicates an unproductive decomposition of the nickelalactone, low yields of methyl acrylate (max. 33%); no catalysis cycle was described. In the case of use of LiI, at best only traces of methyl acrylate were found. The nickelalactones used bear the ligands diphenylphosphinopropane (dppp), diphenylphosphinoethane (dppe) and tetramethylethylenediamine (TMEDA). The two former lactones were prepared by ligand exchange of the lactone prepared from Ni(COD)2, TMEDA and succinic anhydride, and none of the lactones were synthesized proceeding from CO2 and ethylene in a one-pot reaction (Organometallics 2010, 29, 2199).
Similar results in principle, with better yields, were found by Herrmann and Kühn et al. (ChemSusChem 2011, 4, 1275-1279). There was no description of a catalytic reaction regime here either.
WO 2011/107559 discloses a process for preparing an alkali metal or alkaline earth metal salt of an α,β-ethylenically unsaturated carboxylic acid, wherein a) an alkene, CO2 and a carboxylation catalyst are converted to an alkene/CO2/carboxylation catalyst adduct, b) the adduct is decomposed to release the carboxylation catalyst with an auxiliary base to give the auxiliary base salt of the α,β-ethylenically unsaturated carboxylic acid, c) the auxiliary base salt of the α,β-ethylenically unsaturated carboxylic acid is reacted to release the auxiliary base with an alkali metal or alkaline earth metal base to give the alkali metal or alkaline earth metal salt of the α,β-ethylenically unsaturated carboxylic acid. The process achieves the cleavage of the intermediate adduct by means of an auxiliary base, for example of a tertiary amine, in order to prepare, in a first step, the ammonium salt of the α,β-ethylenically unsaturated carboxylic acid, which overcomes the fundamental thermodynamic limitation. In a second step, for example with aqueous sodium hydroxide solution, the ammonium cation is exchanged for sodium, in order thus to obtain the sodium salt of the α,β-ethylenically unsaturated carboxylic acid. This two-stage reaction regime is complex. Moreover, the cleavage of the lactone is slow and thus reduces the space-time yield of such a process considerably.