Acrylates from Alkenes and CO2, the Stuff That Dreams Are Made of

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Acrylates from Alkenes and CO2, the Stuff That Dreams Are Made of Michael Limbach* CaRLa (Catalysis Research Laboratory), Heidelberg, Germany BASF SE, Synthesis & Homogeneous Catalysis, Ludwigshafen, Germany *Corresponding author: e-mail address: [email protected]

Contents 1. Introduction 1.1 Thermodynamics and Rationale 1.2 State-of-the-Art Functionalization of Alkenes and Alkynes with CO2 2. Shedding Light into the Dark: Lactone Formation 2.1 Lactone Formation: Mechanistic Intermezzo 3. Lactone Cleavage and Final Ligand Exchange 3.1 Lactone Cleavage with Auxiliaries to Force a β-H Elimination 3.2 Mechanistic Course of the β-H Elimination from Neutral Nickelalactones 3.3 Lactone Cleavage with Brønsted Bases 3.4 Acrylate/Ethylene Exchange 4. Catalytic Reactions 5. Catalysts Gone Astray 6. Conclusions and Outlook Acknowledgments References

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1. INTRODUCTION 1.1 Thermodynamics and Rationale The exploitation of carbon dioxide (CO2) for the production of world-scale chemicals, such as formic acid,1b has industrial potential, as CO2 is a cheap and abundantly available C1 building block.2 Nevertheless, only a few reactions and catalysts enable the straightforward catalytic functionalization of industrially viable starting materials, such as alkanes and alkenes with CO2, to industrially relevant target molecules. Advances in Organometallic Chemistry ISSN 0065-3055 http://dx.doi.org/10.1016/bs.adomc.2015.03.001

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2015 Elsevier Inc. All rights reserved.

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Among those, there is a small set of “dream reactions,”3 i.e., economically highly attractive transformations, which do not yet exist due to major technological or scientific hurdles (Fig. 1): the direct carboxylation of alkanes,
0 ¼ + 55:0kJ=mol , the twofold carboxsuch as methane to acetic acid ΔGR
0 ¼ + 77:4kJ=mol , or the direct ylation of butane to adipic acid ΔGR synthesis of acrylic acid and its derivatives from alkenes and CO2
0 ΔGR ¼ + 42:7kJ=mol . In the latter transformation, addition of auxiliaries like alkyl halides (e.g., RX ¼ MeI) would lead to alkyl acrylates, but does
0 ¼ + 21:0kJ=mol . not alter the unfavorable thermodynamic balance ΔGR All those transformations are highly endergonic and do not occur spontaneously. In order to shift the thermodynamic equilibria to the product side, the free acids have to be removed from the reaction with a base via their salts, i.e., acetates, adipates, or acrylates. It has to be pointed out that—albeit now the reaction becomes feasible from a thermodynamic point of view 0 (ΔGR ¼  56:2 kJ=mol for sodium acrylate)—an economic penalty comes now into play: seldom enough are the salts, the desired commercial products but the acids. Salt cleavage adds often significant costs and by-products. For the hydrogenation of CO2 to formates, innovative base-recycling concepts have been developed1 and might prove useful for other acid/base pairs, too.

Figure 1 Dream reactions in the context of CO2 functionalization and their thermodynamic feasibility. A dream reaction is an economically highly attractive transformation, which is currently unfeasible due to a major scientific and/or technological challenge.

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Acrylates and their downstream products are manufactured globally on a multimillion ton level as they are ubiquitous in daily life as hygiene products, coatings, adhesives, and food preservatives. Sodium acrylate is the most important industrial acrylic acid salt and is used as monomer for superabsorbent polymers. The combined thermodynamic need and market demand for the salt instead of the free acid, made sodium acrylate an ideal target molecule for our investigation.

1.2 State-of-the-Art Functionalization of Alkenes and Alkynes with CO2 The hydrocarboxylation of activated unsaturated hydrocarbons with CO24 is an established methodology in organic chemistry to yield either α,β- (for alkynes),5 β,γ-unsaturated α-branched (for allenes and 1,3 alkadienes),6 or α-branched carboxylic acid derivatives (for styrenes),7 but requires the stoichiometric use of reductants (i.e., AlEt3, hydrosilanes, Et2Zn, RMgX) or directing groups in the substrate (Fig. 2A).7c The catalytic carboxylation of unsaturated hydrocarbons with CO2 to α,β-unsaturated carboxylates (i.e., acrylates) has been a topic of academic as well as industrial research for three decades, since seminal work of Hoberg8 and Yamamoto9 in the early 1980s. Although Hoberg et al. reported the catalytic nickel-catalyzed reaction of alkenes and isocyanates (isoelectronic to CO2) to acrylamides,10 there has been no catalyst for the direct carboxylation of CO2 and alkenes, neither based on nickel nor based on other metals from the nickel (Pd, Pt)11–20 or iron triad,21 titanium,22 molybdenum,23 tungsten,24 zirconium,25 or rhodium.26 Basic obstacles for a catalytic transformation remained: (a) the ender
0 ¼ + 42:7 kJ=mol , (b) the high gonic nature of the overall reaction ΔGR activation barrier for the proposed β-hydride elimination from a nickelalactone or other Hoberg-type complexes (ΔG ¼ 164 kJ/mol),27,28 and (c) the limitation to a small set of ligands paired with unproductively low reaction temperatures down to 70 °C (Fig. 2B and C).29 Apart from the mentioned Hoberg complexes, the mechanistic course of the reaction has been purely speculative and was poorly supported with experimental findings. This is why despite of the maturity of the technology toward formiates,1 the development toward acrylates has been remaining in its infancies over several decades.

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Figure 2 State-of-the art hydrocarboxylation of alkenes/alkynes with CO2 (A), an early mechanistic proposal for the reductive carboxylation of ethylene (B), and recent kinetic considerations in a putative catalytic cycle (C).

2. SHEDDING LIGHT INTO THE DARK: LACTONE FORMATION 2.1 Lactone Formation: Mechanistic Intermezzo Hoberg et al. and others30–33 have revealed metallalactones, in particular nickelalactones (Hoberg complexes),8 to be stable and isolable intermediates of the potential catalytic coupling of ethylene and CO2 (Fig. 2B). The reaction has been described with numerous ligands,34 solvents, and substrates (alkenes, alkadienes, alkynes, allenes)35 and the experimental efforts have early on been supported by quantum mechanical studies: for a monodentate DBU-model ligand (DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene),27

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bipyridine (bipy),28 and recently for chelating bisphosphines, phosphites, or bisamines.36 It is widely accepted that the reaction starts from an ethylene complex, since it is found to be more stable in calculations than the metallalactone and its formation prior to lactone formation has been observed experimentally (Fig. 3). For the more oxophilic molybdenum, lactone formation occurs via prior formation of a stable CO2–ethylene complex by ligand exchange and by subsequent coupling of CO2 and ethylene.37,38 Lactones of other metals than nickel resulting from the direct coupling of CO2, and alkenes, alkynes, alkadienes, or allenes have only been described for an allene (a single palladalactone),39 nevertheless, as a substance class they are known.12,14 We have focused our experimental activities on bidentate ligands, such as bisphosphines. In a systematic study of the oxidative coupling of ethylene and CO2 with Ni(COD)2 as a Ni(0)-precursor and bidentate phosphines bearing either phenyl- or tert-butyl substituents at the phosphorus atoms and differing in the length of the carbon bridge –CH2(CH2)n– in the ligand backbone (n ¼ 0–2), we observed the following trends (Fig. 3A): (1) Sterically, nondemanding bidentate ligands as bis(diphenylphosphino) methane (dppm) or certain monodentate ligands form aggregated complexes (e.g., dimers, trimers), as already discussed by Langer and Walther et al.,40–44 and (2) the higher homologues of dppm, such as 1,2-bis (diphenylphosphino)ethane (dppe) and 1,3-bis(diphenylphosphino)propane

Figure 3 Experimental findings (A) and competing mechanisms for lactone formation for the dtbpe-ligand (B).

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(dppp), rapidly form the corresponding known tetracoordinate Ni(0) species Ni(dppe)245 and Ni(dppp)2 instead of the expected lactones [Ni ((CH2)2CO2)(dppe)] and [Ni((CH2)2CO2)(dppp)].46 This does not at all mean that those ligands do not form catalytically active species (as has been shown later), but for our first systematic studies they turned out to be “difficult” for the mentioned reasons. We rationalized that this coordinative saturation of the metal might be avoided by switching to more bulky tert-butyl substituents at the phosphorus atoms (density functional theory (DFT) calculations predicted the formation of tetracoordinated species with such ligands to be clearly endergonic ΔG > 50 kJ/mol). Indeed, ligands such as di-tert-butylphosphinoethane (dtbpe) and di-tert-butylphosphinopropane (dtbpp) resulted in immediate formation of the corresponding ethylene complexes, which formed in the presence of CO2, the desired nickelalactones. Only [Ni((CH2)2CO2)(dtbpe)] was stable enough to be isolated and stored as a solid for several months without decomposition and external gas pressure. Other lactones (dtbpm, dtbpp) easily expelled CO2 to revert to the initial ethylene complexes. Lactone formation in tetrahydrofuran (THF) was slow with those ligands, but significantly accelerated in chlorobenzene. For nickel(0) complexes with bidentate phosphine ligands, we have identified two mechanistic borderline cases for lactone formation: an “inner-sphere mechanism” and an “outer-sphere mechanism” (Fig. 3B)36:a In general, in the gas phase, the “inner-sphere” mechanism is favored, but this changes for typical organic solvents such as THF. The “outer-sphere” mechanism is significantly more favorable for sterically hindered ligands, and the activation barriers for ligands such as bipyridine (bipy) become essentially identical for both mechanisms. More electron-rich ligands lead, in general, to lower barriers for both mechanisms. However, while steric hindrance does not affect the “outer sphere” mechanism, it leads to much higher barriers for the “inner sphere” mechanism. Although it is in general not completely clear, which mechanism is favored for what reason, probably things are most easily understood for the homologous series dtbpm, dtbpe, and dtbpp: increasing the bite angle leads to larger steric bulk and the “inner-sphere” barrier systematically increases while the “outer sphere” barrier is much less affected. For the inner-sphere mechanism, the insertion is believed to occur in one step, where the CO2 molecule concertedly coordinates to nickel and inserts into a NidC bond (G{ ¼ 124 kJ/mol). This is comparable to the mechanistic course proposed by Buntine et al. for the reaction with DBU as a ligand, where two DBU ligands according to calculations stay coordinated.27

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Depending on the ligand and the level of theory, we found a weak associative precoordination of CO2, which, however, certainly does not indicate a stable compound of any significance. The attack of CO2 on the coordinated ethylene in the “outer sphere” without precoordination of CO2 (G{ ¼ 110 kJ/mol) is perhaps most easily understood as the reverse reaction of the decarboxylation of the β-H-agostic, formally zwitterionic intermediate B1 (see Fig. 3B). A similar mechanism for attack of CO2 at amide and hydroxide groups of nickel compounds with subsequent insertion of CO2 into the NidN/NidO bonds via zwitterionic intermediates has been computed recently.47 Carboxylation of Ni(II) and Pd(II) allyl complexes has also been proposed to proceed via zwitterionic intermediates resulting from the attack of CO2 at the terminal carbon of the η1-coordinated allyl group.48–53 B1 is isoelectronic to the thermodynamic insertion product in cationic olefin polymerization such as in the Brookhart systems,54 and is fairly stable, if solvation is taken into
account. This means that a second, lower barrier Gs{ ¼ 82kJ=mol needs to be overcome for recoordination of the carboxylate unit to nickel. For other ligands, such as dmpm, dmpe, bipy, and dtbpm, B1 is not stable and the “outer-sphere” transition state TS–B1–C1 leads directly to C1 (see Fig. 4). A third mechanistic option with precoordination of CO2 to a vacant coordination side created by the dissociation of one arm of the bidentate ligand seems to be unlikely (transition state for dtbpm ΔΔG{ ¼ 15 kJ/mol higher in free energy than that of the “inner-sphere mechanism”).36a Alternatively, the role of a η2 side-on bound Ni–CO2 complex55 as entry into a potential catalytic cycle has been debated previously (Fig. 5). Similarly

Figure 4 Structures of the transition states involved in lactone formation in solution. From left to right: TS–B1–C1, TS–A–B1, and TS–A–C1. Bond lengths in pm. Hydrogen atoms on the dtbpe-ligand are omitted for clarity.

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Figure 5 The elusive side-on bound Ni–CO2 complex is—at its best—innocent.

to Aresta et al.28 and different from Dedieu et al.,56 we conclude that an η2bound CO2 complex of the catalytically active [Ni(dtbpe)] fragment is at best innocent. Based on the kinetic barriers to form a lactone either from an alkene or CO2 complex, the alkene precursor is clearly preferred (190.6 vs. 108.8 kJ/mol).

3. LACTONE CLEAVAGE AND FINAL LIGAND EXCHANGE 3.1 Lactone Cleavage with Auxiliaries to Force a β-H Elimination Nickelalactones do not easily convert into other catalytically active species in the sense of a β-H elimination. Therefore, a variety of stoichiometric auxiliaries have been investigated not only such as electrophiles (i.e., alkyl halides31–33,57–59 or protons58,60), Lewis acids,29,58,59,61 a combination of both62 but also physical measures such as heat60,63–65 or even ultrasound (cf. Fig. 6D–G).60 All attempts in common is that the fate of the organometallic species involved in the reaction was ill-defined and so far no catalytic reaction has been reported for this route to overcome both of the abovementioned fundamental thermodynamic and kinetic hurdles (see Figs. 1 and 2C). The relevance and interplay of various catalytically active species in the sense of the elusive β-H elimination from nickelalactones as discussed in literature depend clearly on ligand, substrate, and reaction

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Figure 6 Manifold attempts to cleave nickelalactones at certain (A) and/or uncertain fate of the organometallic species (B).

conditions66: In dimethylformamide (DMF), according to Walther and coworkers, 1,10 -bis(diphenylphosphino)ferrocenyl (dppf ) or 1,10 -bis (diisopropylphosphino)ferrocenyl (dippf )-ligated lactones decomposed at temperatures >40 °C,67 whereas temperatures as high as 140 °C are needed to extrude CO2 from a bipyridine-ligated nickelalactone in the solid state.33 No direct interconversion of nickelalactones to their corresponding π-complexes and vice versa has been reported so far and only Hoberg found indirect evidence for a thermally induced β-H elimination by isolation of cinnamic acid after acidolysis of a crude reaction mixture (styrene, CO2, Ni(COD)2, DBU, THF, >85 °C, 24 h, cf. Fig. 6G).68 A formal β0 -H elimination was proposed based on the isolation of β,γ-unsaturated carboxylic acids in good yields upon cleavage of P,N-ligated lactones (i.e., 2-[2-(dicyclohexylphosphino)ethyl]-pyridine as ligand) with nonaqueous HCl in Et2O at room temperature.58,59 Such lactones even released α,βunsaturated carboxylic acids upon exposure of the crude reaction mixture to the Lewis acidic BeCl2 prior to acidolysis58,29b. Hoberg rationalized that partial ligand decoordination by protonation of the P,N-ligand’s pyridine-N would lead to a 14-electron species, which is prone to hydride elimination.

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A similarly induced β-H elimination might explain Hoberg’s experimentally observed BeCl2-mediated ring contraction between at the time postulated 5- and 4-membered nickelalactones with the same P,N-ligand (Fig. 6).58,59 Transient hydridic species have been observed by Herrmann et al. by 1H NMR in the course of the cleavage of a TMEDA-ligated nickelalactone (TMEDA, N,N,N 0 ,N 0 -tetramethylethylenediamine) with electrophiles57b, and point clearly to a β-H elimination. Moreover, Bernskoetter and coworkers found experimental evidence for a Lewis acid-mediated β-H elimination as they observed an equilibrium between a neutral dppf-ligated 4-membered nickelalactone and its 5-membered isomer (Fig. 6C).69 A similar equilibrium was observed between bis-(dicyclohexylphosphino)ethane (dcpe)-ligated 4- and 5-membered lactones generated from the corresponding neutral species and NaBAr3 F .57 Further, indirect evidence for the β-H elimination31,62 from a nickelalactone and a subsequent transfer of the hydrogen from the metal to the oxygen of the acrylate moiety have been reported by Bernskoetter et al. (Fig. 6B): they observed the isomerization of a 5-membered neutral dcpe-ligated lactone at 55 °C within 1 day.63 We have observed a similar equilibration for a dtbpe-ligated nickelalactone but only after activation with MeOTf; finally, the methyl cation was bound to the oxygen (Fig. 6A).57 It should be mentioned that Yamamoto et al. have reported as early as in 1987 the ring contraction of a 6-membered dcpeligated lactone to its β-branched 5-membered isomer.70 This ring contraction is expected to occur via β-H elimination, rotation of the acrylate moiety and reinsertion, as we confirmed by calculations for the methyl case.57 So far, it is not clear if the lack of reactivity of neutral nickelalactones and the activation by Lewis acids is solely a kinetic or thermodynamic effect or if both apply. Cationic lactones were accessible by protonation of an acrylic acid π-complex (dtbpe-ligand, H(Et2O)2BArF3 ),57 as was a η2-acrylate complex from a 4-membered nickelalactone (dcpe ligand, 2 d, room temperature) by deprotonation with the sterically hindered phosphazene base tertbutyliminotri(pyrrolidino)phosphorane (BTPP, cf. Fig. 6C).67 However, catalysis was not demonstrated in any of the mentioned cases. This might also be a consequence of the incompatibility of strong bases such as BTPP or Lewis acids and CO2 under reaction conditions.

3.2 Mechanistic Course of the β-H Elimination from Neutral Nickelalactones We have identified three pathways for the β-H elimination of neutral dtbpeligated nickelalactones that connect 5-membered (C1) and 4-membered

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(C2) species (cf. Fig. 7). One involves decoordination of the oxygen of the lactone to B1 followed by rotation of the acrylate moiety (TS–B1–C2: Gss{ ¼ 165kJ=mol), and is very similar to that described for methylated nickelalactones.57 This path corresponds to the reaction responsible for branching in olefin polymerization54,71 and is strongly disfavored if solvation effects are not taken into account. The other possibility is a path where the oxygen stays coordinated and the trigonal bipyramidal intermediate B2 is formed. The hydride in B2 is trans to the oxygen. From this intermediate, both carbons can insert into the NidH bond, to give either C2 or B1, where the latter barrier is higher (TS–B1–B2: G{ ¼ 113 kJ/mol). The third path involves formation of the κ1-O-coordinated hydride complex B3. Again, the acrylate can insert into the NidH bond to give either C2 or B1, the barrier now being slightly higher for C2 (TS–B3–C2: G{ ¼ 109 kJ/mol). Overall, the rearrangements, where oxygen stays coordinated to nickel (via either intermediate B2 or B3), are similar in activation barrier and are clearly preferred over the direct, zwitterionic pathway. Both

Figure 7 Mechanistic course of lactone interconversion and spontaneous rearrangement of nickelalactones.

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of these processes are computed to have activation barriers which are slightly too high to be accessible at room temperature. Finally, the question arises if and how an acrylic acid π-complex can be formed. Obviously, β-H elimination with ring contraction is not productive since the hydrogen is still bound to a carbon atom. Possibly, the investigated mechanisms are understood best by studying the reverse reaction, starting from the acrylic acid π-complex C3 (see Fig. 7). The π-bound acrylic acid can protonate the nickel atom to generate (intermediate) β-H-agostic complexes that rearrange to the 5- (TS–B1–C3: G{ ¼ 131 kJ/mol) and 4-membered (TS–C2–C3–I: Gs{ ¼ 138kJ=mol) lactones. Furthermore, acrylic acid can directly protonate its own carbon atoms, leading again— via β-H-agostic intermediates—to the 4- and 5-membered nickelalactones. This is not unreasonable for protonation of the β-carbon (leading to the 4-membered ring C2), as the transition state for proton transfer is a 5-membered ring (TS–C2–C3–II: G{ ¼ 154 kJ/mol). The third variant for hydrogen transfer to the oxygen is described best as an internal deprotonation of intermediate B3 where the carboxylate deprotonates the hydride to give the highly unstable intermediate B4. This is similar to the mechanism proposed for complexes bearing one DBU ligand.27 The transition state TS–B4–B5 (G{ ¼ 133 kJ/mol) refers to a rotation of the κ1-O-coordinated acrylic acid moiety to give the η2-C,Ocoordinated acrylic acid complex B5, which easily rearranges to the favored η2-C,C binding mode. In conclusion, β-H elimination giving rise to equilibria between neutral 4- and 5-membered lactones C1 and C2 might be feasible at room temperature. But the investigated pathways do not lead to the coordinated acrylic acid complex nor at elevated temperatures. According to these results, the lack of reactivity between acrylic acid complexes and lactones observed for bidentate nickel complexes, which in literature, has been attributed to a high kinetic barrier for β-H elimination and is more precisely due to a missing low energy path for a H-transfer from either the carbon or the nickel atom to oxygen.

3.3 Lactone Cleavage with Brønsted Bases The idea to cleave the kinetically inert nickelalactones with a base to make the overall reaction exergonic is easy and appealing: the reaction free energy in THF is around ΔG ¼ 21 kJ/mol. This means if a base is used as cleaving auxiliary, for quantitative deprotonation (ΔG  10 kJ/mol), the base

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should have a conjugate acid that is less acidic than acrylic acid by at least 6 units on the pKa scale. In water, this would mean that typical amine bases are just capable of making the overall reaction quantitative.72 Taking into account, concentration effects (high concentrations of base, ethylene and CO2, low concentration of acrylate) will make the reaction even more exergonic. In aprotic solvents, where acrylic acid is much less acidic relative to neutral bases, the situation becomes more difficult: In DMSO, protonated DBU is only about ΔpKa ¼ 1.6 less acidic than acetic acid (in water, the difference, ΔpKa, between acetic acid and acrylic acid is 0.5).69,73 The strongest neutral base that has been tested for the deprotonation of nickelalactones was BTPP (see above), the corresponding acid of which should then be approximately 5–6 pKa units less acidic then acetic acid in DMSO.70 Even though among the manifold mechanistic scenarios considered for the nickelacycle to nickel acrylate transformation, Buntine et al.’s DFT investigations27 discard pathways via deprotonation at the nickelalactone’s C1 and C2 carbons by a base-like DBU, experimentally in our hands strong anionic alkali metal bases such as alkoxides (NaOtBu) or hexamethyldisilylamides (NaHMDS) converted [Ni((CH2)2CO2)(dtbpe)] in PhCl at room temperature and within minutes in 90% yield to [Ni(η2-CH2]CHCO2Na)(dtbpe)] (cf. Fig. 8A). Not only is the fundamental thermodynamic limitation overcome by switching to Na-acrylate (ΔG ¼ 59 kJ/mol) but also is the kinetic barrier for metallacycle cleavage significantly reduced (98 kJ/mol). A similar reaction with NaOMe or aq. NaOH required prolonged reaction time and elevated temperature, while less basic reagents like NaOPh and others did not lead to conversion of lactone. This reaction is most likely affected by abstraction of one of the lactone’s fairly acidic α-protons next to the carbonyl group by a base. Interestingly, the cation was found to play a crucial role in this transformation: when instead of NaOMe the quarternary ammonium salt [NBu4OMe] was used, [Ni((CH2)2CO2)(dtbpe)] was not converted even after a significantly longer reaction time of 72 h (see Fig. 8A). Addition of an alternative sodium source to [NBu4OMe], such as sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate ðNaBAr3 F Þ, led again to a fast lactone cleavage to [Ni(η2-CH2] CHCO2Na)(dtbpe)] (47% yield in 72 h). The Lewis acidity and coordination ability of the sodium cation seem to be necessary to stabilize the carboxylate which is formed during the course of the elimination reaction. Significant activity of neutral bases was only observed in the case of the phosphazene superbase BTPP (pKa ¼ 28.35)70 in combination with NaBAr3 F as external Na+ source (40% yield after 72 h).

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Figure 8 Productive lactone cleavage with Brønstedt bases (A) and/or additives and final ligand exchange (B).

Calculations were carried out with the COSMO-RS model to account for the solvent effects of PhCl: for very reactive species like alkoxide monomers or dissociated Na+ and RO ions, deprotonation next to the lactone carbonyl group is clearly feasible. The formation of such ions, however, is endergonic by more than 100 kJ/mol, and cubane-like alkoxide tetramers40 have a computed free binding energy of 90 kJ/mol per monomer. We therefore conclude that (a) the base (both the alkali metal and the alkoxide) plays an important role in all lactone cleavage reactions studied so far, that (b) energy barriers are lower than those for β-hydride elimination mechanisms and for nickelalactone formation, and thus that (c) lactone formation and deprotonation in α-position to the lactone carbonyl group appear to be the most viable route from CO2 and ethylene to coordinated acrylate.

3.4 Acrylate/Ethylene Exchange The final mandatory step of the desired catalytic cycle is a ligand exchange reaction of in situ formed [Ni(η2-CH2]CHCO2Na)(dtbpe)] with ethylene to liberate Na-acrylate and to deliver [Ni(η2-C2H4)(dtbpe)] to reinitiate the cycle (Fig. 8B). Indeed, [Ni(η2-CH2]CHCO2H)(dtbpe)], a dimeric

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π-complex with η2-coordinated acrylic acid, failed to undergo an exchange reaction even at a pressure of as high as 30 bar ethylene, whereas the π-complex with η2-coordinated deprotonated acrylic acid [Ni(η2-CH2] CHCO2Na)(dtbpe)] gave by 1H NMR spectroscopy, an almost equimolar amount of Na-acrylate at only 8 bar of ethylene.74 Quantum mechanics (QM) calculations indicate that the reactivity difference between both π-complexes toward ethylene is likely a consequence of the endergonic character of this reaction in the case of [Ni(η2-CH2] CHCO2H)(dtbpe)] (ΔG ¼ 24 kJ/mol), due to a stronger stabilization by metal to ligand back donation. In contrast, the acrylate for ethylene exchange is exergonic (ΔG ¼ 12 kJ/mol) for the ionic complex [Ni(η2CH2]CHCO2Na)(dtbpe)] with computed barriers of 79 and 78 kJ/mol (vs. 88 and 99 kJ/mol for the protonated complex). In fact, dtbpe turned out to be a ligand enabling the isolation of all relevant intermediates and the study of all elementary reactions in combination with NaOtBu as suitable base, and this was the key to puzzle together the first catalytic cycle.

4. CATALYTIC REACTIONS Up to date, there are only three reports on catalytic systems for the carboxylation of alkenes with CO2. Two of three originate from our lab, among them the first catalytic system at all early in 2012. It comprises a two-stage setup: lactone formation in a CO2-rich regime and lactone cleavage in a CO2-poor environment (see Fig. 9).71 The separation into two mechanistic half-cycles was necessary as the alkoxides used for lactone cleavage irreversibly form fairly stable carbonic acid half-esters with CO275: while the conversion of [Ni(η2-C2H4)(dtbpe)] into nickelalactone [Ni((CH2)2CO2)(dtbpe)] proceeds quickly at a fairly high pressure of CO2 (40 bar), the transformation of [Ni((CH2)2CO2)(dtbpe)] into [Ni(η2CH2]CHCO2Na)(dtbpe)] and the final ligand exchange reaction of [Ni(η2-CH2]CHCO2Na)(dtbpe)] with ethylene to Na-acrylate were performed in its absence. Following this procedure in consecutive cycles, we were able to obtain a yield of 1,020% in Na-acrylate, which represents 10 catalytic turnovers (TON ¼ 10). This was the first example of a clearly catalytic reaction for this chemistry at all. The Na-acrylate formed in the reaction was finally separated by simple aqueous extraction and its identity was proven by HPLC-MS and NMR spectroscopy. Noteworthy, the only organic product found in the organic and aqueous phase was the targeted sodium acrylate. Even though the catalytic system is not yet efficient by

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Figure 9 The first catalytic cycle at all. The incompatibility of NaOtBu and CO2 required the separation of CO2-rich lactone formation and CO2-poor lactone cleavage.

industry standards (TOF ¼ 1/d), we consider it to be a very good starting point for further process optimization. A catalytic cycle was enabled as (a) we targeted the exergonic reaction to an acrylic acid salt instead the highly endergonic formation of acrylic acid itself (ΔG ¼ 2136a vs. 59 kJ/mol36a in THF) and (b) overcame the kinetically unfeasible β-H elimination from the nickelalactone (ΔG{ ¼ 103 kJ/mol for dtbpe)71 by its deprotonation in α-position with activation barriers that are feasible at room temperature. Recently, a catalytic cycle with a “hard” Lewis acid as cleavage agent in combination with an amine base to trap excess acid liberated from the anion of the Lewis acid has been disclosed by Vogt et al. and a TON of up to 21 was demonstrated (Fig. 10).76 Based on the seminal findings of Rieger et al., the authors assumed that a “hard” Lewis acid would compete with the carboxy group in a nickelalactone for binding the metal and thus induce a β-H elimination reaction (cf. Fig. 6E and F).31 This is especially true for the combination of the “hard” lithium cation and the soft iodide anion. Accordingly, without additionally added base such as triethylamine (50 equiv.), a significant amount of Li-propionate and Ni(II)I2 formed for a dppe-ligated lactone (Fig. 10A). Crucial for a catalytic reaction with this ligand was the addition of overstoichiometric amounts of Zn (25 equiv.) to reduce Ni(II)

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Figure 10 One-pot catalytic cycles leading to acrylates from CO2 and ethylene.

to Ni(0). This turns out to be a principal limitation to the reaction and enters the door to multiple catalyst deactivation pathways.57 Fine-tuning of ligand as well as CO2, and ethylene pressure yielded a TON of up to 21 within 3 days (dcpp, 50 equiv. Et3N, 100 equiv. Zn, PhCl, 50 °C). The addition of Zn was not mandatory for certain ligands (e.g., for dcpp, dcpb, dcpe), but the highest possible TON of 21 was only obtained with a reductant due to the already mentioned catalyst deactivation.57 Simultaneously, we revisited our approach for the two-stage reaction to find a base strong enough for the deprotonation of the Hoberg complexes but not so nucleophilic that it would deactivate under CO2 pressure: we found the surprisingly clean reaction of a dtbpe-ligated lactone to an acrylate π-complex in the presence of an excess (10 equiv.) of sodium 2-fluorophenoxide, whereas in the presence of only 2 equiv. of this base quantitative decarboxylation to the ethylene complex was the predominant reaction (see Figs. 8A and 10B). In contrast to alcoholates (which form carbonic acid half-esters),72 or strong N-bases such as DBU (which form carbaminates),77 the less nucleophilic phenoxide anion enables a reaction under CO2 pressure. Based on this finding, we found a robust nickel catalyst and reaction setup for the one-pot, direct carboxylation of activated alkenes, such as ethylene, styrenes, and 1,3-dienes with CO2 (Fig. 10B).78 The α,β-unsaturated carboxylic acid salts are formed in high yields and selectivity

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Figure 11 Scope and limitations of base (A) and substrate (B) for the catalytic one-pot carboxylation of olefins with CO2.

for the linear product (Fig. 11). Whereas monodentate phosphines (i.e., PPh3, PtBu3) and small bite-angle ligands (i.e., dtbpm, dppm) failed in the reaction with sodium 3-fluorophenoxide (50 equiv.) under CO2 and ethylene pressure (10 and 5 bar), the electron-rich P-stereogenic bisphosphine ligands, (S)-BINAPINE,79 (S,S,R,R)-TangPhos,80 (R,R,S,S)-DuanPhos,81 and (R,R)-BenzP*,82 gave high TONs up to 16. A polar, aprotic solvents like ethers (THF, dioxane) or even toluene led to higher reactivity; in chlorobenzene, oxidative addition to form Ni(II) turned out to be a limiting side reaction.66 Even though BenzP* was not the most active ligand at 80 °C, it turned out to be robust at elevated temperatures (TON 10 at 80 °C vs. 35 at 100 °C with sodium 2-fluorophenoxide). At 120 °C, the reductive decarboxylation of the nickelalactone was pronounced and the reaction became sluggish.63,71 The electronic and steric influence of substituents on the phenoxide’s core was crucial for reactivity (Fig. 11A): At 80 °C, sodium phenoxide (50 equiv. with respect to Ni(COD)2) as well as its derivatives bearing substituents with +I effect in ortho-position (i.e., sodium 2-methyl- and 2,6-dimethyl-phenoxide) gave comparably low TONs of ca. 4. In a series of fluorophenoxides (substituent with –I effect), the meta- or even better ortho-substituted derivative led to an increase in TON but not the parasubstituted one (TON 8, 10, and 2, respectively). This trend holds true for DuanPhos (TON 16 and 21, respectively), albeit at a lower level.

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The activity of the phenoxide seems to correlate with its pKa value in H2O with a maximum for sodium 2-fluorophenoxide (pKa 8.82).83 Obviously, a base successful in the reaction is sufficiently Brønstedt basic to deprotonate the nickelalactone, and of reasonably poor nucleophilicity to avoid its neutralization by reacting with CO2. Addition of finely powdered zinc (100 equiv.) had a beneficial effect on the reaction with BenzP* (TON 69 vs. 39). A huge excess of base (300 equiv.) increased TON further (107 vs. 69 for BenzP*) but was not pronounced for all of the ligands. Among a series of activated alkenes, i.e., styrenes, Michael acceptors, and 1,3-butadienes, CO2 was only incorporated once into the product (Fig. 11B). (E)-configured starting alkenes yielded exclusively (E,E)configured products. Thus, 1,3-butadiene and (E)-piperylene gave exclusively the linear monocarboxylated α,β,γ,δ-unsaturated carboxylic acids salts (TON 116 and 90, respectively). One or more so two alkyl substituents on the double bonds significantly reduced reactivity (cyclohexadiene, isoprene, 2,3-dimethylbutadiene, TONs from 7 to 64). In case of isoprene, where there are two terminal double bonds for a potential reaction with CO2, both products were formed in roughly equimolar amounts (TONs 7 and 9). The reaction of styrene and CO2 under optimized reaction conditions yielded only sodium (E)-cinnamate (TON 12).64 Electron-donating or -withdrawing groups in 4-position of the styrene frame (i.e., R ¼ OMe or CF3) led to a reactivity drop represented by a TON < 10. Terminal, unactivated alkenes or internal ones did not react to give the corresponding acrylates, most likely due to their lower tendency to form the initial BenzP*–Ni(0)–alkene complex (e.g., cyclopentene, norbornene, (Z)-3hexene), or the corresponding lactones (e.g., propylene, 1-hexene). Apart from ethylene (high TON of 107), the reaction also tolerates different functional groups. This is especially true for 1,3-butadienes: while n-butylvinylether gave only traces of sodium 3-methoxy-2-propenoate, the corresponding (E)-1-methoxy-1,3-butadiene yielded sodium (E,E)5-methoxy-2,4-pentadienoate with a reasonably high TON of 99. Similarly, an (E,Z)-mixture of methyl 2,4-pentadienoate yielded a mixture of muconic acid isomers with an overall TON of 75. 2-Vinylpyridine was the only substrate which led to the salt of an α-branched α,β-unsaturated acid, albeit the TON was low (TON 2). Out of the three different double bonds of the natural product myrcene, only the least substituted one was transformed with very high (E)-selectivity to the corresponding linear α,β-unsaturated carboxylic acid derivative (TON 11).

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5. CATALYSTS GONE ASTRAY The organometallic chemistry of nickel is “rich,” i.e., besides the outlined mechanistic course via catalytically active intermediates, there are manifold detours, dead-ends, decomposition pathways, and whole avenue of side reactions—depending on reaction conditions (temperature, pressure), solubility, ligands, catalyst precursors, and many more.28,65 For instance, do N-ligands like 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)8,84 and especially bipyridines34a yield metallalactones of poor solubility in THF at low reaction rates. They also seem to facilitate the disproportionation of CO2. Bidentate P-ligands, such as 1,2-bis (dicyclohexylphosphino)ethane (dcpe), bear a convenient 31P NMR spectroscopic handle for mechanistic studies and nickelalactones derived from those ligands show a much higher solubility in common solvents. But already Hoberg et al. reported that bidentate phosphine ligands, such as dcpe at Ni(0) complexes, require special attention on the stoichiometry of CO2 to avoid disproportionation of CO2 to the corresponding carbonato- and biscarbonyl complex.35f,85,86 According to our mechanistic understanding, none of those acts as catalytically active species without stoichiometric addition of a reductant (Fig. 12). We have already pointed out that uncrowded, good donor ligands such as dppe form tetracoordinated Ni(0) complexes such as Ni(dppe)2 in a fast reaction instead of the catalytically active ethylene complex. Small biteangle ligands such as dppm lead to binuclear tetracoordinated Ni(0) complexes. But even if the ethylene complexes form, instead of reacting with

Figure 12 Catalysts gone astray.

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CO2 they can incorporate further alkene molecules to yield 7- instead of 5-membered lactones,87 and finally pentenoates instead of acrylates. The 4-membered nickelalactones which originate via isomerization from their 5-membered congeners over time incorporate a second molecule of CO2 and thus lead to metallasuccinates,29 and incorporation of additional alkene and CO2 leads to long-chain dicarboxylic acids.88 The isolatable and wellcharacterized Ni(II)CO2 complex [Ni((CH2)2CO2)(dtbpe)] forms at temperatures above 55 °C in reasonable yield (50% after 6 h), besides significant amounts of the ligand oxidation products dtbpe-dioxide and its monoxide as already reported by Hillhouse et al.89 The alcohols liberated from the alkoxides during nickelalactone deprotonation can principally add to the various Ni(0) species present and from Ni(II)-bisalkoxide complexes.

6. CONCLUSIONS AND OUTLOOK After more than 30 years of intensive research efforts in industry and academia and limited progress in the field, in the last 4 years, various catalysts emerged for the reaction of alkenes and CO2 to acrylates. Even though for the moment published data are only available for nickel, we are confident that further catalysts based on other metals will develop. Key to the development of the first catalyst was an intensive mechanistic understanding, both experimentally and theoretically. We do not want to go that far to state that the development of a catalyst for this important transformation has been delayed for so many decades as most studies have focused on phenomenological observations, i.e., the presence of product. This is per se not a bad strategy for the optimization of existing reactions, but it was not helpful for the targeted development of an absolutely new reaction—despite of the remarkable chemical intuition of our congeners in this field. There is still a long way to go for an industrial application of the new methodology. Especially, space–time yield and catalyst efficiency have to be improved. Questions like separation of the catalytically active species from the product and regeneration remain major topics for future research as well as the search for other catalytically active metals—the dream has just started.

ACKNOWLEDGMENTS M.L. works at CaRLa of Heidelberg University, being cofinanced by Heidelberg University, the state of Baden-W€ urttemberg and BASF SE. Support from BMBF (grant 01RC1015A) is gratefully acknowledged. I want to thank multitude of postdoctoral fellows, Ph.D. students, and cooperation partners at CaRLa who have been working over the years on this

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challenging reaction. Their names are acknowledged in the corresponding references. With their intellectual sharpness, passion, enthusiasm, and resilience paired with hard word they have made our joint dream come true.

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