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Carboxylation reactions involving carbon dioxide insertion into palladium–carbon σ-bonds
Journal of Organometallic Chemistry 751 (2014) 213e220
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Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem
Review
Carboxylation reactions involving carbon dioxide insertion into palladiumecarbon s-bonds Magnus T. Johnson, Ola F. Wendt* Centre for Analysis and Synthesis, Department of Chemistry, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2013 Received in revised form 23 August 2013 Accepted 26 August 2013
The catalytic incorporation of CO2 into value-added chemicals to form e.g. carboxylic acids and carbonates is highly interesting due to the abundance, non-toxicity and low cost of carbon dioxide. Currently, palladium is one of the most exploited transition metals in homogeneous catalysis and its reactivity with carbon dioxide has therefore been studied to a significant extent. All work in the field that is related to the reactivity of the PdeC bond with CO2 to form new CeC bonds is included in this review. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Palladium Carbon dioxide Insertion Green chemistry Carboxylic acids
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .213 Reactions with different types of PdeC bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .214 2.1. Reactions involving aryl PdeC bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 2.2. Reactions involving vinyl PdeC bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 2.3. Reactions involving allyl PdeC bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 2.4. Reactions involving alkyl PdeC bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
1. Introduction Carbon dioxide is non-toxic, highly abundant and cheap and could therefore serve as an excellent C1-feedstock [1]. With rising CO2 content in the atmosphere and CO2 trading systems in place there may even be an economic reward for using CO2 for chemicals production, but one should keep in mind that such production is less than 5% of the total use of fossil material, the lion share being used for energy production. Still, the use of CO2 could be environmentally beneficial, as a greener alternative to many other C1chemicals used today, like phosgene and carbon monoxide [2,3]. Reductive reactions could also play an important role in a future energy system where CO2 is recycled in a similar fashion as in
* Corresponding author. E-mail address: [email protected] (O.F. Wendt). 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.08.045
Nature. There are both thermodynamic and kinetic problems associated with the use of CO2 and to address the former it has to be reacted with high-energy co-reactants, such as dihydrogen or reactive hydrocarbons. To overcome the kinetic barrier, catalysis is clearly the most attractive solution and the one that Nature uses in e.g. the photosynthetic dark reactions [4]. Palladium is currently one of the most widely used metals in catalysis, not the least highlighted by its outstanding ability to cross-couple organic electrophiles and nucleophiles [5]. One key feature of palladium is its ability to shift in oxidation state between Pd(0)/Pd(II) with relative ease. This has led to significant interest in exploiting these benefits also for other types of catalytic applications. The work on carboxylation reactions involving CO2 and palladium has increased in intensity over the last several years, and here we provide a review on catalytic and stoichiometric reactions of CO2 where the carbon dioxide is inserted (or proposed to be inserted) into a PdeC
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Scheme 1.
Scheme 2.
bond. The work on palladium-mediated electroreductive carboxylation reactions of aryl-, vinyl- and benzyl halides and triflates does not necessarily include reactivity of CO2 with the PdeC bond, but it has strongly contributed to the field and is therefore included in this review. However, the important work involving insertion into the palladiumeoxygen bond to form especially (a) cyclic carbonates has been reviewed recently [6] and will thus not be reviewed here. We have chosen to sub-divide the review based on the type of hydrocarbyl ligand bonded to palladium in the reaction and/or proposed during catalysis. More general reviews of the field have appeared [2,3,7e9]. 2. Reactions with different types of PdeC bonds 2.1. Reactions involving aryl PdeC bonds 2.1.1. Thermal carboxylations The carboxylation of aromatic substrates by palladium complexes using CO2 was first reported by Fujiwara and coworkers in 1984 [10]. Through direct activation of CeH bonds using Pd(NO3)2 or Pd(OAc)2, anisole, furane, thiophene and benzene were successfully carboxylated at 150 C to give their corresponding carboxylic acids using 30 atm CO2 and 20 h reaction time. With Pd(NO3)2, the carboxylation of anisole resulted in the isomeric ratio 21/11/68 of the o-, m- and p-isomers. Reactions with furane and thiophene resulted in furan-2-carboxylic acid and thiophene-2carboxylic acid respectively. In the case of benzene and using tBuOOH an oxidant, the yield of the acid relative to palladium could be increased to 127% (using excess of benzene), indicating a catalytic potential of palladium in this reaction. When a mixture of
Pd(NO3)2 and Fe(NO3)3$9H2O was used, 66% benzoic acid, 8% biphenyl, 417% nitrobenzene and 60% picric acid were formed. Clearly, the present synthetic value is limited and similar systems using carbon monoxide are presently far more efficient in this reaction. One traditional drawback of using CO2 instead of highly toxic CO gas for the carboxylation of arenes is that aryl halides require prefunctionalisation by the use of lithium or magnesium reagents. However, Correa and Martín [11] recently carboxylated aryl bromides directly using 5 mol% Pd(OAc)2, 10 mol% of a substituted biphenyl monophosphine ligand (tBu-XPhos) and 2 equivalents ZnEt2 in dimethyl acetamide and hexane solvent, cf. Scheme 1. A variety of substrates were carboxylated in moderate to good yields, without affecting functional groups such as acetals, esters, anilines and thioethers. A specific example of an epoxidecontaining substrate was unexpectedly reported to be susceptible to carboxylation without opening of the ring. The solvent DMA appeared crucial to the reaction outcome as the hydrodehalogenation product or the ethyl-substituted arene were otherwise formed in various distributions. Diethyl zinc was used as a reducing agent and ultimately the zinc carboxylates were obtained which upon acidic workup liberated the carboxylic acids. The best results were obtained with 10 atm CO2. The authors propose a mechanism involving an oxidative addition of the aryl halide, CO2-insertion, transmetallation to eliminate the zinc carboxylate, and finally reductive elimination to regenerate the Pd(0) catalyst. The tentatively proposed step involving the insertion of CO2 into a Pd-aryl s-bond was not established previously. Notably, such insertions into PdePh bonds have been shown to be unviable [12]. Clearly, a detailed mechanistic investigation remains
Scheme 3.
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215
Reducing aryl triflates in the absence of palladium leads to OeS bond cleavage, forming phenoxide and ultimately phenol as the reduction product, even in the presence of CO2 [14]. By using palladium catalysis, Jutand found a successful route around this problem in that the AreO bond was cleaved instead, thus forming the aryl intermediate required for the carboxylation to take place, vide supra. In the palladium-catalysed reactions, the reduction consistently takes place through an overall two-electron transfer, while in the corresponding nickel-catalysed reactions [20] it proceeds through a one-electron transfer. 2.2. Reactions involving vinyl PdeC bonds Scheme 4.
to be done, but a control experiment with PhZnBr supposedly outrules this as the carboxylating species. 2.1.2. Electroreductive carboxylations Through the combination of electrochemical methods and catalysis, the reactivity of aryl halides and triflates with electrophiles such as CO2 can be reversed. An interesting reaction is the palladium-catalysed electroreductive carboxylation of aryl halides, which was first reported by Torii et al., cf. Scheme 2 [13]. Similarly, aryl triflates can be carboxylated in good yields, e.g. 2-naphtyl triflate can be converted to the corresponding carboxylic acid in 95% isolated yield [14]. Interesting to note is that since aryl triflates can be easily synthesised from phenols [15e18], this is a method to indirectly carboxylate phenols via the corresponding triflate. The result is a substitution of the hydroxyl group, as compared to the well-known KolbeeSchmitt reaction where the phenol is selectively carboxylated ortho to the hydroxyl group (never on the i-carbon). In a mechanistic investigation, Amatore, Jutand and coworkers [19] concluded that the electrocatalytic carboxylation of aryl halides proceeded through a two-electron reduction of ArPdX(PPh3) to the short-lived aryl anion which immediately formed the corresponding carboxylate by reaction with CO2. The full mechanism is shown in Scheme 3. The success of the reaction is based on the fact that the nucleophilic aryl anion can be obtained at much less negative reduction potentials using palladium than with the aryl halide itself. In its extension, this also means that electron-rich halides such as p-bromoanisole which require a large negative potential, even exceeding that of CO2, can be carboxylated via the milder reduction of the Pd(II)-aryl species. In essence, the kinetically demanding reduction of the aryl halide to the short lived anion is made possible by the preceding oxidative addition on palladium. The resulting Pd(II)-species turns out to have a much lower barrier for reduction to the aryl anion which immediately is trapped as the carboxylate. Under the right conditions, this process is faster than the competing reverse reaction e reductive elimination.
The carboxylation of vinyl triflates [21] can be achieved through electroreductive methods, cf. Scheme 4. This reaction is performed at room temperature and is faster than that of the corresponding aryl triflates, due to the faster oxidative addition to Pd0(PPh3)2Cle of this substrate [18]. A smaller negative potential was required for the reaction to occur with a palladium catalyst and CO2, as compared to the direct reduction of vinyl triflate to form the corresponding ketone. b-bromostyrene could also be carboxylated to form 2benzylmalonic acid in 36% yield via formation of the corresponding cinnamate, cf. Scheme 5 [13]. When cinnamate was used as the starting material, the yield was increased to 61%, indicating that the first step is limiting. 2.3. Reactions involving allyl PdeC bonds It is well-known that h1-allyl complexes of palladium are nucleophilic in character and reversely that h3-allyl ligands are electrophilic [22]. With CO2, the h1-allyl ligand is likely the most reactive of all s-bonded PdeC ligands thus far. The first report on the reactivity between CO2 and what is believed to be an allylic Pd intermediate was reported by Inoue and coworkers in 1976 [23] and later as a full report, cf. Scheme 6 [24]. This is the first reported catalytic CeC bond coupling between CO2 and an organic compound. They showed that under certain conditions, the previously reported dimerisation between isoprene or butadiene could be turned into a carboxylative version with a mixture of products. The solvent and ligand choice were of outmost importance although low yields were typical, and the 5-membered lactone was never obtained in more than 5% yield. The conditions were later somewhat modified which instead gave a 6-membered lactone, (E)-2-ethylidenehept-6-en-5-olide, in 64% yield with open chain esters and acids as the main by-products [25]. Dinjus and coworkers improved this reaction and obtained the 6-membered lactone with 72% selectivity using an immobilized bis-allyl palladium catalyst [26]. Similarly, Behr found that the palladium-catalysed carboxylative coupling of butadiene resulted in a 6membered lactone and proposed that the reaction was likely to
Scheme 5.
Scheme 6.
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Scheme 7.
Scheme 8.
occur through the h1,h3-bisallyl intermediate [27]. The work on transition metal-catalysed carbon dioxide chemistry to give lactones has been thoroughly and recently reviewed by Behr [28]. A few years later, Santi and Marchi also suggested the s-allyl moiety as the carboxylating Pd-intermediate based on the fact that the insertion was observed into a cis-k2P-1,2-bis(diphenylphosphino) ethane-ligated metal centre, to form the corresponding carboxylate [29]. Concurrently, Ito et.al. reported a similar reaction where 1,3butadiene was reacted with CO2 and a base using various palladium catalysts to typically result in a mixture of carboxylates [30]. Once the equilibrium between the h3- and h1-allyls was established [31,32], the carboxylation reaction was believed to proceed through the h1-complex. For example, (h1,h3-C3H5)2PdPR3 (R ¼ CH3, C6H11) was believed to be the reactive intermediate in the reaction with CO2 and it was found that carboxylation occurs readily at 20 to 30 C in toluene to form the corresponding butenoate [33]. The first equilibrium in Scheme 7 was previously reported by Shaw [34] and the carboxylation step by Ito et al. in 1980 [35]. Although the sintermediate was not isolated, this reactivity supports a mechanism in which the reactivity towards CO2 occurs at the g-carbon of the s-allyl complex. The necessity of the s-bonded intermediate was further supported by Behr who showed that reactivity of binuclear m-bis-h3allyl complex could be induced by the addition of one equivalent of phosphine, thus making one of the allylic moieties undergo transformation to h1-coordination and become susceptible towards carboxylation [36]. The earlier work on allyl carboxylation was later utilised by Nicholas and Shi in the catalytic carboxylation of tributyl allyl stannanes to produce the corresponding tin butenoates, Scheme 8 [37]. The minor a,b-isomer is supposedly formed during the carboxylation step, since no isomerisation was observed if the starting material or b,g-isomer was refluxed in THF with Pd(PPh3)4. With triphenyl allyl tin, only 30% conversion was reached to form
the corresponding isomers in 7:3 ratio, with no improvement after increased reaction times and temperatures. Notably, tetrabutyltin, tetraphenyltin, tributyl vinyl tin, and tributyl benzyl tin, were unreactive towards CO2 in the presence of Pd(PPh3)4. In a continuation of this work, the coupling capabilities of palladium could be combined with the Pd-allyl reactivity towards CO2, to generate allylic esters in good yields [38]. Using palladium(0) complexes such as Pd(PPh3)4 and Pd(PBu3)4, allyl chlorides were carboxylatively coupled with allyl stannanes, Scheme 9. However, the reaction was subject to significant scrambling, and almost a statistical product mixture was obtained. Neither with this procedure, benzyl-, phenyl-, vinyl-, nor phenylacetylene-tributylstannanes could be carboxylated to form the coupled products. The reaction likely proceeds through an oxidative addition of the allyl chloride followed by transmetallation to form the bis-allyl complex that equilibrates with the nucleophilic h1-isomer. Supposedly, this is then carboxylated, the product reductively eliminated and the catalyst regenerated. Recently, Feng et al. reported the previously unsuccessful carboxylative coupling between benzyl chloride and tributyl allyl stannane using palladium nanoparticles [39]. In this work, TBAB was required as an additive to avoid catalyst deactivation. Since the benzyl butenoate was formed exclusively as opposed to the allyl benzoate, the reaction likely proceeds through carboxylation at the allyl ligand and not at the proposed p-benzyl intermediate. Further, when the reaction is performed in the absence of CO2, the obtained allylic hexadiene implies fluxionality of the h3-benzyl intermediate, Scheme 10. The carboxylation occurs much faster thus explaining the regiospecificity of the reaction, in line with other results, vide infra. Johansson et al. reported the catalytic carboxylation of allyl stannanes using PCPPd complexes, cf. Scheme 11 [40]. In this system it was sufficient with 4 atm pressure instead of the previous 33 atm, albeit with lower yields. Furthermore, while the earlier system gave a product mixture selective towards the b-unsaturated
Scheme 9.
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217
Scheme 10.
Scheme 11.
carboxylate, this method was specific. For steric reasons, the catalytic procedure required phenylphosphine-substituted complexes, since the corresponding tert-butylphosphine-substituted complex proved unsusceptible towards transmetallation with tributyl allyl tin. In a stoichiometric study, the isolated tert-butylphosphine substituted PCPPd-allyl complex inserted CO2 quickly, but it was inactive under catalytic conditions. The only observable species under catalytic conditions was the butenoate complex, with the overall conclusion that the transmetallation is the rate-determining step. Later, Hazari and coworkers extended the work of Nicholas and Wendt to employ allyl boranes as nucleophiles and to obtain the corresponding butenoates in good yields [41]. In this work, Nheterocyclic carbenes were found to result in higher catalytic activity compared to the conventional phosphines. In addition, a range of stannanes substituted in the 2-allyl position could be carboxylated under milder conditions than previously reported.
In 2010, Wendt, Ahlquist and coworkers reported a computational and experimental, mechanistic investigation of the CO2carboxylation of the palladium-allyl ligand [42]. To unambiguously determine at which position of the allyl group the reaction occurs, a PCP palladiumecrotyl complex was synthesised. By stoichiometrically reacting CO2 with an 80:20 E/Z isomeric mixture, the previously proposed reactivity at C3 could be confirmed by isolation of a single product with a terminal olefin, as shown in Scheme 12. In an independent contribution, Hazari and coworkers reported the mechanism for the bis-allyl system previously described [43]. Through successful isolation and characterisation of the complex h1,h3-(2-methallyl)2PdPPh3, its reactivity towards CO2 could be investigated. Since no reactivity was observed within a series of complexes of mono-h3-allyl complexes, the prerequisite of h1-coordination was recognised. As shown in Scheme 13, DFT calculations provided an analogous reaction path as the one found in the pincer-ligated system, with the major difference that the
Scheme 12.
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Scheme 13.
highest point in the energy profile was the transition state for the electrophilic attack. For the pincer system the highest barrier was in the rearrangement of the ligand from olefin to O-bound and this could possibly be explained by the higher cost in energy to reorganise into a five-coordinate geometry in the vicinity of the sterically crowded pincer ligand. Another interesting result from the Hazari group, concerns the reactivity of an unusual bridging palladium(I)-allyl dimer, which stoichiometrically showed nucleophilic reactivity towards CO2, cf. Scheme 14 [44,45]. With phosphine ligands, the carboxylate complexes could not be transmetallated using tributyl allyl stannane. Reversely, when substituting with NHC-ligands, this became possible and the catalytic carboxylation of both tributyl allyl stannanes and pinacol allyl boranes were possible. The reaction with 2-methallyl is faster than with allyl ligands. With respect to reaction rate, the insertion into the 2-methallyl ligand occurs faster in the order PEt3 < PPh3 < NHC, and while the trend is generally similar, the allyl ligand does not undergo carboxylation with the PPh3-ligand. Presently, this is the mildest method of palladium-catalysed carboxylation of allylstannanes and -boranes, showing high activity at 1 atm of CO2 and room temperature, with 2.5% catalyst. In a combined experimental and computational study [46], it was found that both bridging allyl groups are required for carboxylation to take place, since neither the monocarboxylate product nor the bridging monoallyl with a chloride inserted CO2. This is similar to the mononuclear bisallyl-system that also requires both allyl groups present to render one of them nucleophilic. The mechanism is proposed to occur by nucleophilic attack at the CO2 by one of the allyl groups in a manner similar to the h1-allyls (vide supra). In the presence of a weakly coordinating ligand, the doubly bridged bis-allyl complex is believed to be in equilibrium with the
Scheme 14.
Scheme 15.
mononuclear Pd0 intermediate and a bis-allyl complex that reacts with CO2 before recombining to form the product. In a computational study on the mechanism by Lin and coworkers [47], a pathway involving nucleophilic attack at one of the terminal carbons of the bridging allyl ligand, forming a zwitter-ionic intermediate that is stabilised by agostic interactions, is proposed. This intermediate then rearranges via a new m-O,C-intermediate to form the product. However, the calculated barrier in this mechanism is higher than the values obtained by Hazari, both experimentally and computationally. Recently, Iwasawa and coworkers reported a selective method of producing b,g-unsaturated carboxylic acids from 1,3-dienes using a PSiP-pincer-ligated palladium catalyst and triethyl aluminium, Scheme 15 [48]. A variety of dienes were carboxylated with a few hundred turnovers. As opposed to the reaction in Scheme 6, this reaction does not generate any coupled dienes. A possible explanation to this could be that the pincer-ligated metal centre only has one open coordination site which prevents the formation of the bis-allyl complex that likely would be required as an intermediate for the diene coupling. Allenes can easily form allyl complexes by insertion into a palladium-hydride intermediate and a few papers report on the direct carboxylation of allenes. Allenes can also produce carboxylic esters and acids through palladium-catalysed carbonylation reactions but these will not be covered in this review [49]. The first report in this area describes the use of an adduct of bis(h3-allyl) palladium treated with bisdicyclohexylphosphinoethane [50] as a catalyst to carboxylate allene to result in a mixture of esters, a lactone, some oligomers and polymers, cf. Chart 1. Selectivity problems were encountered and the similarity of the lactone with the cyclocooligomerisation product of CO2 with alkynes suggested a path where the allene initially isomerises to propyne. This was ruled out using propyne as a starting material and no further mechanistic studies were performed. In the second report methoxyallene is reacted with CO2 in the presence of a catalyst derived from Pd2(dba)3$CHCl3 and 1-(2pyridyl)-2-(di-n-butylphosphino)ethane (nBu2PCH2CH2Py), as shown in Scheme 16 [51]. The reaction is stereo- and regiospecific and produces the pentanolide in 64% yield. Strong solvent effects were observed and acetonitrile appeared crucial to a successful outcome of the reaction. As a co-oligomer was not obtained with 1,2-pentadiene using the same conditions as in Ref. [50], the methoxy-substituent appears to play an important role in the reaction. More recently, Iwasawa and coworkers [52] reported an efficient carboxylation reaction catalysed by a PSiP-pincer-ligated palladium hydride complex, cf. Scheme 17. As nucleophilic h1-allyl ligands on palladium are typically highly reactive with CO2 towards forming carboxylates, allenes are thus also possible to carboxylate
Chart 1.
M.T. Johnson, O.F. Wendt / Journal of Organometallic Chemistry 751 (2014) 213e220
219
Scheme 16.
catalytically through a b-insertion reaction. After carboxylation, the PdeH species was regenerated using triethyl aluminium. Allenes of a wide variety were carboxylated in good yields, all >72%, and obtained as the diethylcarboxylate which after acidic workup gave the carboxylic acid. The proposed mechanism is shown in Scheme 18. The electrophilic attack occurs at the g-carbon as shown in the substitution pattern of the carboxylate although the full mechanism of the allylic carboxylation on pincer palladium complexes was elucidated later [42]. The possibility of carboxylation taking place at an allylic aluminium intermediate was dismissed as unlikely after metal exchange control experiments were performed. 2.4. Reactions involving alkyl PdeC bonds The chemistry of alkyl palladium complexes is rather limited and mainly includes methyl complexes as the longer alkyl groups are prone to undergo b-elimination. To our knowledge, the only currently stoichiometrically established insertion reaction of CO2 into an aliphatic palladiumecarbon s-bond was reported by Johansson and Wendt, cf. Scheme 19 [12]. By exploiting the high trans-influence exerted by the aryl ring on the methyl group, the PdeMe bond is sufficiently destabilised to react with CO2. This destabilisation is underpinned by the fact that this bond, at the time of publication, was the longest reported PdeC bond in a mononuclear complex.
Scheme 17.
Scheme 19.
The reaction was complete within 48 h at 80 C in C6D6. By employing dimethyl zinc to transmetallate the methyl group, the carboxylation reaction was made catalytic. A kinetic and computational study of the CO2-insertion shown in Scheme 19 was consistent with a mechanism in which the methyl group acts as a nucleophile on the CO2ecarbon. The DFT calculations further supported an SE2 mechanism in which the attack is performed on a completely non-coordinated CO2 [42]. A few years later, Dong reported a similar reaction using palladium and nickel complexes as precatalysts to carboxylate various aryl and alkyl zinc substrates [53]. They propose a different mechanism involving nucleophilic attack at a coordinated carbon dioxide of Aresta type [54]. In 1997 Fujiwara et al. reported the catalytic carboxylation of methane using CO [55]. The CO could be replaced by CO2 to produce acetic acid in 16.5 turnovers. Another example of a reaction involving palladium methyl complexes and carbon dioxide is the formation of hydrocarbonate complexes in C6D6/H2O mixtures. Pushkar and Wendt reported the reaction between (dppe)PdMe2 and (tmeda)PdMe2, respectively, with CO2 in wet benzene [56]. Rosenthal and coworkers recently reported similar reactivity using NHC-ligated dimethyl complexes [57]. In both of the above examples, reactions in water-free conditions were unsuccessful. This implies that the hydrocarbonate is formed either directly through reaction with carbonic acid formed in situ from water and CO2, or that the methyl complex reacts with water to form the corresponding hydroxide which consecutively would insert CO2. The carbonic acid route was shown more plausible since the hydroxide route would require a fast and reversible CeH activation of methane [56]. References
Scheme 18.
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Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem
Review
Carboxylation reactions involving carbon dioxide insertion into palladiumecarbon s-bonds Magnus T. Johnson, Ola F. Wendt* Centre for Analysis and Synthesis, Department of Chemistry, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2013 Received in revised form 23 August 2013 Accepted 26 August 2013
The catalytic incorporation of CO2 into value-added chemicals to form e.g. carboxylic acids and carbonates is highly interesting due to the abundance, non-toxicity and low cost of carbon dioxide. Currently, palladium is one of the most exploited transition metals in homogeneous catalysis and its reactivity with carbon dioxide has therefore been studied to a significant extent. All work in the field that is related to the reactivity of the PdeC bond with CO2 to form new CeC bonds is included in this review. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Palladium Carbon dioxide Insertion Green chemistry Carboxylic acids
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .213 Reactions with different types of PdeC bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .214 2.1. Reactions involving aryl PdeC bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 2.2. Reactions involving vinyl PdeC bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 2.3. Reactions involving allyl PdeC bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 2.4. Reactions involving alkyl PdeC bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
1. Introduction Carbon dioxide is non-toxic, highly abundant and cheap and could therefore serve as an excellent C1-feedstock [1]. With rising CO2 content in the atmosphere and CO2 trading systems in place there may even be an economic reward for using CO2 for chemicals production, but one should keep in mind that such production is less than 5% of the total use of fossil material, the lion share being used for energy production. Still, the use of CO2 could be environmentally beneficial, as a greener alternative to many other C1chemicals used today, like phosgene and carbon monoxide [2,3]. Reductive reactions could also play an important role in a future energy system where CO2 is recycled in a similar fashion as in
* Corresponding author. E-mail address: [email protected] (O.F. Wendt). 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.08.045
Nature. There are both thermodynamic and kinetic problems associated with the use of CO2 and to address the former it has to be reacted with high-energy co-reactants, such as dihydrogen or reactive hydrocarbons. To overcome the kinetic barrier, catalysis is clearly the most attractive solution and the one that Nature uses in e.g. the photosynthetic dark reactions [4]. Palladium is currently one of the most widely used metals in catalysis, not the least highlighted by its outstanding ability to cross-couple organic electrophiles and nucleophiles [5]. One key feature of palladium is its ability to shift in oxidation state between Pd(0)/Pd(II) with relative ease. This has led to significant interest in exploiting these benefits also for other types of catalytic applications. The work on carboxylation reactions involving CO2 and palladium has increased in intensity over the last several years, and here we provide a review on catalytic and stoichiometric reactions of CO2 where the carbon dioxide is inserted (or proposed to be inserted) into a PdeC
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Scheme 1.
Scheme 2.
bond. The work on palladium-mediated electroreductive carboxylation reactions of aryl-, vinyl- and benzyl halides and triflates does not necessarily include reactivity of CO2 with the PdeC bond, but it has strongly contributed to the field and is therefore included in this review. However, the important work involving insertion into the palladiumeoxygen bond to form especially (a) cyclic carbonates has been reviewed recently [6] and will thus not be reviewed here. We have chosen to sub-divide the review based on the type of hydrocarbyl ligand bonded to palladium in the reaction and/or proposed during catalysis. More general reviews of the field have appeared [2,3,7e9]. 2. Reactions with different types of PdeC bonds 2.1. Reactions involving aryl PdeC bonds 2.1.1. Thermal carboxylations The carboxylation of aromatic substrates by palladium complexes using CO2 was first reported by Fujiwara and coworkers in 1984 [10]. Through direct activation of CeH bonds using Pd(NO3)2 or Pd(OAc)2, anisole, furane, thiophene and benzene were successfully carboxylated at 150 C to give their corresponding carboxylic acids using 30 atm CO2 and 20 h reaction time. With Pd(NO3)2, the carboxylation of anisole resulted in the isomeric ratio 21/11/68 of the o-, m- and p-isomers. Reactions with furane and thiophene resulted in furan-2-carboxylic acid and thiophene-2carboxylic acid respectively. In the case of benzene and using tBuOOH an oxidant, the yield of the acid relative to palladium could be increased to 127% (using excess of benzene), indicating a catalytic potential of palladium in this reaction. When a mixture of
Pd(NO3)2 and Fe(NO3)3$9H2O was used, 66% benzoic acid, 8% biphenyl, 417% nitrobenzene and 60% picric acid were formed. Clearly, the present synthetic value is limited and similar systems using carbon monoxide are presently far more efficient in this reaction. One traditional drawback of using CO2 instead of highly toxic CO gas for the carboxylation of arenes is that aryl halides require prefunctionalisation by the use of lithium or magnesium reagents. However, Correa and Martín [11] recently carboxylated aryl bromides directly using 5 mol% Pd(OAc)2, 10 mol% of a substituted biphenyl monophosphine ligand (tBu-XPhos) and 2 equivalents ZnEt2 in dimethyl acetamide and hexane solvent, cf. Scheme 1. A variety of substrates were carboxylated in moderate to good yields, without affecting functional groups such as acetals, esters, anilines and thioethers. A specific example of an epoxidecontaining substrate was unexpectedly reported to be susceptible to carboxylation without opening of the ring. The solvent DMA appeared crucial to the reaction outcome as the hydrodehalogenation product or the ethyl-substituted arene were otherwise formed in various distributions. Diethyl zinc was used as a reducing agent and ultimately the zinc carboxylates were obtained which upon acidic workup liberated the carboxylic acids. The best results were obtained with 10 atm CO2. The authors propose a mechanism involving an oxidative addition of the aryl halide, CO2-insertion, transmetallation to eliminate the zinc carboxylate, and finally reductive elimination to regenerate the Pd(0) catalyst. The tentatively proposed step involving the insertion of CO2 into a Pd-aryl s-bond was not established previously. Notably, such insertions into PdePh bonds have been shown to be unviable [12]. Clearly, a detailed mechanistic investigation remains
Scheme 3.
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215
Reducing aryl triflates in the absence of palladium leads to OeS bond cleavage, forming phenoxide and ultimately phenol as the reduction product, even in the presence of CO2 [14]. By using palladium catalysis, Jutand found a successful route around this problem in that the AreO bond was cleaved instead, thus forming the aryl intermediate required for the carboxylation to take place, vide supra. In the palladium-catalysed reactions, the reduction consistently takes place through an overall two-electron transfer, while in the corresponding nickel-catalysed reactions [20] it proceeds through a one-electron transfer. 2.2. Reactions involving vinyl PdeC bonds Scheme 4.
to be done, but a control experiment with PhZnBr supposedly outrules this as the carboxylating species. 2.1.2. Electroreductive carboxylations Through the combination of electrochemical methods and catalysis, the reactivity of aryl halides and triflates with electrophiles such as CO2 can be reversed. An interesting reaction is the palladium-catalysed electroreductive carboxylation of aryl halides, which was first reported by Torii et al., cf. Scheme 2 [13]. Similarly, aryl triflates can be carboxylated in good yields, e.g. 2-naphtyl triflate can be converted to the corresponding carboxylic acid in 95% isolated yield [14]. Interesting to note is that since aryl triflates can be easily synthesised from phenols [15e18], this is a method to indirectly carboxylate phenols via the corresponding triflate. The result is a substitution of the hydroxyl group, as compared to the well-known KolbeeSchmitt reaction where the phenol is selectively carboxylated ortho to the hydroxyl group (never on the i-carbon). In a mechanistic investigation, Amatore, Jutand and coworkers [19] concluded that the electrocatalytic carboxylation of aryl halides proceeded through a two-electron reduction of ArPdX(PPh3) to the short-lived aryl anion which immediately formed the corresponding carboxylate by reaction with CO2. The full mechanism is shown in Scheme 3. The success of the reaction is based on the fact that the nucleophilic aryl anion can be obtained at much less negative reduction potentials using palladium than with the aryl halide itself. In its extension, this also means that electron-rich halides such as p-bromoanisole which require a large negative potential, even exceeding that of CO2, can be carboxylated via the milder reduction of the Pd(II)-aryl species. In essence, the kinetically demanding reduction of the aryl halide to the short lived anion is made possible by the preceding oxidative addition on palladium. The resulting Pd(II)-species turns out to have a much lower barrier for reduction to the aryl anion which immediately is trapped as the carboxylate. Under the right conditions, this process is faster than the competing reverse reaction e reductive elimination.
The carboxylation of vinyl triflates [21] can be achieved through electroreductive methods, cf. Scheme 4. This reaction is performed at room temperature and is faster than that of the corresponding aryl triflates, due to the faster oxidative addition to Pd0(PPh3)2Cle of this substrate [18]. A smaller negative potential was required for the reaction to occur with a palladium catalyst and CO2, as compared to the direct reduction of vinyl triflate to form the corresponding ketone. b-bromostyrene could also be carboxylated to form 2benzylmalonic acid in 36% yield via formation of the corresponding cinnamate, cf. Scheme 5 [13]. When cinnamate was used as the starting material, the yield was increased to 61%, indicating that the first step is limiting. 2.3. Reactions involving allyl PdeC bonds It is well-known that h1-allyl complexes of palladium are nucleophilic in character and reversely that h3-allyl ligands are electrophilic [22]. With CO2, the h1-allyl ligand is likely the most reactive of all s-bonded PdeC ligands thus far. The first report on the reactivity between CO2 and what is believed to be an allylic Pd intermediate was reported by Inoue and coworkers in 1976 [23] and later as a full report, cf. Scheme 6 [24]. This is the first reported catalytic CeC bond coupling between CO2 and an organic compound. They showed that under certain conditions, the previously reported dimerisation between isoprene or butadiene could be turned into a carboxylative version with a mixture of products. The solvent and ligand choice were of outmost importance although low yields were typical, and the 5-membered lactone was never obtained in more than 5% yield. The conditions were later somewhat modified which instead gave a 6-membered lactone, (E)-2-ethylidenehept-6-en-5-olide, in 64% yield with open chain esters and acids as the main by-products [25]. Dinjus and coworkers improved this reaction and obtained the 6-membered lactone with 72% selectivity using an immobilized bis-allyl palladium catalyst [26]. Similarly, Behr found that the palladium-catalysed carboxylative coupling of butadiene resulted in a 6membered lactone and proposed that the reaction was likely to
Scheme 5.
Scheme 6.
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Scheme 7.
Scheme 8.
occur through the h1,h3-bisallyl intermediate [27]. The work on transition metal-catalysed carbon dioxide chemistry to give lactones has been thoroughly and recently reviewed by Behr [28]. A few years later, Santi and Marchi also suggested the s-allyl moiety as the carboxylating Pd-intermediate based on the fact that the insertion was observed into a cis-k2P-1,2-bis(diphenylphosphino) ethane-ligated metal centre, to form the corresponding carboxylate [29]. Concurrently, Ito et.al. reported a similar reaction where 1,3butadiene was reacted with CO2 and a base using various palladium catalysts to typically result in a mixture of carboxylates [30]. Once the equilibrium between the h3- and h1-allyls was established [31,32], the carboxylation reaction was believed to proceed through the h1-complex. For example, (h1,h3-C3H5)2PdPR3 (R ¼ CH3, C6H11) was believed to be the reactive intermediate in the reaction with CO2 and it was found that carboxylation occurs readily at 20 to 30 C in toluene to form the corresponding butenoate [33]. The first equilibrium in Scheme 7 was previously reported by Shaw [34] and the carboxylation step by Ito et al. in 1980 [35]. Although the sintermediate was not isolated, this reactivity supports a mechanism in which the reactivity towards CO2 occurs at the g-carbon of the s-allyl complex. The necessity of the s-bonded intermediate was further supported by Behr who showed that reactivity of binuclear m-bis-h3allyl complex could be induced by the addition of one equivalent of phosphine, thus making one of the allylic moieties undergo transformation to h1-coordination and become susceptible towards carboxylation [36]. The earlier work on allyl carboxylation was later utilised by Nicholas and Shi in the catalytic carboxylation of tributyl allyl stannanes to produce the corresponding tin butenoates, Scheme 8 [37]. The minor a,b-isomer is supposedly formed during the carboxylation step, since no isomerisation was observed if the starting material or b,g-isomer was refluxed in THF with Pd(PPh3)4. With triphenyl allyl tin, only 30% conversion was reached to form
the corresponding isomers in 7:3 ratio, with no improvement after increased reaction times and temperatures. Notably, tetrabutyltin, tetraphenyltin, tributyl vinyl tin, and tributyl benzyl tin, were unreactive towards CO2 in the presence of Pd(PPh3)4. In a continuation of this work, the coupling capabilities of palladium could be combined with the Pd-allyl reactivity towards CO2, to generate allylic esters in good yields [38]. Using palladium(0) complexes such as Pd(PPh3)4 and Pd(PBu3)4, allyl chlorides were carboxylatively coupled with allyl stannanes, Scheme 9. However, the reaction was subject to significant scrambling, and almost a statistical product mixture was obtained. Neither with this procedure, benzyl-, phenyl-, vinyl-, nor phenylacetylene-tributylstannanes could be carboxylated to form the coupled products. The reaction likely proceeds through an oxidative addition of the allyl chloride followed by transmetallation to form the bis-allyl complex that equilibrates with the nucleophilic h1-isomer. Supposedly, this is then carboxylated, the product reductively eliminated and the catalyst regenerated. Recently, Feng et al. reported the previously unsuccessful carboxylative coupling between benzyl chloride and tributyl allyl stannane using palladium nanoparticles [39]. In this work, TBAB was required as an additive to avoid catalyst deactivation. Since the benzyl butenoate was formed exclusively as opposed to the allyl benzoate, the reaction likely proceeds through carboxylation at the allyl ligand and not at the proposed p-benzyl intermediate. Further, when the reaction is performed in the absence of CO2, the obtained allylic hexadiene implies fluxionality of the h3-benzyl intermediate, Scheme 10. The carboxylation occurs much faster thus explaining the regiospecificity of the reaction, in line with other results, vide infra. Johansson et al. reported the catalytic carboxylation of allyl stannanes using PCPPd complexes, cf. Scheme 11 [40]. In this system it was sufficient with 4 atm pressure instead of the previous 33 atm, albeit with lower yields. Furthermore, while the earlier system gave a product mixture selective towards the b-unsaturated
Scheme 9.
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217
Scheme 10.
Scheme 11.
carboxylate, this method was specific. For steric reasons, the catalytic procedure required phenylphosphine-substituted complexes, since the corresponding tert-butylphosphine-substituted complex proved unsusceptible towards transmetallation with tributyl allyl tin. In a stoichiometric study, the isolated tert-butylphosphine substituted PCPPd-allyl complex inserted CO2 quickly, but it was inactive under catalytic conditions. The only observable species under catalytic conditions was the butenoate complex, with the overall conclusion that the transmetallation is the rate-determining step. Later, Hazari and coworkers extended the work of Nicholas and Wendt to employ allyl boranes as nucleophiles and to obtain the corresponding butenoates in good yields [41]. In this work, Nheterocyclic carbenes were found to result in higher catalytic activity compared to the conventional phosphines. In addition, a range of stannanes substituted in the 2-allyl position could be carboxylated under milder conditions than previously reported.
In 2010, Wendt, Ahlquist and coworkers reported a computational and experimental, mechanistic investigation of the CO2carboxylation of the palladium-allyl ligand [42]. To unambiguously determine at which position of the allyl group the reaction occurs, a PCP palladiumecrotyl complex was synthesised. By stoichiometrically reacting CO2 with an 80:20 E/Z isomeric mixture, the previously proposed reactivity at C3 could be confirmed by isolation of a single product with a terminal olefin, as shown in Scheme 12. In an independent contribution, Hazari and coworkers reported the mechanism for the bis-allyl system previously described [43]. Through successful isolation and characterisation of the complex h1,h3-(2-methallyl)2PdPPh3, its reactivity towards CO2 could be investigated. Since no reactivity was observed within a series of complexes of mono-h3-allyl complexes, the prerequisite of h1-coordination was recognised. As shown in Scheme 13, DFT calculations provided an analogous reaction path as the one found in the pincer-ligated system, with the major difference that the
Scheme 12.
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Scheme 13.
highest point in the energy profile was the transition state for the electrophilic attack. For the pincer system the highest barrier was in the rearrangement of the ligand from olefin to O-bound and this could possibly be explained by the higher cost in energy to reorganise into a five-coordinate geometry in the vicinity of the sterically crowded pincer ligand. Another interesting result from the Hazari group, concerns the reactivity of an unusual bridging palladium(I)-allyl dimer, which stoichiometrically showed nucleophilic reactivity towards CO2, cf. Scheme 14 [44,45]. With phosphine ligands, the carboxylate complexes could not be transmetallated using tributyl allyl stannane. Reversely, when substituting with NHC-ligands, this became possible and the catalytic carboxylation of both tributyl allyl stannanes and pinacol allyl boranes were possible. The reaction with 2-methallyl is faster than with allyl ligands. With respect to reaction rate, the insertion into the 2-methallyl ligand occurs faster in the order PEt3 < PPh3 < NHC, and while the trend is generally similar, the allyl ligand does not undergo carboxylation with the PPh3-ligand. Presently, this is the mildest method of palladium-catalysed carboxylation of allylstannanes and -boranes, showing high activity at 1 atm of CO2 and room temperature, with 2.5% catalyst. In a combined experimental and computational study [46], it was found that both bridging allyl groups are required for carboxylation to take place, since neither the monocarboxylate product nor the bridging monoallyl with a chloride inserted CO2. This is similar to the mononuclear bisallyl-system that also requires both allyl groups present to render one of them nucleophilic. The mechanism is proposed to occur by nucleophilic attack at the CO2 by one of the allyl groups in a manner similar to the h1-allyls (vide supra). In the presence of a weakly coordinating ligand, the doubly bridged bis-allyl complex is believed to be in equilibrium with the
Scheme 14.
Scheme 15.
mononuclear Pd0 intermediate and a bis-allyl complex that reacts with CO2 before recombining to form the product. In a computational study on the mechanism by Lin and coworkers [47], a pathway involving nucleophilic attack at one of the terminal carbons of the bridging allyl ligand, forming a zwitter-ionic intermediate that is stabilised by agostic interactions, is proposed. This intermediate then rearranges via a new m-O,C-intermediate to form the product. However, the calculated barrier in this mechanism is higher than the values obtained by Hazari, both experimentally and computationally. Recently, Iwasawa and coworkers reported a selective method of producing b,g-unsaturated carboxylic acids from 1,3-dienes using a PSiP-pincer-ligated palladium catalyst and triethyl aluminium, Scheme 15 [48]. A variety of dienes were carboxylated with a few hundred turnovers. As opposed to the reaction in Scheme 6, this reaction does not generate any coupled dienes. A possible explanation to this could be that the pincer-ligated metal centre only has one open coordination site which prevents the formation of the bis-allyl complex that likely would be required as an intermediate for the diene coupling. Allenes can easily form allyl complexes by insertion into a palladium-hydride intermediate and a few papers report on the direct carboxylation of allenes. Allenes can also produce carboxylic esters and acids through palladium-catalysed carbonylation reactions but these will not be covered in this review [49]. The first report in this area describes the use of an adduct of bis(h3-allyl) palladium treated with bisdicyclohexylphosphinoethane [50] as a catalyst to carboxylate allene to result in a mixture of esters, a lactone, some oligomers and polymers, cf. Chart 1. Selectivity problems were encountered and the similarity of the lactone with the cyclocooligomerisation product of CO2 with alkynes suggested a path where the allene initially isomerises to propyne. This was ruled out using propyne as a starting material and no further mechanistic studies were performed. In the second report methoxyallene is reacted with CO2 in the presence of a catalyst derived from Pd2(dba)3$CHCl3 and 1-(2pyridyl)-2-(di-n-butylphosphino)ethane (nBu2PCH2CH2Py), as shown in Scheme 16 [51]. The reaction is stereo- and regiospecific and produces the pentanolide in 64% yield. Strong solvent effects were observed and acetonitrile appeared crucial to a successful outcome of the reaction. As a co-oligomer was not obtained with 1,2-pentadiene using the same conditions as in Ref. [50], the methoxy-substituent appears to play an important role in the reaction. More recently, Iwasawa and coworkers [52] reported an efficient carboxylation reaction catalysed by a PSiP-pincer-ligated palladium hydride complex, cf. Scheme 17. As nucleophilic h1-allyl ligands on palladium are typically highly reactive with CO2 towards forming carboxylates, allenes are thus also possible to carboxylate
Chart 1.
M.T. Johnson, O.F. Wendt / Journal of Organometallic Chemistry 751 (2014) 213e220
219
Scheme 16.
catalytically through a b-insertion reaction. After carboxylation, the PdeH species was regenerated using triethyl aluminium. Allenes of a wide variety were carboxylated in good yields, all >72%, and obtained as the diethylcarboxylate which after acidic workup gave the carboxylic acid. The proposed mechanism is shown in Scheme 18. The electrophilic attack occurs at the g-carbon as shown in the substitution pattern of the carboxylate although the full mechanism of the allylic carboxylation on pincer palladium complexes was elucidated later [42]. The possibility of carboxylation taking place at an allylic aluminium intermediate was dismissed as unlikely after metal exchange control experiments were performed. 2.4. Reactions involving alkyl PdeC bonds The chemistry of alkyl palladium complexes is rather limited and mainly includes methyl complexes as the longer alkyl groups are prone to undergo b-elimination. To our knowledge, the only currently stoichiometrically established insertion reaction of CO2 into an aliphatic palladiumecarbon s-bond was reported by Johansson and Wendt, cf. Scheme 19 [12]. By exploiting the high trans-influence exerted by the aryl ring on the methyl group, the PdeMe bond is sufficiently destabilised to react with CO2. This destabilisation is underpinned by the fact that this bond, at the time of publication, was the longest reported PdeC bond in a mononuclear complex.
Scheme 17.
Scheme 19.
The reaction was complete within 48 h at 80 C in C6D6. By employing dimethyl zinc to transmetallate the methyl group, the carboxylation reaction was made catalytic. A kinetic and computational study of the CO2-insertion shown in Scheme 19 was consistent with a mechanism in which the methyl group acts as a nucleophile on the CO2ecarbon. The DFT calculations further supported an SE2 mechanism in which the attack is performed on a completely non-coordinated CO2 [42]. A few years later, Dong reported a similar reaction using palladium and nickel complexes as precatalysts to carboxylate various aryl and alkyl zinc substrates [53]. They propose a different mechanism involving nucleophilic attack at a coordinated carbon dioxide of Aresta type [54]. In 1997 Fujiwara et al. reported the catalytic carboxylation of methane using CO [55]. The CO could be replaced by CO2 to produce acetic acid in 16.5 turnovers. Another example of a reaction involving palladium methyl complexes and carbon dioxide is the formation of hydrocarbonate complexes in C6D6/H2O mixtures. Pushkar and Wendt reported the reaction between (dppe)PdMe2 and (tmeda)PdMe2, respectively, with CO2 in wet benzene [56]. Rosenthal and coworkers recently reported similar reactivity using NHC-ligated dimethyl complexes [57]. In both of the above examples, reactions in water-free conditions were unsuccessful. This implies that the hydrocarbonate is formed either directly through reaction with carbonic acid formed in situ from water and CO2, or that the methyl complex reacts with water to form the corresponding hydroxide which consecutively would insert CO2. The carbonic acid route was shown more plausible since the hydroxide route would require a fast and reversible CeH activation of methane [56]. References
Scheme 18.
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