Hydrocarboxylation mechanism

Ans. (a) Yes hydrocarboxylation mechanism, C-H bond breaking precedes the rate-determining step, (b) Slight differences reflecting the difference in the rate of nucleophilic substitutions of CH3I and C2H5I. (c) No C-H bond cleavage not involved in mechanism. 

The reaction is also called hydrocarboxylation. According to a later modification, the alkene first reacts with carbon monoxide in the presence of the acid to form an acyl cation, which then is hydrolyzed with water to give the carboxylic acid.97 The advantage of this two-step synthesis is that it requires only medium pressure (100 atm). Aqueous HF (85-95%) gave good results in the carboxylation of alkenes and cycloalkenes.98 Phosphoric acid is also effective in the carboxylation of terminal alkenes and isobutylene, but it causes substantial oligomerization as well.99 100 Neocarboxylic acids are manufactured industrially with this process (see Section 7.2.4). The addition may also be performed with formic acid as the source of CO (Koch-Haaf reaction).101 102 The mechanism involves carbocation formation via protonation of the alkene97 103 [Eq. (7.10)]. It then reacts with carbon monoxide... 

The metal hydride mechanism has been written particularly for hydrocarboxylation reactions with a palladium catalyst.67,68 In the reactions of propene in the presence of (PhaP dCk, the acyl complex (18) was isolated from the reaction mixture, and also shown to be a catalyst for the reaction. 

Cobalt is the catalyst of choice for the hydrocarboxylation of butadiene to adipic esters.89 The reaction is carried out in two steps, the first of which yields methyl-3-butenoate. This product can either be isolated or carried on to dimethyl adipate at high temperatures (Scheme 11). The first hydrocarboxylation occurs by the metal carboxylate insertion mechanism (vide supra). 

The regioselectivity of the Pd(II)-catalysed hydrocarboxylation of styrene has been elucidated and two different mechanisms have been suggested to account for the differences... 

To date, mechanistic studies into the carbonylations of secondary alcohols with the same type of rhodium/RI catalyst system have used 2-propanol as a model substrate. At least part of the reason for this has been to minimize the expected complexities of the product analyses. The carbonylation of 2-propanol gives mixtures of n- and isobutyric acids. Two studies have been (24b, 32) reported with this system. The first of these (32) concluded that the reactivity could be described in terms of the same nucleophilic mechanism as has been described above, despite the fact that the reaction rates at 200°C were approximately 140 times faster than predicted by this type of chemistry (24b). Other data also indicated that this SN2-type reactivity was probably not the sole contributor to the reaction scheme. For example, the authors were not able to adequately explain either the effect of reaction conditions on product distribution or the activation parameters. They also did not consider the possible contribution of a hydrocarboxylation pathway, which is known to be extremely efficient in analogous systems (55). For these reasons, a second study into the carbonylation of 2-propanol was initiated (24b, 57). 

Scheme 8. Suggested mechanism for the contribution of hydrocarboxylation during the nickel-catalyzed carbonylation of higher alcohols.

The reaction of an aUcene (or aUcyne), CO, and H2O to directly produce a carboxylic acid is called Reppe carbony-lation chemistry or, more recently, hydrocarboxylation (see Reppe Reaction). An excellent review of palladium-catalyzed Reppe carbonylation systems has been published recently by Kiss, and coverage of this important material will not be repeated here. This catalytic reaction has been known for quite some time, although the stoichiometric Ni(CO)4-based carbonylation of acetylene was the first commercial carbonylation process implemented (equation 13). The extreme toxicity of Ni(CO)4, however, has limited practical applications (see Nickel Organometallic Chemistry). Co, Rh, and Pd catalysts have certainly replaced Ni(CO)4 in smaller-scale laboratory reactions, though for historical reasons a number of the fim-damental mechanisms discussed in this section are based on Ni(CO)4. 

The HCo(CO)4-catalyzed hydrocarboxylation of alkenes has also been known for a long time. The mechanism is analogous to that presented for hydroformyla-tion (Scheme 1), except that H2O is used instead of H2. Hydrocarboxylation is generally slower than hydro-formylation, and it is believed that the concentrations of the intermediate species are quite low relative to those seen for hydroformylation. Pyridine has a rateenhancing effect that is believed to be due to the facile cleavage of the (acyl)Co(CO)4 intermediate. This reaction forms [pyridine-acyl] + [Co(CO)4] , which is more rapidly hydrolyzed by water to form the product carboxylic acid and HCo(CO)4. 

One of the first mechanistic proposals for the hydrocarboxylation of alkenes catalyzed by nickel-carbonyl complexes came from Heck in 1963 and is shown in Scheme 24. An alternate possibility suggested by Heck was that HX could add to the alkene, producing an alkyl halide that would then undergo an oxidative addition to the metal center, analogous to the acetic acid mechanism (Scheme 19). Studies of Rh- and Ir-catalyzed hydrocarboxylation reactions have demonstrated that for these metals, the HX addition mechanism, shown in Scheme 24, dominates with ethylene or other short-chain alkene substrates. Once again, HI is the best promoter for this catalytic reaction as long as there are not any other ligands present that are susceptible to acid attack (e g. phosphines). 

Fig. 1-29. A mechanism of hydrocarboxylation with hydroxycarbonyl and iS-(hydroxycarbonyl)-alkyl complexes as intermediates (Rf = fluoroalkyl) (adapted from [214]).

Many catalytic reactions described in this book depend on carbon monoxide and hydrogen as feedstock chemicals. Hydroformylation (CO + H2) and simple hydrogenation (H2) are typical examples. In many cases carbon monoxide undergoes side reactions, among which the water-gas shift reaction is well studied in terms of the mechanism. This explains why carbon monoxide in the presence of water (e. g., aqueous media) can be used to hydrogenate substrates such as olefins, nitroaromatics, and other unsaturated organic compounds. In a number of industrial processes (e. g., the hydrocarboxylation of ethylene), however, this is an unwanted side reaction. 

Mechanism of RhCl(PPh3)3-catalyzed hydrogenation 14.3.3.1. Nonreactivity in hydrocarboxylation ... 

Side reactions such as double-bond migration and others are observed, similar to hydroformylation. Mechanistically, hydrocarboxylation is related to hydroformylation up until the metal acyl formation stage13. The presence of an acidic compound shifts the reaction towards formation of carboxylic acid derivatives and suppresses reductive elimination which forms aldehydes. The mechanism of the final steps is unclear13. 

Rhenium A hydrocarboxylation with high selectivity for anti-Markovnikov addition and predominant (Z)-enol ester product is mediated by ReBr(CO)5 (1 mol%, 110°C, 15 h) [168]. The 7i-activation mechanism proposed by the authors does not fit to the observed anti-Markovnikov selectivity. Iridium The precursor complex [ IrCl(cod) 2] (1 mol%) combined with P(OMe)3 (4 mol%) and Na2C03 (2 mol%) produces a catalyst that adds carboxylic acids to terminal alkynes (toluene, 100°C, 15 h) to give a mixture isomers with variable selectivities, although the Markovnikov product is usually formed in excess (ca 5 1) [169]. The complex []IrCl(cod) 2] (1 mol%) in the presence of Na2C03 (0.6 equiv) is also a catalyst for the transvinylation of vinylacetate with diverse alcohols [170]. 

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