Hydrocarboxylation reactions

If cobalt carbonylpyridine catalyst systems are used, the formation of unbranched carboxylic acids is strongly favored not only by reaction of a-olefins but also by reaction of olefins with internal double bonds ( contrathermo-dynamic double-bond isomerization) [59]. The cobalt carbonylpyridine catalyst of the hydrocarboxylation reaction resembles the cobalt carbonyl-terf-phos-phine catalysts of the hydroformylation reaction. The reactivity of the cobalt-pyridine system in the hydrocarboxylation reaction is remarkable higher than the cobalt-phosphine system in the hydroformylation reaction, especially in the case of olefins with internal double bonds. This reaction had not found an industrial application until now. 

The hydroformylation reaction has been the subject of excellent reviews (for example I, 6-8) therefore, the object of this particular treatise is not to provide comprehensive coverage of all aspects. The basic chemistry is presented, along with recent developments of interest as reported in the literature, although not in chronological order. Stereochemical studies (6) are included only when pertinent to another point under consideration. Carbonylations or hydrocarboxylation reactions which produce ketones, esters, acids, esters, or amides are not included (/). Also not included is the so-called Reppe" synthesis, which is represented by Eq. (1). 

The hydrocarboxylation reaction of alkenes and alkynes is one which utilizes carbon monoxide to produce carboxylic acid derivatives. The source of hydrogen is a protic solvent (equation 35) dihydrogen is not usually added to the reaction. There are a number of variations to this reaction, since the solvent can be water, alcohols, amines, acids, etc. The catalysts can be Group VIII-X transition metals, but cobalt, rhodium, nickel, palladium and platinum have found the most use. 

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. 

Although the hydrocarboxylation of 1 -alkenes is not of interest for the synthesis of more complex organic molecules, the information obtained from the hydrocarboxylation reactions with various catalysts can be applied to the synthesis and reactions of other alkene substrates. 

The hydrocarboxylation reaction of simple alkenes and alkynes in the presence of primary or secondary amines or ammonia yields amides (equations 48 and 49). The fact that formamides can be used in place of amines suggests that a key intermediate in the reaction is the hydride metal carboxamide (20). 

Other asymmetric synthetic processes used for the manufacturing of (S)-(+)-naproxen can also be applied to the production of (S)-(+)-ibuprofen these include the Rh-phosphite catalyzed hydro-formylation,37 hydrocyanation,25 and hydrocarboxylation reactions.24... 

There are some relatively small-volume but value-added chemicals that are commercially manufactured by carbonylation or hydrocarboxylation reactions. A few examples with some details are given in Table 4.2. 

TABLE 4.2 Chemicals Manufactured by Carbonylation or Hydrocarboxylation Reactions... 

Practically, all of the above reactions have been realized, with different metals and conditions. In determining the scope of this review, we have attempted to focus our attention on the nature of the transformations at the metal center, especially with regard to oxidation state and formation of the initial alkyl-, alkoxy-, or carboalkoxy-metal bond from saturated precursors. Therefore, while it appears that hydrocarboxylation reactions make some contribution to the total reactivity in a variety of alcohol carbonylation systems, we feel that the mechanistic aspects of this topic would be better covered separately. So, except for noting where this chemistry makes probable contributions, it will not be discussed here. Similarly, homologation reactions, which are believed to usually proceed by way of aldehyde intermediates, will be discussed only as they pertain to the incorporation of the CO into the metal-carbon bonds, that is, the factors governing the subsequent hydrogenation reactions will not be covered. 

The hydrocarboxylation reactions discussed above have been proposed to involve direct addition of water to the metal center prior to elimination of the product, analogous to the oxidative addition of hydrogen to a metal center at the end of a hydroformylation catalytic cycle. Another class of hydrocarboxylation reactions is more analogous to the haUde-promoted Monsanto acetic acid process, where one has a reductive elimination of an acyl halide species that is rapidly hydrolyzed with free water to generate the carboxylic acid and HX. 

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). 

Hydrocarboxylation reactions generally do not have very high regioselectivities when carried out with C4 or higher alkenes, due to alkene isomerization side reactions catalyzed by both acid and metal. Thus, many of the reactions done industrially involve substrates such as acetylene and ethylene where isomerization side reactions will not present any problems. 

Moreover, hydrocarboxylation reactions have been expanded to include functionalized olefins as substrates, leading to arylpropionic acids [16], fluorinated acids [17], silylated esters [18], and y5-amino acids [19] as products (Structures 1-6). 

Thus, ibuprofen and naproxen can be synthesized in 64-89% yield and 83-91 % optical purity via palladium(Il) chloride catalyzed hydrocarboxylation of the corresponding styrenes in the presence of (-)-(/ )- or ( + )-(5)-2,2 -(l,l -binaphthyl)phosphoric acid. The hydrocarboxylation reaction is completely regiospeciftc and proceeds at room temperature under 1.01 bar carbon monoxide pressure25. 

Hydrocarboxylation of acetylene. Italian chemists have prepared a very active catalyst for hydrocarboxylation reactions from palladium(II) chloride and 2 eq. of thiourea. When acetylene, carbon monoxide, and oxygen are passed into a solution of the catalyst in methanol at room temperature, dimethyl maleate is... 

The rate of the hydrocarboxylation reaction depends on the nature of the olefin, just as in the case of the hydroformylation reaction. The rate of formation of acids from olefins decreases in the following order 1-hexene >... 

The HSAB concept can also be applied to the related hydrocarboxylation reaction, in which carboxylic acids are produced from olefins, CO, water, and small amounts of hydrogen. With hard cobalt/tert-amine catalysts, the products are the hard carboxylic acids, whereas rhodium catalysts give mainly aldehydes. Rhodiiun makes the intermediate acyl complexes softer, and in the subsequent elimination step H2, which is softer than H2O, gives aldehyde as product. 

The q -(Si-H)Pd(0) 43 was found to catalyze the hydrocarboxylation reaction of allenes, indicating that 43 worked as the palladium hydride complex 13 in solution via reversible oxidative addition/reductive elimination of the Si-H bond after dissociation of PPhj (Scheme 9.12) [21]. This result prompted us to investigate a... 

Scheme 18 Rh(I) mono- and bimetallic catalysts (46, 49, 50, 51) used for the intramolecular hydrocarboxylation reaction to generate lactones...

These hydrocarboxylation reactions have been conducted v ith terminal alkenes containing a series of functional groups. As summarized in Equation 17.40, these reactions occur in the presence of keto, cyano, formyl, acetoxy, carboxylic acid, and amide functionality. In addition, these reactions have been conducted with polybutadienes containing pendant vinyl groups to form polymers possessing pendant carboxylic acid functionality (Equation 17.41). 

Cobalt, nickel, iron, ruthenium, and rhodium carbonyls as well as palladium complexes are catalysts for hydrocarboxylation reactions and therefore reactions of olefins and acetylenes with CO and water, and also other carbonylation reactions. Analogously to hydroformylation reactions, better catalytic properties are shown by metal hydrido carbonyls having strong acidic properties. As in hydroformylation reactions, phosphine-carbonyl complexes of these metals are particularly active. Solvents for such reactions are alcohols, ketones, esters, pyridine, and acidic aqueous solutions. Stoichiometric carbonylation reaction by means of [Ni(CO)4] proceeds at atmospheric pressure at 308-353 K. In the presence of catalytic amounts of nickel carbonyl, this reaction is carried out at 390-490 K and 3 MPa. In the case of carbonylation which utilizes catalytic amounts of cobalt carbonyl, higher temperatures (up to 530 K) and higher pressures (3-90 MPa) are applied. Alkoxylcarbonylation reactions generally proceed under more drastic conditions than corresponding hydrocarboxylation reactions. 

The catalytic hydrocarbonylation and hydrocarboxylation of olefins, alkynes, and other TT-bonded compounds are reactions of important industrial potential.Various transition metal complexes, such as palladium, rhodium, ruthenium, or nickel complexes, have widely been used in combination with phosphines and other types of ligands as catalysts in most carbonylation reactions. The reactions of alkenes, alkynes, and other related substrates with carbon monoxide in the presence of group VIII metals and a source of proton affords various carboxylic acids or carboxylic acid derivatives.f f f f f While many metals have successfully been employed as catalysts in these reactions, they often lead to mixtures of products under drastic experimental conditions.f i f f f In the last twenty years, palladium complexes are the most frequently and successfully used catalysts for regio-, stereo-, and enantioselective hydrocarbonylation and hydrocarboxylation reactions.f ... 

The hydroesteriflcation and hydrocarboxylation reactions catalyzed by transition metals and their complexes demonstrate the versatility of these processes. Palladium complexes are particularly useful catalysts for these reactions. Indeed, the hydrocarbonylation of a large variety of substrates has been selectively achieved by using palladium catalysts in homogeneous, heterogeneous, or biphasic systems. The results obtained for the thiocar-bonylation showed excellent regio- and stereoselectivity control for most substrates. It is anticipated that the prochiral nature of most of the reactants and products will open a new venue for the asymmetric synthesis of acids, esters, and thioesters of substantial development in the future. 

Unless stated otherwise, the substrates were used in oxidation reactions. Substrates were used in hydrocarboxylation reactions. 

Interestingly, these hydrocarboxylation reactions also occur to some extent in metal-free systems, but the reaction efficiency can be improved significantly by the use of metal catalysts or promoters [18]. Among the variety of different transition metal catalysts, multicopper(II) compounds were usually the most active ones [18, 20], leading to product yields that are circa two to five times superior to those in the metal-free systems. The water-soluble tefracopper(II) complex [Cu4(/x4-0)(/u,3-tea)4 ( u,3-BOH)4][BF4]2 (6) was formerly used as a model catalyst in the hydrocarboxylations of C2-Q alkanes [18, 31]. Since then, the reactions have been optimized further [19-21] and extended to other alkanes and multicopper catalysts, namely including the dimer 2 [22], the trimer 5 [13], the tetramer 7 [14], and the polymers 11 [12], 12 [12], 13 [14], and 15 [15] (Table 3.1). Interestingly, in contrast to alkane oxidation, the hydrocarboxylation reactions do not require an acid cocatalyst. 

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