Alcohols hydrocarboxylation

As a unique reaction of Pd(II), the oxidative carbonylation of alkenes is possible with Pd(ll) salts. Oxidative carbonylation is mechanistically different from the hydrocarboxylation of alkenes catalyzed by Pd(0), which is treated in Chapter 4, Section 7.1. The oxidative carbonylation in alcohol can be understood in the following way. The reaction starts by the formation of the alkoxy-carbonylpalladium 218. Carbopalladation of alkene (alkene insertion) with 218 gives 219. Then elimination of /3-hydrogen of this intermediate 219 proceeds to... 

It has been known since the early 1950s that butadiene reacts with CO to form aldehydes and ketones that could be treated further to give adipic acid (131). Processes for producing adipic acid from butadiene and carbon monoxide [630-08-0] have been explored since around 1970 by a number of companies, especially ARCO, Asahi, BASF, British Petroleum, Du Pont, Monsanto, and Shell. BASF has developed a process sufficiendy advanced to consider commercialization (132). There are two main variations, one a carboalkoxylation and the other a hydrocarboxylation. These differ in whether an alcohol, such as methanol [67-56-1is used to produce intermediate pentenoates (133), or water is used for the production of intermediate pentenoic acids (134). The former is a two-step process which uses high pressure, >31 MPa (306 atm), and moderate temperatures (100—150°C) (132—135). Butadiene,... 

Most ring syntheses of this type are of modern origin. The cobalt or rhodium carbonyl catalyzed hydrocarboxylation of unsaturated alcohols, amines or amides provides access to tetrahydrofuranones, pyrrolidones or succinimides, although appreciable amounts of the corresponding six-membered heterocycle may also be formed (Scheme 55a) (73JOM(47)28l). Hydrocarboxylation of 4-pentyn-2-ol with nickel carbonyl yields 3-methylenetetrahy-drofuranone (Scheme 55b). Carbonylation of Schiff bases yields 2-arylphthalimidines (Scheme 55c). The hydroformylation of o-nitrostyrene, subsequent reduction of the nitro group and cyclization leads to the formation of skatole (Scheme 55d) (81CC82). 

The acid-catalyzed hydrocarboxylation of olefins (the Koch reaction) can be performed in a number of ways.565 In one method, the olefin is treated with carbon monoxide and water at 100 to 350°C and 500 to 1000 atm pressure with a mineral-acid catalyst. However, the reaction can also be performed under milder conditions. If the olefin is first treated with CO and catalyst and then water added, the reaction can be accomplished at 0 to 50°C and 1 to 100 atm. If formic acid is used as the source of both the CO and the water, the reaction can be carried out at room temperature and atmospheric pressure.566 The formic acid procedure is called the Koch-Haaf reaction (the Koch-Haaf reaction can also be applied to alcohols, see 0-103). Nearly all olefins can be hydrocarboxylated by one or more of these procedures. However, conjugated dienes are polymerized instead. 

Cyclization of olefinic acids 5-23 Hydrocarboxylation of unsaturated alcohols... 

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. 

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. 

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). 

Finally, it should be apparent that the nature of the reaction media has a profound effect on the reactivity of this system, and that, particularly for secondary alcohols, generation of olefins (and metal hydrides) occurs quite easily. Since this involves only the organic equilibria, this situation is not unique to rhodium chemistry. Unless great care has been taken to eliminate possible contributions of the hydrido/olefin pathway to the total reaction scheme, then, the hydrocarboxylation route should probably be considered to be a contributing reaction with other catalytic systems. 

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

The salt production can be circumvented by performing the selective Pd/ tppts-catalysed carbonylation of benzyl alcohol in an acidic aqueous biphasic system (Fig. 1.36) [106]. This methodology was also applied to the synthesis of ibuprofen (see earlier) by biphasic carbonylation of l-(4-isobutylphenyl)ethanol [107] and to the biphasic hydrocarboxylation of olefins [108]. 

Asymmetric Hydrocarboxylation. The title reagent was used in the first example of an asymmetric hydrocarboxylation (eq 1). With the a-methylstyrene, the straight chain isomer was formed. The regiospecificity was much less pronounced, however, for other alkenic substrates. The influence of some reaction variables on the reaction shown in eq 1 was studied. For example, the presence of a solvent such as THF or benzene, the alcohol source, the effect of CO pressure, the effect of substitution on the phenyl ring, the PdC /DIOP molar ratio, or the presence of PPha along with DIOP, were varied to improve the optical yield. ... 

Hydrocarboxylation of the Ce-Cs a-olefins with cobaltcarbonyl/pyridine catalysts at 200 °C and 20 MPa gives predominantly the linear carboxylic acids. The acids and their esters are used as additives for lubricants. The Ce-Cio a-olefins are hydroformylated to odd-numbered linear primary alcohols, which are converted to polyvinylchloride (PVC) plasticizers with phthalic anhydride. Oligomerization of (preferably) 1 -decene, applying BF3 catalysts, gives oligomers used as synthetic lubricants known as poly-a-olefins (PAO) or synthetic hydrocarbons (SHC) [11, 12]. The C10-C12 a-olefins can be epoxidized by peracids this opens up a route to bifunctional derivatives or ethoxylates as nonionic surfactants [13]. 

The synthesis of an ester by addition of carbon monoxide and an alcohol to an alkene, i.e. hydroesterification, has a fairly obvious relationship to the hydrocarboxylation described in Section 4.1.5, where water replaces the alcohol and a carboxylic acid is formed. Not surprisingly, therefore, the same types of catalysts, [Co2(CO)s], [H2PtCl6] and [Pd(PPh3)2Ch], are effective for both reactions. Unfortunately, the reaction usually requires very high pressures (200 bar) and necessitates the use of an autoclave. By varying the catalyst and reaction conditions a variety of linear, branched and cyclic alkenes can be carbonylated under these conditions to give the product in good yield (equation 32). Improved selectivity to the linear ester can be obtained by addition of SnCU to the catalyst system. 

Alkenes undergo hydrocarboxylation and bisalkoxycarbonylation by varying the other reaction components besides PdClj and CO. Using a trisulfonated triphenylphosphine the hydrocarboxylation can be performed in a biphasic system. The addition of PhjP=S and CuCl and presence of oxygen in an alcoholic solvent are conducive to bisalkoxycarbonylation resulting in the formation of succinic esters. ... 

Hydrocarboxylation is the formal addition of hydrogen and a carboxylic group to double or triple bonds to form carboxylic acids or their derivatives. It is achieved by transition metal catalyzed conversion of unsaturated substrates with carbon monoxide in the presence of water, alcohols, or other acidic reagents. Ester formation is also called hydroesterification or hydrocarb(o)alkoxylation . The transition metal catalyst precursors are nickel, iron or cobalt carbonyls or salts of nickel, iron, cobalt, rhodium, palladium, platinum, or other metals4 5. 

---