Palladium carbonylative processes

Novel palladium catalysts show marked improvements in both yields and selectivities, compared to nickel carbonyl catalysts utilized in eadier commercial carbonylation processes (83,84). The palladium catalysts are also expected to be less hazardous. 

Carbonylation of methanol to acetic acid is fully discussed in Chapter 9. Another carbonylation process using a phosphine ligand to control the course of the reaction is a highly atom efficient route to the widely used monomer methyl methacrylate (Scheme 4.19). In this process the catalyst is based on palladium acetate and the phosphine ligand, bisphenyl(6-methyl-2-pyridyl) phosphine. This catalyst is remarkably (>99.5%) selective for the 2-carbonylation of propyne under the relatively mild conditions of <100 °C and 60 bar pressure. 

In reactions closely related to the carbonylation processes described above, the dimeric azoarene palladium complexes (78) can be transformed efficiently in two steps into 3-imino-2-phenylindazolines (Scheme 95).162... 

Under appropriate conditions, alcohols and amines can undergo an oxidative double carbonylation process, with formation of oxalate esters (Eq. 34), oxamate esters (Eq. 35) or oxamides (Eq. 36). These reactions are usually catalyzed by Pd(II) species and take place trough the intermediate formation of bis(alkoxycarbonyl)palladium, (alkoxycarbonyl)(carbamoyl)palladium or bis(carbamoyl)palladium complexes, as shown in Scheme 29 (NuH, Nu H = alcohol or amine) [227,231,267,293-300]. 

Since Wakamatsu serendipitously discovered amidocarbonylation while performing the cobalt-catalyzed hydroformyla-tion of olefins in 1971, this unique carbonylation reaction, affording a-amino acids directly from aldehydes, has been extensively studied.More recently, palladium-catalyzed processes have been developed to expand the scope of this reaction.The Pd-catalyzed amidocarbonylation has been applied to aldehydes,aryl halides, and imines. As a related reaction, lactamization " of aryl halides catalyzed by a rhodium complex has also been developed. 

The Suzuki coupling reaction of 2-haloselenophenes with boronic acids catalyzed by palladium salt is a new route to prepare 2-arylselenophenes and 2,5-diarylselenophenes in good yields. In addition, 2-arylselenophenyl ketones were also obtained by this protocol from 2-iodoselenophene and boronic acids via a carbonylative process [130],... 

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. 

Carbonylation of olefins and aryl halides to the corresponding esters or amides find in ILs an ideal media for the palladium-catalysed process. 

In the palladium-catalyzed carbonylation process, allyl formate, prepared by the reaction of allyl alcohol with formic acid, oxidatively adds to Pd(0) species with the C-0 bond cleavage to give allyl palladium formate. The CO insertion into the allylpalladium bond produces butenoyl palladium formate, which reductively ehminates butenoic formic anhydride with regeneration of the catalytically active Pd(0) species. Spontaneous decarbonylation of the mixed anhydride yields 3-butenoic acid, which isomerizes to 2-butenoic acid [61]. The process to give the butenoic acid proceeds only under CO pressure, suggesting that the CO insertion into the allyl-Pd bond is favored under CO pressure. When the reaction is carried out under normal pressure of CO, decarboxylation of the formate to give palladium hydride takes place. Reductive elimination of the allylpalladium hydride yields hydrogenation product of the allyl moiety [62]. 

Grigg, R., Kennewell, P. and Teasdale, A.J. (1992) Palladium catalysed cascade cycUsation-carbonylation processes. Rate enhancement by T1(I) salts. Tetrahedron Lett., 33, 7789-92. 

In 2000, a palladium-catalyzed decarboxylative carbonylation of 5-vinyloxazolidin-2-ones was studied by Knight and coworkers [44]. By a palladium-catalyzed decarboxylative carbonylation process, 5-vinyloxazohdin-2-ones, which are prepared... 

In addition, reductive elimination of palladium and nickel complexes to form esters (Equations 8.67 and 8.68), amides, and tiiioesters has been reported. -" The reductive eliminations of esters and amides were observed during mechanistic studies on the palladium-catalyzed formation of esters and amides from aryl halides, carbon monoxide, and alcohols or amines. This catalytic process is presented in Qiapter 17 (carbonylation processes). The reductive elimination of thioesters from nickel complexes were studied, in part, to understand the C-S bond-forming process of acetyl coenzyme A synthase. Prior to this work, an iron-mediated synthesis of p-lactams had been reported that appears to occur by reductive elimination to form the amide C-N bond of the lactam. ... 

Careful mechanistic studies on the carbonylation process have been reported by Yamamoto, including studies with isolated model compounds. - These studies have revealed several reaction pathways, and the particular pathway depends on whether the electrophile is an aryl or benzyl halide and whether the nucleophile is an amine or an alkoxide. The existing experimental data suggest that the latter pathway b in Scheme 17.29 involving insertion of CO into the palladium-aryl bond to form a benzoylpalladium halide intermediate occurs. These complexes have been isolated with PMe and PPhj as ligand and have been shown to form the ester product upon reaction with an alcohol and amine base and to form the amide products upon reaction with amine alone. 

As part of Pd gets washed to the solution, it remains unclear whether Pd adsorbed on carbon or Pd washed to the solution is the real catalyst of carbonylation. Probably both palladium species contribute to carbonylation process. By all means the mechanism of carbonylation seems to be similar to liquid-phase carbonylation in the presence of transition metal complexes (6). It involves the following steps ... 

Other organic processes facilitated by metal carbonyl clusters include a palladium carbonyl catalysed Diels-Alder reaction the selective reduction of aromatic nitro compounds using rhodium and ruthenium phosphine-carbonyls aza- and oxa-carbonylations of allyl phosphates by rhodium carbonyls Michael reactions of alkoxy-alkenones using iron... 

Before discussing the double carbonylation processes it may be helpful to understand the mechanism of the single carbonylation of aryl halides into carboxylic acid derivatives (Heck processes). The first step in the catalytic process is oxidative addition of an aryl halide to Pd(0) species formed from a catalyst precursor to yield an arylpal-ladium halide intermediate (A) in Scheme 1. Insertion of carbon monoxide into the aryl-palladium bond in A gives an acylpalladium halide complex (B). Attack of a nucleophile such as alcohol, amine, and water assisted by a base on the acylpalladium complex yields carboxylic ester, amide, and carboxylic acid, although details of the mechanism have not been unequivocally established. The palladium(O) species regenerated in the process further undergoes oxidative addition to carry out the catalytic cycle (Scheme 1). 

Detailed mechanistic studies on the elementary processes putatively involved in the catalytic double carbonylation process established that the latter mechanism involving the reductive elimination of the bis-acyl ligands most reasonably accounts for the features of the double carbonylation. Scheme 2 presents the proposed mechanisms for generation of the a-keto amide and amide in the reaction of an aryl halide with CO in the presence of a secondary amine and a catalytic amount of a palladium complex (yide infra). Table 1 summarizes the representative results of the double carbonylation with various aryl halides. 

Alkyl halides are usually considered to be less suitable for double carbonylation because of the possibility of the direct reaction of alkyl halides with nucleophiles and of instability of alkyl-transition metal complexes involved in the catalytic process. However, allylic halides were found amenable to double carbonylation promoted by zerovalent palladium complex. It is well known that allylic halides undergo ready oxidative addition with a Pd(0) species to produce Tj -allylpalladium halide complexes. Thus, it was reasoned that the double carbonylation process might be realized if CO insertion into the aUyl-palladium bond proceeds before attack of amine on the 17 -allylpaUadium halide takes place. On the basis of fundamental studies on the behavior of i7 -allylpalladium halide complexes with CO and secondary amines, double carbonylation processes of substituted aUyl halides to give a-keto amides in high yields have recently been achieved (Eqs. 15 and... 

As we mentioned in Sect. A, the first catalytic double carbonylation reaction was found to take place by using cobalt carbonyl complex as catalyst. In fact, there are a few other transition metals that can promote the double carbonylation process in addition to palladium complexes. The following is a brief description of the double carbonylation process promoted by transition metals other than palladium. 

---