Decarbonylation oxidative addition intermediates

Dissociation of carbon monoxide from an acylcobalt tetracarbonyl yield a 16-electron acylcobalt tricarbonyl complex which is the active intermediate in the decarbonylation, oxidative addition, and in various ligand substitution reactions. 

An intermediate acylnickel halide is first formed by oxidative addition of acyl halides to zero-valent nickel. This intermediate can attack unsaturated ligands with subsequent proton attack from water. It can give rise to benzyl- or benzoin-type coupling products, partially decarbonylate to give ketones, or react with organic halides to give ketones as well. Protonation of certain complexes can give aldehydes. Nickel chloride also acts as catalyst for Friedel-Crafts-type reactions. 

Since formamide is a weak nucleophile, the use of imidazole or 4-dimethylaminopyridine (DMAP) is necessary for acyl transfer to formamide via an activated amide (imidazolide) or acylpyridinium ion. As Scheme 22 illustrates, the reaction starts with the oxidative addition of aryl bromide 152 to Pd(0) species, followed by CO insertion to form acyl-Pd complex 154. Imidazole receives the aroyl group to form imidazolide 155 and liberates HPdBr species. Then, imidazolide 155 reacts with formamide to form imide 156. Finally, decarbonylation of imide 156 gives amide 157. In fact, the formations of imidazolide intermediate 155 and imide 156 as well as the subsequent slow transformation of imide 156 to amide 157 by releasing CO were observed. This mechanism can accommodate the CO pressure variations observed during the first few hours of aminocarbonylation. When the reaction temperature (120 °C) was reached, a fast drop of pressure occurred. This corresponds to the formation of the intermediary imide 156. Then, the increase of pressure after 3 h of reaction was observed. This phenomenon corresponds to the release of CO from imide 156 to form amide 157. ... 

There is general agreement on the mechanism for the stoichiometric decarbonylation of acid chlorides (9,14,15,16). The overall mechanism is shown by Equation set 2 where X = Cl. The stoichiometric decarbonylation reaction results from initial oxidative addition of the acid chloride to RhCl(PPh3)2 (Equation 2b, X = Cl). RhCl(PPh3)2 is a very reactive, low-concentration intermediate which is likely to be solvated (see Equation 2a) (17). 

Equations 2c and 2d show the acyl-alkyl migration and reductive elimination steps, respectively. There is good evidence that this same mechanistic scheme applies to the decarbonylation of aldehydes (see Equation set 2, X = H), although in this case reaction intermediates have not been isolated (3, 5, 9, 18). Additionally, evidence exists that the rate-determining step is oxidative addition for aldehyde decarbonylation (see Equation 2b, X = H) (3, 9, 18). Several recent reports have shown that for some special aldehydes, oxidative addition of the carbonyl-hydrogen bond indeed does occur using rhodium(I) complexes (8,19). In these studies a stable chelate was formed after oxidative addition that enabled isolation and characterization of the products (8, 19). 

Firstly, an oxidative addition of the aldehyde to the Rh(I) species takes place to form the intermediate Rh(III)-species 37, followed by retro-CO-insertion. In the reductive elimination, the decarbonylated product 26 and the catalyst species 39 are formed the latter can lose CO to regenerate the catalytically active Rh(I)-species. 

Palladium chloride and metallic palladium are useful for carbonylating olefinic and acetylenic compounds. Further, palladium is active for decarbonylation of aldehydes and acyl halides. Homogeneous decarbonylation of aldehydes and acyl halides and carbonylation of alkyl halides were carried out smoothly using rhodium complexes. An acyl-rhodium complex, thought to be an intermediate in decarbonylation, was isolated by the oxidative addition of acyl halide to chlorotris(triphenylphosphine)rhodium. The mechanisms of these carbonylation and decarbonylation reactions are discussed. 

For example, the acyl-aryloxo bond cleavage (type b) is shown by the reaction of Ni(cod)2 with phenyl propionate in the presence of PPhs (Scheme 3.34) or 2,2 -bipyridine [65]. The reaction products are ethylene, phenol, and (car-bonyl)nickel complex. Formation of these products is conveniently understood by initial oxidative addition of EtC(0)-0Ph followed by decarbonylation, )S-hydrogen elimination and reductive elimination, though (acyl)(aryloxo)nickel(II) intermediate is not isolated. However, such an intermediate is isolated by the selective insertion of CO into the (alkyl)(aryloxo)nickel (or palladium) complexes, which smoothly affords esters by reductive elimination promoted by electron deficient olehns. The results suggest that the oxidative addition involving C-0 bond cleavage is essentially reversible. 

The stoichiometric decarbonylation reaction begins with the oxidative addition of acid chloride to RhCl(PPh3)2 (Equation 7b), which is presumably a solvent-stabilized, very reactive intermediate/ Tolman " and Halpern " have presented kinetic evidence for the importance of RhCl(PPh3)2 in the catalytic hydrogenation of olefins by RhCl(PPh3)3. In addition, the solvated species, RhCl(S)(PPh3)2 (where S = DMF, acetonitrile), was observed in the stoichiometric decarbonylation of aldehydes vide infra). 

The only Rh(III) intermediate that has been isolated from the reaction sequence is given below. This compound was prepared by reacting an aldehyde, 8-quinoline carboxaldehyde, with RhCl(PPh3)3. The ability of the aldehyde to form a chelate after oxidative addition has occurred (termed chelate trapping by the author) imparted sufficient stability to the compound to allow isolation and characterization. Prolonged heating in refluxing xylene yields the expected decarbonylation products. Other examples of oxidative addition of aldehydes to Rh(I) complexes are presented in Chapter 7, Section 4. 

Important reactive metal-containing intermediates (e.g., metal-benzyne complexes, MtC02" ) and processes (e.g., decarbonylation, oxidation, reductive addition) of practical interest have been characterized using various mass spectral methods, and provide insight as to the mechanisms of organometallic reactions. 

On the other hand, the oxidative addition of aliphatic acid chlorides occurs in the absence of alkyne, but the oxidative addition complex could not be isolated due to fast decarbonylation followed by facile /1-hydrogen elimination. The decarbonylation of carboxylic acid was reported with palladium catalysts as well [47-56], In general, the reactions to acid anhydride as the intermediate need relatively high temperatures. 

The mechanism as proposed by Tsuji and co-workers proceeds first by oxidative addition to form an acylpalladium(II) chloride spedes. Exchange of hydrogen on the palladium and release of HCl produces the acylpalladium(II) hydride, which undergoes reductive elimination to the aldehyde. Without hydrogen present the intermediate, under forcing conditions, can undergo decarbonylation to the olefin via /3-hydiide elimination of the alkylpalladium intermediate or to chlorobenzene via the reductive elimination of the arylpalladium chloride. 

The mechanism (Scheme 48) ° is expected to proceed through the acylpalladium species much as in the Rosenmund reduction. Indeed, the acyl complex 56 from oxidative addition of vinyl chloride with Pd(CO)(Ph3P)3 was isolated (Scheme 49). " The reaction of acid chlorides with the same catalyst provides aldehydes. However, aliphatic acid chlorides do not reduce effectively. The phosphine ligands present in the Heck acylpalladium intermediate are thought to be the cause, allowing decarbonylation and elimination to occur. Interestingly, the formylation will not occur with the Rosenmund catalyst. 

The complexity of this reaction scheme already points out the necessary time-balance between oxidative addition, CO insertion versus deinsertion (decarbonylation), acylpalladation, carbopaUadation, and dehydropalladation events to have a selective acylpallada-tion-type reaction. This has been more recently addressed by selecting acyl chlorides that could not readily undergo dehydropalladation, hence stabilizing the organopalladium intermediates to favor their intramolecular reaction with alkenes. For instance, Pd(OAc)2 catalyzes the reaction of benzoyl chloride derivatives with enol ethers (R = OEt, Scheme 3) to give the corresponding acylpalladation products typically in 40-77% yields. " ... 

Under certain conditions in the presence of other substrates, particularly of bases, facile decarbonylation of the aroylpalladium halides occurs to form arylpalladium halides as intermediates, which are usually prepared directly by oxidative addition of aryl... 

Aldehydes undergo a decarbonylation reaction by action of a transition metal complex [17]. For example, benzaldehyde was decarbonylated with Wilkinson complex to furnish benzene along with a rhodium carbonyl complex 47 (Scheme 7.14) [17e,f]. Oxidative addition of the aldehydic C-H bond to rhodium, migratory deinsertion of CO, and reductive elimination operate in sequence for the decarbonylation reaction. Decanal was also decarbonylated to furnish a mixture of nonane and nonene, which were produced via the alkylpalladium intermediate 48 (Scheme 7.15) [17d]. 

Besides halides and triflates, other electrophiles can be applied to Heck reactions. The first classical alternative was diazonium salts. Reactions proceed in the absence of phosphine (partly due to the fact that phosphines result in uncontrolled decomposition of the diazonium salt). The Heck reaction using these species can be useful in cases when mild conditions are required. Alternatively, iodonium salts behave in a similar manner to diazonium salts and show better tolerance to bases. " The reactions take place at ambient temperature and so are once again most useful in situations when mild conditions are required. Some main group metallie eompounds such as lead(IV) and thallium(III) have also been shown to undergo Heck-type chemistry and can be useful in specific cases. Of particular interest is the fact that acid chlorides and anhydrides can be employed in Heck chemistry, the use which was pioneered by Blaser and Spencer in 1982. " The process involves oxidative addition of palladium into the C-X bond followed by decarbonylation to yield the intermediate ArPdX species, de Vries has exploited this reaction, demonstrating the use of benzoic anhydride (105) as an effective arylating agent. ... 

Oxidative addition of a low-valent nickel catalyst to a C—N bond yields a reactive intermediate C—Ni—N, which undergoes carboamination with an alkyne via simultaneous C—C and C—N bond formation. However, this type of transformation has not been explored in detail, despite its usefulness for the preparation of nitrogen-containing compounds, because the C—N bond is less reactive toward oxidative addition of transition metals than are C—O and C—S bonds. In this context, Kajita et al. found that oxidative addition of phthalimide 19 to Ni(0), followed by decarbonylation, yields azanickelacycle 20, which then reacts with alkynes to furnish isoquinolones 21 (Scheme 12.9) [12]. For example, the reaction of iV-pyridylphthalimide with 4-octyne successfully afforded isoquinolone derivative 22. iV-Pyrrolylphthalimides... 

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