Stoichiometric Decarbonylation of Aldehydes

The mechanism of aldehyde decarbonylation is thought to follow the established mechanism for acyl halide decarbonylation discussed in the previous section (Equation 7, where A = H). Several observations support this idea, even though intermediates are much more labile than those of the acid chloride system. 

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. 

The stereochemistry of aldehyde decarbonylation has received much attention. Walborski and Allen have shown that the decarbonylation of optically active aldehydes proceeds with 93% retention of configuration,as shown in Equation 11  

Retention of configuation at the a-carbon was also observed with acid chlorides vide supra), but aldehydes give a product of higher optical purity 

Decarbonylation of deuteroaldehydes has been used for specific deuteration of compounds. The incorporation of deuterium into the products as given in reaction 12 occurs with high yield. 

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The rhodium complex [RhCl(PPh3)3] readily brings about stoichiometric decarbonylation of aldehydes, acyl halides and diketones. A typical aldehyde decarbonylation is illustrated by equation (69). a,3-Unsaturated aldehydes are decarbonylated stereospecifically (equation 70), while with chiral aldehydes the stereochemistry is largely retained (equation 71). ° ... 

A considerable amount of work has been performed over the last 15 years to determine the mechanism of acid chloride decarbonylation with RhCl(PPh3)3/ " Although the discovery of aldehyde decarbonylation preceded that of acid chlorides/ much more time has been spent on the acid chloride system because it is more easily studied. Many intermediates have been isolated and characterized (see Table 1). Even though the mechanism of the catalytic reaction is not well understood, the mechanism for the stoichiometric decarbonylation of acid chlorides has been proposed. However, the generally accepted mechanism has recently been challenged/ In this section, we will first review the stoichiometric decarbonylation mechanism for acid chlorides, followed by the stoichiometric decarbonylation of aldehydes. Finally, the mechanism of catalytic decarbonlyation of acid chlorides and aldehydes will be discussed. 

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 results discussed above lead to the conclusion that the mechanism of decarbonylation of aldehydes is very similar to that postulated for the decarbonylation of acid chlorides. However, kinetic studies of the reaction show that a different rate -limiting step is operative with aldehydes. With acid chlorides, the rate-limiting step is thought to be migration or reductive elimination, depending on the R-group. " A detailed kinetic study on the stoichiometric decarbonylation of aldehydes with RhCl(PPh3)3 has... 

As discussed in the previous sections, the stoichiometric decarbonylation of aldehydes is effectively carried out by using RhCl(PPh3)3. It would... 

Decarbonylation of aldehydes and acid halides is an important synthetic reaction (i, 2) and using various transition-metal complexes as stoichiometric or catalytic reagents for this process has... 

First attempts for decarbonylation of aldehydes were conducted with transition-metal complexes in stoichiometric or near-stoichiometric reactions. Later on, also active catalysts and milder reaction protocols could be identified. The reaction proceeds best with electrophilic aldehydes and nucleophilic metal centers [5]. 

Decarbonylation of aldehydes is frequently used in synthetic organic chemistry [19]. Aromatic aldehydes and enals, but also saturated aldehydes, have been shortened by one C atom with this transformation. In most cases, rhodium complexes were used in a stoichiometric reaction, but also catalytic transformations have been described [20]. It was found that RhCl3-3H20 modified with dppp was less air-sensitive than [Rh(COD)Cl]2 (COD = 1,5-cyclooctadiene) modified with dppp or the Wilkinson complex and therefore better suited for lab-scale experiments [21]. While using the Wilkinson complex, strictly oxygen-free conditions were essential for the success. Besides homogeneous rhodium catalysts, also supported complexes were suggested recently [22]. The reaction in ionic liquids is a possibility to recycle the precious rhodium complex [23]. 

The decarbonylation of aldehydes is a remarkable reaction in the framework of hydroformylation. The reaction is mediated by metals usually applied in hydroformylation. In most cases, severe conditions are necessary in order to remove the formyl group from all kinds of aldehydes. The reaction may be responsible for some unusual results in hydroformylation, such as a posteriori isomerization of aldehydes. Noteworthy, the decarbonylation has developed from a curiosity in stoichiometric organometallic chemistry to a well-established method in organic synthesis. Only recently, a very smooth protocol for the dehydrocarbonylation (dehydroformylation) was disclosed. It may have a large impact on the synthesis of complicated natural compounds without using any protective group chemistry. 

Since the early 2000s, different sources of CO have been explored and applied to carbonylation reactions for laboratory organic synthesis. For example, the use a stoichiometric amount of metal-carbonyl complexes, thermolysis of formic acid at high temperature, and the use of aldehydes via decarbonylation have been investigated. For the use of metal-carbonyl complexes and formaldehyde as carbonyl source, it has been shown that microwave irradiation greatly accelerates the process. ... 

The decarbonylation of acyl halides and aldehydes proceeds under mild conditions to give aryl halides ArX and arenes ArH with a stoichiometric amount of RhCl(Ph3P)3. At the same time, RhCl(CO)(Ph3P)2 is formed which is inactive at moderate temperatures, and the reaction is stoichiometric [245-248],... 

The insertion of CO into M—C bonds to give acyls is reversible (Section 21-5). However, CO can be irreversibly removed from organic molecules these reactions involve acyl intermediates. Thus aldehydes, acyl, and aroyl halides can be decarbo-nylated either stoichiometrically or catalytically by complexes such as RhCl(PPh3)3 or RhCl(CO)(PPh3)2. An example of a stoichiometric decarbonylation is... 

Employing a stoichiometric quantity of diphenylphospho-ryl azide as a carbon monoxide receptor allows the reaction to proceed at a lower temperature. The azide reacts with frani -[RhCl(CO)(PPh3)2] to form a cyanate at room temperature. Alkene formation is suppressed when this reagent is employed, but higher aldehydes are not decarbonylated in its presence. ... 

One of the standard methods for the preparation of aldehydes involves the reduction of acid halides. A variety of stoichiometric reducing systems are available for this transfomiation, which include NaAlH(OBu-r)3, LiAlHfOBu-O.i, NaBHfOMe). Catalytic hydrogenation with H2 and Pd on carbon is also a popular method. In contrast, methods based on the radical reduction of acyl halides are synthetically less important. Radical reduction methods involve generation and subsequent hydrogen abstraction as key steps, which is complicated by decarbonylation of the intermediate acyl radicals. The first example in Scheme 4-1 shows that this competitive reaction is temperature dependent, where an acyl radical is generated from an acyl phenyl selenide via the abstraction of a phenylseleno group by tributyltin radical [5]. 

Aldehydes are stoichiometrically decarbonylated by reaction with (XL) under mild conditions (77, 98,110,113). Aromatic aldehydes yield aromatic hydrocarbons whereas aliphatic aldehydes form saturated hydrocarbons and olefins. The latter minor products can be considered to arise from a reverse hydroformylation reaction. The initial step of this reaction is probably the oxidative addition of an aldehyde C—H bond to the rhodium(I) complex. However, a stable adduct of this type has not yet been reported. The driving force in these reactions is derived from the stability of the carbonyl (LXIX). 

In the foregoing, the formation of organic molecules on transition metal complexes is explained by stepwise processes of oxidative addition, insertion, and reductive elimination. One typical example, which can be clearly explained in this way, are the carbonylation and decarbonylation reactions catalyzed by rhodium complexes 10-137). Tsuji and Ohno found that RhCl(PPh3)3 decarbonylates aldehydes and acyl halides under mild conditions stoichiometrically. Also this complex and RhCl(CO) (PPh3)2 are active for the catalytic decarbonylation at high temperature. 

Chiral aldehydes can be decarbonylated under full retention of the configuration, but occasionally partial racemization may take place [10]. In general, decarbonylation with stoichiometric rhodium-arylphosphine complexes can be achieved at ambient temperature, but usually the catalytic version requires more severe conditions. Goldman et al. [11] discovered that trialkylphosphines form significantly more active catalysts, as exemplified with the binuclear complex [Rh(PMe3)(CO)Cl]2. The complex operates even at room temperature. A similar effect was also noted with iridium-phosphine catalysts [12]. 

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